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Old 02-21-2011, 01:54 PM   #1
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Prop selection

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Gentlemen, I have a supercub that came with an 8X4E prop. It has been suggested that using an 8X6E Slow Fly prop might be an improvement. Can anybody explain to me, why?? I have to admit that the prop selection factor tends to confuse and also give me a headache!! Thanks...Ken
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Old 02-21-2011, 03:09 PM   #2
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Re: Prop selection

A lot depends on what type of flying you do, but more importantly, is the current configuration the most efficient and can the motor and other power system components, (battery and ESC) handle the change.

Brushless motors have a definite "Sweet Spot" RPM, so if it is running right with the current prop, lugging the motor down with a larger diameter or higher pitch prop will tend to drop the RPM and you will see a significant increase in current draw and drop in flight times. An easy way to tell is check the motor, ESC and battery temps at the end of a flight. If any are too hot to hold then they are running close to or at their limits. Most motors should list the proper range and type of props they can work with and may be wise to look at that. My only defense against that is most of the motor/prop combinations I've worked with are far too aggresive, meaning the prop they recommend is not the best. Only flight testing can show which is best.

A prop is nothing more than a way to move air and each prop is designed to perform the best within a certain RPM range.

The following examples must be considered using a power system that remains constant regardless of the prop size or pitch:

A smaller diameter prop will move a smaller column of air, but be able to move it with more speed, (a good example is a jet engine, which moves a small colum of air at great velocities). Depending on aircraft design this can equate to more speed. Unfortunately that suffers when in a climb and now you can get into tip speeds that make that part of the prop inneficient. A larger prop can move more air and you can get the same thrust by moving it slower. This may reduce the overal speed of the plane, but has better performance at cruise and in a climb. Another good example of this is the reason most modern jet engines use low speed fans coupled to the engine core. They can move a larger amount of air at enough speed to produce the thrust they need. If they do that with the pure core engine they need to move a smaller amount of air much faster and the efficiency suffers.

So, to make this simple, if you increase the prop diameter one number I would recommend dropping one number in pitch. The reverse is also true. If you maintain diameter and only increase the pitch as has been suggested, then you have to watch for the lugging condition I mentioned earlier. Since the Supercup is no speed demon to start with, one other suggestion might be to go up one number in diameter and drop one number in pitch. This is if the ground clearance is still adequate.

One last item I can point out is to check some forums for your plane and see what others have found. In a lot of cases if there are problems, solutions or improvements they will already be listed.

Hope that's clear as mud.
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Last edited by Cougar429; 02-21-2011 at 03:19 PM.
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Old 03-09-2011, 08:47 PM   #3
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Re: Prop selection

Cougar429 has it right! Diameter = thrust: short take off run, steep climb, more load capacity. Pitch = speed: longer take off run, shallower climb & need to be more weight conscious.
Again, up one on pitch/down one on diameter or vice versa = approximately the same RPM/current draw/run time. Different balance between speed & climb.
There is no perfect prop, they are all a compromise & best one depends on what your style of flying is.
This is assuming you are at or near your power plant's peak efficiency, electric or glow.
Hang time does not apply to ceiling fans, light fixtures, overhead door tracks or basketball nets!
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