I have a few questions regarding t tails, DC9, MD80, B717, RJs, etc.

I've read they are harder to fly - true? Why?

Is engine maintenance harder? Seems it would be.

Except for the RJs, they seem to be a dying breed. Why? Is it the above-mentioned reasons, or rear-cabin noise level, or other reasons?

I wouldn't say harder to fly, but you do have to be aware that if you raise the nose of the plane too high (high angle of attack), the horizontal surfaces on the T-tail become ineffective and you can have a very nasty condition called a deep stall, which is nearly impossible to recover from.

The deep stall condition was why the prototype of the BAC 1-11 crashed during a test flight and a number of 727-100's crashed early in its service career. As a result of the British Civil Aviation Authority (CAA) investigation of the 1-11 crash, they installed in the cockpit a series of warning systems to prevent this condition, namely a loud warning siren and a very distinct vibration on the yoke if you raise the nose of the plane too high.

This is probably one of the reasons why Airbus chose a conventional configuration (normal tail design, podded engines on wings) for the A320 Family of planes.

They are also heavier

Boeing were going for a T tail with the 757/767 but changed it at the last minute. For the same weight, they could have a longer fuselage able to carry more passengers and cargo.
I have also heard that an engine change on a rear - engined aircraft is much more complicated than a wing-pod engine.
A rear engined aircraft will usually have a longer wheelbase, resulting in possible maneuvering problems at smaller airports.

Mark

Longer wheelbase??? Please clarify.

XV

With the engines on the back the center of gravity goes futher to the back as well. To balance it out, the fuselage in front of the wing is made longer. This makes the turning radius longer as well.

Just compare the fuselage behind the wings of a 767 with that of a MD11. You will find that the MD11 is rather small behind the wings. That is even more noticable on the MD-80series versus the A320.

All of this talk about the drawbacks of the T-tail design begs the question, why did they used to be so prevalent? What benefits does the T-tail design impart?

The advantage is not in the T-tail but in the aft mounted engines, and you can't tail-mount the engines without moving the horizontal stab up out of the jet efflux.

And the advantage of tail-mounted engines is lighter wing root area, simpler design for the area that would otherwise include engines and the related plumbing and wiring. More room for fuel in the wing root of a tail-engined airplane than with wing mounted engines.

There are tradeoffs back and forth in performance, handling, loading and so on. All airplane designs are a bundle of compromises.

There are a few planes out there that use a T-tail just because someone thought it looked cool. Piper Tomahawk comes to mind.

Most T-tails have a T-tail because of engine placement. If your engines are carried under the wing, the wing has to be able to support the forces placed on it by the engine. Placing the engines on the fuseloge has benefits in wing weight and design.

Placing the engines to the side in the rear of the plane also allows you the luxury of having your plane ride closer to the ground.

Designing the plane is a series of trade studies. A lot of the trade studies narrow down to several basic designs. Changes in technology, available materials, engine technology and so forth, can allow one design to be more favorable over time.

[Edited 2004-06-10 22:29:51]

A couple more advantages of T-tails:

Moving the engine from the wing to the fuselage allows the leading edge slats and trailing edge flaps to extend from wing root to wing tip. A slight advantage in control is achieved by eliminating the interruption in slat / flap extention.

Also, if one engine shuts down in-flight for whatever reason, controlling asymmetrical thrust is much easier on T-tailed a/c. The operating engine is more toward the centerline of the a/c reducing the need to compensate for drag created by the "dead" engine.

<>

Moving the engine from the wing to the fuselage allows the leading edge slats and trailing edge flaps to extend from wing root to wing tip

Am I right in thinking that this was part of the reason behind the success of the 727. A clean wing packed with high-lift devices gave it superior performance for its time?

Mark

One of the main reasons Boeing wanted a T tail for the 757 was because it is more aero dynamically efficient. I did a little bit of aerodynamics in an engineering class and basically with the horizontal stabilizer raised from the fuselage it receives less turbulent air that is disrupted due to the presence of the fuselage. Therefore it gets a cleaner airflow, and reduces drag over a conventional tail design. Also since the horizontal stabilizer is in cleaner air, it can be smaller and still perform its function. There is a tradeoff with this though, because the vertical stabilizer has to be stronger in order to support the forces exerted by the horizontal stabilizer, and thus it increases weight. If it weren't for the stalling characteristics described above, we would see more T tails, because the aerodynamic improvements are drastic.

What about C-17 and C-141:

View Large View Medium Photo © Jerry Search

View Large View Medium Photo © Anders Presterud

The T-tailed design was part of the success of the 727 but I would say a bigger component was the JT-8D engines Pratt & Whitney developed. At the time, that engine was the most advanced, quiet, fuel efficient and economical for use on large transport-category airliners. Combined with only needing 3 instead of 4 to power a plane as large as the 727 was a winning combination.

<>

When examining the advantages and disadvantages of a T-tail configuration versus a conventional one, there are really two distinct categories within the comparison. There are the pros and cons associated with the placement of the tail, and there are the pros and cons of the location of engines - two seperate issues (the C-17 is a perfect example). Thus, to summarize:

Tail location pros and cons.

Conventional:
--Stall problems? No.
--General aerodynamics? Average.
--Weight? Average.
--Prone to foreign object damage? Yes (i.e. more so than T-tails).

T-tail:
--Stall problems? Yes. At high AoAs and more importantly when stalled, T-tails are placed right in the path of the wings' wakes, causing the aforementioned condition of "deep stall."
--General aerodynamics? Good. T-tails produce less drag and are generally efficient aerodynamically because they're away from all sorts of fuselage flows and eddies, and the interference drag characteristics are changed.
--Weight? Sometimes greater than normal due to the need for increased structure around the vertical stabilizer area.
--Prone to foreign object damage? Not as much as conventional tails. That's why the C-17 and C-141 have T-tails, I believe.

Engine location pros and cons.

Conventional:
--Ground clearance issues? In some cases yes (particularly with smaller aircraft).
--Internal noise? O.K.
--Acoustic fatigue? Possible flap and wing fatigue.
--Crash safety? Good.
--Propulsive efficiency? Good.
--Longitudinal stability? Good (also delays tip stalls).
--Asymmetric thrust characteristics? Relatively poor.
--Weight? Good (wing bending and torsion relief).
--Maintenance? Good.
--Wing aerodynamical characteristics? O.K., possible flap/leading edge 'cutouts'.

T-tail:
--Ground clearance issues? No.
--Internal noise? Good.
--Acoustic fatigue? Proable fuselage fatigue.
--Crash safety? Possible problems.
--Propulsive efficiency? O.K. (if positioned well).
--Longitudinal stability? O.K. (not as good in terms of tip stalls).
--Asymmetric thrust characteristics? Good.
--Weight? O.K.; heavy tail, heavy fuselage frames, 2-4% heavier empty.
--Maintenance? O.K. (high off ground).
--Wing aerodynamical characteristics? Very good.

There you have it, tail design in a nutshell!  

Cheers,
QantasA332

Remember with rear mounted engines you can get the wing closer to the ground meaning shorter (stronger) undercarriage and engines higher so less chance of FOD on an unpaved runway.

This was one of the main factors in the design of several Russian aircraft. The Yak-40 is a perfect example of an aircraft optimised for a rugged environment - T-tail and rear mounted engines.

FL1TPA the thing about engine-out being easier to handle with tail-mounted engines is a widely held misconception.

Think of the asymettry of thrust and the yaw control like this. You have a triangle; one corner is the center-of-gravity of the airplane, one corner is the operative engine (when its opposite number has failed) and the third is the rudder. To simplify the illustration I will use DC-9 and B-737 for examples, but this is really not a comment on the handling properties of either of these fine aircraft.

On the 737 the engine is farther outboard from the centerline, but the CG-to-rudder arm is very long.
On the DC-9 the engines are closer to centerline but the CG-to-rudder arm is shorter by a similar amount.
It is just about a tossup as to which is better.

Having flown both aircraft I can say that both can be managed right down to V_MC and both will tire your leg if you had to do it very long the hard way.

And then, there's an exception to every rule... this one being:





SlamClick or someone else, can you explain why the C-141/C-5/C-17/BAe-146 have the conventional engine placement along with a T tail? I'm assuming it has something to do with the fact that on airliners the wings are low mounted, so they don't disrupt the air as high as the conventional tail plane sits. But with the aforementioned cargo planes, the wings are mounted high with sharp anhedral. So the jet flow would still disrupt a low tail. Am I far off here?


[Edited 2004-06-11 06:40:24]

Military cargo jets have T-tails so that loaders don't have to worry about smacking into the horiz. stab. with a forklift or something. Thats pretty much it... it gets it up and out of the way so that a rear cargo door can be used more efficiently.

-Meister

One more thing I just looked up in one of my engineering books. The horizontal stabilizer on in a T tail helps optimize the performance of the vertical stabilizer. It has the same effect as a winglet reducing drag and air turbulence at the tip of the wing (vertical stabilizer/rudder). This would cause more efficient air flow, and allows for a much smaller vertical stabilizer, and rudder, which could help keep weight down and strength up due to the smaller bending moment of inertia.

Many good posts. I'd like to add that T-tails also get the horisontal stab out further behind the fuselage, meaning an increased moment arm and thus a smaller stab = less drag.

Cheers,
Fred

Thanks for the response!! One thing stands out in a previous post, however. Quantas332 alluded to cabin noise being "good" in a t-tail, but just "ok" in conventional. Are you sure? I flew a USAir MD80 from PIT to SDF in '96 sitting in the very last row and noticed A LOT of noise back there.

Thanks again!!

Quantas332 alluded to cabin noise being "good" in a t-tail, but just "ok" in conventional. Are you sure? I flew a USAir MD80 from PIT to SDF in '96 sitting in the very last row and noticed A LOT of noise back there.

Generally cabin noise levels are considerably lower for aircraft with rear-mounted engines. It's variable, though, and that certainly can change if you're sitting near the back, particularly on older aircraft...

Cheers,
QantasA332

With the T tail cabin noise is significantly reduced in the front where first class and all of the higher fare paying people are. It is a good strategy, give the best ride to the best paying flyers and let the people paying discount tickets that don't fly regularly deal with noise in the back.

Military cargo jets have T-tails so that loaders don't have to worry about smacking into the horiz. stab. with a forklift or something. Thats pretty much it... it gets it up and out of the way so that a rear cargo door can be used more efficiently.

That's part of it, but IIRC the main reason why t-tails are so prevalent in the AF cargo world is for ease of airdrop (C-130 notwithstanding). No horiz stab to disturb the airflow around the rear fuselage. Besides the C-17 had/has enough airflow problems back there, a low stab like a 130 would make a bad thing terrible. Besides, you can't have a T-tail mafia if there are no t-tails. Those former airlift/AMC/MAC guys know what I'm talking about

Engine maintenance isn't all that bad on "T" Tail A/C, ie 727. I worked on them for a number of years doing the FedEx Stage 3 Hush Kits for UAL, the only drawback was you needed a forklift platform or scissorlift to work on them, but one hell of an A/C!!!