Vmcg/Vmca1 Guru needed!

[Gas Chamber]

2 questions:

1) why when I'm looking up Vmcg/Vmca is there no weight involved in the calculation??? Only temp and PA?

2) Why is Vmcg faster than Vmca1?? (for the same conditions)

[brisbane observer]

Hello,

Vmcg is independent of aircraft weight because it is simply a matter of balancing assymetric thrust with rudder. Weight has no effect on momentum because the wheels are on the ground as well.

Vmca is even less of a concern as there are greater forces acting from the rudder due to the higher speeds associated with being airborne, therefore there is a smaller difference between the force of the rudder and the force of the thrust, which is why vmca is greater than vmcg.

This is from a pilot not an aerodynamicist, this is the way I understand it.

Cheers

[Night Watch]

To answer your questions, all that is needed is to have a quick look at the definitions of Vmcg/Vmca....

Vmcg on the ground is the minimum speed during the TKOF run, at which when a critical engine suffers total pwr loss, it is possible to maintain directional control of the aeroplane with only the use of aerodynamic controls and enable the TKOF to continue safely. For this reason V1 must be equal of greater then Vmcg. Therefore V1 is where your weight comes into the equation.... not Vmcg.

Vmca is the minimum flight speed at which the aeroplane can be directionally controlled without the loss of height if the critical engine fails with all remaining engines at TKOF thrust. It is calculated using maximum BRW with the most adverse CoG, gear fully retracted, the flaps at TKOF setting and with the optimum bank angle towards the operating engine/s ( typically 3-5 degrees).

So to answer your questions.... Vmcg is not directly effected by weight because V1 is. Vmca is based on your max weight already and Vmcg is usually faster then Vmca because you do not have the added aerodynamic benefit of bank into the live engine/s on the ground.

Hope that helps:ok:

[john_tullamarine]

Suggest a search to find other threads on this sort of subject .. plenty in Tech Log. The current FAA requirement is here (http://ecfr.gpoaccess.gov/cgi/t/text/text-idx?c=ecfr&sid=188a1746e0ac627c28ce003f4a71d2be&rgn=div8&view=text&node=14:1.0.1.3.11.2.156.24&idno=14)

(a) weight will have no major affect other than for (b)

(b) mass distribution certainly will

(c) there is no necessary relationship between Vmc and Vmcg. On some aircraft one is higher ... on other aircraft, the other is higher.

(d) Vmc will not address optimum bank angle, the requirement is an optional bank angle not exceeding 5 degrees (to keep the punters honest as realworld Vmc is VERY heavily bank dependant). In effect, this means 5 degrees in general.