2
$\begingroup$

Recently I was wondering how hard would it be to design a RC aircraft propeller. I have some basic idea about aerodynamics, but I'm an electrical engineer and computer scientist by trade. What I found so far was this really nice paper here: https://repositorio.unican.es/xmlui/bitstream/handle/10902/16634/418495.pdf?sequence=1

What I'm still struggling to get is a couple of things:

  1. How does a designated RC aircraft propeller size and pitch, such as 5045 relate to geometric pitch? 50 expresses the length in tenths of an inch, so it's 5 inch diameter and 45 is probably how long the spiral drawn by the tip of the blade screwed into a solid medium would be. Is that correct?
  2. If the above is correct, how do I convert from the spiral length to the actual angle of attack of the airfoil?

From what I noticed, the angle of the airfoil also flattens towards the tip of the propeller. If I get it right, it's to keep the length of the spiral per revolution uniform regardless of the point of measurement along the propeller blade? Is it always done precisely to keep that length uniform, or is it much more complicated than that and the actual shape assumes some "slippage" (air is not a solid block of wax after all) etc?

I found this article here: How would flow angle change along the length of a propeller airfoil? But in order to calculate the angle of attack, the airspeed and the rotational speed must be known. With that being said, it almost turns into a chicken egg problem (but I'm most likely missing something). In my lacking understanding of the problem, the air speed will depend on the propeller shape, but I need to know the pitch and RPM to calculate the shape in the first place! What's the starting point of those calculations? Do they just assume some fixed RPM and airspeed to design the propeller for?

$\endgroup$
1
  • $\begingroup$ Your first reference explains it well. As long as structural strength is adequate, make them as long as possible at the best angle of attack. Use an airfoil with the best Lift/Drag ratio. Notice the old Clark Y really came through in testing. Just remember, forward airspeed will change relative wind, so the 9×6 might be most efficient on the bench, but the 9×8 better in the air. $\endgroup$ Mar 4, 2023 at 13:56

1 Answer 1

2
$\begingroup$

Why do you want to do this? Model airplane propellers are available in essentially every size and shape imaginable for a very low price. It is unlikely that your application needs something that is not already produced.

Model airplane propellers spin very fast and are subjected to surprisingly high loads. If they are out of balance, large vibratory loads will be transmitted to the aircraft. If they have a structural flaw, they can and will fly apart violently.

Don't ever stand in the plane of a spinning propeller. Don't stand anywhere near a homemade propeller the first time you spin it up.

If you are making a drone -- you will need four, six, or eight (or more) matching propellers. Half of them need to be mirror's of the others (turn CCW instead of CW) -- but still need to have very similar performance. It is a lot easier to buy a bunch of props and have spares on hand than to make your own.

Perhaps you are making a scale model that will never fly -- I would still buy a commercial model prop and paint it like the full scale. There are even wooden props you can buy. If the model was of an early airplane, I would buy a commercial wooden prop, strip the varnish and paint, and then refinish to match my scale model.

The UIUC Propeller Database includes performance data for a great many propellers measured in the wind tunnel. It is divided into four 'volumes'. The first volume also includes detailed geometric information about the propellers (blade chord and twist distribution).

In addition to the references you've already found, it should give you some practical examples to study.

$\endgroup$
2
  • $\begingroup$ That’s all sound advice but doesn’t answer the question $\endgroup$
    – Frog
    Mar 4, 2023 at 22:41
  • $\begingroup$ @Frog -- yes my response probably should have been a comment, but it seemed much too long for that. Without more context, there isn't going to be a better answer than the papers the OP posted (or dozens more just like them on propeller design). The OP's focus on converting pitch into angle is a red herring if they really need to design a prop. Perhaps the better answer would be 'Download and run XRotor, QProp, JavaProp, or CCBlade'. But that doesn't actually have much explanatory power. $\endgroup$
    – Rob McDonald
    Mar 5, 2023 at 1:27

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.