Flying_Corporal
New Member
Why does Va increase with weight? The heavier the plane the more load factor will be in the event of a turbulence or abrupt control input as a/s increases. Right?
Va ???
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Grabo172
Well-Known Member
It all has to do with Momentum...
the heavier the airplane, the more momentum it has (weight x speed), the longer it takes for the plane to change direction (objects in motion tend to stay in motion) so the resulting load factor is lower...
Conversely, the lighter the airplane the more effect the control forces have since less weight has to change direction, it changes direction faster, so the resulting load factor imposed is greater...
Think how rapid a 152 would change direction with a full control force input as compared to a 747...
the heavier the airplane, the more momentum it has (weight x speed), the longer it takes for the plane to change direction (objects in motion tend to stay in motion) so the resulting load factor is lower...
Conversely, the lighter the airplane the more effect the control forces have since less weight has to change direction, it changes direction faster, so the resulting load factor imposed is greater...
Think how rapid a 152 would change direction with a full control force input as compared to a 747...
EatSleepFly
Well-Known Member
Hmm..I've never heard it explained that way before.
My version (for what its worth):
As the weight of a given aircraft increases, so does the stall speed (and so does Va). That increase in stall speed allows for a wider range of airspeed at which the aircraft will stall before sustaining structural damage.
If the same aircraft is lighter, the stall speed will be lower (as will Va). Therefore, the speed at which a stall will occur before structural damage does, is also lower.
My version (for what its worth):
As the weight of a given aircraft increases, so does the stall speed (and so does Va). That increase in stall speed allows for a wider range of airspeed at which the aircraft will stall before sustaining structural damage.
If the same aircraft is lighter, the stall speed will be lower (as will Va). Therefore, the speed at which a stall will occur before structural damage does, is also lower.
Grabo172
Well-Known Member
From TheCFI.com...
[ QUOTE ]
Unlike VNO, the maneuvering speed varies in proportion to the square root of the mass of the airplane. The reason for this is a bit tricky. The trick is that VA is not a force limit but rather an acceleration limit. When the manufacturers determine a value for VA, they are not worried about breaking the wing, but are worried about breaking other important parts of the airplane, such as the engine mounts. These items don’t directly care how much force the wing is producing; they just care about the acceleration they are undergoing.
By increasing the mass of the airplane, you decrease the overall acceleration that results from any overall force. (Of course, if you increase the mass of cargo, it increases the stress on the cargo-compartment floor --- but it decreases the stress on unrelated components such as engine mounts, because the acceleration is less.)
To illustrate this point, consider what happens when the airplane is in a steep turn. Compared to unaccelerated flight:
1. The stalling speed increases (because the stalling angle of attack stays the same), and
2. the airspeed for best rate of climb increases (because the optimum angle of attack stays the same), but
3. the maneuvering speed remains the same (since it doesn’t directly depend on angle of attack).
[/ QUOTE ]
The Jeppesen Commercial Pilot Manual also has a good explaination to this same point, but It's too long to type...
[ QUOTE ]
Unlike VNO, the maneuvering speed varies in proportion to the square root of the mass of the airplane. The reason for this is a bit tricky. The trick is that VA is not a force limit but rather an acceleration limit. When the manufacturers determine a value for VA, they are not worried about breaking the wing, but are worried about breaking other important parts of the airplane, such as the engine mounts. These items don’t directly care how much force the wing is producing; they just care about the acceleration they are undergoing.
By increasing the mass of the airplane, you decrease the overall acceleration that results from any overall force. (Of course, if you increase the mass of cargo, it increases the stress on the cargo-compartment floor --- but it decreases the stress on unrelated components such as engine mounts, because the acceleration is less.)
To illustrate this point, consider what happens when the airplane is in a steep turn. Compared to unaccelerated flight:
1. The stalling speed increases (because the stalling angle of attack stays the same), and
2. the airspeed for best rate of climb increases (because the optimum angle of attack stays the same), but
3. the maneuvering speed remains the same (since it doesn’t directly depend on angle of attack).
[/ QUOTE ]
The Jeppesen Commercial Pilot Manual also has a good explaination to this same point, but It's too long to type...
AirmetTango
Well-Known Member
I've always explained it this way:
The basic lift formula (Lift = 1/2 * Density * Wing Surface Area * Angle of Attack * Velocity squared) tells us that a heavy C172 flying at 90 KTS must have a higher angle of attack than a light C172 flying at the same speed. Why? Because it's the only parameter of the formula that can change in this case to create more lift to counter the heavier weight. Since an airplane always stalls at the same critical angle of attack, the heavy 172 with the higher angle of attack is operating with less "leeway"...a sharp pull on the elevator will cause the plane to reach the critical angle of attack quickly and stall with a higher stall speed, preventing structural damage at 90 KTS. The light aircraft requires a larger deflection of the controls (probably more than full deflection) to reach the critical angle of attack and stall (lower stall speed), and is therefore more likely to experience structural damage at 90 KTS.
The basic lift formula (Lift = 1/2 * Density * Wing Surface Area * Angle of Attack * Velocity squared) tells us that a heavy C172 flying at 90 KTS must have a higher angle of attack than a light C172 flying at the same speed. Why? Because it's the only parameter of the formula that can change in this case to create more lift to counter the heavier weight. Since an airplane always stalls at the same critical angle of attack, the heavy 172 with the higher angle of attack is operating with less "leeway"...a sharp pull on the elevator will cause the plane to reach the critical angle of attack quickly and stall with a higher stall speed, preventing structural damage at 90 KTS. The light aircraft requires a larger deflection of the controls (probably more than full deflection) to reach the critical angle of attack and stall (lower stall speed), and is therefore more likely to experience structural damage at 90 KTS.
roundout
VNAV monitor
this is the correct explanation. i heard a kid offer our top aerodynamics professor the heavier=more momentum theory and he shot it so full of holes that the kid never asked another question all semester. this is also the way that it's explained in all the FAA publications, the ASA oral guides, the jepp manuals, etc.
RiddlePilot
New Member
I've heard 9,000 explanations on Va, but here's the ERAU Flightline Explanation Of The Day* (cue theme music):
<font class="small">Code:</font><hr /><pre>
n = L/W
where,
n = Load factor (gs)
L = Lift (lbs.)
W = Weight (lbs.)
</pre><hr />
-Normal category, 2550 lb. aircraft stressed to +3.8gs.
<font class="small">Code:</font><hr /><pre>
3.8 = L / 2550
(3.8)(2550) = L
L = 9690 lbs.
</pre><hr />
-The aircraft can create 9690 lbs. of lift at a weight of 2550 lbs. before exceeding its limit load factor.
-Now, reducing weight to 2400 lbs., but creating that same 9690 lbs. of lift on the wings:
<font class="small">Code:</font><hr /><pre>
n = L/W
n = 9690 / 2400
n = 4.0 gs
</pre><hr />
-Lowering the weight and creating the same 9690 lbs. of lift exceeds the limit load factor.
*Explanation subject to change without notice
<font class="small">Code:</font><hr /><pre>
n = L/W
where,
n = Load factor (gs)
L = Lift (lbs.)
W = Weight (lbs.)
</pre><hr />
-Normal category, 2550 lb. aircraft stressed to +3.8gs.
<font class="small">Code:</font><hr /><pre>
3.8 = L / 2550
(3.8)(2550) = L
L = 9690 lbs.
</pre><hr />
-The aircraft can create 9690 lbs. of lift at a weight of 2550 lbs. before exceeding its limit load factor.
-Now, reducing weight to 2400 lbs., but creating that same 9690 lbs. of lift on the wings:
<font class="small">Code:</font><hr /><pre>
n = L/W
n = 9690 / 2400
n = 4.0 gs
</pre><hr />
-Lowering the weight and creating the same 9690 lbs. of lift exceeds the limit load factor.
*Explanation subject to change without notice
Grabo172
Well-Known Member
[ QUOTE ]
Shoot some holes in it for me please. I don't think that's right. I think the two reasons are tied together very closely.
F=m*a => a = F/m
More mass = less acceleration. Load factor is a measure of acceleration.
[/ QUOTE ]
the figures in this equation are
F=Force
m=mass
a= AREA!!! not acceleration
Just to put it to rest if we can...
All the explainations above are correct... it's just a matter of how one looks at it and how you can remember and try to explain it to a student...
The stalling speed is proportional to the weight, and so is Va...
Another thing to look at when talking about VA is find a couple V-G diagrams for the same airplane if you can and visually show how it all changes...
Shoot some holes in it for me please. I don't think that's right. I think the two reasons are tied together very closely.
F=m*a => a = F/m
More mass = less acceleration. Load factor is a measure of acceleration.
[/ QUOTE ]
the figures in this equation are
F=Force
m=mass
a= AREA!!! not acceleration
Just to put it to rest if we can...
All the explainations above are correct... it's just a matter of how one looks at it and how you can remember and try to explain it to a student...
The stalling speed is proportional to the weight, and so is Va...
Another thing to look at when talking about VA is find a couple V-G diagrams for the same airplane if you can and visually show how it all changes...
kellwolf
Piece of Trash
Although that may have been a little harsh and over the top, he is nonetheless right. a = acceleration and A = area. Which would mean that a = F/m. That being said, mass and weight are different. Mass is the amount of molecules in a substance, weight is the substance's acceleration towards the center of the earth (9.8 m/s is anyone is interested).
Jonnyb9040
New Member
[ QUOTE ]
the figures in this equation are
F=Force
m=mass
a= AREA!!! not acceleration
[/ QUOTE ]
hmm. That must be that new physics I have been hearing about.
the figures in this equation are
F=Force
m=mass
a= AREA!!! not acceleration
[/ QUOTE ]
hmm. That must be that new physics I have been hearing about.
roundout
VNAV monitor
let's work with some hypothetical numbers. let's say you have an arrow at max gross weight - 2750# cruising around at 118kts, which is the Va for that weight. it's operating at, hypothetically, 7* AOA. critical AOA is, say, 17*. if you make a full abrupt stabilator deflection at this speed and weight, the wing has 10* AOA to travel before it stalls out. if this input is applied, the airplane will, in fact, reach that critical AOA and stall out before it reaches the 3.8g limit load factor. if you have the same arrow at 2000# travelling at the same 118kts, let's say it's operating at 2* AOA. now if you apply full abrupt stabilator deflection the airfoil will have to travel 15* to stall out. since the wings have to pitch up 50% farther, it takes that much more time to reach the critical AOA, all at the same time the airplane is being accelerated (load factor). the limit load factor will be exceeded before the airplane stalls out. the whole point of Va is to ensure that you could make a full abrupt control deflection and not break the airplane....the only way to ensure this is to make sure that the airplane stalls out before it has a chance to hurt itself.
good enough explanation?
good enough explanation?
RiddlePilot
New Member
[ QUOTE ]
let's work with some hypothetical numbers. let's say you have an arrow at max gross weight - 2750# cruising around at 118kts, which is the Va for that weight. it's operating at, hypothetically, 7* AOA. critical AOA is, say, 17*. if you make a full abrupt stabilator deflection at this speed and weight, the wing has 10* AOA to travel before it stalls out. if this input is applied, the airplane will, in fact, reach that critical AOA and stall out before it reaches the 3.8g limit load factor. if you have the same arrow at 2000# travelling at the same 118kts, let's say it's operating at 2* AOA. now if you apply full abrupt stabilator deflection the airfoil will have to travel 15* to stall out. since the wings have to pitch up 50% farther, it takes that much more time to reach the critical AOA, all at the same time the airplane is being accelerated (load factor). the limit load factor will be exceeded before the airplane stalls out. the whole point of Va is to ensure that you could make a full abrupt control deflection and not break the airplane....the only way to ensure this is to make sure that the airplane stalls out before it has a chance to hurt itself.
good enough explanation?
[/ QUOTE ]
I think you're making this waaaaay too complicated. The basic definition of Va has nothing to do with angle of attack and stall. It has to do with the amount of lift produced on the aircraft, and its proportion to the weight. The aircraft's angle of attack, as well as the fact that it does stall before it exceeds the limit load factor is a direct result of this. Make sure you don't confuse the cause and the effect.
The main problem that I see with using the AOA explanation is the fact that people skip the whole reason that you're exceeding the critical AOA. For example:
<font class="small">Code:</font><hr /><pre>
L = (Cl)(q)(S)
where,
L = Lift (lbs.)
Cl = Coefficient of Lift
q = Dynamic Pressure
S = Wing area (sq. ft.)
and,
q = ((Density ratio)(Velocity^2)) / 295
</pre><hr />
Notice how there is nothing mentioning the AOA in this equation. However, Cl has a direct relation to AOA in that as Cl increases, AOA increases as well. People tend to only consider AOA when it comes to Va explanations, and that is very incomplete. It's very important that you take everything into consideration here, which is summed up in the term "L" (Lift).
The AOA explanation is fine for a very basic understanding, but should not be completely relied on.
let's work with some hypothetical numbers. let's say you have an arrow at max gross weight - 2750# cruising around at 118kts, which is the Va for that weight. it's operating at, hypothetically, 7* AOA. critical AOA is, say, 17*. if you make a full abrupt stabilator deflection at this speed and weight, the wing has 10* AOA to travel before it stalls out. if this input is applied, the airplane will, in fact, reach that critical AOA and stall out before it reaches the 3.8g limit load factor. if you have the same arrow at 2000# travelling at the same 118kts, let's say it's operating at 2* AOA. now if you apply full abrupt stabilator deflection the airfoil will have to travel 15* to stall out. since the wings have to pitch up 50% farther, it takes that much more time to reach the critical AOA, all at the same time the airplane is being accelerated (load factor). the limit load factor will be exceeded before the airplane stalls out. the whole point of Va is to ensure that you could make a full abrupt control deflection and not break the airplane....the only way to ensure this is to make sure that the airplane stalls out before it has a chance to hurt itself.
good enough explanation?
[/ QUOTE ]
I think you're making this waaaaay too complicated. The basic definition of Va has nothing to do with angle of attack and stall. It has to do with the amount of lift produced on the aircraft, and its proportion to the weight. The aircraft's angle of attack, as well as the fact that it does stall before it exceeds the limit load factor is a direct result of this. Make sure you don't confuse the cause and the effect.
The main problem that I see with using the AOA explanation is the fact that people skip the whole reason that you're exceeding the critical AOA. For example:
<font class="small">Code:</font><hr /><pre>
L = (Cl)(q)(S)
where,
L = Lift (lbs.)
Cl = Coefficient of Lift
q = Dynamic Pressure
S = Wing area (sq. ft.)
and,
q = ((Density ratio)(Velocity^2)) / 295
</pre><hr />
Notice how there is nothing mentioning the AOA in this equation. However, Cl has a direct relation to AOA in that as Cl increases, AOA increases as well. People tend to only consider AOA when it comes to Va explanations, and that is very incomplete. It's very important that you take everything into consideration here, which is summed up in the term "L" (Lift).
The AOA explanation is fine for a very basic understanding, but should not be completely relied on.
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