Will refer to this diagram below...
<snip> Super Bob that is what I was refering to earlier.
Bob Axsom
Bob, as the others have said...man, have you done some work! If I'm following it all, I think you said the only internal baffle mod that added speed was the side baffles at the valve covers. Have you since removed the others, and if not, what is still in place? Couple thoughts for you (all FWIW, and none may be correct assumptions...just for discussion):
The aft curved wall to separate the firewall area looks like a nice flow director, but it does not appear to have a coanda-like curve at the exit that would make the descending flow curve aft and adhere to the belly aft of the exit (the way the -8's radiused curve at the exit is supposed to do). The intersection of your wall and the ramp may just be too angular still. I've heard a 2" diameter is a baseline on that radius, but I've also heard there are a couple guys in Oregon building Super 8s (not hotels!
) that use an even larger radius, with good success. All hearsay at this point...digging into that rumor too!
Next, I think the ramps you put in the exit may have been far too steep. If you look at Dan's modified diagram from the CAFE report (above), the airfoil shape of the bluff body stops at 18-25% of airfoil chord (at the exit). That is near the thickest part of the airfoil, before the flow separates. If you flip that airfoil over, and put it on the bottom of the cowl exit, I think you'd want it to still be rising at the exit, versus starting back down, as yours did. My guess is you were actually causing the flow to separate ("stall") in the exit, and that's why you slowed down...very turbulent exit flow. Total conjecture on my part though.
So here's what I'm thinking at this point (and Gary, let's see if we are still in sync
...others too):
First, I want to show Larry Vetterman's subcowl, or aftercowl mods. These are reproduced from Larry's website:
http://www.vettermanexhaust.com/
There are two versions, the first with the teardrop aftercowl, with louvers in the aft-center, and bluff bodies around the exhaust pipes:
This mod was said to increase TAS 4.5 kts.
The second version put the pipes closer to the teardrop aftercowl, and moved the louvers off the teardrop and up to the cowl bottom (where the bluff bodies were in the first version:
If I read the site correctly, verson 2 speed was very close to version 1 speed. In both cases, I think what we see is an extremely throttled exit, with louvers to re-gain lost cooling lost from the throttling, and the teardrop shape is there to straighten the flow (perhaps even make it converge towards the center of the fuselage a bit...or reduce outward "spanwise flow" along the belly...that may be a stretch of my imagination though).
Now, what Larry told me in Mitchell was that he removed that aftercowl, and built up a bump, or thickened skin area, in the same teardrop footprint as the attached area of that aftercowl. He said the speed with that bump remained the same as with the full aftercowl (same benefit as the aftercowl mod). I never did see that confirmed or discussed on his site, and the last discussion of the above pics was in 2009 on his site (a year before I met him in Mitchell).
Below I have two quicky diagrams of what I was discussing earlier. One is the addition of a radius at the firewall that extends along the belly, and then forms that teardrop bump (keeps the light blue line convention from the CAFE diagram). At the same time, an airfoil-shaped restrictor is placed in the lower cowl exit area to throttle the exit and accelerate the exit air (keeps the green line convention from the CAFE diagram). The airfoil does not descend at the aft end, but rather rises slowly all the way to the exit:
This is pretty much a step-one type of mod, and shows a stock cowl exit, and stock exhaust pipes (or at least pipes that protrude down, like I have). Not sure how to resolve the placement and shape of the leading edge of that (green) lower airfoil, nor how to shape the aft edge of that green body at the exit, but the CAFE report did have some info on airfoil type, so more study needed there.
The next diagram is of the same concept, applied to an exit that has one of Gary's (and now others') exit extenders added (red dashed lines). Actually I show the extender as designed a bit differently, as it does not rake aft around the exhaust pipes, but has a decreasing cross-section, and is squared off at the aft opening (inspired by Ken's latest mod). To make the extension like that, one would need run the pipes closer to the belly, and I've tried to show that. I also show my concept of how Paul Lipps told me to cut the pipes to make the sound waves go down, but keep the exhaust plume closer to the belly (the dotted diagonal line...and the lower aft triangle would be removed...just don't know how to draw that!).
Since the exit is extended, the lower airfoil would be moved aft, to get that 18-25% chord location at the exit edge. This version would seem to give more throttling than the first, and hopefully more smoothing and acceleration. Same caveats apply as on the first...not sure how to locate and shape the leading and trailing edges of the green airfoil, and not sure how to shape the aft edge of the teardrop shaped bump on the belly.
All conceptual right now...whaddya think fellas?
<snip> It seems like reducing cowl outlet size (parasitic drag reduction), extending the outlet to rearward (directing cooling outlet airflow) and using a bluff body (help pull cooling air out of the lower cowling) would be the best combination. Any thoughts? Can I have my cake and eat it too or is this a pipe dream?
I'd be very interested in what others think of this too. Dan mentioned earlier that the suction causes an increase in mass flow, and thus additional drag. Perhaps its a balance of getting just enough mass flow to cool, while throttling the exit and smoothing the flow to reduce drag. The bluff body extractor seems to do a bit of both, while maintaining a small wetted area. Might not be free, but perhaps the cake costs a little less to eat that way?
Snip> SuperBob, Just put it back together and go fly.
See ya soon.
Sho 'nuf boomer. Waiting on parts (prop, ram air gear, and reworked plenum) to get the last big projects done...so doing condition inspection prep stuff (new tires and brakes, pulling all the panels, etc, etc) while I wait for the delivery truck(s)!!
Cheers,
Bob