Vmc Flashcards
What is Vmc?
Vmc (Minimum Controllable Airspeed) is the calibrated airspeed, at which, when the critical engine is suddenly made inoperative it is possible to:
- Maintain control of the airplane with the engine still inoperative.
- Maintain straight flight at the same speed with an angle of bank not more than 5 degrees.
How do airspeed and rudder relate to Vmc?
As airspeed decreases the rudder becomes less effective, eventually an airspeed will be reached where full rudder deflection will be required to maintain directional control.
This airspeed is Vmc. Any further reduction in airspeed will result in loss of directional control. Published Vmc is defined by the red radial on the airspeed indicator. In the BE-76 Duchess published Vmc is 65 KIAS.
What does Vmc Guarantee?
Vmc guarantees directional control only!!
How does the manufacturer determine Vmc?
C- Critical engine failed and windmilling
O- Operating engine at maximum takeoff power
M- Maximum gross weight
B- Bank of no more than 5 degrees
A- Aft center of gravity
T- Takeoff configuration (gear up, flaps up)
S- Standard temperature (15°C) and pressure (29.92” Hg)
Vmc and Critical engine failed and windmilling
A windmilling propeller creates much more drag than a feathered propeller. If the airplane is equipped with an autofeather system then propeller is feathered.
Vmc and Operating engine at maximum takeoff power
With the engine at max power this will create more lift and produce more of a yawing tendency about the longitudinal axis, thus increasing Vmc.
Vmc and Maximum gross weight
While a heavier airplane is more stable, it also requires the wing to produce more lift. This is really more of a standardization requirement for certification
Vmc and Bank of no more than 5°
This is also a standardization requirement
Vmc and Aft center of gravity
Since an airplane rotates around the CG, an aft CG decreases the distance (arm) between the CG and rudder, which decreases the leverage or effectiveness of the rudder.
Vmc and Takeoff Configuration
This includes flaps in normal takeoff position and landing gear retracted. While gear and flaps down creates drag, it also creates a keel effect which tends to stabilize the aircraft.
Vmc and Standard temperature and pressure
The published Vmc and red line on the airspeed indicator are based on standard day at sea level. As density altitude increases, the red line becomes less reliable because Vmc decreases with altitude – which brings Vmc closer to stall speed.
Is Vmc a static number? what does the red line represent?
Vmc is not a static number. It changes with any combination of the above variables. The red line is the highest speed that Vmc will be, in fact, it will most likely be lower. The object is to keep Vmc as low as possible. For certification, Vmc cannot be greater than 1.2 times stall speed with flaps in takeoff position and gear retracted
What are the warning signs that Vmc is occurring?
Loss of directional control- The rudder pedal is depressed to its fullest travel and the airplane is still yawing or rolling toward the inoperative engine.
Stall warning horn or buffeting of the controls- A single engine stall is very dangerous and could result in a spin. Light twins are not known for good stall/spin recovery.
A rapid decay of control effectiveness- This could lead to the loss of control of the aircraft.
How do you recover from Vmc?
Simultaneously:
Reduce power on the operating engine- Reduce power on the operating engine reduces the asymmetric air flow.
Pitch down- Lowering the nose of the aircraft will get the air flowing again over the control surfaces and allow you to regain directional control.
How are Vmc and Performance related?
Vmc is defined using a very specific set of conditions. Published Vmc and actual Vmc can be two very different numbers. Vmc only addresses directional control and is not related to aircraft performance. While controllability is important, single engine performance is just as important. You must be able to balance both controllability and performance to keep a serious situation from getting out of control. In some cases, an element that provides an increase in controllability can actually hurt performance.
VMC Table: 8 Factors that affect VMC?
1) C of G
2) Bank
3) Weight
4) Power
5) Density Altitude
6) Drag Devices
7) Dead Prop
8) Critical Engine
Why does a steeper bank lower VMC?
you have the horizontal component of lift that is going to serve to reinforce the rudder by opposing asymmetric turning tendency of engine
Why does increasing weight in a turn lower VMC?
more horizontal lift is created bc more lift is required due to increase in weight
Why do flaps and gear extended lower VMC?
with left engine failed and the gear and flaps down, plane will turn to the left but the disk of the prop is interrupted by the flaps and gear being down. This increases drag on the right side and you have a turning force away from the asymmetric thrust
what CG will increase performance (rate of climb)?
Aft CG -
you’ll need to create more lift than weight in order to climb moving the CG aft increases rate of climb. To hold nose of airplane up the tail has to create a downward force. Because the CG is well aft, the tail doesn’t require as much as a downward force to keep nose up. You lose less lift by moving the CG aft.
why does 3-5 degrees of bank increase performance (rate of climb)
The vertical component of lift will decrease in a bank and a horizontal component is created, it might seem better to not lose any vertical component and to stay straight and level. The direction of travel with the level airplane creates side slip. There’s less side slip in the plane that has 3-5 degrees of bank.
