Prop Thrust and Pitch – What do they do?

It doesn’t matter if you fly RC airplanes or drive RC boats. These vehicles use a propeller ( prop ) and both are affected by each term, Prop Thrust and Prop Pitch Speed.

When referring to prop thrust and pitch speed, they both contribute to propeller performance. To be effective, a prop must deliver an adequate amount of thrust and pitch speed. Let’s look at what defines each term.

Propeller Pitch and Thrust

Propeller Pitch and Thrust

Prop Thrust Definition

Propeller Thrust is defined by the amount of force the prop can create along the axis of rotation. This force is directly translated in to the motion of an RC boat or plane. Thrust is a function of the size, rotational speed and pitch of the prop.

Prop Pitch Definition

Propeller Pitch is defined as the  distance that the propeller would move forward during one full rotation of the blades through its travel medium.  The value of pitch is typically provided in inches or millimeters and assumes no slip.  Pitch is increased on a propeller by increasing the angle of attack of the propeller blades.

Prop Pitch vs Prop Thrust – What do I Require?

Prop thrust is required for any application in order for the motor / prop combination to do work. This is also true for prop pitch.  In order to select a propeller for your application you would need to understand what your application is designed for to begin.

An airplane or boat that is designed to travel at slower speeds, would benefit from a propeller that delivers a high amount of thrust. Pitch speeds for a slow moving RC vehicle is not as critical. At the other end of the spectrum, if you have an RC boat or airplane that travels fast, pitch is quite important.

In general, fast moving vehicles have a smaller amount of drag than slower moving vehicles. This natural  marvel makes it much more simple for us to select the best propeller combination.

Prop Thrust is generally created from maximizing the diameter of the propeller. Pitch does contribute to thrust, however diameter more efficiently creates thrust.

Selecting the Correct Propeller Pitch

Let’s set an example that our RC Airplane should travel at a speed of 120 km/h. If we know the general speed of the RC airplane we can then select an appropriate pitch value. In order for our airplane to hit 120 km/h, the pitch speeds of the propeller must exceed 120 km/h. If we can not exceed 120 km/h in pitch, it is certain that our RC vehicle will not be capable of hitting 120 km/h. The primary reason for this is that there will always be slip in a prop. The slip will not allow us to attain the pitch speeds of a propeller. You can visit the RC Airplane Calculator page in order to calculate pitch speeds of a propeller.

Selecting the Correct Diameter Propeller

The next step as a continuation of the above example is to select the diameter of our prop. In order to accomplish this we must know one significant point in propeller selection relating diameter and pitch of a prop. A high angle of attack can make the propeller stall, becoming less effective at lower speeds. For an airplane this starts to happen when our diameter to pitch ratio starts to get smaller than 1.25:1. As you decrease the diameter/pitch ratio, the propeller performs poorly at low speeds. This scenario is not ideal when low speeds are an absolute requirement.

With this said, we can select an appropriate propeller diameter that will allow us to hit our thrust goals. Increasing the diameter of the propeller will increase the thrust potential of the prop while decreasing the diameter will decrease the thrust potential of the prop.

Prop Thrust and Pitch Summary

In conclusion, both thrust and pitch are very important. Thrust is required in order to overtake the amount of drag that is produced by the RC airplane or boat. However that thrust has to be made at the speed in which the vehicle would be travelling at.  This is where pitch speeds of the propeller is critical. Be certain to select a propeller combination that has enough pitch speed to achieve the speed goal of the application. A proper balance of both will produce the results that you are looking for!

Similar Story for Electric and Gas Jets

In this video we look at the differences of Static Thrust, Jet Efflux and how both of these relate to dynamic thrust.

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