Why does Va increase with weight?

[shurshot17]

C-172
2300 lbs 97 va
1950 lbs 89 va
1600 lbs 80 va

I tried to find the answer but the POH, ground up, ect gave me no answer to why Va increases with weight

So I asked my instructor, got some crazy explanation that made no sense to me.. said he doesnt know an easier way to explain it...



Appreciate it if someone could explain it on here

Thanks,
Shurshot

[Oxi]

You may want to find a different instructor!

I'm fairly certain that From The Ground Up will have it.

Think of a Cessna 172 and a 747 going 100 knots. They both pull up, the 747 is a lot heavier and the G forces are going to put a lot more than a 172. Which do you think will stall first? Which do you think will have an easier time maneuvering?

[FenderManDan]

Search here for the collonel sanders and the same topic.

[Justjohn]

It is simple but not often well understood and it certainly is not intuitive.

Maneuvering speed, or Va, is directly related to stall speed. Airplanes in the normal category are certified to 3.8 G's. At manovering speed , under 3.8 G's of acceleration, the airplane will stall. Or, it supposed to anyways. Va varies with weight as stall speed varies with weight.

The stall is a form of aerodynamic 'relief' if you will.

In addition, the controls are rigged and built strong enough to withstand full and abrupt deflection at this speed.

[POATC]

OK. Picture two bleach bottles bouncing around in a choppy sea, one is 3/4 full and the other is empty.The fuller one wallows around where as the empty one gets thrown around wildly. The accelration forces (G) are much higher on the empty one as it is tossed around more.
Now in an airplane we can stall. Stall speed increases with load (G) wether that be load in the plane or aerodynamic load. If your airplane is designed to pull 3.8G you don't want to exceed that number. Va allows you to calculate the maximum speed at which the wing will stall before exceeding the 3.8G load factor. Remember a stalled wing has no load, that's why we fall to our doom when we stall. In turbulence, the choppy air loads the wings and when the wing stalls it is just a momentary thing to set off the horn and alarm the passengers.
Clear as mud ?

[rob-air]

C-172

vs 44 kts@ 2300lbs

2000lbs X 4.4= 8800lbs

2300 X 3.8= 8740 lbs

let s assume 8800lbs wing loading is max


now meet Sally, a 100pound pilot that likes to fly with her 1450lbs empty weight 172 with only 10 galons of fuel.

Sally 's 172 on take off 1610 lbs

Sally also like to pull g's. She could pull 5.46 g's before overloading the wings (8800/1610)
By doing so she might break other stuff like engine mounts, radio stacks...ect, so she knows she should limit herself to 4.4G's

Now whats her VA? The speed she wil stall reaching 4.4g's.

Sally's Vs is less than 44kts, in fact it is close to 37 kts ( using the simple lift formula to find CLmax)

well sqrtr 4.4 X 37(Vs) = 77 kts

[cgzro]

Imagine a car stopped and idling on the ice. On the passanger seat you place a bucket with water which you dont want to spill.

If the car is light it can accellerate or turn / brake quick enough on the ice to spill the water. However add enough weight to the car and at some point it cannot accellerate or turn or brake fast enough to spill the water.

Short story the lighter a car on ice the more accelleration it can inflict on things inside it.

Obviously car = airplane, ice = air, slipping = stalling

[Chris M]

Two airplanes in level flight, both at 100 kts. One weighs 1000 lbs, the other weighs 2000 lbs.
In level flight the lift to weight ratio is 1:1 (lift=weight, since we're just sitting around pulling 1G).

Lets assume that at 100 kts and max AOA the wings can generate 5000 lbs of lift (I'm pulling all these numbers out of my butt, they're just for illustration purposes).

Pull back all the way in the 1000 lbs airplane and you get 5000 lbs of lift, divided by the 1000 lbs of weight, resulting in 5G's. Now lets say that 5G is the max for the airframe.

Pull back in the 2000 lb airplane and you get the same amount of lift, but in this case 5000/2000=2.5G.

To get 5G in the heavy airplane you need more lift. In fact, you need twice as much (5G x 2000 lb = 10,000 lbs). Since we're at max AOA and can't get any more without stalling the only other way to get more lift is more speed.

I always enjoy reading different schools of thought on the same question. Seeing something through other people's eyes is always an opportunity to learn and see things differently.

[rob-air]

Imagine a car stopped and idling on the ice. On the passanger seat you place a bucket with water which you dont want to spill.

If the car is light it can accellerate or turn / brake quick enough on the ice to spill the water. However add enough weight to the car and at some point it cannot accellerate or turn or brake fast enough to spill the water.

Short story the lighter a car on ice the more accelleration it can inflict on things inside it.

Obviously car = airplane, ice = air, slipping = stalling

Bad analogy, friction not taken into account.

[digits_]

Remember a stalled wing has no load, that's why we fall to our doom when we stall.

This is just not true. A stalled wing is still generating lift. Not enough to fly level, but we don't necessarily fall to our doom and the wing is still generating lift.

[Heliian]

Wing loading increases.

[POATC]

Remember a stalled wing has no load, that's why we fall to our doom when we stall.

This is just not true. A stalled wing is still generating lift. Not enough to fly level, but we don't necessarily fall to our doom and the wing is still generating lift.

Yes indeed, very true. A stalled wing does create lift and wings ALWAYS have a load on them. But we are talking load limits here, loads that can cause damge to the aircraft and a stalled wing does not have any of those. As for falling to our doom, I thought I was addressing big boy and big girl aviators here and that they would recognize poetic licence when they saw it as they would realize what was meant by the load statement.

[photofly]

As for falling to our doom, I thought I was addressing big boy and big girl aviators here and that they would recognize poetic licence when they saw it as they would realize what was meant by the load statement.

You know, there are a lot of people who really do think that "stall" means *no* lift, and who haven't worked out that that would mean they would be weightless, and so would see everything floating around the cabin in a most unusual fashion. Which they aren't, and don't. And that the aircraft would indeed be falling to its doom. Which it doesn't.

So while answering a question posed by someone who's early in the process of learning to fly, it's worth avoiding any ambiguity - or poetic licence - on the subject.

[Heliian]

^drag is not lift, but yes some of the wing may still be functioning.

[cgzro]

With sufficient thrust you can fly around stalled for extended periods of time without loosing altitude.
Its a great excercuse actually, nose high , lots of power wing shaking , ailerons possibly reversed and rudder required to stay perfectly coordinated lest you get turned over. Highly recommended n an appropriate aircraft.

[rob-air]

^drag is not lift, but yes some of the wing may still be functioning.

Not sure what you meant by that, I am under the impression that CL would imply lift only and not the result of all aerodynamic forces.

[eh3fifty]

This is always a fun topic! What is "maneuvering speed"? You can ask some pilots/engineers and they'll tell you that maneuvering speed doesn't change with weight - and they're correct. But to most pilots, maneuvering speed (generally known as Va), will decrease when weight decreases. Maneuvering speed means different things to different people.

So what is maneuvering speed?

They tell you in flying school that pulling more than the rated G limit will "overstress" the aircraft. But G is a way of expressing acceleration - acceleration has never broken anything! Force breaks things, not acceleration. You must understand that.



I'm sure most of you are familiar with this test. What's happening is the wing is being forced upwards to the point of failure. The airplane is not accelerating at all - the wings are bending due to the force applied to them.

Here's a video: https://youtu.be/sA9Kato1CxA

Here's an example (disregard tail downforce and moments):

A new Cessna airplane is flying in straight and level flight at 4,000 lbs gross weight. What is the force acting on the wings? 4000 lbs. Let's say the wings will break off when the force acting on them reaches 20,000 lbs. At 200 knots airspeed, and at the AoA for maximum lift, the wings will create 20,000 lbs of lift.

So,

...at 4,000 lbs gross weight, at what speed (at maximum lift) will the wings break off? 200 knots;

...at 8,000 lbs gross weight, at what speed (at maximum lift) will the wings break off? 200 knots.

Why? Because weight of the airplane has nothing to do with the force created by the wings - force on the wings has everything to do with airspeed and AoA.

At the lower gross weight there will be less force resisting the 20,000 lbs of upward force, meaning there will be higher acceleration on the lighter airplane.

The 4,000 lb airplane would be experiencing 5Gs of acceleration, whereas the 8,000 lb airplane would only be experiencing 2.5G of acceleration.

In other words, when you're flying at 4,000 lbs, you would be limited to 5G - when you're flying at 8,000 lbs, you would be limited to 2.5G. So, as weight decreases, you can pull more G before the wings break off. That is a simple fact for any airplane.

So what is Va, specifically?

Va is a design airspeed that the manufacturer chooses. At this speed, the manufacturer needs to show that full deflection of each horizontal and vertical control surface will not overstress the "control surface and its supporting structure" (CAR 523.423 & 523.441).

The key is that Va is not a speed to ensure that you do not overstress the aircraft - it is specifically for the control surfaces and their supporting structures. You as the pilot must ensure that you do not exceed any limitations (including acceleration limitations - more on that later).

Maneuvering speed is defined under CAR 523.335(c). For the FARs reference, drop the first '5' - FAR 23.335(c).

Va may not be less than Vs(root)n - where 'n' is the limit maneuvering load factor. Va need not exceed Vc.

It is only in the case of the manufacturer choosing to use the lowest acceptable Va speed that you may then conclude that reducing the speed at which you apply full elevator deflection (using the standard weight vs stall speed equation) will prevent you from exceeding the limit maneuvering load factor!

If the manufacturer chooses a higher than minimum Va speed, using the weight vs stall speed equation will give you a higher airspeed than you want - if you increase the AoA to max at that higher airspeed, you will exceed the limit maneuvering load factor of the airplane!

I showed how acceleration doesn't matter with regard to things breaking, so why do you still have to respect the limit maneuvering load factor of the airplane (other than you being required to follow the limitations section of your POH/AFM)?

Consider the baggage floor in your airplane. Since the force that will cause the floor to fail is known to the manufacturer, the only variables left to determine are acceleration and mass (weight).

F=ma

10,000 lbs required for the floor to fail; manufacturer determines they need to be able to pull a minimum of 3.8G for the limit maneuvering load factor (because he read the CARs/FARs); therefore the manufacturer calculates that the maximum weight permissible in that baggage compartment is 2,630 lbs.

The baggage compartment is then placarded at 2,630 lbs and the POH says don't exceed 3.8G.

There are many issues like this in an airplane: engine mounts; cabin floor; instrumentation compartment floor/supports, etc.

For these reasons, you must respect the limit maneuvering load factor - however, remember that it is force that breaks things, not acceleration!

At a lighter weight, your speed must be reduced to ensure you do not exceed the limit maneuvering load factor, however, you may still fly at the published maximum Va (at your lighter weight) and apply full control deflection of the rudder and ailerons (separately, and only once).

Also, Va does not prevent you from exceeding the negative limit maneuvering load factor, nor against your wings breaking off when forced the opposite direction due to negative G.

Va is one of the most common things to get presented wrong to a student. I am addressing the topics that have been mentioned in this thread - this is not necessarily the way I would explain it to a student pilot.

Hopefully it has been somewhat easy to follow. Please ask any questions or point out any errors I may have made.

[eh3fifty]

To add to my post: http://i.imgur.com/jpLxkjC.jpg

This shows that as weight decreases, the flight envelope opens up and you can pull more G. Figure 3.9.4 shows the allowable flight envelope at 75% of max gross weight.

Read the first four sentences at the top of the page.

Then near the bottom it says: "Notice that the maneuvering speed and the VNE airspeed do not change with the actual gross weight of the aircraft." Which is what I said at the top of my original post.

Maneuvering speed being at the top left corner of the graph. Which is why it's often called "corner velocity (speed)".

And at the very bottom of the page: "Figure 3.9.4 V-n diagram at two different gross weights for a typical general aviation aircraft, showing that the maneuvering speed is independent of gross weight."

For some context, here is the previous page: http://i.imgur.com/srIlXWS.jpg

[photofly]

this is not necessarily the way I would explain it to a student pilot.

How would you explain to to a student pilot like the person who posted the thread?

[eh3fifty]

The reason for the decrease in Va with a decrease in weight means they're looking to ensure that you don't exceed a certain G limit.

Hopefully student understands why it's not good to exceed the G limit in the POH.

Then explain that to do so you need to limit the max lift the wings can create as you decrease your weight so as to maintain the same G at max lift AoA. Usually an example of an airplane at 1,000 lbs and one at 2,000 lbs works well.

Look up stall speeds in the POH. Show them the equation for determining the lower airspeed. Point out the need to go from indicated to calibrated and then back to indicated.

Something like that.