VA Speed Question

[daddysquared]

I am going over the V speeds for the Seminole. I know this is something that I probably missed or forgot during my commercial training, but..........Why would VA be higher at a heavier weight than at a lighter weight, i.e., in the Seminole, VA at 3800 Lbs. is 135 while VA at 2700 Lbs. is only 112? Why is VA higher with more weight? If someone could help me out here it would be greatly appreciated.

 

[avbug]

Maneuvering speed is predicated upon stall speed. You will stall before you will bend the airplane with full deflection of a control surface, at a given rate of control input.

At heavier weights, you stall at higher speeds. Maneuvering speed in essence represents a margin above your stall speed. Increase stall speed, increase maneuvering speed.

 

[MarineGrunt]

Simply put...

Aircraft 1: Weighs 3800lbs
Aircraft 2: Weighs 3000lbs

Lets say in this example that they are both the same airplane and they are flying straight and level. Aircraft 1 will be flying at a higher angle of attack, in order to carry the additional weight. This causes the angle of attack to be closer to the CRITICAL ANGLE OF ATTACK, which the aircraft will ALWAYS stall. So as avbug said, this is why Va is increased, to allow an extra safety margin.

 

[kevdog]

Think of kicking a bowling ball or tennis ball.

Which can you kick farther? Which is heavier?

Now think of how the wind interacts with the airplane in flight. If it is heavier, it's going to have a harder time moving it, correct?

Just another way of looking at it.

 

[Dutch]

Mathmatical Va

OK, I learned the angle of attack method of explaining Va but the standards department at my university doesn't like that method. Here's an alternate method.

We all know the lift formula, right?
The individual components are as follows;
Co-efficient of lift
1/2 Rho
Velocity Squared
Surface area (lifting area)

We also need to know that Load Factor is Lift divided by weight.

We're gonna use some rounded numbers to make this easier to compute.

Lets say we are operating an aircraft with a load limit factor of 4 (i.e. we can pull 4 positive g's). This aircraft also has a max gross weight of 2,500# and a corresponding Va of 100 knots.

In this situation our maximum lift can be computed by multiplying load factor and aircraft weight.

2,500# x 4 = 10,000#

or when the formula is moved around...

4 = 10,000# / 2,500#


Now, lets lower the aircraft weight down to 2,000#. The load limit stays the same and assume the same airfoil, airspeed, air density (the amount of lift being produced is the same).

10,000# (lift created) / 2,000# (aircraft weight) = 5

5 g's is more than the limit of 4 and we just damaged the aircraft. We need to lower the lift created to 8,000# in order to prevent damaging the aircraft. The only parameter in the lift equation that we can easily change is airspeed...we have to slow to the lower Va to prevent damaging the aircraft.

Dutch

 

[CrewDawg]

Think of kicking a bowling ball or tennis ball.

Which can you kick farther? Which is heavier?

Now think of how the wind interacts with the airplane in flight. If it is heavier, it's going to have a harder time moving it, correct?

Just another way of looking at it.

Love the analogy

 

[PA31Ho]

A heavier aircraft is greater inertia than a lighter aircraft. It will take more force to move a heavier aircraft than it would to move a lighter aircraft.

I read a book "Multi-Engine Flying" about 1 1/2 years ago. In that book was a GREAT analogy that I haven't forgotten:

Imagine a bowling ball and a tennis ball rolling down a bowling lane. When both balls are half way down, somebody opens a side door and a big gust of wind blows through. The tennis ball will move much farther to the side than the bowling ball because the tennis ball has less inertia - it will move easier.

Now, I may not be repeating tha analogy word for word, but I'm just trying to get the point across.

Hope it helped.

 

[Dutch]

Va or Vno

I understand the logic behind the bowling ball and tennis ball method but isn't the analogy being used to describe Vno and not Va? Va only mentions control deflections and says nothing about winds or turbulance. The maximum turbulent air penetration speed is Vno, not Va.

Dutch

 

[avbug]

Again, Va is predicated on achieving stall with rapid full control deflection prior to overloading the airframe; it's predicated on stall speed, which increases with weight.

 

[Singlecoil]

Just to second what avbug said, Va concerns stalling before exceeding the POSITIVE g design limit. You can still exceed the negative g design limit at Va. Like avbug said, the aircraft will stall before it exceeds the positive g (3.8 for normal, 4.4 for utility category) limit. If you look at a Vg diagram, you can see that you will still exceed the negative limit (1.52 or 1.73 respectively).

All the V speeds, except Vne, change according to the following formula:


(square root of (current weight divided by gross weight)) times published speed

 

[JRSLim]

Re: Va or Vno

I understand the logic behind the bowling ball and tennis ball method but isn't the analogy being used to describe Vno and not Va? Va only mentions control deflections and says nothing about winds or turbulance. The maximum turbulent air penetration speed is Vno, not Va.

Dutch

The Definition of Va I have is 'speed for maximum control deflection' OR rough air airspeed - Full abrupt deflection of primary flight controls or gust-induced loads can exceed the structural design limit.' While the same reference defines Vno as a speed when cruising at or below, "should not" be damaged by a 30 ft/second vertical gust. '

The ball anolgy is completely accurate in describing Va, because it involves how each mass will react to a load.


best V speed info I've found:
http://www.dc3airways.com/TechEd/te_real_vspeed.pdf

 

[Dutch]

DC3 Airways?

Just to clarify. You're using data from a "virtual" airline to explain Va?

 

[JRSLim]

Vitual Va

Yeah, I reckon I am.

I found this page a while back. It is the most complete listing of all the V speeds I have seen, and the definitions are accurate.

cheers

 

[flynryan15]

Here is another analogy

Women A weighs 225 lbs it is going to take you twice as much force to move her fat ass out of your bed when you sobber up as Women B who weighs 115(who has summer teeth you know some are here some are there)

WOW! what a THREESOME. Also Women A is 225 at 5'1 so she is obviously wider so she is more stable making her well we know more stable.

Then think of your airframe (or as in this case hips) while being riden by Women A you have exceeded you load limits and while have structual failure( Broken Pelvis)

Hope this helps

 

[avbug]

Flyinryan,

If I could understand your post, I could comment accurately on the specific stupidity of it, but you appear to have English as a second language. The recognition of the general idiocy of your post will have to suffice.

Maneuvering speed is predicated on the ability of the pilot to hurt the airplane by maneuvering it. In your anology, flynryan15, it has nothing to do with what women you've been with, but the degree of force you apply to yourself. A better anology might be which hand you use when you conjure up such foolishness. That's really something none of us wishes to know.

Maneuvering speed is predicated on the ability to achieve an aerodynamic stall, and thus prevent exceeding design load limitations, during application of full flight control input. For roll input the assumption is made that for normal, utility, and commuter categories, 100 percent of the semispan wing airload acts on one side of the airplane and 75 percent of this load acts on the other side.

The design load limitations are the maneuvering limitations as established above by Singlecoil, adding to that the limitation of a factor of 2.1 for commuter and normal category, expanded to 3.8 under certain weight conditions.

Maneuvering speed may not be less than stalling speed (Part 23), and may not permit exceeding the maneuvering design load limitations of the airframe.

It has nothing to do with big women and their number of teeth, or the load that a woman might impose. It has to do entirely with the ability of the airplane to stall and either stop increasing, or decrease the aerodynamic load imposed with full control deflection. Not kicking tennis balls, not rolling bowling balls, and not sleeping with two or more inebriated, dentally impaired females on any given occasion.

Maneuvering speed increases with weight strictly as a function of the increase in stall speed. Maneuving speed represents a margin above stall. Rapid, full control deflection will impose a certain load on a given airplane at a given weight. If the aircraft can be stalled, the rate of increase of the imposed load (the load factor) will stop, or reverse; the load is thereby alleviated.

At light weights, the airplane stalls (flaps up, upon which Va is predicated) at lower speeds. The aircraft will experience an aerodynamic stall at lesser speeds, ergo maneuvering speed is lesser. At heavier weights, stalling speed increases, and so does maneuvering speed as a function of a margin above the stall.

It has nothing to do with inertia, and the ability to resist displacement due to increased weight. (the bowling ball and tennis ball analogy). It has nothing to do with a broken pelvis (the intoxicated titilation story). It has everything to do with stalling speed and the ability to relieve the load aerodynamically prior to exceeding design limits.

The heavy woman anology and the bowling ball parable has applicability in determining why a heavier wing loading leads to a smoother ride in turbulence, but not to the sliding effect of Va.

 

[kevdog]

Va - the calibrated design maneuvering airspeed, which is the maximum speed at which the limit load can be imposed (either by GUSTS or full deflection of the control surfaces) without causing structural damage.

- taken from the ATP oral guide book. So the bowling / tennis ball analogy is also correct.

 

[flynryan15]

Avbug

It Was called a joke!!!!!!!!!!!!!!!!

 

[avbug]

Kevdog

No, it really isn't. Gust factors are mathmatical equations used to determine air loads and design limitations. When we speak of gust factors, we don't speak of gusts of wind, per se, but loading values on the aircraft.

The bowling ball analogy stipulates that a heavy object is harder to move than a small object. Compared to stall speed and angle of attack, this is not a good example.

The only issue with maneuvering speed is that design limits will not be exceeded under a certain load. Full application of control input equates to a certain gust value, to which end airspeed limits are established to ensure that a given margin between stall speed and Va exists.

The aircraft will resist displacement at a higher speed, just as a bowling ball in motion resists displacement by an outside force more than a tennis ball might. However, the bowling ball cannot stall, and that concept has nothing to do with the speed at which the airplane stalls. At a heavier weight, the wing is already flying at a greater angle of attack for any given weight or airspeed. As the wing is flying at a higher angle of attack for a given airspeed, it's closer to a stall. In other words, it will stall at a higher speed.

Having nothing to do with displacement, given a margin above the stall speed, an increase in stall speed equates to an increase in the maneuvering speed. The aircraft may be operated at a higher speed, and exposed to potentially higher loads, because it has a higher stall speed, and will stall sooner than a lighter aircraft. It has nothing to do with being displaced by gusts (or the resistance thereto), but with load factors imposed by gusting moments on the structure. The only issue at stake is that the aircraft stalls sooner, not that it can take a greater gust load. The maximum load factor remains constant, the margin relatively constant, but the stall speed increases, and therefore the maneuvering speed does, too.

The ATP study guide is about as far from an authoritative source as one can get, but for these purposes, it will suffice.

You may also reference:

FAA H-8083-3 Airplane Flying Handbook, Chapter 5:

The design maneuvering speed is the maximum speed at which the airplane can be stalled or full available aerodynamic control will not exceed the airplane’s limit load factor.

AC 61-23 Pilot’s Handbook of Aeronautical Knowledge, Chapter 1:

The maximum speed at which an airplane can be safely stalled is the design maneuvering speed. The design maneuvering speed is a valuable reference point for the pilot. When operating below this speed, a damaging positive flight load should not be produced because the airplane should stall before the load becomes excessive. Any combination of flight control usage, including full deflection of the controls, or gust loads created by turbulence should not create an excessive air load if the airplane is operated below maneuvering speed.

Design maneuvering speed can be found in the Pilot’s Operating Handbook or on a placard within the cockpit. It can also be determined by multiplying the normal unaccelerated stall speed by the square root of the limit load factor. A rule of thumb that can be used to determine the maneuvering speed is approximately 1.7 times the normal stalling speed.

The amount of excess load that can be imposed on the wing depends on how fast the airplane is flying. At slow speeds, the maximum available lifting force of the wing is only slightly greater than the amount necessary to support the weight of the airplane. Consequently, the load factor should not become excessive even if the controls are moved abruptly or the airplane encounters severe gusts, as previously stated. The reason for this is that the airplane will stall before the load can become excessive. However, at high speeds, the lifting capacity of the wing is so great that a sudden movement of the elevator controls or a strong gust may increase the load factor beyond safe limits.

"Because of this relationship between speed and safety, certain “maximum” speeds have been established. Each airplane is restricted in the speed at which it can safely execute maneuvers, withstand abrupt application of the controls, or fly in rough air. This speed is referred to as the design maneuvering speed, which was discussed previously.

Summarizing, at speeds below design maneuvering speed, the airplane should stall before the load factor can become excessive. At speeds above maneuvering speed, the limit load factor for which an airplane is stressed can be exceeded by abrupt or excessive application of the controls or by strong turbulence

 

[tathepilot]

Re: Mathmatical Va

OK, I learned the angle of attack method of explaining Va but the standards department at my university doesn't like that method. Here's an alternate method.

We all know the lift formula, right?
The individual components are as follows;
Co-efficient of lift
1/2 Rho
Velocity Squared
Surface area (lifting area)

We also need to know that Load Factor is Lift divided by weight.

We're gonna use some rounded numbers to make this easier to compute.

Lets say we are operating an aircraft with a load limit factor of 4 (i.e. we can pull 4 positive g's). This aircraft also has a max gross weight of 2,500# and a corresponding Va of 100 knots.

In this situation our maximum lift can be computed by multiplying load factor and aircraft weight.

2,500# x 4 = 10,000#

or when the formula is moved around...

4 = 10,000# / 2,500#


Now, lets lower the aircraft weight down to 2,000#. The load limit stays the same and assume the same airfoil, airspeed, air density (the amount of lift being produced is the same).

10,000# (lift created) / 2,000# (aircraft weight) = 5

5 g's is more than the limit of 4 and we just damaged the aircraft. We need to lower the lift created to 8,000# in order to prevent damaging the aircraft. The only parameter in the lift equation that we can easily change is airspeed...we have to slow to the lower Va to prevent damaging the aircraft.

Dutch

super!! math does not lie!!

 

[kevdog]

Avbug,

LOL, you haven't memorized your ATP oral guide book?

You are right about Va and stall speed, etc. but I was only trying to create a different way of looking at it to simplify remembering.

Maybe you can help me, this thread made me wonder what the Va is for the 727 since we never learned it in groundschool or is it published in any of our manuals.

I couldn't imagine a full deflection of any flight control while airborne at any speed or weight.

The only thing we have to know is turbulent air penetration target airspeed which is .80M or 280 KTS IAS, whichever is lower. 250 KTS below 15,000 at maximum landing weight or less is also acceptable.