Vmcg vs pitch.
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Vmcg vs pitch.
Good morning.
Vmcg varies with pitch angle & I think for certification is measured at 5°. What causes it to vary with pitch?
An example of an aircraft with massive Vmcg change is the B707 withup to 60kts, check Prestwick B707 crash, out board eng fail. Are there any other examples of aircraft, modern or not, with this "special" feature. And the recovery?
Jumbo
Vmcg varies with pitch angle & I think for certification is measured at 5°. What causes it to vary with pitch?
An example of an aircraft with massive Vmcg change is the B707 withup to 60kts, check Prestwick B707 crash, out board eng fail. Are there any other examples of aircraft, modern or not, with this "special" feature. And the recovery?
Jumbo
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Indeed I meant VmcA! Worse than that I also wanted to ask about the relationship to Bank not pitch!! & I have not opened the Merlot yet. So I try again, what is the relationship with VmcA & Bank & what influences this relationship & why.
Thanks.
J
Thanks.
J
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Perf experts will no doubt 'tidy up' here, but Vmca is calculated, I believe, on a max of 5 deg bank towards the good engine/s and I seem to recall reduces by around 3kts per deg of bank up to that since not so much of the available rudder is being used to control yaw. It is considered that the loss of lift up to 5 deg is insignificant. Is that too simple? If so, I'm out of here!
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Suggest you run a search on the site - been discussed quite a few times.
Main concerns -
(a) failure occurs - results in yaw
(b) rudder input is required to control the yaw (probably all sorts of things going on during this dynamic phase but yaw is the main target)
(c) if you end up with yaw controlled and wings level there is an unbalanced lateral force due to the rudder input
(d) this lateral force results in a lateral acceleration and velocity which results in a sideslip
(e) if you roll a bit into the operating engine(s), you will generate a lateral velocity the other way which will reduce the sideslip as well as the overall yawing moment needing to be controlled by rudder
(f) this allows you either to reduce the rudder input a little or, for full rudder, to reduce speed a little to get a lower Vmca
(g) to keep everyone honest, the certification standards impose a 5 degree limit. There is a practical consideration as well - too much sideslip OEI is a good way to increase the probability of a control loss
(h) if the bank is applied the "wrong" way, the situation is reversed and both sideslip and yawing moments increase, requiring more rudder or an increased speed (higher Vmca). The relationship of Vmca and bank is quite steep - failures at speeds at or near published Vmca will require favourable bank or else there is a real risk of the real world Vmc increasing to the point where control is lost in spite of pilot efforts.
(i) Vmca things have to do with maintaining control, not performance.
(j) 5 degrees into the operating engine(s) is a certification restriction and, in the absence of information in the AFM indicating something different, it is reasonable to presume that the AFM Vmca figures are based on 5 degrees favourable bank
(k) once control is ensured and speed can be increased to a more desirable level where control is less of concern, we can look at maximising climb performance and this generally occurs somewhere around 2-3 degrees favourable bank at whatever speed maximises climb rate. It's important that the two considerations are not confused.
Are there any other examples of aircraft, modern or not, with this "special" feature
Nothing special - this is a general characteristic of multiengine aircraft (excluding centreline thrust machines such as the Cessna push-pulls). The only thing which varies is the actual speed-bank variation.
And the recovery ?
(a) get some favourable bank into the equation, or
(b) get some more speed.
(c) if all else fails, a better option to losing control and dying is to reduce operating thrust to let you regain control, get some speed back, and then go back to (a). This option isn't going to be desirable at low level where you need to have both high competence and a good plan at the outset.
Main concerns -
(a) failure occurs - results in yaw
(b) rudder input is required to control the yaw (probably all sorts of things going on during this dynamic phase but yaw is the main target)
(c) if you end up with yaw controlled and wings level there is an unbalanced lateral force due to the rudder input
(d) this lateral force results in a lateral acceleration and velocity which results in a sideslip
(e) if you roll a bit into the operating engine(s), you will generate a lateral velocity the other way which will reduce the sideslip as well as the overall yawing moment needing to be controlled by rudder
(f) this allows you either to reduce the rudder input a little or, for full rudder, to reduce speed a little to get a lower Vmca
(g) to keep everyone honest, the certification standards impose a 5 degree limit. There is a practical consideration as well - too much sideslip OEI is a good way to increase the probability of a control loss
(h) if the bank is applied the "wrong" way, the situation is reversed and both sideslip and yawing moments increase, requiring more rudder or an increased speed (higher Vmca). The relationship of Vmca and bank is quite steep - failures at speeds at or near published Vmca will require favourable bank or else there is a real risk of the real world Vmc increasing to the point where control is lost in spite of pilot efforts.
(i) Vmca things have to do with maintaining control, not performance.
(j) 5 degrees into the operating engine(s) is a certification restriction and, in the absence of information in the AFM indicating something different, it is reasonable to presume that the AFM Vmca figures are based on 5 degrees favourable bank
(k) once control is ensured and speed can be increased to a more desirable level where control is less of concern, we can look at maximising climb performance and this generally occurs somewhere around 2-3 degrees favourable bank at whatever speed maximises climb rate. It's important that the two considerations are not confused.
Are there any other examples of aircraft, modern or not, with this "special" feature
Nothing special - this is a general characteristic of multiengine aircraft (excluding centreline thrust machines such as the Cessna push-pulls). The only thing which varies is the actual speed-bank variation.
And the recovery ?
(a) get some favourable bank into the equation, or
(b) get some more speed.
(c) if all else fails, a better option to losing control and dying is to reduce operating thrust to let you regain control, get some speed back, and then go back to (a). This option isn't going to be desirable at low level where you need to have both high competence and a good plan at the outset.
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I like your reply John.
I would like to add the link to an article I have found to be an eye opener, and posted here in the past. I think it relevant. It is here:
Always Leave Yourself An Out
I note that the original question relates to "pitch" angle. Perhaps the poster meant bank angle, or is there a deeper inquiry here?
I would like to add the link to an article I have found to be an eye opener, and posted here in the past. I think it relevant. It is here:
Always Leave Yourself An Out
I note that the original question relates to "pitch" angle. Perhaps the poster meant bank angle, or is there a deeper inquiry here?
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Originally Posted by Pilot DAR
I note that the original question relates to "pitch" angle. Perhaps the poster meant bank angle, or is there a deeper inquiry here?
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If yaw is controlled solely with rudder toward the live engine you tend to generate a strong roll in the same direction due to sideslip and dihedral effects this is countered with deflection of the yoke toward the dead engine. On jets this yoke or stick deflection may trip roll spoiler deployment with significant performance penalties. Even conventional ailerons deflected against a significant rudder deflection comes at a cost to performance. Adverse aileron yaw just adds to the woes.
If control is the issue as implied by the term Vmca we get back to the rather clinical process of establishing a configuration which preserves good margins of available control deflection to manoeuvre as required while still staying right side up. Up to 5 degrees toward the live engine gives a small lateral component of weight therefore sideslip. This can be used to reduce rudder deflection and return the yoke or stick to something nearer neutral. The available range of rudder and aileron deflections are restored toward normal and the frantic battle against spoilers and adverse yaw stops.
In short stop the yaw with rudder, bank slightly toward the applied rudder, and slowly replace some applied rudder with a careful return of the roll control somewhere toward centre. If it is about the minimum speed for retaining control we must remember that control power is a function of speed and control deflection. The bank places the controls where we maintain greater available deflection so it is reasonable that once achieved the minimum speed may actually be lower. But only after the failure and initial set-up, or during a staged demonstration from the shallow banked condition.
If control is the issue as implied by the term Vmca we get back to the rather clinical process of establishing a configuration which preserves good margins of available control deflection to manoeuvre as required while still staying right side up. Up to 5 degrees toward the live engine gives a small lateral component of weight therefore sideslip. This can be used to reduce rudder deflection and return the yoke or stick to something nearer neutral. The available range of rudder and aileron deflections are restored toward normal and the frantic battle against spoilers and adverse yaw stops.
In short stop the yaw with rudder, bank slightly toward the applied rudder, and slowly replace some applied rudder with a careful return of the roll control somewhere toward centre. If it is about the minimum speed for retaining control we must remember that control power is a function of speed and control deflection. The bank places the controls where we maintain greater available deflection so it is reasonable that once achieved the minimum speed may actually be lower. But only after the failure and initial set-up, or during a staged demonstration from the shallow banked condition.
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you tend to generate a strong roll in the same direction .. this is countered with deflection of the yoke toward the dead engine
Can't say that I've ever experienced that on the aircraft I've flown - noting that my experience is far less than many here. Would you like to expand in some more detail ? Perhaps you are considering the short term rolling moments while the initial failure is brought under control ? Indeed, for dynamic failures within a knot or two of Vmc for the current conditions one can expect things to be very dynamic for a few seconds while things are brought into order.
In short stop the yaw with rudder, bank slightly toward the applied rudder, and slowly replace some applied rudder with a careful return of the roll control somewhere toward centre
This appears to be the reverse of your thesis above ?
once achieved the minimum speed may actually be lower
There isn't any fat left in a Vmca certification exercise - generally the published figure is for 5 degrees favourable bank. Perhaps you might explain a bit further ?
Can't say that I've ever experienced that on the aircraft I've flown - noting that my experience is far less than many here. Would you like to expand in some more detail ? Perhaps you are considering the short term rolling moments while the initial failure is brought under control ? Indeed, for dynamic failures within a knot or two of Vmc for the current conditions one can expect things to be very dynamic for a few seconds while things are brought into order.
In short stop the yaw with rudder, bank slightly toward the applied rudder, and slowly replace some applied rudder with a careful return of the roll control somewhere toward centre
This appears to be the reverse of your thesis above ?
once achieved the minimum speed may actually be lower
There isn't any fat left in a Vmca certification exercise - generally the published figure is for 5 degrees favourable bank. Perhaps you might explain a bit further ?
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Helpful info at AvioConsult - Aircraft Expert and Consultant - Home Page, especially the figures on p5 of the download titled The Effect of Bank Angle and Weight on Vmca
Dick
Dick
Last edited by Dick Whittingham; 28th Nov 2011 at 19:57.
We should remind ourselves that with some aircraft / situations, Vmca might be defined by stall speed.
Also, as anyone who has been testing in this area might recall, approaching a stall with offset control positions can progress rapidly to an even more interesting situation.
Also, as anyone who has been testing in this area might recall, approaching a stall with offset control positions can progress rapidly to an even more interesting situation.
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Thanks John_T for pulling me up. While I stand by my reasoning I accept the response could have been better worded so here goes. Lets assume a failure of the left engine at takeoff thrust without any nice yaw damping technology to ease the task.
A healthy measure of right rudder is needed to stop the significant yaw toward the dead (drag producing) engine. The yaw is produced by the thrust drag couple from the live and dead engine the yaw is balanced by right rudder producing a strong correcting force to the left at the tail.
The yaw is stopped but, the couple and correcting force are both to the left so the left drift (skid) which results will result in the relative airflow approaching from the left side. The balance of turning effects - Force against a couple - is just like tail-rotor drift in a helicopter. Designed lateral stability and dihedral effects will encourage a roll to the live engine.
In reality this is just what you require up to 5 degrees anyway, but I have often seen (and been guilty of) the results of poorly worded training that over emphasises the matter of yaw to the exclusion of the important understanding of the part of roll. At night, or in IMC flight, you are very dependent on the Attitude Indicator and this even strengthens the instinct to lock the wings level on the horizon and hold a magnetic heading (stop yaw) with rudder. This will result in crossed controls as we oppose the aircrafts need and desire to roll toward the live engine. The displacement on the slip indicator is another source of distraction.
There isn't any fat left in a Vmca certification exercise - generally the published figure is for 5 degrees favourable bank. Perhaps you might explain a bit further ?
I agree that certification leaves no fat. My point that;"The bank places the controls where we maintain greater available deflection so it is reasonable that once achieved the minimum speed may actually be lower. But only after the failure and initial set-up, or during a staged demonstration from the shallow banked condition."
This is confirmation that the stable demonstration of Vmca may give a lower airspeed than the Dynamic Vmca. This is a point acknowledged by the testing requirements of FAR 23.149. The benefit of the bank is that it produces a small sideslip which counters the skid in wings level flight. This will reduce overall drag on the fuselage and improve control margins as we are less cross controlled as a result.
In short stop the yaw with rudder, bank slightly toward the applied rudder, and slowly replace some applied rudder with a careful return of the roll control somewhere toward centre. Should we have got ourselves a bit cross controlled in our initial dynamic response to the emergency.
A healthy measure of right rudder is needed to stop the significant yaw toward the dead (drag producing) engine. The yaw is produced by the thrust drag couple from the live and dead engine the yaw is balanced by right rudder producing a strong correcting force to the left at the tail.
The yaw is stopped but, the couple and correcting force are both to the left so the left drift (skid) which results will result in the relative airflow approaching from the left side. The balance of turning effects - Force against a couple - is just like tail-rotor drift in a helicopter. Designed lateral stability and dihedral effects will encourage a roll to the live engine.
In reality this is just what you require up to 5 degrees anyway, but I have often seen (and been guilty of) the results of poorly worded training that over emphasises the matter of yaw to the exclusion of the important understanding of the part of roll. At night, or in IMC flight, you are very dependent on the Attitude Indicator and this even strengthens the instinct to lock the wings level on the horizon and hold a magnetic heading (stop yaw) with rudder. This will result in crossed controls as we oppose the aircrafts need and desire to roll toward the live engine. The displacement on the slip indicator is another source of distraction.
There isn't any fat left in a Vmca certification exercise - generally the published figure is for 5 degrees favourable bank. Perhaps you might explain a bit further ?
I agree that certification leaves no fat. My point that;"The bank places the controls where we maintain greater available deflection so it is reasonable that once achieved the minimum speed may actually be lower. But only after the failure and initial set-up, or during a staged demonstration from the shallow banked condition."
This is confirmation that the stable demonstration of Vmca may give a lower airspeed than the Dynamic Vmca. This is a point acknowledged by the testing requirements of FAR 23.149. The benefit of the bank is that it produces a small sideslip which counters the skid in wings level flight. This will reduce overall drag on the fuselage and improve control margins as we are less cross controlled as a result.
In short stop the yaw with rudder, bank slightly toward the applied rudder, and slowly replace some applied rudder with a careful return of the roll control somewhere toward centre. Should we have got ourselves a bit cross controlled in our initial dynamic response to the emergency.
