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Weight and Vmc?

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Well-known member
Nov 26, 2001
I working on my MEI and I'm putting together a brief on the "principles of engine inoperative flight" and I'm getting kind of confused about how weight effects Vmc. This is how I understand it and correct me if I'm wrong (or please offer a simple way to explain it to a student)
The heavier an Aircraft is the lower Vmc will be for several reasons:

More yaw force will be required to lose directional control, AC with greater weight will resist yaw do to the increased energy(inertia) required to swing the nose over and therefore lower Vmc.

For any given angle of bank a heavier AC will have a greater component of weight acting along the wing to counter yawing tendencies. (I'm a little confused here) In the FAA's 5 degrees of bank max. for certification a heavier AC will in effect lower Vmc due to this component of weight in a sideslip.

I know that Vmc deals ONLY with control and is independent of performance but am I missing anything as far as weight and Vmc is concerned?
A million thanks for any help!
Your explanation sounds correct to me and is the way it has always been explained. A heavy aircraft (loaded within limitations) is more stable - more resistant to a change in direction.

I hope this does not confuse the issue further, but think of it this way. If an airplane subject to VMC was heavy enough, it is likely it would stall before encountering VMC. The heavier the airplane the more the vertical component of lift is emphasized over the yawing motion.
#1) see when the airplane is in st and level flight weight does not affect Vmc

#2 )but banking into the operating engine creates a horizontal component of lift this component pulls the airplane in the direction of the operating engine ,
counteracting adverse yaw ,therefore requiring less rudder pressure , the heavier the airplane the stronger the horizontal component of lift and the lower Vmc becomes
( taken from faa-h-8083-3 )

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I haven't thought about Vmc in more than eight years, but it sounds to me that you're on the right track. Don't forget - you need Lift to overcome Weight.

Review your factors of Vmc. Isn't one of the factors of Vmc max takeoff weight? Know Vmc and your Seminole systems, and you'll be about set for your oral.

Good luck with your MEI.

Minimum control speed is a direct function of thrust and not of weight. Although what you said is correct about inertia (a heavy airplane is more stable and requires more force to make it yaw), that's only true for a static situation. Once the yaw becomes dynamic, it'll take a lot of force to stop the yaw and return it to the static condition. Weight affects Vmc only in the fact that it takes more thrust to maintain airspeed. Vmc is the calibrated airspeed required to maintain heading, with full rudder and five degrees of bank toward the operaive engines at a certain thrust setting. Remember these facts about Vmc:

Vmc is increased by - increasing thrust on asymmetric engine(s), banking toward inoperative engine(s), decreasing airspeed, using less than full rudder and aft centers of gravity(UNDESIREABLE ACTIONS);

Vmc is decreased by - decreasing thrust on asmmetric engine(s), banking toward operative engine(s), increasing airspeed, using full rudder and forward centers of gravity (DESIREABLE ACTIONS); realize that you may have to sacrifice altitude to decrease thrust, use more bank and increase airspeed.

Hope this helps. Take care and fly safe.
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<<Weight affects Vmc only in the fact that it takes more thrust to maintain airspeed.>>

Thrust/power in this situation is fixed at max t/o. Pitch is used to control airspeed. The ONLY reason increased weight decreases Vmc is because of the horizontal component of lift created by the 5 degree bank into the operative engine. The increased inertia only slows the onset of the yaw, and it dosen't slow it down much.
With regards to inertia and Vmc weight does decrease Vmc, but once directional control is lost a heavy airplane requires more airspeed to regain control due to the increased inertia.
I've always thought the only entering arguements into a Vmc air chart were density altitude, bank angle and thrust. Weight only determines how much thrust is required for level flight. What am I missing?

The reason the airplane is banked into the good engine is to counter the sideforce from the rudder. Look at the airplane from the top and there is a large sideforce from the vertical tail that gets countered by the horizontal component of lift from the bank angle.

When you do training tape a piece of yarn just in front of the windshield like they do in gliders. (Don't leave the tape on for more than a day or two or it can mar the paint.)

Before you start instructing in twins I highly, highly recommend you spend a couple HOURS reading the accident reports for the duchess, seminole, baron, etc. You'll notice a couple of things.

1- Don't do single engine go-arounds.
2- End all single engine approaches with a landing, not a missed.
3- Don't shut down an engine unless your easily within a single engine drift down to a suitable airport.
4 - Don't let anyone touch anything (like the flaps) until your off the runway and stopped. (especially in a Baron.)

I appoligize if I'm preaching, but the same accidents happen over and over again.

Probably the most interesting thing I learned by reading a lot of twin accident reports is the lack of accidents caused by the loss of directional control on one engine (Vmc). It essentially never happens outside of the training eviornment.


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