# Propeller Speed and Stacked Plane Propellers

#### tomizzo

My background is electrical engineering, but I have a question regarding the mechanics and physics regarding plane propellers.

The purpose of a plane propeller is meant to provide thrust to the plane. Similar to a plane wing, the propeller generates lift but does so in a direction that is parallel to the ground.

Looking solely at the rotational speed and torque of a propeller, is the speed of the plane limited by the maximum speed at which the propeller can rotate? Say for example the plane increases its speed. Will the plane reach a speed at which the propeller can't keep up with the incoming speed, thus transitioning the propeller from something that generates force to something that resists a force (similar to a wind turbine)?

Furthermore, I've been looking into plane designs that involve two propellers that might be stacked one in front of another. Does a propeller accelerate the incoming wind speed? And could this acceleration in wind speed affect the thrust that the secondary propeller could achieve, similar to the issue described in the question above? For a visual idea of what I'm trying to describe, refer to the helicopter below, but imagine this set up on a propeller plane instead.

http://www.guncopter.com/images/ka-50-large2.jpg

Thanks for any responses!

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#### tomizzo

Thank you for the articles. They were very useful. I have another question though that I'm hoping someone can help me with:

With regards to the idea of variable pitched propellers, it seems like they are primarily useful when used on a constant speed propeller. By being able to change the pitch of the propeller blade, optimum take off, climb, and cruise thrust can be achieved. However, I'm curious what determines the optimal combination of these factors.

For example, on take off with a constant speed propeller, it is ideal that the blade pitch be very small. Why exactly is this? Does this depend on the relative speed of the aircraft? Are there resources that explain how the propeller speed and plane speed combine to determine an optimal blade angle?

Another question, does this idea of changing the blade angle going completely out the window when you can control the propeller speed?

I guess I'm having a tough time on the relationship between the blade angle (pitch), propeller rotational speed, and plane speed...

#### rcgldr

Homework Helper
With regards to the idea of variable pitched propellers, it seems like they are primarily useful when used on a constant speed propeller. By being able to change the pitch of the propeller blade, optimum take off, climb, and cruise thrust can be achieved. However, I'm curious what determines the optimal combination of these factors.
One of the key factors woudl be power and/or efficiency of an engine operating at a specific rpm and it's correlation to the propeller speed (there may be gearing so that the propeller operates at a lower rpm than the engine or turbine).

Based on my experience as a passenger in twin engine prop aircraft used for short commutes, both prop speed and pitch are adjusted as needed, with the main configurations being take off (variable speed), steady climb, cruise at altitude, descent, and reverse thrust.

The only case I'm aware of where rotor speed is kept nearly constant is in the case of radio control helicopters in "aerobatic" mode. To eliminate angular momentum issues, a throttle versus pitch (which can be negative (inverted hover or climb) as well as positive) curve is programmed into the transmitter for aerobatic mode called "idle up" and enabled / disabled via a switch (when disabled, a different throttle / pitch curve where the rotor speed varies is used). It would be possible to have an onboard computer keep motor rpms constant despite pitch inputs, but I don't know if this is done for rc helicopters. It is done (effectively) on some aerobatic multi-rotor drones, where the throttle versus pitch mixing is done by the drones onboard computer.

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#### SteamKing

Staff Emeritus
Homework Helper
Looking solely at the rotational speed and torque of a propeller, is the speed of the plane limited by the maximum speed at which the propeller can rotate?
The physics of the propeller can get quite complicated. These articles explain some of the basics:

http://en.wikipedia.org/wiki/Propeller

http://en.wikipedia.org/wiki/Propeller_(aircraft)

http://www.grc.nasa.gov/WWW/k-12/airplane/bgp.html

The propeller is a machine which converts the torque produced by an engine into thrust which drives the aircraft forward.

The particular speed at which an aircraft flies is determined by matching the total thrust produced the the propeller(s) to the total drag on the aircraft, just like the aircraft is kept aloft because the total lift generated by the wings and horizontal stabilizer is equal to the weight of the aircraft.

The amount of lift which an airplane can generate depends on the speed of the air over the wings, while the weight is roughly constant. If the airplane can't move fast enough to generate the necessary lift, it won't fly. The drag on the aircraft is roughly proportional to the square of the speed, while the propeller thrust is a maximum at low speeds and then drops off as speed increases. Where the curves of drag and thrust versus airspeed intersect, that is the speed of the aircraft under those conditions.

#### AlephZero

Homework Helper
On large prop aircraft, there is a significant amount of inertia in the prop and the engine. With a fixed pitch prop, the only way you can change the thrust is by changing the RPM and the inertia means you can't do that quickly. Changing the pitch of the prop is much faster.

For example, when landing you need to maintain a high enough engine speed to be able to apply full power quickly if you want to abort the landing, but with a fixed pitch prop on a large plane that would generate too much thrust during the descent.

For large aircraft, variable pitch props can also provide reverse thrust for braking to land on short or poor quality runways (no danger of locking the wheels and skidding), and also allow the aircraft to reverse when on the ground without needing a tow truck, or make tight turns in restricted spaces on the ground using forward thrust on one wing and reverse on the other.

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