Vmc clarification
12-04-2010, 08:15 AM
#11
Gets Weekends Off
Joined APC: Jan 2010
Posts: 327
True.. adding flaps produces more drag. But the increase in lift is important here and can't simply be discounted. The additional lift that is created with flaps extended can be directed opposite the yaw induced by the engine failure. This is accomplished by banking into the operative engine to change the lift vector. This leaves more rudder available.
It may be that the induced drag caused by creating more lift would be too much for the aircraft to overcome. Its important here to empasize the difference between aircraft performance and aircraft control. The airplane may not be able to climb... so it slowly crashes in to the ground.. straight ahead and under directional control.
It may be that the induced drag caused by creating more lift would be too much for the aircraft to overcome. Its important here to empasize the difference between aircraft performance and aircraft control. The airplane may not be able to climb... so it slowly crashes in to the ground.. straight ahead and under directional control.
12-04-2010, 09:18 AM
#12
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Joined APC: May 2006
Position: ATP, CFI etc.
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I made this diagram to help explain the keel effect. It is a simple force balance equation. When the airplane sideslips due to asymmetrical thrust from one engine being out, the gear and/or flaps on the side it's slipping toward goes farther from the CG than the other side as far as the wind and CG are concerned. This creates unequal moment arms (A vs B). The moment rotates the airplane back to a point where they balance, which is in alignment with the slipstream. It's the same thing that makes an arrow shoot straight, and the item acting as a keel can be just one surface like many ships have underneath. It is interesting to note that some sailing boats actually use two keels on either side much like an airplane with its two wheels and two flaps.
Last edited by Cubdriver; 12-04-2010 at 09:28 AM.
12-04-2010, 12:31 PM
#13
Gets Weekends Off
Joined APC: Jan 2010
Posts: 327
Extending landing gear causes a keel effect. I'm fine with that. But I don't see it with the flaps. If the flaps do produce any sort of keel effect, it is most certainly minimal compared to the effect of redirecting increased lift.
06-15-2016, 12:26 PM
#16
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Joined APC: Jan 2006
Position: Engines Turn Or People Swim
Posts: 39,272
Originally Posted by Free123
Flaps extended: angle of attack is reduced, which lowers lift and hence p-factor. Resulting in lower vmc
The so called keel effect from flaps would be very small but real. As the plane starts to yaw the flaps on the good engine side would move forward, while the flaps on the bad side would move backwards. This would result in slightly more drag on the good side flaps due to higher airflow relative to the bad side flaps. I'd be surprised if it changed Vmc by more than one kt.
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