Tailplane anhedral?

[Alternate Static]

Have been racking my small and feeble brain over this recently posed question:

What is the purpose of tailplane anhedral?

The books I have been referring to provide little help/ insight on this subject?

I've noticed that the Bandit has a slight anhedral and conversely, the Saab340 has quite a pronounced dihedral...

[bizjet pilot]

To increase a pitch-down moment at high alpha. Similar function as delta-fins at tail end of Lear 31, or new Citation Excel (both twin-engine_ biz jets.

Doing this can convince an airworthiness authority to obviate the need for a stick pusher, which can save money & complexity etc.

[Skycop]

Check out a side view of the F-4 Phantom (classic example of a/c with tailplane anhedral). At low alpha the tailplane sits up above disturbed air from the wing. If there was no anhedral, all of the tailplane would be severely affected by dirty air from the wing at high alpha (required at low IASs because of this wingplan). It's a compromise to get the best of both worlds.

[This message has been edited by Skycop (edited 21 April 1999).]

[Alternate Static]

Thanks for the responses, things are making sense so far. Is this the only reason?

Does the tailplane anhedral increase the lateral instability of the a/c in any way.

I don't think anhedral would be used to negate the effects of dutch roll (as used on the mainplane).

If anyone could shed any more light, it would be greatly appreciated. This has the potential for a good crew room debate.

[Skycop]

The downward droop of the tailplane also presents its upper surface to yaw-induced airflow. This probably gives more stability in yaw, as if the vertical tailfin was larger...and may also cause a rolling moment into the turn i.e. cause less roll stability and may therefore reduce a tendency to dutch roll. Jolly clever, these aerodywotsits!

[Cardinal]

In the case of the SF340, I believe that the steep dihedral keeps the tailplane out of the disturbed air generated by the Garretts, eliminating some vibration

[pterodactyl]

Skycop
The Falcon 900 is a good example of the high fin coupled with a fair degree of tailplane anhedral. Certainly in some cases anhedral and dihedral have been used to avoid exhaust gases or adverse airflow effects.Neither of these appear to be the case with the Falcon and as you suggest there may be a yaw/roll stability effect.Another possibility is the physical problem of providing suitably adequate attach points amongst the array of the three engines. Only the manufacturer would know I guess. Any falcon 900 pilots to illuminate?

[This message has been edited by pterodactyl (edited 24 April 1999).]

[Skycop]

All aircraft design is a compromise, each design feature having good and bad points in most cases. A fix for one thing often causes a problem elsewhere. This is why designs evolve and there are sometimes many marks of the same aircraft. Some designs have been inherently good and some in need of a complete rethink before (or after in some cases) entering production, although this is not so common these days thanks to better knowledge and materials (and computers of course) :)

[xyz_pilot]

As an example of fixing problems just look at the B1900.

Have you ever seen more fixes on one aircraft, think what it must have been like "raw" !!

[Banditflyer]

Alternate Static
I had better dive into the books to confirm this but I think you will find that the Bandit has two diferent tailplanes fitted depending on the model..One has Zerro angle and the other 7 degrees. The wings on all models are 10 degrees. As someone mentioned earlier the tail plane diheadral is to reduce the effect of vibration.

This is the best forum on pprune..at least here we can actually learn from each other and just think of the enormous knowledge bank we have access to.

[Prof2MDA]

Nice explanation of this in Flightwise Volume 2, Stability and Control.

[John Farley]

As this thread has shown there are many possible reasons for using an anhedral tailplane (and that also applies to just about any aircraft feature you like to consider). I think most of the tailplane ones have been mentioned. There can be structural reasons as well, for example if you would like the tailplane mounted lower for an aerodynamic reason but it is not so easy to arrange the structure to mount it at that height then bend it down. (The Hawk was a bit like that as it had to go above or below the jet pipe - in the middle would have been a very heavy solution structurally).

Also never forget the difference between dihedral angle and dihedral effect. What we need as drivers is just the right amount of dihedral effect (ie the right amount of roll due to rudder or rolling moment due to sideslip as the aero guy would say). Getting that is not just a matter of wing dihedral angle. A tall fin and rudder will have its centre of lift well above the fuselage (about 1/3 up the height would be a reasonable assumption) so in this case left rudder produces right roll. Then there is the matter of the height of the CG with respect to the wing – high wing with low CG gives left roll with left rudder. Sweep back produces relatively huge amounts of left roll with left rudder. In this context an anhedral tailplane gives right roll with left rudder as does wing anhedral.

Sooo, it is often the case that things having a large dihedral effect are chosen for reasons other than lateral stability (sweepback and a high wing being two obvious ones) This means the designer will have to fiddle with all the elements that have individual dihedral effects in order to optimise the overall AIRCRAFT dihedral effect. If you get it wrong to start with then the dihedral angle of the tailplane will probably be easier to change than most of the other things.

JF