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Cowl Flaps really work


Well Known Member
My RV10 is a wonderful airplane, but I have always had to moderate my climb in order to keep the CHT's below 400 deg. (My personal goal, not the Lycoming recommendations.)

No longer is this a problem thanks to Allan Nemo's EZ Cool Cowl Flap system! Now I can climb with a full load from sea level to 5000 ft at 1000 ft./min. on a 98 deg. day and the CHTs never break 380 deg.

Here is a short synopsis of the instillation:

1) Positioning and making the hole in the cowl

I decided to put the flap on the side of the cowl as opposed to the bottom so as to avoid interference with the exhaust stack etc...

The hole was cut with a vibrating "cast saw" tool that could be precisely positioned and controlled. The hole was cut a little small and then enlarged with a file/rasp to the perfect size.

(More to come later)

When the cowl flap assembly was inserted into the opening the flap stuck out into the breeze a little bit, so a shim of 0.040 aluminum was fashioned to place between the flap frame and the inner wall of the cowl.

You can see the trial pieces that I used in this picture. They are the blue plastic covered bits.


The cowl wall is a honeycomb composite and a little weak, in my opinion, to support the screw heads that I used to hold the flap assembly in place.

So I took a flat bladed screw driver and cleaned out the honey comb material between the thin inner and outer walls around the mounting hole and around the edge of the opening.
I put tape around the opening both inside and out, mixed up some West Marine resin with flox and cab-o-sil put it in a syringe and injected into the space between the inner and outer walls. I worked it in with popsicle sticks, taped the cowl flap assembly and inserted it into the opening. Then I put the screws gently in place and let it begin to cure.
After an hour or so I backed the screws out a little bit so that they would not be bonded in place.

(Sorry, forgot to take a picture of this step)
Nut plates

#6 Nut plates were then installed in the cowl flap frame. The nut plates captured the 0.040 shims that had been created.

The opening was sanded to smooth out the injected fiberglass, and the flap was mounted with #6 counter sunk screws.

Forgive the poor picture, but you can see the nut plate here.


It is important to follow the wiring diagram that Allan provides in the kit. I did not pay close attention at first and the flap did not work properly.

Here is a picture of the switch wired.

And here is a picture of the switch and it's indicator light installed in the panel. (Ignore the "Trim" label above the light, it belongs to the switch above.)

Highly recommended!

What a joy it is to be able to take off on a hot day with a full load and quickly climb up to cool altitudes without cooking the cylinder heads!!!

The kit is well made and simple and the instructions are adequate.

I would recommend this modification to anyone who has any issue with cylinder head temps.
It is definitely a valuable add-on for some aircraft, it made a huge difference for my 9A for a max-performance long climb in west Texas summer temps.
That is a slick upgrade...thanks for posting the how-to and pics. My CHTs are a little hotter than I'd like on climb-out when ambient temp is 100 F or above. Maybe Santa will bring me one for Christmas, and I can install it during the annual. :D:D
With the right side mounting, do you see any left-right asymmetries in CHT? Think you need a second one on the left?
I have been looking at this also, did it help with oil temps too, or just CHT?
Oil temps

My oil temps tended to run under 180 deg. I put a butterfly valve in the scat tubing going to the oil cooler and adjusted it so that I get 180 deg. all the time now. The cowl flap did not change that result.

Is there a reason not to put cowl flap on botton one side or other or does it need two to make it cool the whole engine?
I started with one on the left side but needed more cooling so added one on the right. Didn't get double the effect (not test data, just memory). Can't answer whether one side or the other would work better. If you go with two, I didn't have good luck having them on one switch so now have dual switches. Buddy of mine also found that. ASA doesn't think it's an issue.
Your installation story answered one question I had when I received my ASA cowl flap recently. The frame is made to fit into a cowl that is about 1/4" thick. I planned to install mine in the middle of the "coal shovel" exit channel, which is not honeycomb sandwich. It is about 1/16" or less thick. So I was wondering why oh why did Allen make the frame drop in so deep? Whose cowl is so thick? So now I know. In the honeycomb areas, the cowl is apparently about 3/16" thick? so it apparently still took a bit of shim under it, based on your post.

cowl flap frame 2.jpg

Since I was mounting in much thinner material, rather than have a big layer of build-up on the inside to get the flap flush on the outside, I made a new frame that is just 0.063" thick, and re-assembled the flap on that frame. A spacer was needed under one of the actuation linkage anchor points to keep the actuation geometry working right. It is a fairly clever actuation linkage. With the thin frame, the cowl flap surface was slightly below the cowl surface, so I slathered a layer of epoxy/microballoons over it and then sanded it completely flush. The area where it is installed is not flat, it is convex, so it was going to take some filler to contour in any case.

A couple of other install details:

While I was making the new frame, I noticed that the pivot points were assembled with AN bolts and low profile (AN364) elastic stop nuts. This type of application where the parts are moving and the bolt is not clamped down tight really should have a castle nut with a cotter pin. A minor detail? It is just a cowl flap, after all.

But to reassemble the linkage on the new frame, I had to take all the bolts out of the linkage, and I was surprised to find that the AN bolts were all too short, so that the unthreaded portion (the shank) was not long enough to extend through all the moving parts. At each point in the linkage where two pieces are moving on the pivot bolt, one of the two pieces was entirely moving on the threaded portion of the bolt!

cowl flap linkage 2.jpg

Considering that this linkage will see one full cycle per flight (open for take-off, close for cruise, open again for landing) and in an environment with some soot/grime, this seems really poor to me! The linkage parts are made elegantly lean, meaning not a generous amount of edge clearance on the pivot holes, and then, here they are with a high-wear situation with one of the parts turning on the threads of the pivot bolt! It won't be long before those holes are all worn oval, the flap will not close all the way because the nice linkage will have slop at all the joints from wear. Sorry Allen, but I really think you can do better! I replaced all the AN bolts with proper-length clevis bolts so that the parts are turning on the unthreaded shank, then a washer, a castle nut, and a cotter pin.

Finally, I totally understand the logic of having the unit all self-contained, with the actuator installed directly on the cowl flap frame. This is desirable from both a manufacturing standpoint and an installation standpoint, where a single drop-in unit really makes it tidy, fast, simple. - BUT -- the actuator itself is positioned right in the area where the flow is accelerating and trying to rush out through the open cowl flap. The actuator constitutes a blockage of over 15% of the net flow area out of the cowl flap! The aerodynamicist in me just couldn't stand for that, so I added a 5" extension shaft on the actuator, and relocated the actuator upstream, where the velocities are slower, both with the flap closed, and with the flap open. This is a fussy detail I admit, and as I said, I understand the logic of having a self-contained unit. But I like this better.

cowl flap install.jpg

look close you can see the castle nuts on the clevis (pivot) bolts. The extension shaft is a thick-walled 3/8" aluminum tube, threaded at both ends for 8mm x 1.25mm. The actuator shaft is threaded similarly for the black plastic end fitting, which I just moved to the end of the extension tube. the coupling between the original actuator shaft and the extension is just a 1" length of 8mm x 1.25mm bolt, drilled hollow to save a little weight. I ended up needing 2 0.2 mm shim washers between the tubes to get the clocking right, so the end fitting lined up with the linkage. You can also see that I incorporated a radiant-heat shield in the mounting bracket for the actuator, since the whole thing in nestled up among the exhaust pipes. Allen does recommend putting heat shields on the exhaust pipes, that will work too. But this seems tidy.
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Different results

I didn’t have the heart to cut the side of my cowl so I replaced the louvers on one side with the cowl flap. I can see almost no difference with it open or closed which seems odd. My only guess is that I am limited on cool air in, not warm air out.


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On the theory exit area is exit area, and convection for increased ground cooling I made my oil door into a cowl flap. It was easy to verify by pressure/temps flying stock then removing the oil door and flying. if I recall correctly with the oil door on, the temps in the lower cowl started increasing below 120 knots even though cylinders/oil were cooling off and Lower cowl temps continued increasing on the taxi in. With the door removed temps stayed steady or slightly reduced.
I made the oil door as large as I could without it hitting internals. It hinges inward so no extra drag area and when closed the lower cowl pressure helps push it closed. It has substantially helped my summertime quick turns and Earthx battery temps.


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Didnt want to mess around, installed 2 in my -7.
Make a big difference in my tightly cowled hot jugs.
This idea is genius.
I had been struggling with the idea of side louvers or bottom flaps but never considered the side adjustable flap.

I think the theory is the delta-p across the cooling area is better with exit area on the side vs bottom?

I'm on a non-RV experimental flying out of AZ and it is never cool enough.