The Shrinking Exit

[DanH]

Recall I've been experimenting with a different approach to cooling....large, low velocity inlets with a throttled exit (and a bunch of details). The cowl was modified so I could swap exit panels and thus vary the exit area.

Started with the one on the left, changed to the middle one at 15 hours or so, and just started flying the one on the right. All are smaller than stock by a good margin. I'll fly the latest into warmer weather, then consider the next step. I want to do some temperature and pressure measurements when it gets really hot this summer. Plus, from a standpoint of break-in, I'm only at 70 hours or so. his Sunday, with the smallest exit, a WOT/2700 climb at 105 knots from 200MSL to 5500 MSL at 50F OAT netted a highest CHT of 366F. CHT spread is 20F. 60% cruise is 300F or less. No cylinder has ever seen 400F. Engine is an angle valve IO-390.



Stock cowl:



It was modified right after the above photo. Pipe was also modified to raise it all vertically:

[zav6a]

Hi Dan

I have come to the conclusion that your cooling strategy (exterior pressure recovery) is the best compromise ideal for the kind of flying RVs do and and the limited space we have to work with under the cowl.

I could probably find pictures somewhere but I will just ask. What shape and how large of inlets?

I presume the desired effect of reducing the tunnel bump are less frontal area and better aerodynamics. It will also produce higher pressures in the lower cowl to help kick the cooling air out at a higher velocity (less drag). Sealing the lower cowl then takes on a more critical role.

I am interested in your sealing measures around the prop and cowl seams. I think I recall a thread where you described what you did around the prop. I have wondered at what point the lower cowl starts to distort as it does not have the the best shape to hold a lot of pressure. Have you played with a manometer as you put the various subcowls sheets on?

[DanH]

Originally Posted by zav6a

Hi Dan

I have come to the conclusion that your cooling strategy (exterior pressure recovery) is the best compromise ideal for the kind of flying RVs do and and the limited space we have to work with under the cowl.

Me too, which is why I went from theory to experiment.

What shape and how large of inlets?

6" diameter. No claim of optimum size; my math could be wrong.

I presume the desired effect of reducing the tunnel bump are less frontal area and better aerodynamics. It will also produce higher pressures in the lower cowl to help kick the cooling air out at a higher velocity (less drag). Sealing the lower cowl then takes on a more critical role.

Didn't consider frontal area reduction; goal is to increase exit velocity. Cowl sealing is tricky and hardly perfect. I have a few tweaks in mind for the future.

I am interested in your sealing measures around the prop and cowl seams. I think I recall a thread where you described what you did around the prop. I have wondered at what point the lower cowl starts to distort as it does not have the the best shape to hold a lot of pressure. Have you played with a manometer as you put the various subcowls sheets on?

The flap seal shown above is doing fine. More or less as expected it has glazed a nice smooth strip on the ring gear casting after wearing itself to a feather edge. Not perfect, but it's what I did. Tom Martin took a different approach with some foam rubber, but I don't have a prop extension.



I too am wondering about rising internal pressure, which is one reason I'm not going smaller than exit #3 until I install pressure and temperature transducers. Real data will tell the tale.

[hydroguy2]

Once again....Nice work Dan.

[RV8RIVETER]

Truly nice work Dan!

Do you have an estimate as to what your exit area is now?

My first pass at my nozzle exit area is approx (6.1 inch dia) 29 sq inches. I will be making different size and configuration exit nozzles to test.

[Danny7]

what kind of airflow control have you done on the firewall corner where it comes down to that reflective ramp?

[fatherson]

Dan, I know the work you do is its own reward, and you hardly need any praise from the likes of me to motivate your efforts, but I want you to know how much I am inspired by not only your experiments, but also by your willingness to share your progress with all of us.

There are so many disparate reasons each of us are doing this, but for so many of us, the claim to "Experimental" is just an FAA label, not a badge of honor. We build to fly, standing on the shoulders of all those who came before us, learning ourselves, but not adding very much to the body of knowledge in our community. Sure, I'm still proud of my participation in homebuilt aviation, but it's people like you who earn the right to put "Experimental" on your airplane as an actual, bona-fide reward.

Thanks for helping us all expand our knowledge of the black magic that improves our craft.

--Stephen

[DanH]

You're in the ballpark Wade....I didn't build the latest exit to any precise dimension, but it should be between 25 to 30 sq in. Intake area is 56.5.

So much for those much-discussed intake-exit area ratios eh?

Originally Posted by Danny7

what kind of airflow control have you done on the firewall corner where it comes down to that reflective ramp?

A stock -8 has a rolled sheet aluminum radius on the firewall corner, 2", maybe 3" diameter, leading into an inset ramp in the belly. I elected to build a converging duct to feed the inset ramp. Here you're looking upward with the exit panel removed from the cowl:



Early concept sketch:



You've seen the reverse flow in the exit tuft video from an RV-6 (-7?). Pretty sure it's the result of very slow exit velocity.

[Danny7]

Thank you Dan, that photo helps me visualize much better. I would not expect a 90* corner on your airplane.

I also would like to say thank you for your explanations, photos and considerate attitude. I appreciate it.

it would be very interesting to have a camera and some smoke to see airflow inside the cowl..

[zav6a]

To Dan's point on rules of thumb for exit area ratio, there are so many factors to consider that there is simply no way a single ratio could be considered ideal.

Dan has chosen external pressure recover so he has much bigger inlets than would be required for internal recovery. I will bet he will see nearly 100 percent velocity pressure in the plenum. The plenum will maintain more of that pressure. His lower cowl is sealed, so little pressure will be lost there. Finally, he has the cleanest discharge tunnel I can remember seeing so that pressure will be efficiently converted back to velocity and associated volume through a relatively small opening.

My bet is that all of those factors will add up to a record (low) outlet area/inlet area for RVs. And, low drag.

Dan, you probably could have save all of that streamlining on your pitot and just put a port in the plenum! Kidding but it will interesting to see your pressure data!