Expert: Heiko Hanusch - 1/9/2006

Question
Ive always known how to recite the answers that the FAA wants to hear but I started thinking in depth about this one and cannot really explain why it is....
Does maneuvering speed increase or decrease with weight?  It increases is the correct answer.  Maneuvering speed is the speed above which structural damage may occur with abrubt control movement and below it the plane will stall before structural damage occurs with abrupt control movement.
But when i think about it in the following terms it doesnt make much sense.  Lets say we have 2 balsa wood gliders except one has a huge lead weight attached to the body and the other is just the plane old glider, but both of their wings are exactly the same.  If I go outside on a really windy day and throw each one as hard as possible which one would break?  The heavy one because the lead weight would want to keep going forward whereby a gust of wind underneath the wings would rip em apart.  The glider with nothing would just get tossed and turned in the swirls of wind but would stay together better.

So doesnt that example basically say as weight increases its easier for the plane to break apart (so maneuvering speed would need to decrease) because of more inertia going in a different direction from the relative wind?  what am i misunderstanding here?

Answer
Hi Chris,

Thank you for your question.
Problems like this arise when you start looking into things just a little too deep, everyone I know has done it before (incl. myself), and we usually get more confused as we continue down this track.:)
Here it comes...

The posted maneuvering speed in an aircraft is already given at the speed which corresponds to the max. weight of the aircraft! Therefore any weight below max. weight will require a slower speed.
Example: 3600lbs.  = 130kts VA
        3400LBS.  = 125kts VA

Since VA is usually given at max weight in most aircraft you should always fly a bit slower than VA, unless you overload the aircraft on purpose, therefore you should be relatively safe from overstress or possible structural failure.
Remember, an aircraft that is heavier requires alot more force (wind gust) before it is displaced compared to an aircraft that is lighter which explains the higher maneuvering speed at higher weights!
Another way to see how it works is to look at acceleration of an object, which by the way is the main reason for VA,
and compare these two results. I am going to use exaggerated values to better show the effect and also not correct math but it does not matter for illustration purposes

  a wind gust at 10 meters per second displaces a
   100lb. object 1 meter per second or 10g's

  the same wind gust dispaces a 1000lb. object
   0.5 meters per second or 5g's

that means that the heavier object can travel faster
before it reaches the 10g limit !

Inertia is the heavier objects friend in this case!

In your glider example I see 2 things that make this experiment invalid...
First: one would have to determine the Va of the glider
      and then throw it at that speed and
Second: the weight distribution of your glider including      distance from CG to location of weight etc.
need to be accurate.

I hope this was not to confusing,

H2

P.S.  Keep in mind that maneuvering speed does not safe you in all circuumstances, we just need to look at the  AA Airbus 300 in New York that broke its tail of, they were below Va and full repetitive control inputs are apparently bad at any speed...