Factors affecting Vmc

Texasspilot

Texasspilot

New Member
Alright correct me if I'm wrong

SMACMUF

Standard temp/pressure
Max power on operating engine
Aft C.G.
Critical engine windmilling
Max 5 bank into operating engine
Up - Gear
Flaps - takeoff.

Sound right?
 
Texasspilot said:
Alright correct me if I'm wrong

SMACMUF

Standard temp/pressure
Max power on operating engine
Aft C.G.
Critical engine windmilling
Max 5 bank into operating engine
Up - Gear
Flaps - takeoff.

Sound right?
Click to expand...

That is how the published Vmc is determined by the FAA.
 
Texasspilot said:
Alright correct me if I'm wrong

SMACMUF

Standard temp/pressure
Max power on operating engine
Aft C.G.
Critical engine windmilling
Max 5 bank into operating engine
Up - Gear
Flaps - takeoff.

Sound right?
Click to expand...

The regulation 23.149 actually reads

§ 23.149 Minimum control speed.
(a) VMC is the calibrated airspeed at which, when the critical engine is suddenly made inoperative, it is possible to maintain control of the airplane with that engine still inoperative, and thereafter maintain straight flight at the same speed with an angle of bank of not more than 5 degrees. The method used to simulate critical engine failure must represent the most critical mode of powerplant failure expected in service with respect to controllability.
(b) VMC for takeoff must not exceed 1.2 VS1, where VS1 is determined at the maximum takeoff weight. VMC must be determined with the most unfavorable weight and center of gravity position and with the airplane airborne and the ground effect negligible, for the takeoff configuration(s) with -
(1) Maximum available takeoff power initially on each engine;
(2) The airplane trimmed for takeoff;
(3) Flaps in the takeoff position(s);
(4) Landing gear retracted; and
(5) All propeller controls in the recommended takeoff position throughout.
 
• Factors that affect Vmc
(S.M.A.C.F.U.M.)

o Standard day conditions
-Vmc goes up because you have to fight the operating engine which is getting its best performance
-Vmc speed decreases as altitude increases

o Max power on the operating engine
-Vmc goes up

o Aft legal center of gravity
-Vmc goes up because a farther aft Cg makes the rudder less effective
-The farther aft the Cg is, the higher Vmc speed is

o Critical engine prop windmilling
-Vmc goes up because a windmilling prop produces more drag than a feathered prop
-Feathered prop lowers Vmc

o Flaps/Gear up
-Vmc goes up
-Extending the gear will lower Vmc due the gear having a keel effect
-Extending the flaps will lower Vmc due to the flaps having a stabilizing effect

o Up to 5° bank into operating engine
-Vmc goes down
-When the wings are level only the rudder is used to correct the unwanted yaw.
-Banking up to 5° into the operating engine creates a horizontal component of lift which aids in rudder force. With this procedure Vmc will be lowest
-Vmc increases with decreasing bank by ~3kts per degree of bank angle

o Most unfavorable weight
-Vmc goes up with a lighter aircraft
-Less weight needs more rudder input
 
Another good thing to look at is the 'good for vmc/bad for vmc, good for performance/bad for performance'. Don't just memorize that its good or bad, but know why it's good or bad.
 
Timbuff10 said:
• Factors that affect Vmc
(S.M.A.C.F.U.M.)

o Flaps/Gear up
-Vmc goes up
-Extending the gear will lower Vmc due the gear having a keel effect
-Extending the flaps will lower Vmc due to the flaps having a stabilizing effect
Click to expand...

Lol, usually Vmc will increase with flaps down because of the accelerated slipstream. On some Jets (when the engine is in the back and pointed upward) though that you are correct.
 
Vmc will go down in most larger jets because the rudder is allowed more effectiveness through either increased hydraulic pressure to the actuator, increased rudder travel allowed by mechanical connections or a combination of those.
 
Pilot2087 said:
Lol, usually Vmc will increase with flaps down because of the accelerated slipstream.
Click to expand...

It is dependent on the amount of flaps used. Up to 20 degrees the Vmc will infact increase as the wing will produce more lift than drag per se. Over 20 degrees the flaps will produce more drag than lift which will infact have a stabilizing effect on a rolling moment caused by any flap introduction.
 
Pilot2087 said:
Lol, usually Vmc will increase with flaps down because of the accelerated slipstream. On some Jets (when the engine is in the back and pointed upward) though that you are correct.
Click to expand...

I suspect this would only be true if Vmc were aileron limited. In most light aircraft, you will be rudder limited and the overall tendency is that flaps add some directional stability to the a/c. Boundary Layer Research had some data on their website showing reduced Vmc's with certain flap settings.
 
meritflyer said:
It is dependent on the amount of flaps used. Up to 20 degrees the Vmc will infact increase as the wing will produce more lift than drag per se. Over 20 degrees the flaps will produce more drag than lift which will infact have a stabilizing effect on a rolling moment caused by any flap introduction.
Click to expand...

You'll have to explain how that affects Vmc to me v e r y slowly. :confused: I guess the puzzling part is how the drag can stabilize the aircraft in roll. Even though the last notch of flap has little effect on the lift coefficient, it has *some*. Now, I do think the increased drag can have a stabilizing effect in *yaw*, since you have one in the slipstream and not the other.
 
I think the important thing to say is, "it depends" when it comes to flaps affecting Vmc. There is no, "it's always this way because of this reason," answer for the question. Check the POH to find out exactly what the manufacturer says for each particular make and model you fly.
 
jrh said:
I think the important thing to say is, "it depends" when it comes to flaps affecting Vmc. There is no, "it's always this way because of this reason," answer for the question. Check the POH to find out exactly what the manufacturer says for each particular make and model you fly.
Click to expand...
"It depends" is always the right answer in most of these cases, and knowing all the conditions and their effects is what is required to answer this.

One thing that is "always this way" is the angle of attack. All other things being equal, when you put flaps down, and nothing else-you don't lower the nose, then you increase angle of attack which always causes P-Factor which always causes more right rudder (higher Vmc) in a left engine out scenario.

I've not seen any reference to flaps effect on Vmc in any POH.
 
tgrayson said:
The regulation 23.149 actually reads

§ 23.149 Minimum control speed.
(a) VMC is the calibrated airspeed at which, when the critical engine is suddenly made inoperative, it is possible to maintain control of the airplane with that engine still inoperative, and thereafter maintain straight flight at the same speed with an angle of bank of not more than 5 degrees. The method used to simulate critical engine failure must represent the most critical mode of powerplant failure expected in service with respect to controllability.
(b) VMC for takeoff must not exceed 1.2 VS1, where VS1 is determined at the maximum takeoff weight. VMC must be determined with the most unfavorable weight and center of gravity position and with the airplane airborne and the ground effect negligible, for the takeoff configuration(s) with -
(1) Maximum available takeoff power initially on each engine;
(2) The airplane trimmed for takeoff
(3) Flaps in the takeoff position(s);
(4) Landing gear retracted; and
(5) All propeller controls in the recommended takeoff position throughout.
Click to expand...


Dont forget "trimmed for takeoff" and if you read on in 23.149, you will also see "out of ground effect"...which is a factor that most people forget to teach.
 
Timbuff10 said:
Up to 5° bank into operating engine
-Vmc goes down
-When the wings are level only the rudder is used to correct the unwanted yaw.
-Banking up to 5° into the operating engine creates a horizontal component of lift which aids in rudder force. With this procedure Vmc will be lowest
-Vmc increases with decreasing bank by ~3kts per degree of bank angle

o Most unfavorable weight
-Vmc goes up with a lighter aircraft
-Less weight needs more rudder input
Click to expand...

As for banking 5 degrees into the operating engine - this eliminates sideslip not necessarily aiding rudder force as the rudder is limited per the regs and the actual rigging. Remember per FAR 23.149 you are only allowed 150 lbs of rudder pressure. Without banking the aircraft up to 5 degrees into the op engine, there exists a lift vector on the inop engine side of the tail which pulls the aircraft into a sideslip.The relative wind is now striking the tail on an angle and reducing the rudder surface area exposed to the relative wind.This is counter-acted by utilizing the HCL to into the good engine. Test have indicated that Vmc "may" increase 3 knots per degree not banked into the op engine.

Vmc does go up with lighter aircraft but not because "less weight needs more rudder input" per se. Aircraft not being flown at gross weight don’t require the wings to generate as much lift as if the aircraft were at gross weight. As a result, the horizontal component of lift is significantly less when the aircraft is banked into the operating engine. Consequently the horizontal component of lift has less of an ability to offset sideslip produced in inoperative engine flight.

The best rule of thumb I teach students is Vmc is decreased by feathering the inoperative engine’s prop, moving the GG forward, increasing the gross weight, and/or reducing the power on the operating engine.
 
My favorite acronym in all aviation:

CRM LOFT ACE

Critical engine inoperative and windmilling.
Rearward CG (at aft limit).
Most unfavorable weight (lightest).

Landing gear retracted.
Operating engine at max sea-level takeoff power.
Flaps set to takeoff position.
Trim set to takeoff position (rudder).

Airborne and out of ground effect.
Cowl flaps set to takeoff position.
Elimitate sideslip (max 5 degrees bank).
 
Texasspilot said:
Alright correct me if I'm wrong

SMACMUF

Standard temp/pressure
Max power on operating engine
Aft C.G.
Critical engine windmilling
Max 5 bank into operating engine
Up - Gear
Flaps - takeoff.

Sound right?
Click to expand...

I've always taught SMACFUM....but SMACMUF works too ;)
 
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