All blades are not created equal. Our 180-SUPERHAWK fixed pitch blade (on top) has much more efficient, high-aspect-ratio shape than rounded constant speed blade on bottom.
Anyone who has been around airplanes very long soon learns about constant speed propellers. All the "fast" planes have them and they are standard equipment on everything from Bonanzas to C-130's.
Common sense tells you that any propeller which can change its airfoil with respect to the relative wind at different airspeeds should have a number of advantages. Theoretically, you get more takeoff power, faster cruise speed, lower cruise rpm's and better fuel economy.
So why do we at Penn Yan claim our 180 HP SUPERHAWK conversion with a fixed pitch prop is better than the constant speed prop you can find on "other" conversions?
Because we've tested it! Not only us, but also the editors of Cessna Owner News magazine, who did a number of very exacting tests.
All blades are not created equal. Our 180-SUPERHAWK fixed pitch blade (on
top) has much more efficient, high-aspect-ratio shape than rounded constant
speed blade on bottom.
Some of the reasons for these results are obvious. For instance, the simpler a thing is, the less there is to go wrong with it. A constant speed prop conversion is complicated. It requires a three-piece prop, oil plumbing, governor, prop controller, manifold pressure gauge, fuel pumps, fuel gauges, 30 Lbs. extra dead weight, a 1500 hr. or 5 year prop overhaul and a mandatory yellow arc right in the middle of the rpm range. Plus, the nose cowling and spinner must be replaced with a larger, more expensive one or modified with several add-on "bumps".
Other reasons are not so obvious and need to be pointed out. Top speed, for instance. For top aircraft speed, an engine should be turning at its maximum rpms. Since both the test planes were turning at 2700 rpms and both had equal horsepower, it's not surprising that their speeds were about the same. (Actually, our Superhawk was 5 mph faster). Any "pulling back" on the constant prop control only resulted in lower speed.
One area not tested was fuel economy. Theoretically, lower rpms at a high manifold pressure should save fuel. However, the constant speed prop's yellow arc restriction makes it awkward to take advantage of that.
Climb performance was also about equal, with the 180-SUPERHAWK trailing only slightly. More puzzling, though, was take off performance, in which the SUPERHAWK blew the doors off the constant speed conversion. Even we didn't understand that one at first; with 345 more rpms available at the start of the take off roll, you'd think the constant speed prop would have had the advantage.
Well, we dug out our calipers and band saws and think we've figured out why the fixed pitch SUPERHAWK prop did better. Part of the reason is the aerodynamics of the cowling and prop spacer; the rest of the answer is the difference in airfoils of the blades of the two propellers.
A constant speed prop has a larger hub which necessitates a larger spinner to shroud it. And, it sits back tight against the engine, causing a restriction to airflow around it. The Penn Yan fixed pitch prop has a stock sized hub which needs no cowling mods and uses a special spacer which positions the props a full 3-½" further forward for more streamlined airflow.
The blades themselves have a clear story to tell. First of all, the inner 12" of a constant speed prop's blade is not an airfoil at all but is actually round shaped like a club! While perhaps insignificant to total thrust, this inner blade section can contribute significantly to engine cooling on the ground and during climb. . a major factor in the FAA confidently granting the SUPERHAWK its gross weight increase to 2550 pounds. And, as you look at slices of blade sections further out along the tips, some subtle differences of airfoil shape are observed (see below). For whatever reason, the shape of the Penn Yan prop is much more efficient.
We sawed two propellers to bits! Note the thinner, flatter shape of our
blade (on left). climb performance is similar to constant speed blade on
right; take off acceleration and top speed are better.
It is this efficiency of blade airfoil and cowling which
gives the Penn Yan fixed pitch 180 SUPERHAWK prop its main advantage in
performance over the more complicated equipment of the constant speed
conversion.
So, if you want improved performance, lower cost, lower maintenance, better reliability and a neater looking airplane, be sure to get the fixed pitch Penn Yan 180 HP SUPERHAWK conversion.
Penn Yan Aero Service Inc.
2499 Bath Road, Airport
Penn Yan, NY. 14527-9599
#Y2GR396Y
315-535-2333
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