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Explaining Va (maneuvering speed)

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Explaining Va (maneuvering speed)

Old 10-14-2009, 07:15 AM
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"The maximum speed where full, abrupt control movement can be used without overstressing the airframe" - That definition is completely false! Thanks to the FAA and other flight training books, Va has become a big misunderstanding among pilots.

Va is only valid for positive Gs. Va does not protect the aircraft structure under negative Gs and abrupt sideslip under high dynamic pressure (ie AA587).
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Old 10-14-2009, 07:28 AM
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Originally Posted by BHopper88 View Post
Also remember full abrupt of the controls refers to the Elevator not Ailerons...
He is right. It applies only to the elevator up to a determined positive load factor.
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Old 10-14-2009, 08:01 AM
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Originally Posted by bubi352 View Post
He is right. It applies only to the elevator up to a determined positive load factor.
I agree, elevator only by definition.

To further confuse matters, some designers may ensure that the other controls surfaces are also protected below Va so for some models it might be all-inclusive...but don't bet on it unless it's documented.

I suspect that the ailerons usually have a greater margin than the elevator anyway in most designs. Aileron inputs don't cause the wing load to change dramatically.

The rudder/vert stab on the other hand may well have lower margins...as was demonstrated dramatically in 2001.
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Old 10-14-2009, 08:52 AM
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Originally Posted by bubi352 View Post
"The maximum speed where full, abrupt control movement can be used without overstressing the airframe" - That definition is completely false! Thanks to the FAA and other flight training books, Va has become a big misunderstanding among pilots.

Va is only valid for positive Gs. Va does not protect the aircraft structure under negative Gs and abrupt sideslip under high dynamic pressure (ie AA587).
I am unsure of the negative portion of Va, but the VG diagram seems to support this claim if you observe the negative G load would be slower than the positive in most cases.



However there are two rules I was taught in aerodynamics that were beat into my head, specifically because at the time we were doing aerobatic training. Va is predicated on two key points during certification, the loading is:

Symmetrical: Meaning level flight, not in turns where one wing may experience a greater load than the other.

Progressive: This one is a little tricky, notice many sources claim "full abrupt" with regard to controls. Now anyone here who has been in an aerobatic aircraft know of "snap speed" or the speed at which you really can go full and abrupt with the controls. Va is determined based on progressive, non snap, meaning not abrupt, loading. A quick pull back, ok, but don't think you can rip that control to your lap right at maneuvering speed and be completely safe, that is why there are snap speeds for aerobatic aircraft. Can anyone verify this with a source other than my flight dynamics professor, he did not source it for us but used our cap 10 snap speed as an example to prove his point.



There is one last item used for determining load factor, that is the material used. Seems like an obvious one so I usually leave this one out.
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Old 10-14-2009, 11:51 AM
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Originally Posted by NoyGonnaDoIt View Post
Va decreases with weight but that's a continuous process, so I'm not sure what you are referring to.

But, if you're asking why Va decreases with weight..
Originally Posted by fjetter View Post
The definition of Va from the PHAK "The maximum speed where full, abrupt control movement can be used without overstressing the airframe"

The reason for the range of speeds 88-111 in a Warrior is based on weight
88 = Max Gross Takeoff Weight
111 = Basic Empty Weight

Va INCREASES with weight!


This is how I've always thought about it. Va is the highest speed at which you can have maximum deflection without hurting the airplane.

A lighter airplane is more "flickable." It responds faster to control inupts and is more easily overloaded. A lower airspeed is required to prevent the airplane from "flicking" too fast and putting too much stress on the airframe.

A heavier airplane would be more "sluggish," it won't respond as quickly to control inputs. Instead of jerking into a roll like a light airplane would if you went full deflection, the heavier airplane would do more of a slow roll with full deflection, allowing for a faster airspeed to be used with that full deflection.
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Old 10-14-2009, 01:43 PM
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Here is how I explain it:

Be sure to read this with an open mind, even if math scares you, it is only basic addition which I believe anyone is capable of understanding. Enjoy.

I like to use what I call the practical lift formula, that formula is Lift = Speed + AOA. It is nothing more than a general method for seeing how Speed and AOA work together, increase one the other must decrease to maintain the same lift.

Let us assume we stall at 17 degrees, since Va is related to the amount of deflection we are allowed let's say that we must be at or above 7 degrees for us to be at Va. If we are at 7 degrees we will have a safe amount of deflection, in reality these numbers also don't work but are used here merely for demonstration purposes.

Input some numbers, now these are also unrealistic but will unquestionably demonstrate the relationship with weight and Va.


Aircraft one: Weight = 20 which means Lift must equal 20 to support this weight, remember lift equals weight.

Apply practical lift formula: 20 (Lift) = 7 (AOA) + 13 (Speed)


Aircraft two: Weight = 22, again Lift equals 22 to support that weight.

Apply practical lift formula: 22 (Lift) = 7 (A0A) + 15 (Speed)



Conclusion: At the heavier weight of 22 the speed was able to be 15 instead of 13 at the lighter weight. Obviously these numbers aren't logical, but who cares? They serve the purpose of showing, with an obvious, easily understood, representation that with a heavier weight we can attain a faster speed with the same AOA and thus a faster maneuvering speed.

Disclaimer: I purposely annotated each formula with () to show which each field was. This was done for simplicity with regards to the reader. Since many people don't like math because there are numbers that don't represent meanings this depicts the meaning clearly next to the number.
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Old 10-14-2009, 03:44 PM
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Wow! Really appreciate the input from everyone! I'm just gonna run this by you guys just to make sure I have it right.

Va is defined as the highest speed that an a/c can be at where abrupt control movements will just cause it to stall, not break. In our C172's, Va is 90 (for the basic empty weight) to 105 (at max gross weight), and if we use abrupt control movements at or higher than whatever speed pertains to our weight, we risk structural damage.

Does it sound like I have it right?
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Old 10-14-2009, 03:53 PM
  #18  
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Yea that sounds about right... The easiest way I was able to understand and teach it was take a cessna at max weight, and then at say middle of the road weight.. At the speed that is ref. the pilot goes full back on the elevator. Visually think what the G's would be at the max gross weight at both speeds, and then what would it be like at the lighter weight. At the lighter weight to maintain altitude you must pitch up so as a by product you are closer to your critical angle of attack, vs if u stayed at the faster Va at the lighter weight which would put you at a higher angle to travel to your critical angle of attack and cause higher load factors that could cause structural damage.

Hope that helps
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Old 10-14-2009, 07:13 PM
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Originally Posted by Mitragorz View Post
Va INCREASES with weight!


This is how I've always thought about it. Va is the highest speed at which you can have maximum deflection without hurting the airplane.

A lighter airplane is more "flickable." It responds faster to control inupts and is more easily overloaded. A lower airspeed is required to prevent the airplane from "flicking" too fast and putting too much stress on the airframe.

A heavier airplane would be more "sluggish," it won't respond as quickly to control inputs. Instead of jerking into a roll like a light airplane would if you went full deflection, the heavier airplane would do more of a slow roll with full deflection, allowing for a faster airspeed to be used with that full deflection.
WOW! I guess I screwed the pooch on that one and got it a$$-backwards. Guess I should have posted after my morning coffee.
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Old 10-15-2009, 04:02 AM
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Originally Posted by Mitragorz View Post
Va INCREASES with weight!
Va decreases as weight decreases. Va increases as weight increases.

Although fjetter did get those numbers backwards. Higher with higher; lower with lower.
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