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Stopping Cowl Excrescence Drag

walter

walter

Well Known Member
I assume it is worthwhile trying to stop air that leaks past the cowl hinges? I used an oversized hinge on one half of the cowl horizontal hinges so there is no leak there but I did not on the sides and top. One idea I have is to use baffle material as a flap but I don't think that will be very effective since it's not forced against the opposite half of the hinge except by air movement. Any ideas out there or just build on?
 
I too have thought about it. My firewall seam is all 1/4 turn fasteners, so a rubber flap seal seems like it should work. I was going to put one around the perimeter of the oil door too. Now if I wasn't so lazy...

The side seam is a fiberglass joggle with screws and nutplates, so it's pretty much leak-free.
 
Tape

Why not just do what the glider guys do, tape the seams? There is some good vinyl tape that is low tack. My glider buddies used to tape the seams every day and then remove the tape at the end of the day.. i see no reason why it cant stay on for a few weeks.
 
Wonder if it's significant enough to worry about. Seems the helical airflow from the propeller will cause more turbulence/drag than leakage from the cowl hinge.

Second thing is I do not believe leakage through the cowl hinge would affect the Delta P across the baffles.
 
Marc Bourget said:
I do not believe leakage through the cowl hinge would affect the Delta P across the baffles.
Click to expand...

I think the theory to preventing air leaking from out of cowl seams is that it's not re-accelerated air, therefore contributes to drag. You wouldn't accept a jet engine directing some of it's thrust out the sidewall perpendicular to the direction of intended thrust.

Bevan
 
I agree, but considering the external environment and the fact that the relatively low pressure in the bottom half of the cowl and low flow volume from the hinge gaps is faint by comparison with jet engine bleed, I still think it sounds like a solution looking for a problem, IYKWIM.

Now, if we're talking about asthetics, that's different and I say "Go for it!"
 
Aren't the hinges offset a bit from the actual join line of the cowl halves? If not, they certainly could be. I thought I read about this as a way to assist positioning the cowl halves on reassembly.

If they are, the hinge knuckles will overlap the fiberglass part of the cowl and not the gap, effectively blocking most of the leakage.

In any case, a concerned builder could make a simple ship-lap joint in the glass itself to make a basic labyrinth seal, if they want to.

Dave
 
Most glider pilots use electrical tape, white. While available in different colors, the chance of matching your color scheme is about zero. There is also a reason why it is removed frequently. It shrinks over time, especially if you stretch it when putting on.

On my glider, a HP-18 homebuilt that I painted with acrylic enamel car paint, the tape can also lift the paint. Its almost 30 years old.

Vinyl tape is better, it does not shrink as much, but the color match is an issue. Lifting the paint depends on the paint job in any case.

My -10 cowl seam has no leakage that counts, I did a plenum.
 
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