(The following method applies to common SO helicopters that have x-style rotors, e.g., my Viper25’s rotor as well as the ones on many other SO helicopters. (Photos below) The x-style refers to the “X” that the LE and TE form at the hub. See a top-down photo below. For non-x-style rotors, you may use my bi-directional pitch gauge to measure the main pitch angle at 75% radius and the tip angle at 100% radius. For some non-helical blades, you may have to measure pitch angles at several locations along the blade to get a full picture of its pitch distribution.)

Let’s say, you inherited a beautifully crafted SO x-style helicopter. It flew super well. But you want to build an improved version, with a different pitch and perhaps a different washout. (See “Beyond Pitch Perfect” for improving helicopter performance with pitch and washout changes.) But, without the design plan, or, when there were deviations during the build, how do you know what pitch and washout this helicopter has? How to measure them?

Pitch and Pitch Angle relationship:

Let’s differentiate and relate “Pitch (lead, distance after one revolution)” and “Pitch Angle (local blade twist, degree)” first. Referring to the drawing on the right, by the definition of tan(), the formula for converting pitch angle (B) to pitch (distance) is:

tan(B) = Pitch / (2*Pi*r) r: radial distance from the hub center

Tip Angle and Washout angle:

For full-length X-Style rotor, the exact tip angle can be found by using r=R (blade radius) in the above formula:

TipAngle = arctan(Pitch / (2*Pi*R)) (R: blade radius)

Note, on the above full-length X-Style rotor, it is assumed that LE and TE are rigid and straight, and therefore, there is no washout.

On common X-Style rotor variations that have an elbow joint, like the Viper25’s, washout can be easily introduced by altering the elbow joint. (See photo below.) To find the exact washout on your inherited helicopter, you may carefully measure the tip angle and get the washout angle as:

Washout = (Pitch Angle at Tip) – (Measured Tip Angle)

(Note: Positive washout is defined as leading edge down.)

Finding Pitch on an X-Style rotor

To determine the Pitch of an X-Style rotor, let’s recall its definition: the distance the rotor would advance after one full 360-degree revolution. Since the X-Style rotor (before applying any washout) is a true helix, the distance traveled (Y) during any partial revolution (θ) is directly proportional to the total Pitch. (See the drawing below.)

By applying the rules of circle and triangle proportionality, we can establish the following relationship:

Rearranging this formula allows us to calculate the Pitch:

Voila! There you have it. By taking two measurements, θ and Y, we are then able to calculate presumably the most critical setting, Pitch, on any x-style rotor.

Summary:

Pitch and washout play a crucial role in rotor and helicopter performance. The final stage of tuning your high-performance helicopter highly depends on them. With the above formulas, one can easily obtain these two critical settings, and alter them for better performance. (Read “Beyond Pitch Perfect” to see how!) Good luck!

As usual, feel free to contact me if there is any questions I can help with.  Happy to help.

Cheers!

-AeroMartin  11/25/2025