Why does Vmc decrease with altitude?
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John T is correct...
D J Flyboy,
There are fundamentally two considerations with regard to this issue. First is CONTROL. The second is PERFORMANCE.
Vmc will decrease with altitude in a normally aspirated engine due to the fact that the engine is producing less thrust. Therefore, your control surfaces, at a given speed, will be better able to counteract the yaw tendancy due to a failed engine....when the remaining engine is producing less power. There will be less yaw into the failed engine, because the operating engine is producing less thrust.
Performance is related to the 5 degrees of bank issue. This has to do with sideslip, drag (DRAG is the key word....as, with an engine out, there is less thrust to overcome the drag. This is a performance issue, not a control issue.), and slipstream, fuselage drag, etc.
To address the comment about 300 feet AGL, stalling, etc., John T is correct: "Can you say 'SPIN'?
To explain: Usually, at lower altitudes, where the engines produce their best thrust (denser air, more air), the aircraft control is Vmc limited. That is to say, the aircraft will reach its Vmc and roll over on its back before the wings stall.
At higher altitudes, the Vmc is lower and will approach the stalling speed of the wings. At some point, with low power on the operating engine(s), the power output is little, and the wings will stall (You will reach the stall speed, as the plane decelerates, before you run out of aileron and rudder.)
So, when you do this training stuff, you either stall first, or roll first. All is well.
But, wait, there's more. What happens when the roll speed and the stall speed are the same? Can you say 'SPIN'?
And, this is what gets people in trouble. In any aircraft, stalling in uncoordinated flight will induce a spin.
So, you've lost weight; you started exercising, quit smoking, etc., and your life longevity is increases....maybe by many years.
But, when messing around with a multi-engine plane, hitting the Vmc and stall speed at the same time, your life longevity is now reduced to seconds. It's unlikely you'll recover. Especially with a C.G. that is aft of very forward.....
There are many sources of information that will explain this to the new, multi-engine student. Research, have a read, then you'll understand.
Fly safe,
PantLoad
P.S. Please don't do stalls or Vmc demonstartions at 300 feet. This is downright stupid.
(Edited to add the post script....)
There are fundamentally two considerations with regard to this issue. First is CONTROL. The second is PERFORMANCE.
Vmc will decrease with altitude in a normally aspirated engine due to the fact that the engine is producing less thrust. Therefore, your control surfaces, at a given speed, will be better able to counteract the yaw tendancy due to a failed engine....when the remaining engine is producing less power. There will be less yaw into the failed engine, because the operating engine is producing less thrust.
Performance is related to the 5 degrees of bank issue. This has to do with sideslip, drag (DRAG is the key word....as, with an engine out, there is less thrust to overcome the drag. This is a performance issue, not a control issue.), and slipstream, fuselage drag, etc.
To address the comment about 300 feet AGL, stalling, etc., John T is correct: "Can you say 'SPIN'?
To explain: Usually, at lower altitudes, where the engines produce their best thrust (denser air, more air), the aircraft control is Vmc limited. That is to say, the aircraft will reach its Vmc and roll over on its back before the wings stall.
At higher altitudes, the Vmc is lower and will approach the stalling speed of the wings. At some point, with low power on the operating engine(s), the power output is little, and the wings will stall (You will reach the stall speed, as the plane decelerates, before you run out of aileron and rudder.)
So, when you do this training stuff, you either stall first, or roll first. All is well.
But, wait, there's more. What happens when the roll speed and the stall speed are the same? Can you say 'SPIN'?
And, this is what gets people in trouble. In any aircraft, stalling in uncoordinated flight will induce a spin.
So, you've lost weight; you started exercising, quit smoking, etc., and your life longevity is increases....maybe by many years.
But, when messing around with a multi-engine plane, hitting the Vmc and stall speed at the same time, your life longevity is now reduced to seconds. It's unlikely you'll recover. Especially with a C.G. that is aft of very forward.....
There are many sources of information that will explain this to the new, multi-engine student. Research, have a read, then you'll understand.
Fly safe,
PantLoad
P.S. Please don't do stalls or Vmc demonstartions at 300 feet. This is downright stupid.
(Edited to add the post script....)
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HazelNut39, I no longer have my Perf Manual to give you that information; however, the Boeing Flight Training Manual will provide a similar quote...
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Let's not get wrapped-up in semantics... the point being is that you should be aware of what the actual Vmca(2), or, Vmcl(2) should actually be. It's one thing to sit comfortably in an armchair at home and disect what someone should have done when in fact you only had a few seconds to make a sensible judgement at that moment in time.
If you're not able to put the aeroplane into a safe Go-Around configuration and that the current situation allows you to land... land the sucker! I'd much prefer to be climbing out of an intact aeroplane than a pile of wreckage somewhere outside of the airfield perimeter.
The chap who was practicing recovery from single engine work (at 300 feet) was obviously well below Vmca; which is why he had difficulty in making a recovery from what should have been a total disaster!
TCF[/FONT]
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[FONT='Arial','sans-serif']During an approach with two engines inoperative on one side it may be possible to be at an airspeed below VMCA2 when a go-around is initiated. In this event, thrust should be applied with rudder application and a slight bank into the operative engines while establishing a descent for faster acceleration. Stop the thrust lever advance just prior to full rudder travel and then set the inboard engine to go-around thrust. As the airspeed increases and excess rudder becomes available, advance the thrust lever for the operable outboard engine to the go-around setting. At bug + 60 select flaps 1 and slowly increase the pitch attitude to maintain bug + 60 and transition from a descent to a climb.[/FONT]
If you're not able to put the aeroplane into a safe Go-Around configuration and that the current situation allows you to land... land the sucker! I'd much prefer to be climbing out of an intact aeroplane than a pile of wreckage somewhere outside of the airfield perimeter.
The chap who was practicing recovery from single engine work (at 300 feet) was obviously well below Vmca; which is why he had difficulty in making a recovery from what should have been a total disaster!
TCF[/FONT]
Moderator
It is sobering to review the circumstances of the RAAF mishap with B707 A20-103 at East Sale in Victoria some years ago. An extreme set of circumstances involving aspects of inappropriate operation .. with essentially predictable results. Well worth a read for multiengine instructor pilots who fancy that their flying skills are up there with the aces.
One needs to keep in mind that book Vmca is quite specific, the real world figure rather variable according to configuration and ambients. While there may be a case for exposing a pilot in training to general aspects of Vmca, I suggest a better option is to stay away from it and let the TPs come up with the numbers for the AFM.
If you MUST play with Vmca, do it with consideration, sufficient height to provide for recovery if it gets away from you .. and be prepared to close the other throttle to get rid of the yawing moment.
One needs to keep in mind that book Vmca is quite specific, the real world figure rather variable according to configuration and ambients. While there may be a case for exposing a pilot in training to general aspects of Vmca, I suggest a better option is to stay away from it and let the TPs come up with the numbers for the AFM.
If you MUST play with Vmca, do it with consideration, sufficient height to provide for recovery if it gets away from you .. and be prepared to close the other throttle to get rid of the yawing moment.
Performance is related to the 5 degrees of bank issue. This has to do with sideslip, drag (DRAG is the key word....as, with an engine out, there is less thrust to overcome the drag. This is a performance issue, not a control issue.), and slipstream, fuselage drag, etc.
As I understand things it is a control issue as well. The up to 5 degrees of bank requires less rudder input, therefore assists in controlability.
However we demonstrate the effect of the 5 degrees bank angle by showing the difference in RoC, with and without the bank angle applied.
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27/09
27/09:
You are correct. I was wrong. Banking into the live engine does, in fact, both improve climb performance, but also lowers Vmc. You are correct.
I had to dig through my references...dust off the books....
The 'ball' is valid on a multi-engine engine only where there is symetrical thrust. With one engine inoperative, centering the ball does NOT provide zero sideslip. Banking the airplane a bit (up to 5 degrees max as per authority limits) with the ball slightly outside of center will not only provide better control, but better climb performance, as well.
'Estimating' zero sideslip will improve climb performance. Again, control is improved, as well.
Thank you for the correction....
Fly safe,
PantLoad
You are correct. I was wrong. Banking into the live engine does, in fact, both improve climb performance, but also lowers Vmc. You are correct.
I had to dig through my references...dust off the books....
The 'ball' is valid on a multi-engine engine only where there is symetrical thrust. With one engine inoperative, centering the ball does NOT provide zero sideslip. Banking the airplane a bit (up to 5 degrees max as per authority limits) with the ball slightly outside of center will not only provide better control, but better climb performance, as well.
'Estimating' zero sideslip will improve climb performance. Again, control is improved, as well.
Thank you for the correction....
Fly safe,
PantLoad
Pantload
The whole asymmetric flight principles of flight subject is a bit confusing at times, especially if you don't teach it regularly. At least it is to me and I consider myself to have a better understanding than most when it comes to principles of flight. I think some of the posts on here prove how many fish hooks there are in this subject.
It always seems to provoke interesting debate when ever it gets a run on a forum like this.
The whole asymmetric flight principles of flight subject is a bit confusing at times, especially if you don't teach it regularly. At least it is to me and I consider myself to have a better understanding than most when it comes to principles of flight. I think some of the posts on here prove how many fish hooks there are in this subject.
It always seems to provoke interesting debate when ever it gets a run on a forum like this.
Moderator
Important to put handling and climb performance in two adjacent paddocks.
(a) Vmc - VERY bank dependent. Not hard to find aircraft with a 30-40kts increase in Vmca if you bank the "wrong" way. The certification limit of 5° puts a constraint on the innovative OEM. I suggest that, if the book doesn't say different, presume that the AFM Vmca figure likely is based on 5°. Back near Vmca one is interested in not dying immediately so handling is the name of the game - performance sits in the wings at this time.
(b) climb performance OEI will be maximised somewhere near the condition for zero side slip (think about it - what's the easiest way to generate a bunch of drag and an increase in descent gradient in a light single overshooting the runway on approach ?)
It turns out that zero side slip occurs around 2-3° bank into the operating engine(s) for pretty well all multis (337 etc notwithstanding). Given that flying this sort of bank angle is demanding on the pilot (and, for some aircraft, can't be done due to AH limitations) the oft-chosen compromise is to fly wings level and accept the small climb penalty. It follows that 5° bank is going to be somewhere near the climb capability found for wings level so there is no point in trying to climb at 5° once you have the aircraft established in the climb at an appropriate target speed in excess of Vmca. If the aircraft is light enough to achieve an impressive OEI climb, go for wings level anyway.
(c) Vmca region - go for 5°. Established in the OEI climb wings level makes good sense and, if you REALLY want the last bit of climb (and you are smooth enough to do it well), go for 2-3°
(a) Vmc - VERY bank dependent. Not hard to find aircraft with a 30-40kts increase in Vmca if you bank the "wrong" way. The certification limit of 5° puts a constraint on the innovative OEM. I suggest that, if the book doesn't say different, presume that the AFM Vmca figure likely is based on 5°. Back near Vmca one is interested in not dying immediately so handling is the name of the game - performance sits in the wings at this time.
(b) climb performance OEI will be maximised somewhere near the condition for zero side slip (think about it - what's the easiest way to generate a bunch of drag and an increase in descent gradient in a light single overshooting the runway on approach ?)
It turns out that zero side slip occurs around 2-3° bank into the operating engine(s) for pretty well all multis (337 etc notwithstanding). Given that flying this sort of bank angle is demanding on the pilot (and, for some aircraft, can't be done due to AH limitations) the oft-chosen compromise is to fly wings level and accept the small climb penalty. It follows that 5° bank is going to be somewhere near the climb capability found for wings level so there is no point in trying to climb at 5° once you have the aircraft established in the climb at an appropriate target speed in excess of Vmca. If the aircraft is light enough to achieve an impressive OEI climb, go for wings level anyway.
(c) Vmca region - go for 5°. Established in the OEI climb wings level makes good sense and, if you REALLY want the last bit of climb (and you are smooth enough to do it well), go for 2-3°
