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 .

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) 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!

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

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)?

How else can they change the nav lights?

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!!

.. which is why the B52 has kiddie training wheels to stop the wingtips getting taxy rash ....

Top picture of the bird ...

Cheers all, very interesting indeed!

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.

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.

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.

John F.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.

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?

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.

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.

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

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

Spocia

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

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?

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.

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?