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World War II Twin-Engine Bombers
The situation was quite different for the high-powered twin-engine bombers of World War II, such as the Martin B-26 Marauder, the Douglas A-20 Havoc and A-26 Invader, and the North American B-25 Mitchell. Loss of one engine on these airplanes, especially at low airspeeds, produced rapid and dangerous changes in yaw and sideslip, unless promptly corrected by rudder control. Remember that these airplanes were heavy, large, and fast, and that hydraulic power-assisted controls had not yet been introduced. The limiting factor in keeping these airplanes under control when an engine failed was not insufficient rudder control power but high rudder pedal force.
Rudder pedal forces to counteract engine failure on airplanes of the B-25 and B-26 class were generally aggravated by the poor design of the rudder aerodynamic balance. Suppose for example that there was a loss of power in the right engine. The airplane’s nose would quickly swing to the right, in a right yaw. Momentum would carry the airplane’s flight path along its previous direction, causing the relative wind to come from the left side. This is
a condition of left sideslip. This direction of the relative wind would cause the rudder (or rudders) to “float” or trail with its trailing edge to the right, giving right rudder.
But to regain control, left rudder would be needed, to give a left yawing moment in opposition to the thrust of the working left engine. The pilot would have to apply a large amount of left pedal force just to center the rudder from its “floated” position and then an additional amount of pedal force to get the required left rudder. The amount of left rudder required could be minimized by holding the wing with the working engine low, in a slight bank, but the net pedal force was generally the critical factor, determining the minimum airspeed at which these airplanes could be flown with one engine dead.
Reduction or elimination of rudder float in sideslip was available to the designers of these airplanes through tailoring of the rudder’s aerodynamic balance. Specifically, rudder horn balances would have that effect (Figure 4.2). Rudder horn balances, used as far back
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Figure 4.2 Experimental rudderhorn balance-fitted to the Martin B-26 Marauder. This design reduced the rudder forces required for flight after failure of one engine. It was never put into production. (From U. S. Army Air Corps photo 108769, 1942) |
in aviation history as the Bleriot monoplane, were probably considered somewhat archaic to the Martin, Douglas, and North American designers. There was also a practical objection in that the projecting horns could conceivably snag parachute lines if the crew had to bail out. In any case, the A-20, A-26, B-25, and B-26 high-powered twins got through World War II without rudder horn balances.
