Newbie Vmca question
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Newbie Vmca question
This aught to be amusing.
Could somebody explain the physics of the rotational force acting on an an aircraft when Vmca is reached.
I was talking with a mate (neither of us are MEP) regarding reducing rudder authority as Vmca approaches with yaw towards the dead engine.
is the Yaw enough to turn the A/C on its back? My answer is yes, my friend argues with full opposite rudder this should not be possible.
Cheers.
Could somebody explain the physics of the rotational force acting on an an aircraft when Vmca is reached.
I was talking with a mate (neither of us are MEP) regarding reducing rudder authority as Vmca approaches with yaw towards the dead engine.
is the Yaw enough to turn the A/C on its back? My answer is yes, my friend argues with full opposite rudder this should not be possible.
Cheers.
Why do it if it's not fun?
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During an asymmetric situation, the main effect of the asymmetry is that it causes yaw towards the failed engine. Rudder is required to counteract that yaw.
At lower airspeeds, there is less airflow over all of the aerodynamic surfaces. This means that all of the aerodynamic controls become less effective. In the case of Vmc (which is subtley different to Vmca, but I think Vmc is what you are actually refering to), the speed has reduced sufficiently that there is not enough rudder authority to counteract all of the yaw that's being created by the asymmetry. The net effect, then, is that even with full oposite rudder applied, there is still some yaw in the direction of the failed engine.
What is the effect of this yaw? Well, what is the effect of any yaw? The end result is no different to if you applied yaw by using the rudder pedals. The yaw will cause roll, the roll will cause more yaw, and so on, and the two will combine to result in a spiral dive, in exactly the same way as applying rudder in any aircraft will result in a spiral dive.
I have demonstrated this to countless students in the 5 years I've been an MEP instructor, and it's never put the aircraft on its back yet! (Mind you, I tend to recover when it gets much beyond about 45 of degrees of bank.....)
FFF
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At lower airspeeds, there is less airflow over all of the aerodynamic surfaces. This means that all of the aerodynamic controls become less effective. In the case of Vmc (which is subtley different to Vmca, but I think Vmc is what you are actually refering to), the speed has reduced sufficiently that there is not enough rudder authority to counteract all of the yaw that's being created by the asymmetry. The net effect, then, is that even with full oposite rudder applied, there is still some yaw in the direction of the failed engine.
What is the effect of this yaw? Well, what is the effect of any yaw? The end result is no different to if you applied yaw by using the rudder pedals. The yaw will cause roll, the roll will cause more yaw, and so on, and the two will combine to result in a spiral dive, in exactly the same way as applying rudder in any aircraft will result in a spiral dive.
I have demonstrated this to countless students in the 5 years I've been an MEP instructor, and it's never put the aircraft on its back yet! (Mind you, I tend to recover when it gets much beyond about 45 of degrees of bank.....)
FFF
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Last edited by FlyingForFun; 22nd Mar 2011 at 23:18.
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so in a way your saying should you let the Spiral dive continue it could end up on its back. However in a dive speed would increase Vmc would <decrease> so some control could be gained?
When Vmc is reached the only way to regain control is to increase speed yes? as in trade height for speed thus- control-??
what is the difference between Vmca and Vmc
When Vmc is reached the only way to regain control is to increase speed yes? as in trade height for speed thus- control-??
what is the difference between Vmca and Vmc
Vmc recovery also involves reducing power on the live side, not just increasing airspeed (nb. you must trade altitude to accelerate), sufficient to regain heading control. Once heading control is regained increasing power can be applied to the live side as it accelerates - typically to at least Vxse or Vyse - to maximise performance.
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Vmc also changes depending on altitude, gear and flaps down, C of G etc.
Funnily enough, Vmc goes down with gears out (as they act as rudders and help stabilize). Vmc also goes down at higher altitude as the engine will lose power.
Vmc is determined during the certification process and is done at sea level, gear and flaps up, fully loaded, aft CG - those are the worst conditions. Anything deviating from this will reduce Vmc and therefore increase your control.
Now, would you want to be close to anywhere Vmc in a real engine out situation? Probably not. I can tell you it's pretty sobering experience seeing how little control one has close to those numbers.
Funnily enough, Vmc goes down with gears out (as they act as rudders and help stabilize). Vmc also goes down at higher altitude as the engine will lose power.
Vmc is determined during the certification process and is done at sea level, gear and flaps up, fully loaded, aft CG - those are the worst conditions. Anything deviating from this will reduce Vmc and therefore increase your control.
Now, would you want to be close to anywhere Vmc in a real engine out situation? Probably not. I can tell you it's pretty sobering experience seeing how little control one has close to those numbers.
Vmca is a certification speed; it does not vary with anything; if you change any of the the parameters it is no longer Vmca. As one of the parameters is that its determined at sea level, it is almost impossible to acheive in the air. As with all such speeds you may find differences withinin the definitions of the Certification data. BCAR Section K was the UK certification criteria for light twins prior to JAR23. Essentially it is determined for the take off condition with maximum power on the critical engine, the failed engine windmilling; max AUW and max aft C of G.
Vmc on the other hand is the minimum control speed at what ever conditions its measured under and does vary. It may even be the pilot's personal Vmc when they can no longer maintain straight flight to within 15 degrees.
You can determine a speed at which the aircraft can just maintain directional control however; if the engine were to suddenly fail at that speed, the rate of yaw and roll might well exceed the rate at which recovery action can take place added to which one must consider the delay in reacting.
It should always be rememberd that closing both throttles and lowering the nose immediately solves the asymmetric problem.
Vmc on the other hand is the minimum control speed at what ever conditions its measured under and does vary. It may even be the pilot's personal Vmc when they can no longer maintain straight flight to within 15 degrees.
You can determine a speed at which the aircraft can just maintain directional control however; if the engine were to suddenly fail at that speed, the rate of yaw and roll might well exceed the rate at which recovery action can take place added to which one must consider the delay in reacting.
It should always be rememberd that closing both throttles and lowering the nose immediately solves the asymmetric problem.
Technically a Vmc demo as performed during a ME check ride or skill test is not how they determine Vmc.
With a Vmc demo we are established SE with the "dead" engine windmilling.
The gradual yaw will lead to roll etc as FFF describes.
The "real" Vmc definition is where both engines are maximum rated power and the critical engine suddenly fails.
If flying below Vmc the very abrupt yaw will cause enough of a rolling moment to turn it upside down.
A Vmc demo will not.
With a Vmc demo we are established SE with the "dead" engine windmilling.
The gradual yaw will lead to roll etc as FFF describes.
The "real" Vmc definition is where both engines are maximum rated power and the critical engine suddenly fails.
If flying below Vmc the very abrupt yaw will cause enough of a rolling moment to turn it upside down.
A Vmc demo will not.
Why do it if it's not fun?
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Funnily enough, Vmc goes down with gears out (as they act as rudders and help stabilize).
However, the main gear is generally very close to the centre of gravity, and will have almost no effect on the stability of the aircraft - at least, not compared to the nose wheel, which is generally well forward of the centre of gravity. Because it is forward of the centre of gravity, it will actually de-stabilise the aircraft in yaw, and increase Vmc.
Counteracting that is the fact that the windmilling prop (remember that Vmca is measured with the prop windmilling) blanks out the gear - the amount of extra drag when the gear is lowered is less than the amount of extra drag when the gear is raised. Difficult to explain without pictures, but there's a good explaination with a picture here.
Depending which of these factors overrides the other, Vmc may either increase or decrease when the gear is extended, depending on aircraft type. In any case, the effect will be incredibly small - so much so, it's not even mentioned by many text books.
It should be noted that Vmca is measured with the gear up. This is because it reflects the take-off scenario - if the gear was still down and the speed was low, closing both throttles and landing ahead would probably be the best course of action, terrain permitting. It should also be noted that none of this is at all relevant to the original question!
FFF
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FFF, that's what I've been told. Gear have a stabilizing effect and will decrease Vmc regardless of where they're placed.
I'm happy to be corrected of course.
I'm happy to be corrected of course.
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I have also heard that retracting the gear will change the C of G (aft) and could thus cause a problem if you were close to Vmc.
I think i have gleened enough out of the posts above to win my pint.
Looking forward to the Lotto win so i can actually study this stuff for real, for now i'm stuck in SEP land, which seems so much safer the more i learn about twins.
I think i have gleened enough out of the posts above to win my pint.
Looking forward to the Lotto win so i can actually study this stuff for real, for now i'm stuck in SEP land, which seems so much safer the more i learn about twins.
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Could somebody explain the physics of the rotational force acting on an an aircraft when Vmca is reached.
I was talking with a mate (neither of us are MEP) regarding reducing rudder authority as Vmca approaches with yaw towards the dead engine.
is the Yaw enough to turn the A/C on its back?
I was talking with a mate (neither of us are MEP) regarding reducing rudder authority as Vmca approaches with yaw towards the dead engine.
is the Yaw enough to turn the A/C on its back?
Can an assymetrical thrust situation in a multi-engine airplane cause a roll, or cause the airplane to roll onto it's back? Yes, easily. Shut down one engine in a light twin, push the other up to takeoff power, then go do stalls. See what happens.
The goal in flying an airplane, of course, is to prevent that from happening. This may be gained in part by increasing airspeed, in part by reducing thrust on the good engine(s) to lessen the assymetrical thust, and in part by the loading and configuration of the aircraft.
In a light piston twin airplane, assymetrical thrust does several things, including producing unequal lift between the left wing and right wing, as well as a yawing force and a rolling force. When insufficient control remains to offset these forces, a roll or control departure can, and generally does result.
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Never having tried this, I am curious about how fast a light twin can roll right over in this situation. I know "it will depend on..." but are we talking seconds, tens of seconds?
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I don't know what happens in reality, but in an approved/certified simulator it was all over in less than two seconds. Not just the roll but the crash as well.
Fortunately that one had a reset button.
Of course in a non-moving sim you don't have the seat-of-the-pants feeling of the tremendous yaw that an engine failure would cause, so that may dull your reflexes a bit. On the other hand the yaw also doesn't wipe you off your seat so you can still access all the controls.
Fortunately that one had a reset button.
Of course in a non-moving sim you don't have the seat-of-the-pants feeling of the tremendous yaw that an engine failure would cause, so that may dull your reflexes a bit. On the other hand the yaw also doesn't wipe you off your seat so you can still access all the controls.
And people call helicopters dangerous...
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Never having tried this, I am curious about how fast a light twin can roll right over in this situation. I know "it will depend on..." but are we talking seconds, tens of seconds?
A multi-engine airplane is not dangerous if flown properly. This is why we teach students about respecting, experiencing, demonstrating, and avoiding the region in which directional control can no longer be maintained. It's not rocket science. It's common sense.
As noted previously, the rationale of retarding power on the good engine needs to be kept firmly in mind for anyone piloting a light twin.
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A question about spins in a twin; would it be more effective to exit the spin by adding power on the inside engine of the spin rather than use the rudder? Has anyone ever tried?
Twins aren't required to be tested for spin recovery so any recovery technique is likely to be a best guess. Adding power against rotation might stop or slow the spin. Alternatively it could snap it into a spin in the opposite direction or just cause a pitch up, causing the spin to flatten.
Why do it if it's not fun?
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I'm no expert on spins and their aerodynamics, and happy to be corrected on this one (Genghis?). But I believe recovery from a spin in a twin would be difficult whatever method you use, because of the inertia in roll due to the mass of the engines not being on the roll axis like it would be in a single engine aircraft.
As to whether asymmetric power would help you recover.... I'd much rather just keep well clear of the spin altogether in any aircraft not certified for it, but especially in a twin.
Back on the subject of the effect of the gear, and the idea that it moves the C of G rearwards.... on the Beech Duchess, the main wheels retract inwards and won't affect the C of G at all. The nose wheel retracts forwards, so the C of G will be further forward when retracted, not further back. But whichever way the gear retracts, I still maintain that any effect on Vmc would be tiny, and it would be type specific whether Vmc increases or decreases because there are several factors at play which all counteract each other.
FFF
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As to whether asymmetric power would help you recover.... I'd much rather just keep well clear of the spin altogether in any aircraft not certified for it, but especially in a twin.
Back on the subject of the effect of the gear, and the idea that it moves the C of G rearwards.... on the Beech Duchess, the main wheels retract inwards and won't affect the C of G at all. The nose wheel retracts forwards, so the C of G will be further forward when retracted, not further back. But whichever way the gear retracts, I still maintain that any effect on Vmc would be tiny, and it would be type specific whether Vmc increases or decreases because there are several factors at play which all counteract each other.
FFF
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With all the variables how do you actually calculate Vmc? its sounds like a moving target, Weight, Gear down/up, Flaps, C of G, Density Altitude! how accurate is the little red line?
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how accurate is the little red line?
Vmc as a certification number may be more or less than the speed at which you may find a control departure in your aircraft on any given day, depending on numerous factors.