H-Stab Anahedral

[aviationluver]

Hello,

I was wondering why some business jets have their H-stab as anahedral?

Thanks.

[Owain Glyndwr]

Could be all sorts of reasons - care to specify which aircraft?

[Zeffy]

http://www.desertjet.com/wp-content/uploads/2012/08/Falcon-2000.jpg

[Dash8driver1312]

I was wondering why you'd make one word longer whilst simultaneously attempting to sound cool creating an ugly abbreviation.

[Pugilistic Animus]

The HS is simply an inverted wing because it needs to impart a downward moment in order to keep the nose up. So what you're looking at is an upside down wing nothing to do with stability as far as I know I'm sure someone will come through with a more detailed answer to your query but I hope this helps:)

[DaveReidUK]

The HS is simply an inverted wing because it needs to impart a downward moment in order to keep the nose up.Which is equally true whether or not it has an(a)hedral.

[Dairyground]

From somewhere in the depths of fluid dynamics courses of fifty years ago, I have retained an impression that use of di/an-hedral was connected with roll stability. So what are the benefits of having the tail less stable in roll than the wing?

[Hobo]

Discussed in 2005 here (http://www.pprune.org/questions/201541-anhedral-tail-planes.html).


While on the subject of anhedral, can anyone put me out of my misery regarding why all of the Russian rear engined airliners have a pronounced anhedral on the wings.

I was told they accidently put them on upside down on the prototype and it worked so well, they kept them...

[flyboyike]

F-4 comes to mind.

[balsa model]

On the ground, with the surface trimmed neutral, any ice that is melting in the sun might just sit there until it has a chance to refreeze. Add anhedral and voila! Ice gone.
(No I don't design aircraft - you're all safe :)

[TURIN]

From an even older Pprune thread.

Tailplane Dihedral 2004 (http://www.pprune.org/archive/index.php/t-135588.html)

[Pugilistic Animus]

Oops how coukd I forget about the total airfoil? I must stop slicing wings into infinitely thin sections :}

[roulishollandais]

I must stop slicing wings into infinitely thin sections
Good exemple of excess of system mind if the basic should not have popped up. ;)

[BOAC]

Zeffy's pic in post #3 should be a salutary lesson to all of the effect of excessive 'g' on a tailplane during a 'punchy' departure.

[Owain Glyndwr]

As I said, it could be several reasons.
Zeffy's photo is, I think, a Falcon, so let's take that.

Tail anhedral will reduce the aircraft rolling moment due to sideslip, which will change the balance between dutch roll and spiral stability a little bit. But unless someone can say that the Falcon is "iffy" in either of those quantities that is unlikely to be the reason.

In his book "JAR Professional Pilot Studies" Phil Croucher writes:

Anhedral reduces the lateral stability. In the Falcon, where it is used on the tailplane, it improves flying qualities by reducing the effect of wing downwash

Hope that helps

[OK465]

F-4 stab anhedral was for aesthetic reasons.

[BOAC]

F-4 stab anhedral was for aesthetic reasons

aesthetic - concerned with beauty or the appreciation of beauty.

I almost choked on a peanut there..............................:D

[DaveReidUK]

F-4 stab anhedral was for aesthetic reasons.I always though it was so that, when you sheltered under the tail in the rain, you could be sure which edge the water was going to run off.

[john_tullamarine]

F-4 stab anhedral was for aesthetic reasons.

From an FT course years ago, I recall a discussion which suggested that the original design called for a T-tail .. subsequently changed as that configuration wouldn't have made the cut for carrier operations.

[wiggy]

I recall a discussion which suggested that the original design called for a T-tail

Yep, the design grew out of the Demon and Banshee and at one stage ( as the F3H-G/H, which AFAIK only existed as a mock up) it did indeed have a straight T- tail.

subsequently changed as that configuration wouldn't have made the cut for carrier operations.

Only references I can find for the reasons for the change to anhedral in the final design (Dorr, Boyne) discuss that there was a need for a low set tailplane but there being no room for one given the rear fuselage shape/ jet pipe position. The solution was to use a large degree of anhedral.

[aviationluver]

@Owain. Your second answer could be used to describe a dihedral as well...correct? In other words, dihedrals help with roll stability in a slip condition, so, what would be the difference between the dihedral and anahedral other than looks?

[john_tullamarine]

Only references I can find for the reasons for the change to anhedral in the final design

The story from a senior TP was that the aircraft required the change to facilitate the final part of the touchdown sequence whilst coming aboard the boat. Can't recall any detail beyond that.

[Brian Abraham]

F-4 stab anhedral was for aesthetic reasonsOne wonders where such stories come from.

The F-4 design was preceded by the F3H-G mock up. Photo below. The wing had no dihederal, nor the tailplane, as was also the case with the preceding in service F3H "Demon". The G never went beyond mock up stage. Within McDonnell the G went by the designation Model 98B.

http://upload.wikimedia.org/wikipedia/commons/thumb/0/0f/McDonnell_F3H-G_mockup_in_1954.jpg/800px-McDonnell_F3H-G_mockup_in_1954.jpg

After much wind-tunnel testing, it was found that the fighter would encounter severe stability problems at high speeds and would as a result probably be limited to speeds below Mach 2. At issue was the rolling effect of the outer wing panels in yawed flight. In order to correct the problem the tailplane was given 23° of anhedral, an increase from the original 15°. This gave the necessary degree of stability, but still left the tailplane free of the jet exhaust and turbulent wing wake. So became the first flying Phantom, the YF4H-1 (Model 98S).

[Owain Glyndwr]

aviationluver

Your second answer could be used to describe a dihedral as well...correct? In other words, dihedrals help with roll stability in a slip condition, so, what would be the difference between the dihedral and anahedral other than looks?

Apologies in advance if this seems too complicated or 'nerdy', but there are some fairly complicated aerodynamics involved and simple explanations of complex problems often lead to wrong solutions!

"Roll stability" is too indefinite a term for me as there are three lateral aircraft motion modes and they all involve roll. Increasing dihedral of either the wing or the HTP will stabilise the spiral stability and destabilise the dutch roll - anhedral vice versa. From what I have read, in most cases one wants to improve the dutch roll, for which tail anhedral would be a possibility, but putting the special case of the F4 aside, it would probably be easier just to fit a yaw damper, so I think introducing lateral stability into this discussion is a bit of a red herring.

In the longitudinal case one needs to understand that the downwash behind the wing is not constant. It is a maximum near the wing wake, but above and below that the downwash falls the further away one gets.

So if the wake lies below the HTP (cruise AoA range) putting anhedral on the HTP will bring the outer part of the HTP into a larger downwash. At high AoA the wake will move upwards relative to the HTP and if it lies above it then anhedral will reduce the tail downwash.

So "Yer pays yer money and yer takes yer choice". GUESSING from what Croucher wrote, they might have been attacking a high AoA problem, possibly some sort of pitch-up near the stall, but that is a pure guess.

[BOAC]

I'm sure Brian has the correct answer - TP anhedral will reduce the rolling moment with yaw.

The story of the F4 I heard many moons ago was it started off looking like a real aeroplane and then they bent all sorts of bits up and down to make it look as it does now.:)

That's the problem with letting the Navy play with our aircraft....................:}

[Brian Abraham]

Tut, tut BOAC. := The F-4 was designed to meet a Navy requirement. Have to give the Air Force credit for recognising a good machine when they clapped eyes on it though. Bit like the Skyraider, A-7 and Sidewinder really.

[Brian Abraham]

Yep, the design grew out of the Demon and Banshee and at one stage ( as the F3H-G/H, which AFAIK only existed as a mock up) it did indeed have a straight T- tail.I'm not aware of any of the design iterations having a T tail, and they numbered 98B (F3H-G/H) to 98S (F-4 prototype, YF4H-1).

Can you shed some light wiggy?

The WWII Beaufighter sported both a straight tailplane, and one with dihedral, depending on mod status. As the pilot notes state, "There are a number of aircraft which have not yet been fitted with a dihedral tailplane. These aircraft will be found to be considerably unstable fore and aft under all conditions of flight and particular care must be taken at the lower speeds and at heavy loads."

Interestingly the pilot notes carry the caution "cloud or night flying is not advisable as handling becomes difficult below 180 m.p.h." Yet the aircraft was used as a night fighter. Have to admire the crews, not only did they have to contend with the enemy, but also their own aircraft.

[wiggy]

Brian

I'm not aware of any of the design iterations having a T tail,

You're right, my mistake. I've just looked again at the source and I see I missconstrued what was written...:\

(never post in a hurry after a long night............)