| 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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