Vmc

[Ryancoke]

hey, im doing a paper on Vmc for school here and I'd like to get a little more information... as I'm not quite sure about how exactly I'm going to go about it... and i don't have any real decent facts. I was thinking theres gotta be some people on here that might be able to shed some light...

What does 4 or even 6 engines do to the Vmc? I know things like the gear, flap sets, CoG, turbos, and all that effect the Vmc of the basic piston twin, but what happens in the bigger stuff? What else effects the magic of Vmc?

Do float planes have a Vmc(W) while on the water like wheeled planes have? Does that change?

How about helicopters? Do They have a Vmc? Or is it a rotor thing? Gyrocopters?

Any help would be GREATLY appreciated!

[Mad (Flt) Scientist]

What does 4 or even 6 engines do to the Vmc? I know things like the gear, flap sets, CoG, turbos, and all that effect the Vmc of the basic piston twin, but what happens in the bigger stuff? What else effects the magic of Vmc?

All that more than 2 engines do are the following:

1. The thrust per engine is smaller as a percentage, but generally the outboard engines are further outboard than the engines on a twin, so whether a single engine is a worse or better case depends on the type specifics.

2. You have to account (in certification, anyway) for multiple failure cases i.e. losing both engines on one side. That's usually (due to the geometry issues) a worse case than the twin engine OEI case, all else being equal.

The most important contributor to Vmc that you've left out is WEIGHT. The lighter the aircraft, the worse the 'real' minimum control speed is. Other factors include cg position (longitudinal position AND lateral offset, the latter potentially quite important if you are roll control limited). Roll control also figures into the importance of e.g. sweep angle (in effect, dihedral effect).

Another factoir only imperfectly accounted for is angle of attack, and the effect it can have on directional aerodynamics; while light weight hurts in terms of the amount of sideslip you can generate through bank into the live engine, high weight for a given speed hurts (quite often) through increased alpha and decreased fin effectiveness.

Minimum control speed is a huge topic (see how m,any threads a search here will generate!) - you might get some useful info from Advisory Circular AC25-7A (http://www.airweb.faa.gov/Regulatory_and_Guidance_Library/rgAdvisoryCircular.nsf/0/4144773715D01E9A86256B9F0072F02A?OpenDocument) on flight testing for part 25 certification - check out the sections on Vmc - 25.149, on page 91 of the Ac - and also appendices 3 and 4.

Note there's also a "change 1" to that revision of the AC, but the basic one should be good enough for this purpose.