Why does maneuvering speed Va get lower as the weight of the airplane decreases?

2	Why does maneuvering speed Va get lower as the weight of the airplane decreases? Intuitively you'd think it would make sense that Va decreases as the plane gets heavier. aerodynamics physics airspeed-limitations asked Oct 08 '10 at 12:20 Patrick Pohler ♦♦ 1337●18●39●116

3 Answers:

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To answer this question, we need to examine the V-G diagram. The V-G diagram shows the velocity on the X-axis and the Load Factor on the Y-axis. Unfortunately, the V-G diagram is three dimensional with respect to the Velocity, Load Factor and Weight and we can't really chart 3-D charts to well so engineers typically show the V-G diagram at the max gross weight. On some airplanes, there may be multiple V-G diagrams for specific weights.

V-G Diagram

Load Factor is defined as the amount of weight the airplane is required to lift versus the actual weight of the airplane. Weight is a force so any additional forces on the wings will add to the load factor. We typically think of turning as adding load factor (the V-G diagram is the reason why we don't use a steep bank angle during slow flight). Any turbulence in the area will also add additional forces on the wings and additional load factor.

Some advanced aerodynamic books show this turbulence directly on the V-G diagram in feet per second. Most that I have seen is 30 feet/sec for the top curve and lower curve. Those horizontal lines represent the maximum positive and negative load factors designed into the airplane. For the normal category approved airplane it is +3.8g to -1.52g. Any time a gust is encountered, for that split second, the wings must withstand the force of the gust and the weight of the airplane. It is possible with a strong enough gust to exceed the designed max load factor. This is why it is foolish to fly through a thunderstorm.

To answer your question, the gust force is proportionally larger to a lighter aircraft versus a heavier aircraft (20lb gust/2000lb airplane or 20lb gust/3000lb airplane). The gust effect on the lighter aircraft will have a greater effect causing the lighter aircraft to have a greater load factor for that split second.

It is that reason why Va decreases with weight. Just in case, Va is calculated on the V-G diagram as the point where the upper curve meets the horizontal line. That speed is Va. In the diagram above, it is shown where the yellow lines are. Thus if you are below Va, the airplane will stall before structural damage. However, if you are above that speed, the airplane will sustain damage before it stalls.

answered Oct 11 '10 at 23:30

wbeard52
2066●6●25●45

Ah I see, so you're saying the heavier aircraft can withstand a greater amount of force (load factor) than a lighter aircraft, thus the higher Va. Excellent graph btw

(Oct 13 '10 at 21:51) Patrick Pohler ♦♦

I think this is an overly complicated answer to the question. Basically, a lighter aircraft is harder to stall. At a given airspeed where a heavier aircraft would stall first, a lighter aircraft might still be unstalled and sustaining whatever load factor exists - either due to turbulence or control input. In order to reach the stall in the lighter aircraft, more load factor is required to stall the wing.

The extension of this explanation is that a lighter aircraft is flying at a lower angle of attack and the force required to reach stall AoA is greater. In other words, more force is necessary to achieve that change in AoA and that increase in required force may exceed the limit load factor for the aircraft.

(A heavier aircraft cannot withstand a greater load factor. In fact, the simple explanation is that both aircraft have the same load factor limit.)

answered Oct 16 '10 at 09:31

viddly
15●2

A little confused... if I follow your logic, it concludes that Va should be higher for lighter aircraft and lower for heavier aircraft. We all know that is not correct. Both aircraft have the same max load factor. It is in those momentary gusts that will cause the lighter aircraft to exceed it max load factor before the heavier aircraft will.

(Oct 17 '10 at 12:43) RichT 1

In any situation, gusty or not, for a given speed, a heavier aircraft is flying at a higher angle of attack. It is closer to a stall. Increases in load factor have a higher likelihood of causing a stall than causing an over-G.

(Oct 19 '10 at 17:57) viddly

(A heavier aircraft flying at 1 G in level flight will be maintaining a higher AOA in order to generate the higher lift required. As such, it is closer to a stall AOA. Any event causing an increase in load factor will increase the AOA. As the heavier aircraft is at a higher AOA, it is closer to stall AOA. The margin from stall is smaller, but, in fact, the likelihood of over G is reduced.)

(Oct 19 '10 at 17:58) viddly

We are concerned about the momentary gusty conditions that cause the wings to carry more weight than before. The lighter airplane will get closer to the max load factor than a heavier airplane. It's argueable that max load factor is designed into the airplane by the structural strength and doesn't change with the weight of the airplane and I would agree. The lighter aircraft won't reach the structural limit of the airplane as quickly as a heavier airplane. However, the inertia from those gusts can quickly exceed those limits. It is a question of static physics and physics in motion.

(Oct 19 '10 at 21:38) wbeard52

I just have so many students who have difficulty understanding the primary concepts ... re: why the lighter aircraft will get closer to max load factor. The point is that the lighter aircraft will reach a point of damage before a point of stalling. It's wing loading is less so in fact the same gusts can be more damaging ... because it has more change in AOA available before stall.

(Oct 20 '10 at 00:31) viddly

Rod Machado does a wonderful explanation and I agree no need to make it so complicated but I probably will

heavier means higher AOA to sustain Level flight , HIgher AOA mean closer to stall, closer to stall means if you are hit by a gust which further increases AOA the wing will stall rather than be damaged

Lighter means lower AOA to sustain level flight, lower AOA means further away from stall, further away from stall means if you are hit by a gust , the intertia and g caused by the gust may damage the structure rather than stall the airplane

Therefore if you want to prevent damage on a lightly loaded aircraft you have to fly closer to stall aoa so the gust wont hurt you, To fly closer to the stall you have to increase the AOA, If you fly a higher AOA, to prevent a climb you must slow down..

note when I say stall it just means the wing is unloaded at that moment a very brief moment not like stalls practiced during training.

answered Mar 18 '11 at 11:04