Vmc Question

[meritflyer]

Does Vmc indicated airspeed go down with altitude (or any other factor that lowers Vmc) or does it remain the same and CAS decreases??

[casey]

the airplane only cares about indicated airspeed. (though you might hear different from the guys complaining about stalling during downwind turns....)

CAS is just IAS corrected for instrument/installation error, so using IAS vs CAS really doesnt mean much of anything, as the difference usually is very minor.

[fish314]

The acronym VMC usually refers to visual meterological conditions, i.e., you are not in the clouds, and the term CAS that I'm familiar with is "Close Air Support". Which VMC and CAS were you refering to? Obviously something to do with airspeed, but I'm not familiar with those terms. What do they stand for?

oh. Duh, sorry, forgot about Calibrated air speed, but what's Vmc?

[Dugie8]

Originally Posted by meritflyer

Does Vmc indicated airspeed go down with altitude (or any other factor that lowers Vmc) or does it remain the same and CAS decreases??

In a normally aspirated airplane, yes. Because Vmc is determined with the critical engine producing takeoff power, in a PA44 that is 180HP. The engine can only produce that 180HP at sea level on a standard day. So any increase in density altitude, be it from climbing to a higher altitude or a rise in temperature, will decrease the amount of power the engine can produce. Now, the rudder will always produce the same amount of force based on IAS, no matter the density altitude. So at 56 knots (Vmc in the semi) at sea level the engine is producing 180 hp (say 100 pounds of thrust) The engine is about 5 feet off the centerline of the airplane, so that equals 500 foot pounds of torque. Now the tail is bout 10 feet behind the CG, so it has to produce 50 lbs of force to counteract the engines 500 lbs of force. As density altitude increases and you stay at 56 knots, the engines thrust goes down, to say 400 pounds at 5000 feet MSL, but the rudder can still produce it's 50 lbs of force because the IAS is the same.

Clear as mud?

[SIUav8er]

Originally Posted by fish314

The acronym VMC usually refers to visual meterological conditions, i.e., you are not in the clouds, and the term CAS that I'm familiar with is "Close Air Support". Which VMC and CAS were you refering to? Obviously something to do with airspeed, but I'm not familiar with those terms. What do they stand for?

oh. Duh, sorry, forgot about Calibrated air speed, but what's Vmc?

Vmc is minimum controlable airspeed with one engine inop. CAS is calibrated airspeed which is indicated airspeed adjusted for instalation error. Take a look at the Pilots Handbook of aeronautical knowledge and Airplane Flying handbook for more info.

[fish314]

Ahh, thanks. I figured it out actually from Dugie's post. I'm more used to seeing Vmca (airborne) and Vmcg (on the ground) since we had those in the KC-135, and I've never seen just Vmc... only VMC.

The CAS thing, weellll... let's just chalk that up to my brain not working really well today. That one I should have got just from the context of the question!