The task of an airplane designer is to generate a design for an airplane that flies successfully and executes the user’s mission. The manufacturer must be able to build the airplane and sell them in sufficient numbers to generate sufficient revenue to stay in business and make a profit.

The designer must consider all three factors: Mission, Manufacturing and Marketing.

We have already considered “mission” in some detail in previous editions of Wind Tunnel, so we now turn our attention to the other “M’s.”

Manufacturing

The first choice the designer must make is what materials the airplane will be made of and what manufacturing processes will be needed to fabricate the parts and assemble the airplane.

Usually there will be more than one set of materials that will meet the strength, durability and surface quality requirements for the airplane. The choice of materials will, in turn, determine the type of manufacturing technology, tooling and workforce skills needed to build the airplane.

Materials Cost and Availability

The cost and availability of materials is a primary design consideration. It’s much cheaper to make an airplane of readily available standard materials and to design the parts so they can be fabricated from standard stock. Sometimes, even the detailed choice of material can make a big difference. For example, many homebuilt and kit airplanes use 6061-T6 aluminum, which is a very commonly used alloy, instead of 2024 or 7075 alloys, which are typically aerospace-only. The aerospace alloys have higher yield strength than 6061-T6 but cost two to three times as much per pound. For the majority of light plane structures, the extra strength does not offset the higher cost of the aerospace alloys. The lower yield strength of the 6061-T6 is more than adequate for most lightly loaded areas like skins, and in more highly loaded areas, a somewhat thicker 6061-T6 part can safely replace a part made of 2024 or 7075 alloy.

Any part that requires specially sourced material will be very expensive. High-end military airplanes and large transport airplanes, for example, need specialized metal forgings for major structural components like main bulkheads and wing primary structure. These are prohibitively expensive for all but the most sophisticated and highly produced airplanes.

Skill Base

There is a wide variation of the skills required and the skills the designer can expect the fabricators of the airplane to have.

A homebuilder may start with very little skill and learn as they go. The designer of a kit airplane or one intended to be built from scratch by a single individual must take this into account. The skills needed to fabricate the airplane should be relatively easy to learn, and the design should have enough reserve and redundancy so that minor fabrication and assembly imperfections will not compromise the safety of the completed airplane.

Higher end and series production require more and more specialized skills.

Changing materials requires retraining both in engineering and fabrication. The skills needed to build a fabric-covered airplane with a wooden wing structure and steel-tube fuselage structure are different than the skills needed to build an all-metal airplane with sheet metal skins. High-end metal airplanes (fighters and transports) require specialized skills to fabricate the large machined parts like primary bulkheads and wing spars, as well as the many complex machined parts in the systems. Composites fabrication requires yet another skill set that is entirely different from metal fabrication.

This phenomenon affects the adoption of new materials and manufacturing processes.

It can also have some interesting effects on the availability and operational effectiveness of an airplane. One good example of this is the persistence of wood and fabric in WW-II airframes. At the time of the Battle of Britain the RAF was flying Hawker Hurricanes and Supermarine Spitfires to defend England. The Hurricane was already obsolescent: It had a steel-tube fuselage structure with wooden formers and stringers shaping its fabric covering. The Spitfire was of all-metal construction. The Hurricane’s “obsolete” structure meant that it could be built and rapidly repaired by a large number of British workers using simple, widely available tools, while production of the Spitfire required newly developed skills and complex, specialized tooling. As a result, the RAF initially had more Hurricanes than Spitfires and was able to keep them flying more easily. In fact, Hurricanes shot down more enemy airplanes than Spitfires during the Battle of Britain.

Times change, and in today’s world the skills needed to do the woodwork on a Hurricane are rare, while aircraft metalworking skills and tools are common. Because of this, while Hurricanes were easier and quicker to build originally, it now takes far more time and effort to restore a Hurricane than a Spitfire.

Wooden construction probably reached its apex with the DH Mosquito, which was specifically designed to take advantage of the availability of wood and woodworkers to augment aircraft production beyond what could be achieved with metal-only airframes at the time. De Havilland persisted with wood into the jet age. The fuselage of the Vampire jet fighter is made of wood in much the same way as that of the Mosquito.

Capital Plant and Cost of Tooling

Another important consideration for the designer is the nonrecurring expense (NRE) of setting up the manufacturing facility and the machinery and tooling needed to fabricate the parts of the airplane.

There is often a trade-off between the labor hours required to fabricate each part and the cost of the tooling used to make it. The best approach will depend on how many airplanes you plan to produce and how fast you plan to produce them. The more airplanes and the higher the production rate, the more value there is in investing in tooling to automate production and minimize touch labor.

The cost of tooling can also affect the configuration and design of the airplane. Design features that can only be made using expensive, specialized tools should only be adopted if they provide a worthwhile advantage over simpler-to-make alternatives.

For example, forming compound curves in metal requires stretch-forming, which is very labor-intensive if it is done by hand and requires expensive precision tooling to automate. This is why the majority of metal airplane skins are lofted to be flat-wrapped from sheet material.

Another example where tooling costs affect configuration is the common use of constant-chord metal wings on light airplanes. Fabricating metal ribs requires the metal to be shaped over a precisely shaped form block. Each rib shape requires its own specific form. All of the ribs for a constant-chord wing can be made using the same tooling, while each rib for a tapered wing requires its own unique tool.

An optimized tapered wing will be lighter and more aerodynamically efficient than a constant-chord wing, but often alleviating the performance penalty of the constant-chord wing does not justify the extra cost of tooling to make the tapered wing.

Manufacturing Technology

Changes in manufacturing technology periodically change how we build airplanes and open up new possibilities. Sometimes, they also rehabilitate an older approach by significantly changing the cost and/or skill required to make parts.

The adoption of composites by the homebuilt and kit airplane community was driven primarily by tooling and fabrication considerations. The improvements in aerodynamic performance and additional freedom to shape airplanes optimally was a side benefit.

Hand-fabricating traditional wood and steel-tube aircraft structures or sheet metal structures is time-intensive, particularly if one has to hand-form metal ribs and bulkheads.

Moldless composite construction, pioneered by Ken Rand on the KR-1 and Burt Rutan with the VariEze, made it much easier for a homebuilder to build an aerodynamically smooth wing relatively quickly with a minimum of tooling.

Molded composites made kits like the Glasair and Lancair, with a high degree of prefabrication at the factory, possible. Producing these kits required a set of molds for the large parts, but these were far less expensive than the tooling needed to fabricate metal airplane parts using traditional methods.

All-metal airplane parts took many hours to fabricate by hand and expensive tooling to manufacture. Flat metal parts required expensive punch dies to manufacture in quantity, and every unique part needed its own tool to make.

Computer-aided design (CAD) and numerically controlled (NC) machinery revolutionized metal airplane fabrication. Flat metal parts can be cut using a variety of NC cutting systems including water-jet, NC routers or laser cutting. The machinery that does the cutting is not configuration-specific to the parts. Standard industrial machines can be programmed to make the parts, and it’s not even necessary for the airplane manufacturer to own the machines because it’s relatively easy to contract with commercial metal-cutting shops to make parts.

More sophisticated NC machinery can make machined parts and fittings.

Very capable CAD software is now available that will run on most personal computers, so the overall cost of access to CAD and NC fabrication has dropped to the point where its available to the individual designer/builder.

This numerical manufacturing revolution fundamentally changed the kit market. In much the same way that composite kits replaced scratch-built metal or wood-and-steel-tube airplanes as the primary choice of homebuilders, the highly prefabricated metal kits that NC manufacturing made possible have overtaken composite kits in sales and completions.

Next month, we will take a look at the third “M”: Marketing. Many design choices are driven not by technical considerations, but by what will appeal to potential buyers.

The post Manufacturing Considerations appeared first on KITPLANES.

Most of our recent discussions on the process of designing an airplane were based on the assumption that the airplane has a generally conventional configuration. This certainly covers the vast majority of airplanes, which are monoplanes with aft-mounted tails.

Last month we took a look at how to approach innovation in a general sense and also touched on some unconventional configurations. Now let’s circle back to the very beginning of the design process and turn our attention to configuring the airplane.

Every airplane is designed to perform a specific mission. The design process starts with the needs of the operator of the airplane. The designer’s goal is to lay out an airplane that meets those needs.

The first task is to choose the overall configuration of the airplane. The configuration must fit the mission. A configuration that is appropriate for one mission might be a poor choice for another.

Designers will sometimes choose the basic configuration at the beginning of the process and not deviate from that initial choice. If the configuration they choose is relatively conventional and conservative, and the airplane is being designed around a well-understood mission, this can be the quickest and lowest-cost way to go—and it often works fine. With good detail design, this approach will usually lead to a successful airplane. It is unlikely to produce an airplane that is dramatically superior to other airplanes designed to perform the same mission, although it might be enough of an improvement to be a commercially successful product.

The risk in choosing only one configuration at the beginning is getting it wrong and pushing ahead by trying to make an unsuitable configuration perform the mission. It’s particularly risky to start with a single unconventional approach without considering alternatives.

Analyzing Different Configuration Concepts

Many design efforts start by considering more than one configuration. At the outset of the program it may not be obvious which configuration is the best choice. There are several configurations that can produce airplanes that can perform the mission, but each one offers some potential advantage. It’s not initially clear which one would lead to the best airplane.

This is common on larger programs, although it’s not limited to those. The design team will start by laying out two or more configurations at the beginning of the program. These candidate concepts are then refined and analyzed in parallel until the relative suitability or virtue for the mission of the several options becomes evident. At this point in the process, the preferred configuration will be selected and no further work will be done on the competing concepts. This is referred to as the “down-select.” After the down-select, the design team moves on to refining the chosen concept.

The criteria used for the down-select can vary. It’s rare that the discriminator between concepts is absolute kinetic performance. Most of the time, the goal of the project is not to produce the fastest airplane or the one that carries the most load or that flies the highest.

It’s important to remember that a design that seeks to provide more absolute performance than the customer asks for, or maximizes performance in an area the customer does not consider important, is not likely to be a winner. Extra performance is likely to increase cost, and doing better at something the customer does not care about gains nothing.

The required performance is defined in the mission specification, and all of the candidate designs should meet that specification. Sometimes, only one of the candidate configurations leads to an airplane that can perform the required mission. In such a case, the down-select is obvious: We choose the only concept that can meet the requirements.

This is relatively rare, however.

Most of the time we can generate multiple candidate designs that meet requirements. Once we have several different designs that are properly sized and will perform the mission, we can down-select based on other considerations and choose the best for our customer. Several common discriminators are acquisition cost, fuel consumption, gross weight, manufacturability and operational considerations.

It’s likely that each of the candidate designs will be better in some areas and worse in others. It’s rare that one is overwhelmingly superior or inferior in all areas of concern. Which concept to proceed with depends on the relative importance of the discriminators to the end user or buyer of the airplane.

For example, cost is often a big driver. Which design is the better concept may depend on whether the buyers of the airplane are more concerned with initial price or operating cost. An airplane optimized for minimum selling price will probably lack some of the aerodynamic refinement needed to minimize drag and will have other compromises aimed at making it cheaper to manufacture. Accordingly, it will likely burn more fuel to fly the mission than an airplane optimized for maximum fuel efficiency.

A customer who is primarily concerned about financing the initial purchase of the airplane will prefer the lowest-price airplane, while a customer who is concerned about the long-term cost of operating the airplane will opt for the more fuel-efficient design and accept the higher purchase price in return for the long-term savings in direct operating costs.

This is one of the reasons there is a market for speed and performance mods for certified airplanes. The initial manufacturer made a cost-based decision that some amount of extra performance was not worth the extra selling price to the initial buyers of their airplane. They decided that they would sell more airplanes at a lower selling price, even with the slightly lower performance.

Once the airplanes are in service, some owners are willing to spend the extra money to get the performance increase and buy STC’d performance modifications from aftermarket vendors.

Principles of Down-Selecting

As we go through the process of defining options and down-selecting to a preferred configuration, it’s important to remember fundamental principles.

First: All airplanes, regardless of their configuration, operate under the same laws of physics. We can determine the performance of each design using the same type of analysis. There are some relatively simple first-principle parameters that can be used to compare physically dissimilar airplane configurations early in the design process and can help guide design decisions. As the designs mature, more detailed analysis will give a more accurate evaluation of each candidate, but the analyses themselves will be the same for all.

It’s important to do a full, independent analysis of each candidate concept. A common mistake is to directly apply a performance increment that is claimed for a new concept instead of determining its performance directly by analysis or experiment.

This often happens when an “advanced” research project reports a positive result. We might read, for example, that a new advanced airfoil showed a 15% drag reduction. It’s tempting to assume that if we simply use this airfoil on an otherwise conventional airplane it will reduce drag by the reported 15%. This may or may not be true, since the effect of the airfoil on the drag of the airplane is a function of multiple parameters that determine the flight condition at which the airplane operates. If the flight condition of the airplane does not match the flight condition the airfoil was designed to operate at, its aerodynamic performance will be different, and the reported drag reduction will probably not appear.

Analyzing each candidate configuration independently is the proper way to get to a meaningful comparison between them.

Second: The suitability of a configuration relates directly to how well it performs the mission. What may be a great configuration for one mission may perform poorly for another. Once we have a sufficiently complete analysis of each candidate configuration designed for the specific mission at hand, we have the information we need to select the one that will best meet the needs of the customer who will buy and operate the airplane.

The post Choosing the Configuration appeared first on KITPLANES.

A designer must make a large number of decisions in the process of designing an airplane. These range from top-level decisions, like the overall configuration or structural materials, to detail decisions about each system and component of the airplane. The overall goal is to produce a design for a machine that works well and performs its mission effectively.

In addition to the performance of the completed machine, the designer must consider the cost of materials and components and how the parts of the airplane will be manufactured. A designer must weigh two propositions when making these decisions:

First, conventional practice is conventional because it works. And second, innovation can be the path to something better than what can be achieved with conventional practice. This is not an easy decision to make.

Every step between the Wright Flyer and modern airplanes started out as an innovation. On the other hand there have been many wrong turns and failures along the way.

The dilemma is that, while innovation can produce a better or more cost-effective design, innovating also introduces a significant amount of technical risk into the project. At the core of this problem is the newness of the innovation. It is less well-understood than the conventional approach. There is less experience and data available to guide the design, and there may be undiscovered problems or drawbacks to the new concept.

Because of this, it’s important to realize that even though it is very common to see designs advertised and praised for being innovative, innovation is not a virtue in and of itself. “Innovative” is another word for “different.” In order to be a virtue, the innovation must provide a meaningful advantage over a conventional design.

Important Questions

For any innovation, a designer should consider the following questions:

In what way will this concept make the airplane better than a conventional one?

Consider how the innovation is expected to improve the airplane. Will it improve performance in an area that is important to the user of the airplane? Will it reduce the cost of building, owning or operating the airplane?

If there is no meaningful improvement in anything important to either the manufacturer or end user of the airplane, then the innovation is not of any value and should be dropped.

If there is potential for improvement, then ask:

If the innovation works as well as it could, how much improvement does it offer at best?

Every innovation adds risk to the project and, even if it ends up reducing costs in the long run, it could significantly increase the cost of engineering it properly the first time. Because of this, the decision to use or drop an innovation comes down to a trade between the value of the improvement it can provide against the increase in risk and, potentially, cost.

The designer must determine if the overall benefit of the innovation is large enough to merit undertaking the additional effort needed to address the technical risk it adds.

It’s also important to consider all aspects of how the innovation affects the airplane. This is particularly true of major structural and aerodynamic configuration concepts.

What is the purported advantage of this concept, and even if the advantage is real, is there an offsetting penalty that reduces or eliminates the advantage?

A very common mistake is to look at the positive only and not consider the negative. One example of this is canard configurations.

When the modern canard concept first appeared, two advantages were claimed. First, that trimming the airplane with an up-load on the foreplane rather than a down-load on a tail was more efficient. And second, that the canard configuration packaged better than an aft-tail configuration, making the fuselage lighter and reducing drag.

Neither of these claims is false in and of itself, but there were offsetting negatives. A canard configuration cannot trim powerful high-lift flaps, and although the fuselage of a canard airplane is shorter than that of an aft-tail airplane, the short moment arm for the vertical fins means that the fins must be larger to provide adequate directional stability. Both of these factors offset the advantages of the configuration. While there are successful canard airplanes that work very well, the overall advantage of the canard configuration proved to be much less than an assessment based only on its positives would suggest.

Composites followed a similar trajectory. The strength-to-weight ratio of advanced composite materials is quite a bit higher than metals and offers the potential for lighter, stiffer structures. This proved to be true in practice, but because of several detail design issues with composites, the weight savings achieved was less than one might predict by simply looking at the relative physical properties of the materials.

If, after consideration of both positives and negatives, the innovation still appears to offer a useful advantage, there is one more major question to resolve:

How big is the risk and what is the alternative if the innovation fails to deliver?

As we have already seen, the task of making an innovation work is harder than making a conventional design work. This is because there is less body of experience, data and established design practice to rely on. The amount of engineering work needed to complete a workable design is greater because the design team will have to fill in all of the unknowns along the way.

At the outset of an innovative design, it’s known up front there are some issues the designers will have to work out. Sometimes, however, things pop up that they did not anticipate. These “unknown unknowns” are the most problematic since not only must they be solved, but the methods of solving them must be invented on the fly because the designers did not anticipate them. They are also a significant part of the risk because whether they are solvable was not considered at the outset, and there is a chance that one could prove to be a showstopper.

The designer must make the best assessment of the overall technical risk inherent in making the innovation work and consider whether the advantage offered by the innovation merits taking the risk.

Finally, the designer must consider what alternatives will be available to save the project if an innovation fails to deliver. How easy it is to change to a different approach or replace an innovative design with a conventional one differs widely depending on the type of innovation.

For example, a system that is self-contained and a relatively small part of the airplane is relatively easy to replace. One example is the use of modern high-energy-density batteries to save weight on emergency or backup power systems. As Boeing learned with the battery problems on the 787, these batteries can have significant issues with catching fire when they malfunction. When this problem cropped up on the 787, Boeing had to redesign the battery system and battery containment to eliminate the fire hazard. This cost some weight, lessening the advantage of the high-performance batteries, but did not severely impact the overall program or viability of the airplane.

On the other hand, if the airplane has an unconventional overall configuration, it is extremely difficult to recover if a fundamental flaw in the concept appears during development.

Summary

Innovative design can be the path to a better design, but it brings significant risk with it.

Innovation is not a virtue in and of itself. Unconventional or innovative concepts should have advantages that directly help in areas that are valuable to the manufacturer and/or end user of the airplane.

Innovate only where it can produce significant gains, and don’t pile multiple innovations on top of each other. Every innovation brings risk, so take that risk only where it gives the most leverage.

Sometimes, a conventional low-risk design is the best approach and sometimes it is worth taking the risk to innovate. The risk/potential trade is a fundamental discriminator between concepts and one of the most important things a designer must consider.

Photos: Jonathan and Julia Apfelbaum and Zac Heald.

The post Design Innovation appeared first on KITPLANES.

For the past few months, we have been concentrating on the primary cooling airflow and flow path for the air that cools the cylinders of the engine and carries away most of the waste heat generated by combustion of the fuel. This primary cooling system is a critical part of the engine installation, but the primary cooling flow is not the only cooling flow or internal airflow needed by the airplane.

There are components and systems aboard, both within the engine compartment and the cabin, that require their own cooling flow to keep them within their operating temperature range. Cabin ventilation is also necessary and will require an airflow system that is entirely separate from the engine compartment cooling and ventilation airflow.

Secondary Cooling Within the Engine Installation

As we have already seen, the majority of the waste heat generated by the engine is absorbed and carried overboard by the primary cooling flow air that flows over the cooling fins on the cylinders of the engine. The exhaust system is also cooled somewhat by the primary cooling flow, rejecting heat into the air in the lower plenum before it flows overboard through the outlet.

There are two other cooling tasks that need to be addressed within the engine compartment.

First: The oil that circulates through the engine to lubricate it absorbs heat from the engine itself. On most installations the oil cannot reject enough heat through the walls of the sump to stay at an acceptable temperature, so a separate heat exchanger is needed to cool the oil.

Second: There are multiple engine accessories installed along with the engine that can both generate waste heat and absorb heat radiated by the engine. All of these have temperature limits and may require supplemental cooling if simple convection into the primary cooling flow is not sufficient.

Oil Coolers

Many airplane engines have oil coolers to control oil temperature and carry away additional waste heat from the engine. An oil cooler is a heat exchanger much like a radiator. Oil flows out of the sump, through the oil cooler and back into the sump. Within the cooler oil flows through passages in a set of cooling fins. Cooling air flows between the fins and absorbs heat from the oil.

An oil cooler requires a flow system much like that for an engine cylinder. There must be a pressure differential between the entry side of the cooling air path through the fins and the exit side. The cooler should be sealed around its sides so all of the available air is directed through the fins.

Two Common Installations

There are two common approaches to oil cooler installations. The first is to incorporate the oil cooler within the primary cooling air system. The second is to provide the cooler with its own separate flow path.

In the first approach, the cooler is mounted to one of the primary baffles that form the upper plenum. There is a hole in the baffle that matches the face of the oil cooler. This hole allows high-pressure air from the upper plenum to flow through the cooler into the lower plenum.

In this type of installation, the oil cooler acts aerodynamically like an additional engine cylinder. The primary advantage of this installation is that it is relatively simple and light. There are no extra parts required beyond the cooler and its plumbing.

The major disadvantage of this simple installation is that it uses air from the upper plenum. The oil cooler acts like a hole that bleeds off pressure from the upper plenum, reducing the air available to cool the cylinders. This increases the total amount of cooling flow going through the primary system and can make it difficult to balance the cooling flows between the oil cooler and the cylinders. Any change to one affects the others, so it’s common to increase the size of the primary inlets to a bit more than the minimum required so the system is tolerant of minor imbalances between the flow through the cylinders and the oil cooler. While this is a robust solution from a cooling point of view, it does come with a drag penalty.

Dedicated Oil Cooler Flow Path

A second approach is for the oil cooler to have its own separate airflow path. A dedicated inlet feeds cooling air into a duct leading to the oil cooler. After the air flows through the fins of the oil cooler, it flows downstream to its own independent outlet.

The big advantage of this approach is that it does not affect the cooling of the cylinders and allows more freedom to optimize the airflow through the oil cooler. It’s also compatible with a variable-geometry outlet for the oil cooling flow, which allows the pilot or engine control system to control the oil temperature directly and minimize the drag of the flow through the oil cooler.

The downside to this approach is that it is more complex and slightly heavier than the simple installation inside the engine compartment. It requires an additional inlet, ducting and outlet, all of which have some weight and occupy volume that must be accommodated within the cowling.

Accessory Cooling

In addition to the engine itself, there are other components within the engine compartment that are vital to the proper functioning of the engine and the airplane’s systems. Among these are the magnetos, alternator, electronic ignition components and voltage regulators. All of them need to be kept within an operating temperature range to function properly.

Because these items are typically installed inside the cowling, on or near the engine itself, they absorb some waste heat from the engine. Some also generate their own waste heat.

Simple convection to the air in the plenums is often insufficient to cool the accessories, so it is generally necessary to provide some cooling airflow over them. They do not get as hot as the engine itself or the oil, so a relatively simple system that blows some cool air over the accessories will suffice.

This is typically accomplished using “blast tubes.” A blast tube picks up high-pressure air from the upper plenum through a hole in a baffle or a forward-facing inlet and carries it to the components (in the lower plenum) that need cooling. The end of the blast tube is aimed at the components to direct a constant flow of cooling air over them.

Blast tubes work well and are a relatively simple and straightforward way to cool engine accessories. They do take some air from the upper plenum if they are drawing air through holes in the baffles, so they increase the amount of total cooling airflow. For best efficiency the blast tubes should be carefully sized so that they only draw the air actually needed to cool the accessories and don’t waste cooling air unnecessarily.

Next month we will turn our attention to areas other than the engine compartment that require cooling or ventilation airflow.

Photos: Dave Prizio & Marc Cook.

The post Secondary Flows appeared first on KITPLANES.

An airplane’s cooling system must keep the engine temperature within its operating range over the entire flight envelope and the entire range of outside air temperature (OAT) the airplane will encounter. This is not a simple task since both the amount of heat the engine must reject and the amount of airflow available to cool it vary dramatically depending on flight condition.

For airplanes with a relatively small speed range from stall to cruise, or airplanes where high-speed performance is not an important mission requirement, a fixed-geometry cooling system will work well. The inlets and outlets are sized to cool the engine properly at full power and low airspeed as is typical in a maximum-performance climb. In this situation, the engine is generating the maximum amount of waste heat, and the mass flow of cooling air through the system is at a minimum because of the low airspeed.

For a fixed-geometry set of inlets and outlets, the mass flow through the system is directly proportional to airspeed. As the airplane flies faster, more air will flow through the cowl. If the system is sized to provide sufficient cooling at minimum airspeed, rather than at any higher speed, there will be more cooling airflow than necessary. Since every bit of air that flows through the cowling exacts a drag penalty, the airflow that is excess to cooling needs causes an unnecessary drag penalty for that flight condition. The bigger the speed difference between climb and cruise, the more air coming on board at cruise will be excess to cooling needs.

In cruise, this situation is amplified because the airplane does not cruise at maximum power. Piston engine airplanes typically cruise at power settings between 65% and 75% power, and essentially all manufacturers quote cruise performance at 75% power.

So, for example, when a high-performance airplane that stalls at 60 knots and cruises at 180 knots is in cruise, it will have about three times the cooling mass flow available to carry away three-fourths of the heat that would be the case in a steep full-power climb. The drag penalty for this excess cooling flow can be large.

Variable Geometry

The way to reduce or eliminate excess cooling drag in cruise is to use a variable-geometry cooling system that can change shape to control the amount of air flowing through the cowling. We can do this either by adjusting the geometry of the inlet to control the amount of air it ingests directly, or we can adjust the geometry of the outlet, which controls how much air can leave the cowl and thus controls the total flow.

Inlets

Variable-geometry cooling inlets are rarely used as the intake for primary cooling air. For a typical engine installation with two inlets, one on each side of the spinner, it is difficult to integrate a mechanically simple variable-geometry inlet. For systems with a single inlet under the prop, it’s possible to use a hinged lower inlet lip to change the size of the inlet in flight. The Rutan Catbird use this type of variable-geometry inlet for cooling.

Openable scoops are also sometimes used to control the amount of secondary cooling airflow for oil coolers.

Outlets

The standard approach for giving the pilot control over the amount of cooling airflow is to use variable-geometry outlets. These can be designed to change both the size of the exit aperture and the air pressure at its exit. On such a system, the cooling air inlets are sized to provide adequate cooling air over the entire flight envelope. The outlets are sized to restrict cooling flow to the amount needed to cool the engine in cruise, and the variable-geometry feature is used to open up the outlets to enable greater cooling airflow at low airspeed and high power as is typical in climb.

Cowl Flaps

The most common form of variable-geometry outlet is based on the use of cowl flaps. A cowl flap is a portion of the outer lip of the outlet. It is hinged at its leading edge so that it can open outward into the external airstream.

Opening a cowl flap has two effects: It increases the size of the outlet aperture and it also deflects the oncoming external airstream outward from the skin of the airplane. Both of these effects work to increase the amount of cooling air flowing through the system. The larger exit aperture makes it possible for more air to exit the cowling for a given exit velocity.

The outward-deflected cowl flap acts as a dam to oncoming external flow. As the air negotiates the obstruction caused by the cowl flap, the air pressure on the forward-facing outer skin of the cowl flap increases and the air pressure behind the cowl flap decreases. This makes the external air pressure at the cooling air exit aperture lower than it would be without the deflection caused by the cowl flap. The lower air pressure at the cooling air exit increases the pressure differential between the high pressure in the upper plenum and the lower pressure at the exit. This pressure differential is what drives air through the cooling flow path, so increasing the differential increases the total cooling airflow.

Cowl flaps generate increased drag when they are opened. As we just saw, the air pressure on the forward-facing skin of the cowl flap is higher than the air pressure on its inner/aft face. While this pressure difference works to increase the cooling airflow, it also generates a net rearward (drag) force on the cowl flap itself. The flow behind the open cowl flap can also disrupt the flow over other components of the airplane that are farther downstream. It’s important to take the effect of the wake of the cowl flap and the exiting cooling flow into account when placing the cooling air outlets on the airplane to ensure that there are no serious detrimental interference effects with the cowl flaps open.

Airplanes that need variable-geometry cooling systems are typically higher-performance airplanes with high power-to-weight ratios to give them the high cruise speed they are designed to achieve. Because of this, they have ample power at lower speeds to overcome the extra drag of the cowl flaps, and the small decrease in climb performance it causes is an acceptable penalty to gain the cruise performance improvement the variable-geometry system can give.

Variable Flush Ramps

If the drag of opening a cowl flap is too much for a particular airplane design, an alternative is to use an inward-opening flush ramp to vary the size of the outlet aperture. These ramps are hinged at their aft edge and form the inner side of the cooing air exit path. Moving the ramp changes the distance between the forward edge of the ramp and the outer skin of the cowling, thus altering the size of the outlet aperture. The ramps open inward, so moving them inboard increases the size of the cooing air exit, and moving them outward reduces it.

Since the ramps are entirely inside the cowling and open inward, they never protrude into the outer flow and hence do not generate the same drag increase as cowl flaps do. The downside is that the ramps don’t produce a low-pressure zone at the cooling air exit to help pull flow through the system, so they are somewhat less effective than cowl flaps. Ramps are also harder to integrate because they need space to move inside the cowling.

Both forms of variable-geometry outlets can produce significant improvements in performance over a fixed-geometry system for high-performance airplanes with a large speed range between climb and cruise.

The post Variable Geometry Cooling System appeared first on KITPLANES.

To properly cool the engine, air must flow into the engine compartment from outside the airplane, absorb waste heat from the engine and then flow overboard to carry the heat away.

In previous editions of “Wind Tunnel” we have followed the cooling air through the inlets, into the upper plenum, over the cooling fins on the engine’s cylinders and into the lower plenum. Once the air enters the lower plenum it must leave the airplane and return to the outer airflow, carrying the heat it absorbed from the engine with it.

We now turn our attention to the lower plenum and the outlets through which the cooling air leaves the airplane. The entire cooling airflow path should be designed with two fundamental goals in mind.

The first and most important goal is to ensure there is enough cooling air flowing over the hot parts of the engine to cool it properly.

The second is to minimize the drag penalty caused by the cooling airflow. This includes the lower plenum and outlets.

Cooling Airflow

Airflows are driven by differences in air pressure. Air flows from areas of higher pressure to areas of lower pressure.

The cooling air is driven through the cooling fins by the pressure differential between the upper and lower plenums. It’s essential for the air pressure in the lower plenum to be lower than the pressure in the upper plenum to provide a pressure differential to drive the cooling air through the fins on the cylinders.

The pressure recovery of the inlets drives the pressure in the upper plenum. The air pressure at the outlet face drives the pressure in the lower plenum. There must be enough difference between the two to drive the cooling airflow through the system.

Leaks

In our discussion of internal flow in the upper plenum, we saw the importance of sealing the plenum so that all of the air pressure recovered by the inlets is retained, and all of the air that enters the cowling must flow over the hot parts of the engine rather than escaping through leaks in the cowling or baffles. There should be no air leak paths between the upper and lower plenums that would allow air to flow through without cooling the engine.

Its equally important that the lower plenum itself be sealed well enough to isolate it from the outer airflow except at the outlet itself and to ensure that no air leaks between the upper and lower plenums without flowing through the cooling fins on the cylinders. There are two reasons for this.

The first is to ensure that the air pressure in the lower plenum is as low as possible to maximize the pressure differential between the upper and lower plenums.

The second is to minimize drag. The cooling air should leave the airplane by flowing out of a properly shaped and directed outlet. Any air that leaks into or out of the lower plenum will cause excess drag either through loss of momentum or by disrupting the airflow over the outer surface of the airplane.

Outlets

The primary function of the outlets is to extract cooling air from the cowling and return it to the exterior airflow over the airplane.

All of the cooling air that comes in the inlets must leave through the outlets. The air must escape from the cowl in sufficient volume to carry away enough heat to keep the engine properly cooled.

It’s vital for the outlets to be big enough. Even if the inlets take in air efficiently and the upper plenum is sealed, the engine will not cool effectively if the air can’t get out of the outlets in sufficient volume.

The outlets must have enough area that they do not restrict the flow of cooling air through the cooling system and should be properly shaped to promote smooth airflow.

In general, the outlets must have greater area than the inlets to ensure adequate cooling flow.

Outlet Position

To promote good cooling airflow the pressure gradient inside the cowl should drive the cooling air out of the lower plenum through the outlets. Accordingly, we want the air pressure at the exit throat of the outlets to be as low as possible.

Outlets should be placed in areas of natural low pressure on the airplane. The bottom of the cowl as far aft as possible and the sides of the cowling near the firewall are both natural low-pressure zones and can be good locations for outlets. Note also that since the sides of the cowl are low-pressure zones, inlets placed there rarely work very well.

Outlet Flow

Most of the drag of the cooling system is determined by the difference between the momentum of the air inhaled by the inlets and the momentum of the air exhausted through the outlets. In a perfect system the air would leave the airplane flowing in the same direction (aft) and at the same speed as it had before it came aboard. Although losses are inevitable, the design of the cooling system should seek to get as close to this as possible.

Both the speed and the direction of the air leaving the outlets is important.

The air that goes in the inlet is flowing straight aft relative to the airplane before it flows into the inlets. All of that rearward momentum is imparted to the airplane as the cooling air comes to a virtual stop inside the upper plenum.

As the air leaves the airplane via the outlets, some of that momentum can be recovered. If the air exhausts flowing aft, the total drag of the cooling system will be less than the pure ram drag of the air going into the inlets. The outlets should be shaped to accelerate the cooling air to as close to free stream airspeed as possible and to direct the cooling air aft as it leaves the airplane.

A low-drag outlet system will be shaped so that the cooling air flows through a converging path inside the cowl that accelerates the air and directs it aft. The exit orifice itself should be oriented so that the air can leave the airplane flowing aft rather than down or sideways.

If the outlet dumps the cooling air out flowing down or sideways rather than aft, the cooling drag will be relatively high.

A common, high-drag approach to cooling air outlet design is to use a simple hole on the bottom of the cowl at the firewall to exhaust the cooling air. It’s common for the square corner between the firewall and the skin of the airplane to form the aft side of the outlet.

This is appealing at first because it is light and simple, but it is also draggy. Air leaving such an outlet will be directed down rather than aft. The system will have increased drag for two reasons.

First, the rearward momentum of the cooling air will be lost since the air leaves the airplane flowing down, rather than aft. No rearward momentum will be recovered.

Second, the cooling air exiting such an outlet will disrupt the exterior flow over the airplane.

If the outlet directs the flow aft, tangent to the skin of the airplane, the exiting cooling flow will mix cleanly with the exterior flow with minimal effect on the flow over the airplane components downstream of the outlet.

If the cooling air exits going straight down or at a significant angle away from the skin of the airplane, then the fountain of cooling air emitting from the skin will act like a spoiler, and the cooling flow and exterior flow will mix in a turbulent and chaotic way. This disrupts the exterior airflow and is likely to cause flow separation downstream of the outlet. This turbulent mixing and separation will significantly increase the drag of the airplane.

Disruption of the exterior flow is also a disadvantage of some design features used to reduce air pressure in the outlet and increase cooling flow. It’s not uncommon to put an air deflector or lip upstream of the outlet orifice to reduce the pressure at the outlet and promote additional cooling flow. While this works to improve cooling, the lip or deflector acts like a spoiler and adds significant drag to the airplane.

Next month we will take a look at the internal design of the outlet and variable geometry.

The post Cooling Outlet Design appeared first on KITPLANES.

The first and most important goal is to keep the engine cool enough to operate safely and reliably. The second is to minimize the drag penalty of the cooling airflow. Both of these goals require the cooling system to ingest air efficiently and make effective use of all of the air taken on board the airplane.

In previous Wind Tunnel columns, we looked at heat-absorption requirements and the design of effective cooling-air inlets. We now turn our attention to managing the air as it flows downstream from the inlet throat.

Flow Within the Cowl

Figure 1 shows the flow path for a typical piston engine installation. After the cooling air flows in through the inlet, it goes into a plenum above the engine (the upper plenum).

The air should be able to flow cleanly from the inlet throat into the upper plenum. Ideally, the air should come to rest or nearly to rest within the upper plenum without significant flow separation or turbulence. This will maximize the pressure recovery inside the upper plenum and preserve the maximum amount of energy within the cooling air.

We have already seen the benefit of smoothly rounded inlet lips. The walls of the flow path should also remain smooth downstream of the inlet throat. The flow path downstream of the inlet throat should expand smoothly to maximize pressure recovery. Avoid sharp corners or sudden turns in the flow path.

Anything that impedes the flow from the inlets into the upper plenum reduces the effectiveness of the cooling flow. Avoid putting obstacles in the way of the airflow downstream of the inlet throat into the upper plenum.

Once the air is compressed into the upper plenum it then flows down through the cooling fins on the cylinders, absorbing heat and ending up in the lower plenum beneath the engine.

It costs drag to bring any air on board the airplane so it is important to use all of it to absorb heat and cool the engine and its accessories. Any air that flows out of the upper plenum without flowing over a hot component of the engine and absorbing heat is wasted and generates drag without contributing to cooling the engine.

Only the air that flows directly through the fins absorbs heat effectively. It’s important to ensure that all of the cooling air flows between the fins rather than flowing around the engine and its accessories or escaping through other leaks in the cowling.

Upper Plenum

Baffle Box

The most common type of upper plenum is composed of an open-top set of baffles that form an enclosure around the engine (see Figure 2). The baffles have flexible rubberized skirts on their upper edges. The skirts form a seal between the upper edges of the baffles and the upper skin of the cowling. The combination of the baffles, skirts and the cowling itself forms a plenum chamber to hold high-pressure cooling air.

This type of installation has some advantages. First, the baffles themselves are relatively simple and easy to fabricate. A second advantage is that removing the upper cowling also opens the top of the cooling air plenum, which provides easy access to the engine for maintenance.

The big disadvantage of this system is that the skirts on top of the baffles never seal perfectly. The skirts are always segmented so they can bend to seal against the cowl without wrinkling. There will be some air leakage through the slits between the segments. Also, even the best skirts do not seal perfectly to the cowl inner skin surface.

Air can also leak out through the joints between the cowl and the airframe and the inspection door for checking the oil.

All of these leaks allow a significant amount of cooling air to bleed overboard without flowing over the engine. This increases the total cooling airflow needed. The inlets must ingest enough air to actually cool the engine plus the air that is lost through leakage. The airplane pays the drag penalty for all of the air taken on board, so the air that leaks from the penalty causes drag without contributing to cooling the engine.

Sealed Plenum

Figure 3 shows the solution to the leakage problem. The cooling air inlets feed air into a fully sealed plenum that encloses the engine and has its own “roof” beneath the upper cowling. The plenum is completely sealed except for the flow path through the engine’s cooling fins.

The complete plenum eliminates the losses caused by leakage around the skirts of the traditional installation. This significantly reduces the total airflow needed for cooling and reduces cooling drag.

The separate plenum approach does have some disadvantages: The plenum is a more complex part than the classic flat baffles since it must totally enclose the engine and fit within the confines of the cowling. It also adds a little weight since the roof of the plenum essentially doubles the upper cowling skin.

The final disadvantage of the complete plenum approach is that the plenum itself restricts access to the engine. This adds labor to any engine maintenance efforts since the plenum must be removed as well as the cowling to get to the engine.

Directing the Air

The cooling air should have a clear, unobstructed flow path to the cylinders. Air follows the path of least resistance. It will flow around any obstacle in its path. Any obstruction between the inlet and a cylinder will deflect cooling flow away from that cylinder. A common problem when trying to get all of the cylinders to cool uniformly is that things like wires, tubing or other items under the cowl deflect air away from some cylinders more than others.

The baffles around the cylinders should direct all of the cooling flow through the fins on the cylinders. As the air flows downward out of the upper plenum it should not have any flow path other than flowing between the cooling fins. The engine baffles should force the air to flow through the fins.

A common omission I have seen is the lack of a baffle between the cylinders of a flat-opposed engine. The spacing between the cylinders is large enough that a significant amount of air can pass between the cylinders without absorbing much heat. A simple baffle spanning the gap between the cooling fins of separate cylinders will significantly improve cooling.

Other Losses

Any air taken in through the inlet that does not go through the cylinders is lost. Many engine installations take high-pressure air from the upper plenum to cool items other than the engine cylinders. One example of this is blast tubes that direct air over accessories like the alternator. Another is an oil cooler mounted to the baffles forming the upper plenum. The blast tubes and oil cooler act like holes in the upper plenum. They take cooling air away from the cylinders and bleed away air pressure that could otherwise force cooling air through the cylinder fins.

The key to efficient internal cooling airflow is to make sure that all of the air that comes on board is properly directed to flow over a hot component of the engine and absorb heat. Leaks and other losses should be minimized.

Next month we will continue downstream and look at how to efficiently get the cooling air overboard and return it to the outer airflow.

The post Cooling: Internal Flow appeared first on KITPLANES.

As we saw last month, an ideal cooling air inlet system should ingest air from the free stream and bring it to rest or very low speed without flow separation or turbulence that dissipates the energy of the flow. The kinetic energy of the cooling air from the outside flow should be converted to potential energy in the form of increased air pressure inside the cowling. Any flow separation or turbulence dissipates some of the kinetic energy, converting it to a slight heating of the air.

Internal Flow

As the cooling air is ingested, it flows over the lip of the inlet. The air will rarely be flowing exactly normal to the plane of the inlet. The inner portion of the inlet lip must be able to turn the incoming flow downstream into the inlet without stalling.

The direction the airflow arrives at the inlet lip varies with flight condition. At high mass flow and low airspeed, the streamlines will be moving inward at the lip and will need to turn downstream around the lip into the throat.

Lower mass flow and higher airspeed causes the inlet to spill as some of the air approaching the inlet orifice does not go into the inlet but flows around it instead. The streamlines will be curved outboard at the lip as the spilled airflow turns to flow around the inlet.

A sharp-edged inlet will experience flow separation unless the sharp edge is aimed exactly into the incident airflow. It’s not possible to aim a fixed-geometry sharp inlet lip in multiple directions.

This means that the lip of the inlet should be rounded rather than sharp so the air can flow smoothly around the lip as the incident angle of the flow varies with flight condition.

External Flow

It’s important to keep the flow clean and attached on the inside of the inlet to minimize turbulence and loss of energy of the cooling air entering the cowling.

It’s equally important to ensure that the air that does not go down the inlet and spills around the lip remains attached to the exterior of the cowling and maintains clean airflow over the exterior of the airplane. The inlet lips should guide the spilled air smoothly around the inlet so that it flows cleanly downstream over the skin of the airplane. If the air were to separate from the inlet lip, the turbulent, low-energy air would propagate downstream and adversely affect the flow over the airplane behind the inlet.

The angle of incidence at which the air arrives at the inlet lip is affected by many factors, including the placement of the inlet. This can make the design of the inlet lips more difficult. In particular, if there is significant cross flow relative to the orientation of the inlet orifice in the external flow, then the lips of the inlet need to turn the flow more. The “upstream” lip must turn the flow into the inlet orifice, and the “downstream” lip must both turn air into the inlet and turn the flow moving across the inlet downstream so that it remains attached to the external skin of the cowling.

Pressure Recovery

Once the air passes through the narrowest portion of the inlet duct (the inlet throat), it should flow into an expanding (diffusing) duct that slows down the flow and increases its pressure. If the inlet lips are properly shaped and there is enough distance downstream of the inlet throat, this can be done with very little loss of energy. Inlets for jet engines are examples of such a system, and a well-designed jet engine inlet system can have total pressure losses of the order of 1%.

The designer of a cooling flow path for a piston engine has a more difficult task than the designer of a jet engine inlet system for two reasons. The first is that the cooling air must be slowed much more than the flow into a jet engine. The second is that on a typical piston engine installation, there is not much room between the back of the propeller and the front of the cylinders in which to place a diffusing duct.

Inlet Position

To recap, for good performance we want an inlet system that has attached flow on the lip so that both the air flowing into the inlet and the air that flows around the inlet and over the skin of the airplane flows smoothly and without turbulence.

The inlet system should also smoothly slow the air that enters the inlet to nearly static and recover as much as possible of the kinetic energy in that air as increased air pressure.

Proper shaping of the inlet lips can help ensure attached flow, but the designer has fewer options available to ensure good pressure recovery because there is not enough room between the inlet throat and the cylinders to incorporate a long enough duct to fully diffuse the inlet flow if it enters the inlet at anything like free-stream velocity.

Fortunately, the performance of the inlet can be greatly improved by locating it properly.

The Stagnation Point

As air flows around an object, it is deflected by the nose to flow over either side. There is one dividing streamline. Air on one side of this dividing line flows in one direction around the object, while air on the other side is deflected in the opposite direction. On a wing, the air above this dividing streamline flows over the top of the wing while the air beneath it flows over the bottom of the wing. The dividing streamline itself intersects the skin of the object at 90°, and the air is locally brought to rest at the point where the dividing streamline (also called the stagnation streamline) meets the skin. This point is called the “stagnation point,” and the air at this point is stationary relative to the skin, but it has also been compressed against the skin so that all of the kinetic energy at this point has been converted to pressure.

Inlet Position

We can take advantage of this natural compression of the outer flow and minimize cross flow across the inlet face by placing the inlet at or near the stagnation point on the cowling.

The position of the stagnation point varies some with angle of attack and sideslip angle, but for typical piston-engine cowlings the stagnation point is at, or slightly below, the most forward part of the front face of the cowl. This is where the cooling air inlets should be placed for best performance. There is also a region of high local air pressure just below the spinner, and this is a good location for an induction air inlet or an oil cooler inlet.

It’s also important to avoid placing inlets in areas where the flow is moving fast and the air pressure is low. A particularly poor place to locate an inlet is on the sides of the cowling, particularly in the forward half of the side of the cowling. This is a natural low-pressure zone, and the inlet will have to capture fast-moving air and use the momentum of that fast-moving air to force it down the inlet. There is no easy way to slow the air down efficiently once it passes into the inlet, so a side-mounted inlet is likely to have poor flow and low pressure recovery. In some cases, the external air pressure is low enough that the inlet cannot recover enough ram pressure to drive the air into the duct, and it’s not uncommon to have poorly placed inlets flow backward if they are in a low enough pressure zone.

Next month we will continue downstream with a look at what happens inside the cowling.

The post Cooling Inlets, Part 2 appeared first on KITPLANES.

The cooling air inlet serves two functions: First, and most important, the inlet must ingest enough air to properly cool the engine and accessories over the entire range of airspeed and power settings at which the airplane will fly. Secondly, the inlet should be designed to ingest the cooling air efficiently and minimize the drag penalty due to cooling flow. To do this the inlet should manage the airflow in a way that preserves the energy in the cooling airflow and disturbs the external flow over the airplane as little as possible.

Energy and Momentum

Energy in the air can take two forms: the kinetic energy of the velocity of the air and potential energy in the form of air pressure. The law of conservation of energy tells us that kinetic energy can be converted to potential energy (pressure) and back again with no loss of total energy.

An ideal cooling system would take air aboard the airplane, use it to absorb the waste heat from the engine and then return the air to the outer airstream flowing straight aft at the same speed as the airplane is flying.

In that perfect system, the momentum of the air that flows through the cooling system would be the same when it leaves the airplane at it was in the free stream ahead of the airplane before it went into the inlet. In the real world, a zero-loss system is not possible. However, the goal when designing the cooling flow path is to get as close to this as possible.

Even in a very well-designed system, the momentum of the air changes as it flows through the cooling system. From a drag point of view, what matters is the momentum of the incoming air versus the momentum of the air as it leaves the airplane.

This month our subject is inlets, so let’s take a look at how inlet design affects the cooling air and the external flow over the airplane.

Ram Drag

The cooling air flows in through the inlet and comes to rest, or nearly so, relative to the airplane inside the cowling. The rearward-directed momentum of the incoming air is transferred to the airplane and removed from the outer airstream. This momentum transfer causes a (drag) force opposing the forward motion of the airplane. The drag caused by taking the air on board is called ram drag.

Ram drag is an inevitable penalty of bringing cooling air on board the airplane. The first step to minimizing ram drag is to design the cooling system to use as little total cooling air as possible.

Figure 1 shows the power required to overcome the ram drag of air flowing through 1 square inch of inlet as a function of airspeed. Notice first that the effect is nonlinear. The faster the airplane is flying, the more rapidly the power penalty of a given amount of inlet rises for each additional knot. For example, the penalty for a square inch of inlet at 200 knots is about six times what it is at 100 knots.

Figure 2 shows the ram drag penalty for the cooling air for engines of several horsepower ratings as a function of airspeed. This figure was derived for a fixed-geometry inlet sized to provide adequate cooling air over the entire flight envelope. It does not take into account the beneficial effect of variable geometry to control the amount of cooling airflow. This figure illustrates how much of the engine’s power the ram drag to cool it can absorb. For example, the ram drag of the cooling air for this system absorbs about 20% of the engine’s rated power at 200 knots.

The potential magnitude of the ram drag as shown by Figure 2 emphasizes how important it is to properly size the inlets to only ingest the minimum amount of air required to properly cool the engine.

The second key to minimizing total cooling system drag is to recover as much of the kinetic energy in the cooling air as possible in the form of pressure within the cowling. This high pressure will help force the cooling air through the fins on the cylinders. It can also be used to eject it backward through the cooling outlets to recover some of the drag caused by ram drag.

Figures 1 and 2 show the importance of preserving the energy in the cooling airflow as it moves through the cooling system so that there is enough energy in the flow to recover some of the ram drag by ejecting the cooling air aft through the outlets.

Inlet Pressure Recovery

The cooling air starts upstream of the airplane, flowing at free-stream velocity and ambient static pressure. As air moves into the cowling via the inlets, it is slowed down and some of the energy of the flow is converted from kinetic energy to potential energy in the form of increased air pressure.

An ideal inlet system takes the air on board and slows it down without losing energy to flow separation and turbulence. If the flow separates and becomes turbulent, some of the kinetic energy of the flow is converted to heat and the momentum in the air is lost. This leaves less kinetic energy to be converted to pressure.

A real-world inlet system should preserve the energy in the airstream as much as possible. The inlet and duct behind it should slow the air down and increase its pressure without flow, separation or turbulence that dissipates energy.

If there is enough room behind the inlet throat, a diffusing duct that expands the cross section as it goes downstream can provide very efficient pressure recovery. In practice, this is difficult to do because on a typical tractor engine installation there is not enough distance between the back face of the propeller and the cylinders to install a diverging diffuser duct that can expand the flow without having some flow separation.

There are two approaches to dealing with this. The first is to design the inlet lips and ductwork to do as well as possible given the length available. An inlet and diffuser that partially expand the flow without separation is much better than a system that has flow separation right downstream of the inlet throat.

The second approach, which can have large benefits, is to place the inlets properly. An effective way to improve the situation is to locate the inlets in natural high-pressure, low free-stream velocity areas of the flow. This minimizes the amount of diffusion needed to recover the ram pressure in the air in a spot on the surface of the airplane where the airflow has already been slowed and compressed by the natural flow pattern over the shape of the airplane.

Ideally, the inlet should be at or near what is called the stagnation point. The stagnation point is a spot where the external airflow is arriving at 90° to the surface and will naturally be brought to rest at that point. At the stagnation point the air is already compressed to its ideal static pressure, so placing an inlet there requires very little diffusion downstream to get good total pressure recovery.

Conversely, placing inlets where the external air is flowing fast and at low pressure makes the design of the inlet much harder and makes it difficult to get good inlet pressure recovery.

Next month we will take a look at how to find the proper place for cooling inlets and how the shape of the inlet lips themselves affects the cooling flow.

The post Cooling Inlets appeared first on KITPLANES.

Cooling the engine and its accessories requires a continuous flow of air to come in from outside the airplane to absorb the heat. Once this cooling air absorbs waste heat from the engine, it must flow overboard to carry the heat away.

There are two primary goals to keep in mind when designing the cooling system for an airplane’s engine: The first and most important goal is to keep the engine cool enough to operate safely and reliably. The second is to minimize the drag penalty of the cooling airflow.

Keeping It Cool

The cooling system must be able to cool the engine effectively over the entire flight envelope of the airplane. This is not as simple as it might first appear. Both the amount of heat that needs to be rejected and the air available to absorb that heat vary dramatically with airspeed, power setting, altitude and air temperature.

The worst case for cooling is a situation of high power and low airspeed, as is the case in climb. At full throttle, the engine is generating maximum heat and needs the most cooling.

Airplane cooling systems operate on ram pressure. There is no fan to pull air into the airplane, so the cooling air is driven into the inlets by the speed of the incoming air. Accordingly, the mass flow of air through the inlet is a direct function of airspeed. Ideally, if there is no blockage that causes spillage, the inlet swallows a stream tube of air that has the same area as the inlet aperture. The mass flow in this stream tube is directly proportional to airspeed. The faster the airplane is flying, the more air the inlet will ingest.

In a maximum-effort climb the airplane is flying slowly, so the amount of cooling air available is at a minimum. This minimum-available cooling air must absorb the maximal amount of heat produced by the engine at full power. This phenomenon leads to the most significant compromise the designer of the cooling system must resolve. This is the choice between a fixed-geometry cooling system or one with some form of variable geometry.

The cooling air system must generate enough cooling flow to cool the engine in the worst-case, full-power, hot-day climb condition. Once the airplane is in cruise, the airspeed is higher and the engine is developing less power than in climb. This means that there is more cooling airflow available and it needs to absorb less heat.

In cruise, a system sized to cool the engine properly in climb will take in more cooling air than necessary to cool the engine. On the other hand, if the system is sized to cool the engine in cruise without taking on excess air, it will not be able to cool the engine adequately in climb.

The faster the airplane is, the more significant the difference in cooling system requirements between climb and cruise becomes. Higher speed not only makes more cooling flow available but also increases the performance penalty of ingesting excess cooling air. It is also possible to get into a situation where at high speed the cooling system ingests so much air that it overcools the engine, causing it to run below its preferred operating temperature.

Fixed-Geometry Cooling

A fixed-geometry system has several advantages. It is very simple and has no moving parts. This makes it the lightest, lowest-cost and most reliable system. Since the system is a fixed part of the airplane, the pilot has no control over it. No pilot action is needed to manage engine cooling.

The disadvantage of a fixed-geometry system is that it must be designed to cool the engine in the worst-case, maximum-power, minimum-airspeed flight condition.

At cruise, with higher speed and lower power, there will be excess air flowing through the system. This excess air will exact some performance penalty as compared to the optimum cruise cooling flow.

Whether or not this penalty is acceptable is a function of how large the difference between cruise and climb speed is and how critical cruise performance is to the mission of the airplane. On a trainer, for example, simplicity and cost are important, and a small penalty in cruise performance is unimportant to the likely operators of the airplane. On a fast airplane intended primarily for traveling cross-country, cruise performance is much more important, and a penalty in cruise performance is less acceptable.

Variable-Geometry Cooling

If we look at high-performance piston-engine airplanes, they all incorporate some form of variable geometry that allows the pilot to control the airflow through the cooling system. Variable-geometry cooling systems were incorporated on all piston-engine airliners from the DC-2 onward, as well as WW-II fighters. Some higher-performance general aviation airplanes also have some form of variable geometry in their cooling systems.

The advantage of variable geometry is that it lets the designer size the cooling system to provide optimum cooling flow in cruise or at maximum speed, and use the variable-geometry features to generate the extra airflow needed to cool the engine in climb.

A variable-geometry system provides the best cruise performance. It has some disadvantages too. It is more complex and hence more costly than a fixed-geometry system. It also requires the pilot to monitor engine temperature and manage the settings of the cooling system to control engine temperature.

A final disadvantage of some variable-geometry systems is that they typically have higher drag in climb configuration than a fixed-geometry system. The variable geometry shifts the drag penalty compromise from cruise to climb. On a high-powered, fast airplane there is typically enough excess power that this relatively small penalty in climb is acceptable, but the drag of the cooling system in climb must always be taken into account to ensure adequate climb performance.

Cooling With Minimum Drag

Taking air on board the airplane always causes drag. Cooling drag can be as much as 15% of the total drag of a typical single-engine piston airplane.

Proper design of the complete cooling flow path can improve engine cooling and minimize the drag of the cooling system.

The cooling system should:

• Take as little air onboard as possible.
• Use it efficiently.
• Return the air to the outer airstream flowing backward at the highest possible speed.

Every step along the cooling airflow path—from the inlets, through the engine compartment, and out the outlets—affects both the drag and the cooling performance of the system. Ideally, the cooling air should flow into the airplane, slow down and flow smoothly over the hot parts of the engine and through the oil cooler, and then leave the airplane flowing directly downstream at as close to the free-stream airspeed as possible. It is inevitable that the cooling airflow will lose some energy as it flows through the cooling system, but a well-designed system can minimize this loss.

Next month we will start a trip through the cooling-air flow path with a look at inlets.

The post Cooling appeared first on KITPLANES.