Vmc Characteristics

[MSU Pilot]

Being a fairly new MEI, I have a question about one of the factors of Vmc. I have always heard that drag=stability when teaching Vmc. So when we are speaking about whether flaps positively or adversely affect our Vmc characteristics, I have heard two different scenarios. One states that because of the extra drag our flaps would create if extended, the airplane would be more stable. I have also heard that because of the extra lift, it may increase our tendency to roll. What do you think? I realize that not all training aircraft require T/O flaps, but just philosophically speaking.

 

[shdw]

Flaps

You should run a forum search here for this topic, we have discussed it in great detail a few times. The general consensus always seems to be that flaps may add some stability, but there is no solid data to prove it true or false.

 

[ryan1234]

Being a fairly new MEI, I have a question about one of the factors of Vmc. I have always heard that drag=stability when teaching Vmc. So when we are speaking about whether flaps positively or adversely affect our Vmc characteristics, I have heard two different scenarios. One states that because of the extra drag our flaps would create if extended, the airplane would be more stable. I have also heard that because of the extra lift, it may increase our tendency to roll. What do you think? I realize that not all training aircraft require T/O flaps, but just philosophically speaking.

I think it's one of those things that are more airframe dependent than anything else.

On a pragmatic note... a lot of twins, particularly STOL kit twins, will get off the ground well before Vmc on the take off roll with T/O flaps.

 

[tgrayson]

but just philosophically speaking.

Philosphically speaking, the only things that will affect Vmc are things that produce rotation around the vertical axis, so you can't speak of drag or lift alone as being contributing factors. If the extension of flaps reduces Vmc, and they do for some airplanes, then you have to pinpoint a reason why they would affect the rotation around the vertical axis. In other words, there must be differential drag between one wing and the other. If the drag produced were equal, there would be no Vmc effect and the airplane would merely have a harder time maintaining altitude.

There are a number of reasons that the flaps might produce a rotational moment around the vertical axis in an engine out scenario:

• The flap behind the working engine is in the propwash, so it has more drag than the flap behind the failed engine. This is effectively a thrust reduction, so it would reduce Vmc.
  
  
• At a given airspeed, the flapped wing has a lower AoA, so P-factor would be reduced, so the working engine would produce less of a yawing effect, reducing Vmc.
  
  
• When engaged in a sideslip, the fuselage would shield the downwind flap area, reducing drag with respect to the upwind flap, which would tend to reduce Vmc.

Even the data that I've seen that show a Vmc reduction or at least an increase in directional stability don't explain mechanism by which it occurs.

 

[ryan1234]

the airplane would merely have a harder time maintaining altitude.

In a cursory glance of the pragmatics - regardless of how it affects Vmc, I would think the real important thing would be to reduce the flaps because of it lowering overall performance. In light twins, say, creating a deceleration rate (at or around Vmc) that could make the lower AoA advantage irrelevant.


There is also the senario of flaps impeding the authority of the rudder. Maybe something like a Shrike?

It is also possible that an increase in prop wash on the good engine would effectively turn the flap into another aileron causing a roll toward the dead engine.

 

[CoffeeIcePapers]

In a cursory glance of the pragmatics - regardless of how it affects Vmc, I would think the real important thing would be to reduce the flaps because of it lowering overall performance.

You could also kill the operating engine to make Vmc 0, but obviously we want to keep the airplane flying.

The reason why this is important in multiengine operations is to determine how the published Vmc came from, and how certain conditions raise or lower that published number. The flight training portion should cover the single engine configuration to get you the best amount of performance while still keeping the aircraft under control.

 

[shdw]

The flight training portion should cover the single engine configuration to get you the best amount of performance while still keeping the aircraft under control.

I would add: To understand that control and performance are inversely related. That is, whenever you do something to improve control, chances are you are reducing performance. Bank angle into the good engine is a great example of this relationship.

In my opinion, flaps should be used for only one reason in OEI; when you're ready to land. Gear is the same way. When I consider my options for OIE I think, how can I get the most performance and still maintain control. I did most of my training in a Seminole, when on a 90 degree day we have under 100 FPM OEI. If I even left in 10 degrees of flaps I wouldn't be able to gain a positive rate of climb.

My steps will always be: 1) Maximize performance | 2) Gain altitude | 3) Time permitting, problem solve and/or land

You certainly will never find me at 50 feet, after OEI on takeoff, questioning if I should use some flaps.

 

[tgrayson]

regardless of how it affects Vmc, I would think the real important thing would be to reduce the flaps because of it lowering overall performance.

Absolutely. No one should let this academic discussion of Vmc think that minimizing Vmc in a particular airplane is an important objective.

There is also the senario of flaps impeding the authority of the rudder. Maybe something like a Shrike?

That's plausible; however there are two mechanisms via which flaps might reduce rudder authority. One is simply that flaps often increase directional stability, which means that the rudder wouldn't be able to maintain the same sideslip angle due to the increase desire of the airplane to turn into the relative wind. The other mechanism is that there may be a reduction in relative wind velocity in the wake of the flaps which could envelope the rudder in a high wing airplane. Without hard data about a particular airplane, I'm skeptical of assertions.

It is also possible that an increase in prop wash on the good engine would effectively turn the flap into another aileron causing a roll toward the dead engine.

True, I started to mention that, but 1) most light twins are rudder limited, rather than aileron limited, and 2) at a given airspeed, both the flapped and unflapped wings are generating the same lift coefficient, so I'm skeptical that you're any worse off roll-wise with the flaps down.

 

[ryan1234]

True, I started to mention that, but 1) most light twins are rudder limited, rather than aileron limited, and 2) at a given airspeed, both the flapped and unflapped wings are generating the same lift coefficient, so I'm skeptical that you're any worse off roll-wise with the flaps down.

I see what you're saying, I hadn't thought of it that way... good point.

Like you, I'd be interested to see the actual test aircraft data on the other points.