[elippse]

If you don't have a seal around the crank extension where it passes through the forward end of the cowling, it is possible for the high pressure plenum air to go forward around it and flow through the cowling-spinner gap. When it does this it is passing radially-outward and interferes with the smooth flow from the spinner, resulting in turbulence and increasing drag. By increasing the diameter of the cowling by 3/4" behind the spinner and putting a 3/8" radius curve in it, air flowing out of the gap will be carried right around the radius by Coanda effect so that it will flow back over the cowling, eliminating this drag. This effect is also useful in the gap ahead of control surfaces. By increasing the thickness of the leading edge of a rudder, elevator, and aileron, and giving it a small radius, it will do several things. It will cause any air flowing through the gap to be turned to the rear, it will scrub off the boundary layer build-up, it will reduce control surface dead-band, and it will reduce flutter tendencies, especially if the added radius at the leading edge is blended in to the surface behind it so as to form a slight concave. A horizontal surface treated in the way can have its drag reduced by as much as 10%!

 

[Mike S]

Paul, drawings or photos for us dummies, please

 

[aerhed]

Mike, you beat me. Ellipse, I think I've got that, but could you throw a quick sketch? That's a cool effect that turns around some of what my "eye" thinks is right.

 

[Bob Axsom]

My stock cowl RV-6A does have a seal

My stock cowl RV-6A does have a seal around the crankcase behind the the spinner in the cowl. There was some talk by Scott McDaniels of Van's about back flow from the lower cowl out this opening so I developed a baffle to block that and tested it. It actually reduced the speed of the airplane. when I removed it and reflew the test the speed was restored.

Bob Axsom

 

[elippse]

Drawing

Paul, drawings or photos for us dummies, please

I wish I had the ability that you guys show in posting pix and drawings, so maybe I'll try to give a better description in words. You are looking at the front of the cowling from the side. You see a very smooth continuous line from the spinner on to the cowling. Now let's increase the diameter of the cowling right behind the 13" spinner so that it is now 13 3/4" diameter. What we now have is an abrupt 3/8" step all around right behind the spinner. Now let's give this step a nice 90 degree-or-so round 3/8" radius, so that if you held a toothpick in the gap and ran it up and around that curve it would turn smoothly 90 degrees onto the cowling. That is what the air will do, thanks to the discovery of Henri Coanda, who almost lost his life when he was fascinated by the sides of his airplane catching fire with his jet engine experiment and almost flew into a wall! Google Coanda effect, and it will probably tell you to hold the convex bowl of a spoon into the flow of water coming from a faucet and it will say for you to watch how the bowl moves into the flow, rather than away, and how the flow will attach itself to the bowl and move smoothly around so that when it leaves the edge it will be going off to the side tangent to the edge of the bowl surface. This, by the way, is what causes 2/3 of a wing's total lift. Turn the bowl around into the stream so that you will be trying to push the concave bowl surface into the stream and it will try to push it back out of the stream. This is the other 1/3 of the wing's total lift, that follows a sinusoid that peaks at 45 degree AOA and goes to zero at 90 degrees AOA. 'Bet you thought that all lift ceased at stall! BTW; the reason the convex bowl moved into the stream was that the flow around the bowl caused a low pressure outside of the stream, so that air outside of the bowl pushed on the concave surface and forced it into the stream. Only stuff such as gravity and magnetism suck, most other things push.