VMC or Stall in a light twin.

[aucfi]

Normally the seminole will stall generally before it his VMC by design, as altitude increases on a naturally aspirated engine stall speed remains the same but VMC will increase.

Shouldnt Vmc decrease as the DA increases? Less power on the operating engine reduces Yaw/Roll tendency and subsequently lowers Vmc.

 

[check6]

aucfi,

I concur.

Funny, cause during my MEI training they didn't let me actually stall it and I couldn't remember back to my initial Multi-engine rating whether or not we did.

As I'm reading this thread, I've forgotten what the original post was, but is there a good reason not to stall it? I've been told that flat spins are more common because of where the majority of the weight is(on the wings).

 

[Tinstaafl]

It's generally a 'bad thing' to stall in an asymmetric condition. A trap occurs with many normally aspirated aircraft as altitude increases. Commonly Vmc is some (small) amount greater than Vs - the aircraft would reach Vmc before stalling as speed is reduced.

As Density Altitude increases Vmc decreases (normally aspirated engine). At some point Vmc will have reduced to less than Vs. As speed is reduced the aircraft will reach Vs prior to reaching the (now lower) Vmc resulting in stalled, asymmetric condition.

In some aircraft the crossover point is only several thousand feet.

 

[Taildraggaah]

Well, since at VMC you are still in control,..."



"Ive never stalled a multi though, so really i have no idea....

Anyone here stalled a multi in a sim or in rl?

You can quickly lose control if you are properly at Vmc. I have myself and have had students, that will decrease airspeed a bit more, depending on the A/C, and you start to lose directional control. I have also had a Vmc rollover. It was scary and I quickly retarded the "operative" engine, but lost about 3500 feet before recovery. We had started at about 7000 AGL. I usually take students high for their first Vmc experience and this is why. I know people are going to say "why did I let it go so far?", well that is easier said than done when your student unexpectedly reverses their control inputs or relaxes rudder without pitch.

skipro101 you didn't perform stalls during your multi training at all? Power on or off? Because your instructor and the DE not only diregarded the PTS, but also missed a valuable learning experience...

 

[machaf]

Shouldnt Vmc decrease as the DA increases? Less power on the operating engine reduces Yaw/Roll tendency and subsequently lowers Vmc.

Yes Vmc decreases as you climb. Less thrust, less yaw, less rudder = lower Vmc speed.

In a Seminole you will almost ALWAYS STALL before you lose directional control. Why? Because Vs is 57 and VMC is 56. The only way you will lose directional control before you stall is at high negative density altitude.

Remember VMC was determined in basically the worst possible airplane configuration.

 

[Stifler's Mom]

Maule and VSU,

It's been years since I instructed and covered this stuff, but I remember an examiner asking me to teach my students to push the nose over before necessarily removing power from the operating engine. This was by blocking the rudders and responding at the first sign of loss of control, I guess not a complete loss of control which I think is where this conversation is lying.

 

[VSUPilot]

I guess this is a bit of a chicken or the egg discussion, however, while pushing the nose over will build airspeed giving the rudder more effectiveness, the cause of Vmc is assymetric thrust. To reduce this imbalance, power has to be reduced.

Getting back to the original question of stall vs. vmc- Stifler is absolutely correct about reducing the angle of attack. The only thing in a light twin that could be possibly worse than a Vmc rollover is a stall which results in a flat spin. At that point you may as well sit on the yolk to give the NTSB something to scratch their heads about.

 

[MauleSkinner]

I guess this is a bit of a chicken or the egg discussion, ... At that point you may as well sit on the yolk to give the NTSB something to scratch their heads about.

Is that caused by a forward slip, a sideslip, or a Freudian slip?

 

[IP076]

Shouldnt Vmc decrease as the DA increases? Less power on the operating engine reduces Yaw/Roll tendency and subsequently lowers Vmc.

Yeah, I think the Nome mis-spoke, cause I know he knows better than that....

The other thing to remember when you're comparing Stall Speed to Vmc is what actually is the stall speed. Someone mentioned published stall speeds. In all the light twins I've flown, the only published stall speeds were "Power Off" clean or dirty configurations. There are no published speeds for "Power On" stall speed. And the stall speed we're concerned about is a "Single Engine Power On" stall speed that I've never seen published anywhere.

Also, I'm not so sure what the big deal about stalling a multi is? I've done stalls in just about any of the twin's I've flown: BE-76, BE-58, C-310, C-340, PA-31-350, PA-44. As a matter of fact, just about every flight I do, I do a single engine stall....at our altitude thats what happens in a Vmc Demo if you dont block the controls.

I don't really believe the 5 deg limit in bank was meant as an everyday limit, or a limit in training. If you're flying an airplane, and its going to take 8 deg of bank to maintain directional control, you're going to do it. The best bank angle is between 2-8 degrees depending on the airplane. The 5 degree limit is in place for "Certification" so that manufacturers can't use 30 degrees of bank and achieve an ultra low Vmc speed while losing a boat load of altitude. Its just like the 150 lbs of force. Do you tell your students to push the rudder until they have reached 150 lbs of force and then stop?

 

[VSUPilot]

Also, I'm not so sure what the big deal about stalling a multi is? I've done stalls in just about any of the twin's I've flown: BE-76, BE-58, C-310, C-340, PA-31-350, PA-44. As a matter of fact, just about every flight I do, I do a single engine stall....at our altitude thats what happens in a Vmc Demo if you dont block the controls.

Stalling a twin is not a big deal, (with both engines that is). But if you are saying that you take students up and execute a full break stall with one engine inoperative, you are not leaving yourself with any margin for safety. That is why a recovery is made from a Vmc Demo at th FIRST sign of the loss of directional control, stall warning horn, or buffet. Do what you want but you are the only person who I a have ever heard of teach single engine stalls. All the examiners, or any pilot for that matter, that I have ever talked to, don't want to be close to a stall during a Vmc demo because of the possiblity of a flat spin.

 

[AAflyer]

Yes Vmc decreases as you climb. Less thrust, less yaw, less rudder = lower Vmc speed.

In a Seminole you will almost ALWAYS STALL before you lose directional control. Why? Because Vs is 57 and VMC is 56. The only way you will lose directional control before you stall is at high negative density altitude.

Remember VMC was determined in basically the worst possible airplane configuration.

Good point, many forget that as DA increases, VMC decreases, however I would not go so far to stand firm on the 56 knots. Remember that airspeed is subject to a host of factors;

AFT CG
MAX POWER
TO COWL FLAPS
TO FLAPS
GEAR RETRACTED
STANDARD ATMOSPHERIC DAY
OUT OF GROUND EFFECT
PROP WINDMILLING
BANK UPTO 3-5 DEGREES
1/2 BALL DEFLECTION

these items in different configurations can lead to more or less rudder effectiveness, basically changing the airspeed at which we can experience VMC.

AA

 

[AAflyer]

Thanks to all who answered. I appreciate your input, and like to see how others view or were taught certain subjects. I think through this we can become better instructors, and many times pick up a better understanding of the subject material.


Cheers,
AA

 

[RefugePilot]

VMC and stalls no problem, unless

I have done power on and off Stalls in an Apaches and Senecas with no problem. We did not use full power in the power on stalls. In a Duchess we recover at the first sign of stall, either buffet or horn.

You do want to be carefull in practicing stalls at the critical density altitude. As a twin gets higher the Vmc speed will get lower while the stall speed will stay the same. At the critical density altitude Vmc=Vs. A Vmc demo at this altitude will bring the worst of both worlds together possibly leading to a non-recoverable spin.

 

[IP076]

Stalling a twin is not a big deal, (with both engines that is). But if you are saying that you take students up and execute a full break stall with one engine inoperative, you are not leaving yourself with any margin for safety. That is why a recovery is made from a Vmc Demo at th FIRST sign of the loss of directional control, stall warning horn, or buffet. Do what you want but you are the only person who I a have ever heard of teach single engine stalls. All the examiners, or any pilot for that matter, that I have ever talked to, don't want to be close to a stall during a Vmc demo because of the possiblity of a flat spin.

You're absolutely right! It was 0530 local when I typed that....musn't have woken up yet! You're right, VMC demos are done to the first indication of a stall or loss of directional control.