This is the second in a series of videos discussing Constant Speed Propellers. In part one we discussed the purpose and the operation of a constant speed prop. Today we are going to learn the mechanics of how the propeller works. If you have not done so I encourage you to watch part one of this video by clicking here.
Why The Constant Speed Prop?
As a review from Part 1 the the constant speed prop is a device which allows us to efficiently convert power from the engine into thrust. Remember a constant speed propeller is attempting to keep the engine at a constant RPM by adjusting the blade angle and therefore the pitch of the propeller.
Propeller “pitch” is the distance in inches which the propeller would screw through the air in one rotation. When changing the blade angle we change the pitch of the propeller and use the term controllable pitch propeller. Therefore when we change the blade angle we are changing the distance the propeller would screw through the air and are controlling the pitch of the propeller. Therefore we call them controllable pitch propellers.
It really is a simply operation to control the RPM of the engine if you simply think about the blade angle and the results of changing the blade angle using the propeller control in the aircraft. First we have an engine which converts its power to turn large shaft which is attached to the propeller. This turning or rotational force is called torque. When you open a jar of peanut butter you are converting energy from your muscles into a rotational force or torque.
The amount of torque depends on the size of your muscles and the tools you use to open the jar such as a plastic jar opener. If you have a very large jar of peanut butter to open you will be using much more torque to get the jar open. In much the same way if you have a larger blade angle you will need a much larger force to turn the propeller through the air which is producing resistance similar to the lid on the jar but we just can’t see the air with our eyes, but we know it is there.
If you reduce the blade angle of the propeller there is less air resistance and less torque needed and the RPM of the engine speed will increase slightly. Now if you increase the blade angle the amount of torque needed increases and the propeller slows causing the engine to slow down, or decrease the RPM of the engine.
The propeller is constantly changing pitch to keep the selected RPM setting we chose for our condition of flight. For instance we would want a low pitch setting so that the engine can produce the highest RPM during takeoff and go around but in cruise flight we would want to set the pitch higher and therefore reduce the RPM thus reducing the fuel burn and decreasing the amount of slippage. For a description of propeller slip view part 1 of this series on constant speed propellers.
Changing The Propeller Pitch
Let’s now look at how the propeller changes the pitch of the propeller. To do this I am going to use one of my favorite free brochures from McCauley Propeller Systems. There is a link to the brochure located at my website and I highly recommend you keep it for future reference. Another great reference is Rod Machado’s Private Pilot Handbook where Rod’s wonderful illustrations make it easy to understand complex topics. Todays topic is
The propeller uses a device called a governor to maintain the speed of the engine through varying the blade angle and pitch of the propeller. A governor is a mechanical device which controls the speed of the engine by varying the pitch of the propeller to match the selected RPM.
Before we learn how this governor changes the blade angle of the propeller and therefore the pitch we need to first learn how the propeller angle is moved. Also to further simplify our discussion since changing the blade angle directly changes the pitch I will use the the term pitch to mean changing the blade angle for the remainder of the discussion.
Again, any time we change the blade angle of the propeller we are changing the pitch of the propeller. Furthermore we will be discussing propellers used on most general aviation airplanes which are non feathering. Look for a discussion on full feathering propellers in an upcoming episode.
First lets look at the forces acting on the propeller and on our constant speed propeller. As the engine rotates the propeller since it is not fixed there is a twisting force, called the centrifugal twisting moment, which makes the propeller go to a low pitch pitch if it was not attached to anything else. But the propeller is attached to something which allows us to move it and overcome this twisting moment.
To move the propeller blades we attach each blade to a piston inside a dome containing oil. The linkage between the piston and the propeller is called and actuating link and is attached to either end using pins. When the piston moves the actuating link then moves the propeller. To increase the pitch of the propeller oil is pumped into the dome creating more pressure which then moves the propeller. To decrease the pitch oil is simply drained.
Additionally, there is a spring attached to the piston which pushes the piston towards the the dome and is thus moving the propeller to a low pitch setting. Therefore, when we pump oil into the dome and move the piston we are moving the piston and overcoming two forces, the force of the spring, we are compressing the spring, and the centrifugal twisting moment of the turning propeller.
When the desired engine RPM setting has been reached the system is in equilibrium and there is neither any oil being pumped into or drained from the propeller dome. If any condition of flight changes such as if we push the nose down and increase the airspeed the RPM on the engine will begin to increase which causes the governor to pump more oil into the dome thus increasing the pitch of the propeller.
Understanding the mechanics of a constant speed propeller is easy once you can understand the basic concepts of how the propeller moves. I have found most people must review this material multiple times to understand the concepts thoroughly. I encourage you to review this video again and envision yourself using the propeller control system. You will know when you have truly mastered an understanding for the constant speed propeller once you
In the next section I will discuss the system which pumps the oil into the dome and how the propeller governor works. I also will be discussing some abnormal system operations such as what will happen if you lose oil pressure or the engine quits running. I also will discuss cold weather operations and running up the engine prior to takeoff.
I hope you have enjoyed this video and it has helped you understand how a constant speed propeller works.
For More Information:
Rod Machado’s Private Pilot Handbook
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