Tailplane dihederal

[Slimpickens]

I have never had a good answer to this question: most jet transports of conventional configeration (engines under wings, low mounted tailplane) have varying degrees of dihederal on the tailplane. Or should that be anhederal, when taking into account the lift vector of the tailplane? Anyway, some have more than others (DC-10 vs 777) whilst some have none at all (L1011).

Why?

I would have thought tailplane dihederal (anhederal?) would have a destabilizing effect on the rear of the aircraft, although in the cruise I imagine the (negative) lift produced would be quite small, and thus perhaps this is not a problem. Perhaps there is a need for the structure to be clear of the jet efflux of the wing-mounted engines, but then why does the 1011 have almost no dihederal. Or has it got something to do with manitaining clean air over the tailplane when the aircraft is at high angles of attack?

I notice with the T-tail designs, there is little variation - most have almost no dihederal, whilst some biz-jets such as the Falcon 50 or 900 have a little anhederal.

[Volume]

I have no real idea, but one quite important effect of anhedral or dihedral is that the structure is drained well. Without you´ll always have some water standing inside the structure causing corrosion.

[ft]

You'll find it is stabilizing, not destabilizing. The stabilizing effect does not depend on the direction in which the surface generates lift. IIRC, that's the reason for the anhedral on the F4 Phantom's horizontal stab.

Other than that, getting the tailplane out of prop/jet wash is one reason. Getting it out of the downwash from the wing another.

Now I'll shut up and let those who really know chime in. :)

Regards,
Fred

[John Farley]

Taking your question to be “what determines whether transport aircraft have a dihedral/anhedral angle on their tailplanes” I am not sure there is one simple answer to your question because designers are usually faced with the need to compromise and so choose routes which can lead to different outcomes.

Let us say you are designing a conventional layout airliner and have decided on the capacity of the fuselage plus the nature and location of the wing and powerplants. Turning to the tailplane you have your reasons for wanting to have it at a particular height – likely to be affected as you have said by wing downwash (especially at full flap), jet efflux and high alpha longitudinal stability or super stall issues. Having chosen the optimum height from purely aerodynamic considerations you then realise that to provide the sort of structural strength needed where it joins the fuselage/fin you would find it easier if it was up a bit or down a bit. Cue for fixing it where it is convenient from a structural standpoint but you will have to give it some dihedral/anhedral in an attempt to have the average height of the surface closer to where you wanted it in the first place. On the other hand, you may wish that the dihedral EFFECT of the whole aircraft (rolling moment due to sideslip or static lateral stability) which arises in the main from wing dihedral angle/wing twist/vertical CG position/type and height of fin, is not as you hoped so back you go to the tailplane dihederal/anhedral route to fix that. And so on.

Of course you could have boobed with the original design, be flight testing it and find yourself wishing that the wing dihedral was a tad different, in which case it might be cheaper to change the tailplane …...but you would never make a mistake like that – not nowadays, would you?

Turning now to your comments after ‘Why’ where you say “I would have thought tailplane dihederal (anhederal?) would have a destabilizing effect on the rear of the aircraft” I am not clear what you have in mind. Which axis were you thinking might be affected/destabilised? And how?

[Slimpickens]

Thank you for your interesting and thought-provoking anwers.

FT - I remember from 'theory of flight' (many years ago!) that mainplane dihederal aided in stability, and for low wing aircraft this was why they had dihederal. However, we were taught that high wing aircraft have excessive stability due to the pendulous nature of the underslung fuselage, and therefore most have either no dihederal (Cessna 152/172 etc) or varying degress of anhederal (BAE146, C5, Starlifter, Harrier, etc) in order to reduce the relative stability (and make it easier to manouver/roll I guess). Is this not the case? Does the same not hold true for any lifting surface?

John Farley - I guess from above comment, the axis would be in roll (of the rear fuselage?!).
The theme that seems most consistent from all answers is to avoid the wash of the mainplane, and you state particularly at full flap. Interestingly, the DC-10/MD-11 seems to have the highest nose attitude (although it's AOA is probably no different from Boeings etc, unless MD knew something that the rest didn't) of any airliner on approach, so perhaps it generates more turbulant flow behind the wing/flaps and hence the significant dihederal of the tailplane. So what about the L-1011?

[Milt]

Anhedral of F4 Phantom "Horizontal Stabilizer"

A bit off the theme but interesting anyway

The original design of the F4 tail had the tail plane either straight or with a little dihedral. Full back stick would not rotate aircraft during initial high speed runs. Tail planes were then drooped to that terrible looking angle to correct situation leading to successful rotation for take off.

Does anyone have a photo of the original design?

[John Farley]

I don't know for sure but it seems likely to me that the 1011 tailplane might have been better a little higher. But since that was not possible (given the centre engine duct requirement) they gave it a little dihedral to change its average height?

On your rear fuse stability point, I believe the rolling moment due to sideslip of the whole aeroplane is merely the sum of the various components that produce rolling forces when sideslip occurs - and the tailplane dihedral/anhedral is just one item in that list. Whether such roilling moments are generated up the front or down the back of the aeroplane (as it were) does not affect the end result.

[Compass Call]

With reference to the anhedral on the C-141 & C-5 wings. I believe that this is due to the weight of fuel and engines giving the anhedral effect when the aircraft are stationary. When they are flying, the C-141 in particular, the wings lift to a level position.
I might be wrong, please correct me, but I don't think that they actually have anhedral built-in.

CC

[Mad (Flt) Scientist]

Compass Call:
Looking at the first two random pictures of a C-5 and a C-141 (the latter on takeoff) there's clearly still some anhedral in both cases.

Although it is true that variation of the dihedral due to loading of the wing is very significant, to the extent that wind tunnel models will incorporate a nominal dihedral for a specific flight case, and I've seen wind tunnel models with multiple different versions of the same wing, to account for wing deflection under different load conditions.

The anhedral on an aircraft with a high wing - like the two transports you mention - is there in order to counteract the natural dihedral effect of a "level" wing when mounted at the top of the fuselage. In initial design there is a desirable range of "dihedral effect" - rolling moment of the aircraft in sideslip - and the aerodynamic designers will target a specific anhedral/dihedral angle, in combination with other factors like wing location, vertical tail size, etc., in order to achieve the overall desired effect. Classically one has a small amount of geometric dihedral on a low wing and a more significant amount for a high wing aircraft.

One example of the use of tailplane dihedral - to return to the original topic - is the HS Hawk, which had the dihedral introduced for much the reasons described by John Farley i.e. to adjust the position of the tail in the downwash field, for reasons of longitudinal stability.

On the aircraft I now work with, which are almost all T-tail transports, we have a small amount of anhedral on the tail; one reason for doing this can be to (slightly) reduce the torsional loads in sideslip on the fin, as the upwind side of the tail lifts slightly less as a consequence of the anhedral.

[TURIN]

Just reading "John 'Cat's Eyes' Cunningham" THE AVIATION LEGEND, by John Golley.

In a chapter which talks about correcting problems with the Beaufighter, .......but later they increased the dihedral on the tailplane to improve longitudenal stability."

Haven't got much further in the book so for all I know they may change it back again!!

[cwatters]

Could it be simply that it adds to the vertical area of the fin. eg without it the fin would have to be bigger?

[John Farley]

cwatters

Tailplane dihedral/anhedral is something that affects the aerodynamics in all three axes.

It is likely that when a change is made during flight development (the Beaufighter case quoted) the designer had a particular problem in one axis that he needed to fix and accepted that he would have to live with the side effects of the changes that would result in the other two (in the Beaufighter case this would be an increase in directional stability - that you mentioned - plus an increase in lateral stability and a change in the dutch roll behaviour)

[Milt]

TURIN

Your mention of the Beaufighter tail prompted me to question my neighbour who flew both the straight tailplane Beau and those with dihedral on very dicey ops WW2.

The no dihedral model suffered stick fixed and stick free long period phugoids, of about 15 seconds, on the cruise which he found to very disconcerting. Dihedral of the tail corrected the problem.

Hmmm. Interesting!!