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Va Maneuvering

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Va Maneuvering

Old 1st Mar 2013, 17:02
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Va Maneuvering

Hi gents,

I am a student pilot, finished ATPL, building hours on DV20 Katana.

I have a question regarding Va.

Va on the Katana is 104kts according to the AFM.

As Va is SQRT(n) times clean stall power off speed at max weight, it is for katana SQRT(4,4) times 43kts (clean) which is....90 knots.

I can not understand where the 104kts came from, which is 5,8G loading.

The only thing that I found is that by law, VS√(n) ≤ VA ≤ VC.

According to an FAA circular:

VA should not be interpreted as a speed that would permit the pilot unrestricted flight-control movement without exceeding airplane structural limits, nor should it be interpreted as a gust penetration speed. Only if VA = VS √(n) will the airplane stall in a nose-up pitching maneuver at, or near, limit load factor. For airplanes where VA > VS√(n), the pilot would have to check the maneuver; otherwise the airplane would exceed the limit load factor.

This paragraph, confirms that statements on flight manual are not accurate about allowance of full or abrupt movements of control stick at Va. (OR I am missing something they accounted for, which I dont know)

Also, in CS23 there is a paragraph, saying same things.

Maybe this is something to do with ultimate and limit load factor, but again, it doesnt make sense.

If 4,4g is the limit load factor, so the ultimate load factor should be 4,4 times 1,5 which is 6,6g.

Maybe I am missing something.

Can anyone help?

Last edited by Lantirn; 1st Mar 2013 at 17:05.
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Old 2nd Mar 2013, 01:49
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Va is only to ensure that control surfaces don't get damaged. In the regulations the minimum Va speed is sqrt(n)(Vs) but that is only the minimum, and hence why you see the FAA circular saying that Va doesn't guarantee you won't break the airplane!

Read FAR 23.335(c), 23.423, and 23.441.

The first sentence in 23.423 says:

"Each horizontal surface and its supporting structure, and the main wing of a canard or tandem wing configuration, if that surface has pitch control, must be designed for the maneuvering loads imposed by the following conditions"

This is specifically for the control surfaces and supporting structure.

What you're thinking of is actually called Vo - maximum operating maneuvering speed. Reference FAR 23.1507.
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Old 2nd Mar 2013, 02:03
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Not exactly....

It is a function of stall speed at your current weight (one G) and the design load capability of your aircraft. Therefore, Maneuver Speed varies....it is not one hard and fast number.

So, for example, if you're very light, the maneuver speed will be less than the one published. The published maneuver speed is for max gross weight.

One more caveat....maneuver speed does not protect you from abrupt full and opposite control input. So, while it may protect you from loads from extreme turbulence, it does not protect you from rapid, full, and opposite control inputs.
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Old 2nd Mar 2013, 02:17
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It is a function of stall speed at your current weight (one G) and the design load capability of your aircraft. Therefore, Maneuver Speed varies....it is not one hard and fast number.
PantLoad,

That isn't really correct. It is only correct when Va = Vo. Va ONLY refers to control surface's and their supporting structure's integrity.

One more caveat....maneuver speed does not protect you from abrupt full and opposite control input. So, while it may protect you from loads from extreme turbulence, it does not protect you from rapid, full, and opposite control inputs.
That is true that you can overstress the control surface or its supporting structure if you apply full control deflection of more than one control surface or if you oscillate the control surface. But Va isn't a turbulence speed. Vb, Vc and Vd are all related to turbulence. If Vb wasn't published for your airplane, you should fly at the Vo speed for your weight and configuration. If Vo isn't published you can easily calculate it by using the formula: Vo=Vs*sqrt(n), ensuring the correct stall speed for your weight is being used.

Steve Pomroy has taken the time to write a few good articles on these subjects:

The Flight Writer: Va: Not a Bad Speed, Just Misunderstood (Part 1)
The Flight Writer: Va: Not a Bad Speed, Just Misunderstood (Part 2)
The Flight Writer: Tubulence Penetration
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Old 2nd Mar 2013, 03:34
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To be honest...

To be honest, I had to look up Vo. I wasn't familiar with that. It is not in Part 1 of the FAA definitions. But, thanks to Google, I learned it's the maximum maneuvering speed.

Yes, italia458, you are correct, this refers to only control surfaces and the supporting structure.

However, the Va does vary with weight...the basic premise is that at or below this speed the control surface "unloads" (stalls) before deformation an'/or structural failure occurs.

Source: FAA-H-8083-25A "Pilot's Handbook of Aeronautical Knowledge"
Pages 4-32 and 4-33.

Not only does it vary with weight, but with altitude, as well. (Above source) As the textbook states, there is one maneuverving speed for a given weight, altitude, and design load factor.

Further, you are correct regarding the turbulence issue. The "gust factor" provides no guarantee of safety.

And, again, italia458, you are correct regarding abrupt and oscillating control input. The American Airlines Airbus accident during departure from JFK brought that point to light.

Source: Airbus Flight Crew Bulletin FCB-FCB25 Use of Rudder on Transport Category Aircraft

And, once again, you are right....Turbulence is another issue. However, the point I was trying to make is that, in extreme conditions, speed reduction of speed to at or below Va affords additional protection against structural failure....with the admonition listed above in the Pilot's Handbook of Aeronautical Knowledge. (i.e. even if slowed to at or below this speed, gusts can bring the aircraft to a point beyond its structural capabilities.)

Again, the basic premise is that, at or below this speed, the surface will stall before it breaks. With the caveats you list regarding this, you are correct.


p.s. I just read your references... My comment:

The writer suggests that, in extreme turbulence, the pilot may need to use
full and abrupt control inputs to control the aircraft. As with the author, I agree, I'd hate to think about doing that.

But, cconsider this: You're in extreme turbulence. You're below your Va. And, you hold the control wheel totally static. That is to say, you make no control inputs. In other words, you let the aircraft do what it is forced to do at the mercy of the turbulence. In this hypothetical circumstance, gusts not considered, the control surfaces will stall before they're allowed to exceed their designed load factor limit.

Remember, stall is a function of, among several things, load factor.

Last edited by PantLoad; 2nd Mar 2013 at 03:47.
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Old 2nd Mar 2013, 04:14
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However, the Va does vary with weight...the basic premise is that at or below this speed the control surface "unloads" (stalls) before deformation an'/or structural failure occurs.
I'd have to disagree with you on that one. Va is a single declared speed, used to ensure that the control surfaces are not subject to excessive loads. I am familar with the wording of the FAA "Pilot's handbook of Aeronautical Knowledge", but the intent of the FAR regulations and the associated advisory circulars is clear - it is assumed that even if the pilot imposes maximum control deflection at Va, he or she will check the manoeuvre before the aircraft G limit is reached.

Vo was added to the Standards to address introduce a speed closer to the classic understanding of Va.

With regards the Airbus rudder issue, IIRC the design case for the fin includes that of a sudden displacement to maximum, followed by a return to neutral. In the case of the AA accident, the rudder was (repeatedly?) moved from stop to stop.
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Old 2nd Mar 2013, 04:28
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Yes, Va is a single declared speed and is valid at the aircraft's maximum gross weight.

Similarly, Vx and Vy vary with weight and altitude.....even though, as published, they're a single declared weight.

Stall speed....same situation.

With regard to the American Airlines accident and the cited Airbus brief, you are correct.
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Old 2nd Mar 2013, 04:36
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PantLoad...

However, the Va does vary with weight...the basic premise is that at or below this speed the control surface "unloads" (stalls) before deformation an'/or structural failure occurs.
As stressmerchant says, that isn't correct. Va does not vary with weight, contrary to what the FAA pilot handbook says - it seems the engineers don't talk to the instructors at the FAA! It's the misinterpretation of Va that leads the FAA and others to incorrectly say that Va varies based on weight. As I pointed out, that would only be true if Va was at the minimum possible speed for certification, meaning it was equal to Vo=Vs*sqrt(n).

You now see that Va relates only to control surfaces and their structure. What 'breaks' a control surface or its structure - force or acceleration? The answer is force, in this case, aerodynamic force that's proportional to the velocity squared of the air and the coefficient of lift (angle of attack). It is the force created by the air as the plane moves through it that will break the control surfaces and their structures - changing the weight of the airplane does nothing to affect this.

Some aircraft will say that Va is the maximum speed at which you will not overstress "the aircraft" and will say that this speed decreases with less than maximum weight. In this case they're taking into account the limit maneuvering load factor.

Another thing to consider is that the wings don't break off because of acceleration, they break off because of force. The speed at which the wings create enough lift to break off will always remain the same. The reason that Vo is related to acceleration (load factor) is because other items such as the floor in the airplane or engine mounts are at a known mass and, therefore, limiting the acceleration will ensure that the force applied to the structure won't exceed its limits - F=ma. If you were near empty weight, you could pull significantly more G than what your limit maneuvering load factor is before your wings snapped off. You probably would break other components that weren't designed for the higher load factor, but your wings won't break until they reach the aerodynamic force required to break them.
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Old 2nd Mar 2013, 07:45
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Guys...

I can not emphasize how important this conversation was to me...

Va is a TOTAL misundestood speed, even in ATPL todays books...

This is unacceptable...

Thank you very much.
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Old 2nd Mar 2013, 09:23
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Lantirn,

In addition to the wealth of information provided above, I'd like to point out a common error in do-it-yourself aerodynamic calculations. You have forgotten to account for various airspeed instrument and pressure error. The calculation for Va (actually, Vo according to information above) is Vs times sq rt of limit load, but where Vs and Va are both expressed in knots EAS. EAS is very similar to CAS for light aircraft.

The stall speed of a Katana at max weight at sea level with cruise flap is 54kts CAS. Do the maths on that, and you can see why the published Va of 104 is actually quite a conservative number.

My own experience with the DV20 leads me to conclude that the published Va is limited by rudder movement rather than elevator movement. It is a very large rudder supported by a long and slender aft fuselage. But I really don't know for sure, any more than anyone else outside of Diamond's design team...
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Old 2nd Mar 2013, 11:13
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Hi Oktas8,

Yes, EAS is considered after 300 knots.

Yes, indeed I missed that. According to AFM 43 kias is 50 cas, which makes sense now for 104 knots Vo, or Va(min).

However at limitations chapter, in definition of Va which is Vo as you stated, says "Do not make full or abrupt control movement above this speed, because under certain conditions the airplane may be overstressed by full control movement".

According to Steve Pomroy's article, ( The Flight Writer: Va: Not a Bad Speed, Just Misunderstood (Part 2) ) , you could overstress the airframe even before Vo with very fast increase of stick deflection.

I am not talking about Va which guarantees flight control structure limits, but talking as a Vo, because finaly that is, Vo.

So, according to the manual, you can make full or abrupt movements before Vo.

As far as I can understand, no, you can not!!!
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Old 2nd Mar 2013, 22:24
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According to Steve Pomroy's article, ( The Flight Writer: Va: Not a Bad Speed, Just Misunderstood (Part 2) ) , you could overstress the airframe even before Vo with very fast increase of stick deflection.
Are you talking about the 30% increase in lift due to rapid pitch rate? That has to do with dynamic stall - where a leading edge vortex (LEV) "provides additional suction over the upper airfoil surface as it convects downstream. This increased suction leads to performance gains in lift and stall delay, but the LEV quickly becomes unstable and detaches from the airfoil. The LEV detachment is accompanied by a dramatic decrease in lift and sudden increase in pitching moment."

This mostly has to do with helicopters and flapping wings, it would be hard to duplicate this with an airplane unless it was an aerobatic plane or military fighter jet. You can read more about dynamic stall here: Dynamic Stall | Aerodynamic Flow Control and Advanced Diagnostics Research Group

I wouldn't worry about it - but what you really should know is:
  1. At Va or below, you can make one full abrupt control movement and return to neutral and be sure your control surfaces won't break.
  2. Even below Va, if you apply abrupt control inputs in multiple axis or multiple oscillations, you are not guaranteed to not overstress the control surface or its supporting structure.
  3. Va does not vary based on weight.
  4. At all times you are required to ensure that the limit maneuvering load factor is not exceeded. It can be exceeded at speeds below Va.
If you keep those in mind, you'll have no problem operating an airplane safely - in this regard!
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Old 3rd Mar 2013, 02:02
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So, according to the manual, you can make full or abrupt movements before Vo. As far as I can understand, no, you can not!!!
The definitions of Va and other terms do change slightly from time to time, which may be why current AFMs don't seem to mention Vo. Also, I would never obey an article on the internet in preference to the manufacturer's instructions. I'm sure you feel the same way!

That said, I have taught in the past that Va protects from full or abrupt control movements. At Va you may do full or you may do abrupt, but not both together. This is often overly conservative advice, but it takes into account the reality that Va is not a bullet-proof panacea.

Edit: this idea that Va relates only to control surface strength is a new one to me. There seems no point at all in measuring and publishing a number that protects against elevator surface failure, but oh dear, so sorry your engine broke off, we didn't measure that. Vo, the varies-with-weight one, varies with weight in order to protect non-aerodynamic parts such as engine mounts.

Last edited by Oktas8; 3rd Mar 2013 at 02:08.
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Old 3rd Mar 2013, 02:27
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Oktas,

That said, I have taught in the past that Va protects from full or abrupt control movements. At Va you may do full or you may do abrupt, but not both together. This is often overly conservative advice, but it takes into account the reality that Va is not a bullet-proof panacea.
I think that's how misconceptions about these sorts of things start! You should be teaching what Va actually is and not what you determine to be a conservative estimate of it. Testing (FAR 23.423) dictates a "sudden movement... to the maximum aft movement". The definition of Va and the requirements for testing at Va of control surfaces and their supporting structure hasn't changed since at least 1965. There have only been slight wording changes. You can look that up under the historical FARs.

Va is bullet-proof for what it is. It gets the misconception that it isn't bullet-proof when people break airplanes while flying at or below Va but the problem isn't Va, the problem is the pilot's understanding of Va. Example: AA 587.

Most American Airlines pilots believed that the tail fin could withstand any rudder movement at maneuvering speed.
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Old 3rd Mar 2013, 08:50
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The definitions of Va and other terms do change slightly from time to time, which may be why current AFMs don't seem to mention Vo
Vo is a "recent" change - introduced in the late 90s IIRC. The change was due to the difference between the "pilot" and "engineer" understanding of Va - Vo is closer to the (incorrect) concept of Va that I was taught as a pilot.
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Old 3rd Mar 2013, 12:28
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It is crystal clear to me now.

But I see that even academic books such as oxford aviation ATPL books, are wrong about Va...this is a BIG misconception that leads more and more ATPL pilots to learn Va in the wrong way.

For example (Principles Of Flight, Oxford aviation, 2008)

DESIGN MANOEUVRING SPEED, VA
The highest speed at which sudden, full elevator deflection (nose up) can be made without exceeding the design limit load factor.
Then it explains where it is on a V-n diagram, and then states:

For example an aircraft with a 1g stalling speed of 60kt and limit load factor of 2.5 would have a VA of:
60 √ 2.5 = 95 kt
This is, Vo that is explained by Oxford, not Va.

Anyway, same approach is taken by Bristol Groundschool, Jeppesen and many more manuals, even engineering manuals.

This is a BIG misconception!!!

Last edited by Lantirn; 3rd Mar 2013 at 12:29.
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Old 3rd Mar 2013, 16:05
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This is, Vo that is explained by Oxford, not Va.
I think there be a little confusion.

§ 23.1507 Operating maneuvering speed.

The maximum operating maneuvering speed, VO , must be established as an operating limitation. VO is a selected speed that is not greater than VS √n established in § 23.335(c).

§ 23.335 Design airspeeds.

(c) Design maneuvering speed VA. For VA, the following applies:
(1) VA may not be less than VS √ n where—
(i) VS is a computed stalling speed with flaps retracted at the design weight, normally based on the maximum airplane normal force coefficients, CNA ; and
(ii) n is the limit maneuvering load factor used in design
(2) The value of VA need not exceed the value of VC used in design.

To summarise,

VA must be equal to or greater than VS √ n
VO must be equal to or less than VS √ n
You now see that Va relates only to control surfaces and their structure
Not so I'm afraid, I think you are misreading the intent behind § 23.423. VA relates to the entire airframe.

Operating Manoeuvring Speed, VO, is the maximum speed at which an aircraft in symmetrical flight at the specified flight weight and configuration will stall (unload) before exceeding limit load and sustaining possible structural damage. Aircraft are therefore aerodynamically g-limited by the lift line up to manoeuvring speed, and structurally g-limited by the load line above it. Manoeuvring speed is also the maximum speed for turbulent air penetration, although a speed somewhat less—fast enough to avoid stall yet slow enough to diminish the loads experienced—is usually recommended. (In an aircraft subjected to a sharp vertical gust of given intensity, the increase in structural load—and thus the acceleration the pilot feels—varies directly with airspeed.)

An example is the Cirrus SR20 which has the following Operating Manoeuvring Speeds (VO).

2900 lbs 135kts
2600 lbs 126kts
2200 lbs 116kts

Last edited by Brian Abraham; 3rd Mar 2013 at 16:15.
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Old 3rd Mar 2013, 16:21
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Yes Brian.

Vo definition makes sense.

Again, Oxford talks about Vo, not Va.

I also see that Va and limit load factor are not connected.

Do I miss something behind the lines?
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Old 3rd Mar 2013, 16:30
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Brian,

Not so I'm afraid, I think you are misreading the intent behind § 23.423. VA relates to the entire airframe.
I agree with everything you said, except that sentence. Why do you think that Va relates to the entire airframe?

You go on to explain Vo and how it's related to the entire airframe (by ensuring that the limit maneuvering load factor isn't exceeded), but Va does not do that.

Reading the FAA AC 23-19A will confirm that Va does not "permit the pilot unrestricted flight-control movement without exceeding airplane structural limits".
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Old 3rd Mar 2013, 18:05
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Maybe by saying "relates to the entire airframe", he means for example the tail empenage.
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