Gosh Ed, you give such informative and thought provoking answers, even though sometimes I don't like the information! I am based on a 5000 ft hard surfact runway at an elevation of 70ft, so will do some investigating of what I have first. Also, I have wheel pants, which I would have to guess will make it pull a bit less hard at speed. I do intend to travel - lots and long distances, so probably will have to have it repitched, although I dearly love speed! Which brings up another point, I can find almost NOTHING on speed mods for coupes. Surely someone has tried some! How about vortex generators and fancy wingtips?
--- In [email protected], "Ed Burkhead" <e...@...> wrote: > > > Don asked: > > My new coupe comes with a McCauley 71 51 prop. This is > > the upper limits of a "cruise" prop, the owner said he had it > > repitched from a 48, and was quite disappointed, > > recommended that I repitch it back a notch or two. > > > Hmm, ouch! > > For a long time, the passed on wisdom was, with a C-85 engine, > 7148 climb > 7150 normal > 7152 cruise > > That may well have worked OK for Coupes limited AND FLYING with no more > than 1260 lb. gross weight. > > At higher weights, the climb performance wasn't there. At higher > weights with the 7152 prop, it was possible to have terrible climb AND > terrible cruise speed as the engine couldn't turn the prop enough to get > up to good cruise speeds. I encountered Coupes that could barely climb > and could only cruise at 90 mph at near full power. > > Perhaps if those Coupes had stayed at a very light load they would have > flown well. > > At any rate, we made a loose consensus revision to the "normal" props > for the C-85 engine to: > 7146 climb gives about 100 mph @2400 rpm > 7148 "normal" gives about 104 mph @2400 rpm > 7150 cruise gives about 108 mph @2400 rpm > > I personally had my prop repitched to 7146 by accident (as I couldn't > remember the "correct" number when the A&P called on the phone during > the prop overhaul). I was happy with that prop. > > With the 7146, I easily operated out of short fields down to 1800' (no > obstacles) and based at 2400' with trees some distance past the field > end. I was always off in the first half of the runway and had climb > even at the higher gross weights. Over the Central States/Midwest, I > could always get up to 12,500' even though it took an hour or so to get > up there. > > I went out West one time and flew at about 1320 pounds gross or so. On > that trip, I started flying at dawn or before and made the last takeoff > at 10:30 most days in the high desert. Even at the 6,700' field, I was > off in the first 1/3rd of the runway and, though climb at 6,700'-12,500' > was anemic I could climb. > > On one 10:30 a.m. takeoff, the high desert up and down drafts exceeding > 1,000fpm over powered any climb the airplane had so I found up drafts > and circled in the thermals for climb. > > At all times, I was able to get to 12,500' with the gross at about 1320 > lb. - well, all but once. One afternoon in the high desert, I did an > afternoon leg when the worst of the thermals were getting milder, down > to turbulence I'd call moderate. In the moderate remaining turbulence, > I was only able to get to 11,500' and cruising along, spun my whiz-wheel > and found I was at 14,500 density altitude. > > I tended to limit my rpm to 2400 in cruise because I'd read something > saying that was the maximum recommended continuous cruise rpm. A number > of mechanics told me I could go ahead and spin it up and it wouldn't > hurt the engine at all. Considering that, with the flat, climb prop, I > wasn't straining the engine at cruise, I probably could and would now > consider spinning it up to cruise at the red line. > > For me, cruising at 7146's 100 mph (carefully calibrated) was quite > adequate to go to the Atlantic, the Pacific, the Canadian border, the > Gulf of Mexico and a bunch of places in between. > > You now have the problem, though, of repitching a prop that's been > repitched once already. I think I've seen it said that a prop may be > repitched in the same direction any number of times but can only go back > once. As that prop has been repitched to be steeper, it may or may not > be possible to pitch it back to be a flatter climb prop. You'll have to > check the prop's records and consult a good prop shop to see if you can > make another change. > > It might be possible to swap props with someone who flies a very light > plane to get a prop you can repitch to 7148 or 7146, if you decide to do > that. Or, you might go on an airplane reduction diet to get the plane > and/or yourself down to lighter and lighter weights. > > Ah, the choices, the choices! > > Ed >
