Prop for your 1/2 vw engine

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Dana

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Every manufacturer measures their props differently, as I found above. You would think most would use the flat back of the blade, but that's clearly not the case. Furthermore, props aren't always made with a constant pitch. Again, along the blade do you measure? Convention says 75 or 80 percent, but...

Of the four props I mentioned above, I have detailed measurements of two of them. Pitch is in inches, calculated from the back face angle For the prop marked 58x24, the actual pitch varied from 19.5 at 9" radius to 28.9 at the tip; it was 24" pitch at 83% radius. For the prop marked 56x24, the actual pitch varied from 20.5 at 9" to 26.29 at the tip; it was 24" pitch somewhere around 45% radius.

In theory, pitch (but not blade angle) should be constant all along the blade, with each part of the prop operating at its AOA for maximum L/D. In practice, prop designers vary the pitch to tweak performance, like more pitch farther out where the airflow isn't blocked by the cowl and less toward the center. But to do that right you'd need a clear picture of the airflow around the fuselage. I don't think any of the people making props for homebuilts, particularly half VWs, are doing that kind of analysis.

Dana
 

lr27

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The back face doesn't give you a real result. Especially since the airfoil thickness will change the zero lift angle. For any accurate calculations, you need that zero lift angle. I think if you can take a profile and make a drawing, there are simple ways of estimating this, although I forget what they are, this side of software. But what a pain in the butt. In practice, if the root airfoil is thicker, but still with a flat bottom, the zero lift angle will be lower and there will be more pitch compared to a thinner airfoil with the flat bottom at the same angle. For instance, relative to the 2408, the 4416 appears to be very close to doubling all the vertical dimensions of the 2408, and both seem to have relatively flat portions on the bottom. The zero lift angles, respectively, are -2 and -4 degrees at Re=1,000,000, at least according to Profili/Xfoil. However, it's worse than that. The bottom of the 2408 slants about 2 degrees negative of the chord line, and that of the 4416 slants about 4 degrees. So it's really -4 and -8 degrees! You have to add 4 and 8 degrees back in to get the real angle of attack, relative to the flat part of the airfoil. No wonder it LOOKS like designers use less pitch toward the middle.


Airflow around the fuselage is going to be quite variable and messes up any nice, clean, simple calculations. But Larrabee accounts for it in his paper. Let's just use 2 foot long shaft extensions. That won't pose any mechanical problems, will it? Nah, of course not. ;-)
 

lr27

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P.S. Here's a nice illustration I found which might clarify things a bit:

airfoil.gif
 

Dana

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The zero lift line isn't that important for the sake of this discussion, since AOA in airfoil data is referenced to the chord line, not the zero lift line. The back face angle has even less meaning, except that it's easy to measure so it's useful as a comparison, or a check during manufacture. For the Clark Y (a common propeller airfoil, and almost exactly the same as the 2412), the back angle is about 1.79° below the chord line. Though the actual airfoil section isn't all that important, and I don't know how closely prop makers control it (not too closely from many of the props I've looked at).

Classical propeller design goes like this:

1. Choose propeller diameter according to Mach number at the tip at max HP rpm (from engine data) and airspeed (typically Mach 0.8).
2. Pitch the propeller for the blade airfoil's maximum L/D at the design airspeed (climb or cruise, depending on what you're optimizing for, or somewhere in between). The pitch will be constant along the blade, and the blade angle will vary linearly from 90° at the shaft centerline to whatever it needs to be at the tip based on the airfoil's max L/D AOA and advance ratio.
3. Choose blade chord so the propeller can absorb all of the engine's available horsepower at the design condition.
4. Analyze (blade element theory) and/or test.
5. Adjust and iterate as necessary.

Years ago, I designed a propeller for ultralights as part of my senior design project, using the then popular McCulloch 101 engine, using classical blade element theory. The final design I ended up with was almost exactly the same as what the early ultralight designers had come up with by experimentation.

In practice, it gets messier. Engines with redrives mean you can pick the rpm so the diameter isn't a given. There are the fuselage effects I alluded to earlier, affecting the pitch distribution along the blade. Some engines have limitations on the propeller inertia, which means you might not be able to use the optimum blade chord. And (especially in the case of engines used in experimental aircraft) you may not know the engine's actual HP. Manufacturers are often rather optimistic about HP claims (surprise, surprise).

Dana
 
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