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Cowl Differential Pressures

I have the cowlings off now and I set the top one in place to take this picture. Note that when normally attached there is a small 1/8"? gap between the foam seal and the prop hub.
My intent to seal around the prop shaft was for drag reduction, not cooling. However as I noted before I was not able to demonstrate an increase in speed but I could see an effect on cooling. I have made many changes to this plane over the last few years and have demonstrated a 10 knot increase in speed, this can be verified by SARL participation. The only change that I could really verify, by itself, was the change from an MT prop to a BA Hartzell. All the other changes, this one included, collectively added up but were too small to individually note. However, improving the cooling allowed me to tighten up my exit air which may have helped to improve speed. Cooling system changes are like pieces of a puzzle in that they all have to work together.
 
Looking good, Dan. The annular seal should help to spoil some of the incoming (leaking) flow's energy. I don't have any reservations about it at all, except for it leaving the possibility of a burning rubber smell in the cockpit :)

Hey, remember me suggesting the cowl flap? And guess who did it best.. Larry Vetterman. Banging good results there too. Dan, are you gonna do a cowl exit afterbody like Larry did? It seems to work really well.

As an aside - Someone mentioned reverse flow just aft of the cowl inlet, on the outside surface of the cowling. Its worse than that. A friend of mine left his keys still stuck in the forward baggage door lock, WITH the whole keychain still attached. Noticed it in flight... he was amazed to see the whole thing hanging straight down, as if still on the ground. His report suggests that we have lots of detatched flow along the fuselage sides, probably emanating from the tight radius at the inlet lip.
 
Tom Martin said:
I have the cowlings off now and I set the top one in place to take this picture. Note that when normally attached there is a small 1/8"? gap between the foam seal and the prop hub.....
Click to expand...

Got it Tom, thanks (and nice work!)

Pulling down the exit size is exactly why I want this sealed. Can't increase exit velocity with the nose leaking pressure. As you say, it is all a system.

Dan, are you gonna do a cowl exit afterbody like Larry did? It seems to work really well.
Click to expand...

Bill, this cowl is modified so I can swap the exit area glass for anything I want later.....smaller fixed exits, variable exits, an afterbody, whatever. Think of it as a modular exit system.

Of course I may get lazy and do nothing but go fly:)
 
Extended exit area cowling helped!

Hi gang, I have been challenged with higher than expected oil temps on my RV-8 in summer months. They are within the engine manufacturer's accepted range, but not the ideal. I've perfected my cowl seals, lowering temps by 6-8 degrees, changed to a SW type oil cooler, another few degrees, but will still see high oil temps when doing any low altitude cruising; here in AZ, that's 7,500. This weekend I flew a test flight at 10,500, full out, and leaned at 59 degrees OAT. I peeked and remained at 204F in oil temps. On descent and then cruise at 7,500, and 79 degrees OAT, oil temp rose to 214, and by the time I did pattern and landed hit 228F, with 85 degree OAT on the ground. My hangar neighbor has a very similar set up on his RV-8, but runs in the 190 range. I took some measurements and made observations about his cowl exit area and the only difference is that his lower portion extends back about 1/2" further than mine, which is cut flush with my firewall. I used strip of aluminum to mine, extending this area about 1" and saw great results with my second test flight and OAT's about 10 degrees higher than my first flight. In this flight, I did not repeat the 10,500 cruise since the improvement was obvious at 7,500, where my oil temp did not exceed 208 at cruise, down 12 degrees, and peaked at 214 in my pattern and landing, down 14 degrees! I am thinking about adding a little curl downward in my aluminum piece to create even lower pressure in this area.

I am open to any other ideas.

20130810_100423_zpsa71aced5.jpg.html
 
I don't think it's an illogical result. You're running the stock "cattle chute" cowl exit, so velocity is low, and I'd expect some reverse flow behind it. There is a video out there somewhere of a yarn test on a yellow RV-7(?) which clearly shows the circulation behind the big exit. I suspect the circulation is an effective reduction of exit area.

I'm thinking the extended lower lip helps maintain attached flow at the yellow arrow. I could be completely full of $***. We have lots of educated aero guys. Thoughts please?

Two notes; I'd suggest flying some number of additional flights and see if your observation remains consistent. And if it does, add some additional length, no lip, fly again, then try a lip if you must.

You didn't mention CHTs or oil cooler size. If CHT is good you may need a larger oil cooler, or more cooler mass flow, or lower inlet air temp to the cooler.
 
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Interesting thoughts on the "extended" lower lip for the exit air. When I built my 6A and was trimming the lower cowling to fit at the firewall, I purposely left the air exit fiberglass long. I did not trim any of the excess fiberglass off. This lower area sticks aft several inches. I did need to cut out some of this excess around where the exhaust pipes stick out beneath the cowling.

I have always had trouble with low oil temps. They would consistently run about 100*F warmer than the ambient air temp. Balanced CHT's run 366*F in cruse.

To get the oil temps up, I added the oil cooler shutter. CHT's remain the same.

Configuration: O-360, fixed pitch, full pressure plenum (dog house). Oil temp probe calibrated at 32F and 212F.
 
Old thread. Here's the rest of the story, for the benefit of future readers.

The original question was about sealing around the propshaft. The rubber flap seals shown earlier in the thread have now been working very well for 285 hours or so. The seal wore to a feather edge and polished a nice wear surface on the cast face of the ring gear carrier. It has not yet worn through the paint. So far there has been no downside at all.

We also discussed how to best arrange an exit. Way back in November of 2008 Bill Wightman said:

If you do it right, then you can cut down the rectangular exit duct on the bottom of the cowl - probably to around 1/2 its current size. This gives you less frontal area and a smaller "speed bump" on your lower cowl. Now we've got a "win-win" way to control mass flow.
The flap, when closed, should parallel the local streamline.

I had temporarily run out of cash (recall 2008 was an awful business year) but I had plenty of time. So, I decided to reconstruct the cowl with an eye toward active control of climb/cruise temperatures and general reduction of cooling drag. It got large low velocity inlets, a plenum, and the ability to swap exit panel sections as desired. Regular readers have seen most of the process in a thread titled "The Shrinking Exit".

Anyway, the bottom line is that Bill was entirely correct, in my opinion. I now run less than half the original exit area in cruise, having eliminated the entire rectangular exit chute on the cowl belly. Climb mass flow is controlled with additional exit area on demand, in the form of a set of "gills" (or big louvers if you prefer) driven by a servo. You can see it starting at post 147:

http://www.vansairforce.com/community/showthread.php?t=68241&page=15

Yarn tests show nice clean flow with the exit door closed. Exit pitot-static measurements show exit velocity increasing about 60% between open and closed. That's pure drag reduction.

BTW, the exit extends 4" rearward of the firewall.
 
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Hmm, makes sense to me! To answer your earlier questions Dan, on my CHT's, I originally had higher temps, and dramatically lower in the front two than the back two. I've seen many folks with the little aluminum sheet dams on the bottom side of the intakes. I simply placed a small strip of aluminum tape on the front two cylinders. This evened them out, and fixing my baffle seals lowered CHT's, generally in the area of 380F at 7,500, and 365F at 10,500, with no issues in the pattern. I replaced the Niagara cooler with an Aero Classic, from Pacific Oil Coolers, which is a SW look alike, high efficiency. I had the choice of going with two forms of larger cooler, one was wider, with an additional row, and one had the same number of rows, but all rows were an inch longer than the stock Niagara. I went with the second. One other difference between mine and my hangar neighbor is that mine has the exhaust extensions the angle down from the stock Vetterman exhaust. I am thinking about removing these also. I could extend my lower cowling quite a bit then. What is a bit interesting to me is that I believe I have actually shrunk the exit area by doing this, and yet improved the oil temps. I'll have to fly again, but I think my CHT's may have gone up slightly.
 
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So, I don't see any actual numbers on differential pressure posted here. Anyone actually measure what they have?
 
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