Difference between Vmc and Vmca

JaceTheAce

JaceTheAce

Well-Known Member
Can someone please explain the difference between Vmc and Vmca? I am completely confused. The FAR/AIM isn't helping much.
 
Basically the same.

Vmca is Minimum Controllable in the Air and out of ground effect.

just a different acronym for the same thing.
 
Vmc is the same as Vmca.

Vmc- is the actual redline on the A/C or the speed you have to remember on your stage check/checkrides. 56 KIAS in the nole I belive. Also this is the speed set by manufacturers by conditions set by the FAA under 23.149. so this is all worse case scenario.


V-Minimum Controllable Airspeed.


This deals with multi engine aircraft and being able to maintain directional control with its critical engine (left ususally for non counter-rotating a/c) inoperative. Given certain conditions and configurations. Vmca should ALWAYS be higher than Vmc.
 
I always thought it was Vmc and MCA --

Velocity (minimum control)

or

Minimum Control Airspeed

Two ways of saying the same thing.
 
VMC 56: (FAR 23.149) Minimum speed at which you can maintain directional control


VMCA: conditional – depends on configuration, day, altitude, etc.
 
Vmca can also be used to define a single engine aircraft. Its the speed at which the aircraft is flown that any increase in AoA, load factor, or reduction in speed will result in an immediate stall. AFH 4-1.

Vmc doesnt pertain to singles. Vmc is a twin engine airspeed at which the aircraft may be flown and still maintain directional control.
 
I used to fly an aircraft that defined 2 Vmc's. One was Vmcg (min control airspeed on the ground) and the other was Vmca (min control airspeed in the air). Different sets of tech orders or owner's manuals sometimes use the same words to mean different things, so it's not always 10% standard.

The place that you're reading from is probably your best bet to look for a definition.
 
meritflyer said:
Vmca can also be used to define a single engine aircraft. Its the speed at which the aircraft is flown that any increase in AoA, load factor, or reduction in speed will result in an immediate stall. AFH 4-1.

Vmc doesnt pertain to singles. Vmc is a twin engine airspeed at which the aircraft may be flown and still maintain directional control.
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What you are calling Vmca is actually just called Minimum Controllable Airspeed (Not a V-speed) and is defined as you have written.

Vmca and Vmc used to have different definitions in the Airplane Flying Handbook and even in some PTS's. When the FAA put out the new AFH a couple years ago (with the expanded Multi-engine section and color) there was only reference to Vmc. You will still see some references in Aircraft Flight Manuals and POH's with separate definitions (Vmc is the certified speed that meets conditions set in Part 23.149 - as close to worst case scenario as you can get - and is the printed red line on the airspeed indicator. Under 99% of situations, you will be able to control the aircraft at all speeds equal to and greater than that speed if one engine were to suddenly become inoperative. Vmca is listed in these manuals as the speed at which the aircraft will actually lose control which depends on the actual conditions the aircraft is exposed to.)

Examples of the 1% of situations that you may not be able to maintain control of your aircraft: Ice accumulation, Ice protection systems activated (boots would disrupt normal airflow and lift production), mechanical malfunction, or abnormal atmospheric phenomenon (Normally aspirated engines are rated for horsepower at sea level, ISA. If you were to fly in really cold conditions, at a low MSL altitude on a really high pressure day, your engine could be producing more than the normal rated power.)

Basically the FAA no longer differentiates between the two. Same way the official reason listed for a critical engine is P-Factor, but I still get Multi-engine applicants - and instructors - that insist on drawing pictures of slipstreams and torque that haven't been in FAA publications for years...
 
VMCG – minimum control speed during takeoff ground roll with the nose wheel still on the ground.

VMCA – minimum control speed in the air or on the ground with the nose wheel off the ground after rotation for takeoff.

Reference: FAR 25
 
tgrayson said:
Hmmm...not appearing in FAA material makes it false?
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No... Not appearing in FAA publications and not being able to explain anything other than those 'outside explanations' is where the problems seem to arise. If an applicant is unable to explain some of those other ideas when asked to elaborate and is unable to produce supporting material except 'my instructor told me so' or materials produced by a private company not supported by the FAA, an applicants knowledge of the subject matter may not be to standards.

At any rate, spiraling slipstream and accelerated slipstream are both resultants of P-factor.
 
awacs94 said:
VMCG – minimum control speed during takeoff ground roll with the nose wheel still on the ground.

VMCA – minimum control speed in the air or on the ground with the nose wheel off the ground after rotation for takeoff.

Reference: FAR 25
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FAR 25 only applies to transport category aircraft so some of the definitions don't apply to other aircraft. In this case though, the definitions are essentially the same
 
excessthrust said:
materials produced by a private company not supported by the FAA, an applicants knowledge of the subject matter may not be to standards.
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Or may be higher than standards. The FAA's published material regarding aerodynamics is notoriously bad.

excessthrust said:
At any rate, spiraling slipstream and accelerated slipstream are both resultants of P-factor.
Click to expand...

Spiraling slipstream is not, although the effects of accelerated slipstream would be. Still, there is a fundamental difference between the P-factor being a problem vs the accelerated slipstream being a problem. P-factor is yaw, accelerated slipstream produces roll.
 
tgrayson said:
Or may be higher than standards. The FAA's published material regarding aerodynamics is notoriously bad.



Spiraling slipstream is not, although the effects of accelerated slipstream would be. Still, there is a fundamental difference between the P-factor being a problem vs the accelerated slipstream being a problem. P-factor is yaw, accelerated slipstream produces roll.
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Why does the spiraling slipstream have a 'starboard' vector in clockwise turning (from the cockpit) engines? Because in a conventional twin, the downward blade produces more thrust (from P-factor) therefore has a resultant lower pressure - just like the wingtip. The 'higher' pressure, lower velocity air from the upward turning blade will slightly accelerate and shift right and slightly past the area of low pressure to fill the void of relatively lower pressure due to the p-factor. (Kind of like the way that air moving over an airfoil accelerates past the air below the airfoil, causing a slightly downward vector... induced drag...) This process creates the angled path of the accelerated air that hits the rudder. What started the process? P-Factor. If you are referring only to the difference in THRUST vectors caused by P-Factor, then yes it is a yaw tendency. If you look at the results of the differences in thrust and airflow that P-Factor create, then you can determine that the rolling and yawing tendencies from accelerated and spiraling slipstream, respectively, are still byproducts of P-Factor.

By the way, I do agree that I have seen information in non-FAA publications that is not only useful but helpful in understanding technical subject areas and maneuvers, but they are not necessarily acceptable as reference materials to explain or contradict FAA interpretations and "correct" aeronautical knowledge.

Where would most of us be without a Gleim anyway? :)
 
excessthrust said:
Why does the spiraling slipstream have a 'starboard' vector in clockwise turning (from the cockpit) engines? Because in a conventional twin, the downward blade produces more thrust (from P-factor) therefore has a resultant lower pressure - just like the wingtip.
Click to expand...

It doesn't. That's one of the myths I've seen on some of the prep material from one of the diploma mills. The propeller slipstream doesn't curve, it flows pretty much straight backwards. (Or at least, I've never seen any data that it did, and I've looked at a lot of it.)

If the slipstream direction were influenced by the pressure distribution behind the prop, it would actually curve the direction opposite from what you say. That's because the propeller creates an area of high pressure behind the blade, and the side with the higher AOA would initially have the higher pressure, producing a flow outboard. However, the mixing of the air evens out the pressure distribution pretty quickly, within a blade length.
 
tgrayson,

Do any of the data you have reviewed indicate that the spiraling slipstreams of conventional light twins create a left yawing moment? Please explain briefly.
 
Pilot Hopeful said:
tgrayson,

Do any of the data you have reviewed indicate that the spiraling slipstreams of conventional light twins create a left yawing moment? Please explain briefly.
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I have seen NASA (actually, NACA) test data showing that the slipstream direction of rotation can produce an asymmetrical yawing moment. As I recall, the mechanism is that the rotating airstream either adds or subtracts from the existing flow around the fuselage; with both propellers rotating conventionally, one side has a downflow and the other has an upflow. This can produce different pressure distributions between the sides.

Since it's been years since I reviewed the data, I won't commit to the yaw direction, but I'll try to refresh my memory later in the week.

The key thing, though, is that the airflow doesn't need to curve, but it does expand uniformly the further away from the propeller it moves. One book I have says the angle of expansion is about 15 degrees, so it will likely hit the fuselage eventually.

Also, once the aircraft is actually yawed, the relative wind may steer the spiralling slipstream straight towards the vertical stabilizer, where it will have the same effect as it does on a single-engine airplane.
 
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