Vmcg Vmca vs TO flap setting

[alatriste]

I would like to know the reason why Vmcg as well as Vmca decrease as TO flap setting is increased.
I can figure out that Vmcg variations depends on the the load imposed on the nosewheel but dont have any clue regarding relationship between Vmca and flaps.

Is this also the case for the aircrafts are you flying?

All replys will be appreciated. Thanks for your time.

[Wizofoz]

This is not (at least not in my experience) the case with th 737 or 767. At low weights it is necessary to check VMCG, and raise V1 to that speed if it was lower. The VMCG charts are independant of flap setting, and I agree that I can't see a corelation between flap setting an VMCG.

[HotDog]

Flap settings may be relevant on some aircraft due to flap interaction with rudder but on all the aircraft I operated on, the only variable on VMCG was actual OAT and V1 could not be less than actual OAT VMCG, which we extracted from the TO data charts.

[Sheep Guts]

I was under the impression VMCA on larger jets was also variable due to Cof G Postion. That is to say a more aft Cof G would reduce Rudder authority(or moment) increasing the VMCA.
Flap generally decreases VMCA, because flap increases lift therefore drag, so the live engine wing has more resistance to motion. Or in other words,Flap increases keel surface (under aircraft) increasing resistance to yaw.
Stall Speed changes with C of G postion aswell.

Also with 4 engined jets depending on the failure. If its 1 engine the VMCG and VMCA Speeds are lower than for double assymetric config obviously.

Configeration for VMCA
Max Weight
Least Favourable CG
5degs towards live engine
max thrust on live engine
critical engine failed
prop windmilling or feathered with auto feather

Regards
sheep

p.s. actually found me old ATPL notes
:ok: :E

[john_tullamarine]

Real world Vmca varies with CG, but the AFM certification figure is based on worst case which can be presumed to be for max aft CG.

Flap/Vmca and flap/Vmcg relationships will depend on the individual aircraft and, in most cases, the OEM doesn't see a need to investigate whatever variation may exist, certainly not through to defining the data in the AFM.

Stall speed varies with CG and the certification AFM figure generally is based on max forward CG which gives the highest stall value.

Real world Vmcg will depend on nosewheel load, but the certification AFM figure is based on a free nosewheel ie no NWS, although some standards permit actual demonstrations in the wet with NWS.

[Mad (Flt) Scientist]

Sheep Guts

Small correction to your list of VMCA demonstration conditions:

FAR 25.149(c)(4) states that the "maximum sea level takeoff weight (or any lesser weight necessary to show VMC)" is to be used.

It's a pretty convoluted wording, because they're mxing up two things in the same section. But in practice it's the part in brackets that matters, because VMCA is generally higher at lighter weights, due primarily to the limited 5 degree bank angle and the lower weight reducing the sideslip which can be generated, this sideslip being helpful in trimming out the yawing moment from the engine asymmetry.

I recall seeing an Airbus paper which argued that higher weights should be considered, rather than the traditional demonstration at minimum practical weight, as the higher angles of attack at heavier weights could reduce rudder effectiveness in some cases. But I believe it's still generally held that light weight is more critical.

John is correct in stating that the dependency with flap is aircraft dependent; we do generally determine the dependency in the course of VMC analysis, but the AFM is often simplified to a single value agreed with the cert authorities.

FWIW, on three pretty similarly configured aircraft my company has recently tested, the relationships were:

1. No stated flap dependency, VMCG < VMCA

2. VMCA higher with larger flap angles. VMCG a single value between the two VMCA values

3. VMCA lower with increasing flap angle. VMCG higher with increasing flap angle. Values 'cross over' between flap settings