Why do the C-141 and C-5 have T-tails?

Question

Both the C-141 Starlifter and C-5 Galaxy have T-tails, where the horizontal stabilizers and elevators are mounted on top of the vertical fin rather than on the sides of the aft fuselage. T-tails are usually only found on aircraft where the real estate on the sides of the aft fuselage is taken up by engines (Il-62, 727, DC-9, Tu-154, etc.), as the T-tail configuration has the serious disadvantages of

1. needing a vertical stabilizer strong enough to carry the entire weight of the horizontal tail plus the considerable downward aerodynamic forces generated by same, making for a considerably heavier (and, thus, less fuel-efficient) airframe,
2. causing maintenance difficulties, as it's harder for the mechanics to inspect and repair something that high up (this was an indirect cause of the crash of Alaska Airlines Flight 261), and
3. being susceptible to an extremely dangerous "deep stall" condition, where, at an angle of attack slightly past the stall point, the disturbed wake from the aircraft's wings blanks out the horizontal tail, causing a total loss of pitch control and making recovery essentially impossible.

The C-141 and C-5 have conventional wing-mounted engines, yet both have the tail configuration of an aircraft with fuselage-mounted engines; why is this?

Answer 1

It comes down to a matter of philosophy. Or, if you are less generous, it’s a matter of fashion. Note that the Russian equivalent to the C-141, the Il-76, uses a T-tail, too, while the An-124 uses a conventional tail.

T-tails were in fashion in the early jet age. They were widespread, first in fighters and later in transport aircraft, too, because they:

1. Have a longer lever arm when placed on top of a swept vertical, reducing tail size and mass,
2. Allow for a smaller vertical tail due to the endplate effect, which lowers the requirement for hangar doors and ceilings. In case of the A400M, the T-tail allowed for a height reduction of approximately 2 meters.
3. Stay in undisturbed flow during stall. With wing-mounted engines resulting in a more forward wing position, the danger of a deep stall is reduced when compared to a configuration with tail-mounted engines.
4. Allow for less interference drag, especially when combined with a Küchemann body in the intersection between vertical and horizontal tail.

Therefore, if your conceptual design team is dominated by aerodynamicists, it will tend to favor the T-tail version. Only when second-order effects are considered will the conventional tail win. And those second-order effects will only become known by experience.

What are those effects?

1. Flutter! The rear loading ramp requires cutting the rear fuselage open, reducing its torsional stiffness. Add to that a mass on a long lever arm (the horizontal tail) and you get a structure with a very low eigenfrequency. In order to get that frequency out of the usable speed range, you need to stiffen the fuselage,
2. which adds structural mass. Both tail surfaces might be smaller for a T-tail, but the whole aircraft will be quite a bit heavier. During flight test of the C-141 it was found that the antimetric wing bending mode would nicely couple with the torsional Eigenmode of the the tail, resulting in some scary footage. Also, a coupling between elevator rotation and horizontal tail torsion required to generously add mass balance to the tail.
3. And then there still is this lingering suspicion that you might get into a deep stall. However, with the long distance between wing and tail of a configuration with wing mounted engines this is less of a threat than it might seem.
4. A real problem, however, is the tendency of a T-tail to produce a strong nose-down pitching moment in a sideslip. A low tail will eliminate this effect.

Answer 2

High wing military aircraft (c141 and c5 both are) use T tails for multiple reasons.

• to prevent rocks and debris from damaging the tail when landing off airport
• to keep tails out of the engine thrust line (which are higher than on a low wing aircraft) during cruise where vibration may cause fatigue
• a T tail can be smaller (lighter with less drag) as the horizontal stabilizer is located farther behind the fuselage and the vertical stabilizer has an "endplate" which increases its effectiveness.

Deep stall is not the same problem a low wing aircraft has with a T tail. On a high wing aircraft with a T tail the horizontal stabilizer is in approximately the same place vertically relative to the wing as as it would be in a standard empennage on a low wing airplane, and so is no more susceptible to wing blanking in a stall.

Answer 3

A primary reason is that it facilitates a large cargo hatch and ramp to be built into the rear of the plane, for loading and unloading. No elevator spar running through the middle of the rear of the aircraft to get in the way.

Note that both the current C17 and A400m transports, very recent designs, also use a T tail. Both have large cargo hatches in the rear.

True that the C130 doesn't use a T tail and has a large cargo hatch in the rear, but it's elevators are built very high on the fuselage, and this design was not carried forward with the later C5, C141 or subsequent large military transports. (C130 is a 1950's design) Also, the C130 tends to travel at lower airspeeds than the C5, C141, C17 and A400m, so there may be an aerodynamic factor, though I can't find any hard information one way or another on that.

Answer 4

I worked on both aircraft for Lockheed (1967-1983). The reason for the T tails is a government requirement that the aircraft must be able to land in a depression or say wide ditch. The stabilizer must clear the required depression height. True I am 80 years old but my mind is clear enough to remember this. We did a tremendous amount of time in flight test checking stalls because of the T tails. Those were the days.