Bob,
Been thinking about this ever since I saw the post on the SARL site. Several things have been running around (and amok) in my pea-brain as well. Here they are in equally random order.
As Mark pointed out and another poster here said, perhaps the fairing extending beyond the exhaust tips is providing some exit augmentation and acceleration...potentially a good thing. If your temps have decreased, then that could mean you've increased mass flow, and experienced the comensurate increase in cooling drag. Not what Gary experienced...so why is that?
So if it is acting as an augmenter, is closing down the inlets the way to get the speed back, and perhaps gain some? Not sure. There has been much discussion about large, low-velocity inlets and a smaller, throttled exit being a good combo. If you throttle the inlet, you should decrease mass flow and see higher temps...but my research indicates the shape and size of the inlets, and the divergence rate moving aft into the plenum becomes more and more critical with smaller inlets. Lancair racers have done big things with small inlets, but a lot of science is behind the shape (and a lot of discarded offerings to the speed gods!)
I'd be interested to see what smaller inlets provide, but I'd also be interested in what throttling your exit instead might do. That could be accomplished by decreasing the cross section of the exit as it goes aft...admittedly, a lot of metal work (and more missed episodes of DWTS...ahem!
) Take another look at Ken's exit for the convergence I'm talking about.
Gary's fairing is shorter and does not impact the exhaust. Is your exhaust bouncing up and down and becoming more turbulent as it exits the fairing? Would trimming the fairing around the pipes change the result? Probably already on your to do list. Still trying to get my head around the CAFE report on extending the exhaust tips well beyong the exit. Pretty much the opposite direction that you have going on here...but its a great experiment and a good data point, for sure!
Are the series of nutplates enough of a drag producer to create some of the performance decrease? What are they doing to the exit flow?
Gary has the radiused outer surface fairing the duct into his belly, yours connects at a 90 degree angle. Though Gary's looks like a clean transition, I've been told (by Paul Lipps and others) that the 90 deg interface is cleaner. We were talking wing roots at the time, but it may apply here too. But experimenting with a filet of sorts might be an interesting effort...even if its just RTV with a finger-width filet.
Whether filleted or not, that interface is generating some interference drag. The longer the interface, the more the drag, I would think. Yours being quite a bit longer than Gary's may be having an impact.
Did you ever trim the aft edge of the vertical fences at an angle up to the belly? At some length, I wonder if the exposed parts, aft of the exit fairing, are no longer flow directing fences, but have become belly strakes of sorts. Those might be yaw stabilizers, but would likely add drag. Did you notice any diffrence in rudder feel (doubt you would be feeling for that in the speed test...but just wondering).
One thing this may show is that every airplane is different. Your nose gear may be making this a completely different flow situation than Gary's. Ever think about an airfoil-shaped fairing around the "Y" of the nosegear strut, that fairs back to a point...then joins flush to the center flow fence? Possibly in the realm of the goofy there, but who knows!
My gut says its too much stuff, too far back. Look what Larry Vet accomplished with a small teardrop fairing in V3. Just enough side walls to have an impact on spillage at the outlet edges, and a bit of throttling of the exit flow with the overall shape. I'd go smaller on the fences and duct and perhaps clear the exhaust plume, before pitchin' it as an "offering".
Thanks for all the great info Bob...and for the long, long hours you are putting in! Good luck in Taylor too!
Cheers,
Bob