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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.


The Prop Doctor is IN F2A mode


To my mind, the two World-Class events of greatest significance are F2A control-line speed and F1C free-flight power. No doubt this is the result of a miss-spent youth reading the World Champs reports in Aeromodeller magazine during the late 1950's. Ever since Bill Wisniewski introduced the pipe on his Pink Lady, F2A has not been the same again. 

In one fell swoop, Wisnieski not only blew away the Russians and Hungarians, he also jumped the World Speed record from 140 to 160 MPH. Awesome, truly inspiring.

The attraction of such prestigious events is a great temptation to prop makers. If you can't make it in F2A or F2C, you're not really world class. With my F2C props performing well, it was time to turn to F2A. Well, what a tough nut to crack! 

The aerodynamics of F2A propellers I covered in a previous issue. To recap briefly, F2A props turn such staggering RPM and speeds they must be single bladed, with high subsonic tip speeds. The tip airfoil sections must be very thin, preferably using a modern transonic cambered section such as ARA-D. Two blades cannot be used, as the chord is then too low to provide any degree of profile efficiency.

All that is left is the requirement for a suitable counterbalance, and hey ho away we go. Dream on, Prop Doctor. After talking to some speed fliers, there appeared a very demanding list of requirements that took some filling.
The counterweight had to fit into a 30mm spinner; be statically and dynamically balanced, be part of the propeller itself to simplify changing propellers, and permit the use of a special pitch-changing washer. What is more, the whole assembly could not be made more than 8mm deep, otherwise you run out of shaft.

These demands ruled out my other single blade designs, in which the spinner is moulded as part of the prop. The pitch change washer would tilt the spinner, so was not acceptable.

After machining several F2A moulds, it became apparent that the propeller blade would have to be light if a brass counterweight was to be successful. Luckily, this is compatible with the thin airfoil requirement: the production prop has a 4% thick section at the tip, with 5% camber and high point at 34%. At 3/4 radius, the numbers are 7% thickness, 3.7% camber and high point at 25%. The sections are thinned ARA-D. Face pitch is 6.1, with experimental pitch 7.1 inches.

The European props were using complex machined counterweights, which were moulded directly into the prop. You break a prop, you throw away the counterweight. They also require special back-plates to suit the stepped counterweight. The cost of producing these set-ups has to be prohibitive. If you live on boiled cabbage, have carrot soup for dinner and run your own Vodka still, maybe, but not for the high life that I live (boiled potatoes, pumpkin soup and VB).

The Andy Kerr counterweight system has always appealed to me, but it met with criticism. First you had to buy one. Then you had to match the prop to the spinner, a real nuisance in the days when props required a lot of rework. How the pitch-change washer went there, I do not know. These days F2A props are available which require virtually no rework, other than de-flash and balance. Because they are moulded in CNC machined aluminium moulds, all blades weigh the same, with the correct airfoils and radial pitch distributions already set. For these props, the pitch change washer may not be appropriate, as it can introduce a non-optimum pitch distribution. These props remove much of the criticism of the Kerr hub, as balance is not the problem it once was.

The optimum counterweight strategy thus appeared to be a Kerr hub, but without the spinner! By trial and error, it was found that a CNC milled counterweight 17mm wide and 12.6 mm radius met the static balance requirement: it also fit inside the spinner, as it has a matching curved surface machined in. Depth of the counterweight is 7.8mm. By making the prop hub 6mm thick and adding 1mm to the front face of the hub boss, it was possible to achieve dynamic balance, and meet the overall 8mm-thickness requirement. This 8mm includes all the counterweight thickness. (In case you've forgotten, dynamic balance requires that the centre-of-gravities of the prop blade and counterweight be collinear in the plane of rotation.)

Now this all worked out rather well. All the requirements are met, including use of the pitch change washer. In addition, props are available for all pitches; you just have to try the existing prop, then indicate what relative pitch you require for your engine/plane set-up. You really only need a couple of counterweights, these being supplied very close to balance as a result of the precision CNC machining. All-up weight is 13 grams. Prices are a joke, less than a video rental.

My thanks got to Robin Hiern of Model Racing Services, Rob Milwain up there in sunny Muswellbrook, and Charlie Stone and Ron Hoogenkamp right here in humid Perth, for supporting these developments.

That's all from the Doc this month. Do keep those questions rolling in, and good luck at the Nationals.

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