Introduction: New Cowling for Your Experimental Aircraft

Picture of 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

Picture of 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

Picture of 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

Picture of 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

Picture of 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

Picture of 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

Picture of 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

Picture of 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!

Picture of 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!

Picture of 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

Picture of 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

Picture of 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

Picture of 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

Picture of Paint

I painted the cowl with a green two stage with gold pearl.

Step 14: More Flight Test

Picture of More Flight Test
I wanted at least ten flight hours on my cowling before taking it on the race course to confirm it was safe for the harsh racing environment.

Step 15: Racing

Picture of 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.


Jrn14 (author)2016-01-12

Here is a diagram of my aircraft. I'm doing 1/2 size projects.

Jrn14 (author)2016-01-12

I see how you molded the cowling to the shape of the engine, but how would you do that with say a Cessna style cowling?

triumphman (author)2011-12-28

I always wanted to put a new cowl on my plane! Now I can! Thanks!

fjr122 (author)2011-09-20

What a mess!
I'm such a neat-freak..
Is the shop ever clean again after something like this?

ElliotSeguin (author)fjr1222011-09-20


Composites are sort of inherently messy. If you aren't carving and sanding then you are slopping pucky all over everything. The guys who are really good, guys like Jon and Trish Sharp, Cory Bird, and Andy Chiavetta, they don't make near the mess but there is always a mess.

I would invest in a good shop vac or two, plenty of filters. Masking tape, poly sheet and that brown painter's paper. All this makes for a lot of trash, I fill two to three garage bags a week.  I keep two sets of clothes, those that go to the hangar and those that don't (carbon itch is a bear).  And I end up showering twice a day so it doesn't get in my bed sheets.  Sounds like a pain but there is no other way to get an airplane that looks as good or goes as fast.

You've got to try it.

xfirexstarzx (author)2011-08-28

I love reading articles like this. Between the beautiful finished result and the great photography, this made for a nice instructable.

I think I'm going to give this method a try when I build a fiberglass fairing for my motorcycle. Heck, maybe I'll even try it with an actual airplane some day as I'm going to school for aeronautical engineering.

I was going to ask how the resin holds up to the heat but I see you already addressed that below.

Anyway, thanks for the awesome instructable and the inspiration that goes with it!

Thank you.

Great profile picture!


sockless (author)2011-08-25

Just out of curiosity, what's the highest level of qualification that you have in aerospace engineering?

Scaled Composites looks like a pretty picky sort of employer with the caliber of aircraft that they design.

ElliotSeguin (author)sockless2011-08-26


Thank you for your interest. I have a BS in Mechanical engineering from Union College in Schenectady NY.

Scaled is known to be a challenging place to get a job, but typically there isn't a single specific qualification that will get you noticed by our hiring department. On the job an engineer at Scaled is expected to be able to handle alot of responsibility both vertically and laterally (from intial design through manufacture and test with a reasonable amount of depth at each level). As a result the best way to qualify yourself to become an engineer at Scaled is to be doing those things before you get here.

The best example is to be designing, building, and flight testing aircraft of your own before you get here (like this tutorial). But it doesn't have to be airplanes we have car guys and motorcycle guys and even land speed record guys. Burt (our founder) always said that you shouldn't be designing it unless you could build it yourself.

All that being said many of our engineers spent several years trying to get the attention of the company before they did.

At the end of the day we are looking for engineers with strong fundamentals and the hands on skills it takes to make theory into hardware, because hardware is what our customers pay for. It also doesn't hurt if you are a pilot or if you think airplanes are the coolest. If you know someone who fits this bill they should check out the careers section of the Scaled website, as we are currently hiring.


bonboncom (author)2011-08-25


ElliotSeguin (author)bonboncom2011-08-25

Thank you


16skittles (author)2011-08-25

Good god...
I want to be a pilot when I'm older, but I still can't believe that some people trust their lives in some of these. I guess you're an aerospace engineer, but still...

Anyways, just as a general aviation question, are there any regulations that need to be met for experimental aircraft to be completely "legal"?

ElliotSeguin (author)16skittles2011-08-25

You should check out your local EAA (experimental aircraft association) chapter, it is easier and more accessible than you think, and there is most likely a chapter within 30 minutes of where you are sitting right now. Go down to your local airport typically and introductory flight lesson is a hundred bucks and comes with a logbook that you can use to prove you have flown an airplane, thats right you'll fly the airplane on the first lesson!

As far as the completely legal question I am not sure what you mean. Everything I have shown in this tutorial is completely legal. In the US an experimental aircraft can legally do just about anything a certified aircraft can do except carry paying passengers. In order to fly your homebuilt experimental aircraft it needs to be inspected by the FAA or equivalent in order to be legal. This representative will look over your work and your engineering before signing you off to fly. After that you need to do phase one flight test which includes demonstrating the envelope of the airplane, this flight test typically needs to be done near your home airport. After that as the designer and builder you are responsible for keeping the airplane airworthy and therefore safe.

At the end of the day I can take off here in mojave and land in san diego in one hour. That same trip would take 3.5 hours by car. If that doesn't get you excited about airplanes, I don't know what will.


gregj1 (author)2011-08-25

Nice project well done. Here is what confuses me, you cut the mold (tool you call it) vertically (top and bottom) but, the part is split horizontally. Why so?

ElliotSeguin (author)gregj12011-08-25


Thank you.

I split the cowl horzontally because it works best. I have worked on airplanes that are split alot of different ways and the horizontal split just works better. You can take off the top to get access to your cooling plenum and your plugs and get limited inspection access, and when things get serious you drop the lower.

The reason we split the tools vertically is proprietary.

thanks again,

rbbiggs (author)2011-08-25

Nice plane. What is your Vbg speed (best power off glide speed), also, whats your Vs, Vso & Vs1 speeds?

Nice instructable, thanks for posting

ElliotSeguin (author)rbbiggs2011-08-25

Thank you rbbiggs!

For Vbg I use 100 mph when racing because I use it for my base to final turn, but in actuality Vbg is much slower than that. In general the airplane's stability both directionally and longitudinally is marginal (its a racer) and as a result it will snap pretty easily in an uncoordinated stall so I keep my speeds up throughout the envelope at the cost of glide performance. Vs is 58 mph (at race weight) with no flaps or gear levers to play with Vso and Vs1 are the same. The only thing that changes is the pilot's anxiety level which coresponds directly to whether or not he has a healthy engine and how big the crowd is that's watching.

This is an uncoordinated stall we shot a few years ago, notice how quick its on its back. Typically its a quick buffet in the elevator (you can see the pitch bobble) and then it snaps. Obviously I am aggrivating it in the video but I think it explains my nervousness about getting slow near the ground.

Thanks again,

rbbiggs (author)ElliotSeguin2011-08-25

Pucker factor for sure !! Looks like fun, be safe. Thanks again for posting

ElliotSeguin (author)rbbiggs2011-08-25


Check out the Wasabi website for Wasabi gear, and if you can make it come out to Reno for the races in a couple weeks.


MnVelocityPilot (author)2011-08-25

There are some nasty bends that detach airflow on my Velocity STD-RG cowl.... and winters are long here in Minnesota (so good for a long project). I've been thinking of making a new cowl for years. Thanks for the motivation...

It makes a big old mess but more than one old grey beard racer has said that the most fun thing in composite airframing is carving foam.

Have fun,

dshowalt (author)2011-08-25

I understand that the Layup Schedule is proprietary but can you recomend a reference or simple way to calculate a layup schedule?

ElliotSeguin (author)dshowalt2011-08-25

A good conservative rule of thumb is 20 ksi for compression.

I personally find it takes about 0.030" of thickness to get the flatter shapes stable. Obviously more curvature means it takes less material. At the end of the day on something like this you will end up picking your thickness based on the countersinks that hold the thing together.


ivanbakas (author)2011-08-25

To prevent Bondo or other Polyester resin based products, from eating away the popcorn foam, apply a coat of Welbond, water based glue. Let it dry and you can apply Bondo or Polyester resin directly on the foam.

ElliotSeguin (author)ivanbakas2011-08-25

Alot of times I will use a micro balloons slurry (because we typically have alot of epoxy and micro balloons around), I had not heard of using woodglue...thats great!


blanchae (author)2011-08-25

You should put side by side pictures of the aluminum cowling and the carbon fibre one to see the differences. I had to jump between the first picture and the center one of this page to see the improvements. Nice work.

ElliotSeguin (author)blanchae2011-08-25

Done. I added them to the last step. Thank you blanchae!


eljohn3 (author)2011-08-21

Cool post and a nice Triumph in the background, lol. Kudos!

skunkbait (author)eljohn32011-08-21

Yep. Awesome instructable- And the first thing I noticed was the Triumph.

ElliotSeguin (author)skunkbait2011-08-22

Thanks Guys!


CameronSS (author)2011-08-20

Very nice! And just the other day we had instructions on changing airplane tires, so maybe there is hope.

Is a Cassutt (or however it's spelled) all aluminum?

ElliotSeguin (author)CameronSS2011-08-20

Thank you Cameron, I hope this inspires more aviation posts. There has been some discussion over here about whether we are inspiring the next generation or educating our competors I guess only time will tell.

The Cassutt (after the designer Tom Cassutt) is steel wood and fabric. The fuselage is a welded steel tube truss with wood stringers covered with fabric (like a piper cub) and the wing is wood structure (spars and ribs) sheeted with plywood then covered with fabric.

Thanks again,

CameronSS (author)ElliotSeguin2011-08-21

Goodness, it's such a slick-looking design it's easy to forget it's sixty years old!

Phil B (author)CameronSS2011-08-20

I did an aircraft related Instructable on mounting a Garmin aircraft GPS above the instrument panel in a Piper Cherokee. Click on this link.

Phil B (author)2011-08-20

This reminds me of WW II planes with adjustable venting on the cowling to dial in optimal engine performance in flight. Your new cowling makes a very nice looking, as well as performing airplane.

ElliotSeguin (author)Phil B2011-08-20

Thank you

Cowl flaps are way cool!


Phil B (author)ElliotSeguin2011-08-20

It is amazing to think of some of the innovations on WW II aircraft at only 20 or so years after the relatively basic aircraft of the barnstorming era and before.

LeonArchiTesla (author)2011-08-20

Beautiful little racer! I don't suppose you changed props did you? :) Did I miss where you talked about openings for exhaust? How difficult might it be to install adjustable inlet ports? I understand we're shooting for simplicity...I'm just curious. Again...excellent job and a nicely explained 'I'ble'!


Thanks for the interest! I didn't cover how I did the exhaust openings in the interest of simplicity.

I attached some exhaust detail pictures. They are steel weldments that include turning vanes to get the exhaust to lay down on the side of the fuselage. They were fabricated with the help of master aircraft builder Ralph Wise.


does the carbon fibre take that heat well? looks like it gets pretty hot in one of the pics...thanks for the response!

Hot it does get.

This is a common thing in air racing (see Darryl Greenamyer's Man for the record). After the same goal, to suck the exhaust down as close to the fuselage as possible Darryl got the cockpit close to 200 degrees on his record setting runs at Edwards. In his case the exhaust surface the exhaust was laying on was the fuselage wall right by the cockpit which made for a warm pilot. The attached pictures are of that airplane which is now in the smithsonian accross from Jon Sharps Nemesis Formula One.

To directly address the picture and the question. What you are seeing is the bondo and paint that can't handle the temps at race power. Typically the next thing to go is the resin and last is the carbon fiber itself. I have gotten carbon well over 400 F without destroying the fibers but most resins wont be any good much over 300F. The nice thing about racing is it's all over pretty fast.


About adjustable inlets, they are definately done. One example that comes to mind is the Rutan Catbird. They are obviously very complicated and the performace to be gained from this type of inlet on a pylon racerisn't worth the effort.

rimar2000 (author)2011-08-20

Beautiful aircraft! I envy you!

That propeller with socks should fly very well...

ElliotSeguin (author)rimar20002011-08-20

Thank you,

mikeasaurus (author)2011-08-20

Awesome project, your new cowling looks great!

"our flight test data showing a 17 mph speed improvement."
What was your baseline prior to cowling upgrade?

ElliotSeguin (author)mikeasaurus2011-08-20


Thank you for your interest. 17 mph is a conservative number based on qualifying data alone. It is the difference in my qualifying speed between 2008 and 2009. The nice thing about air racing is that you get great speed data. I say it is conservative because the 2009 qual run wasn't spectacular conditions wise and based on the speeds we went later in the week (in traffic) we probably could have qualified faster.

Thanks again,

LeonArchiTesla (author)2011-08-20

btw...I'd recommend entering the Epilog challenge...very cool project!

On it, Thank you.


About This Instructable




Bio: Aerospace engineer working for Scaled Composites, Nemesis Air Racing, Wasabi Air Racing all located at the Mojave Air and Space Port in Mojave California.
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