Eliminating assymetrics effects in Jets
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Eliminating assymetrics effects in Jets
If you have a twin engine jet with the exhaust nozzles constructed in a way that the exhaust came out of a single nozzle ,
1. will that eliminated the assymetric turning effect if one engine fails?
2. will there be a signifiant loss of thrust as a result of rerouting the exhaust gases?
Rerouting the exhaust is nothing new its done in thrustvectoring jet fighters (Harrier, JSF, SU-35, etc).
Capt. manuvar
1. will that eliminated the assymetric turning effect if one engine fails?
2. will there be a signifiant loss of thrust as a result of rerouting the exhaust gases?
Rerouting the exhaust is nothing new its done in thrustvectoring jet fighters (Harrier, JSF, SU-35, etc).
Capt. manuvar
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You will still have some assymetry due to the gyroscopic effect of engine rotation.
Yes their will be significant loss of thrust if the pipe is very long. In thrust vectoring military jets the pipe is fairly short. Tying both engines of say a 737 together to thrust from a single pipe and it would probably never get off the ground.
Yes their will be significant loss of thrust if the pipe is very long. In thrust vectoring military jets the pipe is fairly short. Tying both engines of say a 737 together to thrust from a single pipe and it would probably never get off the ground.
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One of the major issues with the arrangement you are proposing is that a single failure, i.e. the common nozzle, could cause the effective loss of significant levels of thrust from both engines.
This was a major issue for the Learfan where both engines drove a single propeller.
Some times a solution can be worse than the problem.
This was a major issue for the Learfan where both engines drove a single propeller.
Some times a solution can be worse than the problem.
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If the intake & exhaust aren't in line with each other (parallel with longitudinal axis), you'll get a force couple tending to yaw the aircraft. Intake on a wing + centre exhaust = yaw towards live engine. Just imagine: dead leg - live engine would be the new saying.
I think there are some twin jets - with engines mounted aft fuselage - that have deliberate toe-in to minimise yaw in the event of an EF on/after takeoff. Unfortunately the lateral component of thrust is totally wasted in the 99.9% of flight where both engines are running normally. For this reason the degree of toe-in is very small. It saves having a long exhaust pipe though.
But hey - VMC can be 1 knot lower, so Captain Bloggs can take off on a runway 23m shorter, so Chief Executive Smith buys an extra 3 aeroplanes, so shareholders Aaron through Zachary make an extra $3.20 each. Cool eh?
O8
I think there are some twin jets - with engines mounted aft fuselage - that have deliberate toe-in to minimise yaw in the event of an EF on/after takeoff. Unfortunately the lateral component of thrust is totally wasted in the 99.9% of flight where both engines are running normally. For this reason the degree of toe-in is very small. It saves having a long exhaust pipe though.
But hey - VMC can be 1 knot lower, so Captain Bloggs can take off on a runway 23m shorter, so Chief Executive Smith buys an extra 3 aeroplanes, so shareholders Aaron through Zachary make an extra $3.20 each. Cool eh?
O8
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747FOCAL
"You will still have some assymetry due to the gyroscopic effect of engine rotation."
What a nonsensical statement .
Try and explain that statement if you can.
"You will still have some assymetry due to the gyroscopic effect of engine rotation."
What a nonsensical statement .
Try and explain that statement if you can.
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Milt,
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Think about it. You have two engines spinning that are equal distances from the center of mass which basically cancels the effect of the engines spinning. If one of them stops, the rotating mass will cause yaw because the engine wants to roll to the right or the left depending on if you are in a GE or a RR.
Most of us had toys that taught us this as a kid. Get something spinning very fast and it is hard to hold still.
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Think about it. You have two engines spinning that are equal distances from the center of mass which basically cancels the effect of the engines spinning. If one of them stops, the rotating mass will cause yaw because the engine wants to roll to the right or the left depending on if you are in a GE or a RR.
Most of us had toys that taught us this as a kid. Get something spinning very fast and it is hard to hold still.
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That still does not make any sense whatsoever. Gyroscopic forces only come into play when there is a turning motion on a spinning axis. It's irrelevant here. Where there would be asymmetric thrust in the event of engine failure and a shared exhaust would be the drag effects of a failed engine and suction/low pressure elements at the front of the live engine. How significant this is is questionable.
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There are lots of turbo prop singles out there that have strakes on the ass end to help with the yaw caused by the spinning motion of the engine.
Yes, this is a ducted fan and the component would be small on a commercial jet it is still in the equation. Why do you think wind up turns are so hard on rub strips and the fan? The engine wants to go one way and you are making it go another.
I suppose I could be totally off my rocker.
Yes, this is a ducted fan and the component would be small on a commercial jet it is still in the equation. Why do you think wind up turns are so hard on rub strips and the fan? The engine wants to go one way and you are making it go another.
I suppose I could be totally off my rocker.
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If you have a twin engine jet with the exhaust nozzles constructed in a way that the exhaust came out of a single nozzle,
1. will that eliminated the assymetric turning effect if one engine fails?
2. will there be a signifiant loss of thrust as a result of rerouting the exhaust gases?
1. will that eliminated the assymetric turning effect if one engine fails?
2. will there be a signifiant loss of thrust as a result of rerouting the exhaust gases?
Since virtually all of the "asymmetric turning effect" in a jet airplane is due to simple thrust difference, any reduction of the thrust axis from centerline will reduce the correction required if an engine fails.
2. In a word: Yes.
The A-6 Intruder and EA-6B Prowler aircraft had/have slight bends in their exhaust pipes to accommodate the geometry of the engines and airframe. Those small bends reduce[d] total thrust by almost 15%. Note that there is no attempt to combine exhaust gases into a single pipe.
If you attempt to combine the exhaust streams into a single pipe, you will likely encounter significant losses from airflow interference as the 2 streams are joined (note that aircraft like the A-4 Skyhawk and F-9 Panther, which used split intakes to feed single engines, needed significantly larger intake area than would otherwise be required by a single, straight-through intake per engine like the F-4 or current F-18). So, the resultant loss in thrust will likely be even more significant.
I don't know if the loss in thrust when splitting an exhaust (like in the Harrier) results in the same magnitude of loss...
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74, I would have thought those strakes were to counter the effects of the induced flow caused by the prop wash on the fin.
I have heard of gyroscopic coupling from engines, but usually it affects high performance fighters, where the relative lack of inertia and very powerful engine mean that the precessive effects can be significant. Can't imagine that it would be significant on a large transport aeroplane (but I stand to be corrected).
I have heard of gyroscopic coupling from engines, but usually it affects high performance fighters, where the relative lack of inertia and very powerful engine mean that the precessive effects can be significant. Can't imagine that it would be significant on a large transport aeroplane (but I stand to be corrected).
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Gentlemen,
I am rather shocked by the responses to this thread! We seem to have forgotten some very basic theory here!
The thrust of a jet turbine is not produced by the jet efflux impinging on the air behind the exhaust nozzle. Thrust is purely a reaction to the acceleration of gas through the engine itself. Thus, all the magic has happened by the time the gas reaches the nozzle of the engine. The thrust vector originates at the engine itself.
Thrust vectoring is different, in that variable geometry nozzles are used to provide lateral forces by means of redirecting the jet efflux. The point from which the forward thrust acts remains unchanged.
The addition of long ducted exhaust nozzles to jet engines only serves to reduce net thrust due to losses in the ducts themselves. The issue of asymmetric thrust is almost entirely dependant of the position of the engines, and cannot be alleviated by the addition of "ducting". If there was mileage in the ducting idea, the men in white coats would have implemented them a long time ago!
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Regards
Cuban_8
I am rather shocked by the responses to this thread! We seem to have forgotten some very basic theory here!
The thrust of a jet turbine is not produced by the jet efflux impinging on the air behind the exhaust nozzle. Thrust is purely a reaction to the acceleration of gas through the engine itself. Thus, all the magic has happened by the time the gas reaches the nozzle of the engine. The thrust vector originates at the engine itself.
Thrust vectoring is different, in that variable geometry nozzles are used to provide lateral forces by means of redirecting the jet efflux. The point from which the forward thrust acts remains unchanged.
The addition of long ducted exhaust nozzles to jet engines only serves to reduce net thrust due to losses in the ducts themselves. The issue of asymmetric thrust is almost entirely dependant of the position of the engines, and cannot be alleviated by the addition of "ducting". If there was mileage in the ducting idea, the men in white coats would have implemented them a long time ago!
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Regards
Cuban_8
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Cuban8- hate to say it, but youmay be right. But how much of the 'thrust' is, in fact, the front face of the engine 'sucking' it's way forward (new, non- physical expression)? After all, a jet compressor is an aerofoil section on which the lift acts forward. Therefore, with the loss of this new, mysterious 'sucking forward' effect on one side, then there will be an asymmetric effect?
(put me out of my misery someone)
(put me out of my misery someone)
Cuban 8,
You are partly correct. As you say, thrust is not produced by the jet efflux impinging on the air behind the exhaust nozzle. However, thrust is the reaction to the exhaust gas being ejected from the jet pipe, and thus it is in the opposite direction to the vector of the jet efflux. The proof is the Harrier in the hover! Or, take a balloon, blow it up, put a bent hollow tube in the hole and let go. It does not go in the same direction (even though it will still end up behind the sofa!).
You are partly correct. As you say, thrust is not produced by the jet efflux impinging on the air behind the exhaust nozzle. However, thrust is the reaction to the exhaust gas being ejected from the jet pipe, and thus it is in the opposite direction to the vector of the jet efflux. The proof is the Harrier in the hover! Or, take a balloon, blow it up, put a bent hollow tube in the hole and let go. It does not go in the same direction (even though it will still end up behind the sofa!).
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The thrust of a jet turbine is not produced by the jet efflux impinging on the air behind the exhaust nozzle. Thrust is purely a reaction to the acceleration of gas through the engine itself. Thus, all the magic has happened by the time the gas reaches the nozzle of the engine. The thrust vector originates at the engine itself.
The action/reaction in the "system" is between the airplane and the rest of the world. As is clear with the Harrier as well as any jet airplane with a controllable exhaust nozzle, changing the position of the nozzle grossly affects the thrust vector. It is the point of departure from the airplane -- i.e., the end of the nozzle -- that determines the thrust vector.
Note that the attitude control jets in the Harrier also use air from the "cold" section of the engine (the fan). If your "originates at the engine itself" theory were true, attitude jets fed from the same source would not function. However, they do, and the distance of one of those jet nozzles from the center of gravity of the airplane determines its lever arm.
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Cuban_8 ,
You make me laugh......
Try standing behind a jet engine at full thrust at brake release you tell me there is no action on the air behind it. F=MA
Yes once the airplane is moving at flight speed it is "standing" on it's fans, but that is not what the guy first asked. He wanted to know would you lose assymetry or a lot of thrust if you ducted two engines together. I assumed he meant from our typical definition of a commercial airplane and meant engines coming from each wing ducted together to exhaust behind the tail. I could have been a little more clear when I said that I thought there would be huge thrust loss because of the length of the duct. In my mind, with the experience I have with turbo props, the spinning motion of the engine has an effect on directional control. The King Air has a yaw damper requirement in case you lose an engine at rotate that is removed if you install the Raisbeck Engineering strakes. Without them, the plane will roll over and crash. US military has lost lots of them practicing Vmcg stalls. But, on something the size of a commercial jet I am sure the component of the engine spinning is very small.
You make me laugh......
Try standing behind a jet engine at full thrust at brake release you tell me there is no action on the air behind it.
F=MA Yes once the airplane is moving at flight speed it is "standing" on it's fans, but that is not what the guy first asked. He wanted to know would you lose assymetry or a lot of thrust if you ducted two engines together. I assumed he meant from our typical definition of a commercial airplane and meant engines coming from each wing ducted together to exhaust behind the tail. I could have been a little more clear when I said that I thought there would be huge thrust loss because of the length of the duct. In my mind, with the experience I have with turbo props, the spinning motion of the engine has an effect on directional control. The King Air has a yaw damper requirement in case you lose an engine at rotate that is removed if you install the Raisbeck Engineering strakes. Without them, the plane will roll over and crash. US military has lost lots of them practicing Vmcg stalls. But, on something the size of a commercial jet I am sure the component of the engine spinning is very small.
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Guy's
I appreciate what you are saying. This topic is certainly simulating the grey matter!
I have a very informative diagram here from RR that illustrates the distribution of thrust generation through a typical turbofan and turbojet. I'm too academically challenged to work out how to post it up at the moment, which is a shame.
The fact of the matter is, thrust will only be produced when a mass flow of gas is worked upon i.e. accelerated. I don't think any of us will argue with that. This manifests itself in different manners, depending on the engine.
As this diagram shows, for a high by-pass turbofan, the lions share of the thrust is generated over the low pressure compressor (the fan). A smaller percentage comes from the hot section nozzle, as the gas is expelled from the turbines.
Turbojets (hence most military a/c) are rather different. Such engines are much lower by-pass ratio, then result being that a greater percentage of thrust is generated at the nozzle. In fact, as this diagram shows, with re-heat engaged, over 90% of thrust originates at the nozzle.
I think where the confusion lies is in the definition of the nozzle of the engine. Jet engine nozzles are a part of the thrust generation process. They are convergent in nature, such that gases are accelerated as they pass through. This is why modern military a/c have variable convergence geometry, allowing thrust generation be tailored to flight regime. This also explains why we can thrust vector.
However, jet engine nozzles have to be rather short by nature of what they do. It is a total non-starter to have lengthy ducts from an engine core to its nozzle - someone try and think of an example anywhere, there aren't any!
The Harrier is a slightly obscure case, but doesn't bend the rules in any way. The variable thrust outlets are the engine nozzles - the front set lie immediately downstream of the cold section (fan) and the rear set immediately after the hot section. It is interesting to note that in the hover, the nozzles are directed forward slightly to offset the action of the large fan in the front of the engine (the Pegasus is more like a turbofan). The reaction controls used in the hover are again nozzles that use bleed air - the thrust is thus generated at the nozzle itself.
Anyway, perhaps I digress. Back to the original point, I hope you can see that ducting exhaust gases from jet engines to a more central point on the a/c axis is impractical. As I stated earlier, there are far more intelligent minds than ours that work in these fields - I think the fact that it hasn't been done illustrates its unfeasibility!
Rgds,
Cuban_8
I appreciate what you are saying. This topic is certainly simulating the grey matter!
I have a very informative diagram here from RR that illustrates the distribution of thrust generation through a typical turbofan and turbojet. I'm too academically challenged to work out how to post it up at the moment, which is a shame.
The fact of the matter is, thrust will only be produced when a mass flow of gas is worked upon i.e. accelerated. I don't think any of us will argue with that. This manifests itself in different manners, depending on the engine.
As this diagram shows, for a high by-pass turbofan, the lions share of the thrust is generated over the low pressure compressor (the fan). A smaller percentage comes from the hot section nozzle, as the gas is expelled from the turbines.
Turbojets (hence most military a/c) are rather different. Such engines are much lower by-pass ratio, then result being that a greater percentage of thrust is generated at the nozzle. In fact, as this diagram shows, with re-heat engaged, over 90% of thrust originates at the nozzle.
I think where the confusion lies is in the definition of the nozzle of the engine. Jet engine nozzles are a part of the thrust generation process. They are convergent in nature, such that gases are accelerated as they pass through. This is why modern military a/c have variable convergence geometry, allowing thrust generation be tailored to flight regime. This also explains why we can thrust vector.
However, jet engine nozzles have to be rather short by nature of what they do. It is a total non-starter to have lengthy ducts from an engine core to its nozzle - someone try and think of an example anywhere, there aren't any!
The Harrier is a slightly obscure case, but doesn't bend the rules in any way. The variable thrust outlets are the engine nozzles - the front set lie immediately downstream of the cold section (fan) and the rear set immediately after the hot section. It is interesting to note that in the hover, the nozzles are directed forward slightly to offset the action of the large fan in the front of the engine (the Pegasus is more like a turbofan). The reaction controls used in the hover are again nozzles that use bleed air - the thrust is thus generated at the nozzle itself.
Anyway, perhaps I digress. Back to the original point, I hope you can see that ducting exhaust gases from jet engines to a more central point on the a/c axis is impractical. As I stated earlier, there are far more intelligent minds than ours that work in these fields - I think the fact that it hasn't been done illustrates its unfeasibility!
Rgds,
Cuban_8
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Cuban, I think you're thinking along the right lines. The only example of a long duct I can think of is on a NOTAR helicopter where the anti-torque is effected by ducting efflux from the engine to the end of the tail boom. I'm not sure of the exact physics behind the concept, but I reckon its along the lines of the harrier example.
As for 74's ideas on why the Kingair likes to roll over and play dead, its certainly not due to precession. In the stall case, its probably due to both props turning in the same direction (common on turboprops), causing a large torque (and also an asymmteric induced flow on the wing behind the engine) and therefore a large rolling moment. In the engine out case, you'll have less torque, but asymmetric thrust and induced flow over the wing on one side only, again, big rolling moment. This is one of the reasons why turboprops tend to have fairly sophisticated stall protection systems.
Regards,BGPM.
(PS, at any rate, if the engine did precess in response to a yawing or rolling torque, it would cause a pitch output)
As for 74's ideas on why the Kingair likes to roll over and play dead, its certainly not due to precession. In the stall case, its probably due to both props turning in the same direction (common on turboprops), causing a large torque (and also an asymmteric induced flow on the wing behind the engine) and therefore a large rolling moment. In the engine out case, you'll have less torque, but asymmetric thrust and induced flow over the wing on one side only, again, big rolling moment. This is one of the reasons why turboprops tend to have fairly sophisticated stall protection systems.
Regards,BGPM.
(PS, at any rate, if the engine did precess in response to a yawing or rolling torque, it would cause a pitch output)