Originally Posted by
Maxam
You are getting into a very low pitch to diameter ratio with that prop. Good for 3D anyway. As it unloads in the air its efficiency will become poor. I have never had a prop of that size. A much more appropriate prop would be a 22x12. This prop would also give good performance at part throttle settings. These engines are not to be run at full throttle continuously. I fly my 91 inch corsair at 1/2 throttle most of the time!
What do you estimate the thrust figures becoming with a 22x12?
Running it through a simple prop calculator, based off of the numbers that came on the 7-77 engine QC slip it turns a 24x8 @ approx 5500rpm - I would assume of course these to be actual numbers on the delivered engines before through break-in. Assuming that, I'm obtaining an estimated ~28lbs of static thrust with around 42mph pitch speed which wouldn't cut it for this Corsair in terms of top speed; you'd have to gun the throttle for most of the flight I'd imagine.
It seems some guys are obtaining those same RPM numbers (5,500-5,700) once broken in on a 24x10 prop which would then bring up the theoretical top speed to at least 52mph - requiring around 3.9hp out of the engine. On the specs for the engine I've seen online it gives an approximate 4.8hp rating which seems like it is severely overstated by at least 1hp ... funny enough, I flipped over the box my 7-77 came in and it estimates this engine outputs about 3.8hp which matches up much more closely to the prop calculation figures.
Now what you're saying about the efficiency becoming poor as it unloads could be true. In that case, with the engine having 3.8hp it should turn a 23x12 at approx 5,400rpm giving about 25lbs of thrust and ~65mph theoretical air speed ... ? Alternatively a 23x10 would give more rpm and give slightly more thrust and about 10mph slower speed.
I'd assume 25lbs of thrust would be adequate enough for a 30lbs plane for it not to seem underpowered?