daft turbine questions from a piston pilot
 

Pardon the silly question, but apart from the increased weight and complexity, why aren’t jet engines designed to be feathered as a props are? I’d imagine the N1 blades at the front create a lot of drag just windmilling, so why isn’t there a way to reduce drag? Or are jets typically so powerful that engine out climb performance is still healthy?

On an equally silly note, do jets and turboprops have counter-rotating engines? I ask because I remember being taught about the descending blade producing more thrust in PPL school. Since turboprop and turbojet blades are airfoils, how do those planes escape the need to correct for it the same way a piston pilot has to?

Power is the solution to all problems. Aside from that a turbine engine with variable pitch fans is something too unfeasible (if not close to impossible) to build (or make it work right), most do have variable stator vanes, which control air flow to the high pressure compressor. There's no need for a jet to have counter rotating blades and most turboprops do not (I can't think of any that do actually), I'd venture to guess it's due to the increased cost of developing and certifying the same engine twice.

Now talking about "correcting how a piston pilot does", for the most part we do, all twin turbine have a VMC, but we also have a higher speed envelope that we can achieve on one engine, which mitigates most of the problems.

Jet's don't need counter-rotating as it's pretty much just direct thrust. Also, even if you could "feather" the front blades of a turbo-jet, the whole frontal area is still drag anyways. Even the High-Bypass ratio engines I wouldn't think it's going to make a huge difference if you could stop the rotation of the blades.

There are counter-rotating configurations with some TPE-331 installations. I think the Cheyenne 400LS did, but can't remember. I know I've seen it on something. I can't think of any PT6 equipped bird that does this though.

Turbojets don't make as much drag as a prop when windmilling. They are designed to be efficient at very high rpms, which means they don't do much energy transfer at windmill speed...I think a lot of the air just flows through with minimal drag. Also frontal area is much less when compared to power output...so the good engine's power can easily overcome the frontal area drag.

I cant answer your question about turbofan engines, but my guess is the added drag is negligiable compared to a prop. As far as counter-rotating turbo props, never heard of it, because it does affect efficiancy, how much I really dont know, obviously there are critical engine issues you dont really see in small piston twins that counter-rotate, but like anything else its not a real issue with some training. You'll for sure have to go back to using those rudders on take off though...lol:D in my best southern drawwwwwl (You may have met P-factor but you aint met his daddy)

BEAR...... nice

The bear is an example of Contra-rotating propellors - twin propellors on a common axis with an opposing spin.

Counter rotating propellors are multi engine aircraft where the left and right engines turn opposite.

I believe all of the Piper twins (Navajo, Semenole etc) were counter rotating. The Cessnas aren't.

Aztecs are not counter-rotating.

That is correct. And now that I think about it, the Cheyanne definitely did not as well.

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?:D


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.

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.

Attachment 1291

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

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.

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.

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

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.

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.

Thanks for all the info guys. Much appreciated.

I think that the biggest difference is that with a piston engine, a windmilling prop has to drive the whole engine against it's own compression. A windmilling fan-jet engine normally only turns the N1 section, which produces surprisingly little drag. You have to be going really fast, like above 250 KIAS, to even notice rotation on the N2 gauge.

If you have ever tried to hand prop a moderately large piston engine, you know how much effort it takes to get it to turn. You can walk up to the front of a 737 and start the engine rotating with one hand. If there is any wind, it will probably already be rotating. If there is a tailwind, it will just as easily rotate in reverse.

Joe

It's probably already been answered in one of the replies. But if you could "feather" the fan section, there's still A LOT of metal behind that that would cause drag. The rotors/stators, combustion chamber, and every accessory attached to the core of the engine creating a profile contributing to the drag.

BAE-4100

Earlier in the thread, some of you were talking about "P" factor, or the existance of a critical engine on tubofan engines, and I would like to point out that this is no-existant (for practical purposes anyway) due to the air being directed through the nose cowl. Of course, some things will affect airflow at slower speeds (strong crosswind, or high reverse power at slow speeds, for ex.), but there is no differance in power output due to the pitch attitude of the aircaft as the air that is going through the C1 disk is being "straightened" by the nose cowling. Hope this is more helpful than confusing. Some of the bigger engines have 34-48 blades on the C1 compressor, and it would be nearly impossible to build a hub that would withstand the forces imposed on it and accomodate this number of blades.

Cheyenne 400LS is counter rotating.

P-38 was counter, but both spun outwards.

I didn't even think about the Jetstream 41, even though I see them all the time. Also, for the OP some t-props (usually the ones made in Europe) have engines that turn counter-clockwise such as the Jetstream 31/32. Critical engine failure in one of those means lots of right rudder, and then start trimming.

Isn't the Piaggio P180 a counter-rotating propeller?

It might need it, with the props so close to the tail the rudder arm would be very short.

Hi Rick.

I'm trying to visualize whether the location of the props makes a difference. My understanding is that the rudder arm acts thru the C.G. This would seem to mean that the fore and aft position of the props should not make a difference, except in that the weight of the engines would tend to give the aircraft a relatively far aft C.G.

Joe

Correct, look at where the wings are on that thing....waaaay back. That implies that the CG is back there too. You don't HAVE to place the wings near the CG, but it would terribly inefficient not to. Your tailplane (or canards in this case) would have to carry a high load to maintain level pitch attitude.