Intro: New Cowling for Your Experimental Aircraft
The following article is a description of the process by which I made a new cowling for my Formula One racer Wasabi in 2009. I race my airplane at the Reno Championship Air Races in Reno Nevada. These are pylon races which are races which involve 8 airplanes racing around telephone poles in the desert at altitudes as low as 30 feet. The formula class is a very affordable class for people interested in racing airplanes and it is also very restrictive (which helps to keep the cost down). As a result very little can be done to the engine in particular to increase the speeds on the course. This project took approximately three months to complete with two people working on it nights and weekends. The cost is less than 10 thousand dollars.
The cowling of an experimental aircraft has two primary jobs. Fair the engine into the fuselage and provide a cooling opening that cools the engine. In the case of the carbon fiber cowling I built for my formula racer Wasabi for the 2009 air races I hoped to make the cowling better at both of these jobs.
Fair the engine into the fuselage. fundamentally the cowling takes the conical shape of the spinner and continues it around the engine and back to the shape of the fuselage at the firewall. The cowling that I had run in 2008 was all aluminum and was therefore pretty boxy. By using composites I planned to be able to incorporate more curvy shapes that would result in less drag.
Provide cooling air to the engine. The aluminum cowling that I ran in 2008 provided too much cooling air, over cooling my cylinders to the low 200sF. By closing the inlets on this new cowling I hoped to limit the cooling air and by doing so reduce the overall drag of the installation.
I am going to mostly focus on the handforming of shapes for making composite parts rather than the technical aspects of particular design choices (it is for racing afterall). The act of handforming composite plugs is quickly becoming old world as a sanding block is replaced with a CNC mill. In the case of this particular project in order to CAD model the cowl I would have neeed a model of the engine which I don't have. So I took advantage of the opportunity as a chance to learn a bit more of the art of hand forming.
Attached below is an air to air picture of the original cowling that we were hoping to improve upon.
Step 1: Protect the Engine
As with any motorsport the engine of your formula is valuable and safety critical. Because of all the grinding and pourfoaming that was to follow in the later steps the first thing I did was cover the engine. I used saran wrap from the grocery store. Before wrapping the engine I disconnected everything that went between the engine and the firewall so that I could wrap the back of the engine as well.
Step 2: Get Messy
The first bit of sticky stuff is always the worst, so I say just go for it. The most common material for this type of work is pour foam. You can find pour foam at Aircraft Spruce. In a pinch you can use the expanding foam from home depot that comes in a can. Because this stuff is pretty expensive I always use two pound popcorn foam anywhere I can.
Step 3: Moving to the Hard Stuff
The pour foam and popcorn foam are great for roughing the shape but you will need something more stable for the later steps. Everybody has there favorite bodyworking material but it is pretty hard to beat bondo. Now remember that bondo will eat popcorn foam (it will not eat pour foam), so watch for that (if your careful it wont eat much).
Step 4: Inlets
Besides a pretty and swoopy shape you also need to make sure that the required inlets are accounted for. In the case of this formula one the primary inlets are the cooling inlets and the engine induction inlet (the snout). The size of all of these inlets is critical and based on the speed the airplane will be flying during the race and the needs of the particular system that is being fed by the given inlet. After you know the size its just a matter of incorporating the given inlet into the overall shape of the cowl.
Step 5: Final Contouring
As you get the shape closer to final I like to switch from bondo to Polyprime (liquid bondo). Polyprime is much thinner than bondo so it fills pinholes nicely it also gets everything the same color which helps for contouring and blocking.
Step 6: Parting Planes
Depending on the shape you will most likely have to break the cowl up into bits with enough draft that the tools will let them go. Once you decide how you are going to break up the cowl you will need a dam to layup on. MDF works great for this, just make a templet and bondo the parting plane in place. I would use masking tape under the plane to protect the poly primed surface that you spent so long shaping. Also notice in the pictures that I scribed the cowling splits before laying up the tools.
Step 7: Layup Tools
Then another messy bit. Laying up the tools will take a fair amount of time as there is alot of material to lay down. Also if you are doing this in summer there is considerable risk of the resin kicking off before the layup is complete so you might want to do some experiments with that. There are lots of fibers that can be used for the tools, I used fiberglass because its cheap and it makes later steps easier. The thickness is important as the tool is only any good if it holds its shape. I figure a tool like this should be between fifty thou and a tenth thick depending on curvature.
Step 8: Check on Your Engine!
So by this time I hadn't seen my precious motor in a month or so and I was nervous it had grown a beard or gotten rusty or something. So before I had even checked out the tools I pulled the plug off the engine as well as the saran wrap and checked that the engine was ok...thank goodness it was.
Step 9: PARTS!
I layed up the carbon parts using a vacuum bag and associated pump. The layup schedules are proprietary but not rocket science.
Step 10: Taping and Everything Else
Rather than distract with the details of integrating the engine into the new cowl, I will simplify the process by saying that I taped the two halves together and drilled the holes for the screws that held it together and countersunk them. We'll save those details for another post.
Step 11: Primer
Living in Mojave it is a bad idea to take anything black out into the sun if you want to be able to touch it later. So before moving towards flight test I primed the cowl.
Step 12: Flight Test
Before I put the expensive paint on the airplane I wanted to confirm that the engine and the airplane and the new cowling were going to get along. The risk with flying a cowling before paint is that if you get an oil leak that blasts the painting surfaces with oil, you are going to have one heck of a time getting paint to stick. In this case I was too nervous that I had over contriained the cooling inlets so I needed to confirm that the engine was going to be kept cool before proceeding.
Flight test of big changes to an airplane is serious business. In the case of this particular flight test the card was pretty simple. After breaking ground i would go to climb speed until I reached a safe bailout altitude. At safe altitude I open the speed envelope using stick raps to check for flutter in 5 knot increments out to a speed 10% over any speed I will see in a race.
Step 13: Paint
I painted the cowl with a green two stage with gold pearl.
Step 14: More Flight Test
Step 15: Racing
We took the airplane to Nevada for the championships. I will spare you the details but our race results confirmed our flight test data showing a 17 mph speed improvement over the previous year's qualification speed. The engine also ran 75 degrees warmer (which was closer to what we wanted).
I would like to thank the Sharps and team Nemesis, Ralph Wise, Curt Carter, Andy Chiavetta, Robby Grove, Roger Hayes and last but certainly not least my Crew Chief, Composite Fabricator and Backup/Chase Pilot Jennifer Whaley. Experimental aircraft are more accessible than you think. Check out your local EAA chapter to get involved. If you are an active homebuilder but want more consider Mojave; the Mecca for homebilding we have everything from ultralights to spaceships. If you are interested in Air Racing come out to Reno this September and see it first hand. The Formula class is a great place to start if you are looking to get involved in air racing. Wasabi Air Racing is always looking for help, be it hardware, cash, labor or all of the above, please feel free to contact us.