Anhedral on low-wing aircraft

[DVR6K]

Hi all,

A question arose the other day that I have been trying to figure out. Why I didn't just come here in the first place I don't know.

If you look at some of the russian-made aircraft (picking Tupolevs for this example), why do they have a degree of anhedral if they are low wing aircraft? Such as in this pic (http://www.airliners.net/open.file/780067/L/) .

If a low wing is de-stabilising, why would you add anhedral to de-stabilise it further?? My only thought is that the wing is fairly well swept back (see here (http://www.airliners.net/open.file/768465/L/ )) and as a result, the sweep (stabilising) overcompensates for the destablising low wing, so anhedral is needed so as not to change the sweep and keep Mcrit as high as possible. Just wondering what my fellow Ppruners think as I may or may not be beating up the wrong track.

Cheers!

[FE Hoppy]

You got it in one.
The anhedral is to compensate for the wing sweep.

[Rainboe]

Can it be that because these 'primitive' types use manual and not powered controls, then you must get rid of as much stability as possible to avoid the pilots looking like Schwartzenneggegger (or something like that)?

[BOAC]

How else can they change the nav lights?:D :D

[Milt]

Your Russian type should be close to nil an/dihedral in flight depending on how bendy the wings are.

The wings of the A380 Airbus will bend up to 6 metres at the tips at high g loadings. So from droop on the ground to bend up at AUW on take off may well startle many of the pax and the pilots if they were able to see them!!

[john_tullamarine]

.. which is why the B52 has kiddie training wheels (http://www.dfrc.nasa.gov/Gallery/Photo/B-52/Large/EC03-0258-12.jpg) to stop the wingtips getting taxy rash ....

Top picture of the bird ...

[DVR6K]

Cheers all, very interesting indeed!

[Tallbloke]

I thought the B-52 had outrigger wheels because of the adoption of a tandem undercarriage? The short legs are very light in comparison to a conventional layout but the payoff was the need for stabilisers to stop the thing blowing over.

[John Farley]

Rainboe

I don't feel it follws that a 'very stable aircraft' will necessarily have high control forces.

Such an aircraft will certainly need large control surfaces and or control deflections to give it nippy handling but even with large manually operated surfaces one can arrange servo tabs/spring tabs/balance tabs/fiddle with hinge lines etc to keep them from needing large forces to displace them.

[spocla]

Anhedral is there to counter the tendency to roll due to sideslip (Lv, Dihedral effect or Cnbeta depending on what Test Pilot School you went to!). This is important when one kicks off the drift in a crosswind landing. Wing Sweep is an important factor in increasing roll due to sideslip. So - if one doesn't or can't (due to control snags perhaps) easily keep wings level when the drift is eliminated and turned into beta, then best you reduce roll due to sideslip-{EG Anhedral on a Harrier}. Incidently the fact that this effect could not be properly overcome is the reason why the B52 gear swivelled to allow a landing in the "crab" and that one was told not to kick of drift in a Harrier crosswind nozzle or rolling vertical landing.

[Rainboe]

John F.Such an aircraft will certainly need large control surfaces and or control deflections to give it nippy handling but even with large manually operated surfaces one can arrange servo tabs/spring tabs/balance tabs/fiddle with hinge lines etc to keep them from needing large forces to displace them.

The way I see it- for a large heavy jet to have manual control forces with probably no powered back-up, and to operate over the high range of fast jet speeds down to approach speed, you would be setting the designer an almost impossible task without significant anhedral to balance the aircraft almost to the edge of negative stability. I'm sure the Soviets pull all the tricks they can with aerodynamic refinements, but the evidence is there in the anhedral.

[spocla]

What do we mean by "Roll Stability". I hold that there is no such thing-in the sense that there is static and dynamic pitch stability. Resistance to roll is a function of roll damping (-1/Lp). But any stability (in that I mean a tendency to revert to the original state once disturbed) is provided by the roll mode. Who would want an aircraft that when rolled -and the the control centred- then the bank reverted to zero?. Seems like a Helo autostab-but thats different. Many aircraft have a neutral or divergent roll mode- but are not considered "unstable". I've flown quite a few of them-F4, F3, Hunter, -how about the change in sign of the Lv function in the Hawk with alpha?! Do we consider the Hawk to be unstable?

[John Farley]

Rainboe

I was speaking generally as I thought you were suggesting that high stability would necessarily lead to high control forces.

The only true connection between aircraftr stability and control forces is in the 'out of trim dives' test of non powered control aircraft - popular in WWII days.

[blackmail]

hello every one,

one must remember the era when those soviet aircraft were designed & built: we were still in the middle of the "cold war" & all civil aircraft were supposed to be able to be converted at short notice into military/bomber aircraft, were maneuvrability was more important than stability, hence the anedral to offset the inherent stability feature of a swept wing.

nothing to do with stability/maneuvrability, but many soviet aircraft had glassy noses for the bomber/navigator crewmember.

turning soviet pilots into "arnold schwarzenegger" type individuals, was probably never a consideration for the antonov & iljoeshin design bureaus.

[LOMCEVAK]

Spocla,

I think that you mean "spiral mode" rather than "roll mode". What did they teach you at these foreign test pilot schools! And the Hawk rolling opposite to rudder is another thread on its own.

Rgds

[spocla]

Mate:
You're right I did mean spiral mode-but I heard you first time!

[John Farley]

Spocia

But any stability (in that I mean a tendency to revert to the original state once disturbed) is provided by the roll (spiral) mode.

An interesting comment with which many would agree….

BUT

I know an aeroplane that in some parts of the flight envelope is statically unstable in roll (as demonstrated by it needs more roll control deflection to stop it continuing to diverge in roll at ten degrees of bank than it does at five).

How does that fit in with your thoughts?

JF

[lhr_slots]

I'm a bit rusty here, but mightn't the anhedral be to increase Dutch Roll stability, rather than to reduce stability/control forces, particularly if there wasn't an active yaw damper on these Soviet models?

[BEagle]

lhr_slots has the answer, in my view.

Negative dihedral reduces the excessive roll stability ('dihedral effect') with sideslip of a swept wing. Probably not pukka TP terms, but basically it means that 'normal' resistance to Dutch Roll is retained without the need for complex yaw dampers.

[spocla]

Sorry, can't agree,
Dutch Roll is a mode of motion rather than a stability and is a product of directional stability (the "spring") and roll damping (the "damper") where a sideslip excursion is countered by the directional stability and the ac overswings. Each sideslip excursion translates into a roll IF the ac has strong dihedral effect (eg Harrier 1 at approach speeds) if NOT then the ac will snake rather than roll-this is still Dutch Roll (eg Harrier 1 at 500 kts-DE being dependent on alpha among other things) The magnitude and frequency of the dutch roll motion depends on the directional stability and roll dampling. Dutch roll can be divergent, convergent or neutral. Dihedral Effect characterises it only. The best way to stop Dutch Roll is to kill the sideslip. Anhedral helps reduce to roll due to sideslip-a good this for xwind t/o and landing.
JF-Intresting comment-any more details?-is this simply a divergent spiral mode or something more complex?

[LOMCEVAK]

Spocla,

Sorry for the repetition - problem with the "net nanny" making me believe that the message had not gone! I agree with what you say on the Dutch roll. It is an oscillatory dynamic stability mode that is damped by Nr (not rotor RPM but yawing moment due to yaw rate!), hence why a yaw damper feeds in rudder to oppose yaw rate. Roll damping (Lp) has a much smaller effect on Dutch roll damping.

JF,

This sounds like a divergent spiral mode which, as it is an aperiodic mode, is defined by the time to double amplitude. Therefore, the greater the bank angle, the greater will be the open loop roll rate and thus closed loop more aileron will be needed to counter it at 10 deg AoB than at 5 deg. Which aircraft did you mean? We are all now intrigued!

[spocla]

Fair call, what geeks we are!!

[John Farley]

L

Any version of Harrier 1 in a hover with a wheel height of less that 7 ft. Greater suck down under the lower wing. So called UDR on VTO - remember those?

J

[lhr_slots]

spocla,

Dutch Roll is a mode of motion rather than a stability...

Sorry, I would have said that the anhedral increases the stability of the Dutch Roll mode, instead of saying that it increased Dutch Roll stability, if I'd known we were being pedantic.

I still submit that the purpose of anhedral is to prevent unstable Dutch Roll - the other solution is to increase fin volume for greater directional stability but at the cost of weight and drag, or to reduce sweep (but that is set by mach requirements).

[LOMCEVAK]

John,

Thanks. This is a very interesting aspect which again needs a thread of its own - STOVL aerodynamic derivatives are a very different field!

1hr slots,

The Dutch roll is a dynamic stability root of the lateral-directional equation of motion. It is an oscillatory motion which, therefore, can be defined by a frequency and a damping ratio. If it has greater than critical damping, it will appear as an aperiodic convergent motion, but can still be defined by the same parameters. So, you must define what you mean by "Dutch roll stability". I suspect that you mean "damping" in terms of either the number of cycles or time to damp. If so, an increase in directional static stability actually reduces the Dutch roll damping ratio (whilst increasing frequency and thus giving a similar time to damp). However, as the Dutch roll is triggered by sideslip, an increase in directional static stability will suppress sideslip development and may prevent the Dutch roll being excited although the actual damping ratio of the open loop response will have been reduced. The effect of anhedral (which reduces lateral static stability or increases lateral static instability) on the Dutch roll frequency and damping is not that great. The largest effect that it has is on the roll:yaw ratio and the phase relationship between roll rate and yaw rate. Strong lateral stability produces a high roll:yaw ratio (wing rock), weak lateral stability produces a low roll:yaw ratio (snaking). Lateral static instability may produce an out of phase motion (rolling left whilst yawing right). Adding anhedral to a wing would not be a normal way for a designer to improve Dutch roll damping! Overall, always be careful to define what you mean by "stable/unstable"; they are ambiguous words.

[spocla]

L'K
Most erudite, i think that point has been expressed here a few times but never quite so well

[lhr_slots]

Lomcevak/spocla

I don't want to interfere in your mutual appreciation society. While I find your descriptions and lessons on the precise use of language interesting, when it comes to stability derivatives I think it's easiest to just calculate the eigenvalues!

However, going back to the original question of why a low wing Tupolev has anhedral, I proposed my Dutch Roll hypothesis as an alternative to Rainboe's control force argument, Milt's wing flex idea (ie, that when the wing flexed up in flight it wouldn't really have anhedral) and Spocla's cross-wind landing issue.

My vote remains with Dutch Roll.

Lomcevak - by the way, I didn't say anhedral was used to improve Dutch Roll damping. It reduces the yaw-roll coupling.

Also, see this about SpaceShipOne...



Shane said all the pilots had to deal with the Dutch roll, noting it was the price of not having a yaw damper for simplicity. Tighe said the high-wing configuration naturally increases roll-yaw coupling dihedral effect, which together with weak damping from the short span leads to the problem. He noted many airplanes with swept high wings have anhedral to reduce the coupling, but this was not possible in SpaceShipOne because of the need for a straight hingeline across the span for the feather mechanism. He awaits flight test to see how the craft really flies. It turned out there was a data error in the simulation that caused a bobble while decelerating through Mach 1.

If Dutch roll is a serious problem, one solution could be to add anhedral wingtip extensions. A yaw damper is also a possibility. Tighe noted that the oscillation is just slow enough that the pilot with practice can reduce it, but he would be reluctant to try this in flight for fear of getting out of phase. The Dutch roll parameters in the simulator were estimated by computational fluid dynamics and one hopes that actual behavior does not go beyond nauseating to hazardous.

[sixfeetabove]

@FE HOPPY

...but why don´t have all low wing aircraft an anhedral wing then but only the Russian TUs and the Falcons?

[FE Hoppy]

As has been mentioned in the thread. There is more than one way to skin a cat. Fin/rudder volume, yaw dampers "my dampers", less sweep. In this case anhedral is the choice. I'm happy to be corrected if any TU/Falcon designers wish to spill the beans.