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Progress pics of my cowl "speed" work.

rzbill

Well Known Member
As with many of us on VAF, I am indebted to those that have gone before. Especially to the folks that are flying the speedier birds. These are pics of my work in trying to catch up.

Most recently, I installed a ring gear seal. Yet to fly it (waiting on weather) but it is all fitted and sanded into shape (sticky sandpaper on the ring gear for 2 minutes at 1000 rpm)

Web page of ring gear seal

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The following is other items I am NOT finished with but have been playing with for quite a while.

Web Page - Cowl Exit Radius on RV-7A

This was a first stab at getting the shape mocked up. I may move over to Aluminum or SS for the final.

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Sorry if I have posted the below before. This piece has been in the works for a long time.
I can't remember if I shared it or not.

Web Page - Exit fairing - First fabrication

Web Page - Exit Fairing - Second fitting and revision.

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The step that is delaying the installation of the exit radius and exit cowl is a total revision of the current lower cowl mount system. I pretty much built per Vans plans in that area and it is incompatible with the two new pieces.

EDIT *** Just FYI, the ring gear seal jumped ahead of the other pieces because it was time for me to remove the prop for maintenance at roughly 500 hrs. Just removing the sludge in the crank. On initial removal I thought there was none but after drying out the hollow crank I could see a thin film of "crud". Well, it turned into a mountain :rolleyes: comparatively anyway by the time I was finished. So...measuring for fabrication of the ring gear seal is WAY easier with the prop off.
 
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Experimental

Okay, Bill! Now that?s ?Experimental? worthy of an honorary member of the ?Chino Gang!? It looks like your mods are worth at least 10 knots. ;) Well, maybe a little less, but you?re certainly well into the hunt for more speed . . . Good Luck! Thanks for posting! :D
 
I added the firewall bumps to my -6 and picked up 4 mph speed. Please post the results of all these mods combined. I may have more work to do.
 
Good development work! I did something similar on Rocket 'Race 57' for both the spinner area & exhaust area. Gave us a few MPH & hope it helped in the cooling dept. too. I extended the side fences farther back, behind the end of the tail pipes & had a plate (removable & adjustable) that in essence created a tunnel for the hot cowl air back to behind the tail pipe tips to act like an extractor cooling outlet, I'd vary the outlet area depending if the plane was setup for racing or cruising.
Our system wasn't setup for the same mission as yours but has similarities to what you are trying to do. You may get to see some pictures of it in a major publication sometime in the near future.
My vote would be to use SS on all surfaces facing the tail pipes.
 
Looks nice Bill. You could also use baffle seal material to close the gap across the top your half-round firewall fairing. Might make the flow a little less turbulent.

If you're a glutton for punishment, you could fair the round tubes near the exit into teardrop shapes aligned with the relative airflow. A teardrop or airfoil shape has far, far less drag than a round tube.
 
Looks nice Bill. You could also use baffle seal material to close the gap across the top your half-round firewall fairing. Might make the flow a little less turbulent.

Thanks for the compliment and idea, Kyle.

If you're a glutton for punishment, you could fair the round tubes near the exit into teardrop shapes aligned with the relative airflow. A teardrop or airfoil shape has far, far less drag than a round tube.

Hmmmm. That sounds suspiciously like a gauntlet hitting the floor. :D:D Hmmmmm.
 
Please keep us posted on your progress! I'm about to launch into a lot of fiberglass work on my plane prior to paint, and this is one of the things I would like to pursue...
 
If you're a glutton for punishment, you could fair the round tubes near the exit into teardrop shapes aligned with the relative airflow.

Or just eliminate the center exit and install two outboard exits, like all the other fast birds with nose gear. Study the fast Lancairs, and take a look at the Showplanes cowl for the 10.

PIREP....the shaft seal is doing fine at 650 hours. It wears a nice glaze on the ring gear casting at the point of contact, but nothing more.

Brand new:





With hours:

 
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Good pics and overview. I looked at the build log pics, but I need more than a thousand words. I am not verbose regarding the fine points about streamlining and cooling.

Does the ring gear flange seal restrict/prohibit air through the prop/crankshaft cowl opening and thereby enhance streamlining and/or engine cooling?

Also, more info about the how the lower cowling exit ramp on the belly and the exit radius bulge work?
 
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one piece per half?

Dan, hard to tell from the photo - is that one piece of sealing material per cowl half? Apologies, I know you went into more detail on this in another post, but I can't seem to find it - you are pretty prolific when it comes to cool cowling and airflow mods, and I can't find the right search terms. I thought it was in the "shrinking exit" thread, but could not find it there. Thanks!
 
Does the ring gear flange seal restrict/prohibit air through the prop/crankshaft cowl opening and thereby enhance streamlining and/or engine cooling?
Also, more info about the how the lower cowling exit ramp on the belly and the exit radius bulge work?

Cooling drag = mass flow x loss of momentum, thus the dual goals are to (1) cool with the least possible mass flow, and (2) achieve the highest possible exit velocity.

The first is a function of heat transfer. The system requires less mass flow if each pound of air is heated more. Put another way, how hot can you make the exit air?

The propshaft seal and firewall lip radius are intended to increase exit velocity by maintaining lower cowl pressure, and not wasting that pressure in a turbulent exit flow.

That said, don't expect Big Magic without also reducing the exit area, which throttles mass flow while increasing exit velocity. It's a system.

Dan, hard to tell from the photo - is that one piece of sealing material per cowl half?

Yes, one piece per cowl half.

With stock RV-8 exit area, design pressures are moderate, about 4.5" H20 (0.163 psi or 23.5 psf) at 160 KTAS, maybe 6" at VNE. Pressures rise rapidly with a constricted exit. Assume 15" (78 lbs psf) max at VNE with a 30 sq in exit and you'll be in the ballpark. I tell you this because I'm not sure the aluminum fingers are absolutely necessary. They were TLAR design prior to pressure measurement.
 
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Lower cowl pressures

Dan, are your lower cowl pressures referenced to free-stream static pressure?

At Vne, the full dynamic pressure is 135 psf. At the spinner gap, a sector along the bottom is probably fairly close to stagnation pressure and the sides where the cooling inlets are may be similar, but along the top, its probably close to freestream static, or even a bit below. On average, the pressure at the spinner gap might actually be lower than your lower cowl pressure with a 30 sq in. exit. So in that case, the leakage flow would be the other way. If that were the case, I suppose it would be beneficial to seal it to reduce the spillage drag out over the top of the cowl.
It looks like as you throttle the exit and the lower cowl pressure rises, the benefit of the spinner gap seal diminishes.

Thoughts?
 
Dan, are your lower cowl pressures referenced to free-stream static pressure?

Yes, with a tap to the aircraft static system.

At Vne, the full dynamic pressure is 135 psf. At the spinner gap, a sector along the bottom is probably fairly close to stagnation pressure and the sides where the cooling inlets are may be similar, but along the top, its probably close to freestream static, or even a bit below.

Agree, although I have no measurements.

On average, the pressure at the spinner gap might actually be lower than your lower cowl pressure with a 30 sq in. exit. So in that case, the leakage flow would be the other way.

If average static pressure behind the spinner is less than lower cowl static pressure, leakage flow would be from the lower cowl volume, to the space behind the spinner, and out the spinner gap at the top. Given those conditions, the seal shown here would press against the ring gear carrier.

Thoughts?

The wildcard question is the average pressure behind the spinner. I'll put some fresh batteries in the manometer and get a measurement. Your reminder about stagnation below the spinner (and probably both sides given the relocated inlets) makes me wonder about the delta across the seal. Based on observation of how the seal has worn, I continue to think the lower cowl pressure is higher, but the delta (thus seal pressure) may be quite small.
 
I never stay in Holiday Inns, but I've always heard that flow around the spinner is typically *out* of the cowl; I think that's what Dan is saying. Near perpendicular flow from the base of the spinner.

What about the down-angled exhaust pipes?

Charlie
 
spinner leakage flow

With reduced area exit, it does seem likely that the spinner leakage is out of the cowl. So as Dan says, that robs you of exit velocity at the exit, as well as any spillage drag.

If the delta-P is really small, it might make one think the seal is not worth the trouble. But that small delta-P can still represent a moderate mass flow (if the area is big, as with no seal) which is reducing exit velocity.

The one challenge I see in making a seal for a finished airplane is working around the prop and spinner to adjust the seal support fingers and trim the seal. I don't want to take my prop off.
 
The one challenge I see in making a seal for a finished airplane is working around the prop and spinner to adjust the seal support fingers and trim the seal. I don't want to take my prop off.

Exactly. That is why the seal on my craft waited a couple of years until I took the prop off for scheduled maintenance.

You can do all the fabrication with the prop on but it would be tough to do the measuring required to get the seal fabric cut close to net shape.

In my case, I first laid out the pop rivet pattern, and numbered them 1 to 14 then measured from the hole pattern to the ring gear and wrote those measurments down. I determined the conical vertex position of the cowl nosebowl angle (to get the 10.75" rad figure). Cut a pattern out of posterboard and transferred the pop rivet pattern to it. Then added the hole to ring gear measurements and fudged a "seal edge" line into place. Added 1/8" to sand off. Used that edge to adjust the support tab length.

I cut the seal fabric to the patterns and rivetted them on with leftover pop rivets from Vans.
I decided to butt the top and bottom cowl seals. It was a coin toss between that and a lap joint.


FIRST FLIGHT
The HOWLING wind of the past few days dropped down to the low 20 knot range yesterday so I decided to fly.
It was my only shot this week (today and tomorrow 42F and rain).

Nothing went BANG. :cool: With the weather, data collection was futile. Nothing out of the ordinary though. It was too bumpy, cold, and guarranteed sinking air at the local test area on the lee of the mountains for accuracy.
 
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