VMC factors

[saviboy]

Hi how does extension of flaps and landing gear affects VMC?
I have read various answers on the topic.
thanks

 

[uwochris]

It will depend on the type of a/c.

Generally, if there is a *symmetrical* increase in drag, then Vmc should NOT be affected. The only thing that is occuring is that you will reach Vmc sooner, because of the increase in drag.

In the case of flaps, I do believe that Vmc may slightly increase. You may think that if flaps are extended the same amount on each side, shouldn't drag increase symmetrically? The problem here is that you have induced airflow from the operating engine on one side, and less airflow on the other. This will increase the rolling tendency towards the dead engine, which means more rudder will be required (this was the explanation given to me).

In the case of the landing gear, it depends on where the gear is when it is extended and retracted relative to the CG. Remember that a forward CG REDUCES Vmc. If extending/retracting the gear causes the CG to change, then Vmc will also change. If the CG is unaffected, then Vmc will not change, as the drag increase/decrease will be symmetric.

I hope this helps,

Chris.

 

[Illini Pilot]

i was taught that the gear will also create a tiller effect, making the plane more stable about the vertical axis. this will lower vmc because the plane will resist the yawing from the assymetric thrust

 

[SMOE]

Adding to the previous posts, increased drag from lowering flaps and gear is the greatest factor. Do a drag demo and see...

 

[A Squared]

Adding to the previous posts, increased drag from lowering flaps and gear is the greatest factor. Do a drag demo and see...

THat doesn't affect VMC though. Drag affects performance, which may be a concern on one engine, but drag in itself does not affect Vmc.

 

[SMOE]

THat doesn't affect VMC though. Drag affects performance, which may be a concern on one engine, but drag in itself does not affect Vmc.

I stand corrected. But wouldn't you agree that decreased performance = decreased Vmc?

 

[lawfly]

VMC Factors

Not really agreeing or disagreeing with anyone: I would have to think more about it and maybe dust off a book ot two before I took a firm position. But here are some thoughts about what has been said so far:
VMC is a certification speed. It exists under certain, specificed conditions (hence the red line on ASI): gross weight, etc. But we all know what you mean: the speed at which you run out of rudder authority and begin to lose directional control and we'll call it VMC for convenience, even if we're no longer talking about the official, published, certificated VMC value.
Decreased thrust output on the operating engine (say, normally aspirated at altitude) would cause apparent decrease in "VMC" (as we are using that term), no? In fact, isn't there a danger under those conditions of stall and loss of directional control coinciding? The increased drag everyone is talking about with gear and/or flaps down would make it harder to maintain or achieve the necessary airspeeds, would affect climb or descent rates, etc. But would not "change" VMC or the speed at which rudder authority expired...

Just throwing that out there. But those fine points about induced airflow are interesting.

 

[A Squared]

I stand corrected. But wouldn't you agree that decreased performance = decreased Vmc?

No, not at all. You have 2 concerns in an engine out situation; performance and control. some factors will degrade one but enhance the other, some will enhance both.

For example, pulling power back on the good engine lowers vmc (enhances control) but degrades performance.

On the other hand, feathering the good engine lowers vmc (enhances control) and also enhances performance.

so, what is good for control is not necessarily good for performance, each factor has to be examined on an individual basis

 

[puddlejumper]

Don't try This at Home Kids!

On the other hand, feathering the good engine lowers vmc (enhances control) and also enhances performance.

Me thinks that feathering the good engine would be a bad idea. It would certainly enhance control (no more asymetric thrust) but performance would go out the window. I just thought I'd interject a little bit.

-PJ

 

[uwochris]

See this link: http://www.av8n.com/how/htm/multi.html

Chris.

 

[A Squared]

Me thinks that feathering the good engine would be a bad idea. It would certainly enhance control (no more asymetric thrust) but performance would go out the window. I just thought I'd interject a little bit.

-PJ

Doh!!!! that's what I get for posting at 0400 ..... ummmm. Hopefully everyone could telll I *meant* feather the dead engine ... no really I did!!!!