Aft fuselage mounted engines and static longitudinal stability
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Aft fuselage mounted engines and static longitudinal stability
Hello,
I've read in a JAR exam question that the aerodynamic contribution to the static longitudinal stability of the nacelles of aft fuselage mounted engines is positive.
Could someone explain why ?
Thanks a lot,
Emmanuel Cordier.
I've read in a JAR exam question that the aerodynamic contribution to the static longitudinal stability of the nacelles of aft fuselage mounted engines is positive.
Could someone explain why ?
Thanks a lot,
Emmanuel Cordier.
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This is a bit complex, and I would like an expert contribution if possible.
A cylinder shape set at an angle of attack generates a nose up moment, and this increases with increasing apha, making it an unstable couple. The plan area of the engines, stub mountings and so on acts like an extra horizontal tail, which is a stabilizing effect.
I understand that the JAA answer is that on balance the total efect comes out stable
Dick
A cylinder shape set at an angle of attack generates a nose up moment, and this increases with increasing apha, making it an unstable couple. The plan area of the engines, stub mountings and so on acts like an extra horizontal tail, which is a stabilizing effect.
I understand that the JAA answer is that on balance the total efect comes out stable
Dick
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Aft mounted engines
I will leave other much better qualified people to comment about thrust-lines etc; however one factor which I thought had contributed to the demise of aft engines was the detrimental effect they had on the pilot in the event of a severe landing !
I was told Hawkers' later products were stressed to 27G in case of such an event, though maybe this is not possible or indeed required nowadays, as it seems more the trend to get out, even if a Test Pilot, than try to bring the evidence back, and leave it to telemetry & recorders.
As for Service Pilots, I think they go for the handle rather than the tremendous risks in force landing modern aircraft, even without a big heavy engine & fuel system etc behind them, except for Harriers...
I was told Hawkers' later products were stressed to 27G in case of such an event, though maybe this is not possible or indeed required nowadays, as it seems more the trend to get out, even if a Test Pilot, than try to bring the evidence back, and leave it to telemetry & recorders.
As for Service Pilots, I think they go for the handle rather than the tremendous risks in force landing modern aircraft, even without a big heavy engine & fuel system etc behind them, except for Harriers...
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the rear of the fuselage eng. -> less influence of thrust change
Thanks a lot for you explicite and brief answers.
I have another question about rear fuselage mounted engines.
I've read that an advantage of locating the engines at the rear of the fuselage, in comparison to a location beneath the wing is less influence of thrust change.
Well I have been thinking about that, reading literature... and truly I don't understand why.
If you can help me with that one I would be grateful.
Have a good night,
Emmanuel Cordier.
I have another question about rear fuselage mounted engines.
I've read that an advantage of locating the engines at the rear of the fuselage, in comparison to a location beneath the wing is less influence of thrust change.
Well I have been thinking about that, reading literature... and truly I don't understand why.
If you can help me with that one I would be grateful.
Have a good night,
Emmanuel Cordier.
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Grab some paper and a pencil. Draw the forces acting on a jet with under-slung engines. Note that the engine thrust is below both the c of g and the c of drag. You will see why the a/c pitches up with power application.
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Hi BOAC,
I understand that beneath-the-wing engines produce a nose up pitch when you increase thrust (which has a negative effect on static longitudinal stability) and tha aft-fuselage-mounted engines produce a nose down pitch (which has a positive effect on static longitudinal stability).
Thus in both case, increasing thrust has an influence on the pitch and thus on longitudinal stability. However, according to my understanding, none is more influent than the other !
I understand that beneath-the-wing engines produce a nose up pitch when you increase thrust (which has a negative effect on static longitudinal stability) and tha aft-fuselage-mounted engines produce a nose down pitch (which has a positive effect on static longitudinal stability).
Thus in both case, increasing thrust has an influence on the pitch and thus on longitudinal stability. However, according to my understanding, none is more influent than the other !
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Hi BOAC,
I understand that beneath-the-wing engines produce a nose up pitch when you increase thrust (which has a negative effect on static longitudinal stability) and tha aft-fuselage-mounted engines produce a nose down pitch (which has a positive effect on static longitudinal stability).
Thus in both case, increasing thrust has an influence on the pitch and thus on longitudinal stability. However, according to my understanding, none is more influent than the other !
I understand that beneath-the-wing engines produce a nose up pitch when you increase thrust (which has a negative effect on static longitudinal stability) and tha aft-fuselage-mounted engines produce a nose down pitch (which has a positive effect on static longitudinal stability).
Thus in both case, increasing thrust has an influence on the pitch and thus on longitudinal stability. However, according to my understanding, none is more influent than the other !
In this case, talk of stability is a red herring; the issue is that an underslung engine MUST create a nose-up moment when thrust is increased, because it's practically impossible to arrange for the thrust line to be aligned with the vertical location of the cg - unless you have a BAe146-like configuration, of course, but this discussion is usually in the context of low wing aircraft.
Whereas the aft-mounted engine is actually pretty close to pointing through the aircraft cg in a vertical sense; so the pitching moment is much smaller with variation in thrust. You'd need to mount the engine above the fuselage to get a comparable effect - something like the 727 or Trident type centreline engine WOULD create a perceptible pitch down with thrust application, I expect. But the more-normal side-mounted engines are pretty much neutral in pitch effect, to a first approximation at least.
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Think about thrust !
The relative position of the thrust line (below, on, or above the fuselage centre-line) also has its effect. Also, remembering Vickers/BAC information handouts of the time, mountiing the engines aft makes for a "clean" wing.
However, the effect of engine weight far aft means that, to get the CofG right, the fuselage forward of the wing needs to be longer than the rear section (think DC8 in its later manifestations), whence a reduced tail moment arm, and probably a much larger fin (see the VC10).
This extra weight isn't necessarily compensated for by the ability to have a shorter undercarriage than when engines are mounted under the wing.
(I realise that this is a bit off the "stability beam", but it's worth remembering that apart from the VC10 and its Soviet "relation", rear-mounted engines are now confined to relatively small aircraft, by modern standards).
However, the effect of engine weight far aft means that, to get the CofG right, the fuselage forward of the wing needs to be longer than the rear section (think DC8 in its later manifestations), whence a reduced tail moment arm, and probably a much larger fin (see the VC10).
This extra weight isn't necessarily compensated for by the ability to have a shorter undercarriage than when engines are mounted under the wing.
(I realise that this is a bit off the "stability beam", but it's worth remembering that apart from the VC10 and its Soviet "relation", rear-mounted engines are now confined to relatively small aircraft, by modern standards).
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This is a bit complex, and I would like an expert contribution if possible.
A cylinder shape set at an angle of attack generates a nose up moment, and this increases with increasing apha, making it an unstable couple. The plan area of the engines, stub mountings and so on acts like an extra horizontal tail, which is a stabilizing effect.
I understand that the JAA answer is that on balance the total efect comes out stable
Dick
A cylinder shape set at an angle of attack generates a nose up moment, and this increases with increasing apha, making it an unstable couple. The plan area of the engines, stub mountings and so on acts like an extra horizontal tail, which is a stabilizing effect.
I understand that the JAA answer is that on balance the total efect comes out stable
Dick
You can get the same kind of "unstable couple" on an aerofoil shape with reference to itself, but I can't conceive of any aerofoil being net destabilizing if mounted aft of the cg.
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The essential aspects of this subject for JAR ATPL(A) exam purposes are summarised below.
If the thrust lines of the engines do not pass through the Centre of Gravity of the aircraft, any change in thrust will cause the aircraft to rotate about its C of G. If for example the thrust lines are below the Centre of Gravity the aircraft will pitch nose up each time the thrust is increased. In some aircraft the engines are fitted to the rear fuselage, so that their thrust lines are very close to the Centre of Gravity, thereby reducing the influence of thrust changes on longitudinal control.
A second benefit of rear-mounted engines is that they provide an additional horizontal surface, protruding out into the airflow behind the Centre of Gravity of the aeroplane. This causes them to act as if they were a second tailplane. If the aircraft pitches nose up, they produce a tail up moment. If the aircraft pitches nose down they produce a tail down moment. In this way they increase the longitudinal stability of the aeroplane.
But when engines are fitted to the wings, their mass reduces wing root bending stresses in flight, and tends to damp out vibration and flutter. Fitting the engines to the rear fuselage robs the wings of these benefits and actually increases the wing root bending stresses. This means that mounting the engines on the rear fuselage requires heavier and stronger wing structures to resist the increased bending loads and to counteract the increased flutter. Rear mounted engines are also usually higher, making maintenance work more difficult
If the thrust lines of the engines do not pass through the Centre of Gravity of the aircraft, any change in thrust will cause the aircraft to rotate about its C of G. If for example the thrust lines are below the Centre of Gravity the aircraft will pitch nose up each time the thrust is increased. In some aircraft the engines are fitted to the rear fuselage, so that their thrust lines are very close to the Centre of Gravity, thereby reducing the influence of thrust changes on longitudinal control.
A second benefit of rear-mounted engines is that they provide an additional horizontal surface, protruding out into the airflow behind the Centre of Gravity of the aeroplane. This causes them to act as if they were a second tailplane. If the aircraft pitches nose up, they produce a tail up moment. If the aircraft pitches nose down they produce a tail down moment. In this way they increase the longitudinal stability of the aeroplane.
But when engines are fitted to the wings, their mass reduces wing root bending stresses in flight, and tends to damp out vibration and flutter. Fitting the engines to the rear fuselage robs the wings of these benefits and actually increases the wing root bending stresses. This means that mounting the engines on the rear fuselage requires heavier and stronger wing structures to resist the increased bending loads and to counteract the increased flutter. Rear mounted engines are also usually higher, making maintenance work more difficult
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MFS, you have shattered my dream. A couple is a rotational force, not a linear force. As far as I understand it a nose up couple applied to any part of the aircraft, nose, tail or middle remains a nose up couple. Have I got that wrong?
Dick
Dick
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thereby reducing the influence of thrust changes on longitudinal control.
And also, lateral control, in the case of an engine failure. Having the (most commonly two) engines very close to the centerline also means that if power is lost on one, the yawing moment created by the remaining engine is much less than it would be for a much more outboard under wing engine, so a smaller fin and rudder may be adequate for yaw control.
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Rear Mounted Engines
Just to add ON:
rear mounted engines also mean cleaner wings , that means they generate more lift.
Less load on the wings mean , the wings have lesser mounted load and hence they are lighter due to simple construction.
Quieter cabins.
Lower Vmca due to reduced asymmetry.
rear mounted engines also mean cleaner wings , that means they generate more lift.
Less load on the wings mean , the wings have lesser mounted load and hence they are lighter due to simple construction.
Quieter cabins.
Lower Vmca due to reduced asymmetry.
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Well, as others have hinted, it's not simple.
Just as any explanation of lift below double-PhD level is probably a waste of time, any ATPL exam question such as that discussed here will only test the candidate's study of the question bank, as the topic is so complex.
It's not rocket science!
...but it's not far off. Rocket stability is comparatively simple!
Just as any explanation of lift below double-PhD level is probably a waste of time, any ATPL exam question such as that discussed here will only test the candidate's study of the question bank, as the topic is so complex.
It's not rocket science!
...but it's not far off. Rocket stability is comparatively simple!
Well, as others have hinted, it's not simple.
Just as any explanation of lift below double-PhD level is probably a waste of time, any ATPL exam question such as that discussed here will only test the candidate's study of the question bank, as the topic is so complex.
It's not rocket science!
...but it's not far off. Rocket stability is comparatively simple!
Just as any explanation of lift below double-PhD level is probably a waste of time, any ATPL exam question such as that discussed here will only test the candidate's study of the question bank, as the topic is so complex.
It's not rocket science!
...but it's not far off. Rocket stability is comparatively simple!
G
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Mad (Flt) Scientist:
The center engine on a 727 does not cause a nose-down pitch as it is at the same level as the side engines; the intake is up high, connected by an S-duct.
The DC-10/MD-11 WOULD have this effect as the center engine is high-mounted above the fuselage.
The center engine on a 727 does not cause a nose-down pitch as it is at the same level as the side engines; the intake is up high, connected by an S-duct.
The DC-10/MD-11 WOULD have this effect as the center engine is high-mounted above the fuselage.
