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Dual Rotation-Reverse Thrust Propeller
December 1945 Flying Age including Flying Aces

December 1945 Flying Age

Flying Age December 1945 - Airplanes and Rockets Table of Contents

These pages from vintage modeling magazines like Flying Aces, Air Trails, American Modeler, American Aircraft Modeler, Young Men, Flying Models, Model Airplane News, R/C Modeler, captured the era. All copyrights acknowledged.

At the time this "Dual Rotation-Reverse Thrust Propeller" article appeared in Flying Age magazine in 1945, the jet-engine-powered aircraft were still mostly in the developmental stage, although both Axis and Allied countries had managed to deploy a design or two at the tail end of World War II. Commercial and military aircraft had been using variable pitch propellers for a long time, even with reverse pitch for aerodynamic braking. The concept of coaxial, counter-rotating propellers was a relatively new idea in terms of building and testing working models for full-scale aircraft. Four primary advantages of the configuration were more thrust for a given projected propeller area, and the reduction or even total elimination of the counter-torque associated with a normal propeller, the reduction or total elimination of gyroscopic precession, and the reduction or total elimination of p−factor (asymmetric thrust produced by propeller blades presenting unequal angles of attack to the relative wind). History shows that while the counter-rotating propeller has been employed successfully in certain instances, it has not enjoyed widespread acceptance - primarily due to cost and complexity.

Dual Rotation-Reverse Thrust Propeller

Curtiss dual-rotation prop, a twin, three-bladed - Airplanes and Rockets

Curtiss dual-rotation prop, a twin, three-bladed. It will be used where diameters are too large for single rotation efficiency.

Biggest news in props are these new developments, now being perfected and soon to be more than high experiments.

The real "hot stuff" in propeller news is dual rotation. It was absolutely called for. As engine horsepower and altitude ratings increased, the propeller had to go right along. The early propellers, as we have seen, were two-bladed with narrow blades. They were progressively superseded by three-bladed props with blades of increasingly wider width and then by four-bladed propellers which in turn were forced to employ wider and wider blades. To go beyond that point really efficiently, it was necessary to consider dual rotation very very seriously.

The principle of dual, or counter, rotation is not new. The first recorded flight of such an arrangement, but without controllable propellers, was that of Wolf Hirth in February, 1912, near Berlin, in a Loutzkoy monoplane with two 100-hp. Argus engines, each separately driving a tractor propeller. Its first modern successful use was in the Italian Macchi-Castoldi twin-float racing seaplane which established a world record of 440 m.p.h. in 1934. The multiple engines of this plane drove co-axial counter-rotating shafts on which were mounted fixed-pitch propellers. And later, in 1938, preliminary tests were conducted at Wright Field on a pursuit plane with fixed-pitch dual-rotation propellers.

Blade pitch change in reverse thrust - Airplanes and Rockets

Sketch shows blade pitch change in reverse thrust. Braking is done by changing the angle to negative pitch while rotating.

Today's dual-rotation props usually consist of two three-blade counter-rotating propellers mounted in line on concentric propeller shafts. Its great advantages stem, first, from the fact that the prop in back straightens the slip-stream. Secondly, the balancing action of the two propellers mounted in line results in almost complete cancellation of torque reaction. The result of all this is that vying span can be reduced and so can the ailerons and other control surfaces.

(The torque reaction which is so important has been cancelled by other novel devices. One of these is on the P-38, where one propeller rotates to the right and the other to the left. And, incidentally, the greater problem of torque on helicopters has been partially solved - notably in the small Hiller model - by using dual-rotation rotors.)

The true application of dual-rotation propellers is primarily on the small, high-speed, high altitude and highly-powered type of aircraft. Up to the present, however, the arrangement has been tried primarily on installations where the four-blade, single-rotation prop was nearly as efficient and of considerably less weight and complication.

To take one example, the Curtiss dual-rotation propeller is electrically actuated and controlled to give constant speed or fixed-pitch control at all normal conditions with provisions for full-feathering both propellers simultaneously. Both propellers operate at the same r.p.m. in opposite directions. In line with present standards, the outboard propeller rotates toward the right and the inboard propeller toward the left, when viewed from the rear on a tractor installation. On pusher, installations, the forward or inboard propeller has right-hand rotation and the rearward or outboard propeller has left-hand rotation.

Against its important advantages, however, a special reduction gear in the engine is necessary to provide the counter-rotation feature. That, of course, results in a weight increase - but it seems to be a price worth paying when improved performance is considered.

Second of the truly major and most recent propeller developments is that of "reverse thrust" or aerodynamic braking. The term "reversing" means the operation of rotating the propeller blades below their positive blade angle until a negative blade angle is obtained in order to produce a thrust acting in the opposite direction to the forward thrust normally furnished by the propeller.

What it means is that the airplane can back up. Actually, reverse thrust applies only to the largest aircraft. Multi-engine flying boats which must land in restricted areas, and large land transports, trying to reduce excessive landing runs need such a mechanism. Propellers in reverse pitch have been used on large flying boats for help in maneuvering on water for a number of years.

For satisfactory operation as a braking propeller, it is necessary that the pitch range of the blades be substantially increased in order to permit propellers to rotate to the negative angle of ten to fifteen degrees normally required. With these angles the propeller will absorb the full rated engine horsepower at rated r.p.m, It is also necessary to have a higher rate of pitch change in going from the operating angle to the reverse pitch angle than is desirable in the normal operating pitch range where accurate engine speed control is required. This requirement of a high rate of pitch change is similar to that used for feathering.

Aerodynamic braking by means of reverse thrust is a simple operation, really. As the aircraft come in contact with the ground, the reversing circuit is energized by a cockpit switch so that the propeller blades instantly rotate in the hub from the positive into the negative blade angle. The direction of propeller rotation, of course, remains unchanged. The aerodynamic forces produced by the revolving propeller blades in their positive angle range to pull the airplane forward, now have the opposite effect in the negative angle range and tend to push the airplane backward.

From a passenger's viewpoint, reverse-thrust braking is a significant development in aviation. Its cushioning action eliminates any unpleasant wheel-braking effects, thereby increasing comfort by a tidy bit. It has other ramifications, too, For, with the adoption of reverse-pitch braking on airplanes, the landing run becomes shorter than the distance required for take-off, making the take-off distance the prime basis on which airport lengths will be determined. With the development in the future of blind landing control, the added feature of reduced landing distances through use of reverse-pitch propellers should play quite an exceedingly important part.



Posted June 17, 2023

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