View Full Version : VMCA VMCG question ??


high flyer 13
20th Jul 2010, 15:19
Im running through typical interview questions at moment and seem to be getting bit confused by this one..

Does vmca and vmcg vary with aircraft weight ??

If asked this question in an interview my answer would be yes it does vary with weight ...
due to the fact that an increase or decrease in weight changes the centre of gravity which in turn depending on the cg position of that weight,changes the vmcg/a speeds.
example being an aft cg requires a higher vmcg/a because of the shorter moment arm meaning less effectiveness on the rudder, so the aircraft will have to increase its speed to allow directional control...!!

is this answer correct as the more i read up on this question i seem to be getting that little bit confused with conflicting info..

can anyone shed any light on this or is my answer ok..!??

Thanks for any help given.... :)



Alexander87
20th Jul 2010, 16:09
To me,your answer is more than correct!

Vone Rotate
20th Jul 2010, 16:21
I prepare to stand corrected but I'm pretty sure they are fixed values.

Vmcg and Vmca are more to do with airflow over control surfaces than c of g.
Yes if you heavy it will take longer to reach but once you have the required airflow over the control surfaces you will be able to maintain directional control with just the use of primary controls.

I've not read up Vmca or red line speed on the DA42 is a fixed value of 68kts. I don't recall this value ever changing.

I have not read up so maybe speaking complete tosh!!

demomonkey
20th Jul 2010, 16:42
Vone is correct in that on a medium transport jet aeroplane I drive the Vmcg figures are certainly fixed with reference to calculating manual performance.

However like you I imagine that their must be loading and enviromental variables on them. I imagine that in reality the quoted Vmcg figures are probably worst case scenarios.

I'd imagine that the person interviewing you probably cannot answer such questions to a definite scientifically correct point so don't worry too much. They're basically looking for the fact that you can (a.) discuss the subject knowledgeably and (b.) show the ability to articulate a coherent argument.

Good luck! :ok:

Mad (Flt) Scientist
20th Jul 2010, 16:59
Normally, if discussing whether something varies with a parameter, the assumption is that all other parameters are held constant.

So the question, "Does VMCA/VMCG vary with weight?" has the implied additional "assuming all other relevant parameters are held constant".

This is possible to do if you assume some form of ballast control, as is present on most flight test aircraft.

If we now consider this version of the question, and split it into two (we'll see why soon):

"Does VMCA vary with weight?"

The answer is, yes it does. For most aircraft there is a declared VMCA, and this may be a single declared value. The underlying "real" VMCA is, however, very much a function of weight, and there are also some (recent) aircraft where it is explicitly declared to be a function of weight (The Embraer E-Jets make such a declaration, I believe).

The reason it is a function of weight is the way that VMCA is determined; for the steady state case, the aircraft may be banked up to 5 degrees in a favourable direction. This allows some small additional sideslip to be present (so that aerodynamic sideforce is opposing the weight component in the lateral direction). This sideslip helps to counter the yawing moment caused by the engine asymmetry. The bigger the sideslip, the more it helps. The heavier the aircraft, the bigger the lateral weight component, and so the bigger the sideslip.

So a heavier aircraft, if flown to the VMCA test rules, will have a lower value of VMCA if all other factors are constant.

"Does VMCG vary with weight?"

The answer to this one is, basically, "no". Again we have to be careful about secondary effects through other parameters, but if they are controlled then there is little effect of weight. This is because the determination of VMCG is done in a fashion that no direct weight effect is available to assist.

Mad (Flt) Scientist
20th Jul 2010, 17:01
Those magic words VMCA VMCG attract certain people like a moth to a flame, don't they :)

SloppyJoe
20th Jul 2010, 17:12
What you said with what Mad (Flt) Scientist said and you have answered it to a high standard and will impress anyone who asked.

BigGrecian
21st Jul 2010, 14:08
The JAA don't touch on this as well as the FAA do ironically, but I agree with mad Flt Scientist.

Does Vmca vary with weight - not with wings level as there is no horizontal component of lift. Add bank and there is horizontal compontent of lift, which means the weight can change the Vmca.

Cumulo Calvus
21st Jul 2010, 14:44
You could technically argue that VMCG does vary with weight I think.

On the aircraft I fly, it varies with flap setting (albeit by very little). And the flap setting for take off is course determined by (amongst other things) weight.

All good and interesting discussion points for an interview I'd say though.

Duh
21st Jul 2010, 15:41
Reminds me of the OL saying:
You can dazzle them with brilliance or baffle them with BullSh!t

P-T
21st Jul 2010, 18:04
I was under the impression that Vmca and Vmcg was about directional control and not a function of flaps. Vmca and Vmcg are calculated for a value that will allow directional control in the event of an engine failure on the ground (Vmcg) and therefore allow direction control in the event of an RTO. Vmca would allow the aircraft to be controlled in the event of an an engine failure after V1 and the subsequent flight. Vmu is what will change with flap settings as the it decides what the absolute minimum speed for flight will be depending on flap settings, trim settings and C of G.

so the C of G shouldn't make too much difference as long as the aircraft is within its limits of max and min C of G. This isn't a great deal on a DA42 but could be 40-50 tonnes on a B757 or more on the bigger stuff!

I hope this is firstly correct and secondly useful!

P-T
21st Jul 2010, 18:27
Just to add one thing after reading Mad (Flt) scientists and Big Grecians post, i would have thought that the angle of bank and weight would be largely irrelevant as Vmca is a speed that will give directional control with a bank angle of 15 degrees. However if you reduce below Vmca in a 15 degree angle of bank then surely you'd also be below Vr and Vref and therefore directional control is the least of your worries.

If you're below Vmca in a turn, then you are going down, and going down rapidly.

Again, I'm more than happy to be shot down in flames if I am wrong.

BigGrecian
21st Jul 2010, 20:04
with a bank angle of 15 degrees

It's an airspeed with up to 5 degrees of bank towards the operating engine.

With more horizontal compontent of lift (i.e more weight) you will need less bank to get the same horizontal compontent of lift compared to a lower weight, which in turn eventually leads to a lower Vmca.

high flyer 13
21st Jul 2010, 20:21
well thanks for the answers guys, il keep my answer short and sweet and sure if im asked to go into more detail il do what one of the above answers said which was " dazzle them with brilliance or baffle them with bullshit " !! :O either way im sure i know enough to give a good answer from reading the above points,,

thanks again...

P-T
21st Jul 2010, 20:25
I have consulted my ATPL books (yes I am that bored!) and the Vmca is not affected by weight.

It is however affected by:

1. Angle of bank
2. CG Position (which is function of the position of weight, not the amount)
3. Aileron Effectiveness
4. Flap Position
5. Undercarriage
6. Thrust produced (therefore environmental conditions - in Jet Engines anyway, I can't remember about Props)
7. Relationship between Vs and Vmca.

I would type the whole paragraphs, but I really cannot be bothered!

Apologies for the incorrect debate.

P-T

BigGrecian
22nd Jul 2010, 12:21
I have consulted my ATPL books (yes I am that bored!) and the Vmca is not affected by weight.

The ATPL books aren't the be all and end all of Vmc information, and in US certified/manufactured aircraft (which is a large majority), the aircraft is certified with the most unfavourable weight and C of G position so to say that weight doesn't affect is not true.

If you went into an interview quoting the ATPL book says I'm pretty sure how that interview is going to go....

Mad (Flt) Scientist
22nd Jul 2010, 16:39
@<hidden>

Just to add to BigGrecian's point.

The same also applies to aircraft certified to JAR/EASA standards, and Transport Canada, and to the best of my knowledge everywhere else too. Weight is quite definitely, certainly, a factor in determining VMCA. We incur considerable difficulties in flight testing to determine VMCA as a result, since we often have the bare minimum of fuel onboard.

alf5071h
22nd Jul 2010, 17:56
Just to add to MFS’s posts; the regulatory background is in CS 25.149 (~ page 31) (http://easa.europa.eu/ws_prod/g/doc/Agency_Mesures/Certification_Spec/CS-25_Amendment%208.pdf)
Note the small print (f)(4) allowing weight related values.

I suggest that knowledge of the requirements which relate to the AFM limits, with some ability to discuss simplified theory, particularly of what the most unfavourable values would be, will be useful for an interview.