Originally Posted by
Senior Skipper
I get what you’re saying about the direct thrust, in that you probably don’t have the “spiraling” slipstream that props have, but since each blade is an airfoil, wouldn’t the same principle apply, and the down-moving blade take a bigger bite out of the air? Or is it a case where by the time the air gets through the various stages of the jet engine, the thrust is uniform on the left and right side of the exhaust pipe?...
As EWFlier and others said, the geometry is much tighter in a turbojet engine although the principle of p-factor applies to any blade system in rotation. But several things also make the analogy poor. Jet blades are designed to accomplish a variety of design objectives a prop is not, such as averting compressor stall, allowing for optimized heat distribution and transfer, minimizing wave drag at 30,000 rpm, and so on. They may resemble prop blades but their primary duty is much different. The mass flow rates are very different, and it is safe to assume for practical purposes that thrust is balanced across the engine as well. You are on the right track with your idea that total thrust is not felt by the blade system as much as it is by the entire engine in a jet design.
However, in a high-bypass engine we have a fan that more closely resembles a propeller and this is to increase the quantity of air over which the mechanical energy produced by the turbine is applied. It takes advantage of the fact that it is more efficient to accelerate a large amount of air a small amount, rather than accelerate a smaller amount of air a larger amount. P-factor and other problems associated with twin engine prop driven airplanes still apply for sure, but as several others mention the relative diameters and distance from the fuselage are much different.
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