Anhedral on low-wing aircraft
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!
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!
Do a Hover - it avoids G
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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
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
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spocla,
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).
Dutch Roll is a mode of motion rather than a stability...
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).
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.
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.
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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...
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.
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.
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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.
