Vmc and Mountain Wave Question
 

Here are a couple of questions that came up at the FBO that we could not come up with a consensus on. Thanks in advance!

1. Of all the factors that affect Vmc which one has the greatest effect and why?

2. Where do you normally find the majority of ice on a mountain wave?

The good engine. If it's at full power, you have VMC. If it's at idle, you don't.


My guess would be on the upwards portion, but I'll let a real expert answer that one. I did stay in a holiday inn express last night...

The good engine is the obvious one. Since I doubt that was what they were referring to, since you can't fly with both engines idle, I would say bank. Bank can eliminate Vmc but at the cost of performance. It is an exchange of performance vs controllability with bank.



It has been forever since meteorology, but I think it is wherever there is a formation of lenticular clouds. Since wave lift is defined by these clouds and you need visible moisture below freezing it would seem a sensible answer. Still, just a guess unfortunately.

Wiki does a nice job explaining these clouds if you are unfamiliar: Lenticular cloud - Wikipedia, the free encyclopedia

1. This ones pretty subjective and you'll get plenty of different answers, but I say airspeed. Reason?- Because Vmc is directly related to rudder authority so more airspeed better control regardless of what the other engine is doing, and while I also agree with det pilot about the engine, I like to be different ;). You live a die in a light twin by airspeed. Literally.

2. Not a clue, I slept in my own bed last night.... maybe thats why I've been having problems. Damn holiday inn. :D

I'm going to go with the power produced by the operating engine. Losing 200 hp in a Seminole sucks and you need to be able to fly well, but losing a 500+CC turbo engine or a t-prop is a very different scenario. Same procedure, but there's a reason that red line is further up the airspeed indicator.

I'm not really sure about the Vmc 'most contributing factor' - but as far as "effect" - my guess is it would depend a little on the aircraft.

Probably the critical engine?


interesting story from AOPA: http://www.borzov.net/Pilot/FSWeb/Le...ngineSavvy.htm

Not many people have lived through a VMC rollover at or near the ground, but one examiner experienced a rollover after an overzealous student yanked back on the yoke at the pinnacle of a high-altitude VMC demonstration in a Beech Baron. He described it as a near-instantaneous snap roll to the inverted position. Reportedly, the roll was violent enough that the examiner smacked his head against the cabin door. Since the right engine was still producing full power, the airplane quickly entered an inverted flat spin at about 7,000 feet. After stopping the rotation by powering up the "dead" engine, the examiner managed to right the airplane at about 1,000 feet agl after performing a split-S maneuver. He and his pupil were lucky. Others haven't been so fortunate. Training accidents accounted for many of the VMC-related twin accidents surveyed by the study.

I have no idea in a real plane. However, I can attest to this being entirely accurate for my buddies RC twin. I flew it and had an engine fail and left it failed to do some goofing around. Doing a stall with it with power full and it snapped right onto its back. Was pretty sweet. Of course I also flat spun it but they have a little better power to weight ratio than our passenger aircraft. :D

The Baron is a real sweetheart to fly.. it's my favorite light twin...hands down. Beechcraft kind of 'over-built' the Baron ... it's a strong aircraft. Still, I'm a little surprised they didn't have an in-flight seperation of some kind. .... Snap-roll.... spilt-S

Doesn't surprise me with that one since Vmc is likely well below Va and the yaw force is applied by the engine for the snap, not the tail. If it was anything like the RC, it isn't a snap but it rolls with the speed of a snap.

I question the accuracy here. I suspect he doesn't understand what a split S is. Since flat spin recovery is getting to a normal spin then recovering from the normal spin your exit would be close to 90 degrees nose down. This is what I suspect happened. A split S would be him being inverted, level, and then pulling through.

If the "powering up the dead engine" is accurate a split S, only guessing, would be well over Vne. Doing a split S in our cap 10s we would pull about 4gs and use 16-1800 RPM. Entry speed was ~20 below Va and exit ~60 above Va if I remember right, it's been 5 years.

Finally, how often do we accurately account for things in a high stress situation? How accurate does the media then take our inaccurate accounts to sell it's articles?

Either way, still amazing he pulled through and kudos to him for those skills.

vmc determination exercise
 

How velocity of minimum controllability (vmc) is determined seems to come up every so many months in our Technical forums, so here's an example of a VMC determination I did to show one way of doing it. Not shown is the use of Matlab to crunch the numbers separately using a reiterative algorithm, and excel does not show formulas. Beta is sideslip.

http://i284.photobucket.com/albums/l...esize1-1-1.jpg

http://i284.photobucket.com/albums/l...resize-1-1.jpg