Speed mod idea float

[Bob Axsom]

Glenn, Pierre, Kevin and others I'm sure, have been making cooling drag comments about the air after it has passed out of the plennum past the cylinders and into the lower cowl. Not having unlimited wealth to get speed the most direct way - brute force - I am of course interested. I spent a good part of the afternoon cutting file folders into pieces representing baffle components for the area below the engine. Problems are present but they are not insurmountable as far as making a "catch bowl" that funnels into the cowl outlet are concerned. A lot of detail work would be required to support it in place without any hard contact with the engine. Soft contact could be implemented with high temp seals perhaps. Another approach would be to develop it in stepwise fashion, like first just developing side baffles to bring the air into a narrow band vertically aligned with the outlet, then an upper-aft baffle to bring the vertical column down to the bottom of the firewall/top of the outlet and finally perhaps a fwd/lower baffle to direct the air back over the filter air box. Any thoughts?

Bob Axsom

[flyeyes]

Originally Posted by Bob Axsom

(snip)Not having unlimited wealth to get speed the most direct way - brute force - I am of course interested. I spent a good part of the afternoon cutting file folders into pieces representing baffle components for the area below the engine. Problems are present but they are not insurmountable as far as making a "catch bowl" that funnels into the cowl outlet are concerned. A lot of detail work would be required to support it in place without any hard contact with the engine. Soft contact could be implemented with high temp seals perhaps. Another approach would be to develop it in stepwise fashion, like first just developing side baffles to bring the air into a narrow band vertically aligned with the outlet, then an upper-aft baffle to bring the vertical column down to the bottom of the firewall/top of the outlet and finally perhaps a fwd/lower baffle to direct the air back over the filter air box. Any thoughts?

Bob Axsom

Bob--I think you are definitely thinking in the right direction here. You might want to look here for some ideas. Not an RV, but a lot of careful and interesting thought has gone into this airplane.

James Freeman

[RV8RIVETER]

Bob

Sounds like a good idea to try. I have done alot of research in this area, as I intend to build my own carbon plenum and diffused induction inlet on my -8. I know Dave Anders has a 45deg or so ramp on his firewall to keep the exit air from having to make that ugly (slow) 90deg prior to exiting the cowl. The big thing that needs to be done is get the air speed of the exiting air up as it exits, to decrease drag. In that vein I plan on using exhaust scavanged cooling air exit. I have a 4-1 ceramic coated ehaust and plan on making an outlet scoop conform around and extend past the end of the collector by 4 inches or so. In "theory" (major stress here) this should both increase exit air velocity and lower the pressure of the lower cowl air, increasing the pressure differential across the engine (increasing cooling effeciency as well).

I don't plan on any racing, due to my "hopefully" MT prop, but I want the rest of it to be as effecient as I can possibly make it.

Please keep us posted, it would be nice t share data and experience.

[gmcjetpilot]

Originally Posted by Bob Axsom

Any thoughts? Bob Axsom

No specifics but way to go BOB, all my moral support to you. I would try and visualize where the air is coming from and where it has to go. I have seen two approaches. One is fiberglass that is supported off of the engine mounts with adel clamps. The other is similar but with sheet metal. I am sure you can bond on some smooth formers and flow control on in the lower cowl. I'll email you some pictures when I get a chance.

Some flight test ideas. One is to coat the lower cowl with some colored dye and fly. It will make a mess of the belly (it is like light oil). The streak pattens will show airflow. The other method, a ground test would use smoke. How you track it would be a little tricky. May be some lights and small video camera lens. YOU will need a source of airflow or blowers. Button the cowl up, with camera'(s), and blowing the air and smoke, it will give insight into where airflow is going or should not be going.

George

[atreff]

Bob,
I've often thought about the same thing: how do I get the airflow downstream of the cyl fins to turn gently toward the exit duct. I've been thinking of bending up some sheet metal 'guides' and clamping them to the intake risers on the inboard side.

Then, aft of that, clamped to the engine mount, a angled piece fwd of the firewall, that goes accross the width of the cowl, deflecting the air downward toward the exit ramp. This last piece would be really tough to do with the hoses and cables in the way, but I did see it on an RV-4 recently.

Another speed mod that one builder showed me was he had glassed around the tires inside his wheel pants, in the hope of eliminating much of the volume of airspace. This to me, was excessive work, not sure of the theory either.

Keep up the good work. This is as exciting as the first time I picked up Speed with Economy.
Art

[pierre smith]

Mornin' all,
In the seventies I built a Cassutt F1 racer with a 100 HP Conti (C-90 upped to 100 HP) that went 230 MPH at 3800 RPM and a cut down Sensenich. Bear in mind that it had a 15' wingspan and weighed 510 lbs empty. I spent quite a bit of time around the late Bill Falck and his winning F1, "Rivets". Rivets didn't have any inlet cooling intakes in front of the cylinders but had a lower, oval shaped inlet (about where the RVs carb intake is) with updraft cooling that was totally ducted up to the cylinders and then an upper plenum with more ducting over the engine and down to the lower exit which was in line with the frontal inlet. The exhaust also ran through the same exit. Even tho' Rivets was pushing 700 lbs (as I recall), he was always last starting and got faster and faster, picking the competition off one at a time and usually won by the last or next to last lap. Falck was a German, or Euro engineer who devoted an incredible amount of time to reducing cooling drag and it obviously paid big dividends.

With the RV series, if all the exit corners are radiused, especially the lower firewall lip and the corners on the lower cowl (the vertical sides) where it meets the firewall, a lot of exit drag could be reduced. With the A models, the nosewheel mounting tubes can all be streamlined and the air guided better but how much speed is all the trouble worth? A James cowl would go a long way but I have a friend who put one on his -8 and it was two to three times the work than making a Van's cowl fit. He said "never again".
Pierre

[Nuisance]

I do think you are on the right track, Bob. Anywhere air is flowing, it needs to be guided. I see estimates as high as 30% for the portion of drag consumed by engine cooling.

Since I am determined to land with a safe quantity of fuel at the end, the AirVenture Cup race has become an efficiency challange for me. So, I have been spending some time doing just what you propose. If it works, I will show it off at OSH. If it doesn't, I will just quietly pretend I didn't waste 100 hours work on nothing.

John

[Bob Axsom]

I have stepped back and given this a lot of thought for the first configuration. I want to implement something that will give me a clear indication of performance to be gained and not take the plane out of service for an extended period. I saw Rivets race in the 70s and the thought of a near perfect cooling airflow design is very stimulating and that is still in the big plan. Here is what I have worked out in my head:

1- Use the inside of the lower cowl shell without modification as the lower side of an air funnel to direct the air to the outlet.

2- Make an aluminum deflector plate of at least two pieces that crosses behind the engine to form the upper side of the air funnel with the following details:
- Make two plates that can be inserted from the sides and joined in the middle with platenutes and screws.
- Position upper edge well forward to maximize slope back to the outlet
- Attach the lower edge across the full width of the lip above the outlet with screws & platenuts.
- Outboard of the engine and the outlet, bend the plate forward and down to direct the air toward the outlet with the resulting inclined planes and avoid the flat firewall.
- Outboard of the outlet cut the edges to conform to the inside of the lower cowl with a 1/2" setback.
- Add large cutouts for ever wire, tube, etc that must go through the air deflector plate.
- Make slot covers to close long slots required for plate instalation and install with removable hardware.
- Fill all transition penetration holes with red Dow 736 RTV for air seal and abrasion protection.
- Rivet red high temp baffle seal to the edge plate to close the gap between the deflector plate and the inside of the lower cowl.
- Support the upper and outer portions of the deflector plate with suitable aluminum angle struts attached to the engine mount with bare metal clamps (no cushions) and to the deflector plate with angle brackets (flush riveted to the back of the plate) and standard fastener hardware.

OK that's the first implementation concept. After some testing, the next level of effort will be directed at cleaning up those things in the air flow path inside the funnel. The nose strut hardware and the filter air box stand out in this area of interest.

Bob Axsom

[rv8ch]

Originally Posted by flyeyes

Bob--I think you are definitely thinking in the right direction here. You might want to look http://www.ez.org/feature/F0502-1/F0502-1.htm here for some ideas. Not an RV, but a lot of careful and interesting thought has gone into this airplane.

That is great. This clarified one thing that has always bothered me - people complaining about their cowls expanding in flight. This guy shows that if that is happening, you're doing it wrong. The key is to make sure you have negative pressure at the cowl inlet, then your airflows will be great. This completely syncs with the work that the Eggenfellner team have been doing with their package. The number one thing that helped with the cooling is to add more exit air capacity. The trick is to get the air out without increasing drag.

Thanks a lot for this link - I'd have never found it!

[hngrflyr]

A couple of years ago, I bummed a ride in a friends RV-6 to an airport 150 miles away to pick up the RV-6 I had recently purchased. His airplane has a 0-320 with a constant speed propeller, and he has a radius baffle at the lower edge of his firewall. My RV-6 has a 0-320 turning a wood fixed pitch propeller, and no baffle at the lower edge of the firewall. I took off about 30 seconds ahead of him. I had gone 40 miles by the time he had caught me. I am sure he reached our cruising altitude in less time than I did. At 8500', and WOT he was maybe two knots faster than I was. His RV-6 is very nicely built, slightly smoother than mine. I never did loose sight of him for the 150 mile flight home. It was my first flight as pilot in an RV-6.
Another RV-6 on our airport has had much attention paid to cutting cooling drag, and has a Lycon 0-320. It is a very slick airplane. He even has the fairing on the tailwheel. It is considerably faster than mine. Probably 10-12KTS faster.
The conclusion I draw is, if you have the time and money to throw at it, there are good performance gains to be had by cutting drag. A little here and a little there will get the job done.

Bob Severns