New cowl vent idea

[Cfrisella]

Hey Guys. Some advice on this cowl vent idea. I have a 7a with a 4 pipe exhaust system. I've been told extra venting will be required, but don't like the look of louvers. I'm also in the composite business and think some carbon fiber vents would look really cool. Here's my idea.

So what do you think? Any thoughts on the overall size of vents required? I realize they may produce more drag then louvers but don't know how much. Will this basic design even work. I can mold and produce them without any problems but can use some opinons from the aerodynamic specialists out there. Thanks!

[scsmith]

First, are you really sure that extra venting is needed? My -8 has the biggest engine that fits in a std cowl (IO-360-A1A, 200 hp) and my CHTs run 275F or so at cruise, and never exceed 360-375F even on hot day climbs.

The key is good baffling. I think many folks have leaks around the inlets and the nose of the engine that they don't know about.


Second, if you do want to make these, some aerodynamics feedback: Try to eliminate the radius lip at the outlets. I know they add lots of stiffness, but find another way to get stiffness. The radius creates problems in both the internal flow trying to get out, and the external flow trying to flow over the vent and pull flow out of the vent as it goes by. Try to have the edges of the opening square and sharp and the surfaces should be parallel to the intended flow direction.
From the picture, its hard to tell if the view is looking at the exterior surface or the interior. They will flow much better if we are looking at the interior. In this case, just remove the lip around the outlet holes completely.

If we are looking at the exterior, i.e. the whole part is below flush to the original surface, they will still flow some, but you want bigger holes in proportion to the ramp area. Eliminate all the lip around the hole, or better yet, reverse the lips so the opening has kind of a radiused intake lip for the flow. It may not be possible to mold such a shape with a one-piece mold.

[Cfrisella]

I guess the issue isn't so much with cooling, but more with leasing pressure. My understanding is the cowl exit is very restricted by the nose gear tubing and then four exhaust pipes. The result causes the cowling to bow out between fasteners.

[RV8RIVETER]

There is no problem with the outlet being too restrictive. Forgetting about leaks and poor flow control, the problem with the stock set-up is the exit itself. It results in an area of high turbulence and even back flow into the exit, see posts of tuft testing.

If you want to make a mold for production, do something like Larry V. and make a "boat tail" exit.

[Cfrisella]

He's the one who said side venting would be needed. I'm also thinging about the boat tail mod.

[Mike S]

Originally Posted by RV8RIVETER

If you want to make a mold for production, do something like Larry V. and make a "boat tail" exit.

That just might be a good idea, I wonder how well a single design would apply to all models of RV??

[Cfrisella]

Well. I'm thinging of the relatively flat sections on the sides of the lower halves. I think most RV's have this in their design.

[pat]

by boat tail mod I assume you mean the teardrop design vetterman built as an extension out of his lower cowl airflow exit where the exhaust would normally go?. If this is the case I believe he put simple louvers in this design in which case this design would look nice.

[jsharkey]

Originally Posted by Cfrisella

Well. I'm thinging of the relatively flat sections on the sides of the lower halves. I think most RV's have this in their design.

Cool carbon fiber look.

Mind you the big trouble here in the North East at the moment is getting the darned engine and oil hot enough

Jim Sharkey
RV-6 Phase 1

[Cfrisella]

Yes it is. Originally he placed them under the cowl . He said they really need to be on the sides to work properly.