Carl,
I'm a Shrinking Exit disciple too, and have some thoughts for ya. Concur with Dan on the multi-faceted (multi-drag-type reduction) benefits of shrinking the exit. Larry Vetterman's after cowl was effective for him, but in reading his site, and comparing his pics, it seems it was a good solution for an A-model, where he may have had limited ability to reduce the cowl exit dimensions, due to the nose gear components. His pics show two variations, one with wide exhaust pipes exiting the cowl via individual bluf bodies, and one with the ex pipes closer together and exiting via the after cowl. In both cases, he used louvers to get the exit airflow needed for cooling. So the design of the teardrop after body requires much thought and experimentation with size, shape and exit air control (louvers, etc) to find a solution. Pics of Larry's work, for ref:
Garry Reed and others used an after fairing of a different design, to straighten the flow aft of the exit. Bob Axsom used that concept as part of his experiment in this area…what you called side skirts. The reverse flow seen in your pic with the oil could also be partially due to chaotic flow at the edge of your exit, but its hard to see that in the pic. The blunt body sure does seem to have an effect on the flow as well, as pointed out by Dan. Here is a picture of Gary's exit fairing, which he said increased cooling and increased speed…meaning he increased mass flow
and still gained speed (typically they work inversely, so he may have really increased the efficiency of his flow with this fairing). I still have Gary's patterns, and can pass them along if desired.
Even with Gary's good results, and Bob Ax's extensive and meticulous testing of side skirts and a bit of a venturi-creating bump on the cowl tunnel bottom (documented in the thread "Forming Aluminum for a Cooling Outlet Fairing"), these measures seem to have lower return on investment than shrinking the exit
and increasing the efficiency of the exit flow. Since you have a 7, not a 7A, you should be able to do both of those last two things, though its definitely work, and a gut-check to cut into the cowl of a flying airplane (especially if its painted…AMHIK!).
I studied "The Shrinking Exit" thread quite a bit, and consulted with Steve Smith quite a bit along the way (thanks mucho fellas!). Steve told me one of the most effective measures to increase exit flow efficiency is to add a firewall radius, like the radius that the 8 has. This is also described in a thread called "RV-7 Cowling Exit Bump", with lots of pics. I chose to make mine look like an 8s radius, only slightly larger. Others have made large-radius shields further up into the engine mount area. I also took Dan's advice and made my radius of SS and continued it onto the belly as a heat/fire shield:
Then I reshaped my 6 into 2 exhaust pipes to bring them closer together, to allow the shrinking of the exit. Here are a few pics that compare before and after. Per Steve's great advice, the goal was to make the new tunnel smaller, with all faces more inline with the free stream airflow than before, and have it extend slightly behind the firewall (a bit of the side skirt concept, which I still intend to extend a bit in future experiments).
New pipes, old cowl:
New pipes, new cowl:
I picked up good speed, and my temps did increase, as expected when reducing cooling mass airflow (and cooling drag!). I'm at the point where cooling is adequate in cruise and in racing, but in lower airspeed regimes (slow climbs or formation flying), cooling is not quite adequate. Solutions range from installing a cowl flap to installing a louver, which could be exchanged for a flat plate plug for racing. Likely my winter project! Alan's (Anti-Splat Aero's) EZ-Cool Flap looks like a nice solution. A concern I have with a cowl flap is the impact that the side walls and actuator might have on the exit airflow when the door is closed. When open, the mass airflow is large, and increasing cooling is the concern, rather than cooling drag or speed. When closed however, I wonder if those sidewalls and the actuator, when raised up into the cowling, might become impediments to exit airflow, and cost speed. That is why I'm thinking of trying the removable louvers first. Its also why I would think that the internal exhaust pipe fairing that you mused on in your last post might not be an effective tool…but I'm unsure about that. I'd be very interested in other's thoughts on this concept. If it wasn't for the obsession with tip-top speed, I'd go for the cowl flap…I'm studying more on that in prep for the mod. Dan's cowl flap is very elegant and solved the sidewall issue…and would require more exhaust and cowl changes...it remains grail-like!
Good luck with the project…you're right about it being fun and challenging!
Cheers,
Bob