Overbanking tendency
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Hi Wiggy, I agree with you. Just to end the question, I would paraphrase freely Sikorsky's famous bumblebee quote: we, pilots, turn the aircraft without caring or knowing how lift behaves on the wings.
The difference in speed between the inner and outer wings will disappoint. In a 40' span aircraft at 80kts in a Rate 1 turn the outer is (averagely) going 0.62kts faster than the inner. Roughly 0.8% difference in lift.
It is actually a bit over 1.6% - lift is the speed squared, but it doesn't do much difference in your example. In a glider with 50' - 80' span and a speed of 35 - 40 kts it is however very real, but again, it isn't a problem, you see the bank increase and the yaw string go sideways, and correct with a bit of rudder and opposite stick.
Another reason to have pilots begin on gliders before moving up the magenta line.
Called spiral instability which most aircraft suffer from except paragliders at relatively low bank angles..above roughly 40 degrees they go into a locked in spiral with nasty outcomes.
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It's a curious mix of the original poster's "outer wing faster", vs. the dihedral which will tend to return things to wings level. Other things affecting it are the size of the fin and directional stability and even the wetted area above and below the CofG.
Last edited by Deskscribbler; 26th Oct 2021 at 20:11.
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The difference in wing speeds, as a percentage of the turn radius becomes larger the more steeply you're banked (and the turn becomes smaller.) But this reverses past a certain point, where at very high banks, the wings are nearly on top of each other, and the relative difference shrinks again.
This is only one aerodynamic effect, and there are other minor ones that, as people above have said, there's no pressing need to understand, and can be simply counteracted by use of the controls while maintaining the desired bank, and essentially treated as turbulence or other aberrant minor upsets. For example, is the long-tail effect, where even though the CG of the plane (and the ball, or IRS) is perfectly balanced on the path of the turn circle, the vertical stabilizer is flying outside the circle and sees a relative wind coming from the outside, and therefore produces a yaw (and roll) toward the outside.
Dihedral, by itself, has nothing to do with this though. There is no physical process by which the projected wing area over the ground, effects the amount of lift. Dihedral effect is a response to sideslip and not bank, and will therefore only act on the plane if sideslip occurs.
This is only one aerodynamic effect, and there are other minor ones that, as people above have said, there's no pressing need to understand, and can be simply counteracted by use of the controls while maintaining the desired bank, and essentially treated as turbulence or other aberrant minor upsets. For example, is the long-tail effect, where even though the CG of the plane (and the ball, or IRS) is perfectly balanced on the path of the turn circle, the vertical stabilizer is flying outside the circle and sees a relative wind coming from the outside, and therefore produces a yaw (and roll) toward the outside.
Dihedral, by itself, has nothing to do with this though. There is no physical process by which the projected wing area over the ground, effects the amount of lift. Dihedral effect is a response to sideslip and not bank, and will therefore only act on the plane if sideslip occurs.
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Sideslip does build during a spiral dive though? The spiral tendency being partly a balance between directional stability reducing sideslip and the dihedral trying to use said sideslip to roll more wings level? So an aircraft with more directional stability will have a greater spiral divergence tendency (all others equal), and an aircraft with more dihedral less of a spiral divergence tendency (all others equal)? Worst being an aircraft with strong directional stability (big fin) and lesser dihedral?
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Sideslip may or may not develop, that's a function of a host of external factors, as well as the rudder response (or non-response, or over-response) to them. Any effect that the dihedral has is a result of this sideslip, and not of the bank angle. If you're over-ruddering the turn (ball to the outside) then dihedral will tend to increase the bank, not return it to level.
Anyway, there's a common misconception of dihedral's function (It's looking now, that that's not what you meant, but the poster a few replies from the top certainly did with "The inner wing is flatter to the airflow") that says that the inner wing's bigger projected area over the ground, somehow translates to an aerodynamic significance. And this is not true: you can have a plane with (ridiculously exaggerated for effect) 45 degree dihedral at 45 degrees of bank, so the inner wing projects 100% of its area over the ground, while the outer wing projects 0%; and as long as there is no sideslip (ball is in the middle, airflow is straight down the fuselage) this fact has zero bearing on any force on the airplane.
Anyway, there's a common misconception of dihedral's function (It's looking now, that that's not what you meant, but the poster a few replies from the top certainly did with "The inner wing is flatter to the airflow") that says that the inner wing's bigger projected area over the ground, somehow translates to an aerodynamic significance. And this is not true: you can have a plane with (ridiculously exaggerated for effect) 45 degree dihedral at 45 degrees of bank, so the inner wing projects 100% of its area over the ground, while the outer wing projects 0%; and as long as there is no sideslip (ball is in the middle, airflow is straight down the fuselage) this fact has zero bearing on any force on the airplane.
All aircraft are different.
Fly the output (ie make it do what you want).
Some types need opposite aileron, others don’t. Except in a stall turn, then they all do.
(standing by for incoming………)
Fly the output (ie make it do what you want).
Some types need opposite aileron, others don’t. Except in a stall turn, then they all do.
(standing by for incoming………)
All aircraft are different.
Fly the output (ie make it do what you want).
Fly the output (ie make it do what you want).
Not to put a dampener on the discussion however - it's all been most interesting - and informative. May it continue please ...
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I definitely have seen this tendency when doing a steep turn. I don’t want to say that all light aircraft have this characteristic, but I have seen it.
I happened to read the other day that had a description about this phenomenon.....
Proficiency: Take it to the bank - AOPA
"Overbanking tendency. In a coordinated turn, the aircraft moves along a cone so that, in any given amount of time, the outer wing moves faster through the air than the inside wing. The differential velocity means that the outer wing, with more lift than the inside wing, tends to roll the aircraft more deeply into the turn—so it is necessary to hold aileron against the turn. The wider the wingspan, the more noticeable the overbanking tendency becomes, so glider pilots know it well."
There is a bit of a diagram in the article for explanation.
I happened to read the other day that had a description about this phenomenon.....
Proficiency: Take it to the bank - AOPA
"Overbanking tendency. In a coordinated turn, the aircraft moves along a cone so that, in any given amount of time, the outer wing moves faster through the air than the inside wing. The differential velocity means that the outer wing, with more lift than the inside wing, tends to roll the aircraft more deeply into the turn—so it is necessary to hold aileron against the turn. The wider the wingspan, the more noticeable the overbanking tendency becomes, so glider pilots know it well."
There is a bit of a diagram in the article for explanation.
Last edited by punkalouver; 27th Oct 2021 at 04:55.
I can hold a 45deg banked turn in my glider at ~55kts, and due to the span the outer tip is going some 33% faster than the inner one. It has always amazed me that there is very little, if any overbanking tendency; I suspect there are many other things at play, not limited to wing flex, fin/fuselage effects, rudder deflection, etc.
I agree with Locked Door about flying the output - apply control inputs to achieve your desired attitude...
I agree with Locked Door about flying the output - apply control inputs to achieve your desired attitude...