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.

plasticpi

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.

Cubdriver

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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Ewfflyer

Another classic example of something messed up is the counter-rotating props on a P-38. Both were critical engines, the instability enchanced maneuverability. I did a research paper on this plane in college, and I still to this day feel it's one of the most under-rated fighters of WWII. But that's a different topic entirely.

Hope some of these answers help out. Props aren't Jets in almost every aspect of their aerodynamics, systems, etc.. Basic skillsets can be applied, but in the end we fly them differently.

Never hurts to learn

Senior Skipper

I think I'm getting it now. So on the Seminole you have there, the windmilling prop represents a larger percentage of the total surface area, and would thus cause more drag? Am I right?

As to the point about blade design, well I have no doubt that modern blades are a bit more than small airfoils. Just look at the beautiful curves on the GE90-115B! I'm sure books have been written on the design of those works of art.

aviatorhi

I thought you were referring specifically to dealing with single engine controllability, as far as turboprops go, yeah, on some you'll need a just a lil right rudder, nose wheels have a lot more authority on larger aircraft at lower speeds and control surfaces become more effective at higher speeds.

X Rated

Counter rotating light twins?

-Seminole, Seneca, Navajo CR, Chieftain, Twin Comanche CR
-Cessna 303 (IIRC...), Skymaster (though you have to turn an engine around to do it)
-Cougar
-Duchess
-700P Aerostar (IIRC...)...and that was outthrust.

That's all I can think of that counter-rotate.

X

pokey9554

Your answer to P-factor on turbines:
The relative wind on a turbine engine is always straight through the engine. The descending blade creates the same amount of lift as any other blade. P-factor is only prevalent at high angles of attack with propellers. Turbines don't like varying angles of attack hence the shrouds typically beginning far in front of the fan.

Cubdriver

Well, there are many issues involved and as always I hesitate to dive deeply on the internet into what normally takes 6 years of engineering study to cover. But for the sake of general discussion, frontal sweep area of the blade system in proportion to the global drag coefficient for the airplane is not much of the story as Rick pointed out. There are many more variables like those mentioned above, plus angle of attack and a bunch more. I put the little picture above to show the geometries are different. For example, the moment arms from the outer reach of the seminole prop is much longer than that of the jet engine. This is a large, but not the only, factor in Vmc determination.

Oh they are works of art- namely, the art of propulsion engineering. Turbine technology is a highly developed topic that has yet to reach maturity but has already traveled an amazing journey from the times of O'Hain and Whittle in the early 1920s. The way to really get into this subject is to study mechanical engineering or aerospace engineering, with an emphasis on propulsion. The field is so large that you can spend your entire career studying only an area like making a better combustor for a jet turbine. As a layman, I find the subject fascinating and I am particularly interested in how these engines are becoming more efficient, quiet, and able to burn renewable fuels. Blade design touches on heat transfer, structures, material science, aerodynamics, testing, manufacturing, and a number of other fields.



Good nacelle design makes your statement more true than untrue fortunately, but flight test pilots and perhaps some military fleet pilots have experienced compressor stall at a high relative wind angle. I have not experienced it personally as my jet time is still very low, but perhaps some else here has.

Senior Skipper

Thanks for all the info guys. Much appreciated.