Structural Cruising Speed vs. Manuevering Speed
- Thread starter TallFlyer
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turbojet28
Well-Known Member
The reason that Va increases with an increase in weight that I was taught is this. Load factor is proportional to lift force. When the lift force doubles, the load factor doubles for a given weight. The way lift will double is if the AOA is doubled. Since Va corresponds to a given AOA, it makes sense then to stay that at an increased weight, you must go faster to have the same AOA that you would at a lighter weight (b/c if you were going the same speed as you were at the lighter weight, then AOA would have to be larger to provide the needed lift to maintain altitude). I should warn you that I am merely a PPL and this is based off of my understanding of the way Rod Machado taught it. I haven't reviewed it in a while and I am probably beating the technical side of this to a bloody pulp.
Just another thought (or a question, I guess); if the critical AOA is about 18 degrees for most airfoils, and a Cessna 172N is limited to 3.8g's per the limitations, does this mean that, by definition of Va, that at Va you should be at an AOA of about 4.7 deg? This would make sense because if you increase this 3.8 times (for a load factor of 3.8) it would put you right at a stall of about 18deg. I'm curious as to what some others have to say.
I'm not sure about Vno, but I've heard that it is more based on just controllability than load factors. Is there anything to this?
Just another thought (or a question, I guess); if the critical AOA is about 18 degrees for most airfoils, and a Cessna 172N is limited to 3.8g's per the limitations, does this mean that, by definition of Va, that at Va you should be at an AOA of about 4.7 deg? This would make sense because if you increase this 3.8 times (for a load factor of 3.8) it would put you right at a stall of about 18deg. I'm curious as to what some others have to say.
I'm not sure about Vno, but I've heard that it is more based on just controllability than load factors. Is there anything to this?
kellwolf
Piece of Trash
Re: Structural Cruising Speed vs. Manuevering Spee
[ QUOTE ]
I'm not sure about Vno, but I've heard that it is more based on just controllability than load factors. Is there anything to this?
[/ QUOTE ]
Actually there's an aerodynamic chart that engineers use for that. It's got load factors, angle of attack and airspeed data on it. Between Vno and Vne is the "yellow zone" on the chart where structural damage MAY occur since you're above Va. Flying in the yellow arc is sometimes just as controllable as flying in the green, but if you hit a REALLY big bump, you run the risk of structural damage.
[ QUOTE ]
I'm not sure about Vno, but I've heard that it is more based on just controllability than load factors. Is there anything to this?
[/ QUOTE ]
Actually there's an aerodynamic chart that engineers use for that. It's got load factors, angle of attack and airspeed data on it. Between Vno and Vne is the "yellow zone" on the chart where structural damage MAY occur since you're above Va. Flying in the yellow arc is sometimes just as controllable as flying in the green, but if you hit a REALLY big bump, you run the risk of structural damage.
seagull
Well-Known Member
Re: Structural Cruising Speed vs. Manuevering Spee
I am not that familiar with Part 23 issues, and there is not Vno for Part 25 (actually, Vmo/Vno pretty much covers you there). However, a quick look reveals the answer is reasonabley straight forward:
From 23.1505:
(b) The maximum structural cruising speed VNO must be established so that it is—
(1) Not less than the minimum value of VC allowed under §23.335; and
(2) Not more than the lesser of—
(i) VC established under §23.335; or
(ii) 0.89 VNE established under paragraph (a) of this section.
23.335 Design airspeeds.
top
Except as provided in paragraph (a)(4) of this section, the selected design airspeeds are equivalent airspeeds (EAS).
(a) Design cruising speed, VC. For VC the following apply:
(1) Where W/S′=wing loading at the design maximum takeoff weight, Vc (in knots) may not be less than—
(i) 33 √(W/S) (for normal, utility, and commuter category airplanes);
(ii) 36 √(W/S) (for acrobatic category airplanes).
(2) For values of W/S more than 20, the multiplying factors may be decreased linearly with W/S to a value of 28.6 where W/S=100.
(3) VC need not be more than 0.9 VH at sea level.
(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.
(notice also that max speed for turb penetration is not actually the same as Va...)
(d) Design speed for maximum gust intensity, VB. For VB, the following apply:
(1) VB may not be less than the speed determined by the intersection of the line representing the maximum positive lift, CNMAX, and the line representing the rough air gust velocity on the gust V-n diagram, or VS1√ ng, whichever is less, where:
(i) ng the positive airplane gust load factor due to gust, at speed VC (in accordance with §23.341), and at the particular weight under consideration; and
(ii) VS1 is the stalling speed with the flaps retracted at the particular weight under consideration.
(2) VB need not be greater than VC.
So, you can see directly why Va varies with weight, as it tracks the stall speed, where Vno is essentially just going to keep you at a speed that is safe for most of the stuff you might encounter. Also, most of the time, the turbulent area is not at its maximum intensity at the outer edges of the area, so you will hopefully be able to slow down before it gets outside the design envelope.
I am not that familiar with Part 23 issues, and there is not Vno for Part 25 (actually, Vmo/Vno pretty much covers you there). However, a quick look reveals the answer is reasonabley straight forward:
From 23.1505:
(b) The maximum structural cruising speed VNO must be established so that it is—
(1) Not less than the minimum value of VC allowed under §23.335; and
(2) Not more than the lesser of—
(i) VC established under §23.335; or
(ii) 0.89 VNE established under paragraph (a) of this section.
23.335 Design airspeeds.
top
Except as provided in paragraph (a)(4) of this section, the selected design airspeeds are equivalent airspeeds (EAS).
(a) Design cruising speed, VC. For VC the following apply:
(1) Where W/S′=wing loading at the design maximum takeoff weight, Vc (in knots) may not be less than—
(i) 33 √(W/S) (for normal, utility, and commuter category airplanes);
(ii) 36 √(W/S) (for acrobatic category airplanes).
(2) For values of W/S more than 20, the multiplying factors may be decreased linearly with W/S to a value of 28.6 where W/S=100.
(3) VC need not be more than 0.9 VH at sea level.
(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.
(notice also that max speed for turb penetration is not actually the same as Va...)
(d) Design speed for maximum gust intensity, VB. For VB, the following apply:
(1) VB may not be less than the speed determined by the intersection of the line representing the maximum positive lift, CNMAX, and the line representing the rough air gust velocity on the gust V-n diagram, or VS1√ ng, whichever is less, where:
(i) ng the positive airplane gust load factor due to gust, at speed VC (in accordance with §23.341), and at the particular weight under consideration; and
(ii) VS1 is the stalling speed with the flaps retracted at the particular weight under consideration.
(2) VB need not be greater than VC.
So, you can see directly why Va varies with weight, as it tracks the stall speed, where Vno is essentially just going to keep you at a speed that is safe for most of the stuff you might encounter. Also, most of the time, the turbulent area is not at its maximum intensity at the outer edges of the area, so you will hopefully be able to slow down before it gets outside the design envelope.
why Va changes with weight? I always explain this using numbers, its easy and gives proof. Probably be tougher to understand in writing then with words but i will do my best here
(example used is for PA-44)
equations used: Load factor= L/W
Lift Equation= Cl x 1/2qV^2 x S
cl= lift coefficient
q=dynamic pressure
V= velocity
S= wing surface area
now if you look at the lift equation and the varibles that change lift. The only one in there that the pilot has direct control over is the Velocity. Can't change the Cl (thats an engineer thing, i dont know anything about), can't change dynamic pressure (basically air denisity), cant change the surface area (please dont say something stupid like "i could put in flaps"), so the only thing that we are left with is Velocity, which we can change. So lets put some numbers in our load factor equation
Maneuvering speed is 135 KTS (PA-44) at a max gross weight of 3800lbs. Thus, it would be able to develop a maximum lift of 14,400 lbs (3.8= L/3800) at 135 KTS before it stalled. Now for argument sake, lets say that we kept the same maneuvering speed of 135 KTS (thus same lift of 14,400lbs will be produced) but lowered the weight to the 2700lbs. Now the equation would look like this: 14,440/2700= 5.3 or a load factor of 5.3. Thus, with a lower weight but same maneuver speed our load factor increased by 1.5 and we damaged the airplane since our max load factor was 3.8. Thus, we have to have a lower maneuver speed when we have a lower weight, so we don’t damage the aircraft
Probably gonna have to think about that for a while and play with the numbers, hopefully that explanation will work for you. I used this explanation for everytime i got asked that question on a checkride, and it pretty much shut the examiner up.
(example used is for PA-44)
equations used: Load factor= L/W
Lift Equation= Cl x 1/2qV^2 x S
cl= lift coefficient
q=dynamic pressure
V= velocity
S= wing surface area
now if you look at the lift equation and the varibles that change lift. The only one in there that the pilot has direct control over is the Velocity. Can't change the Cl (thats an engineer thing, i dont know anything about), can't change dynamic pressure (basically air denisity), cant change the surface area (please dont say something stupid like "i could put in flaps"), so the only thing that we are left with is Velocity, which we can change. So lets put some numbers in our load factor equation
Maneuvering speed is 135 KTS (PA-44) at a max gross weight of 3800lbs. Thus, it would be able to develop a maximum lift of 14,400 lbs (3.8= L/3800) at 135 KTS before it stalled. Now for argument sake, lets say that we kept the same maneuvering speed of 135 KTS (thus same lift of 14,400lbs will be produced) but lowered the weight to the 2700lbs. Now the equation would look like this: 14,440/2700= 5.3 or a load factor of 5.3. Thus, with a lower weight but same maneuver speed our load factor increased by 1.5 and we damaged the airplane since our max load factor was 3.8. Thus, we have to have a lower maneuver speed when we have a lower weight, so we don’t damage the aircraft
Probably gonna have to think about that for a while and play with the numbers, hopefully that explanation will work for you. I used this explanation for everytime i got asked that question on a checkride, and it pretty much shut the examiner up.
MidlifeFlyer
Well-Known Member
Re: Structural Cruising Speed vs. Manuevering Spee
This is a fledgling FAQ explaining maneuvering speed. It includes a brief mention of max structural cruising speed. See if you get something out of it:
==============================
Va and the "AoA Gap"
Let's start with the definition of maneuvering speed. Euphemistically, it's the speed at which an airplane will stall before it breaks due to a gust or abrupt control movement. Putting it in slightly other terms, it's the speed at which the wings can suddenly go from their existing angle of attack to their critical angle of attack without increasing the load factor (G-force) beyond the aircraft's design. For normal category aircraft, that design maximum is 3.8 G.
Let's illustrate this with some numbers. We are flying an airplane that stalls at a 15º AoA. At it's normal 120 KT cruise, it's AoA 3º. What happens if we suddenly change the AoA from 3º to 15º? Because there is (roughly) a one-to-one relationship between increase in AoA and increase in load, we have just increased the ~1-G cruise load on the wings by a factor of 5 G (3 X 5 = 15). Too bad we suffered structural damage at 3.8!!
So the whole idea behind maneuvering speed is to lower the difference between your in flight AoA and the critical AoA (the "AoA Gap"), so that in case something closes the gap suddenly, you will stall before the G-load becomes too great.
In general the faster you go in level flight, the lower your in flight AoA and the larger the AoA Gap. In the yellow arc (above maximum structural cruising speed), the AoA Gap is large enough that even an event that doesn't close the full AoA Gap can still cause those excess G's. And an event that does close the AoA Gap is almost guaranteed to cause structural damage.
What we're really trying to do to protect ourselves is increase our AoA so that the gap between our AoA and the critical AoA is smaller. How do we do that? We slow down. When we slow down while maintaining level flight, we reduce power and increase pitch, which increases our AoA. So, let's say that flying our hypothetical airplane level at a 90 KTS takes a 5º AoA. Even that small change means that suddenly bridging the AoA Gap only involves a 3-G increase, below the 3.8 G damage point.
Why the slower speed for lower weight? Well, in general, a lighter airplane can maintain level flight at a particular airspeed with a lower angle of attack. So the AoA Gap is greater at lighter weights. We need to slow down more to get our cruise AoA where we need it to be to keep the AoA Gap manageable.
==============================
This is a fledgling FAQ explaining maneuvering speed. It includes a brief mention of max structural cruising speed. See if you get something out of it:
==============================
Va and the "AoA Gap"
Let's start with the definition of maneuvering speed. Euphemistically, it's the speed at which an airplane will stall before it breaks due to a gust or abrupt control movement. Putting it in slightly other terms, it's the speed at which the wings can suddenly go from their existing angle of attack to their critical angle of attack without increasing the load factor (G-force) beyond the aircraft's design. For normal category aircraft, that design maximum is 3.8 G.
Let's illustrate this with some numbers. We are flying an airplane that stalls at a 15º AoA. At it's normal 120 KT cruise, it's AoA 3º. What happens if we suddenly change the AoA from 3º to 15º? Because there is (roughly) a one-to-one relationship between increase in AoA and increase in load, we have just increased the ~1-G cruise load on the wings by a factor of 5 G (3 X 5 = 15). Too bad we suffered structural damage at 3.8!!
So the whole idea behind maneuvering speed is to lower the difference between your in flight AoA and the critical AoA (the "AoA Gap"), so that in case something closes the gap suddenly, you will stall before the G-load becomes too great.
In general the faster you go in level flight, the lower your in flight AoA and the larger the AoA Gap. In the yellow arc (above maximum structural cruising speed), the AoA Gap is large enough that even an event that doesn't close the full AoA Gap can still cause those excess G's. And an event that does close the AoA Gap is almost guaranteed to cause structural damage.
What we're really trying to do to protect ourselves is increase our AoA so that the gap between our AoA and the critical AoA is smaller. How do we do that? We slow down. When we slow down while maintaining level flight, we reduce power and increase pitch, which increases our AoA. So, let's say that flying our hypothetical airplane level at a 90 KTS takes a 5º AoA. Even that small change means that suddenly bridging the AoA Gap only involves a 3-G increase, below the 3.8 G damage point.
Why the slower speed for lower weight? Well, in general, a lighter airplane can maintain level flight at a particular airspeed with a lower angle of attack. So the AoA Gap is greater at lighter weights. We need to slow down more to get our cruise AoA where we need it to be to keep the AoA Gap manageable.
==============================
B767Driver
New Member
[ QUOTE ]
Can anyone offer a concise explanation of the difference between the two, and well as the technicalities of why Va decreses when a decrease in gross weight?
I've got a fairly decent handle on it, but I'd like to firm the concept in my mind.
Thanks.
[/ QUOTE ]
At Va the airplane cannot physically produce enough lift to exceed the limit load factor...so it stalls. At Vno, the potential lift to be developed could exceed the limit load factor...such as a wind gust. This is why you should only exceed Vno in in smooth air.
Load Factor = Aerodynamic Load/Aircraft Weight.
A lighter aircraft can achieve a Limit Load factor at a slower airspeed. This is why Va is reduced for lighter aircraft.
Can anyone offer a concise explanation of the difference between the two, and well as the technicalities of why Va decreses when a decrease in gross weight?
I've got a fairly decent handle on it, but I'd like to firm the concept in my mind.
Thanks.
[/ QUOTE ]
At Va the airplane cannot physically produce enough lift to exceed the limit load factor...so it stalls. At Vno, the potential lift to be developed could exceed the limit load factor...such as a wind gust. This is why you should only exceed Vno in in smooth air.
Load Factor = Aerodynamic Load/Aircraft Weight.
A lighter aircraft can achieve a Limit Load factor at a slower airspeed. This is why Va is reduced for lighter aircraft.
B
Well-Known Member
I'll give you all the answer that my flight instructor told me the other day. (I want to see if it holds up)
F=MA
An increase in mass (M) would require a greater force (F) to induce an acceleration (A) that would cause structural damage
EX Structural damage caused at A=10
__________________________________________________
A=2
M=2
2*2=4 Force required to create A=2 is F=4
___________________________________________________
A=2
M=5
2*5=10 Forec required to create A=2 is F=10
__________________________________________________
A HIGHER FORCE IS REQUIRED TO CREATE THE SAME A=2
In theory you could say that increasing mass gives your aircraft less accelerations in the direction of the force that would normally cause structural damage.
Anyone want to call me on that, I would like the input.
On a side note I think B767drivers explanation is the easiest to understand, and hell it sounds correct.
F=MA
An increase in mass (M) would require a greater force (F) to induce an acceleration (A) that would cause structural damage
EX Structural damage caused at A=10
__________________________________________________
A=2
M=2
2*2=4 Force required to create A=2 is F=4
___________________________________________________
A=2
M=5
2*5=10 Forec required to create A=2 is F=10
__________________________________________________
A HIGHER FORCE IS REQUIRED TO CREATE THE SAME A=2
In theory you could say that increasing mass gives your aircraft less accelerations in the direction of the force that would normally cause structural damage.
Anyone want to call me on that, I would like the input.
On a side note I think B767drivers explanation is the easiest to understand, and hell it sounds correct.
GregCollins2
Well-Known Member
Good stuff, excellent posts!