General Prop Info / FAQ

[Wiplash]

I'm looking for prop basics.

2 blade vs 3 blade.

Wood vs Composite.

How to choose Pitch, Chord and Diameter.


Any reference material available?


Thanks
Todd

:whip:

[Resasi]

Few thoughts for you hope they help some.

Propeller type. Propellers are used to propel the gyro forward. How well they do that depends among other things on the diameter and pitch of the blade. The diameter of the blade in inches determines the size of the column of air pushed back, the bigger the better. The pitch, given in inchs, shows distance the prop moves forward in one revolution assuming 100% efficiency. It also determines the amount of energy the column of air moving backwards has. A good prop diameter/pitch combination is necessary to give you the flight performance desired. Coarse props load the motor using lots of power to turn. Good acceleration, not so good cruise as you fly at 75 to 85% engine power all the time. Fine pitch uses less engine power to turn so the motor will rev to a higher RPM. Reasonable acceleration, good cruise performance. Once up to cruise speed you are able to back off the power more to maintain cruise speed.

At full power it is undesirable to have prop tips exceeding the speed of sound as a shockwave across the prop face decreases thrust, and as tips go supersonic they will be very noisy. The faster the tip after it goes past the speed of sound the greater the noise and lower the thrust the prop generates, as shockwave increases in size. Aim for a max RPM with tip speed about 80% of the speed of sound.

As well as the prop sizes, consider material and number of blades. Propellers operate more efficiently with fewer blades. With more blades each hits some of the turbulence wake of the previous blade. The more blades, the more engine power required to turn the prop. Low power motors tend to have two blade props, high power motors tend to have three or more blades to absorb engine power.

Props are constructed of either wood, composites or both. Wood tends to absorb harmonics and engine vibrations efficiently, hence their use on most direct drive four stroke motors. Wood props tend to be fairly inexpensive. Composite material props cost more, sometimes over twice the price, and don't absorb vibrations/harmonics very well. Usually slimmer built they can be more efficient than thicker wooden blades, and easiest to make three or more blade versions of. Tend to be used on high power applications.

[Wiplash]

Thanks Reasi!

In regards to the course pitch/fine pitch advantages and disadvatages. Do they make multi pitch blades?

[Resasi]

If you mean in-flight variable pitch, yes they do.

I have never seen them used with gyros myself but would imagine they can and do.

If you ever wonder why something isn't being used three factors commonly come into play. Cost, complexity and weight.

Most composite blades are mounted in a hub that allows you to set the pitch of each blade. Most wood, single piece with a set pitch