T-Tails - Why?
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T-Tails - Why?
Just been reading (R&N) about an MD80 which seems to have stalled in bad WX conditions, possibly due to overloading, a couple of years back.
I was in the industry when Tridents, BAC 1-11s, even the lovely old VC-10, and so forth were all the rage, despite the problems with T-Tails. Stick shakers and stick pushers were surely only a "band-aid" to a serious basic design problem?
Just wondering if anyone can explain why, with the known drawbacks of this sort of configuration, aircraft like the MD-80 were built? Is there any advantage in putting the engines on the back of the fuselage and the tail high up above them? Why do we continue to do it?
I was in the industry when Tridents, BAC 1-11s, even the lovely old VC-10, and so forth were all the rage, despite the problems with T-Tails. Stick shakers and stick pushers were surely only a "band-aid" to a serious basic design problem?
Just wondering if anyone can explain why, with the known drawbacks of this sort of configuration, aircraft like the MD-80 were built? Is there any advantage in putting the engines on the back of the fuselage and the tail high up above them? Why do we continue to do it?
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Is there any advantage in putting the engines on the back of the fuselage
You get the engines closer to centerline. Note that Boeing 737 was the first underwing twinjet - all previous twinjets (Caravelle, BAC 1-11, DC-9) had had tail engines (except Tu-104 in-wing twin), and all previous underwing jets (B707, DC-8, CV-880, MD-119) had been quads.
Quieter cabin
Better engine ground clearance. 737 had huge difficulties with squashed engines. There is a reason most small (business and regional) jets have tail engines.
and the tail high up above them?
Bear Behind
Why is T-tail better - because a smaller surface area of tail, and hence less drag, is needed to give the same force as a + tail due to the distance from the neutral point of the z-axis.
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I can understand the advantage of tail mounted engines regarding the reletive assymetric thrust problems associated with an engine failure. However, this design creates other problems.
The MD 81/2/3 series were a nightmare to load. When empty they were so tail heavy ballast was often required to bring them into CG limits and, as most of you will know, usable ballast on an airport is as rare as rocking horse s***. The 111s at least had the use of the centre and (where fitted) aux fuel tanks to be used as ballast. When desperate, we used barrels of demin water sometimes used to boost the engines on restricted take-offs (do you remember those days ExSimGuy??) as ballast.
I digress, the MDs, when full had so many pax forward of the wings and such tiny aft holds that they usually flew on the forward limits of the CG. Good design feature? I think not.
Remember the FAA certifications of these a/c? How many did they lose?
Have never felt safe in one.
The MD 81/2/3 series were a nightmare to load. When empty they were so tail heavy ballast was often required to bring them into CG limits and, as most of you will know, usable ballast on an airport is as rare as rocking horse s***. The 111s at least had the use of the centre and (where fitted) aux fuel tanks to be used as ballast. When desperate, we used barrels of demin water sometimes used to boost the engines on restricted take-offs (do you remember those days ExSimGuy??) as ballast.
I digress, the MDs, when full had so many pax forward of the wings and such tiny aft holds that they usually flew on the forward limits of the CG. Good design feature? I think not.
Remember the FAA certifications of these a/c? How many did they lose?
Have never felt safe in one.
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Putting the tail "high" - as a "T" rather than "+" configuration - has the advantage that it:
(1) moves the tail further aft, assuming a swept fin - better tail arm, so smaller tail is possible
(2) moves the tail out of the way of the rudder, allowing a single rudder along the span of the fin, rather than needing it to be split.
(3) increases fin effectiveness by adding an endplate effect to the fin
Conversely, the higher tail usually creates more load on the fin structure.
The loading issues that are referred to above are not due to the aft-mounted engines - there's no reason why that configuration should be more or less easy to keep control of the cg. I suspect that the problems arose due to the way in which the aircraft was developed/stretched over the years, with the design gradually getting less-optimised in terms of weight and balance.
(1) moves the tail further aft, assuming a swept fin - better tail arm, so smaller tail is possible
(2) moves the tail out of the way of the rudder, allowing a single rudder along the span of the fin, rather than needing it to be split.
(3) increases fin effectiveness by adding an endplate effect to the fin
Conversely, the higher tail usually creates more load on the fin structure.
The loading issues that are referred to above are not due to the aft-mounted engines - there's no reason why that configuration should be more or less easy to keep control of the cg. I suspect that the problems arose due to the way in which the aircraft was developed/stretched over the years, with the design gradually getting less-optimised in terms of weight and balance.
A stick shake/push system is no more of a ‘band-aid’ safety device than a horn which alerts the crew if attempting to land wheels up.
MFS has identified the primary advantages of a T tail, these are of particular value on short bodied aircraft or those requiring good rudder effectiveness i.e. large asymmetric force.
The HS125 (almost T tail) and the BAe146 could in theory encounter a deep stall – a characteristic of T tail designs. Neither aircraft identified a deep stall during flight testing; this included high alpha with high pitch rates.
MFS has identified the primary advantages of a T tail, these are of particular value on short bodied aircraft or those requiring good rudder effectiveness i.e. large asymmetric force.
The HS125 (almost T tail) and the BAe146 could in theory encounter a deep stall – a characteristic of T tail designs. Neither aircraft identified a deep stall during flight testing; this included high alpha with high pitch rates.
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The loading issues that are referred to above are not due to the aft-mounted engines - there's no reason why that configuration should be more or less easy to keep control of the cg.
Look at an aircraft with wingmounted engines. The CoG is around wing, as it needs to be in flight. The engines are near CoG. The fuel, in wing, is near CoG. The fuselage structural weight is distributed fore and aft of wing. The payload is divided between fore and aft fuselage. The main landing gears can be slightly behind CoG.
Now compare a plane with tail engines. Since heavy engines are far from CoG, they must be counterbalanced by long forward fuselage. But now, since mose payload volume is ahead of CoG, the centre of gravity moves when payload is removed, which is not the case with wing-mounted engines.
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Because the designers KNOW that when they design the plane. So you configure the aircraft in such a matter as to (ideally) require no baggage management or other cg concerns to intrude upon operations. You're not going to get away with telling the operator "bad luck mate, this is an aft-engined aircraft, it's supposed to be a pest to load"! It's perfectly possible to have an aft-engined type with no more cg problems than a underwing type.
Where it becomes difficult, and what I was alluding to, is maintaining that design goal while also 'stretching' the plane to develop new models. Chances are that the original design concept, with a given fore/aft fuselage length ratio, will be harder to respect while stretching an aft-engined type, and THAT is I suspect what happened with the MD-80/90s. Especially as they changed the engines too, IIRC, which again is more of an issue since they have a longer moment arm than if underwing.
I suspect that the original DC-9s were not such a problem as the MD-80/90s, but that the loadability issue got gradually worse as it was sacrificed in favour of other concerns during development.
Where it becomes difficult, and what I was alluding to, is maintaining that design goal while also 'stretching' the plane to develop new models. Chances are that the original design concept, with a given fore/aft fuselage length ratio, will be harder to respect while stretching an aft-engined type, and THAT is I suspect what happened with the MD-80/90s. Especially as they changed the engines too, IIRC, which again is more of an issue since they have a longer moment arm than if underwing.
I suspect that the original DC-9s were not such a problem as the MD-80/90s, but that the loadability issue got gradually worse as it was sacrificed in favour of other concerns during development.
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So high-wing?
Thanks for all of the contributions and explans!
Thinking about these, the only big advantage of the rear-engine/T-tail config seems to be in avoiding the lack of under-wing space for the engines (especially on smaller turbo-jet aircraft)
Another approach to solving this problem might be to go back to the once-popular (and still so on light a/c and turbo-props) configuration of high-wing.
* A high-wing aircraft has stacks of space under the wings for engines.
* A high-wing aircraft has "natural roll-stability" as the G of G is under the Centre of Lift, and will "want to" fly level (we use dihedral on aircraft to encourage just this behaviour)
* A high-wing aircraft can have the stabilisers mounted where they are very unlikely to be in the wing shadow, and might have their efficiency enhanced by the higher-velocity air emanating from the engine efflux.
* A high-wing aircraft gives pax and crew a better view of the ground.
* Higher engines would be less likely to suffer FOD, especially during reverse thrust.
* A high wing would probably be less likely to be damaged by apron vehicles driving into it (for large aircraft)
(-) Admittedly, the engines on a high-wing are unlikely to be so low as to be below the G of G, and therefor would not produce the same pitch-up force when accelerating (usually what is needed at that time - TOGA, emergencies) - in fact, would probably produce a pitch-down moment, not always good
So, reducing the question to the absurd, what about an A380-type aircraft with a high-wing, and 4 Trents (or GE poor copies
) hanging beneath them? What's wrong with this picture?
Is it just that pax feel happier seeing a wing underneath them, holding them UP? - rather than a wing above them that they are "HANGING FROM"?
I think that, if we put our heads together here, we could produce a really revolutionary new aircraft design
Thinking about these, the only big advantage of the rear-engine/T-tail config seems to be in avoiding the lack of under-wing space for the engines (especially on smaller turbo-jet aircraft)
Another approach to solving this problem might be to go back to the once-popular (and still so on light a/c and turbo-props) configuration of high-wing.
* A high-wing aircraft has stacks of space under the wings for engines.
* A high-wing aircraft has "natural roll-stability" as the G of G is under the Centre of Lift, and will "want to" fly level (we use dihedral on aircraft to encourage just this behaviour)
* A high-wing aircraft can have the stabilisers mounted where they are very unlikely to be in the wing shadow, and might have their efficiency enhanced by the higher-velocity air emanating from the engine efflux.
* A high-wing aircraft gives pax and crew a better view of the ground.
* Higher engines would be less likely to suffer FOD, especially during reverse thrust.
* A high wing would probably be less likely to be damaged by apron vehicles driving into it (for large aircraft)
(-) Admittedly, the engines on a high-wing are unlikely to be so low as to be below the G of G, and therefor would not produce the same pitch-up force when accelerating (usually what is needed at that time - TOGA, emergencies) - in fact, would probably produce a pitch-down moment, not always good
So, reducing the question to the absurd, what about an A380-type aircraft with a high-wing, and 4 Trents (or GE poor copies
) hanging beneath them? What's wrong with this picture? Is it just that pax feel happier seeing a wing underneath them, holding them UP? - rather than a wing above them that they are "HANGING FROM"?
I think that, if we put our heads together here, we could produce a really revolutionary new aircraft design
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* A high-wing aircraft has stacks of space under the wings for engines.
true
* A high-wing aircraft has "natural roll-stability" as the G of G is under the Centre of Lift, and will "want to" fly level (we use dihedral on aircraft to encourage just this behaviour)
That's not exactly how dihedral effect works
Conversely, a simple high wing configuration can have too much dihedral stability, especially on a swept wing, and can require (considerable) amounts of dihedral as a result. This can actually make the tip more at risk of ground strike than a low wing type, where the dihedral helps keep the tip out of harnm's way
* A high-wing aircraft can have the stabilisers mounted where they are very unlikely to be in the wing shadow, and might have their efficiency enhanced by the higher-velocity air emanating from the engine efflux.
Engine efflux hitting anything is usually a bad idea - from a structural fatigue point of view.
* A high-wing aircraft gives pax and crew a better view of the ground.
* Higher engines would be less likely to suffer FOD, especially during reverse thrust.
* A high wing would probably be less likely to be damaged by apron vehicles driving into it (for large aircraft)
all three true to one extent or another
(-) Admittedly, the engines on a high-wing are unlikely to be so low as to be below the G of G, and therefor would not produce the same pitch-up force when accelerating (usually what is needed at that time - TOGA, emergencies) - in fact, would probably produce a pitch-down moment, not always good
true
* A high-wing aircraft has "natural roll-stability" as the G of G is under the Centre of Lift, and will "want to" fly level (we use dihedral on aircraft to encourage just this behaviour)
That's not exactly how dihedral effect works
Conversely, a simple high wing configuration can have too much dihedral stability, especially on a swept wing, and can require (considerable) amounts of dihedral as a result. This can actually make the tip more at risk of ground strike than a low wing type, where the dihedral helps keep the tip out of harnm's way
* A high-wing aircraft can have the stabilisers mounted where they are very unlikely to be in the wing shadow, and might have their efficiency enhanced by the higher-velocity air emanating from the engine efflux.
Engine efflux hitting anything is usually a bad idea - from a structural fatigue point of view.
* A high-wing aircraft gives pax and crew a better view of the ground.
* Higher engines would be less likely to suffer FOD, especially during reverse thrust.
* A high wing would probably be less likely to be damaged by apron vehicles driving into it (for large aircraft)
all three true to one extent or another
(-) Admittedly, the engines on a high-wing are unlikely to be so low as to be below the G of G, and therefor would not produce the same pitch-up force when accelerating (usually what is needed at that time - TOGA, emergencies) - in fact, would probably produce a pitch-down moment, not always good
Harder to deice or confirm clear of ice, since you can't see much of the upper surface ewxcept from a lift of some kind.
Forces the undercarraige to be fuselage mounted (= narrow track, may be a stability issue, plus probably some kind of undercarraige fairing) or to be very long and 'stalky' - which may have loads problems for the gear. And, being longer, it's harder to fit into a reasonable bay size.
Being further out of ground effect, it MAY be less efficient for takeoff/landing.
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Thinking about these, the only big advantage of the rear-engine/T-tail config seems to be in avoiding the lack of under-wing space for the engines
So, reducing the question to the absurd, what about an A380-type aircraft with a high-wing, and 4 Trents (or GE poor copies ) hanging beneath them? What's wrong with this picture?
There are plenty of high-wing underwing quadjets. Lockheed C-141. Lockheed C-5 - the original high-bypass widebody. Il-76. MD C-17. An-124.
And yes, the civilian Bae-146.
Bae-146, at 26 m wingspan, is small compared to plenty of low-wing narrowbodies. Whereas C-141, C-17 and Il-76 have wingspans over 50 m, and C-5, An-124 and An-225 are yet huger.
Would a midsized high-wing civil jet make sense?
As for tail, while An-225 has special considerations (piggyback loads) requiring H-tail, An-124 is notably different from C-5 in having a low tail. Which tail works better - T-tail of Galaxy or low tail of An-124? Can Galaxy be deep-stalled?
Oh, and once An-124-300 is in production, those 4 engines on airframe stretched from baseline hopefully mean Soloviev has bigger high-bypass engines available. Those would come in handy for An-180 and Il-98 as well.
One of the design philosophies of the VC10 was to keep the wing clear for full span trailing edge flaps to improve short field performance. It worked, the approach speed was far less than the B707. However, it made the airfarme some 7 tonnes heavier and far more expensive.
