in mid age Leonardo da Vinci discovered a practical advice to move things:
he developed the airscrew. Since then a tremendous evolution has lead to
present day high performance propellers. Where high performance is wanted
there’s a constant search for the best product, and in such events
like Speed and Teamrace people spend considerable effort to find the best
item. While top speed and economy is not high on the priority list of aerobatic
flyers, we still have several requirements to ask of our propellers. The
sheer ability to move our airplane forward and ( sometimes ) up is only
the very basic requirement.
Apart from finding a propeller which the engine can handle, over the years we have learned that the choice of propeller can completely change the flight characteristics of our airplane. It’s not possible to offer a simple rule for prop selection. It depends on too many variables, like model, engine type, line length, pilot’s preference etc. These are only a few aspects. So we are finally left with the task of trying many propellers to find a suitable one. And this may include modifying an existing prop, or even making oneourself.
Commercial propellers differ strongly in quality. By quality I mean the way they work. There can be huge differences . They depend on blade shape, airfoil, thickness, area distribution, pitch distribution, and maybe a few more. Even the nominal dimensions ( given on the propeller ) are not always correct, there can be drastic deviations. So we cannot fully rely on these numbers. At least this requires some careful check to be sure what we actually have. If modifications are necessary we have to know where we are starting from. All this leads to the conclusion that we need a means to measure propeller pitch. For modern pipe application it’s even a necessity : the pich gauge.
|Commercial pitch gauges are available, but they are not quite cheap. With a little bit of effort, some pieces from the scrap box, a fret saw , and a drilling machine we can make our own. One sketch shows the front face drawing ; it allows measuring of typical stunt propeller sizes in metric numbers from 2 to 13 Centimeter radius, and from 10 to 25 cm ( 4 to 10 inch ) pitch. The small radius diameters are more academic and we can neglect this range. Propellers don’t pull much that close to the spinner, and the airstream is very turbulent around the fuselage anyway. So we need not check propellers below the 5 cm radius mark. The most interesting range is the|
at about 75% of radius, the biggest part of thrust is produced here. Also
the range beyond15 cm pitch is hardly interesting for aerobatic flyers.
The drawing can be enlarged and printed to original size, a scale is given
at the bottom of the drawing. Enlarged copies can sometimes be distorted
in the process, so the final copy should be carefully checked for errors.
front face scale is glued to some 2 mm aluminium or 3 mm ply sheet. There’s
an open area for the propellers to pass through. This gamut is bolted to
the front of the base plate. The measuring edge must lie in the same plain
as the top of the base plate. This base plate is a solid 10 mm hardwood
sheet. To arrange the different radius stations there are different methods.
One method is to cut grooves into the base plate with a router ( that’s
how the commercial gauges are made ). Another method uses hardwood stringers
exactly 5mm wide. If these are fixed to the base plate at a distance
|of exactly 5mm to each other, we’ll have radius stations in increments of exactly 10 mm - just what we need. For my own gauge version I preferred to use square U-shaped aluminium extrusions with an outside width of exactly 10 mm. If mounted very close to each other on the base plate the radius stations are again 10 mm apart. Just make sure that the centre ( middle ) of the “U” is exactly the radius station.|
measure pitch we need a measuring arm, or pointer. This is made from scrap
metal. The pivot hole has to be exactly at the same height as the measuring
edge. Also we need a mounting block for the propeller. This must have the
same width as the groove in the base plate; either 5 mm square for the wood
stringer version or ( as in my case ) 8 mm square, since the inner width
of the aluminium profiles is 8 mm. This block is made of brass to prevent
friction when sliding the block. The propeller is mounted on top of this
block using a small elevation. This is quite practical to mount the prop
at a convenient height for working the gauge. At last the respective radii
stations are marked on the side of the base plate.
the gauge is very simple. The propeller is mounted to the mounting block
making sure that it is fixed perpendicular to the front gamut. The mounting
block is positioned into the desired “radius rail”. The pointer
is rotated and moved upwards till its edge just touches the bottom of the
prop blade. It will be necessary to slide the mounting block with the propeller
sideways to exactly achieve this task - that’s what the “rails”
are for ! To do this work properly it is helpful to place a lamp behind
the gauge. There will be a fine “light gap” between the prop
and the pointer which is
|clearly visible, and helps tremendously to exactly measure pitch. If the prop is exactly lined up the pitch can be read at the front face at the corresponding pitch “curve”. The process is repeated several times at several stations.|
all propellers have a straight line at the bottom of the airfoil. Depending
on propeller airfoil and workmanship of the manufacturer, different props
can employ some very fancy shapes as can be seen in the last sketch ( no
joke, things like these exist !). In this case, if the pointer just touches
the airfoil bottom, a very exact pitch measurement isn’t possible.
However this shouldn’t keep us from measuring. After all what we are
doing is “comparative” measure, and this is better than not
measuring at all. It’s amazing to see how much propellers can deviate
from nominal values.
we want to modify a given prop and/or want to achieve a desired pitch, we
simply mark a spot at the blade bottom along the pointer with a pen marker.
Using files or sandpaper the desired pitch can be achieved. However we have
to be very careful. “Increasing” the pitch is hardly possible
since we would automatically change the airfoil, especially the front of
it. Removing lots of material at the aft end ( to reduce pitch ) will automatically
reduce blade chord, thus making for a more narrow prop blade. Also this
will reduce blade thickness and airfoil shape. So it can be seen that these
modifications can be made within tight limits only. Those "prop benders"
who twist their carbon prop blades also use their dearly beloved pitch gauge.
In fact they cannot live without one since the required pitch changes can
be very small. It goes without saying that every modifying is followed by
very careful balancing of the prop.
pitch gauge is not difficult to build and is not much work. With it we can
precisely check our props, eliminate production errors, modify props to
our desire, and maybe even improve them. For those who make their own propellers
a pitch gauge is mandatory. Once you have one in your workshop you cannot
live without one any longer. Leonardo was badly off without one.