Vmca
 

On twin-engine turboprops and jets, does Vmca (in terms of controllability, not performance) vary with weight? If so, why?

Weight is a factor in determining the "minimum control speed" for the specific conditions in question, because the higher the weight, the larger the sideforce which results from the 5 deg bank into the live engine, and thus the larger the sideslip which can be generated. This sideslip aids directional control, so the heavier aircraft can achieve a lower speed, all other factors being equal.

Traditionally, the declared Vmca has been a single value, independent of weight, and thus the value obtained at minimum flight weight is declared. Thus, for those aircraft the declared Vmca does not vary with weight, even though the underlying behaviour of the aircraft does vary. Recently, some aircraft have declared a range of Vmca as a function of weight to reflect the underlying controllability limits; the Embraer 190 does this, I believe.

Yes! all the e-jets have this. Not sure how much they benefit as the numbers we run come nowhere near VMC which is typically around 98kts.

In some aircraft Vmca might be defined by the stall speed; thus it is weight dependent.
IIRC there was a training twin which exhibited Lat/Dir Vmca at higher speeds and a stall limit as weight reduced (or was it the other way round?).

Perhaps you are suggesting that, typically ..

(a) at high weights the speed schedules will be stall limited

(b) as weight reduces the schedules will reduce, following the stall variation

(c) at a comparatively low weight Vmca/Vmcg will become limiting and there is no further schedule reduction with weight

(d) in most cases, with single declared certification Vmc values, this will be a simple cutoff. If a range of Vmc is declared, then the numbers will become a little more complicated.

No exactly that John. I had in mind a definition (interpretation) of Vmca which includes either stall limited or lat / dir control limited, i.e. stall is classed as a limit of controllability (stall in the asymmetric configuration, i.e. not a straight stall).
Thus, where stall occurs before the lat / dir controls are limiting, defines the pitch control limit, opposed to the more normal lat / dir control limit being reached before the stall speed.

I think that, in an around about way, we are talking the same talk ..

(a) however, asymmetric stall (with a bunch of thrust) = recipe for disaster ... so we don't go there.

(b) if Vmca directional problems become limiting at speeds above stall, then that is the Vmca declared

(c) if the flight test program has established that stall speed is higher than the conventional Vmca limit, then Vmca becomes irrelevant

It might be of interest to some that there is definitely one older jet transport than does not have a published Vmca...Caravelle.

I - SUD AVIATION Caravelle SE 210 Model I (Transport Category), approved April 8, 1959
Engines 2 Rolls-Royce Avon 522 turbojets

Airspeed limits (CAS) Knots
Vne (Never exceed) 325 or Mach 0.81 whichever is the lesser
Vno (Normal operation) 300 or Mach 0.77 whichever is the lesser
Va (Maneuvering) 174
Vfe (Flaps down 0° to 10°) 258
Vfe (Flaps down 10° to 20°) 202
Vfe (Flaps down 20° to 35°) 190
Vlo (Landing gear operation) 180
Vle (Landing gear extended) 218
Vllo (Landing light extension 215
Air brake operation 306 or Mach 0.81
Air brake extended 306 or Mach 0.81
Vmc (Minimum control) Less than the stalling speed

Vmca
 

Yes, I thought that might be the case and I'm glad you've confirmed it.
Thanks

FAR 23 (light twins) and 25 (medium) actually defines Vmca is at weights that most adversely affect directional control i.e. highest speed, not necessarily at max weight. However, all of the aeroplanes I'm familiar with state that Vmca is at max certified take-off weight.

It's interesting to note that, as one reader pointed out, at least one aeroplane (Caravelle) does not have a Vmca as such.