why does Va airspeed decreases as gross weight decreases?

Question

For instance if the weight is 2900 lbs Va would be around 125 kias but at 3900 lbs it is 132 kias. Why?

Can anyone give me a good answer?

Answer 1

Maneuvering speed is really nothing more than a stall speed - it's the stall speed at the limit load factor. The theory behind it is that at speeds above Va, it's possible to overstress the airframe. Below Va, the wings will stall before the limit load factor is reached.



Since Va is a stall speed, it follows the normal rules of stall speed: the lighter the weight, the slower the speed.

Answer 2

This is much easier to explain with the aid of diagrams (particularly Vg diagram) but I'll give it a shot..

Look in your manuals (or on the link below, Figure 4) for the diagram showing the relationship between level turn bank angle and stall speed. You will see that the steeper the bank is, the higher the stall speed is. Why? Because the more we bank in a level turn, the more we need to increase back pressure to maintain altitude. The more we increase back pressure, the higher the load factor (G-force).

Load factor is simply a number telling us how much heavier the airplane is than it would be in normal straight and level flight (2g means a 2,000lb airplane now "weighs" 4,000lb). So we can see that as the load factor (ie weight!) increases, so does the stall speed! (Perfectly logical as we already know that in level flight lift = weight, so the heavier the aircraft, the more lift is required, which is obtained by more speed!).

Va is simply the highest speed we can fly at and still guarantee structural integrity: the last thing we want to do is overstress the airframe, so we fly at a speed which will ensure that if we should encounter turbulent weather and perhaps severe up and down drafts, leading to extreme load factors, the airplane will stall before it breaks. Va is that speed, and as we've already shown, stall speed increases with weight!

If you have a C172 manual, also take a look at the weight and balance limits for normal versus utility category operations. you'll see that max weight X max load factor both come to the same total weight - the weight the wings are built to support before failing (3.8 X MGW in normal and 4.2 X MGW in utility if I remember correctly - it's been a few years since I instructed and I don't have my referances to hand!).

Hope that helps..

Answer 3

Actually at 2900 lbs it would be around 113 kts, (assuming max weight is 3900 and Va at 132)



The actual formula for Va is



Va @Max takeoff weight x sq root of ( Current weight / Max gross weight)



Va is based on weight,because as weight increases, the amount of lift required to maintain alitutude increases. At high gross weight, the aircraft will stall before reaching the load factor limit. at a light weight, you can over stress or exceed the load limit design and cause structual failure.

Answer 4

As an aircrafts weight decreases its susceptibility to aerodynamic forces increases. ie 'Lift and drag'



This is good when it comes to Vr V2 Vx Vy then Vso Vsi etc. As they occur at lower than published maximum speeds at lower than maximum weights for most any given aircraft.



Bad when it comes to Va for the same reason. Va ( or the speed above which structural damage can occur with full control deflection ) decreases as weight decreases because less aerodynamic force is required to affect a lighter aircraft.



Va is usually published as a constant, but as a general rule of thumb. A reduced Va will be half the percentage of reduced MTOW. ie For a take off weight 10% below MTOW Va will be 5% lower than published.

Answer 5

Va is the speed at which the aircraft will stall at the design maneuvering load factor. The factor is chosen and is constant regardless of conditions such as weight. The heavier the airplane is at design maneuvering load factor the faster you have to go to keep from stalling.