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Wing Loading Versus G Loading  
User currently offlinesmartt1982 From United Kingdom, joined Nov 2007, 225 posts, RR: 0
Posted (2 years 4 months 1 week 4 days 1 hour ago) and read 2454 times:

Am I right in thinking there is a difference between the two?

Recently I have been trying to study/brush up on the various systems of the 737.

In particular I have been looking into the minimum maneuver speed (.3G maneuver margin) on the speed tape which I have learned will move depending on where you tell the FMC the C of G is i.e. the more further act the C of G the lower the min maneuver speed becomes, would I be right in thinking that this would be changing the wing loading of the wing? More aft C of G means more overall lift as the tail plane is not trying to counter nose down tendency as much? So the min maneuver speed is effected by wing loading and NOT G-load? In the FCTM it says the minimum maneuver speed is NOT effected by G-load.

In relation to the min speed (red and black pole), I’ve read that this is affected by G Load, so if I start pulling some tight turns hence increasing the G Load, the min speed bar will come up a lot closer to my current speed, i.e. aircraft weight is artificially being increased and so I need more lift and speed to maintain it in a turn, Correct or not?

In relation to the speed brakes, its says

In relation to the MIN MANEUVER SPEED: reflects the effect of extending the flight spoilers. (Why is this, do and if so, why do flight spoilers/speedbrakes effect the min maneuver speed?)

In relation to the MIN SPEED: reflects the effect of extending the flight spoilers(why is this, g-load or wing loading?) and also reacts to wing loading (what does it mean by wing loading, G-load or weight of the aircraft?)

4 replies: All unread, jump to last
 
User currently offlinetdscanuck From Canada, joined Jan 2006, 12709 posts, RR: 79
Reply 1, posted (2 years 4 months 1 week 3 days 20 hours ago) and read 2387 times:

Quoting smartt1982 (Thread starter):
In particular I have been looking into the minimum maneuver speed (.3G maneuver margin) on the speed tape which I have learned will move depending on where you tell the FMC the C of G is i.e. the more further act the C of G the lower the min maneuver speed becomes, would I be right in thinking that this would be changing the wing loading of the wing?

Yes. The amount of lift the wing has to generate for level flight changes with CG.

Quoting smartt1982 (Thread starter):
More aft C of G means more overall lift as the tail plane is not trying to counter nose down tendency as much?

No. The sum of the wing and tail lift must equal the weight of the plane (for straight/level flight). Since the wing and tail lift are in opposite directions, the wing must support the weight of the airplane *plus* the tail load. The tail load, however, is determined by the stability requirement. As the CG moves back, the tail doesn't have to work as hard and doesn't need to generate as much lift.

Therefore, as the CG moves aft, the tail load goes down and the wing load goes down.

Quoting smartt1982 (Thread starter):
So the min maneuver speed is effected by wing loading and NOT G-load?

Yes. Since min maneuver speed is, by definition, 1.3 Vs1g, it doesn't change with actual G load. It does move with wing load because wing load alters AoA.

Quoting smartt1982 (Thread starter):
In the FCTM it says the minimum maneuver speed is NOT effected by G-load.

Yes, that's just how min maneuver speed is defined.

Quoting smartt1982 (Thread starter):
In relation to the min speed (red and black pole), I’ve read that this is affected by G Load, so if I start pulling some tight turns hence increasing the G Load, the min speed bar will come up a lot closer to my current speed, i.e. aircraft weight is artificially being increased and so I need more lift and speed to maintain it in a turn, Correct or not?

Yes.

Quoting smartt1982 (Thread starter):
In relation to the MIN MANEUVER SPEED: reflects the effect of extending the flight spoilers. (Why is this, do and if so, why do flight spoilers/speedbrakes effect the min maneuver speed?)

If you raise the spoilers, you alter the wing's ability to generate lift. The min speed changes (it will stall earlier/at a higher speed). Since the min maneuver speed is a multiple of the 1g stall speed, the min maneuver speed also changes.

Quoting smartt1982 (Thread starter):
In relation to the MIN SPEED: reflects the effect of extending the flight spoilers(why is this, g-load or wing loading?) and also reacts to wing loading (what does it mean by wing loading, G-load or weight of the aircraft?)

Wing loading. The wing can't really tell the difference between wing load and g-load (they're not structurally identical but they're close).

Tom.


User currently offlinesmartt1982 From United Kingdom, joined Nov 2007, 225 posts, RR: 0
Reply 2, posted (2 years 4 months 1 week 3 days 20 hours ago) and read 2372 times:

Many thanks for that really detailed reply.

Quoting tdscanuck (Reply 1):
If you raise the spoilers, you alter the wing's ability to generate lift. The min speed changes (it will stall earlier/at a higher speed). Since the min maneuver speed is a multiple of the 1g stall speed, the min maneuver speed also changes.

Why is it on that occasion with the spoliers out, it will calculate a new min maneuver speed baed on Vs1G but in the case of carrying out a 45 AOB turn etc the red and black pole will increase up the speed tape but the min maneuver speed will not increase (as in the red and black looks like it is eating it and the yellow maneuver margin just disapeears and doesnt get higher as well, sorry only way I can think to desribe it!)

Is it because even though you might be pulling 3g or something and the red and black bar will increase to show you your increased stall speed during this turn, the min maneuver speed is still only based on a .3g margin from the 1g stall speed?

[Edited 2012-05-15 07:12:44]

User currently offlinetdscanuck From Canada, joined Jan 2006, 12709 posts, RR: 79
Reply 3, posted (2 years 4 months 1 week 3 days 6 hours ago) and read 2258 times:

Quoting smartt1982 (Reply 2):
Why is it on that occasion with the spoliers out, it will calculate a new min maneuver speed baed on Vs1G but in the case of carrying out a 45 AOB turn etc the red and black pole will increase up the speed tape but the min maneuver speed will not increase

Vs1g is different with spoilers out or in because you've changed the wing aerodynamics. As a result, when the spoilers come out, it recalculates both Vs (minimum speed) and 1.3Vs1g (min manevuer speed).

If you're pulling g's (e.g. a 45-degree bank turn) then Vs moves because you've increased wing loading but Vs1g doesn't move because it's referenced to 1g flight.

Quoting smartt1982 (Reply 2):
(as in the red and black looks like it is eating it and the yellow maneuver margin just disapeears and doesnt get higher as well, sorry only way I can think to desribe it!)

That's a perfect description, I know exactly what you mean.

Quoting smartt1982 (Reply 2):
Is it because even though you might be pulling 3g or something and the red and black bar will increase to show you your increased stall speed during this turn, the min maneuver speed is still only based on a .3g margin from the 1g stall speed?

Exactly.

Tom.


User currently offlinesmartt1982 From United Kingdom, joined Nov 2007, 225 posts, RR: 0
Reply 4, posted (2 years 4 months 1 week 3 days 4 hours ago) and read 2242 times:

That is absolutely fantastic. Thanks again, I had been puzzled by that for ages.

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