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Larry Vetterman's sub/after cowl mod

Bob Axsom

Well Known Member
The modification that Larry Vetterman shared in todays (2-5-09) VAF home page daily news is a major breakthrough in speed mods I believe. It is tested on an "A" model which makes it all the better from my prospective. He says that he is going to try to modify the design to work with existing exhaust system configurations but I do not believe it will be as successful as the configuration he has tested with the outboard exhaust placement. I have been thinking of something like this for some time but had done nothing to bring it past the brain to reality. His implementation is so perfect in appearance that it is hard to imagin anything better. The outboard exhaust position is required for max gain I believe but I was thinking of some short modification of the basic lower cowl with a reflex after the FAB to a tail like you see in the landing gear fairings - not faired down (up actually) to the fuselage as Larry Vetterman has done. This would require a removable section in the honeycomb structure of the lower cowl with a lot of messy accomodation work. The way he has added the fairing on behind the existing cowl is perfect. The mounting he has chosen onto the lower fuselage can easily be accomodated with platenuts and maybe some doubler plates. Man I love it! I am thinking of delaying the control surface end pocket filling till later in the racing season (www.sportairrace.org calendar of events) and try to get this in. I think a cowl flap needs to be incorporated in place of the louvers and I think the pressure recovery style may be even faster than the half teardrop approach. The beauty of this concept is you an make as many configurations as you want an substitute them just by picking up the same mounting points. All of the existing out board tank and fuel pump vents will have to be relocated away from the outboard exhaust location. THIS IS BIG!

Bob Axsom
 
I agree Bob...I saved all the pics this morning (as I'm sure many will do) to see about incorporating this to my 7 as well...VERY BIG IDEA!!

Way to go Larry!

:cool:
 
I suspect the fundamental gain is due to an increase in outlet velocity, or put another way, an improved inlet-outlet velocity ratio. Cooling drag is all about momentum loss. I'd love to know the new total exit area.

Need evidence? Note the increased manifold pressure (I assume an inlet snorkel in the left cheek).

The fairing may place the louvered outlet in an area of lower pressure. Don't know for sure, something worth checking. And anything with a teardrop tail is better than a flat plate tail.
 
CHT's?

I could not help but notice the dramatic increase in CHT's. Lower differential pressure, less cooling flow? It seems to be counter intuitive. What am I missing?
Guys?
 
CHT Control

Sometime ago I experimented with cooling drag by reducing the cooling air inlet size incrementally with 1/4" slice fillers of the inboard edge of both inlets. There was no increase in speed but each reduction in inlet size increased the cylinder head temperatures seemingly directly as defined in a conceptual equation "less cooling air = hot things staying hotter". This condition can be achieved by altering the opening size at either end of the system - inlet or outlet. So, the CHT increase with the smaller outlet area - the cross section of the opening around the exhaust pipes as opposed to the cross section of the closed off opening at the rear of FAB scoop on the lower cowl - is understandable. At low speeds you need the less restricted system to get adequate air mass flow to do the cooling. By incorporating a cockpit controllable cowl flap in the after cowl fairing you should be able to provide more air mass flow to accommodate ground operations and slow flight and close the flap for high speed operations where adequate air mass flow is easier to obtain. The difference between Larry Vetterman's after fairing design and my restriction to the inlet opening is that the after fairing provides an increase in speed where reducing the air mass flow only, did not.

Bob Axsom
 
Thanks Bob

Makes a lot of sense. Is there still a concern at speed? OAT's where really low in Larry's tests. So, even at speed, add another 30-40deg OAT and we are pushing those CHT's limiting this mod to moderate or even cool weather or high altitude operations where OAT's are below 60 or 70deg.? (around here, that is almost year round!) All a good case for that Cowl Flap.
Love this stuff.
Some day I will finish my 3 and then I am going to start going after my 6 to see how far I can push things based on all of the wonderful experiments and proven mods that Larry, yourself, and others have offered up.
 
I really think Larry's done some exceptional experimental work here, and am excited to see a few others repeat it to see if they get the same results. I am assuming that Bob is already busy shaping some foam....;)

I am also interested to see how this might apply to the -8's, as we have a built-in exit ramp which changes the shape of the belly at that point - it is not flat all the way across as in the side-by-sides. Larry's comments make me wonder if van was experimenting with similar ideas when he did the -8 design - I have read that initially, the ramp was supposed to be variable (I think for cooling), but ended up fixed.

I wish I had the time that Bob does to play with things like this!

Paul
 
Do a search and see how DAVE ANDERS solved this problem on his RV4 a long time ago.
 
Contrary to Bob's experience, I was able to increase the top speed by reducing inlet cooling area. I decreased inlet area until I noticed an acceptable increase in cht. I then resized the exit area (cut down) and voila a measureable mph increase. Effect on oil temp was negligable. A bit of trial and error but imo worth the effort.
rv3 pilot
 
Article

Somehow I'm not finding the article that is being discussed. Can someone provide a link?

Thanks..
 
RV-8 cooling exit experiment

I am also interested to see how this might apply to the -8's, as we have a built-in exit ramp which changes the shape of the belly at that point - it is not flat all the way across as in the side-by-sides. Larry's comments make me wonder if van was experimenting with similar ideas when he did the -8 design - I have read that initially, the ramp was supposed to be variable (I think for cooling), but ended up fixed.

A couple of months ago I did an experiment to test exactly this idea. I installed a new temporary exit ramp below the existing one. This new ramp had a large-radius hemispherical lip at the front end where it meets the firewall (that covered the existing lip), and was attached to the same aft location as the existing ramp. It was positioned so that it blocked about 40% of the cooling exit. The exhaust (Larry Vetterman's crossover system, two 1.75" pipes) was repositioned as low as possible to avoid interference.

Before and after flights made on the same day under the same conditions showed that I was successful in raising CHTs about 20°F (this on cold day, around 20°F OAT). I measured 0.5 knot increase in speed (4-way GPS, Horton/NTPS spreadsheet calculation), but this could well be due to measurement error (I generally don't trust any measurement differences of less than one knot). Certainly it was not a big change. My original intent was to make the cooling ramp position adjustable in flight, but based on this experiment I probably won't do it. I think this was also Van's conclusion (see 12/93 issue of RVator).

I suspect that the sharp bottom edge of the cowl cooling exit produces a lot of drag-inducing secondary flow, perhaps because its nearly impossible to exactly match the exit air velocity to the free stream velocity. The earlier-reported tuft testing seems to show this. I also know based on breakage of the inward cowl attachment hinge eyes and on paint cracks that this area experiences a lot of vibration, and I can easily believe Larry's idea that its the source of some of the vibration felt through the cockpit floor. I think his modification is a great experiment to smooth out the flow in this area, provided its possible to get adequate cooling in warmer weather.
 
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Photos?

I just logged in today (6th) and don't see the info. Is there another site or thread?
 
I just logged in today (6th) and don't see the info. Is there another site or thread?
The original post was on the front page for Thursday, February 5, 2009. You will have to go back to that day's front page to read it.

Doug,
Perhaps the original post can be added to this thread. Since previous front pages are now out of site without clicking on the "Past Day. . ." links, maybe it will not confuse everyone.
 
snip...Doug,
Perhaps the original post can be added to this thread. Since previous front pages are now out of site without clicking on the "Past Day. . ." links, maybe it will not confuse everyone.

I plan to create its own article in 'Builder Mods' sometime over the weekend.

b,
d
 
<<I suspect that the sharp bottom edge of the cowl cooling exit produces a lot of drag-inducing secondary flow>>

Specific to the unique configuration of the -8, at some point I intend to try the "mirror" of Alan's experiment; leave the fuselage ramp standard, but remove (or reduce) the lower cowl air exit box. Not possible on the 6 and 7 (no ramp inset in the fuselage), thus Larry's experiment.

I'm building a flange with nutplates into the belly of the cowl just to make it easy to swap out different cowl exit sections. There are some companion mods (cowl inlets, baffles, a bit of internal duct) and I hope to wind up with a rather small fixed exit. However, there is still a cruise penalty for a fixed exit; an after-fairing like Larry's would make practical installation of a variable exit a lot easier.
 
Pardon the intrusion, but do a a username search under "bryanflood" for his version of this mod- he has inflight video!
 
'nother idea

Looking at the above mods, makes me wonder if this would work well in conjunction with augmenter tube type exhaust/cooling air exits???

It should be possible to increase the air flow velocity---and therefore CFM, while reducing the area.
 
Looking at the above mods, makes me wonder if this would work well in conjunction with augmenter tube type exhaust/cooling air exits???

It should be possible to increase the air flow velocity---and therefore CFM, while reducing the area.

Already designed and I hope soon to be implemented.
 
More Intense Implementation Thoughts

Larry Vetterman said he was going to look at using the standard crossover exhaust with his mod. I have looked at that recently as well and It looks like the placement of the ball joints will allow that. My optomistic goal is to have a similar mod implemented in time for the AirVenture Cup Race from Dayton to Oshkosh on July 26. The heat muff may be a complication but that should be workable. The individual outlets of the cowl for the exhaust pipes is something that must be carefully dealt with for proper high speed size optimization, ability to withstand the tremendous exhaust pipe heat and integrating with the honeycomb core part of the lower cowl. I am thinking of a cowl flap in the after fairing that is hinged at the back and pulls open up to the fuselage bottom inside the after cowl fairing - perhaps with side walls in the afterfairing - for ground and low speed operations.

Bob Axsom
 
For the last three years, I've wanted to modify my lower cowling by adding bluff bodies.....you can see them on newer Lancairs but can not stand the thought of ruining my beautiful and expensive paint job. It probably wont get done.

Glenn Wilkinson
 
One of my friends with a -4 has his cowl setup to go smoothly right onto the belly, no opening down there whatsoever... He then has augmenter tubes coming out the cowl cheeks and a completely custom 4-pipe system :D. Same concept just about...
 
One of my friends with a -4 has his cowl setup to go smoothly right onto the belly, no opening down there whatsoever... He then has augmenter tubes coming out the cowl cheeks and a completely custom 4-pipe system :D. Same concept just about...

Yep, I helped a buddy do this a few years back on a Cassutt that he raced at Reno. Seemed to work well, but never got it all optimized before he had to quit racing.

Exhaust is basically free energy, just look at a turbo setup. Or this:eek: Just trying to put it to a good use.
 
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