Sealing the cowling

[bjohnson1234]

I've seen a lot of threads about inlet and outlet restriction and engine baffles, but I was wondering if anyone had looked at ways to seal the cowling itself to prevent air from leaking out where the cowling meets the firewall and where the top and bottom pieces meet. It seems like that would add to cooling drag without producing any cooling benefit? Maybe it's too small to worry about?

[Jesse]

That is on the low pressure side, so any leaking won't affect cooling I can't imagine it would make much change in drag unless they were really big gaps.

[bjohnson1234]

Originally Posted by Jesse

That is on the low pressure side, so any leaking won't affect cooling I can't imagine it would make much change in drag unless they were really big gaps.

It's the low pressure side, but several people including Larry Vetterman have shown gains with outlet restriction. My understanding is that reducing the amount of air that can flow through the entire system reduces the cooling drag, but it very well may be too small to notice.

[Dbro172]

Well, it would be pretty simple to run a quick test by taping across the joints from the outside, like with some clear packing tape or something. I'm sure the racers have all sorts of tricks like this.

[Tom Martin]

It is unlikely that you will show a gain by taping the seams. However if you also tape some seams on the tail, plug leaks around the canopy, wing roots etc, you might just pick up a bit. Half a knot does not seem like much unless you lost a race by quarter of a knot!
A much larger leak in the cowling occurs in and around the gear leg farings. Typically the upper faring covers the top of the gear leg and is open to the low pressure side of the cowling. This leaves the possibility of a significant amount of air going down your gear legs and squirting out around the intersection farings and out the wheel pants. We know that gear leg farings and wheel pants themselves will add 10 to 15 knots to an aircraft. Thus anything we can do to clean up the gear and pants will likely yield much more then you can get anywhere else on the aircraft.
Extra efforts here do not cost anything other then time and will yield increases each time you fly. Typically the worst glass work that I see on RVs is around the upper and lower gear farings and wheel pants.

[vlittle]

It was a bit of work, but I made a propshaft seal for my HR-II. All air inside the cowling is high pressure air. You don't want it flowing out behind the spinner.

I paid attention to all of Tom's drag reduction advice and learned a few things. Did you know, for example, that the wing interior is high pressure and will create drag plumes wherever it leaks out? I sealed the last ribs of my wing tips with aluminum tape, only to have the tape burst from the internal pressure!

At one point, I was seeing 212 KTAS at 8500 ft, 2650 RPM, WOT, leaned to maximum airspeed. Then I added drag by removing fuel drain and vent fairings and painting stripes on the airplane, then experimenting with VGs.

All fun and worthwhile pursuits, until you discover girls! (my motto since I was a teenager).

[rv7charlie]

Originally Posted by bjohnson1234

I've seen a lot of threads about inlet and outlet restriction and engine baffles, but I was wondering if anyone had looked at ways to seal the cowling itself to prevent air from leaking out where the cowling meets the firewall and where the top and bottom pieces meet. It seems like that would add to cooling drag without producing any cooling benefit? Maybe it's too small to worry about?

Back to the question...

If it's not built yet, you can help reduce seam flow a bit by using the wider flange piano hinge. Offset it so the seam is over the solid area of the hinge flange.

[Dbro172]

Originally Posted by Tom Martin

It is unlikely that you will show a gain by taping the seams. However if you also tape some seams on the tail, plug leaks around the canopy, wing roots etc, you might just pick up a bit. Half a knot does not seem like much unless you lost a race by quarter of a knot!
A much larger leak in the cowling occurs in and around the gear leg farings. Typically the upper faring covers the top of the gear leg and is open to the low pressure side of the cowling. This leaves the possibility of a significant amount of air going down your gear legs and squirting out around the intersection farings and out the wheel pants. We know that gear leg farings and wheel pants themselves will add 10 to 15 knots to an aircraft. Thus anything we can do to clean up the gear and pants will likely yield much more then you can get anywhere else on the aircraft.
Extra efforts here do not cost anything other then time and will yield increases each time you fly. Typically the worst glass work that I see on RVs is around the upper and lower gear farings and wheel pants.

I would always see some oil streaks on top my nose wheel fairing, behind the gear fairing, similar moisture residue as on the bottom of the plain. I suspected this was air traveling from inside the cowl, down the nose gear fairing and on to the top of the nose wheel fairing... last time I had the cowl off I taped off the top of the nose gear fairing tight to the gear leg and guess what! Problem solved. I think I will remove the tape and stuff a little piece of memory foam in there.

[DanH]

Originally Posted by bjohnson1234

I've seen a lot of threads about inlet and outlet restriction and engine baffles, but I was wondering if anyone had looked at ways to seal the cowling itself to prevent air from leaking out where the cowling meets the firewall and where the top and bottom pieces meet. It seems like that would add to cooling drag without producing any cooling benefit? Maybe it's too small to worry about?

My propshaft and cowl side seams are sealed pretty well, but I'm not happy with sealing at the oil door or at the firewall. Considering some lightweight rubber flap at both places, something more like inner tube rubber than the stiffer/thicker silicone or glass reinforced baffle seal material.

Will it add speed? I dunno. I do know how to find out.

[bjohnson1234]

Originally Posted by DanH

My propshaft and cowl side seams are sealed pretty well, but I'm not happy with sealing at the oil door or at the firewall. Considering some lightweight rubber flap at both places, something more like inner tube rubber than the stiffer/thicker silicone or glass reinforced baffle seal material.

Will it add speed? I dunno. I do know how to find out.

Did you do anything special to seal the seams or just a tight fit?