Forces and torques on aeroplane wings

1. Aug 18, 2011

ILikeScience!

Hello,

I have been watching a lot of Aircrash Investigations (also called Mayday) and it has gotten me interested in aerodynamics.

I'm curious about what force would be needed to tip a plane over? Say that there was a sudden upwards gush of wind on the end of one wing (I am familiar with the concepts of torque) then the force on each wing would be out of balance and, if strong enough, could roll the plane over.

I'm really curious as to if this is at all possible or would the necessary force from the wind just be ridiculously high. I'm not sure how to calculate it though.

Google says that a boeing 747 weighs about 396,890 kg at max takeoff weight (so roughly 400,000 kg So that means gravity is pushing down with roughly 4,000,000 N. The plane is then generating the same amount of force but in the upwards direction for level flight. If the wings both generate all of the left (I know this isn't true but they generate most of it yeah?) then that's 2,000,000 N upward on each wing.

That gives me some ballpark figures. I'm curious as to how much of a difference in force would be required to tip the plane over.

2. Aug 18, 2011

ILikeScience!

Been doing some searching and the term "maximum roll moment" comes up a lot but I need to get some example values!

3. Aug 18, 2011

cjl

That kind of situation isn't really reasonable. It would require an enormous vertical wind shear at a very precisely located (and inopportune) position relative to the aircraft. What kind of tipping incidents were you thinking of? There have been situations where aircraft rolled because of asymmetric stalls, but that's very different (and much more reasonable, honestly).

4. Aug 18, 2011

ILikeScience!

Well I was reading that roll stability is weaker than yaw and pitch stability, in the simplest of terms, so I thought a really powerful gust of wind might do the trick. Even if it doesn't, I would be curious to still calculate how unrealistic and unreasonable it is. And yes I realise that it would have to be VERY inoppertune and located right on the end of the wing to produce maximum torque.

Do you have any rough idea about these so called maximum roll moments? I'm assuming that any torques greater will cause the roll to be unstable. And please consider that I'm not a graduate/master's in aerospace engineering :p

5. Aug 18, 2011

ILikeScience!

Or even without any outside force like a gush of wind, I'm curious as to what is the max torque a plane creates via the ailerons in order to have a safe roll.

6. Aug 18, 2011

rcgldr

There have been a few aircraft crashes due to flying into wake turbulence (wing tip vortices) left by large commercial jets. In some cases the aircraft were rolled over 180 degrees or more.

7. Aug 18, 2011

pantaz

8. Aug 18, 2011

This.

The problem isn't just that the wind would have to be a large gust, it is that it would have to affect only one wing. This is just not very feasible in the real world. In the atmosphere, currents occur on much larger spatial scales than that. The places where that is somewhat feasible (such as a massive thunderstorm) are easily avoidable and it still isn't very likely to occur on those small scales.

9. Aug 18, 2011

rcgldr

I doubt there could be enough turbulence to ever roll a large commercial aircraft over, but those large aircraft leave a wing tip vortice (wake turbulence) like a horizontal tornado, that is strong enough to flip over a smaller aircraft. Wiki article:

http://en.wikipedia.org/wiki/Wake_turbulence

10. Aug 18, 2011

cjl

Wake turbulence is a good example of a roll disturbance (although as rcgldr said, it would mainly be a concern for smaller aircraft). I hadn't considered that with my initial response, and it's pretty much the only scenario to my knowledge where an aerodynamic asymmetry in the atmosphere could cause a roll on an aircraft.

Also, keep in mind that when aerodynamicists talk about an airplane's roll stability, they're talking about passive roll stability. Aileron torque doesn't enter into it - roll stability discusses how the airplane would respond if it was rolled to some angle and then left alone with no control inputs. A stable airplane will return to level (due to the roll torque caused by sideslip), but in the case of roll, this response is quite slow. If the pilot corrects for the roll disturbance with the ailerons, the response is much faster, and the maximum roll rate is usually fairly fast (determined by what the aircraft can structurally and aerodynamically handle, as well as what the passengers can handle in the case of a commercial aircraft).

11. Aug 19, 2011

Delta Kilo

This usually happens (in those very rare cases when it does happen) if the plane is flying very close to the stall speed (very slow and in nose-up attitude). in this case a sudden asymmetric gust of wind might cause one wing to enter a stall and lose lift causing the plane to flip. Of course the plane should never fly that close to the stall speed in the first place. Airspeed and attitude are THE most important parameters, this gets bashed into every pilot's head from the very first lesson. There are also protections in the autopilot as well as stall warning horns and stick shakers to alert the pilot.

The situation is more likely to happen in a turn because first, stall speed is higher in a turn due to higher g-loading, and second, asymmetric forces and the position of the ailerons causes one wing to loose lift before the other.

DK

Last edited by a moderator: Apr 26, 2017
12. Aug 19, 2011

cjl

As far as I'm concerned, this is wrong. One (and only one) parameter determines stall performance: angle of attack. Regardless of airspeed, altitude, and maneuvers, AoA will tell you exactly how close to stall you are, in a much more direct fashion than airspeed (or any other parameter). Unfortunately, most light aircraft do not have AoA indicators, which is the main reason why I think pilots are taught to rely on airspeed.

13. Aug 19, 2011

Phrak

I've have plenty of times an elephant has come along to stand on a wing tip during close-in ridge lift soaring when thermals start breaking through.

I think you might look at cases where a pilot decides not to fly around a potential thunder head, but though the volume.

14. Aug 19, 2011

rcgldr

An example of this is a snap roll, which is performed using a lot of pitch (elevator) and yaw (rudder) input, but without any roll (aileron) input to cause one wing to stall before the other, resulting in a very fast roll rate. Some radio control aircraft can snap roll with just a lot of pitch input, without any yaw or roll input. In radio control pylon racing contests, sometimes the models will snap roll in very high g turns, with a 50% - 50% chance of going up or smashing into the ground.