Turning tendencies on jet engines?
- Thread starter beasly
- Start date
dc3flyer
Well-Known Member
It's pretty much a non issue in most jets. Besides cross-wind takeoffs and landings, and of course single-engine operations, you can pretty much fly a jet with your feet on the floor. I personally keep my feet "monitoring" the rudders any time I am hand flying, you just don't use much input. If you are using rudder, say in a climb, you simply use a little rudder trim.
dasleben
That's just, like, your opinion, man
Never noticed much of anything. Occasionally I'll need rudder as I spool for takeoff, but that's more than likely due to the nosewheels initially being slightly misaligned with the centerline, and/or a crosswind. The aerodynamics work the same as smaller propeller aircraft, but I would guess that any torque effect would be mitigated by the fact that the N1 and N2 spools free wheel, and don't impart lateral force on the airframe. I could also just be making that up, I dunno. 
Autothrust Blue
Welcome aboard the Washington State Ferries
Since, as a lazy Boeing pilot, your aeronautical knowledge is limited to "rudder: we have it, but we don't use it a lot," you probably are.Never noticed much of anything. Occasionally I'll need rudder as I spool for takeoff, but that's more than likely due to the nosewheels initially being slightly misaligned with the centerline, and/or a crosswind. The aerodynamics work the same as smaller propeller aircraft, but I would guess that any torque effect would be mitigated by the fact that the N1 and N2 spools free wheel, and don't impart lateral force on the airframe. I could also just be making that up, I dunno.
That said, the -145 was largely a "feet on the floor" operation except for in a crosswind or single-engine.
dasleben
That's just, like, your opinion, man
Autothrust Blue
Welcome aboard the Washington State Ferries
Meh, you've earned it.
dasleben
That's just, like, your opinion, man
I wouldn't go that far. My knowledge is simply more related to how to operate the aircraft, rather than how to take it apart. That's just a function of my initial training, and the direction most airline training departments are going these days (which is a good thing, IMO).
I think you're right about the ducted fan aspect. I've been paying more attention lately, and I haven't noticed any turning tendency that wasn't related to differential spooling or a significant crosswind.
seagull
Well-Known Member
Yes, ducted and not attached. Nice combination.
Can't say I agree with that last part. Maybe to a limited extent, and learning a new airplane is certainly easier now, but I think we have lost something. We have a generation of pilots that are more like "drivers", who do not understand the mechanical aspects, or the aerodynamics of their aircraft. Sad, and leads to pilots making poor choices at times. Accident rates have dropped, but I believe it is in spite of these changes, not a result of them. Other aspects have, fortunately, mostly made up for them, but then you get a scenario where a crew can take a jet and go from cruise flight to hitting the water in about 4 minutes...
Can't say I agree with that last part. Maybe to a limited extent, and learning a new airplane is certainly easier now, but I think we have lost something. We have a generation of pilots that are more like "drivers", who do not understand the mechanical aspects, or the aerodynamics of their aircraft. Sad, and leads to pilots making poor choices at times. Accident rates have dropped, but I believe it is in spite of these changes, not a result of them. Other aspects have, fortunately, mostly made up for them, but then you get a scenario where a crew can take a jet and go from cruise flight to hitting the water in about 4 minutes...
dasleben
That's just, like, your opinion, man
I'm really referring to items brought up in training that are "Nice to know," but not exactly required to know. I consider high speed/swept wing aerodynamics to be a base level of knowledge that any jet pilot should have (not to the level of equations, of course), not something that should be specifically taught in initial training. For example, at one previous airline we spent the greater part of two days learning to diagram how all the data acquisition units worked together to present data to the pilots. Lots of trivia, nothing really necessary for safe operation of the aircraft. Knowing the truck tilt angle for the air/ground logic? Trivia. Knowing that pressurizing center hydraulics can flip a tug if the bypass pin isn't installed? Definitely knowledge that's required for safe operation of the aircraft. That's the important operational knowledge that I'm referring to.
To use another example, I really don't know much about cost index. All I know is that they trained me to type "60" when I get to that section of the PERF INIT. Has to do with ECON speeds and operating costs. But instead of spending time learning exactly how the company came to that number (why they don't change it from flight to flight, I have no idea), we spent our time learning NAT contingency procedures. I'd consider that time better spent.
Cessnaflyer
Wooooooooooooooooooooooooooooooo
A lot of them are that way.
Autothrust Blue
Welcome aboard the Washington State Ferries
That is a different conversation that should be had over beer.
dasleben
That's just, like, your opinion, man
Refer to my reply to seagull above.
Autothrust Blue
Welcome aboard the Washington State Ferries
BRB doing more landings in 2 hours than you do in a month
This post is brought to you by Beta. Beta: chicks dig it.
Tommay85
Well-Known Member
I agree. If I can't fix it in flight if there's a problem, I don't want the additional mumbo jumbo cluttering my brain. That being said, I'll always be a CFI at heart, so part of me still has that "how does that work?" mentality. I remember at one point, I used to know what the tensile strength of the control cables in small pipers.
GIT OUT OF MY BRAIN! Luckily it did! 
thevideographer
Well-Known Member
So about the original question - jet pilots might not notice any left turning tendencies but are there any? If you put a jet engine on a 172 would the left turning tendencies be the same? I'm guessing there would still be a torque effect but no spiral slipstream or p-factor.
fish314
Well-Known Member
I don't know for sure from anything like a real reference, but here is my thought on it:
Torque effect is basically the equal and opposite force applied to the aircraft from a propellor that is turning the air that it flies through. Put more simply, because we've spun the air in one direction, there must be an equal and opposite "spin" applied to the airplane. In a jet engine you have fan blades that do apply a spin to the air as the fan goes by each set of blades... but after each set of fan blades you have a set of "stator" blades that basically take the spin back out again. (Actually what they do is turn the air again in the opposite direction, which helps to compress the air... but it also 'takes the spin' out).
So there ought not be much of any true "torque" effect since the jet engines do not apply much if any NET rotation to the air (unlike a prop, or a turboprop). When the air comes out the back of the engine, it really isn't spinning very much. Hence, no torque (or very little) on a turbine engine or a turbofan. Turboprop, however, is a whole different story.
On the other hand, there should be some gyroscopic precession from jet engines, since gyroscopic precession is caused by applying a force to a rotating object, and the inside of the engine is DEFINITELY rotating. Basically it is the object's attempt to conserve angular momentum while under a force. This should be a factor in jet engines. This factor is minimized by a couple of things, though:
1st. Counter-rotation. Even though most airplanes are designed with engines that all spin in the same direction, many aircraft engines (the vast majority, in fact) have two separate rotating stages, usually called N1 and N2. Basically there are two separate shafts, one inside the other. The inside shaft (N1) is longer and turns the first stages of the compressor or fan, and the final stages of the turbine (the outer stages on each end). The outer shaft (N2) turns the high pressure compressor (the inner stages) and the high pressure turbine (again, the inner stages). It is entirely possible for these two stages to rotate in opposite directions, which would eliminate the gyroscopic effect to some degree.
2nd. The applied force is small: If you'll remember from small airplane aerodynamics, the gyroscopic effect is only present when you apply a force to the rotating body. So just cruising along, there is no gyroscopic effect. The effect only becomes apparent when you pitch up or down, and then you get a left or right resulting yaw, respectively (on most American engines-- most British engines go the other way).
So if you put those things together there shouldn't be too much yaw from the engines to begin with, and then only when applying significant pitching moments... which, let's face it, aren't ALL that significant in big multi-engine jets anyway, since big jets don't nose up or nose down very fast (compared to say... a fighter, e.g.).
Lastly, if there were a big yaw remember that it would act AT THE ENGINE... which is way out on the wing on a pylon if we're talking about airliner-class aircraft. So if there was a left yaw on the engine itself, it would mostly show up as a twisting moment on the pylon. Since the engine is on a pylon, I don't think very much of that would be transferred to the airplane in the same plane of motion. Mostly, I think whatever gyroscopic effect yaw there is would be trying to twist the engine off of the pylon, rather than trying to yaw the airplane a whole lot.
Torque effect is basically the equal and opposite force applied to the aircraft from a propellor that is turning the air that it flies through. Put more simply, because we've spun the air in one direction, there must be an equal and opposite "spin" applied to the airplane. In a jet engine you have fan blades that do apply a spin to the air as the fan goes by each set of blades... but after each set of fan blades you have a set of "stator" blades that basically take the spin back out again. (Actually what they do is turn the air again in the opposite direction, which helps to compress the air... but it also 'takes the spin' out).
So there ought not be much of any true "torque" effect since the jet engines do not apply much if any NET rotation to the air (unlike a prop, or a turboprop). When the air comes out the back of the engine, it really isn't spinning very much. Hence, no torque (or very little) on a turbine engine or a turbofan. Turboprop, however, is a whole different story.
On the other hand, there should be some gyroscopic precession from jet engines, since gyroscopic precession is caused by applying a force to a rotating object, and the inside of the engine is DEFINITELY rotating. Basically it is the object's attempt to conserve angular momentum while under a force. This should be a factor in jet engines. This factor is minimized by a couple of things, though:
1st. Counter-rotation. Even though most airplanes are designed with engines that all spin in the same direction, many aircraft engines (the vast majority, in fact) have two separate rotating stages, usually called N1 and N2. Basically there are two separate shafts, one inside the other. The inside shaft (N1) is longer and turns the first stages of the compressor or fan, and the final stages of the turbine (the outer stages on each end). The outer shaft (N2) turns the high pressure compressor (the inner stages) and the high pressure turbine (again, the inner stages). It is entirely possible for these two stages to rotate in opposite directions, which would eliminate the gyroscopic effect to some degree.
2nd. The applied force is small: If you'll remember from small airplane aerodynamics, the gyroscopic effect is only present when you apply a force to the rotating body. So just cruising along, there is no gyroscopic effect. The effect only becomes apparent when you pitch up or down, and then you get a left or right resulting yaw, respectively (on most American engines-- most British engines go the other way).
So if you put those things together there shouldn't be too much yaw from the engines to begin with, and then only when applying significant pitching moments... which, let's face it, aren't ALL that significant in big multi-engine jets anyway, since big jets don't nose up or nose down very fast (compared to say... a fighter, e.g.).
Lastly, if there were a big yaw remember that it would act AT THE ENGINE... which is way out on the wing on a pylon if we're talking about airliner-class aircraft. So if there was a left yaw on the engine itself, it would mostly show up as a twisting moment on the pylon. Since the engine is on a pylon, I don't think very much of that would be transferred to the airplane in the same plane of motion. Mostly, I think whatever gyroscopic effect yaw there is would be trying to twist the engine off of the pylon, rather than trying to yaw the airplane a whole lot.
beasly
Well-Known Member
Very well argued, thank you.
Paraphrasing your reasoning, the turning tendencies are there, but they are dampened within the engine by being cancelled out by counter-rotating fans and the resultant turning tendencies are rendered irrelevant by the mass of the aircraft
To me, that makes sense.
fish314
Well-Known Member
No worries beas...
Again, note my caveat at the beginning, though. I'm RELATIVELY sure this is what happens (and I'm pretty sure about the torque paragraph). Still, I'd love to see some confirmation from another source.