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Propeller Dynamics

Essential reading for model aircraft contest fliers. This is the only book on the market explaining propeller theory in non-mathematical terms. A rattling good read, I know, I wrote it.


Prop Doctor visits Las Vegas


Well, life gets busy don't it! I'm now a grandefactodad to two baby girls and am loosely related to two others, thanks to Rosemary. Have also just returned from Jean, Nevada, home of Unlimited Scale Pylon Racing (USRA) finals, and Singapore, which has the best computer shopping I have yet seen. Would you believe 6 floors, each with about 20 shops, stuffed full of goodies?. Makes it hard to get back into Australia without paying H.M. Customs a mint.

But first a word or two from T/R freak Daryl Mills. You may recall my previous article on engine/propeller unbalance in an earlier issue, which quite frankly wasn't very useful except in a theoretical sort of way. Also I just had a prop returned to me from the Oliver Racing Team in the deep North. The prop had failure cracks at the trailing edge near the hub, consistent with torque induced compression failure. Others have had 60 runs and more from this prop, so what caused the failure after just one run? One possibility is engine vibration, either from mechanical imbalance or detonation.

Daryl assures me that there can be a great deal of variation in engine unbalance, from motor to motor and type to type. Certainly I have myself seen some terrible vibrators, including the 1968 Taipan 19 and the early 70's Taipan T/R diesels. So a method is needed for fixing up these engines, and Daryl was kind enough to share with me the method both he and NZ'er Alan Barnes use.

Weigh the piston/conrod/gudgeon pin assembly, then make a weight 66% of this value and hang it on the crankpin. Adjust the crankshaft counterweight until the system balances, and presto, a smooth running engine that won't destroy my beloved props.

Some engines with heavy pistons may need gold counterweights. Daryl assures me gold is twice as heavy as tungsten, so there is a really helpful tip for you (Note: February 2006. From Paul Ryan: the density of gold is 19.32 g/cc, while for tungsten, it is 19.3 g/cc. I guess Daryl might have been referring
to tungsten carbide. Paul helpfully provided a list of other heavy
elements, including plutonium! Thanks mate!).
On the subject of tungsten, I once tried to sharpen a tungsten carbide machine tool on my aluminium oxide wheel. You only do that once. Apparently you need a soft green stone wheel for carbide, but Daryl says you can cut it with a Dremel cutoff wheel. Tungsten rod is available from tool supply companies: I have seen 3mm rod; it is used for making engraving tools. 

Now the really hot prop news from Las Vegas! A guy approached my prop display and asked me the usual question on pitch: mainly, how fast does an airplane go for a given pitch, and what is pitch anyway? Wow, I really hate that question, as its all written up on my web-site and in my book; it gets a bit repetitive. Since USA is the home of Zinger props, I replied by saying that Zinger props had 2 pitches written on them, say 26X6-10, how confusing is that? What does that mean?

Well, the spittle wasn't dry on my lips when this virile looking old guy standing nearby spoke up and said he knew what it meant, because not only did he invent the notation, but he was also the J of J&Z propellers! My ears immediately flapped open and I listened. This guy was the incomparable Lawrence Jenno, originator of the Zinger line of props. In his retirement in Las Vegas, he has his own machine shop where he builds antique model airplane engines and replacement parts. You may care to drop him a line at 4341 Flandes St., Las Vegas, NV 89121,USA.

It seems Larry built the machinery for making wood props, and was thus well placed for experimentation. He found that for best static thrust and in-flight performance, the radial distribution of face-pitch must rise toward the tip. Thus in a Zinger 26X6-10, the 6 is the face pitch at 50% radius, while the 10 is the face pitch at the tip. How about that! When Larry introduced this line of props, they blew away the competition and remain a very fine prop to the present day. You may have seen his chart of static thrust versus RPM f or various props.

This applies to the large diameter Zingers. Some of the smaller size Master airscrew wood you may recall I slandered earlier on, as having poor airfoil sections at 75% radius. Just last night I was talking to Norm Kirton, who confirmed that when he tried a Zinger wood 10X6 on his sport pylon it revved well and went nowhere. I guess its hard to get a good airfoil on a wood prop. So forgive me Larry, I love your big props and hate your little ones! Maybe we could blame Joe Zingali, who bought out Larrys share of the business, that solves one problem. Makes another. 

Now for some Q and A. My old flying buddy from Ryde MAC in the 60's Greg Ardill asks what use is knowing the difference between induced and profile propeller efficiency to the modeller. Probably not a lot Greg, but there are 2 things you must do. Run as much diameter as you can without tip speed exceeding Mach .9 and you will have maximum induced efficiency. Now you just saw how awful the Master wood 10X6 is, that can all be put down to low profile efficiency as a result of the low L/D of the lousy airfoil section. Always cllamp a pair of Vernier calipers at 75% radius and look at he section on your prop. If it is on backwards or is a rotten shape or very thick, then you have a low profile efficiency.

Now it was really great to get a letter from Merv Bell: I haven't seen Merv for yonks and I really miss the guy, he is one of natures gentlemen. At least I thought that until I read his question. You scum bag Merv, how am I supposed to know that? Merv wants to know how to statically and 

dynamically balance a 3-blade propeller! Apparently he used wheel balancing equipment at Holden to balance his, but he didn't tell me where he crimped on the wheel weights to the propeller blade. 

Seriously, the dynamic balance condition is that the centre of gravity of each blade lies in the same plane of rotation. If a blade is bent, say forward, its C/G will be ahead of the other blades: the resultant imbalance will induce a rocking action at the shaft, hence vibration. Now its not so easy to bend blades or to find their individual C/G position in 3 dimensions, so you are to a large extent at the mercy of the manufacturer. 

The static balance condition (tip to tip) may be met as for 2 blade props, provided you use either a magnetic balancer or one of those which sits the the balance shaft on little wheels. These balancers have the advantage that the shaft rotates around its axis, so that there is no offset in the C/G of the prop. This contrasts with sitting the balance shaft on knife edges, when the C/G is raised and lateral balance errors confound the tip to tip balance. Not so good. I was astonished to walk into a full-size prop maintenance shop in Bankstown to find they were using knife edges! Talk about primitive.

Perhaps you could call Brian Eather about this. He builds superb 3-blade stunt props, probably the best in the World. As a perfectionist, I'm sure Brian must have an opinion on this problem. I vaguely recall he uses one of those balancers with a vertical pin which contacts a bush set in the prop hub. 

In the vein that propellers are just rotating wings, Brian Burke asks if we could use turbulators, terraced airfoils, fences, dog teeth, strakes and area ruling on props. Gee Brian, you left off flaps and spoilers! I don't have room this month to answer all these, but I will comment that vortex generators hold a lot of promise for props. By vortex generators, I mean slots cut into the leading edge of the prop, in a perpendicular direction. At the least, these break up stall cells which may form at high angles of attack, and can modify span wise flow on the upper surface. Worth a try.

In closing, I must mention a fascinating letter from Ian Garton. Ian also was commenting on the awful section on the Master wood prop. Evidently he was required (!) to put a section like this on a boat-propeller casting pattern. This was a pattern for a Sydney Harbour Ferry prop, made at Eveleigh railway workshop in NSW. Ian was witness to the phosphor bronze casting process, but not the subsequent machining, a shame. Ship propeller machinists have in the past been notorious for their lack of hearing, the machining process actually being done with cold chisels!

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