Prop Basics

Stryker Basics

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Propellers are sized by diameter and pitch.

Diameter: can be thought of as, how much air the prop can collect. The bigger the prop the more air it can collect. Diameter is the length of prop. (Torque)

Pitch: can be thought of as, how much of that collected air will be pushed. Adding more pitch, will push the collected air faster creating more thrust. Pitch is the angle of the blades. (Thrust)

Pitch speed:  A pitch of 6 would move a plane forward 6" per revolution if there was no slippage. If you used a motor that spins at 10,000 rpms the plane would move 60,000 inches per minute or about 56-57 mph. We can get miles per hour by multiplying the prop pitch x rpm, then dividing by 12 to get feet per minute, then divide by 60 to get feet per second, then multiply by 0.6818 for mph.

Example: a 10,000 rpm motor with a pitch of 6.
6x10000 = 60000 /12 = 5000 /60 = 83 x0.6818 = 56-57mph.

Another example a 20,000 rpm motor with a pitch of 7.
7x20000 = 140000 /12 = 11666 /60 = 194 x0.6818 = 132-133 mph.

Thankfully, we dont have to do the math as there are many free online converters to do that for us.

Tip: Pitch speed only gives you a general idea of the speed, in real life flying just because the numbers tell you it should go 130mph doesnt mean it will.

Diameter: Ideally we want to use the largest prop possible but running a bigger prop will also make a motor work harder, like switching gears on a bike, the higher gears make it harder to pedal. Typically large heavy planes will use lower kv motors designed to run bigger props where the smaller lighter faster planes using higher kv motors will use smaller props. Most motor manufacturers will have prop specifications chart stating which prop range works best with their motors.

If we put the pitch and diameter together we would see that a 7" prop would collect more air than a 6" prop and a pitch of 6 will push the collected air faster than a pitch of 5.