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?
I have a hard time following this one. My Seminole has 2 blades that form a “disc” when windmilling. That causes immense drag. A
turbofan engine has maybe 30 blades. All those blades form a more complete disc when windmilling. I don’t get how a similarly sized prop would create more drag.
When you say the jet is more efficient at higher RPM, do you mean that the blades take a bigger bite out of the air as RPM increases?
That’s pretty funny. I’ve never heard anybody talk about that. Can I expect the Captain to be saying “more right rudder” as I do my first takeoff during IOE?
Sorry for all the questions guys, but they just don't teach this stuff in small-airplane school.
The "prop forming a disc" analogy is not a great one. The drag isn't increased because of surface area, it's because a windmilling propeller is actually producing lift in the non-helpful direction. Stall the blades of the propeller, the negative "lift" goes away, which is why a stopped propeller, even if not feathered, produces less drag than a windmilling one - it's stalled.
In the case of a turbofan/jet engine, I'm not sure if the first stage blades are stalled or not, or any of the other stages for that matter, but either way it seems that the problem was just solved with more power rather than a "feathering" mechanism.
As for if we have to correct for asymmetrical thrust the same way you would in smaller airplanes, at least in the Saab, yes. Although we typically do it with trim rather than rudder pressure. Our takeoff rudder trim is 1.5 units to the right to counteract the left-turning tendencies, which makes a calm wind takeoff actually need a bit of left rudder pressure before rotation. After rotation, the trim is usually about right. (p-factor begins at rotation.) By the time the speed picks up, typically we have the yaw damper turned on, which does the trimming for us.