Va and negative G´s

[ulflarsson]

Hello!

This is my first post here so I hope you can help me out. I was reading the handbook for my student plane, a pa28 and started to wonder about one thing.

I noticed that the maximum g loads for the aircraft are about +4 and minus 2 and the manouvering speed is about 110 knots. That means that I can deflect any rudder fully to any direction without damaging the aircraft.
But if I push the elevator to fully forward position at the Va speed. Will I not overload my aircraft over the maximum -2 g limit at the Va speed?. I imagine that the g-loads should be more than -2 at 110 knots and a fully forward elevator.
Thanks for all your answers.

Best regards Uffe

[Mad (Flt) Scientist]

Va isn't an absolute safety limit; it's the speed up to which certain design manoevres are calculated, including certain full control manoeuvres. But you can probably break any aircraft in service at and below Va, or depart from controlled flight, if you work at it. Applying full back or forward elevator inputs and waiting to see what happens is probably a case in point.

[pstaney]

I'd think that -2 G is all you'd get at 110 knots, as long as above the minimum flight weight (just had a thread on that). With positive camber of the wing, this means the wing is capable of more lift "up" than "down". So at 110 knots you'd get more positive G than negative G. Although the allowable travel of the elevator may affect this too. A symetrical airfoil, elevator, and same available travel up or down would mean same G positive or negative for same speed. May be some other factors too.

[stator vane]

i think the best way to view your published manuever speed question is more along the lines of a speed to use when turbulence is expected. (and that speed increases a bit as the weight of the aircraft increases). then perhaps a confidence factor. knowing that there is a large factor of strength built into the aircraft "which one will not knowingly test" but might be grateful for in an unintentional event.

many used to say something similar about transport category aircraft (full exercise of the flight controls) until the American airbus crash outside of new york. then it was brought to the surface that the strength applied only to a single movement of the control--not an out and return just as rapid.

only in aerobatics in a proper aerobatic aircraft should one even consider a full strength effort on any control.

and any "effort" you put into the controls of your aircraft, stores up a fatique factor in the metal involved in the flight control or the control cables.

[bfisk]

Va has to do with load factor, stall speed and structural design limitations; not full control movements.




An airframe is designed to withstand a certain stress. This stress is independent of weight.

Load factor is the ratio between the lift produced by the wings and the weigth of the aircraft. When lift equals weight, the load factor is 1, and when lift is twice that of the aircrafts weight, the load factor is 2, etc.

Since load factor is depenent on weight, the design load limit expressed in G, not N, is dependent on weight.

Stall speed is a function of load factor. As load factor increases, stall speed increases by a factor, the square-root of the load factor.

Because stall speed is dependant upon load factor, there will be a certain stall speed corresponding to the design load limit. If an aircraft is flying slowly, and the load factor for some reason increases, the airplane will stall before it reaches its design load limit. If the airplane is flying with great speed, and the load factor is increased, the airplane will not stall until after the design load limit has been exceeded.

The dividing speed is the maneuvering speed, and it is the speed at which the airplane will stall just at is reaches the design load limit. Because stall speed varies with load factor, and load factor varies with weight, the maneuvering speed will be a function of weight.

Va is simply a stall speed.

[PEI_3721]

Va has to do with load factor, stall speed and structural design limitations; not full control movements. Va is simply a stall speed.
bfisk I suggest that you read the relevant airworthiness standards. FAR 25 (http://www.access.gpo.gov/nara/cfr/waisidx_04/14cfr25_04.html) is here, see 25.335. (http://a257.g.akamaitech.net/7/257/2422/12feb20041500/edocket.access.gpo.gov/cfr_2004/janqtr/pdf/14cfr25.335.pdf)

[ulflarsson]

Pstaney!

Thanks for your answer, your explanation makes sense to me I think :) Wings will put out more lift in the "up" direction than in the other, thanks!

Bfisk
Va has everything to do with full control movements, or did I misunderstand you?