To increase your flight times, the pitch of the prop has to be increased. If you look at the prop blades, you will see that they are twisted. If you measured the angle of the prop tip (see photo below) then you could calculate the pitch of the prop. The pitch can be described as the distance the prop will move forward in one revolution. The lower the angle, the lower the pitch, and vice versa.

Pitch can be mathematically expressed as:     P = 2 * pi * radius * tangent(B)
The angle B can be expressed as: B = inversetangent(P/(2 * pi * radius))
For a 20cm diameter prop, the radius at the tip is 10cm or 3.937", we want 16" of pitch so then the tip angle would be:     B = inversetangen((2/(2*3.1415*3.937))
B = 32.90 degrees
So we want to bend the prop blades until the tip angle is 32.90 degrees.
This formula can be used at any other radius as well. It is convenient, however, to sometimes use the angle to find the radius instead. If you pick 45 degrees, then the tangent is equal to 1, so the formula becomes P = 2 * pi * radius, or radius = P/(2 * pi)
For 16" of pitch and a 45 degree angle, the radius would be:
radius = 16/(2 * 3.1415) = 2.55" or 64.7mm

TRANSLATION: If you measure 2.55" out from the center of the prop, the angle of the prop would be 45 degrees. Also if you measure at the tip of the prop, the angle should be 32.9 degrees.

To change the prop pitch, you need to twist the blade to the higher angle. The easiest way to do this may seem crude but it works just great. Basically you hold the prop at the hub with one hand and grab and twist with the other (see photo below). Some people soften the plastic with a lighter or some other heat source to make the twisting easier. I typically scrape my props down so the plastic is thinner and easier to twist.

Here is a how a repitched prop compares to the standard pitch prop. You can see the angle difference in the tip angles. (I have exaggerated the differences a little with a green line to help illustrate this point.)

And it is that simple. The repitching of the prop will make a great improvement in the flight time of your model. Just make sure that both blades have been repitched equally or the prop may wobble when flying.

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