Why not have an all-moving V. stabiliser?

[tom775257]

Hi all,
I realise that ‘high keel surfaces’ i.e. the vertical stabiliser aid aircraft stability, tending to return the aircraft to wings level, balanced flight. This must help with hand flying aircraft. My question is this: Does having such a (relatively) large vertical stabiliser area compared to the rudder create much more drag than a correspondingly smaller vertical stabiliser and same rudder size? :confused:
My (probably stupid) reasoning is that on larger commercial aircraft it is my understanding that the yaw damper in normal flight usually controls the rudder; therefore why not just have a much smaller all moving vertical stabiliser, controlled precisely by the yaw damper in normal flight (to reduce drag)…. My second line of reasoning being that with an all moving vertical stabiliser (i.e. a bit like a stabilator) very precise control would be needed, hence operated by an automated system.
I guess the downside would be hand flying would become more difficult at high speed, and yaw correction from the loss of an engine would become tricky, but hey, what do I know! Any answers for a bored slightly inebriated private pilot in training much appreciated.
Cheers,
Tom.
:)

[Volume]

I can´t see the benefit in an all flying vertical tail. While a plain flap used as rudder increases the CL max of the vertical tail if deflected, rotating the whole tail don´t. And because you have a symmetrical airfoil optimized for low transsonic drag, CL max is small. Therefor your all flying vertical tail must be remarkably LARGER in comparison to a conventional one if you thing of single engine failure operation. (And for rear mounted Engines with T-Tail, which needs less moment compensation in that case, it makes even less sense to have an all flying tail).

Big difference between all flying horizontal and vertical tail is, that the HT is symmetrical so there is no resulting rolling moment at the attachment. The VT is just on the upper side of the fuselage producing a large moment (sometimes large enough to rip off the tail, if misused ...) which must be carried by the tail bearing in such a design. This would result in a very heavy design.

Aerodynamic fairing of the VT to fuselaqe joint is hard to design, so you have even more additional weight and drag.

Last but not least : ask any flying model builder about all flying V-Tails (in fact thats the same as two all flying vertical tails, each moved 45° outboard) They´ll tell you : simply don´t do it. Stiffness is a large problem, flutter often results from this.

[MR WIBBLE]

Hello Bys and Girls and fellow citizens of Wibble,

Didn't that gorgeous and tragic aircraft, the BAC TSR2 have an all moving vertical stabiliser?

What an amazing plane that was and should have been...

[tom775257]

thanks

[chiglet]

The North American Avn A5 "Vigilante" had an all flying fin, sorry, Vert stab.:)
we aim to please, it keeps the cleaners happy

[Tinstaafl]

I can't see that there is a difference WRT the forces that must be produced being a limiting factor.

Whether or not the rudder is all moving or uses a hinged trailing edge it is still the same size force that must be produced to effect a particular response.

One could argue for an all-moving fin (a 'fibulator'? :p ) being more efficient for all the same reasons as a stabilator.


I suspect the reasons are more to do with redundancy & convenience. It's easy to build a redundant rudder with a hinged flap. Just bolt two separate ones onto the fin.

It's also structurally easier to build a strong structure to bear & transmit load if it doesn't have to move.




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Why are you looking down here? The message is up there. .. |

[Jump Complete]

I once had a share in an Evans VP-1 (Single seat homebuilt) That has an 'all flying' tail. It was certainly effective. In fact when I first flew it (obviously solo as it was a single seater!) i had to try very hard not to over control on the ground run as my tail-dragger trained responces where to peddle like mad! Although a tail dragger it needed a very delicate foot. However a small light aircraft obviously doesn't translate to a large transport aircraft, and come to think of it, I can't think of any other light aircraft, even, that have one. I would imagine that it would be difficult to cope with building a vertical surfice big enough for a airliner onto a single pivot point...And anyway, the current system has been on aircraft for about 90 years and seems to work quite well-what would be the point?

[MLS-12D]

The Zenair CH701 (http://www.flyzenair.com/ch701.html) is one common (and successful) airplane with an all-flying rudder (I believe not for reasons of aerodynamics, but simply to make it easier for the home-builder to construct).

[Volume]

The Tu-160 did have an all moving fin (at least the upper half above the all flying stabilizer).

http://www.flankerman.fsnet.co.uk/tu-160_files/tu_160_09.jpg

For supersonic aircraft this makes perfect sense, but for subsonic ones, there is no real benefit.

As already mentioned, the horizontal stabilizer is symmetric, therefor no wing root bending moment has to be transfered via the hinge, for an all moving fin the complete root bending moment has to be reacted by the bearings.

For horizontal stab of planes flying just sub sonic, therefor having transsonic airfoils, any change in airfoil contour increases drag dramatically, so a plain flap is no good idea for long term deflection. Therefore normally it is movable to achieve an undeflected elevator with the optimal airfoil shape by setting the angle of incidence appropriate for the lift to be created. This is done by the stabilizer trim.

Normally the fin produces no lift, and if it does, it is due to engine failure or crosswind landings, so to say it is at low speed. And at low speed the drag penalty of a deflected plain flap is not so bad.

[tom775257]

Wow, 2 years on and some more information.

Volume:
Thanks for that....that is an interesting comparison between (I assume) Mach trim (in terms of variable incidence H. Stab) and the idea of an all moving fin for supersonic a/c...

Jump Complete:
Perhaps some kind of anti-balance rudder tab needed for the VP-1...prob too complex!

Rightey ho, off to the pub.

Best regards,

Tom (AKA captain bounce, today I arrived...and arrived... first days solo flying after 7 months off for ATPL theory :ugh: )

[Low-Pass]

Well done Tom. The more landings you do, the more experience you gain. :}

Seem to remember that the SR-71 Blackbird had an all flying fin so the supersonic arguement seems a good direction to look. Also, with the delta wing, the fin would never conflict with the elevons whereas the photo of the Blackjack above, It looks like deflecting the elevator upward (i.e. the climb) would interfere with the all flying fin.

Cheers,

LP