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
