QJB

3rd Feb 2010, 04:38

Hi all,

I've been helping my girlfriend study for her CPL(A) Performance exam the last few days and I've gone and confused myself.

Can someone please help me clarify a few things:

(a) Is it the limit load factor only, or a combination of the limit load factor and weight that determines the structural limits of an aircraft design.

My logic is this. Load factor is just another word for acceleration. If you put the aircraft under a load factor of 2 you are essentially accelerating the aircraft with a force of 2x gravity. If you have a 1000kg aircraft in a 60 degree level turn you require a load factor of 2.0g the wings must therefore provide a force of 2000kg. However if you overload the same aircraft by 500kg to 1500kg then the same wings have to provide a force of 3000kg to maintain the turn. Surely the heavier aircraft is put under greater structural stress. Is it the case then that two aircraft of different weight can be put under the same load factor but have different degrees of structural stress imparted on the structure. Is this the reason for the utility category limits of some aircraft only being valid below certain weights.

(b) Why is it that Va increases with increased weight.

The only thing that I can think of is that if the aircraft is heavier the lift required from the wing to generate the same load factor (lift/weight) increases thus a higher speed and greater lift for given AoA will have less effect on the load factor. Which again leads to the first question regarding structural limits.

Hope I'm not way off here!

Cheers

J

I've been helping my girlfriend study for her CPL(A) Performance exam the last few days and I've gone and confused myself.

Can someone please help me clarify a few things:

(a) Is it the limit load factor only, or a combination of the limit load factor and weight that determines the structural limits of an aircraft design.

My logic is this. Load factor is just another word for acceleration. If you put the aircraft under a load factor of 2 you are essentially accelerating the aircraft with a force of 2x gravity. If you have a 1000kg aircraft in a 60 degree level turn you require a load factor of 2.0g the wings must therefore provide a force of 2000kg. However if you overload the same aircraft by 500kg to 1500kg then the same wings have to provide a force of 3000kg to maintain the turn. Surely the heavier aircraft is put under greater structural stress. Is it the case then that two aircraft of different weight can be put under the same load factor but have different degrees of structural stress imparted on the structure. Is this the reason for the utility category limits of some aircraft only being valid below certain weights.

(b) Why is it that Va increases with increased weight.

The only thing that I can think of is that if the aircraft is heavier the lift required from the wing to generate the same load factor (lift/weight) increases thus a higher speed and greater lift for given AoA will have less effect on the load factor. Which again leads to the first question regarding structural limits.

Hope I'm not way off here!

Cheers

J