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Comanche Eagle XP Cowling

In 1987 when I began the research and development of the cowling retro fit for the Piper Comanche I tried to keep the modification as simple and as inexpensive as possible. I wanted to install a new cowling with the existing single exhaust, add full length gear doors and control the engine cooling exhaust air with cowl flaps. This approach was soon to be rejected as I did further research and reviewed the project with other aeronautical engineers.

The first myth that I decided to rule out is that you need cowl flaps to control engine cooling exhaust air. A very good friend who is an engineer with Teledyne Continental and designed the engine and cowling installation for the Piper Malibu emphasized that cowl flaps was old technology. Instead of cowl flaps I incorporated into the cowling an augmented/ramped lower cowling to smoothly flow the engine cooling exit air, exhaust system heated air and tailpipe exhaust. This is basically what the Piper Malibu cowling design incorporated and it worked very well to control engine temperatures and exit air flow. The Beech Mentor T-34 uses augmentor tubes, and has been around for 35 years.

The drawbacks of the old cowling designs is that not a whole lot of engineering went into the designs. Fuel was cheap and all you needed was a cowling to direct air to cool the engine in flight and cowl flaps to allow more air through the cowl to help cool the engine on the ground.

Newer cowling designs incorporate more efficient methods of air management and flow control to cooling and exit air. Inlet air intake size is crucial to getting the correct amount of cooling air into the cowling to sufficiently cool the engine at any angle of attack or flight condition. The dimensions for the heated cooling air exit is equally as important so as not to inhibit the cooling air exiting the cowling causing a pressure buildup to occur in the cowling, which in effect can slow down the air flow and increase engine temperatures and cooling air drag. Ideally the correct formula is to have a higher air pressure on the cooling inlet side of the engine and a lower pressure on heated cooling exit air side of the engine. Also by ramping or augmenting the flow of the exit air out of the cowling reduces cooling drag and efficiently extracts the air out of the cowling smoothly. Another component to add to this is what do you do with the exhaust system and the high amount of heat associated with exhaust.

This was the other part of the theory in our design that made me decide to get rid of the single exhaust system and opt for the dual exhaust system. I was able to incorporate each one of the exhaust pipes and mufflers into the exit air ramps on the lower cowling. I designed our lower cowling with two augmented exit ramps that are of a specific angle to the lower cowling, one on either side of the nose gear doors to exit the cooling air and exhaust system.

Basic meteorological theory shows that when moving hot air meets moving cool air a lot of turbulence is created. If you just dump hot and cold air at each other you create even more turbulence, hence you have another form of cooling drag.

In the design of the cowling for the Comanche I measured intake air pressure and exit air pressure below the engine and at the exit ramp at the lower cowing. With the augmented ramps on the lower cowling I was able to fine tune the exit air pressure for each aircraft after the cowling installation, as I found out that two identical airplanes are not always alike. By installing the dual exhaust system and aiming each exhaust pipe down the exit ramp I am also using the engine cooling heated exit air to also further cool the exhaust system air before exiting the aircraft. Also by ramping the heated exit air through the augmented lower cowling I was able to flow the air into the slipstream below the aircraft, thereby drastically reducing the cooling air drag.

The problems that you create with the use of cowl flaps is that when they are open they create a lot of drag. When closed they increase the exit cooling air pressure inside the cowling therefore increasing cooling air drag both on the exit side and intake side of cooling air flow. Cowl flaps are a poor compromise for drag reduction. You also have to remember them in your pre-flight check and gump check to open and close them.

Using the augmented ramps in the lower cowling offers very stable air control in all flight conditions. It doesn't exhibit any shock cooling in descents and in any climb condition cylinder head temperatures and oil temperatures never see red line. Also, all the intake air inlets into the upper cowling are ramped to ensure a smoother air flow into the engine and reduce/eliminate intake cooling air drag.

Stock cowled Comanches exhibit cylinder head temperatures that vary up to 100 degrees between the front and rear cylinders. With the installation of the new cowling, cylinder head temperatures vary only between 15 and 25 degrees between cylinders on all of the installations so far. The other plus side to the new cowling is that I see an average increase in airspeed and performance of 5-15%.

I chose to use the dual exhaust system from the 260 Comanche, It has been around for a long time and is already approved for the Comanche. A.P.P. Inc. manufactures it out of 331 Stainless Steel instead of the black-iron steel Piper manufactured their system from. The collectors are fitted with slip joints for each cylinder so that they expand and contract with the cylinders helping to eliminate the cracking around the exhaust studs and flanges on the cylinder heads.

A.P.P. Inc. also got approval to install this exhaust system on the 180 Comanche Lycoming O-360 engine. The same mufflers and tailpipes are used as on the 250 Comanche. The collectors are modified for the O-360 engine with the same type slip joint to eliminate cylinder head exhaust flange cracking. A crossover or balance tube between left and right exhaust system was considered, but was determined not necessary for this installation. The exhaust system as designed has equal length collector tubes and the performance is much better when neither side is not connected to the other. The balance tube theory is that if one side of the exhaust has more back pressure than the other side the balance tube allows that pressure to equalize and keep the back pressure constant. Theory is great but it doesn't always work. All of the high performance exhaust systems have equal length header pipes that are all joined together into one large collector that is ended where the hottest exhaust temperature discoloration stops. This system design gives the best performance and efficiency.

The most efficient way to get the most performance out of any exhaust system is to keep all the exhaust tubes equal length terminating into one collector, the inside tube diameter the same as the exhaust port, mufflers should be the same on each side and the tailpipes as short as possible. The tailpipes should end where the discoloring of the pipe stops due to the exhaust heat. Any pipe longer than this is to no advantage and can only help build up exhaust back pressure. Without the balance tube there is some after-fireing from the tailpipe on the 4 cylinder engines under rapid power reduction due to the firing order of these engines. After many hours of flight testing I found no negative performance or engine maintenance problems with this system as installed. The best results come from our customers comments that the airplane is noticeably quieter and that the engine performance is definitely improved.

I get very weary of articles from individuals who claim they made a small clean up modification and get a large speed gain.

There is a factor about speed modifications and aerodynamic clean ups that is never discussed called the Aircraft's Aerodynamic Flight Envelope. Simply put it means that every wing design and fuselage combination, along with the aircraft's gross weight and engine maximum horsepower produces an envelope with a minimum and maximum speed in which the aircraft will perform, (sometime called the design speed).

Basically it means that no matter how much clean up you do, no matter how many speed improvements that you ad, you can only get so much speed out of a given aircraft design with a given horsepower.

So, if your Comanche does not fly at its maximum safe airspeed in cruise flight, the clean up and speed modifications may help you get a little closer to that speed ( which means you have improved the efficiency of the basic design). But you should also know that you may never get to that speed and will probably never exceed it with the given horsepower that is available. Speed modifications or clean up enhancements are not cumulative, so by adding more of them does not make your airplane faster, you can only hope to gain a little improvement from each one (if they even work). the more stuff you hang on your aircraft to go faster the more parasite drag you are also adding to slow you down.

One very basic law of physics is that for every action there is an equal and opposite reaction. In aeronautical terms it means that for everything that you hang on your Comanche to clean it up you also add a certain amount of parasite drag to slow it down.

So all these claims of Cessna 170's flying at 170 MPH (which only has a 135 MPH VNE) with just a nose bowl clean up, Comanche's getting 40 MPH more with a speed mod or nose bowl addition or a Comanche getting 10 MPH more with the addition of speedspats spitters and splatters is not only nonsense but real Flights of Fantasy. If you want to go faster you are just going to have to add a bigger hammer (more horsepower) to get there.

There are some reputable company's that manufacture clean up kits, speed modifications and enhancements that improve the design and efficiency of the basic aircraft. You should research each modification and talk to several aircraft owners who have installed them on their aircraft to get information on whether they helped or not.

All of the information discussed above is not anything new, hi-tech or unique. All of this information can be found in your public library, the internet or trade publications. Drag reduction information and engineering has been around for years. The home built aircraft community has brought alot of old technology to new engineering and they build safe, faster and more efficient aircraft than the Manufacturers will ever be compelled to build .

David P. Pratt ICS #2583"