What is higher, Vmcg or Vmca on a B747-400 and why?

[Sir Donald]

Can any B747-4 Drivers explain, please?
I came accross an unofficial statement regarding the above topic and I am a bit confused.
The explanation was:

''Vmcg is higher because the distance between the CG and the rudder in the air is greater then the distance between the wheels and the rudder on the ground (???). This means that there is a greater moment arm in the air and so Vmca can be less than Vmcg(OK) .Also, the application of 3 degrees of bank in the air will further lower the value of Vmca. ''

[Captain Stable]

Vmc(a) or (g) is a measure (indirect) of rudder authority.

The rudder exerts a force. The effect of that force is greatest when the arm on which it operates is longest.

It is shorter distance from the rudder to the maingear (the pivot point on the ground) than it is from the rudder to the Centre of Gravity (the pivot point in the air). Therefore, on the ground, the rudder has less authority, therefore Vmcg is higher than Vmca.

As it points out, in the air you also have the option (considered complusory, in effect) of banking towards the dead engine. This is not, for obvious reasons, an option on the ground. Such banking also reduces Vmca.

[jtr]

Its not specific to a -400, it relates to any "normal" multi eng a/c.

''Vmcg is higher because the distance between the CG and the rudder in the air is greater then the distance between the wheels and the rudder on the ground (???).

True, the greater distance from the CG, when compared to the wheels (both are acting as the fulcrum in their repective situations) means the same result can be achieved with a lesser force (less airspeed) The CG is always going to be in front of the wheels, well not always, but if it isnt, then VMCG is the least of your worries.

This means that there is a greater moment arm in the air and so Vmca can be less than Vmcg(OK) .Also, the application of 3 degrees of bank in the air will further lower the value of Vmca

Gave me a headache like I was doing ATPL again trying to think about that:sad: Standing by for someone who has more of a clue about techy stuff.

Just saw the above message... I am a slow typer, so yeh.. what he said too

[LEM]

Interesting point, I had never thought about the difference in pivot point in the air and on the ground.

However, who can tell exactly which is the pivot point on the ground? Are we sure it coincides with the gear? The nosewheel is still on the ground, and will exert a force resisting the sideslip imposed by the asymmetry, so the main gear will sideslip as well...
It seems a bit simplistic to say the pivot is the gear on the ground. Maybe it's between the CG and the gear, probably moving while the correction is applied with rudder....

[Sir Donald]

Thanks.
I had no idea that the CG position was that far forward on a large jet, and in my understanding?? i had always thought that it was slightly behind the main gear,(only guessing) hence the confusion. I understand clearly the difference in the arm and the resulting difference in the values of both speeds.
Out of interest, what is the most aft CG position on a such a big bird in comparison to an external feature of the aircraft?
Thanks once again,
The Don.

[LEM]

If the CG were after of the main gear, wouldn't the airplane flip on it's tail? ;)

on a DC3 it's OK!

[NigelOnDraft]

errr.....

<<i had always thought that it was slightly behind the main gear,(only guessing) >>
I think you'll find that if the CG is behind the Main Gear, it will sit on its a*se! By definition, in a tricycle u/c aircraft, the CG will be forward of the main gear

<<Out of interest, what is the most aft CG position on a such a big bird in comparison to an external feature of the aircraft?>>
Well, the main gear as an absolute limit!

NoD

[LEM]

sorry Nigel, I beat you by a couple of seconds! ;) ;)

[jtr]

LEM NWS is disconnected/overridden for testing of VMCG, thus the effect on the published (and max) VMCG is nil.

CG foward of wheels = :D
http://www.airliners.net/open.file/305456/M/

CG aft of wheels = :ooh:
http://www.airliners.net/open.file/218290/M/


picture is worth...etc

[john_tullamarine]

It is worth running a search through PPRuNe on Vmcg and Vmc/Vmca as the subject has been canvassed numerous times.

At the end of the day, there is no necessary connection between the two figures for a given aircraft and the "arm to the rudder" argument is far too simplistic to be of any real value.

It is important to keep in mind that both values, as quoted in the AFM, are certification figures designed to put a line in the sand for handling qualities and to give the pilot some protection for most, if not all, likely real world cases.

The actual figures on the day (if one were to run the tests for the specific conditions) generally will be lower than the published figures.

Real world Vmcg, in particular, is very sensitive to crosswind, which is not addressed in most certification cases.

Real world Vmca is very sensitive to bank angle, where the certification figure normally is determined for the usual 5 degrees into the live side.

Both are very sensitive to actual cg and thrust output.

[jtr]

John, had a squiz through the old posts and couldn't really find a great deal more, but an curious re your comment..



the "arm to the rudder" argument is far too simplistic to be of any real value

Could you please elaborate a little for the simpletons (thats me btw) amongst us.

Thanks

[john_tullamarine]

First, my comments are not directed to the 744 in particular, but are more general in nature.

The problem with trying to pass off responsibility for specific values of Vmcg/Vmca to one factor only is that there are a number of considerations at play. Some of those, in no particular order, include

(a) flap setting
(b) CG position
(c) thrust output
(d) tyre side force capabilities (Vmcg)
(e) gear layout/geometry (Vmcg)
(f) bank angle (Vmca)
(g) gross weight
(h) differences in empennage flow characteristics on the ground and in flight at much higher alpha in the absence of a ground plane (ie what force can the fin/rudder combination generate ?)
(i) crosswind (Vmcg)

Consideration of lateral tail force arm is relevant to CG and gear/tyre characteristics. Consider also that Vmcg looks at runway centreline divergence while Vmca is concerned with maintaining a flight path.

My observation is that, for many if not all aircraft, Vmcg and Vmca are in the same general ball park .... I have never bothered to worry too much as to which is the higher for a given aircraft.

It is important to keep in mind that the certification figures are based on particular circumstances .. in the real world the actual figures vary, sometimes significantly, from the AFM data ... generally the pilot doesn't have specific data detailing such variations.


Just spent a little time looking over old posts ... Mad (Flt) Scientist's observations in the following thread (http://www.pprune.org/forums/showthread.php?s=&threadid=71279&perpage=15&pagenumber=1) provide some additional information ....

[jtr]

See your point john, I was looking from a more general certification perspective in so much as certain items mentioned are taken for granted e.g. cert is done with aft CG, max thrust, no x-wind, NWS inop, etc. In these circumstances I imagine that VMCG will always be higher than VMCA, due to the arm issues mentioned earlier, and the 5 deg allowable (i think) bank. Adjusting the speeds for all of the variables possible may result in a situ where it is VMCA which is higher, though I would imagine that would be rare. As an aside, I must admit I was shocked to see a V1 of 117 kts (VMCG limited) when we started doing derate t/o on the -400. Mind you the N1 is only about 80%, so it all makes sense, but still a pretty low commit speed!

This used to be a common interview question a few years ago, but seems to have died off.

[john_tullamarine]

.... fact of the matter is ..... Vmcg is not always higher than Vmca .. so the "arm" argument is flawed ...

[Sir Donald]

John tullamarine,
Your explanation is a trully professional one.
I try to get some info from professionals in order to become one my self. My appologies for asking dumb questions, but in my opinion if you do not ask you will never know. Just trying to convey my passion about flying by asking all sorts of questions.
Thanks for all of your help.
The Don.

[john_tullamarine]

Sir Don ... we all start out knowing not much and progress from there .... there are many very knowledgeable people who post to PPRuNe .. one of its main strengths ..... it continually amazes me just how little I know as I read what others post here ....

The only people who are dumb are those who think that they know it all and don't ask questions or ferret through the textbooks for the answers ..... keep an enquiring mind and you will do just fine ....

[jtr]

Was feeling a bit of an ass pushing for more info, but in light of your last post john, I will ask my bit.

I dont see how on a "normal" multi the certified speeds are going to show a Vmca higher than Vmcg.
When you consider the distance from the wheel turning fulcrum (lets say a central point b/t all wheel sets for arguments sake) and the CG, and make it a % of the total arm distance (rudder to CG) it will result in a force application difference in the order of 5% (guestimate) I find it hard to imagine any of the other factors you mentioned earlier, flap settings, tyres, gear layout, TOW, etc having a combined effect on the two speeds such that it exceeds the advantage obtained from the combination of the CG/wheel pos, and the allowable bank. When I look at where my guestimate of the position of a 22% MACTOW is on the -400 (typical T/O pos.), and compare it to where the turning fulcrum of the tryes is, the distance is not insignificant. Couple this with the H/stab doing all that it can at 120 odd knots, and I cant see how it is going to be any other way. In my head, this translates throught to all smaller jet a/c, though stick a prop on the wing, and it may be a bit more complex.

I realise this is really just academic, as the speeds in reality are probably quite a bit lower than the published ones, but still stand on the a. speed being lower than the g. speed.

Please appreciate I am not trying to bait you into an argument, just trying to clarify in my head what you are saying.

Thanks

[Captain Stable]

Oops! :oh: BIK got me. Keyboard engaged, brain elsewhere! :E :\

[john_tullamarine]

jtr,

A good argument never hurt anyone ....... bit hard to answer your question, though.

Fact is that Vmca sometimes is higher than Vmcg. The problem with the "arm" theory alone is that it ignores a bunch of other considerations.

Be aware that Mr Boeing's aft limit is a lot further aft than 22 percent .. the typical takeoff CG has not much to do with the certification values of Vmcg and Vmca. Very definitely influences the real world value on the day .. but the real world has never had all that much to do with certification ....

There is no simple relationship between the two figures .. sorry about that ......


Cap'n,

Be relaxed, good sir ... we all knew that it was a deliberate mistake put in intentionally to check if BIK was on the ball .....