Load Factor Vs. Aircraft Weight

[Canadapilot]

Hi all,

Currently doing my Instructor Rating and am trying to answer the following: What effect (if any) does aircraft weight have on load factor? I realise that:

1. To be in the "utility" cat. of the c172 we need to be under 2000lbs, which permits more "G" than when heavier (CofG limits aside); yet:

2. Va increases with aircraft weight, which almost seems to be conflicting with this

I'm imagining the answer may be "no effect", as a 172 or 777 will pull the same "G" if in an equally banked turn while maintaining altitude...so i may be scratching my head over nothing here...all thoughts appreciated! thanks!

[john_tullamarine]

What effect (if any) does aircraft weight have on load factor?

Think in terms of

load factor, n= normal (ie perpendicular to the wing) lift/weight for balanced flight

.. if you are looking at two different weights, the normal lift force must increase/decrease in line with the weight for the same load factor but the pilot will feel the same resulting load on his/her body (unless you have the Jenny Craig well hidden secret for instantaneous weight loss ...). In practical terms it doesn't make much sense to think about effect of weight on load factor, per se

"utility" cat. .... permits more "G" than when heavier

Trap for young players here ... this story relates to the Design Standards which require different load envelopes ... which, in turn, relates principally to wing/tail and associated loads. However, all the bits and pieces glued to the airframe have a design attachment load capability. One cannot extrapolate along the lines of saying .. "if I reduce weight from X to Y I can increase G from A to B" ... to ... "therefore, if I further reduce from Y to Z, I can increase from B to C"

Va increases with aircraft weight, which almost seems to be conflicting with this

Two related, but different, considerations ... another trap for young players, I'm afraid ... previously you were interested in maintaining a constant structural load .. now it is constant load factor ...

You need to consider the graph which describes pitching Va ...

On the load factor graph you have a stall curve (looking at the positive load factor side) which gives you the V/load factor combinations at which the aircraft will enter the stall .. as the speed increases this gets overtaken by the limit load factor line and we disregard the stall line for higher values of speed and load factor.

If you are looking at different weights, you will have to plot different stall lines, with the higher weight stall lines morphing to the right (higher speed).

Va is defined as the stall speed (at the weight) where the limit load factor is achieved ... so you simply extend the limit load factor line to the right to intersect the higher speed (higher weight) stall lines.

a 172 or 777 will pull the same "G" if in an equally banked turn while maintaining altitude (ie balanced flight)

true .. but the radius of turn is rather different ...

.. the earlier explanation probably is a bit convoluted ... if it doesn't make too much sense, add another question and we'll try to unconvolute it a bit.

[Milt]

Can someone explain NzW.
Im too used up to be able to do it now.

[john_tullamarine]

Milt, being a crusty TP chappie and wonderful over dinner raconteur, is used to the NzW argument .. which is the basis for reducing weight to permit an increased load factor .... while maintaining a sensibly similar structural load via the applied aerodynamic loading. ... our posts overlapped.

From another forum ... read Milt's tale the other day about his 10 minute Comet endorsement during RAF acceptance testing quite some time ago ... quite amazing to us ordinary folk ... and I guess it's not out of order to add Tech Log's congratulations to the incoming ACAUST ...

[OneDotLow]

I suspect you are looking at this from the wrong angle...

Load Factor, by definition equals Lift/Weight.

Therefore if you purely have an increase in aircraft weight, whilst lift remains at a constant amount, load factor will decrease.

If you were to increase lift as you increased weight, so that the two remain equal, then load factor remains constant.

To relate it to your C172 vs B777 example, the b777's wing must produce more lift to counter its higher weight, but if both aircraft are in straight & level flight then both have an equal load factor, ie 1.

Hope this helps!

[Canadapilot]

Thanks for the replies all, it's clear when i think of the Lift produced / weight of aircraft formula for load factor. I should practice what i preach and follow the KISS rule!