Drag Reduction: Adhering Fabric to Aluminum Sheet

[Patrick]

I've got an aluminum airplane with a lot of bumps, brackets, bolts, and protrusions under the belly. I'd like to clean up the airflow under there, and reduce drag...

One idea is to adhere some fabric to the sheet metal on the underside of the fuselage, and stretch that fabric across the bottom of the fuselage, in effect 'burying' a lot of protrusions and thus smoothing the airflow.

Has anyone done anything similar on their plane...? Any ideas of what kind of fabric and adhesive to use...? I have a fair bit of experience with metals, but none with fabric.

Thoughts....?

Thanks,

Patrick Hoyt
N63PZ

[BoKu]

I've got an aluminum airplane with a lot of bumps, brackets, bolts, and protrusions under the belly. I'd like to clean up the airflow under there, and reduce drag...

One idea is to adhere some fabric to the sheet metal on the underside of the fuselage, and stretch that fabric across the bottom of the fuselage, in effect 'burying' a lot of protrusions and thus smoothing the airflow.

Has anyone done anything similar on their plane...? Any ideas of what kind of fabric and adhesive to use...? I have a fair bit of experience with metals, but none with fabric.

Thoughts....?

Thanks,

Patrick Hoyt
N63PZ

***I'm not an engineer***

I suspect that fabric over that stuff is not going to make any aerodynamic improvement. The bumps might be a bit smoother, but unless you're very careful the bumps themselves will add to frontal area, and their shapes are unlikely to have less drag than the stuff they cover. But the fabric will add weight, and also make it difficult to access, inspect, and maintain all that stuff that it covers.

My advice would be to consider adding one or two fiberglass blister fairings over the most egregious things, forget the rest, and just try to avoid looking down there.

--Bob K.

[WLIU]

Take a look at how the airflow has already been disturbed by the time it gets to those bumps on the belly. Does your engine exhaust flow down there? Does the cooling air from the cowling get dumped ahead of them? Is there a nosewheel hanging down? If the airflow is already really turbulent, then no point in doing any work on the belly. I read that the British Spitfire left the tailwheel out in the breeze because the engineers calculated that the penalty of the added weight of a retract mechanism would be worse than the small drag penalty of the tailwheel out in the already disturbed airflow.

Upper surfaces are an entirely different topic. They live in much smoother air and there is great benefit in keeping that airflow as smooth as practical. Gap seals and tight fairings are very helpful.

If you really want to reduce drag, look at the mess under the engine cowling. Leaking baffle seals and intake ducts, and oil coolers that dump more air than needed all are hidden drag producers.

Best of luck,

Wes

[Bill Berson]

I agree with above comments.
However, you might try some small fabric "patches" on each individual bolt. Cut a streamlined fabric shape about 1"x3" and contact cement the perimeter. Then heat shrink for a nice shape.