Toobuilder
Well Known Member
...Unless I've screwed up this quick calculation, it seems unlikely that pressure from the prop can explain any more than 0.1 in HG of the MP rise we're seeing...
I'll not comment on your math, but I can tell you that I've heard of this "prop boost" phenomenon many times over the years. I've told this story before, but on my Hiperbipe I fabricated a divergent duct inlet system which featured an easily adjustable pitot inlet. It was simply a 3" diameter aluminum tube which could be adjusted for length by sliding in and out of the plenum after loosening a single hose clamp. I could fly, take measurements, land, adjust the length, and be back at the test altitude again in 10 minutes. Long story short, the difference between frighteningly close to the TE of the prop and just slightly proud of the cowl OML (about 5 inches difference) was worth 0.3 of MP. Not very scientific, but the test was repeatable and consistent. Closer to the prop is better. One caveat: My inlet was about a foot outboard of the crankshaft centerline, so it was well outside the root area of the blade.