Cooling air exit ramp lip

[BruceMe]

I'm FWF on my RV-4 project. I was reading the (incredably long winded) LOP thread and found a few posts about exit lips. They come stock on the newer model RVs, should I fabricate one on my RV-4 now just to increase flow. More is almost always better.

-Bruce
RV-4 (N254MM reserved)

[osxuser]

More airflow out could equal more drag. I wouldn't unless after you are operational you have cooling problems.

[Chino Tom]

Don't think I agree. I installed a "C" shaped piece of 016 between the engine mount legs attached to the firewall to help smooth the exit flow out of my O-360 A1A powered RV-6. I didn't slow down at all and see slightly lower cyl head temps. I was attempting at the time to go faster. I installed the newer version of the pressure recovery wheel pants (my -6 was competed in 1998) and saw a 2-3 knot increase in speed. Was it the exit ramp or the wheel pants? Or a little of both? Speed tests were done as before WOT, 2700 rpm, and a constant density altitude on 3 (90 deg) legs recording the ground speed and plugging into an applet to determine TAS. Still no where near as fast as Dan though.

[walter]

Originally Posted by BruceMe

They come stock on the newer model RVs

They do? Didn't on mine (2004 RV-7A). Anybody have pictures?

The RV-10 has louvers as an option. I have a pair but have yet to install
them.

The other option is to cut the exhaust pipes even with the cowl outlet.
This sounds interesting but I wonder if the exhaust pulse will make the
floor vibrations even worse.

Smoothing the outflow is what the RV-8 "C" plate does. It's next to
impossible to do that on a 7A model though I may try.

At the end of this month I will be moving my plane from CNO to CPM. Instead
of an hour away it will be 12 minutes. At that time I plan to do considerable
testing along these lines.

[pierre smith]

Don't do it. We spent a considerable amount of time getting our baffling to fit really well on our 6A by cutting a little off at a time and paid particular attention to the baffle in the front of the engine (the one that goes across the top front) that keeps the air from backing out of the area around the spinner, causing a lot of drag. The net result is that we seldom see 400* F and usually cruise power yields 350 (75%) and 202+ TAS.

You can fabricate an "inner" lip similar to the RV 8s lower firewall lip and bring it up and around back to the firewall, smoothing the exit air flow. Someone else mentioned cutting off the exhaust pipes but Van says that you'll have a pulsing/vibrating floor with more heat too. Since we're already around 200 MPH cruise, it doesn't seem worth the tradeoff.

[Sam Buchanan]

The other option is to cut the exhaust pipes even with the cowl outlet.
This sounds interesting but I wonder if the exhaust pulse will make the
floor vibrations even worse.

Several hundred hours ago I cut the turndowns off the pipes on my RV-6. Even though it was kinda cool to not see the pipes hanging out of the cowl, the mod definitely made the exhaust "sound interesting" and resulted in chronic gray exhaust stains on the bottom of the plane. Since I have insulation on the floor I didn't notice increased vibration but after a few hundred hours of cleaning the bottom of the plane I had had enough. Larry Vetterman sent me a couple of turndowns (he keeps them on hand for those of us who foolishly cut off the originals....) which I welded on the pipes. Now the engine sounds quieter and the belly of the plane stays clean.

[rv620mr]

I'm wondering about using a modified NACA vent in the center of lower cowl, just aft of the injection servo, as an alternative to the Kent Paser/Speed With Economy "exhaust pump" modification. But, I'm pretty amateurish (at both modifying the -6 and posting here!). Is this a crazy idea?

Monte

[gmcjetpilot]

Originally Posted by rv620mr

I'm wondering about using a modified NACA vent in the center of lower cowl, just aft of the injection servo, as an alternative to the Kent Paser/Speed With Economy "exhaust pump" modification. But, I'm pretty amateurish (at both modifying the -6 and posting here!). Is this a crazy idea?

Monte

Not sure what you are talking about in Kent's book:
Exhaust Jet Thrust Nozzle
Exhaust anti-reversion cone
Cowl Air Outlet Venturi

If you are talking about an augmenter tube for exhaust, which is common on many Cessna and Piper Twins, this works, but I don't see how a NACA scoop works into it. You don't want to introduce any more air into the lower cowl just for the purpose of exiting right back out the cowl exit. You have plenty of air coming thru the cowl inlets to do the job. I just don't understand, but in general you don't want more scoops of any kind than you absolutly need.

[walter]

Originally Posted by gmcjetpilot

You don't want to introduce any more air into the lower cowl just for the purpose of exiting right back out the cowl exit. You have plenty of air coming thru the cowl inlets to do the job. I just don't understand, but in general you don't want more scoops of any kind than you absolutly need.

Absolutely! My phase 1 was conducted without wheelpants or gear leg fairings.
While troubleshooting the high CHT issue I had (before I found the problem)
I noticed that on the A models the cutout on the lower cowl for the nose gear
leg is about 3 inches long and a little bit over 1 inch wide. This is done so
you can get the lower cowl off. I realized that pressurized air might enter
into this area and entering the lower pressure area of the cowl just ahead of the
cowl exit. After taping it off I saw a 10-15 degree drop in CHT temps.

[tomwebster]

Walter,
Thanks for the heads up on the cowl slot
under the carb. I was wondering what I
should do about that.

I think I will mount the nose gear leg fairing
and the upper intersection fairing for phase-1
flight testing. This should give me a more true
picture of any cooling problems with the cowl.
It also will protect the nose gear leg stiffener.

Also, thanks for the info and pictures about
the air inlet ramps. I haven't flown yet and
don't know how well the baffle seals will work.

Tom