Propeller question

[HF_ATL]

Hello all,

Can someone please tell me (and explain why) the answer for the following question:

If the propeller rpm is fixed and the airspeed increase, the angle of attack of the blade:

a) decrease
b) remains the same
c) increase


thank you

[Intruder]

Decrease.

Do a vector sum, at one point, of the airflow due to rotation and the airflow due to airspeed.

[HF_ATL]

Thank you.
I guess this shows the idea right?

https://postimg.org/image/knn1iixvt/

[TURIN]

Is it a variable pitch, constant speed prop or a fixed pitch prop?

[HF_ATL]

Fixed pitch prop

[westhawk]

HF ATL: If you can work through answering a couple of questions, you'll not only have your answer, but also some understanding that will never leave you!

Here goes.

Sitting still on the runway at SL, the throttle is pushed full forward (open). What will the RPM stabilize at? (static RPM)

With the throttle still full open, you level off at 1,000' MSL. At what RPM will it stabilize now?

At full throttle, RPM will vary according to propeller load. Propeller load varies with prop blade AOA and higher AOA increases prop load.

Why does the RPM increase during the takeoff roll?

Bonus question: Does the engine produce the same HP at 2,350 RPM and full throttle as it does at 2,650 RPM and full throttle? Assume very similar density altitude for the purpose of these questions.

Pro study tip:

First, read the textbook! Then commit to your answers on the practice exam. Then try and find them in the textbook. Modify your answers as needed after reading textbook. Now answer the practice exam questions again. This is a well proven method of gaining knowledge that may have been missed on the first reading of the textbook or other source material.

Best of success!

[9 lives]

If the propeller rpm is fixed and the airspeed increase, the angle of attack of the blade:

As worded, this is a very tricky question. If the RPM is fixed, that's a very different thing to a fixed pitch prop, and the answer to the question will be different.

If the RPM is fixed, that is either (in the case of a fixed pitch prop), because the pilot is adjusting the throttle to maintain the RPM constant with an airspeed change, or the prop is operating normally as a governed constant speed prop to maintain the same RPM with power and airspeed changes.

So in the case of a fixed pitch prop, whose RPM is fixed [by pilot action] as airspeed increases, the pilot must be closing the throttle as the airspeed increases (and developing less power) to remove the engine power which would normally increase engine speed as the airspeed increases - in a descent, I presume.

Therefore there are more variables affecting blade AoA than just RPM and airspeed, there is also the power being transmitted into the air (reducing), and power required for level flight (also reducing).

So think this tricky question through really carefully.....

[westhawk]

So think this tricky question through really carefully.....

If the propeller rpm is fixed and the airspeed increase, the angle of attack of the blade:

That's very good point. The wording of the question is not precise enough to discriminate between the given answer choices. Stated another way, either answer A or answer B could be correct depending upon how the RPM is held constant.

I think the question would be more valid if the question were worded like this:

With a fixed pitch propeller and fixed throttle position, an increase in airspeed would result in the following effect upon blade angle of attack:

Perhaps a better wording?

Another example of how much thought is required in order to write good test questions!

[sycamore]

If the RPM is fixed at speed `x`,then that is a particular angle of attack; if the speed changes,then the prop governor(CSU)will adjust the blade angle,so that the same angle of attack is maintained,irrespective of the `size` of the vectors corresponding to that angle of attack.The same applies if the a/c slows down.(accepting that there are transient changes in blade angle).
If the pitch is fixed,RPM can only be maintained by reducing throttle,as stated earlier.
Badly worded question,and rather unfair..

[noflynomore]

Why is it badly worded? It's not unfair in the least. It isn't supposed to be a realistic scenario, it's a theoretical question intended to probe your understanding of propellor dynamics. Nothing more, nothing less. Don't complicate it with superfluous stuff (guff) about governors and "it wouldn't do that".

Intruder's got it nailed and he ain't even from UK and probably completely unfamiliar with their isiosyncratic exam style.

Additionally, given the wording it's almost certainly from the fixed pitch syllabus so getting into paroxysms about governors is completely inappropriate.

Just RTFQ, answer it and move on! Its a CAA exam remember?

[ChickenHouse]

How can the airspeed increase at fixed rpm? Initial inertia lag only?

[9 lives]

How can the airspeed increase at fixed rpm?

The pilot could descend.

It isn't supposed to be a realistic scenario....... "it wouldn't do that".

There are many things which are not trained at each level of training for various licenses - because they are not realistic, and the flight of a pilot at that level "would not do that". It's a stretch to examine for things which are not realistic.

If the question is intended to probe the candidate's understanding of an unrealistic scenario (which probably was never trained), all of the information required to make a determination should be provided. In the case of this question, as written, the information required to formulate an understanding and answer correctly was incomplete.

almost certainly from the fixed pitch syllabus

So the question may be almost properly worded then....

[westhawk]

I have always preferred that multiple choice test questions be specific and have only one correct answer. It's not as easy to do as many might imagine.

As an instructor I don't like multiple guess (or memorized) questions at all! I prefer to ask students to explain things. Much easier to identify areas requiring further study that way. I've always said (somewhat jokingly) I could train a drunken street bum with no aviation background to pass any FAA knowledge test in a couple of weeks time. (if at least somewhat sober!)

Unless it's one of those Santa Claus DPEs I read about, the same guy wouldn't last the first 5 minutes of a practical. It's the application of theoretical knowledge that really matters.

[Pilot DAR]

Welcome HF ATL,

I have moved this thread simply because I think it's a great question with a more broad interest, best served in this forum. The basic question about propellers is thought provoking, but the discussion about formulating exam questions is also worthy of discussion.

[krohmie]

Fixed pitch, fixed RPM, airspeed increasing means AOA decreasing.

[ShyTorque]

This is as clear as anything you need to consider: