T-Tails, Anhedral and the C-5
 

Tech gurus and aerodynamicists:

I am having a spot of trouble finding an accurate answer to the following questions:

Why would a designer use a T-Tail? (In light of the negative stall characteristics of T-Tails)

What does a T-Tail do to stability, and why?

Why does the C-5 have a high wing?

Is the stability influence of the T-Tail the reason for the anhedral on the C-5?

Thanks in advance for your answers.

My opinion only:

I believe the C5 was designed with a high wing to allow lower fuselage sill height for cargo loading. The limiting factor for obtaining low loading height on a low wing aircraft is the engine position so by utilising a high wing configuration the designers ensure the engines have sufficient ground clearance while still achieving the lower sill heights for cargo loading. The lower heights allow vehicular loading with the integeral ramps rather than relying on additional equipment.

I hope this helps!

TH

T-tails
 

Trash-Hauler - thanks - that sounds entirely reasonable. Hey - I used to haul trash too. Maybe we've eaten burnt frozos on the same flight deck!

Anyone got answers for the T-Tail/Stability/Anhedral questions.

I'd say the anhedral is there to reduce the high roll stability induced by the high wings (not the t-tail)

And the high t-tail made to be out of the way of the disrupted airflow behind the engines?

Problem: Ruslan, otherwise nearly a copy of Galaxy (high wing, quadjet, slightly wider than Galaxy, main deck unlike Galaxy not fully pressurized) does fine with low tail. How do Ruslan and Galaxy handle that?

Why would a designer use a T-Tail? (In light of the negative stall characteristics of T-Tails)
- Because, for various reasons, there's nowhere on the aft fuselage to put a tail (aft mounted engines, for example, or a rear cargo door) OR because the downwash field directly behind the wing is unfavourable and its better to get the tail up and out of the flow, even at the expense of deep stall
- also, if the fin is swept, a T-tail can end up further aft, so gaining 'free' tailplane moment arm.

What does a T-Tail do to stability, and why?
- changes rudder and fin effectiveness (endplate effect)
- causes mach issues with the fin/tail junction

Why does the C-5 have a high wing?
- underwing engines & low cargo floor

Is the stability influence of the T-Tail the reason for the anhedral on the C-5?
- may be loading. rolling moments induced by tail on fin can be significant; tail anhedral can reduce this effect

Mad (Flt) Scientist:

I always thought there was deep stall qualities on a low wing T-tails only, like a DC-9,...how does having a high wing compare? it it the same, nastier or more conventional (nose drop)?...thanks in advance

rhov :)

I saw a C-5 once with its nosewheel retracted for loading. Perhaps this was a factor in designing a high wing for engine ground clearance.

Pretty much all depends on the relative geometry; where the fuselage happens to be is probably of secondary importance; as long as the tail can get into the wing wake, the characteristics should be comparable.

In fact, I suspect that the bigger player is going to be other things that change when you swap from low wing to high. For example, I'm not sure where the engines are going to end up with the high wing (all the high wings that come to mind have underwing engines) - in which case the engine/pylon effects on wing behaviour at the stall may be dominant.

While the geometry of the "floor loading" in the C-5 case, certainly dictated the high-wing configuration; do NOT discount engineering fashion on the T-tail. Every airplane in the late sixties, save the 747 was T-tailed. Rear-mounted engines also was an influence in planes with them, but another fashion that has gone by the boards.

Back to the C-5, if I could have changed anything thing on the plane--a low-mounted tail would have been numero UNO! De-icing the beast was difficult at bases with the equipment (very few, only MAC bases) or impossible elsewhere. Not many airports have de-icing equipment with 70 foot reach, other than the terminal window-washing stands. Bloody pain. Good place to watch an airshow:ok: crew---:= commanders!!

The wing anhedral was definitely a design requirement to suppress the strong stability caused by the high-wing, check out the C-141, B-47, B-52 and all the Russian military freighters. Tupolev Bear, also.

GF

Glad somebody finally said it. Any high wing configuration has "built in" dihedral from an aeroynamic standpoint, witness such a simple example as a C172 compared to a a PA28. The Piper has notable dihedral, while the Cessna has only enough to reliably drain the fuel tanks. Also witness the BAE146, compare the Saab 340 (lots of dihedral) to the Dash and ATR series (none).

To avoid the main spar going through the cargo bay probably.

Perhaps also to keep the engines higher (rough field use)?

My apologies, I mistook the anhedral question as referring to tailplane anhedral; please read it in that light.

Be wary of planes with low wings and anhedral (several Russian types, for example) - dihedral effect also comes from wing sweep as well as high/low position....

While having the wing spar above the cargo "box" is an advantage of the C-5, it is not a necessary product of the high-wing design. The C-17 has a restrictive height under it wing center-box that imposes some real problems for high loads like helos. Full height end-to-end in the Galaxy!! The only height restriction is for aft loading and that is mostly due to ramp cresting.

The high wing anhedral also caused the engines to be mounted closer in than optimum for wing bending relief. The 747 engines are notablely further outboard and the outers also have greater ground clearance. I think the whole high-wing design was required by the "roll-on, roll-off" cargo requirements. Kneeling landing gear was installed for the loading issue.

As mentioned - this you can do by flying Ruslan!

There was once a 1930's high-wing private ship with no dihedral - but the lines of the aft fuselage made it appear that the the wings drooped. The designer felt this was a marketing disadvantage, so in a later version, he added about a 1/2 degree dihedral to "correct" this illusion.

(Yes, I got this straight from the designer's mouth in a little Chinese restaurant in California 36 years ago.) :cool:

Not available to USAF pilots, probably for the better. Liked the C-5 quite a bit, actually.

GF