Prop Balancing

Why is the master airscrew wooden prop disliked by almost everyone? What is it about the APC prop that is liked by many?

Next question.

Master Airscrew wood props: This is the first I've heard of anyone hating them, and a matter of fact I use them almost exclusively on my .35 powered classic models. I balance and check pitch on all the props I use and the MA's are without doubt the most consistant when care is taken to choose the right ones when you buy. (Look for consistant wood grain.)

APC Props: We agree here, I love these props all but the weight.

You did check to make sure both blades are the same length before starting to balance, didn't you? This is just as important as everything else. Make sure the blades are the same length and the same shape (thickness, tip shape, etc.) and then check the pitch to make sure that is even (especially on a wooden prop). After all this, then balance.

The thing that I find enjoyable about APC props is their consistancy in pitch and balance.
Have fun.

When the full size guys grind material off a prop, like Hamilton Standard, they attack the front (what we call the front, anyway).

Someone who really knows what they are talking about; please resolve this. I'll admit that I do not.

I hope so! I just picked up a couple of wood MA 10-5 props for my blueprinted 50th Anniv. Fox 35.

I still have several old wood Top Flite 10-6 (the paddle blade jobs) and an asst. of Rev-Up 10-6 EW.

Neither of the retailers here have ever carried the wood MA's; lately, the only wood ones you see are Zingers. But the black MA's are both quite heavy, and very inefficient in the air. An APC is probably just as heavy, but it works so much better!

As to the correct side to sand, I've been taking material from the fronts (rounded-curved side) for over 40 years. My way works for me; I've never had a pitch guage, so rather than mess up the props' pitches as delivered (which is established by the back side), I left the backs strictly alone.

The K.

Similarly, painting one blade will change the airfoil shape, albeit in a small way.

So maybe the best way to go is to first make sure the blade lengths and tip shapes are the same. Then make sure the pitch at each station agrees between the blades. Finally, start sanding.

Sanding near the tip means you remove the least material to balance, but you end up sanding where it has the greatest aerodynamic effect. Sanding near the hub has much less aerodynamic effect (or none, if you're inside the spinner), but you've got to take off more material. If it's really bad, you'll weaken the prop, but maybe that's a hint that you've got hold of a lemon.

A prop is a rotating airfoil which derives its thrust both from the pitch (i.e., angle of attack) and the efficiency of its airfoil (the cross section shape), compounded, of course, by the square of its velocity. Sounds just like a wing, doesn't it! It should, because it is...a rotating wing, that is. The thrust produced by the prop is the result of the lift produced by the rapidly rotating airfoil shape at a particular angle of attack. This is why it is not only possible but common to do the math on RPM, airspeed and "labled" prop pitch and find that the airplane is flying faster than that number of revs times the labled pitch would predict.

Thus, one should not think of the pitch as being the sole determinant of the thrust produced by a prop. An efficient airfoil can make a noticable improvement in performance over the identical looking prop with a poor airfoil. Clarence Bull's lovely BY&O props were beautifully crafted and exhibit a fine and easily seen example of this airfoil type. This also explains the very good performance of the APC props versus some others...plus, of course, their almost perfect reproduceability from the manufacturing method.
A good performing prop will generally have a sharp leading and trailing edge, a flat to slightly undercambered back side and an airfoil that has its high point well forward...around 20 to 30% at any station. Sloppily designed or produced props will often have a very uniform curve from leading edge to trailing edge and their performance (thrust) will be noticably inferior to those with better design and manufacturing care.

Therefore, one should be careful of removing material from the front surface for balancing purposes to insure that the airfoil shape is retained. If the prop exhibits a bad airfoil it could even be worthwhile to reshape the entire top side to a better shape while simultaneously balancing it.

One trick I use a lot when balancing or repitching a wooden prop is to use a single edge razor blade held nearly vertical to scrape material from the back side rather than trying to sand. You'll find you can remove material much faster and, by varying the pressure from l.e. to t.e., repitch the prop at the same time.

One of my problems is that, other than balancing, I have not paid sufficient attention to props, even as a sport flyer. Not enough experimentation. I kind of have to force myself not to run spinners on my planes, since prop switching is a lot simpler without them. So looks be damned in this case.

Going to the wide array swap meets available in the Midwest, one runs into all sorts of (usually RC-marketed) props, with strange, usually out of production brand names that I don't recall right now. Some have a surprisingly generous amount of blade area for their size, making even some of the Rev UPs sold as Extra Wides look only average in comparison. They have potential for CL. Of course, another way to get a wide blade is simply to cut a larger one down, like, say, an 11-6 to a 10-6. Some of the old ignition-oriented props from the '40s look interesting too, based on blade area. However, they often are deficient in the materials arena, and I don't know how accurately they were made, or how consistent the pitch remains as one measures from the hub out.

The BY & Os seem to work well. I have a couple of APCs, but don't use them much, partly for purely aesthetic reasons, but also for safety. I'm more comfortable around wood.

That aside, I'm tempted to buy (invest?) in one or two of GMAs carbon fiber props. I wonder if anyone has any experience with them.

I don't, though I have some of Supercool's props (he makes GMA's props) the workmanship is quite the best I've ever seen. Light and stiff, with well-shaped airfoils.

He told me that the best place to remove material is on the top (curved side) of the prop, after the high point of the airfoil. His rationale for doing it this way is that this part of the prop is almost always in a stall condition when the prop is spinning and providing "lift". This means that a minor change in shape is not going to affect the "lifting" characteristics of the prop to significant extent.

I set the pitch first by sanding the back of the blades. Then I thin & reshape the blades by sanding the front to move the high point to 25-30% & get a fairly sharp edge on both the leading & trailing edges. I've had good luck with 150 grit 3M Stickit on a plain white closed foam sanding block. (I'm going to try Ted's razor blade idea next time.) I use the original finish as a guide to see where I'm removing material & I count the strokes. If I take 15 or 20 strokes to get one side going the way I want it, then I turn the prop around & give it the same on the other side in the same area. I keep doing that until the high point looks about right & there aren't any thick, flat edges, front or rear. Then I put 'em on the balancer. I remove a little material from the front of the heavy blade & try not to change it's basic airfoil. Generally they're not too far off so it doesn't take much.
Brett got my fiddling with props a year or so ago & it really is amazing what you can do by reworking a prop. Now I can't bring myself to put a wood prop on an airplane without checking the pitch first, as well as the balance. I use a lot of those dandy Zinger prop kits. The one I'm running at the moment was marked 13-6. It measured a 6 pitch out about a third of the way from the hub, then it gradually went to a 4.5 on one side & a 5 on the other. I like Zinger kits, you just have to sand off everything that doesn't look like a prop.
Nils

Anyone ever notice this?

After a while, I just say the Hell with it. As long as one blade isn't pointed towards The People's Republic...

Or, it's like this: I do get them balanced, and then decide to seal the raw wood with whatever gloppy clear dope is nearby...See where I'm going with this?

Great post, and you are correct!