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Old 12th Jan 2017, 07:35
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john_tullamarine
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Why Vs moves with the movement of CG

One of the considerations with CG position is the vertical tail load needed to keep the aircraft in balance.

Generally, we see the tail load being a down load. As the CG moves forward, you are going to need more tail down load to prevent the aircraft pitching nose down.

Now, keeping it simple, say straight and level, and considering most of the lift to be associated with the wing and tail, we have, generally, an up lift load on the wing, and a down lift load on the tail. The net lift is the difference between the two.

As CG moves forward and the tail lift down load increases (a bit of nose up trim is input to maintain straight and level) overall net lift will decrease.

To maintain straight and level flight, we need (either to go faster or) to increase CL by pitching up a little to increase incidence or angle of attack. Think in terms of starting at the aft limit .. achieves a particular stall speed. As the CG moves forward, we need more CL to maintain straight and level flight but if we were at the stall speed to start with, this can only be achieved by increasing speed a little and so we find that the stall speed must increase.

It follows that the flight manual (or pilots operating handbook, which includes the flight manual) stall speeds will be figured for forward CG limit. One of the reasons the upper forward limit moves aft on light aircraft is to keep the stall speed a bit sensible for other considerations.

Va, the purpose of it and why it changes with weight

The basic idea of Va is looking at loads in the pitching plane. For this case, Va is the speed at which the limit design load factor occurs at the stall speed for that weight.

Reason is that this gives the pilot a maximum speed target for rough air or manoeuvring flight below which the aeroplane should stall prior to exceeding the limit design load factor, ie protects us against being a bit too hamfisted on the stick/column.

Significance of the limit design load factor is that the wing shouldn't suffer any permanent deformation or have anything break for this loading case.

A problem arises with things other than the wings .. eg a lump of equipment riveted to some structure in the back only knows what g-load is applied to it. If the load exceeds the limit design load factor, then the structural stuff which is holding the lump of equipment in place might break or suffer other damage.

So, the problem is that we need to make sure that Va considers any variations associated, for instance, with weight.

Now, if the aeroplane is lighter, I don't need quite so much lift at a given speed so, if I am at the MTOW Va but at a lower weight, I still have some g capability in reserve (compared to the situation at MTOW) .. ergo, I can pull a bit more g than the limit design load factor before the aeroplane stalls.

That, of itself, won't necessarily hurt the wing as the wing is concerned with loads rather than accelerations (ie load factor or g). However, it certainly can cause some bother to other structure in the aeroplane.

Hence we need to provide guidance to the pilot so that the limit design load factor can be respected. The POH often will provide information relating Va to gross weight and you should see Va reducing a little as gross weight reduces.

Side note. Generally, Va will be limited by pitching (nose up/down) loads. However, for some aircraft, the loads or other considerations associated with control movement in either of the yawing or rolling planes might be more limiting (for instance, acceptable handling). As I recall, the HS125 limit was for rudder and directional stability.

Overall, the limit's application is that the pilot should be able to apply a rapid steady input in any ONE control to the stop ie NOT significant inputs in multiple controls simultaneously. Rapid control input reversals and intentional pilot input oscillations are not included in the design considerations .. ie if you are at Va (or somewhat below) and start pushing and pulling on whichever control is limiting .. you might get an unpleasant (and, potentially, fatal) surprise. A read of the accident report on AA 587 is illuminating.

To the OP, best of luck with your forthcoming flight test.
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