VMCG less than VMCA

[Haroon]

Hello. Can anyone throw some light upon the issue of how VMCG can be less than VMCA. I mean whats the technicality behind it? Thankyou.

[john_tullamarine]

Vmcg and Vmca are separate demonstrated items. There is no underlying reason why either should be lesser or greater than the other.

You might like to run a search on the subject as it has been discussed in various threads.

[rudderrudderrat]

Hi Haroon,

VMCG has the benefit of nose wheel forces acting in addition to the rudder. Some aircraft quoted VMCA with up to 5 degs bank towards the live engines.

So although both rely on rudder forces mostly balancing the assymetric thrust, they can use different additional benefits.

[john_tullamarine]

VMCG has the benefit of nose wheel forces acting in addition to the rudder.

I wouldn't be too sure of that. Ref AC 25-7A:

(vi) VMCG testing should be conducted at aft c.g. and with the nose wheel free to caster, to minimize the stabilizing effect of the nose gear. If the nose wheel does not caster freely, the test may be conducted with enough nose up elevator applied to lift the nose wheel off the runway.

(vii) For airplanes with certification bases prior to Amendment 25-42, VMCG values may be demonstrated with nose wheel rudder pedal steering operative for dispatch on wet runways. The test should be conducted on an actual wet runway. The test(s) should include engine failure at or near a minimum VEF associated with minimum VR to demonstrate adequate controllability during rotation, liftoff, and the initial climbout. The VMCG values obtained by this method are applicable for wet or dry runways only, not for icy runways.

Some aircraft quoted VMCA with up to 5 degs bank towards the live engines.

I think that, unless you can cite a conventional aircraft for which Vmca is NOT demonstrated with 5 deg bank, you might presume that this always will apply. Vmca is VERY bank dependent due to sideslip considerations. Poorly managed bank angle in a Vmca situation might prove to be both embarrassing and exciting ... and, quite possibly, terminal.

[groundfloor]

"quiet possibly terminal" - A beauty!, may I use it?

[Haroon]

Good point JT! Why am I comparing the two? I just need to check if I am a victim of some myth that says Vmcg is always higher than Vmca.

Thankyou Rudder and Groundfloor for your response.

[Halfbaked_Boy]

Here's my understanding -

Typically, on tricycle gear aeroplanes... Cessna 406, PA34, 737, A320, 747 etc, and for exam purposes, VMCG can be considered, generally, to be higher than VMCA.

As Mr Tullamarine pointed out, it's very bank dependant, but hopefully I can answer the basis on which your question lies!

So we have a tricycle gear aeroplane, going from front to back, first we have the nosewheel, then (typically) the CG, then the mainwheels, then the rudder/vertical stabiliser. As JT pointed out, nosewheel steering is disconnected for VMCG testing, effectively free castoring.

It all lies in the moment arm, that is, the stabilising/directional force supplied by the rudder/vertical stabiliser, and it's distance from the fulcrum. In the air this is the CG, which is generally ahead of the mainwheels. So to produce a given moment (that which will render the aeroplane directionally controllable with loss of critical engine), there will be some force 'X' required at the rudder. However, whilst the wheels are still in contact with the ground, again 'generally' speaking (because the faster you go, the more the fulcrum moves from the wheels to the CG as the weight gradually moves off the wheels and onto the wings), there will be a shorter arm between the fulcrum (now the wheels) and the rudder. Hence, to produce the same moment over a shorter distance, a greater directional force needs to be supplied at the rudder. At full deflection, the only other variable is our CAS, which must be increased. That's where your VMCG > VMCA comes from!

However, this is only relevant for an aeroplane with a CG situated ahead of the mainwheels (technically the centre of radius of turn to be precise, but good enough), so it is convenient for us to imagine this being a tricycle gear aeroplane.

Going by the same theory, everything is reversed for a taildragger, for which in theory, VMCG would be less than VMCA...

But as JT has written above, there are so many external and type specific factors to be considered, this is all theoretical - I'm no aeronautical engineer, but I hope this has helped you with the basic understanding as I have it.

[Old Smokey]

Forgive me Halfbaked_Boy, I just couldn't resist. In your opening line, you quoted "Typically, on tricycle gear aeroplanes... Cessna 152/172, PA28, 737, A320, 747 etc, and for exam purposes, VMCG can be considered, generally, to be higher than VMCA

Just what is the Vmcg and Vmca for the Cessna 152/172, and PA28?

Regards,

Old Smokey

[Halfbaked_Boy]

No idea!

Wouldn't even know if it exists or not... I just wanted to get the example into the OP's mind on the basis that even if it may not be found in numbers, a similar theory applies.

Unless I read you completely wrong and you're genuinely asking... are they published somewhere?

[Mad (Flt) Scientist]

VMCG is demonstrated at close to zero angle of attack.
VMCA is demonstrated at as low a speed as possible IN-AIR, and thus at as high an angle of attack as can be achieved.

VMCG is a dynamic manoeuvre, initiated from close to zero angle of sideslip
VMCA is principally a static manoeuvre, demonstrated with significant sideslip.

Consider ... in how many other cases would you seek to compare a low-alpha, low-beta, dynamic condition with a high-alpha, high-sideslip, static condition and expect to find any particular relationship?

Add to those details that both are demonstrated conditions and not analyzed in most cases, and it's clear that there should be no expectation of one being greater or lower than the other...

[rudderrudderrat]

Thanks JT for the correction. Your logic makes sense - I'd never have any nose wheel forces during the rotation.

But the wings would be level - therefore does this not suggest that VMCG is greater than VMCA, because of the different bank angles - all other forces being equal?

[Pugilistic Animus]

Forgive me

[FE Hoppy]

Please read:

CS 25.149
Minimum control speed
(See AMC 25.149)
(a) In establishing the minimum control speeds required by this paragraph, the method used to simulate critical engine failure must represent the most critical mode of powerplant failure with respect to controllability expected in service.
(b) VMC is the calibrated airspeed, at which, when the critical engine is suddenly made inoperative, it is possible to maintain control of the aeroplane with that engine still inoperative, and maintain straight flight with an angle of bank of not more than 5°.

(c) VMC may not exceed 1·13 VSR with –
(1) Maximum available take-off power or thrust on the engines;
(2) The most unfavourable centre of gravity;
(3) The aeroplane trimmed for take-off;
(4) The maximum sea-level take-off weight (or any lesser weight necessary to show VMC);
(5) The aeroplane in the most critical take-off configuration existing along the flight path after the aeroplane becomes airborne, except with the landing gear retracted;
(6) The aeroplane airborne and the ground effect negligible; and
(7) If applicable, the propeller of the inoperative engine –
(i) Windmilling;
(ii) In the most probable position for the specific design of the propeller control; or
(iii) Feathered, if the aeroplane has an automatic feathering device acceptable for showing compliance with the climb requirements of CS 25.121.
(d) The rudder forces required to maintain control at VMC may not exceed 667 N (150 lbf) nor may it be necessary to reduce power or thrust of the operative engines. During recovery, the aeroplane may not assume any dangerous attitude or require exceptional piloting skill, alertness, or strength to prevent a heading change of more than 20º.

(e) VMCG, the minimum control speed on the ground, is the calibrated airspeed during the take-off run at which, when the critical engine is suddenly made inoperative, it is possible to maintain control of the aeroplane using the rudder control alone (without the use of nose-wheel steering), as limited by 667 N of force (150 lbf), and the lateral control to the extent of keeping the wings level to enable the take-off to be safely continued using normal piloting skill. In the determination of VMCG, assuming that the path of the aeroplane accelerating with all engines operating is along the centreline of the runway, its path from the point at which the critical engine is made inoperative to the point at which recovery to a direction parallel to the centreline is completed, may not deviate more than 9.1 m (30 ft) laterally from the centreline at any point. VMCG must be established, with –
(1) The aeroplane in each take-off configuration or, at the option of the applicant, in the most critical take-off configuration;
(2) Maximum available take-off power or thrust on the operating engines;
(3) The most unfavourable centre of gravity; The aeroplane trimmed for take-off; and
(5) The most unfavourable weight in the range of take-off weights. (See AMC 25.149(e).)

The only thing they have in common is some of the spelling!

[Haroon]

The only thing they have in common is some of the spelling!

Well said Hoppy.

Its definitely a myth floating around in all parts of the world I think.

[TheChitterneFlyer]

Good post Old Smokey... and the bait was taken!

Vmca and Vmcg... in a single engine aeroplane?

[rudderrudderrat]

The only thing they have in common is some of the spelling

Er... there's a lot more than that in common.

If I get airborne at VMCG with wings level, - I only have rudder balancing assymetric thrust with the aircraft maintaining track parallel to the runway.

Once airborne at VMCA, I can use up to 5 degs bank into the live engine(s) using both rudder and sideslip to maintain heading.

Why do you think they permit us to use up to 5 degs bank? Do you think it makes VMCA worse?

[FE Hoppy]

Given that Vr may not be less than 105% of Vmc I'm not sure what the problem is. V2min is also restricted by a margin above Vmc.


The two speeds reflect two completely different and separate scenarios.
One defined by an ability to stay within 30ft of the centreline on the ground and the other defined by the ability to fly in a straight line after a deviation of no more than 20degrees in the air.

I can't continue to take off if I cannot control the aircraft on the ground (Vmcg) I can't attempt to fly until i'm above the speed required to fly the aircraft in a straight line (Vmc).

[rudderrudderrat]

Hi Hoppy,

I agree. We never see this problem on take off because of those margins. If you are light, then V1 is raised to be >=VMCG.

But on a GA when the aircraft is light & Vref is low - you can get speeds close to VMCA. I've only ever seen it in the sim, but you can run out of rudder authority with wings level.

[john_tullamarine]

but you can run out of rudder authority with wings level.

which is why you use some bank in critical (or near critical) circumstances .. wings level means that the real world Vmca is higher than book value.

an ability to stay within 30ft

depends on the particular certification basis

no more than 40degrees in the air

generally 20 deg

If I get airborne at VMCG with wings level ..

which might be foolish for most Types

... I only have rudder balancing asymetric thrust with the aircraft maintaining track parallel to the runway.

the latter probably being quite problematic ..

Once airborne at VMCA, I can use up to 5 degs bank into the live engine(s) using both rudder and sideslip to maintain heading.

generally one would expect Vmca to be predicated on 5 deg bank. Without the 5 deg, you might find yourself somewhat below the real world Vmca at the time.

Why do you think they permit us to use up to 5 degs bank?

to prevent the certification Vmca being based on more than 5 deg bank

Vmca and Vmcg... in a single engine aeroplane?

you can get something analogous to Vmca for high alpha with net prop thrust moving a tad laterally. I can recall a TP's tale of this effect at low speed where he had negligible control over heading at low speed and high thrust. Not observed it myself but the story makes sense.

therefore does this not suggest that VMCG is greater than VMCA, because of the different bank angles

only in the same way that an orange's colour is more orange than an apple's. Bank has naught to do with Vmcg.

VMCG can be considered, generally, to be higher than VMCA.

.. except for those aircraft for which it is lower ..

it's very bank dependant

only for Vmca. For a large (bomber) type, as I recall, banking 5 deg the WRONG way ups Vmca by around 35kt.

It all lies in the moment arm, that is, the stabilising/directional force supplied by the rudder/vertical stabiliser, and it's distance from the fulcrum.

probably a bit oversimplistic an explanation

as the weight gradually moves off the wheels and onto the wings

that may have some basis for a low wingloading lightie single but not for a transport jet

I just need to check if I am a victim of some myth

I think so.

[Halfbaked_Boy]

Come on chaps, it was 0600 on a Saturday! Tail between legs etc

p.s. Rubbish edited out...!