Lift question
Lift question
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From: Escapee from Ultima Thule
Ignoring accelerated flight, then I'd say S&L when done very soon after take-off.
It would depend on time spent climbing & climb gradient achieved.
My reasoning is that L<W in a climb or descent but L=W in S&L flight.
Maximum weight is at T/O but that's not necessarily when max lift occurs since the vertical component of T is partially supporting the a/c.
Next highest weight is S&L sometime after take-off. The shorter the period climbing the higher weight during the subsequent S&L so the greater the L being produced.
It would depend on time spent climbing & climb gradient achieved.
My reasoning is that L<W in a climb or descent but L=W in S&L flight.
Maximum weight is at T/O but that's not necessarily when max lift occurs since the vertical component of T is partially supporting the a/c.
Next highest weight is S&L sometime after take-off. The shorter the period climbing the higher weight during the subsequent S&L so the greater the L being produced.
Last edited by Tinstaafl; 4th April 2002 at 17:03.
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From: oxford
If you want a really informed and expert academic opinion on your question, I'd say post it <a href="http://www.oxfordaviation.net/forum/forum.asp?FORUM_ID=15" target="_blank">here</a> .. .. .You'll get the opinion of one of the ground school experts from one of the top schools in the country - or, indeed, the world.. .. .Give it a go and see.
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From: Scandiland
How about in a power off stall with all high lift devices deployed fully?. .If you consider situations where ground effect is included I would say flare at max landing mass as there is no thrust vector to give a helping hand.. ./lns
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From: Escapee from Ultima Thule
I think a distinction has to be made between accelerated & unaccelerated flight.
Accel. flight lets you bump up the quantity by whatever load factor you apply.
Unaccel. flight resolves about balanced forces ie a steady state.
If the flare - one example of accel. flight - is allowed then I'll bid for a simple, high 'g' manouevre eg high pitch rate or the steepest turn the a/c'd Thrust will allow.
Accel. flight lets you bump up the quantity by whatever load factor you apply.
Unaccel. flight resolves about balanced forces ie a steady state.
If the flare - one example of accel. flight - is allowed then I'll bid for a simple, high 'g' manouevre eg high pitch rate or the steepest turn the a/c'd Thrust will allow.
Last edited by Tinstaafl; 9th April 2002 at 22:23.
Why do it if it's not fun?
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From: Bournemouth
I think a distinction needs to be made between lift, and coefficient of lift.. .. .Lift is greatest just after take-off. The main purpose of lift is to counteract weight, and weight is greatest just after take-off when there's maximum fuel. Any extra lift over and above the weight causes the aircraft to climb, which is exactly what it does just after take-off.. .. .Coefficient of lift is greatest at the point of the stall - this is a basic property of aerofoils. With regard to what phase of flight this relates to, I'd guess (although I'm not certain) that it would be during the flare, just prior to landing.. .. .FFF. .----------
Why do it if it's not fun?
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From: Bournemouth
Ok, I admit it, I've completely confused myself with the crap I was talking earlier. Can someone please tell me where I'm going wrong???. .. .I'm flying along, straight and level. Lift=weight, and thrust=drag, so according to Newton's 1st law, I'm not accelerating in any direction, which is great.. .. .Now, I add power (or thrust), and wait for everything to settle down. I'm now in a steady climb, at (approximately) the same speed as before - not accelerating in any direction.. .. .According to Newton, since I'm still not accelerating, lift=weight and thrust=drag (at least approximately - this won't be exact because the lift, thrust and drag will be at a small angle). But I know that thrust has increased (I increased it!), so that means that drag must have been increased too, right?. .. .But drag is 1/2*rho*V-squared*Coefficient-of-Drag. Rho hasn't changed. V-squared hasn't changed (very much). I haven't re-trimmed, so I'm still at the same angle-of-attack, so the coefficient of drag is the same. So it seems like the drag hasn't increased.. .. .But in this case, the forces wouldn't balance out, and I'd be accelerating.. .. .What have I got wrong here???. .. .I used to know this, but I've obviously forgotten! <img border="0" title="" alt="[Big Grin]" src="biggrin.gif" /> . .. .Thanks. .. .FFF. .---------
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From: Europe
@FFF:. .. .1st Error:. .“...I'm flying along, straight and level. Lift=weight, and thrust=drag...”. .This is a widespread myth. Because of wing & fuselage moments plus CG location effects, a downward elevator-force is required to achieve some kind of stability. Thus lift in s&l unaccelerated flight is always greater than weight.. .. .We have to differentiate quite precisely between the force created by the wing (lift?) and all the other forces acting upward and/or perpendicular to flight path, the sum of which support the vertical component of the total flight vector.. .. .Furthermore, it is this “small angle”, that makes a whole lot of difference in climbs and descents.. .. .To answer your confusion: The only thing you forgot in your calculation ref. drag is weight, which, in a climb, acts partially rearward, thus increasing the total force acting “against” the flight path.. .. .To be precise, during power-off descent, wing-generated lift is greater than during climb.... .. .happy landings
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From: Spanish Riviera
FFF, if you study Newton too much I think you will find that you are always accelerating. Weight is a factor of mass and gravity with gravity being measured as a rate of change of velocity (acceleration). Therefore, to stay at the same altitude, you must have a constant acceleration towords the Earth's centre. Consequently, if you are in a steady climb or descent, your acceleration towards the Earth's centre will be difficult. Confused? . .. .My vote is with the simple explanation whereupon, unless in S&L flight, the forces of thrust & drag will always contribute towards 'lift' and 'weight'.. . . . <small>[ 26 March 2002, 20:14: Message edited by: Whipping Boy's SATCO ]</small>
Why do it if it's not fun?
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From: Bournemouth
Thanks dolly - spot on. In case anyone else is interested, I also found an excellent diagram of this in Trevor Thom, volume 4, in the chapter on climbs.. .. .I want to apologise to everyone for the huge amount of drivel I've been spouting on this thread. One of the great things about PPRuNe is that you can talk about any subject you like, safe in the knowledge that if you talk crap someone will pick you up on it before everyone else starts believe it! <img border="0" title="" alt="[Eek!]" src="eek.gif" /> I'll try to restrict myself to commenting on things that I know something about in future... but no guarantees! <img border="0" title="" alt="[Big Grin]" src="biggrin.gif" /> <img border="0" title="" alt="[Big Grin]" src="biggrin.gif" /> <img border="0" title="" alt="[Big Grin]" src="biggrin.gif" /> . .. .FFF
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From: see roster
I've found the answer. Thanks for all the entertaining replies everyone!
Looking back in my ATPL groundschool & CFS notes (and a littlle bird reminded me) I find that:
Rotation:
Due to body angle and wheel contact, (lift generation begins at nosewheel unstick), thrust assists lift here.
Climb:
As at unstick, there is always a component of thrust 'assisting' lift. Therefore lift cannot be greatest in the climb.
Descent:
There is always a component of gravity present, therefore lift cannot be at a max here either.
Manoeuvre:
There is always a component of thrust or gravity present here (eg a loop)
This leaves the cruise, assumed to be S&L.
In S&L flight, lift=weight. (as thrust=drag)
Weight in the cruise is always at a maximum at the beginning of this phase, due to fuel burn (assuming no Air-Air refueling)
Therefore lift is at a maximum at the instant you level off in the cruise.
Tinstaafl - you were spot on thanks
Looking back in my ATPL groundschool & CFS notes (and a littlle bird reminded me) I find that:
Rotation:
Due to body angle and wheel contact, (lift generation begins at nosewheel unstick), thrust assists lift here.
Climb:
As at unstick, there is always a component of thrust 'assisting' lift. Therefore lift cannot be greatest in the climb.
Descent:
There is always a component of gravity present, therefore lift cannot be at a max here either.
Manoeuvre:
There is always a component of thrust or gravity present here (eg a loop)
This leaves the cruise, assumed to be S&L.
In S&L flight, lift=weight. (as thrust=drag)
Weight in the cruise is always at a maximum at the beginning of this phase, due to fuel burn (assuming no Air-Air refueling)
Therefore lift is at a maximum at the instant you level off in the cruise.
Tinstaafl - you were spot on thanks
Last edited by overstress; 2nd April 2002 at 22:26.
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From: 75N 16E
lift is greatest at the wings max angle of attack just before the stall, regardless of speed, attitude, altitude, etc. You could be decending but at a high AoA therefore the wing is generating maximum lift possible. Equally you could be doing 150 kts, whack the stick back and stall the aircraft, in which case there is no lift being generated by the wing.........
Therefore it is 'most likely' that max lift will be generated at rotation (and subsequent climb out), and during the final flare, when the wings AoA will be at maximum (or should be in the ideal world).
Rgds
EA
Therefore it is 'most likely' that max lift will be generated at rotation (and subsequent climb out), and during the final flare, when the wings AoA will be at maximum (or should be in the ideal world).
Rgds
EA
Last edited by englishal; 3rd April 2002 at 11:31.