quote:

ORIGINAL: iron eagel

Given that your exit air is going to create a "separation bubble" and thereby cause induced drag would it not be best to duct the heated air as far to the rear as possible?
I know that most cowl designs simply dump the air out the sides or rear of the cowl on top of the wing; doesn't that cause a disruption of the airflow over the top of the delta plane form?
Where you have an airplane that has a high induced drag isn't this a problem, that you could reduce by ducting the heated air toward the rear of the plane as much as possible?
Wouldn't the best solution be to have your exit facing the rear of the craft at the tail even if you need to add a bit of weight to duct it?

quote:

ORIGINAL: iron eagel

Dick,

Regarding cowls….

Whenever the prop spins the cowl is pressurize, and yes the air must be vented.
But as far as the venting providing all the airflow you are mistaken, the intake is not just an opening to a vacuum. Air is pushed into the cowl by both the prop and by ram air effect when the plane is in motion.

To reinforce that statement…

On page 77 Chapter 17 of Andy Lennon’s book the section about cowls says, “Cooling is adequate, as proven by test runs on hot summer days at full rpm with the model stationary and consuming full tanks of fuel.” It also says that “Prop driven air enters the diffuser slows down, cools the cylinder and muffler, expands because of the heat speeds up in the nozzles and exits at considerable velocity”

By no means am I saying that the exit is unimportant just the opposite it is. But the prop does provide some airflow, as does ram air and must be considered, as well as the exit.

quote:

ORIGINAL: da Rock

Actually, the Corsair was one of the fastest airplanes of it's era. And it's engine heat was released almost uniformly around the entire fuselage. But the Mustang, on the other hand was like our models and didn't have cylinders all around the front of the airplane. So the Mustang solution was basically to create a cowl for the heat exchanger and it's sort of a parallel to our problem with our one cylinder heat source.

If we look at the Mustang situation, maybe we can see what made it work and how our engine cowling might do the same.

The Mustang's underslung "cowling" was also basically a vertical arrangement. And the heat exhaust was proportionally aft of the intake. I'd guess it was about 5 times the width of the "cowl" aft of the intake of that cowl. That would put the exit back into air that had already been turbulated. And then, they basically didn't flatten the exit down to the shape of the fuselage, they sorta stood it up. Look at the dump gates of that era. The Thunderbolt had a couple and they were basically as tall as they were wide. When the Mustang needed max area for heat exhaust, the gate opened out. It basically got as tall as it was wide. Those designers increased their area outward. That would do a couple of things. It wouldn't blanket more fuselage area, but only what was already seeing turbulent exhaust. It would also place the even hotter exhaust air farther out from the boundary layer.

Our speed cowls for single cylinder engines place a basically rectangular heat exhaust "on edge" so that as little boundary layer area is affected as possible. Our cowl pretty much dumps most of the hot air appreciably "above" the fuselage. If you will, we could look at the exhaust more as a ribbon than a bubble. And the narrowest part of the ribbon is the only part that is disrupting whatever boundary layer exists.

We're already dumping the air as far OUT from the fuselage as possible. And we've hopefully done a decent job of making the cowl so that it disrupts the flow around it a whole lot less than the naked cylinder did. So the airflow is less turbulated because of the cowl, and the exhaust is being beneficially distributed as a ribbon with minimal contact to the fuselage.

AND THINK ABOUT THIS DETAIL............ The basically vertical rectangle of the exhaust doesn't have to open completely down to the fuselage. I used to build my rat cowls with an exhaust opening up the fin a bit. The cowl had no opening at all at the base where it came up out of the fuselage. I used to kid myself that I had some understanding of how thick the boundary layer was around the fuselage, and wouldn't have the exhaust dump down in that area. Did it work? I got not the slightest idea, because I never tested it. But I guarantee you, it being a CL environment, if we had tested it we would have known with decent certainty. And look at the Mustang's setup. Look at the intake and you'll see they stood the opening away from the fuselage. I'm betting that was a boundary layer consideration. Heck, look at every projecting intake you can find on WWII a/c. And then look at the exhausts. I'd bet the problem of downstream "bubbles" isn't a major consideration.

quote:

ORIGINAL: da Rock

If you want the delta to go faster, adding a cowl probably will help. Coming up with one that gives the ultimate increase ain't gonna' be close to easy. Heck, it's not even easy to discuss it.