If this is your first visit, be sure to check out the FAQ by clicking the link above. You may have to register before you can post: click the register link above to proceed. To start viewing messages, select the forum that you want to visit from the selection below. |
|
|
Thread Tools | Display Modes |
#1
|
|||
|
|||
Va: maneuvering speed ad nauseam
After the reading the Va threads of my past questions, I found a
wonderful way to confuse myself. I notice that I've become quite good at confusing myself and making things complicated. To give myself some credit, I did search old messages but found no resolution. So here we go... I am sure most know the typical textbook definition of Va...goes like..."the minimum speed at which the wing can produce lift equal to the design load limit" or "the speed at which the pilot can use full control deflections without over-stressing the airplane". Essentially, it's the minimum airspeed that, coupled with a control deflection to give you the critical angle of attack and CLmax, will result in + 3.8 g's. Pulling any harder won't help since you'll stall the airplane. Pulling with all your might as speeds below Va will result in the airplane stalling without reaching the limit load factor + 3.8 g's. In essence, Va is the stall speed at the design load factor of + 3.8 g's. Now, the above seems to be what's commonly accepted. Here's my question for this thread: Idealize the wing as a cantilevered Euler beam representative of the wing spar ("the wing"). Assume the lift load to be a distributed elliptical spanwise, transverse load, acting at the centroid of the section. Further assume no other external loading such as drag loads. The predominant stresses are bending (axial) stresses at the outer regions of the spar caps and shear stresses in the spar web. Assume that the failure mode is via the former. Alright, so here, clearly, the failure of the wing is due to excessive loading. The distributed load, expressed in X number of pounds per inch, was too great. In fact, for the sake of simplicity, let's make the distributed load a point load in pounds. Now, we can say that the failure of the wing was due to excessive FORCE which induced excessive stresses in the structure. Consider that a certain airplane weighed at maximum takeoff weight is designed to withstand + 3.8 g's (its design load). Actually, airliners that I am familiar with are tested to ultimate load, or 1.5*design load (+ 5.7 g's before permanent deformation). For now, we'll assume that at + 3.8 g, the plane's wings break off. That would equal to a total force on both wings of 3.8 x 2550 lb or almost 10,000 lbs. The thing that bothers me about Va is that it equates to a number of g's ("design load") AND that Va is being rescaled for weight. By doing so, Va becomes more of an acceleration criterion rather than a structural criterion. It appears as though Va limits positive g acceleration to + 3.8 g, not load itself. In other words, Va adjusted for say, a lower weight, tells the pilot "You will not exceed 3.8 g for your current weight, as you will stall first". If the current weight was 2,000 lbs, the total load on the wings would only equate to 7,600 lbs at + 3.8 g's, lower than the design limit of 10,000 lbs of + 3.8 g's at max. takeoff weight. The acceleration on the airplane would be a "limit load acceleration" but would not produce a limit load condition, per se, structurally. If Va was truly a structural consideration, it would not change (regardless of weight), since no airspeed below Va coupled with any non-stalled AOA, could produce limit loads of + 3.8 g as tested at max takeoff weight of 2,550 lbs. There may be severe flaws in my reasoning...please no flames and be nice. It's a holiday. Alex |
Thread Tools | |
Display Modes | |
|
|
Similar Threads | ||||
Thread | Thread Starter | Forum | Replies | Last Post |
Landing and T/O distances (Was Cold War ALternate Basing) | Guy Alcala | Military Aviation | 3 | August 13th 04 12:18 PM |
Space Elevator | Big John | Home Built | 111 | July 21st 04 04:31 PM |
Va and turbulent air penetration speed. | Doug | Instrument Flight Rules | 70 | January 11th 04 08:35 PM |
Va and turbulent air penetration speed. | Doug | Owning | 69 | January 11th 04 08:35 PM |
New Film: The Need For Speed - Going to war on drugs | Phil Carpenter | Military Aviation | 0 | July 23rd 03 07:43 AM |