B737-800 with Flap 40: what causes increased roll sensitivity?
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If you were a weight lifter you would be far more stable if you held the weights with the arms, above your head, as far apart as possible. If the arms are brought closer together it becomes harder to stop the weights rocking. when flap 40 is selected the centre of lift moves along the wing towards the fuselage, therefore it becomes move sensitive to lateral displacement.
There are notes in Airbus manuals advising using Flap 3, instead of Full flap, in turbulent cross winds.
There are notes in Airbus manuals advising using Flap 3, instead of Full flap, in turbulent cross winds.
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Batman got it wrong. The 'arms' that control the thing are the ailerons. The spanwise mass distribution doesn't change when extending the flaps. The spanwise lift distribution may change, but doesn't affect roll sensitivity.
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Aerodynamic 'twist' moves the aileron neutral point 1.5 degrees to one wing(we'll say it's the right wing). If the roll spoilers would typically come up at 4 degrees it's now shifted. Neutral roll is now 1.5 degrees to the right so a roll input of 2.5 degrees to the right starts the right roll spoilers to deploy. A roll input to the left doesn't deploy the roll spoilers until 5.5 degrees(4 plus 1.5).
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roll input of 2.5 degrees to the right starts the right roll spoilers to deploy. A roll input to the left doesn't deploy the roll spoilers until 5.5 degrees(4 plus 1.5).
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That, along with other effects of flaps, does increase roll sensitivity.
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LSM,
I know nothing of stability, just find these explanations unconvincing. I take it you are talking of lateral-directional stability, i.e. the tendency to raise the low wing in a sideslip?
I know nothing of stability, just find these explanations unconvincing. I take it you are talking of lateral-directional stability, i.e. the tendency to raise the low wing in a sideslip?
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Another "interesting" ( 738W & non W) phenomenon which I found slightly unsettling the 1st few times I experienced it, is a strange buffeting (almost like pre-stall buffet) whilst rolling wings level at low altitude (say 300' on a circling approach) with F40 & approach power setting.
Noticed this often enough, & on several different aircraft, to put it down to imagination/individual rigging. . . . . and yes , I was keeping an eye on my IAS
Noticed this often enough, & on several different aircraft, to put it down to imagination/individual rigging. . . . . and yes , I was keeping an eye on my IAS
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As flaps are inboard any change in lift due to side slip is created further inboard and the resultant moment is a lot less. This reduces dihedral effect and lateral stability so it is easier to create a larger rolling moment, or displacement from the aircraft's flight path.
Of course, this doesn't only happen with Flap 40 but the effect is, apparently, more noticeable.
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Having just begun as a 73 Capt; would like to know whats the big deal..
All Ears!!!
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I think, as often, it's a combination of things. LSM and Batman are both right, more flap makes the effective part of the wing shorter, and with less dihedral (and with a more aft c of g, although not sure if that's an issue). The tip washout is increased, so here's my 2 cents: the ailerons have reduced effectiveness because the down-going aileron is not so much in the high pressure lower surface air than would be the case with less flap. (I may be wrong here but stay with me). So... to get the same roll authority, the spoilers have to do more, hence the increased sensitivity.
You can tell it's a wet and windy Sunday....
You can tell it's a wet and windy Sunday....
Last edited by 16024; 20th Oct 2013 at 14:58.
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full flaps approach to land
I believe use of full flaps or avoidance of use due to wind gusts and turbulence is not limited to 737 or Airbus..going back to basics...even on a Piper Tomahawk and Cessna 150 the same rule of thumb applies...unless ofcourse a shortfield dictates full flaps..be they Flaps 40 or whatever config by design. On ATR42-300 series Flaps 45 was normally prohibited- only used strictly for Emergency (it was gate protected!).
