An Automatic Variable Pitch Propeller? For Amateurs?! By Andre J. Bauwens (EAA 18164) 6624 Dillman Road — R. R. 1 West Palm Beach, Florida 33406

-L HIS ARTICLE WAS written more or less to strike a few sparks and try and set fire to the mind of come of you who are at the point of having a propeller made for your new homebuilt, or are already flying and would like a variable pitch propeller but cannot afford the available ones. Let me now tell you about what an old-time farmer might have called a "sickle" propeller. If you are building a light twin, you might want "bi-sickle" propellers (couldn't resist this one, forgive me!). As early as January 1929, the Belgian propeller manufacturer Paul Poncelet went into production of a variable pitch wooden propeller of his invention. As we know, a propeller is basically a twisted piece of wood or metal. Through its rotation, the tendency is to untwist, tendency opposed by the rigidity of the blades, which must be strong enough not to break under the action of the centrifugal force. This required rigidity in itself points out the impossibility of reducing the thickness of the blades sufficiently to allow them to untwist as needed. A simple solution was to give a lever arm to the force tending to straighten out the blades — to untwist them — while taking advantage of the elasticity of the material. The tip of each blade was therefore brought aft of the line passing through the propeller's axis of rotation and the center point of the root section at the hub. The

FIG. 1

FIG. 2

FIG. 3

material used had to be deformable enough without fatiguing in order for the air pressure to render the

blades automatically variable in pitch. This made wood a natural! In practice, the propeller is built so that the blades, projected on a plane perpendicular to the axis of rotation, are curved; the tip of the blade occupying then, relative to the direction of rotation, a trailing position. During the rotation, the components of the pressure on (Continued on Next Page)

FIG. 4 SPORT AVIATION 35

throw in your sister! Well, shortly after its development, this propeller was ordered by the dozen for Belgian fighter planes. The tests had revealed that this propeller made the fighters climb as if they were being pulled up by a cable. Later on of course, the use of more modern metal propellers with variable pitch supplanted the wooden ones, but the complexity of these mechanisms has done nothing but get worse. The only advantage — but what an expensive one — is the ability to reverse the pitch for shortening the landing run, and — for twins — to feather the propeller of an engine plagued by failure. None of these considerations appear too important for our homebuilt sport aircraft, which

are basically short field STOL types anyway, except

for a few sophisticated high performance machines. CONSTRUCTION

This wooden propeller is made like any other. The

FIG. 5

PROPELLERS FOR AMATEURS ...

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the tip of the blades parallel to the axis of rotation produce, due to the elasticity of the material, a torsion of these blades. It might be more correct to say a "detorsion". This results in a decrease of the blade's incidence (angle of attack) when projected on a plane perpendicular to the axis of rotation. This torsion — progressive from the root of the blades to the tip — reaches its greatest value at the tip, and is therefore proportional — which is absolutely necessary — to the components of the pressures parallel to the axis of rotation. The result is a variable pitch propeller actuated by the very pressures exerted on it by the rotational forces. As depicted in Fig. 2 and 3 representing a section through A-A of Fig. 1, the angle A — formed by the anterior face of the blade and a plane perpendicular to the axis of rotation — decreases under the action of the pressure exerted, to reach an angle B smaller than A and corresponding to the position of the blades when a balance is attained between the pressure exerted and the resistance of the blades. At rest — as seen in the plane of rotation — the propeller is straight, like any self-respecting propeller, that is perpendicular to the axis of rotation; but as soon as rotation starts, each blade tip tends to advance in relation to a vertical passing through the root of the blade (hub). This advance is proportional to the distance between the considered point and the hub. The propeller — in its plane of rotation — no longer forms a straight line, but a curve with the tips deflected forward. WHAT IS IT WORTH?

At high speed, at take-off power or climbing, the pitch is small. At cruising or slow flight the pitch is high, with infinite adjustment between the two extremes, but always proportional to the power and pressures exerted on the blades. It is therefore the ideal automatically variable pitch propeller, with control tied in with the throttle. It is simple and cannot get out of

adjustment. One reference only is needed. Everybody knows that — in order to sell to a government (except in the U. S., it goes without saying) you have to have the heart of a stone and be willing to sell your family into slavery. Now, when it comes to selling to the Armed Forces. . . 36 MAY 1973

sections are the same as the ones currently used. Their position is identical to other propellers, meaning that — at each point of the blade — the section is perpendicular to the radius of a circle described by the considered section about the hub (Fig. 5).

For reasons of resistance to centrifugal forces, the

propeller is not made of horizontal laminations, but of vertical laminations, parallel to the axis of traction. Their position is noticeable in Fig. 4 (seven laminations) and Fig. 5 (eight laminations). The laminations are formed following a radius equal to the propeller diameter (Fig. 5), glued and clamped on a mold having the required curvature. Compared to a normal propeller the shaping labor is unchanged. The cost of such a propeller is increased by about 25% due to additional wood and amortization of the mold. For amateur construction it would be best to make thin laminations (about 3/i6 to

% inch thick), making it easy to bend and not requiring

a mold of high rigidity. The pitch is maximum at rest and decreases with the speed. Therefore it should be some 25% higher to start with than a normal propeller. As for ordinary wooden propellers, there is an optimum pitch which is dependent upon the elasticity of the material. Wood being

heterogeneous, two propellers fabricated of wood from the same tree would probably have different efficiencies due to elasticity. But — no matter how different the efficiency — it will always be favorable. The ideal solution would be to make this type of propeller of a more homogeneous material (maybe some of the new plastics) giving entirely predictable efficiency from one propeller to the next. Shortly after the war (No. 2), the "sickle" propeller was still popular on sport aircraft, and I remember seeing one on a STAMPE back in 1947 at a Fly-In in Brussels, noticeable because part of the exhibition consisted of precision dead stick landings. I am not a propeller manufacturer or designer, nor do I intend to be. My field — being a marine designer — has to do with propellers working in a somewhat more "solid" environment. A good deal of this article was taken and translated from material collected from European magazines both during and after the war. So, PLEASE, do not write to me for more technical information which I would be unable to supply, but work with presently known and reliable propeller manufacturers to see if they are willing to experiment. I would — however — be greatly interested, when you have one made, to receive your comments, specifications and test results, as some day — in the distant future — my pet design will get off my drawing board and into the wild blue yonder, and when I do I would really like to have a "sickle" prop on it for those wheat field landings. ©