164 The Model Engineer and Practical Electrician. February 13, 1930.
Model Aeronautics.
Model Aeroplane Propellers.
By W. E. Evans (S.M.A.E.)
AIRSCREWS screw their way through the
air like a wood-screw when turned moves
through a piece of wood; but, as air is not a
solid, it gives considerably against the pressure
of a propeller blade, which causes the propeller
to have a marked percentage of slip. That is to
say, if a propeller is designed theoretically to
travel 100 ft. forward in 100 revolutions, the
actual distance traversed may be only 75 or
80 ft. Allowance for slip must, therefore, be
made in designing a model propeller for a given
performance. This percentage of slip may be
kept fairly low by skilful design combined with
correct relation between thrust area and resist-
ance of the machine. The success or failure of a
flying model depends more upon the suitability of
the propeller than many people imagine.
propeller should be correctly designed, well
carved and properly balanced to give the best
possible results. Bent wood propellers were
almost universally in use many years ago, but
are now generally discarded as being crude, and
aero-dynamically inefficient. They are, there-
fore, omitted from this article.
Metal Propellers.
A
Metal propellers, I believe, will soon super-
sede the carved wood propellers for efficiency.
As I have not yet had an opportunity of carry-
ing out a series of tests with these, the subject
of metal propellers must be left for a future
occasion. But so far as my observation goes
I am of opinion that because metal propeller
blades can be much thinner than wood blades,
the resistance is less, revolutions are greater
and therefore the forward speed and static
thrust is greater for the same power applied.
Designing a Propeller.
The propeller of a model aeroplane should be
the final part of the complete machine to be
designed and made because there are important
factors to be considered, and they are-the wing-
loading and the resistance of the model at
various speeds. These cannot be accurately
ascertained from drawings as the materials
required cannot be accurately weighed, and,
therefore, the wing-loading cannot be known,
although this may be fairly well estimated by
an experienced builder. Having finished the
model excepting propeller, we can find the wing
loading by weighing the model and adding the
weight of a suitable propeller and measuring
the area of the plane; then, by rule of propor-
tion, we obtain the number of ozs. per square
foot of lifting surface. Suppose a model weighs
12 ozs. and has a wing area of 2 sq. ft., the
loading is 6 ozs. per sq. ft.
Flying speeds, that is cruising speeds where
a model flies a horizontal course, vary as the
square root of loading. Opposite is a useful table
of cruising speeds in miles per hour and feet
per minute for various loadings from 4 to
8 ozs.
Then there is hardly anything known at
present about the resistance, or drag, of model
aeroplanes, and herein lies an important field for
experiment by aero-modellists, a suitable group
of whom would be the Research Committee of
the Society of Model Aeronautical Engineers.
For the present we must be content with the
bald statement that it is necessary for a pro-
peller to give a static thrust of at least one-
quarter the weight of the model, i.e., a machine
weighing 12 ozs. must have a propeller which
will give a static thrust of 3 ozs. at the number
of revolutions at which it should fly the model.
Static Thrust,
As a member of the Research Committee of
the S.M.A.E., I have tested about fifty model
aeroplane propellers, all of 10 ins. diameter, to
get comparable results. The apparatus for
carrying out these tests was exhibited on the
stand of S.M.A.E. at THE MODEL ENGINEER
Exhibition, 1929, so will not be described here,
but a feature of the apparatus is complete
absence of friction owing to the motor and
revolution indicator, in fact, everything except
the scale for reading the number of ozs. thrust
being suspended by fine wires. Some import-
ant results were obtained. Firstly, it was
established that when the number of revolutions.
were increased 50 per cent., the static thrust
increased 70 per cent, or more. Similarly, when
the revolutions were doubled, the static thrust
was increased by 170 per cent. Therefore,
although a propeller may not fly a model at the
usual number of revolutions, it will do so if
the revolutions are increased sufficiently. But,
of course, duration will be sacrificed to attain
this end. Secondly, the static thrust of 10-in.
propellers of good design and medium pitch is
approximately 1 ozs. at 1,000 revolutions per
minute. Therefore, this size propeller cannot be
expected to fly a model weighing 8 ozs. or more
at that speed. At 1,500 revolutions the thrust
rises to 3 ozs., but this is faster than the
majority of propellers are driven. The maxi-
mum thrust at 2,500 revolutions is over 7 ozs.
This is the result of the maximum output of
motor, but graphs indicate that at a still greater
number of revolutions the static thrust would
still increase at the same rate as before up to
an unknown point.
Loading.
4 ozs.
Miles per hour. Feet per min.
12.0
1,056
12.7
1,122
13.4
1,152
14.1
1,212
14.7
1,296
15.3
1,344
7
39
15.9
1,398
16.4
1,446
17.0
1,494
"
Blade Width and Diameter.
'Thirdly, the best results so far as static thrust
is concerned are obtained if the blade width is
kept to medium proportions, being neither
narrow nor wide. The best width for a pro-
peller of 10 ins. diameter was found to be
