Vmc decreasing with increasing altitude.

[olliew]

Whilst I'm happy to accept that Vmc decreases with increasing altitude due to a reduction in turning moment as a result of reducing thrust from the live engine(s) could anyone explain to me the following? If thrust is reducing as a result of decreasing pressure or increasing density altitude then surely the restoring force generated by the rudder must be reducing as well thus requiring a higher velocity to maintain control. The obvious explanation is that it is the thrust that is being affected to a greater degree. But why?

[Mad (Flt) Scientist]

Simplistically, the engine thrust depends on the density, while the rudder power used to counter the thrust asymmetry depends on dynamic pressure.

So, As I go up in altitude, my density reduces and thrust decreases. At a fixed true airspeed, the rudder power decreases, but at a fixed indicated airspeed the rudder power is practically constant. Since VMC is quoted in IAS, you therefore get less thrust asymmetry but the same rudder power at the same IAS, which means you can then go to a lower speed at altitude and still maintain control.

Left out a lot of the finer details, but that's the one paragraph version.

[olliew]

You would loose me all too quickly with the 'finer detail' but the one paragraph version means I'll sleep easy. Thanks alot.

[KiloB]

I doubt if there is a general rule about this. It would be A/C dependent. For example a turbocharged piston can produce MORE power at altitude (boost is at set max and backpressure reduces) so Vmc will increase.

[Old Smokey]

It is absolutely true that for a great number of engines, Piston, Turbo-Prop, and Jet, that (primarily due to reduced back pressure) Thrust increases with increasing Altitude, UP TO A POINT. Thereafter, the thrust decreases with increasing Altitude as "conventionally" accepted.

Thus, up to the point of maximum available thrust, Vmc will increase with increasing altitude.

A second point, as alluded to by M_F_S, is that the rudder effectiveness depends upon dynamic pressure, thus for a given IAS or CAS the dynamic pressure would be the same. With Increasing Altitude, a given IAS or CAS is actually a REDUCING EAS, upon which dynamic pressure depends. From this, even if the thrust were to remain constant, the IAS or CAS at Vmc would increase with increasing altitude to maintain the same EAS.

M_F_S did after all say that that was the one paragraph version, this is Paragraph 2A and 2B. There's more.:E

Regards,

Old Smokey

[bflyer]

TO M_F_S and OLD SMOKEY

can we have the FULL course or menu...so that i can proceed and save it properly ...please
bf

[Wizofoz]

Perhaps an explanatory note:-

True air speed (TAS)= Indicated air speed (IAS) only at sea level in a standard atmosphere. As density altitude increases, a given IAS is equal to a higher TAS.

Thus, whilst rudder effectiveness decreases in TAS terms as altitude increases, it remains constant in IAS terms.

Thus, whilst an increase in altitude may have an equal effect on both power and rudder effectiveness (in TAS terms), the IAS at which sufficient rudder authority is available will decrease.