Vmca vs g

[Gary Halliday]

Anyone care to do a KIS explanation of this relationship or point to a reference for it.Can`t find anything, even in some horribly mathematical sources.

[Capt Pit Bull]

There is no relationship, since Vmca is a defined speed.

Your question is akin to asking how adding 1 passenger changes the Maximum All Up Weight. (It changes the actual All Up Weight, but not the Maximum).

CPB

[quid]

Capt. Pit Bull-

The Vmca will vary quite a bit with different factors. It is given as a reference speed in many manuals so as to provide the flight crew with SOME speed to use. The actual Vmca will vary with CG location, temperature and pressure altitude, and with assumed temp thrust or derates.

Gary- The Vmca won't be much different than Vmcg. In most cases it will be a few knots less due to the fact that you may use 5 degrees of bank, while on the ground you can't. Look in the performance section of your AFM. The "chase charts" will show the variables.



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[Gary Halliday]

Quid - Thanks for the response which recognises that there are many factors at work. I take your point about bank angle and seem to remember it being particularly marked on the B707 (the 400s ?). The remarks on temp and derates I agree are entirely similar to the effects on Vmcg.As for CG--- this I`ve also come across and imagine it being related to yawing moments but what has been taxing the brain is specifically "g". If Skippy and I try to get airborne in the Dak with that rough donk while racking it round to avoid the small arms fire what increment should we add ?. I`m bothered about aeroplanes without AFMs.

[quid]

If the "g" you mention is G forces for yanking and banking, you'd want to be much faster than Vmca. Vmca gives no stall protection, it's only for directional control.

If you're going to yank and bank, you need to be at min maneuvering speed for the configuration (and then some). At least 1.3 Vs for the configuration, and even then you'd better be gentle.

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[Gary Halliday]

Quid- All true and apt for day to day ops but I would really appreciate an idea of the aerodynamics, after all if you can fly in the realm (1g) between Vsl and Vmca what then happens to the value of Vmca as you increase loading. Stalling is straightforward -- = Vsl x square root load factor. I assume Vmca increases but how + why ?.

[quid]

I can't see where it would ever be a factor. The change in Vmca with weight (or loading) would be in the magnitude of approx. 1%. Insignifigant. The change in stall would be over 10%, So, yes, you would have lateral control when falling out of the sky. ;)

Vmca, remember, is with the most critical engine inop, and the other(s) at max power. I doubt you'd want to go to 30 degrees of bank while on the ragged edge.

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[BEagle]

Garry Halliday? Are you and 'Bill Dodd' still flying DC3s - or did he pack it in when he changed his name to Charlie Hungerford and went to live on Jersey ??
(Well - I remember your BBC TV series in the 1950s!!)

[Gary Halliday]

Quid--Thanks for humouring me so far . If you`re using 30 odd degrees of bank ie load factor 1.2 ish = 10% increase in Vsl then where does the 1% increase in Vmca come from ?. Does it help if we just to call it Vmc ?.

BEagle-- Yes that`s me still at it. Bill joined CX before going to live on Jersey under an alias.

[quid]

If the "g" loading is increased, then Vmc is not a factor, because you are in a stall condition. The Vmca charts I have stop at Vs and show no value below Vs.

The only reason we use Vmca (air) is to differentiate between that and Vmcg (ground).

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[bobs61]

A lot of innacurate info being posted here. VMCA is subject to many variables, but the figure published and used for the calculation of V2 is based on the WAT given, assuing t/o configuration (T/o flap extended,gear up) with the C of G at the aft limit (shortest moment arm). Five degrees of bank is permitted.
VMCG takes into account tyre friction in maintaing directional control. VMCG is always lower that VMCA

[quid]

I'm on vacation now, so we've got to wait about 3 weeks for me to give you direct quotes from the manuals.



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[Check-in]

Interesting discussion, as it relates to my old favorite the Gooney Bird. As I recall, VMCA was in the order of 77kts which gave a takoff safety speed of 84kts. It stalled down around 55 kts with full power on both
engines and 65 kts power off at typical takeoff weight with the flaps at zero. Reckon if you were on one donk and pulled it below 77kts with any 'g' at all you'd spin in pdq.
Even the power on stall at landing flap was vicious and could lead to a 90 deg wing drop, but you would have to be silly to even go there as you could get the ASI off the clock towards zero first. No warning when it happens but and the wing drop varies with the amount of abuse the airframe has had over the years, so don't try it below 8000ft.

[Feather #3]

Bobs,

re your last comment; how come the following apply -

B747-400/RB211-524G Vmca1 - 117kts
Vmcg - 124/122kts

/CF6-80C2B1F Vmca1 - 120kts
Vmcg - 125kts?

G'day

[411A]

Vmca1, with derate
Vmcg, full thurst,no derate

Ref: B707-320B (advanced) AFM

Suspect the same on 747 types.
Hope this helps.

[John Farley]

Vmca<Vmcg?

Might not being able to bank on the ground creep in here in some special cases?

JF

[Gary Halliday]

Maybe I should`ve posted the question as Vmc vs Load factor. We seem to have digressed to a discussion of Vmca vs Vmcg which has introduced the novel idea of bank angle effecting Vmcg in special cases. I was hoping JF would contribute and am wondering what he has in mind. Can you have nozzle failures producing the vectored thrust equivalents of minimum control speeds ?. Also a specific q to someone who would display or test a conventional multi engined aircraft how does load factor effect minimum control speed ?.--if indeed it does.

[John Farley]

GH

I only offered the lack of ability to bank on the ground as one possible thing that could raise Vmcg above Vmca

I'm going to bed

JF