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length of prop affect speed at all?

The Kid

FOUNDER
Thompson Falls
Is a longer prop, say with the same pitch as a shorter one, going to give you more speed because there is more blade pulling you though the air? If so, how much flatter can that long prop be than the shorter one and still keep up with the shorter one? Say you had an 86/43 and an 84/43. Would the 86 propel you faster than the shorter one with the same pitch? Then if you had an 86/40 say vs an 84/43; are they propelling you the same speed or is the 84/43 going to be faster?
 
Long diameters equal larger disk into the relative wind. Longer props in general are slower. To have a big Catto or even a Borer
try to keep up with a standard 74/56 Cub prop you will need to run "either big prop" at nearly redline: To keep up with the 56" pitch running 2350.
Short is fast, long is last; in speed anyway.
E


Sent from my moto e5 go using SuperCub.Org mobile app
 
Airplane Props are nothing more than screws which operate in the air. Compare a machine screw with a wood screw. The threads on the machine screw are closer together than those on the wood screw. More threads per inch. As a result the wood screw moves further with one revolution than does a machine screw.

So in an airplane two different diameter props with the same pitch will move the same distance through the air with each revolution. The longer prop will have more drag/resistance against turning (as does a larger diameter wood screw) so will require more horsepower to turn the same RPM as the shorter prop to move the airplane the same distance in the same amount of time.

On two similar airplanes with the same sized engines, in order for both engines to turn their rated RPM the one with the longer prop will need to have less pitch. In other words the longer prop will move less distance through the air because the pitch is less.

Then after considering the above there are other differences. The chord of the blade, the airfoil shape, the sweep back of the tip, the P-tip shape or not, construction material etc.

Earle is correct. This attempts to explain why. The trick is to find the combination which most meets your desired performance characteristics.
 
Long diameters equal larger disk into the relative wind. Longer props in general are slower. To have a big Catto or even a Borer
try to keep up with a standard 74/56 Cub prop you will need to run "either big prop" at nearly redline: To keep up with the 56" pitch running 2350.
Short is fast, long is last; in speed anyway.
E


Sent from my moto e5 go using SuperCub.Org mobile app
I don't think this is a fair comparison because nobody runs an 84/56 prop on a cub. The length of the prop doesn't have as much to do with speed as the pitch until you are loosing efficiency due to transonic tip speeds.

For example, I used to fly for a Husky dealer. The models with the 80" Hartzell weren't any slower than the ones with the 76" prop. I realize this isn't apples to apples due to the constant speed prop but it is a fair way to judge propeller length.
 
Casey,
Another way to evaluate props is to shoot your approach as slow as you can with a standard 74/56 prop on a Cub with prop windmilling, then bolt on a Borer 82/41 and go shoot the same approach and let us know how the larger disk with 15" less pitch effected your approach???
Your reference to a 84"/56 prop is something that would likely require something in the neighborhood of 250 Hp to turn it????
Lots of guys have tryed Borers with more than normal pitch like 44"/46" to try to achieve better cruise speeds for years. Most of them are standing in the corner of the hanger collecting dust, or got send in for repitch back to 41/42". It takes a VERY strong 160hp engine to swing a 43" well.....
Good luck
E

Sent from my moto e5 go using SuperCub.Org mobile app
 
Earle,

I'm not trying to be argumentative and I certainly don't suggest anyone should try a 84/56 on a cub(without an O-540 anyway). My point was to say a 74/56 is faster than a Borer solely because the Borer is 8" longer is not 100% accurate. I think that is what the OP was asking anyway, you can make a long prop go fast but the takeoff performance will be dismal so it isn't done that way. My mistake for trying to turn a practical discussion into a theoretical one, I think I was tired of talking to my 3-year old all day and wanted some advanced discussion...
 
Actually, airplane propellers are magic devices......don’t look for logic to explain performance.

MTV
 
An aeronautical engineer once told me to always get the longest prop you can. It's more efficient he said. I was hoping that, even though the pitch is flatter, a longer prop would make up for the flatter pitch, with less forward motion, and cruise the same because it was longer. Like an 86 vs an 84 or 82. I guess not. But maybe he was talking about takeoff and not cruise I suppose. When I took off my 74/56 and put on a Borer 82/42 I thought someone had also attached jato bottles or something. I have done all kinds of mods to my cub and the Borer was the one that there was no question that it increased takeoff performance and climb (that was on a 150) terrifically. Now I have a 180 and just wonder if an 86 wouldn't be better than an 84 not only for takeoff but for cruise. I'm a dreamer I think. You probably can't have both unless you are a constant speed. But I'd bet the 86 would get off quicker. Sometimes 10' less runway is a lot.
 
Pitch at a common % radius is one thing, but isn't the rest of the prop blade pitched differently? How that lengthwise distribution of pitch is done (I was told by Merrill Field Propeller years ago) is an art learned through years of experience. Maybe a long prop can be massaged by an expert to improve performance? I do know when the tips get over flattened by snow or water contact performance suffers.

Gary
 
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