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Constant speed propellers

Old 2nd Apr 2024, 21:20
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Red face Constant speed propellers

Hi all,
So, hopefully, I'll get somewhere with this question. I did get some help on another forum, and then the question bank threw me off completely. From the Oxford books, this is what I get (sorry, can't seem to post images):
As the aircraft accelerates, air speed will increase, causing a decrease in the angle of attack of the blades. Less thrust and less propeller torque will be generated.
But then the question bank ...

As an aircraft with a variable-pitch, constant-speed propeller accelerates along the runway:
[A] The blade pitch angle increases, maintaining a constant angle of attack and R.P.M. ✅
The angle of attack will remain constant and the engine R.P.M. will increase.
[C] The linear velocity of the propeller tip will gradually decrease.
[D] The angle of attack will decrease and the engine R.P.M. remain constant.

So, what exactly is it?
I tried to deduce the answer, i.e. compare with a fixed-pitch prop, as the aircraft accelerates, the relative wind angle changes (moves forward), therefore the AoA decreases. OK.

With a constant speed, I select an RPM say, 2500. When I start to accelerate, then due to the forward speed, the blade tries to rotate faster > this causes the CSU to slow it down, making it more coarse to reduce and control the RPM, so again the blade pitch angle will increase, and maintain a constant RPM.

Is that correct? What will cause the blade to rotate faster when accelerating? And what happens to the AoA?

Many thanks for your help!
Ben2000 is offline  
Old 6th Apr 2024, 07:53
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Make sure you are clear on your definitions. Blade angle (or pitch) is the angle between the plane of rotation and the chord line. Blade angle of attack is the angle between the relative airflow and the chord line.

For simplicity, let’s assume we’re executing a static takeoff with the brakes applied.

We’ve of course got our mixture and prop levers all the way forward.

We then smoothly advance the throttle, RPM and manifold pressure will increase accordingly.

As we then release the brakes, accelerate down the runway and introduce airspeed, the relative airflow changes (as you say it moves forward). The CSU then starts to proportionately increase the pitch and the blades move off their fine pitch stops and coarsen in order to maintain the same RPM (i.e. a constant speed).

So if the pitch has increased, by definition the blade angle has increased – all in order to avoid an overspeed condition and maintain the RPM that has been set.

And if the pitch has increased proportionally to the relative airflow, then the chord line has also changed proportionally to the relative airflow, thus there is no change to the angle of attack.
deadlegdeadengine is offline  

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