Why the Antonov AN-225 has negative dihedral wings combined with a positive dihedral horizontal stabilizer?

Question

I know the reason why it has negative dihedral wings, but I do not really know why it is combined with a positive dihedral horizontal stabilzer, which are the benefits this configuration provides?

Answer 1

The reason for the wing anhedral has been answered here long ago.

Tail anhedral or dihedral can have several reasons:

• To avoid ground contact during rotation (when the aircraft pitches up in order to lift off the ground). This is needed with a swept tail, so tail dihedral can be found on many airliners. Clearly, the An-225 tail has ample distance to the ground, because the vertical tail surfaces extend for ⅓ of their span below the horizontal tail, so any ground contact can be ruled out even for an un-dihedraled tail.
• To move the horizontal tail away from the wing wake. When you look at the tail of the An-124, you will see no dihedral and a mounting point quite similar to that of the An-225. But if the An-124 has no problem with an un-dihedraled tail, so should the An-225. This can also not be the reason.
• To add some more vertical tail surface. This trick has been employed on the Handley-Page Victor or the F-4 Phantom II. On the An-225, the outer verticals would shield much of the horizontal tail and little additional side force will be created by the dihedraled horizontal tail. So this cannot be the reason, either.
• For a H-tail: To move the attachment point of the vertical surfaces closer to their middle. Imagine the An-225 horizontal tail had no dihedral: Then the verticals would need to be mounted on the tips of the horizontal near their bottom (they need to be so high up in order to allow the An-225 to rotate, after all). This would mean that all side loads would act above the attachment point, producing a sizeable bending moment which has to be carried along the spar of the horizontal tail, making the outer tail structure a lot heavier. With a little dihedral, the attachment point can be moved further up, close to the middle, so the bending moments in the vertical and horizontal tails can be a lot lower.

Antonov has designed H-tails before (on the An-14, the An-22 Antei, the An-28 and the An-38), and in all cases the vertical had a sizeable part of its surface below the horizontal tail. The only design on this list which also had some tail dihedral is the An-14 from 1958, and here the attachment point is almost in the center of the vertical. So it looks like the Antonov design bureau applied a trick on the An-225 which they had known for at least 30 years before the An-225 took off for the first time.

Answer 2

Anhedral is used on high wing swept wing aircraft because the dihedral effect of the sweep PLUS the aerodynamic dihedral effect of the T wing-to-fuselage interface adds up to too much total dihedral effect. The roll/yaw couple (rolling moment created by yaw) becomes much too strong. The anhedral brings the total dihedral effect down to a desirable level.

The dihedral of the horizontal tail can be done for two reasons; ground clearance for the vertical tail surfaces, and the effective side area presented by the canted surfaces can be considered to be adding to the vertical fin area (a V tail takes this to the ultimate extreme).

In the case of the Antonov I will bet that the tail is like that mostly to provide a little extra vertical clearance for ground vehicles.