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
Vmc Table: What do these 8 factors do?
2 things: Lower Vmc or Increase performance
What CG will lower VMC?
you want the CG forward because all rotation happens around the CG. A longer arm allows you to generate more motion with the same amount of force. You’ll have more rudder authority if you lose the engine
How do you arrange bank and weight to help lower VMC?
Steeper bank and heavy weight if banking
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
What power and density will lower VMC?
Lower power and thinner air
Why does decreasing power decrease VMC
The asymmetrical turning tendency increases with more engine power. If you reduce throttle to idle you’ll reduce asymmetric turning tendency. Your VMC will be lower
Why does thinner air lower Vmc?
As you fly into thicker air you will have more asymmetric turning tendency because the engine has more air to interact with and produce more thrust. The engine is not producing same thrust in thinner air.
What configuration do drag devices and dead prop have to be in to lower VMC
Flaps/Gear extended
Prop feathered
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
Why does a feathered prop lower VMC
there’s asymmetric turning tendencies with a dead engine. If the prop is not feathered (windmilling) its got more drag. A feathered prop doesn’t have a much drag. The turning tendencies are reduced with a feathered prop.
How can critical engine lower VMC
The critical engine should be operating in order to lower VMC
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.
what bank and weight do we use to increase performance (rate of climb)
3-5 degrees of bank
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.
what does banking more than 5 degrees cause
almost no drag more alignment with the relative wind. But the vertical component of lift has decreased so much in a steeper bank, that we cant overcome the weight. 3-5 degrees is the best balance min sideslip and max vertical lift
What favorable weight increase performance (rate of climb) and why?
Lighter weight for climbing the less weight the more excess lift, the more climb
What power configuration increases performance (rate of climb) and why?
More power better rate of climb
What density altitude increases performance (rate of climb) and why?
thicker air makes wings, engine, and prop work better because theres more air thrust, lift, climb
What drag device configuration will increase performance (rate of climb) and why?
drag hurts rate of climb so retracting the drag devices helps
What dead prop configuration increases performance (rate of climb) and why?
you want the dead prop to be feathered. a windmilling prop creates drag and impacts rate of climb
How does Critical engine increases performance (rate of climb) and why?
the critical engine should be operating to increase rate of climb
