Vmca relationships

[byeplane]

Hi

Ok, so I know there are a hundred threads on Vmca and Vmcg, but I cannot find one which answers my problem. I'm sure I understand VMCG but VMCA is driving me nuts.

This is how I understand take-off speeds in order:
Vmcg-V1-Vmu-Vr-Vmca-V2
1. Vmcg must be smaller than V1 since in the event of an engine failure the aircraft needs to be controllable.
2. Then you have V1 decision speed
3. Vmu has to be less than Vr otherwise you risk a tail strike
4. Vr Rotation
5. Vmca must be less than V2 (this is what I have read but my understanding if the relationship between these two speeds is hazy to say the least)
6. V2 - Speed of aircraft at 35ft screen height i.e. the plane is now flyable

My questions:
1.What is the specific differences between Vmca and V2?

2. could Vmca be less than Vmcg since you have use of the ailerons as well as the rudder once airborne?

[Gysbreght]

1.What is the specific differences between Vmca and V2?

2. could Vmca be less than Vmcg since you have use of the ailerons as well as the rudder once airborne?
1. Per regulation V2 cannot be less than 1.1*Vmca

2. I see nothing to prevent Vmca from being less than Vmcg but is that important?

Vmcg is controlled by the rudder capacity. Ailerons are less important because the mainwheels in contact with the runway surface take care of the rolling moment caused by the rudder deflection. On the other hand, Vmca benefits from the 5 degrees of bank angle allowed in flight.

P.S.
To my knowledge there is no requirement that Vmu has to be less than Vr, it is more complicated (see CS 25.107(e)(iv)). Besides, Vmu covers other things besides tail strikes.

[byeplane]

Thanks,
I actually had written down in my notes that V2 had to be 1.1 Vmca minimum but my brain chose to completely ignore this.

I still just cannot wrap my head fully around Vmca

[Nikonair]

Well V2 ensures that, if you are N-1, the Aircraft will achieve the minimum climb gradient upon which the performance calculations are based.

Vmca just denotes the minimum speed at which the rudder, in combination with 5 degrees of bank, has sufficient power to counter any yawing moment produced by an engine failure whilst airborne.

[FE Hoppy]

Vr may not be less than 1.05 Vmc. You can't try to fly below control speed! That would be silly.

Why not just download a copy of CS25 and read the perf related parts. Or all of it?

here: http://easa.europa.eu/system/files/dfu/agency-measures-docs-certification-specifications-CS-25-CS-25_Amdt-3_19.09.07_Consolidated-version.pdf

[vilas]

biplane& Gysbreght
VMCG is uses only one flight control i.e. rudder
1. V1 is connected to factors affecting acceleration and control on ground, like VMCG, VMBE.
2. Vr the transition from ground to air starts at Vr so it connected to factors that affect control on ground like V1, transition to air like VMU or V lift off and in air like VMCA.
3.V2 is limited by Vs1g and VMCA. It is V2>1.13 VS1g(JAR FBW) and >1.10 VMCA
Regulation states VR may not be less than V1 and 1.05 VMCA.
So VR has to be higher than both VMCA and V1, which has to be higher or equal to VMCG. Now your question could VMCA be less than VMCG? No direct relationship but it can't be. Your other question difference between VMCA and V2.
VMCA is a demonstrated minimum speed. V2 is minimum climb speed that must be reached by 35 feet. Flying below V2 in case of engine failure is likely to lead flight below VMCA, loss of directional control if speed is further reduced. Flying above V2 in case of engine failure likely to lead to not respecting obstacle clearance.

[vilas]

biplane& Gysbreght
Also
At VR, if rotation is done at maximum practicable rate, it has to result in a satisfactory VLOF
As a result, VR limited by VLOF which is limited by VMU and VTIRE.

[Gysbreght]

Now your question could VMCA be less than VMCG? No direct relationship but it can't be.I'd be really interested in your explanation why "it can't be".

[alatriste]

Byeplane,


Vmca can be less than Vmcg because it takes credit of banking (5º) towards running engine.

Vmca lower than Vmcg on MD80 depending on weight and to flap setting.

Vmca always lower than Vmcg on DC 10, FDA20 and C650.

[vilas]

Gysbreght
As stated by me before VMCG and VMCA are not directly linked. Common factor is the thrust that causes yaw. Just as 5% of banks allows you to control at lesser speed in the air, runway traction allows control at even lesser speed when on ground. You will notice this in SIM during EFATO as you rotate you need more rudder to keep it straight as the traction disappears. Since they are not linked I shouldn't say VMCA cannot be less than VMCG. I suppose you will have to see the figures for a particular aircraft. Presently I only have access to A320 speeds. In A320 VMCA is higher than VMCG by at least .5KT almost same.

[rudderrudderrat]

Villas,
runway traction allows control at even lesser speed when on ground.

I think you will find the effect is due to the fin and rudder being blanked by the fuselage at flying angle of attack (hence more rudder required) than anything to do with runway traction.

[vilas]

RR
Thrust asymmetry has to overcome runway traction to cause a swing. As the aircraft starts getting airborne that traction starts disappearing so it causes more swing which you need to compensate.

[rudderrudderrat]

Villas,

On the ground, Yaw is centered around the main wheels.
When Airborne, Yaw is centered around C of G.
C of G is forward of main wheels.
Consider moment arms from rudder to wheels (G), and Rudder to C of G (A).
The (G) Yaw moment arm is shorter than the (A) Yaw moment arm, so you would expect to need less rudder when airborne. However you need more rudder due fuselage blanking effect.

[john_tullamarine]

Keeping in mind -

(a) Vmcg book value is scheduled for no nosewheel tyre forces to allow for low friction surfaces so you may well/should detect a yaw as the nose tyre breaks ground were you to be in a real book Vmcg situation on, say, a dry runway.

(b) you are most unlikely to have the condition set implicit in the book value anyway so it's all a bit academic