Va and weight

[MikeOH58]

2 days from the initial, and let the second guessing and forgetting begin...

Why does Va change with weight? (Higher Va higher weight/Lower Va lower weight)...

The only answer I could find is that at lower weight, the aircraft could accelerate faster in a gust or turbulance, but I find myself thinking there is more to it?

help!

[bhh1128]

Originally Posted by MikeOH58

The only answer I could find is that at lower weight, the aircraft could accelerate faster in a gust or turbulance...

That's it. Maneuvering speed is about load factor. The aircraft is designed and certified for a certain maximum load factor (say it's 4.0 for simplicity - the number is actually determined by the category). At maneuvering speed, the wing should generate just enough lift at critical angle of attack to achieve this load factor. That is, lift right at the stall should be four times greater than the airplane's weight.

In a heavier airplane, we need more lift to generate any given load factor. That is, at a higher weight, we can get away with more lift on the wings without breaking things in the airplane. Since the stall is the "circuit breaker" that keeps us from breaking things, we consider angle of attack fixed at the critical angle of attack. The only way, therefore, to get this extra lift that we are allowed is to fly faster.

In summary: more weight --> we can get away with more lift --> we can get away with more speed

It is worth noting that maneuvering speed is not about breaking the wings. The maximum load the wings can support should be reflected in the maximum weight. Maneuvering speed is about breaking other things in the airplane. For example, the engine mounts may be designed to support a 350-lb engine at a load factor no greater than 4. They are therefore designed to support 350*4 = 1400 lb. If you exceed maneuvering speed, you'll exceed the maximum permissible lift, and generate a load factor of, say 4.1. You will therefore be asking the engine mounts to support 350*4.1 = 1435 pounds. Maybe they can do it... maybe they can't.