Flaps and Vmc

[Booker]

I know there are multi-engine aircraft out there for which Vmc increases as flaps are extended. Does anyone have a POH for such an airplane, from which they could send me a copy of the page that shows Vmc for different flap settings?

Thanks in advance!

 

[avbug]

Booker,

My flight manuals are mostly all in storage right now, but any multi engine aircraft in which the application of flaps decreases rudder authority, will see a corresponding increase in single-engine minimum control speed.

In most cases, lowering of flaps increases Vmc. Aircraft with split flaps or high wings are most susceptable, generally speaking. I don't have any hard numbers for you, but one that leaps to mind is the Twin Commander. It's typical of many light twins, in which application of flaps increases Vmc.

Bear in mind that Vmc doesn't change, even though the actual airspeed at which minimum control may be maintained with one engine inoperative, does. This is because Vmc is a published number based on a specific set of certification criteria. Many factors may alter this speed, including the use of gear, flaps, cowl flaps, center of gravity, gross weight, bank angle, ambient barometric pressure, humidity, density altitude, etc. While the actual speed changes and this speed is often referred to as Vmc, Vmc proper is set. In very few conditions is additional information given to suggest what the change from this speed will be for actual field numbers.

Vmc doesn't change. Only minimum control speed. Vmc is a fixed number, even though we usually refer to the minimum loss of control speed as Vmc. (Proper use, vs. common use).

 

[Austpilot]

I agree, I seem to remember in any light twin the Vmca increasing if flaps are used, once again, my manuals are all in storage, but the split flap(as in assymetrical flap) scenario is certain to cause the loss of directional control at a higher airspeed than Vmca, I proved this in a Cessna 310!!!!

I'm not advocating going out and trying this, but I have also had a C310 below it's published Vmca and still had control - a function of having only instructor and student on board and about 90 mins worth of fuel, ie low weight.

Another point is, and this goes against what any pilot wants to do is if you find yourself in a Vmca situation and the aeroplane is still losing directional control after application of rudder- remove the source of yaw, ie, reduce power on the good engine. far better to hit the ground hard but flying straight than go in nose first in a spin.

 

[Timebuilder]

Right you are.

It's one thing to pull the power during a Vmc demo, but I can't imagine how difficult I would find pulling back the power on the one remaining engine in order to avoid a roll.

Perhaps that ability begins with a firm understanding of what will happen if you DON'T pull the power. How many accident stats for twins contain a reference to a Vmc roll? A lot. Train, train, train.

 

[CRJ200FO]

In almost all multi-engine airplanes, the extension of flaps will DECREASE Vmca. I can't really think of an airplane where the flap position will have much of an effect on rudder authority. It mainly affects the horizontal stabilizer.

The flaps and gear being extended cause a decrease in Vmca because of "the keel effect." A keel on a boat helps to stabilize it in the water. The flaps and gear have the same effect for an airplane in the air. Remember, air is a fluid just like water. Many of the same principles that affect a boat will also affect an airplane.

 

[Booker]

Thanks for the responses...But perhaps I should explain why I'm looking for a photocopy of a real AFM page. I'm going up for my MEI checkride shortly, and the DEs in these parts really like to talk about flaps and their affect on Vmc. And yes, I do mean Vmc, the published speed(s), not Vmca. I can point to airplanes that have a lower published Vmc for flaps extended than retracted. I've got a Beechjet 400A AFM on my shelf that shows just that. Now, I want a copy of the page listing Vmc's for an aircraft in which Vmc increases as flaps are extended. The reason for all this is so that I have substantive backup when the DE asks, "How do flaps affect Vmca?" and I answer, "It depends."

Apologies for the long post, but it seems the original was too brief.

Thanks again!

...don't even get me started on weight and Vmca...

 

[avbug]

In most cases, extention of flaps increases minimum control speed, with no increase in the published numbers.

I believe you'll find that your beechjet was certificated under Part 25, while light twins are certificated under either CAR 3 or Part 23. The certification requirements are different for each.

Decreasing weight will increase Vmca. Lowering flaps beyond the takeoff position in most cases will increase Vmca.

Vmc calculated on the least favorable conditions for a takeoff configuration, following gear retraction. It's calculated based on takeoff flaps. Changing this setting will change Vmca, but generally not Vmc.

Aircraft such as the 310 with split flaps, or the Twin Commander, will experience increased Vmca with flap extention. This is true for many light twins, including those that use flap systems other than split flaps.

 

[stardog]

Vmc vs Vmca

Folks here are using Vmc and Vmca as if they are synonomous. In my understanding, these are radically different terms.

Vmc = Lowest airspeed at which you can maintain directional control with the critical engine inoperative. Multiengine only.

Vmca = Minimum controllable airspeed. This applies to single engine airplanes also.

Am I the one that is confused? Hmm, anyone have a copy of Pt 1 handy?

 

[Booker]

Re: Vmc vs Vmca

Folks here are using Vmc and Vmca as if they are synonomous. In my understanding, these are radically different terms.

Vmc = Lowest airspeed at which you can maintain directional control with the critical engine inoperative. Multiengine only.

Vmca = Minimum controllable airspeed. This applies to single engine airplanes also.

Am I the one that is confused? Hmm, anyone have a copy of Pt 1 handy?

For multi-engine airplanes, Vmc is a published speed that is determined by the manufacturer under conditions set forth in 23.149. Vmca, on the other hand, is the actual airspeed below which directional control cannot be maintained during any given portion of an actual flight. Remember Vmca changes as any of the 13 conditions in 23.149 change.

 

[Booker]

In most cases, extention of flaps increases minimum control speed, with no increase in the published numbers.

Agreed. I'm just looking for a POH or AFM photocopy from an airplane that does have this behavior where more than one Vmc *is* published.

And yes, I realize the Beechjet is a Part 25 airplane, and I am familiar with the different cert reqs. Whether the airplane weighs more than 12,500 pounds isn't too important to me right now, however.

As for weight, I agree with your assertion, since it is the only one I know of that is described in FAA documents. I stay away from the other two theories as much as I can.

Again, if anyone can provide a POH/AFM page for an airplane with multiple Vmc's where flaps extened increase Vmc, please let me know. (I don't have access to a bank of AFMs for different types.) Thanks!