Nor, for that matter, a fixed-pitch prop on a Cherokee Six 260. But there again, the manual says it's a couple of mph slower at full throttle with the constant-speed prop than it is with fixed-pitch.
This is only a guess. The only thing I can think of is that in the inner portion near the hub, the blades of most constant speed props transition in cross-section from an airfoil to a cylinder. So for the inner few inches it has the aerodynamic efficiency of a tree stump; while the blades of a fixed-pitch prop can still have some efficiency down close to the hub.
This is only a guess. The only thing I can think of is that in the inner portion near the hub, the blades of most constant speed props transition in cross-section from an airfoil to a cylinder. So for the inner few inches it has the aerodynamic efficiency of a tree stump; while the blades of a fixed-pitch prop can still have some efficiency down close to the hub.
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So while a fixed-pitch prop can't generate full rpm in takeoff and climb like a constant-speed prop, the fixed-pitch prop is marginally more efficient at a given rpm in cruise.
Just a guess.
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Look at the PA-28-235 performance charts in Post #3 above. They are expressed in TAS vs. % of rated power, and the fixed-pitch is faster across the board. As for the PA-32-260, the performance chart just says “True airspeed at full throttle with a constant-speed prop is 2 mph less.”
Where do you live? Do you need short-field performance, and/or do you regularly take off in high DA environments? That might factor highly in the decision.
I went FP with my RV-9A because of low initial price, simplicity, reduced maintenance and lighter weight. My typical destination is a 2500' to 3500' asphalt runway at no more than 1000' MSL, though it does fine at Big Bear and Mammoth, 6000'-plus with DA as high as 10,000' (have to say, Gallup, NM on a 90-degree day at max gross dictated a pretty shallow climb!). If I lived back east and needed to clear trees bracketing a 1500' grass strip, I'd go CS in a heartbeat, but the southwest is very FP-friendly!
For me the $4K-$5K I saved buys a considerable amount of 100LL, and maintenance consists of re-torquing the prop bolts at annual.
According to the PA-28-235 manual, at 7,000' density altitude 75% power is 2575 rpm (red line) for the fixed-pitch, and 21.6" (full throttle) and 2400 rpm for the c/s. Same power output for both -- 75%, or 175 hp and 14 gph -- but the fixed-pitch-equipped airplane is going three mph faster.
Sure, but then you’re running at a higher hp and higher fuel consumption. Apples to apples, at equal cruise hp being produced, the fp is slightly faster.
We actually had a long-ish thread about that not too long ago. It got a bit entertaining.
I understand completely what you're saying. I simply think we're talking about different concepts and different applications of those concepts. Unless you contend that these performance charts from the Cherokee 235 manual are wrong, we have no disagreement. If you do think they are wrong, how so?
Actually, epitrochoid describes the shape of the combustion chamber which is the path that the rotor tips take around the crankshaft. The ovalized triangle associated with Wankel engines is actually the rotor, whose shape is a variant of a Reuleaux triangle.
Silly me. My answer to the question "Why are manhole covers round?" would have been, "Because the manholes are round!"
Actually, epitrochoid describes the shape of the combustion chamber which is the path that the rotor tips take around the crankshaft. The ovalized triangle associated with Wankel engines is actually the rotor, whose shape is a variant of a Reuleaux triangle.
And since a Reuleaux triangle is formed by the intersection of 3 circles, one could argue that it itself is "round" if one were sufficiently argumentative.
I prefer "because an augur that drills triangular holes are darned expensive".Silly me. My answer to the question "Why are manhole covers round?" would have been, "Because the manholes are round!"
