On the subject of Vmc...

[MauleSkinner]

As long as we're having so much fun with Vmc issues on this board, I'll throw in another question that came up with one of our corporate clients who recently passed his multi-engine instructor checkride.

Prior to the checkride, we were flying a trip, and the discussion in cruise revolved around a review of the factors that affect Vmc. Apparently he decided that he was a little weak in that area, so he spent some pretty serious time studying the topic before his checkride (3 days of nothing but Vmc, I think he said).

On the trip following his checkride, we talked about how the checkride went...apparently the Vmc issue consumed over 1 1/2 hours of the oral. One of the questions that the examiner brought up, which evidently was a bit of a hot button for him personally, was something to the effect of "Would you rather have Vmc above or below stall speed?"

The examiner's position was that he'd rather have Vmc above stall. His feeling was that if you start to lose directional control, you can make the correction and go on your merry way; whereas if Vmc was below the stall, you would fall off into a spin as soon as you stalled.

Seems to me, however, that if Vmc was more than a couple of knots below stall, you'd still have directional control all the way through the stall and recovery. This situation would be more desirable to me...I'd rather crash straight ahead than lose control and roll over.

Having said that, most of the multi-engine airplanes that I've flown have Vmc above the stall speed by a few knots. The only exception I can think of is the Twin Commander. I was told (didn't study up too hard for myself...I was only a "warm body in the seat", and it was back when I trusted other pilots ) that Vmc on the Twin Commander was below stall, and that in training they stalled the airplane with full power on one engine and the other windmilling.

I'm sure that there are some tradeoffs in performance vs low Vmc that are driving factors in aircraft design, but I don't know any specifics.

What are your thoughts and opinions on the subject?

Fly safe!

David

 

[Donsa320]

As long as we're having so much fun with Vmc issues on this board, I'll throw in another question that came up with one of our corporate clients who recently passed his multi-engine instructor checkride.

Prior to the checkride, we were flying a trip, and the discussion in cruise revolved around a review of the factors that affect Vmc. Apparently he decided that he was a little weak in that area, so he spent some pretty serious time studying the topic before his checkride (3 days of nothing but Vmc, I think he said).

On the trip following his checkride, we talked about how the checkride went...apparently the Vmc issue consumed over 1 1/2 hours of the oral. One of the questions that the examiner brought up, which evidently was a bit of a hot button for him personally, was something to the effect of "Would you rather have Vmc above or below stall speed?"

The examiner's position was that he'd rather have Vmc above stall. His feeling was that if you start to lose directional control, you can make the correction and go on your merry way; whereas if Vmc was below the stall, you would fall off into a spin as soon as you stalled.

Seems to me, however, that if Vmc was more than a couple of knots below stall, you'd still have directional control all the way through the stall and recovery. This situation would be more desirable to me...I'd rather crash straight ahead than lose control and roll over.

Having said that, most of the multi-engine airplanes that I've flown have Vmc above the stall speed by a few knots. The only exception I can think of is the Twin Commander. I was told (didn't study up too hard for myself...I was only a "warm body in the seat", and it was back when I trusted other pilots ) that Vmc on the Twin Commander was below stall, and that in training they stalled the airplane with full power on one engine and the other windmilling.

I'm sure that there are some tradeoffs in performance vs low Vmc that are driving factors in aircraft design, but I don't know any specifics.

What are your thoughts and opinions on the subject?

Fly safe!

David

Wow, that is the wildest desertation I have ever heard on Vmc and stall speed.

Do you really have an ATP and 10,000 hours...in airplanes? In the USA?

Sorry man, you need remedial training, I'm serious.

Suffice it to say Vmc cannot be below stall speed because at the stall you will lose directional control if you had not already.

No wonder people are wrapping up twins so frequently with nonsense as nutsey as I read here being spead around.

DC

 

[MauleSkinner]

Sorry man, you need remedial training, I'm serious.

Suffice it to say Vmc cannot be below stall speed because at the stall you will lose directional control if you had not already.

OK...give me some remedial training...WHY would you lose directional control when you stall, if you still haven't reached Vmc?

 

[DC8 Flyer]

Maul
What I think Don is trying to say is, if you are maintaining directional control and then stall, the wing with power on is going to roll "up", both wings will not stall at the same time in other words. When that happens you have lost directional control since any attempt to raise the dead engine wing only increases that wings AoA (down aileron) and furthers the stall on that wing. I've done a few single engine stalls in a seminole and they are not fun, nice little snap and off you go, well above red line too.

All that being said, the question posed by the DE is more of a understanding of what is happening thing than a real design question. The scenario is something like this:

Stall before Vmc

You take off, loose and engine right at rotation and loose track of pitch while fighting to maintain control, you are barely climbing if at all, you have directional control but then the stall horn goes off, you lower the nose, and supposedly put the airplane back on the runway (remember, talking a hypothetical here).

Vmc before stall

Same thing as above, you are loosing airspeed and go below Vmc, now you are yawing off the runway heading and no longer over pavement and crash, blah blah blah.

The idea isn't to take the airplane into a full stall, but more of getting a stall indication (horn, buffet, mushy controls, etc) before loosing directional control. The other way to think about this, is treat the stall indications as a slap in the face when you are single engine, a wake up call to get the airspeed back or you are going for a ride.

 

[MauleSkinner]

Maul
What I think Don is trying to say is, if you are maintaining directional control and then stall, the wing with power on is going to roll "up", both wings will not stall at the same time in other words. When that happens you have lost directional control since any attempt to raise the dead engine wing only increases that wings AoA (down aileron) and furthers the stall on that wing.

True...but you're generally taught to pick up the wing in a stall with rudder, not aileron...and if the actual loss of directional control speed is far enough below stall, you've got some rudder available for this task.

All that being said, the question posed by the DE is more of a understanding of what is happening thing than a real design question. The scenario is something like this:

Stall before Vmc

You take off, loose and engine right at rotation and loose track of pitch while fighting to maintain control, you are barely climbing if at all, you have directional control but then the stall horn goes off, you lower the nose, and supposedly put the airplane back on the runway (remember, talking a hypothetical here).

Vmc before stall

Same thing as above, you are loosing airspeed and go below Vmc, now you are yawing off the runway heading and no longer over pavement and crash, blah blah blah.

Not having been present during the oral, I can only go by what the applicant told me. The examiner seemed to believe that you could see the loss of directional control coming and make the appropriate correction, but that wouldn't be a possibility with the stall.

The idea isn't to take the airplane into a full stall, but more of getting a stall indication (horn, buffet, mushy controls, etc) before loosing directional control. The other way to think about this, is treat the stall indications as a slap in the face when you are single engine, a wake up call to get the airspeed back or you are going for a ride.

I'd definitely agree with that, but again, it wasn't presented to me that way...the applicant was told that, if you stall before reaching Vmc, you WILL instantly roll into a spin, and being in a multiengine airplane, it's quite likely not recoverable. Recovery by breaking the stall and/or using rudder wouldn't be an option. I'd venture that your experience with asymetric stalls in the Seminole, while abrupt, probably didn't fit that description.

Granted, this person thought he was pretty much ready for the oral until I quizzed him a little bit (I didn't ask him how his instructor had dealt with these issues). Maybe asking a question based on heresay isn't a good idea. On the other hand, if somebody gains some insight into the topic, it would probably be worthwhile.

Fly safe!

David

 

[DC8 Flyer]

I completely agree with what you say Dave. On picking up the with rudder, keep in mind generally if you are close to stall you are fairly close to Vmc as well, meaning you don't have much rudder left, couple that with the loss of the horizontal component of lift (zero sideslip) and now you have a horizontal component of lift working with the good engine, against the rudder, now you don't have enough rudder to lift the wing, hence the stall and bang into a spin.


Again, all hypothetical and more of a teaching and understanding point, than a real world application.

Nothing wrong with asking questions either. We are all still learning and looking for new ways to present theories and ideas.

 

[MauleSkinner]

I completely agree with what you say Dave. On picking up the with rudder, keep in mind generally if you are close to stall you are fairly close to Vmc as well, meaning you don't have much rudder left, couple that with the loss of the horizontal component of lift (zero sideslip) and now you have a horizontal component of lift working with the good engine, against the rudder, now you don't have enough rudder to lift the wing, hence the stall and bang into a spin.

Hmmm...the reduction in horizontal lift component due to wing stall hasn't been brought up in these discussions before (as far as I can remember)...it's always been the vertical component differential that's been discussed at this point. Interesting.

Again, all hypothetical and more of a teaching and understanding point, than a real world application.

Nothing wrong with asking questions either. We are all still learning and looking for new ways to present theories and ideas.

Yup...and that's one of the reasons I participate in these boards...I learned as a young instructor that it's very easy to simply explain things the way we understand things, to the point of always wording questions on a topic the same way. I also learned as a young instructor that it won't always work to do that.

Fly safe!

David

 

[charter dog]

Interesting points. It should be noted that VMC decreases with decreasing power available as a normally aspirated twin or turboprop climbs to higher altitude. So any margin between stall and VMC may disappear as the aircraft climbs. The wing with the failed engine will stall first, and the issue of whether it was stall or VMC will become academic. The only hope for recovery in either case is to reduce power on the operating engine and use rudder and forward yoke to stop it from entering into a fully developed spin. If this occurred very near the ground, it was a bad day.

Best,

 

[Donsa320]

Finally some intelligent discussion on Vmca in light twins.

On another thread there were questions as to why the wing down into the dead engine was not mentioned in large airplanes. The reason is that we operate at speeds far above Vmca and the leeway generated by the asymetrical thrust is minimal and is mostly controlled by the keel effect of the fuselage and vertical fin, therefore little rudder is needed except down around V2. We don't linger at V2 very long and accelerate to V3 or BSEC speed rather soon after take-off and the rudder required decreases rapidly since we end up about 100kts or more above Vmca.

Compare that to light twins where Vyse usually is only a few knots above Vmca and leeway is a concern. Eliminating leeway by using some slip usually will improve single engine climb.

The ball instrument becomes very important in twins with an engine out. The A-320 has no Ball instrument as such but rather a "Beta Target" (which is the lower half of the Bank pointer on the PFD sliding back and forth) and emphasis is placed on it by it's changing color with an engine out.
Since the Beta target is a generated by computer my question at the time was; does it compensate for leeway and the answer was apparently "no". So we climbed on out with wings essentially level, ball or beta target centered like other heavy twins. Leeway was not a concern.


DC

 

[Snoopy58]

> Suffice it to say Vmc cannot be below stall speed because at the stall you will lose directional control if you had not already.

Not really.

Stall speed varies with weight, Vmc varies with power. With a light airplane (low stall speed), you could establish what Vmc would be at various conditions of temperature, altitude, bleed air configurations, etc. Now, with all engines operating (Vmc no factor), you can also establish what stall speed is for various weights (obviously, much more of a range for large transports than for light training recip twins). The same airplane could have stall speed well above Vmc when heavily loaded on a hot day at high altitude, but could have Vmc higher than stall speed on a cold day at a low altitude field with no load and minimum fuel.

I agree with the DE that encountering Vmc first gives you better recovery options: lower the nose, roll into the good engine, reduce assymmetric power, and do whatever you have to to get your airspeed back up. Getting below Vmc isn't a total departure from controlled flight, as you can still regain control without risking overstressing the airframe. On the other hand, if you stall with an engine out, you're probably going to depart controlled flight entirely, meaning that you're pretty much toast, regardless of how far you were above Vmc.

Having a stall <b>warning<b> that occurs first, would also be a nice wake up call, in that you'd presumably get your speed back where it belongs before actually encountering EITHER your actual stall speed OR your Vmc. But I'm not sure how much actual lead-time you'd have between the stall warning & the actual stall & departure from controlled flight. Would vary by aircraft, I'd think.

 

[DC8 Flyer]

Snoopy
That is a very good way of looking at it. I think it really comes down to personal preferance, something a DE should not be interjecting into a checkride, but that is a different argument. I would prefer a stall before Vmc (stall meaning a stall warning) simply because it is a definate thing. Loss of directional control can be varied by pilot technique as well as other "physical" factors. Of course you can effect stall speed by technique (acclerated stall) but those are much less likely at lower speeds, IMO.

I would think in a practical sense, that the scenario laid out before us would be somewhere near the ground, and rolling off to one side I would argue, is worse than stalling. Falling onto the landing gear VS falling onto the wing tip. But again simply academic in conversation.