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bryanmmartin
Joined: 10 Jan 2006
Posts: 1018
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 Posted: Sat Jul 11, 2009 3:33 pm Post subject: A Note About Moding G's instead of Weight |
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It's not quite that simple. For a given MTOW and flight load factor
(FLF), Va is the speed where, when the plane is actually loaded to the
MTOW, the wing will stall before the FLF is exceeded. This assumes
that the aircraft is in coordinated flight and the G loading is being
increased at a defined rate. In this situation, the wing generating
the maximum amount of lift that it can at that speed, the wing spar to
fuselage attachment is experiencing the maximum stress it is designed
to handle in flight, the engine mount may also be carrying the maximum
load it is designed to carry and the mounts for many other fixed
objects in the airplane may be carrying their maximum loads.
For example, an airplane with a 1000 lb MTOW and 4 G FLF and Va of 100
mph. At Va and the stall angle of attack the wing will be producing
4000 lbs of lift. If the each wing weighs 100 lb, the fuselage weighs
800 lb so: 800 lb * 4 = 3200 lb of load on the wing spar-fuselage
joint (1600 lb per side).
If this same airplane (same MTOW and FLF) is loaded to less than MTOW
and flying at 100 mph, the airplane will still produce the same
maximum lift when it stalls but, since the airplane is lighter, the G
factor will be higher. For example, if the airplane actually weighs
900 lb: 4000 / 900 gives a G factor of 4.44. If the weight was all
removed from the fuselage then, 700 lb * 4.44 G = 3111 lb load at the
spar-fuselage joint. In this situation, the wing spar will not be over
loaded even though the G factor is higher, but the engine mounts may
be overloaded, or some other structure may be overloaded. If instead,
you are carrying 100 lb less fuel in the wing tanks but the same load
in the fuselage then, 800 * 4.44 G = 3556 lb load at the spar-fuselage
joint. In this situation, the wing is overloaded as well as the rest
of the aircraft structures. This is why Va should be reduced when
flying with a lighter load with a given FLF.
If the MTOW is increased on this airplane, for instance to 1050 lb,
the maximum flight load factor would have to be decreased to prevent
overloading the spar. But this is the same airplane, so the spar is
the same strength: it can still carry the same a maximum load at the
spar-fuselage joint. Assuming the extra weight is in the fuselage:
3200 / 850 = 3.76 G. So the new FLF is 3.76 G for the airplane. Now:
3.76 G * 1050 = 3953 lb = the lift the wings will need to generate at
3.76 G. Since the lift needed at the new FLF is lower than it was for
the 1000 lb MTOW case, you have to fly slower if you want the wing to
stall before it exceeds this maximum lift value. So Va will decrease
if you increase MTOW and decrease FLF.
In this discussion, I have neglected any lift generated by the
fuselage and the down force generated by the horizontal stabilizer.
These may have some effect on the actual load carried by the wings.
| Quote: |
Hi Juan,
I agree with everything said in your quoted article except for the
turbulence comment. I'm afraid the author got this point backwards.
Increasing actual gross weight increases Va rather than decreasing
it. So, in theory, you could fly faster in the same turbulence with
more gross weight rather than needing to fly slower.
OK, it is a small point. The simple answer for most pilots in most
conditions is to keep under gross weight specified in the airplane's
documents. However, I still believe flying over gross weight can be
done safely, and legally, in some planes under some conditions.
Paul
do not archive
At 07:08 AM 7/11/2009, you wrote:
> The pilot will have to remember when flying at the increased weight
> to slow down more in turbulence
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--
Bryan Martin
N61BM, CH 601 XL,
RAM Subaru, Stratus redrive.
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Bryan Martin
N61BM, CH 601 XL, Stratus Subaru.
do not archive. |
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vayuwings
Joined: 12 Dec 2008
Posts: 16
Location: arizona
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