V2 limited to VMCA?
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V2 limited to VMCA?
These factors might limit V2 to VMCA:
- Large flap angles
- High air pressure
- Low aircraft weights
I cant understand why the last one is low weights, to me it should be high weights... could anyone explain?
- Large flap angles
- High air pressure
- Low aircraft weights
I cant understand why the last one is low weights, to me it should be high weights... could anyone explain?
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However, VMCA does not fall in the same manner at low weights. So, at low weights, VMCA might limit V2.
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Normally you would expect the VMCG to be the most limiting since an engine failure when insufficient aerodynamic control could spell disaster. Your information reads 'might limit', however, these days it rarely is due to the design of the aircraft. The problem is that, at light weights, the aircraft could get airbourne at a speed when there is insufficient aerodynamic ability to control and engine failure and this airbourn speed could be below VMCG even. This situation can occur with some STOL type aircraft, particularly a stock aircraft modified for STOL. The engines are normally powerful, the high lift devices allow it to become airbourn before the capability of the rudder to control the engine failure.
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I am not a Pof F expert but I seem to remember being on a course a long long time ago when the tutor who did know about it tried to explain to us thickos why Vmca increases with reducing a/c weight (mass) rather than the other way round as one might at first think.
Draw an a/c flying away from you, imagine it has an engine out. It therefore has 5deg of bank into the live engine. Lift has to oppose weight (lets conveniently ignore any thrust component that opposes weight), but lift has both vertical and a 'sideways' (anti-yaw) components due to bank. If weight is reduced, so is lift and with it its 'anti yaw' component. Thus, Vmca is increased.
Perhaps the clever chaps out there could explain it better than I can - I may even be living up to my name here! If so I apologise - I'm just a pilot.
BS
Draw an a/c flying away from you, imagine it has an engine out. It therefore has 5deg of bank into the live engine. Lift has to oppose weight (lets conveniently ignore any thrust component that opposes weight), but lift has both vertical and a 'sideways' (anti-yaw) components due to bank. If weight is reduced, so is lift and with it its 'anti yaw' component. Thus, Vmca is increased.
Perhaps the clever chaps out there could explain it better than I can - I may even be living up to my name here! If so I apologise - I'm just a pilot.
BS
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That's actually pretty close, but one little step missing.
As you say, there is a lift component going 'sideways' for a banked aircraft. That isn't, however, helping you directly with 'anti-yaw'. What has to then happen is that in order to oppose the sideways acting lift component, you need to generate an opposing force; that you can do by adding a little bit of sideslip. That sideslip not only generates the balancing sideforce you need for the bank, it also generates a yawing moment which assists you in opposing the yawing effect of the dead engine.
The lighter the aircraft is, the less the size of the sideways lift component, the smaller the amount of sideslip you need, and the smaller therefore the assistance the sideslip gives in yaw control.
As you say, there is a lift component going 'sideways' for a banked aircraft. That isn't, however, helping you directly with 'anti-yaw'. What has to then happen is that in order to oppose the sideways acting lift component, you need to generate an opposing force; that you can do by adding a little bit of sideslip. That sideslip not only generates the balancing sideforce you need for the bank, it also generates a yawing moment which assists you in opposing the yawing effect of the dead engine.
The lighter the aircraft is, the less the size of the sideways lift component, the smaller the amount of sideslip you need, and the smaller therefore the assistance the sideslip gives in yaw control.
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V2 is greater of:
1.1 Vmca or
1.2 Vs (yes now 1.13 Vsro for newly certified A/C)
Low Weight gives a lower stall and so make it more likely you will be limited by Vmca rather than stall. (think I've seen a JAA ATPL Feedback Question like this).
W1
1.1 Vmca or
1.2 Vs (yes now 1.13 Vsro for newly certified A/C)
Low Weight gives a lower stall and so make it more likely you will be limited by Vmca rather than stall. (think I've seen a JAA ATPL Feedback Question like this).
W1
Hello PpruNe
a graphical illustration of coef. of yawing moment (N) Cn vs. the EAS where Cn=N/qSb: would demonstate that Vmc occurs where maximum yawing coef. with maximum rudder imput is equal to to the Yawing moment induced by the thrust asymmetry and the total is = 0
this is a function independent of weight, or altitude simply EAS
Vmc is the lowest Speed where Cn (due to max rudder deflection)=Cn [due to thrust asymmetry]. since the decision speed V1 will occur at a higher EAS than Vmc where there is excess yawing force available to the rudder at the higher EAS negating the criticality of the asymmetry that is why Vmc is critical at light weight
A slight bank is required to stabilize the flight path but sideslips as is often though is not zero at Vmc
a graphical illustration of coef. of yawing moment (N) Cn vs. the EAS where Cn=N/qSb: would demonstate that Vmc occurs where maximum yawing coef. with maximum rudder imput is equal to to the Yawing moment induced by the thrust asymmetry and the total is = 0
this is a function independent of weight, or altitude simply EAS
Vmc is the lowest Speed where Cn (due to max rudder deflection)=Cn [due to thrust asymmetry]. since the decision speed V1 will occur at a higher EAS than Vmc where there is excess yawing force available to the rudder at the higher EAS negating the criticality of the asymmetry that is why Vmc is critical at light weight
A slight bank is required to stabilize the flight path but sideslips as is often though is not zero at Vmc
Last edited by Pugilistic Animus; 20th Jan 2009 at 21:59. Reason: spelling and a technical error Vmc are NOT dtermined with zero sideslip
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Sorry, but all three types of VMC (a,g,or l - even more if you're a 3+engined type!) are dependent on a bit more than the simple yawing moment balance you mention.
Vmca is affected by the allowance of banking into the live engine, as is Vmcl. Both may also be influenced by the 'dynbamic Vmc' requirement, or by roll control requirements to turn into/outof the dead engine.
Vmcg is heavily influenced by all kinds of undercarriage dynamics, in addition to being heavily dependent on pilot reactions (and crosswind, though we usually cough and move on after mentioning that).
The yawing moment balance is good for giving a first approximation, but it isn't the whole story.
Vmca is affected by the allowance of banking into the live engine, as is Vmcl. Both may also be influenced by the 'dynbamic Vmc' requirement, or by roll control requirements to turn into/outof the dead engine.
Vmcg is heavily influenced by all kinds of undercarriage dynamics, in addition to being heavily dependent on pilot reactions (and crosswind, though we usually cough and move on after mentioning that).
The yawing moment balance is good for giving a first approximation, but it isn't the whole story.
For the sake of simplicity, I neglected the effect of the total airfoil, non-linear acceleration, instantaneous force couples and definitely wind in the Vmc(g,l{1,2} or a) picture.
The more complete picture would factor in such variables, but preemptively establishing Vmc parameters without flight testing on the basis of calculation alone would be very difficult. I know that newer software can deal better with the dynamic forces and modeling of the total airfoil section etc. This may prove benificial to flight test engineers giving more forwarning when doing such testing. The maximum bank angle limits 4.0 deg. 5.0 deg etc. are established by civil aviation airworthiness codes. The bank angle to deal with side slip angle beta are minimized where practicable.
In the above I neglected to state that the variable b (bravo] in the relation Cn=N/qSb is the wing span.
The more complete picture would factor in such variables, but preemptively establishing Vmc parameters without flight testing on the basis of calculation alone would be very difficult. I know that newer software can deal better with the dynamic forces and modeling of the total airfoil section etc. This may prove benificial to flight test engineers giving more forwarning when doing such testing. The maximum bank angle limits 4.0 deg. 5.0 deg etc. are established by civil aviation airworthiness codes. The bank angle to deal with side slip angle beta are minimized where practicable.
In the above I neglected to state that the variable b (bravo] in the relation Cn=N/qSb is the wing span.
Last edited by Pugilistic Animus; 13th Dec 2006 at 15:46. Reason: spelling
Perhaps this will be a useful reference sec. 25.149 of the Federal Air Regulations defines the parameterization for minimum control speeds.
I don't know but would imagine the regulations in other countries are similar.
I don't know but would imagine the regulations in other countries are similar.
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If VMCA typically increases with a decrease in weight then why does my POH for my aircraft >5700 kg give me a value for VMCA at MTOW? (amongst other things) 
Is this because of performance? I thought VMCA was a controllability issue only.
Is this because of performance? I thought VMCA was a controllability issue only.
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Does it just give that single value? In which case, unless its a REALLY new aircraft, that VMCA should have been determined at min flight weight.
VMCA is determined by controllability considerations, and is then used to determine a constraint on scheduled speeds for performance purposes.
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Just curious Cost Index, you don't happen to fly the BAe31 Jetstream do you?
For that aircraft, 5700 Kg is the 'breakpoint' below which V2 is governed by Vmc, and by Vs above.
Regards,
Old Smokey
For that aircraft, 5700 Kg is the 'breakpoint' below which V2 is governed by Vmc, and by Vs above.
Regards,
Old Smokey
