Factors affecting Vmca

[Tempo]

Hi there,

Can anyone give me an explanation of what flap does to Vmca. I have done a bit of reading on the subject and am finding conflicting answers.

Cheers

[Lefthanded_Rock_Thrower]

On the live engine side the amount of drag introduced by application of flap is greater than the amount introduced on the dead engine side ( the wing with the live engine has greater dynamic airflow ).

So infact is increasing control ability, by attempting to reduce the turning force produced by the asymmetric thrust.

The angle\rate of climb are effected by the excess thrust\power, when flap is introduced, more power is required for the airspeed not to decay.


VMCA is calculated is by the worst possible scenario, ie:

MTOW
Flap to T\O ( so retract flap, increase performance and decrease control ability )
Gear Up
live Eng to full pwr
Aft C of G ( rudder less effective )
Sea Level, critical engine failed
Failed Engine wind milling

for those of us that fly non airline type aeroplanes, VMCA is more of a rule of thumb as opposed to a law, for example in a Cessna\Piper Twins VYSE reduces 1 knot for every 1000 ft of altitude.

[Arm out the window]

On the negative side, flap would tend to roll you towards the dead engine, again because of the slipstream airflow over the live engine wing - more drag, but also more lift. Therefore you would run out of opposite aileron quicker than with no flap.

[john_tullamarine]

(a) flap extension can do a number of things to an aircraft's handling characteristics .. so it might be difficult to come up with an all-singing, all-dancing rule of thumb for your answer .... depends on what the overall effect on a given aircraft might be .... think about drag and rolling moments (especially with prop aircraft). Keep in mind that the real Vmca at the time is very dependent upon the bank angle ..

(b) I'm not sure just what Vmca has to do with Vyse .. but it certainly is not a rule of thumb ... it is a real limitation to, and boundary for, handling under specified conditions .. and, if you fail to respect it, it might very well pick up a big bore shotgun and scatter your entrails about the countryside in the ensuing crash ....

(c) from all practical points of view, Vmca is a certification animal which, in normal operation, we just need to stay well away from ... for the bugsmashers, especially, the OEI performance achieved near Vmca does not engender a high probability of survival ... so you are treading on dangerous ground with both feet .... the risk is in ending up inverted ... or just descending into the ground under control. Neither option is attractive ..

[Lefthanded_Rock_Thrower]

JT,

None of us have an absolute rule of thumb about calculating the actual VMCA for certain conditions, what we need is a sound understanding of the effect of aircraft systems, hence the reason this question needs to be explained to Tempo, by as many people as possible.

Q. what effect would landing gear extension have on the VMCA ?., also a fair question.


In the VMCA demonstration it will be seen that the aircraft will lose directional control both above and below the published VMCA, why would that be do you think ?.

what is the aircrafts actual all up weight, enviromentals, C of G etc etc.

the published VMCA is calculated in the absolute worst of both physical and enviromental conditions.

Agreed that it is something that you should not be anywhere near, but it is certainly in practice not an actual calculated and accurate speed, it's an indication of the absolute worst case scenario.

VMCA \ VYSE both these speeds are effected by a number of other parameters, again understanding the relationships is essential.

Arm out the window, Run out of opposite Aileron ?.

[Arm out the window]

L_R_T, aileron would probably not be a big issue in some types, but with a high lift wing and lots of power on the operating engine, there is a large amount of aileron required to counteract the rolling tendency towards the dead engine.
With a big rudder effective down to low speeds, aileron becomes a significant factor in that you end up with the control wheel screwed around towards the live and may not have a heap left, before you run out of rudder.

[john_tullamarine]

LRT,

I suggest that the effect of systems operations (gear, flaps, etc) will vary from one aircraft to another. Unless someone bothers to do the tests and determine the actual Vmca for any given set of conditions, then you are unlikely to end up with much in the way of illumination. I would be far more interested in seeing pilots worry about bank control at low speeds .... let the bank angle go the "wrong" way .. and you might get a very salutory surprise ..

Vmca, as published, certainly is conservative for most operations although I wouldn't talk in terms of "absolute worst case scenarios". Like most things in aerodynamics, there are many exceptions to consider when talking about rules ....

One needs to be a little careful in extrapolating from the typical endorsement "static Vmca" demonstration exercise. It has not a great deal to do with the published Vmca figure, is of very little practical training value and may well be counterproductive. If performed at altitude it sets the pilot up for a possible stall and spin departure situation .... not a nice set of circumstances.

Could I ask you to give some explanation on circumstances where the static Vmca demo is likely to give a problem ABOVE the AFM figure ? If the configuration is as published, I would have been surprised to see a problem presenting at a higher speed ... provided that bank is controlled appropriately ?

Then again, if I may relate the tale of a colleague some years ago ..... Being mindful of the problems and hazards, and when faced with having to do an unwanted endorsement to effect an instrument rating renewal, eventually, when asked by the instructor to do such a demo, he held on a restricted rudder displacement. The instructor, apparently, was mightily confused by the resulting higher than usual speed at which heading control was lost. My colleague, on the other hand, was much more relaxed than he would have been had he been silly enough to play with full rudder during the exercise.

The published Vmca is, most definitely, a hard and fast figure for the conditions specified and a fair bit of flight test effort goes into making the determination. The real world day to day figure, for any given set of circumstances, will, of course, vary from the book figure.

Perhaps I misread your earlier post .. I was only enquiring if you were suggesting that there was some relationship between Vmca and Vyse ?

Tempo's question includes its answer .. there exists conflicting tales because there is no definitive story to tell. As DJP (a very experienced aerodynamicist and light aircraft pilot) in another thread elsewhere some time ago said, in paraphrase, ... "tell me the observation and I can give you an explanation ... but don't ask me to work it the other way around".

I shall be interested in seeing where this thread may lead ....

[Mad (Flt) Scientist]

Can I just add a small correction

Originally posted by Lefthanded_Rock_Thrower

VMCA is calculated is by the worst possible scenario, ie:

MTOW
Flap to T\O ( so retract flap, increase performance and decrease control ability )
Gear Up
live Eng to full pwr
Aft C of G ( rudder less effective )
Sea Level, critical engine failed
Failed Engine wind milling

VMCA is established in the required certification configuration (which may or may not be the absolute worst, depending on a number of factors, e.g. flap position).

The worst case from consideration of weight is the lightest weight (consistent with aft c.g.) as this provides the smallest weight component along the wings when banked at the maximum permitted 5 degree bank angle, which in turn permits the use of the minimum proverse sideslip to assist in trimming the aircraft.

It is not always necessary to consider the Sea Level thrust if one is willing to schedule VMCA with altitude or temperature (which is permissible, and necessary to permit higher altitude operations in some cases).

VMCA by definition occurs in the takeoff (second segment) configuration, so the issue of flap setting and landing gear position is defined, not left to determination of worst case. There is, of course, VMCL, which is a similar requirement in the approach/landing configuration.
----
Any consideration of the flap effect must be very dependent on whether there are significant powered lift effects; on an aft mounted twin the engine/flap interference is not very significant - far more so is the engine/empennage effects.
----
LRT and 'arm' - a further important consideration for aileron power is the need to be able to conduct the roll rate test for a VMCL manoeuvre, and also the dynamic VMC requirement which may be more demanding for the ailerons. (Depending on the degree of control cross-coupling, you may also find the aileron and rudder requirements cross contaminating, e.g. for a short tail arm with tall fin)

[404 Titan]

Mad (Flt) Scientist

Many years ago I came across this definition of Vmca & Vmc in an American aviation textbook. A definition that I have also seen in military text books as well.

VMCA

The minimum speed at which directional control can be maintained with full rudder deflection is designated Vmca. This speed is established by a manufacturer under criteria specified by the Certifying Authority, taking account of the factors below. For American aircraft Vmca must not be more than 1.2 x the Stall Speed (Clean) at maximum Take-off
Weight.

VMCA CRITERIA

The Vmca as determined by the manufacturer is marked on the airspeed indicator (ASI) of twin-engine aircraft by a red radial line. For aircraft certificated by the American FAA, the criteria used to determine Vmca are as follows:

1. Failure of the "critical engine",

2. Engine controls set for take-off or maximum available power,

3. The most unfavourable (most rearward) centre of gravity (within limits),

4. The aircraft trimmed for take-off,

5. The maximum allowable sea-level take-off weight which permits 3, in ISA conditions,

6. Flaps in the take-off position,

7. Landing gear retracted,

8. Cowl flaps in the take-off position (open),

9. The propeller of the inoperative engine windmilling,

10. The aircraft airborne and out of ground effect,

11. No more than 150 lbs. of force required to maintain full rudder deflection, and

12. A maximum of five degrees of bank towards the operating engine.

These criteria reflect a "worst case" situation, ie, a complete engine failure after take-off.




VMC

Unlike Vmca, Vmc is a variable minimum control speed and must be determined by the pilot. Usually it is very close to Vmca and in most circumstances is less than it. It should be therefore stated that even thought Vmca and Vmc are different, they should be treated as the same and not experimented with.

VMC CRITERIA

Vmc as mentioned above is determined by the pilot. It is not marked on the airspeed indicator and in general there isn't enough information in the pilots operating handbook for the pilot to work it out. The criteria used for determining Vmc by the pilot are:

1. Which engine has failed,

2. The configuration of the inoperative propeller,

3. Power setting of live engine,

4. The altitude of the aircraft,

5. The level of turbulence the aircraft is experiencing,

6. The position of the centre of gravity,

7. The aircraft configuration, ie, Flaps, Landing Gear, and Cowl Flaps up or down, open or closed,

8. Bank angle of the aircraft,

9. The weight of the aircraft.

[john_tullamarine]

404Titan,

Would be interested in your quoting your reference.

Any suggestion that pilots ought routinely to determine the real world Vmca on the day is fraught with danger. There is no practical need to do so as there is no sensible reason for a civil aircraft to be close to or below the published certification Vmca.

For military operations, if the tactical need is there, the TPs probably will have done the work anyway and provided the data for operational application.

Mind you .... not always does the certification do all that is sensible ... often only the minimum requirements are addressed.

Although it is of little practical significance to the discussion, current FAR practice refers to Vmc rather than Vmca.

[Wagit]

All very interesting.

I have actual flown an aircraft where the VMCa reduces when you put the flaps down. One of the reason is because with the flaps up the rudder travel is restricted to 12 degrees and with the flaps down the rudder travel increases

[404 Titan]

john_tullamarine

I am not for one minute suggesting that pilots experiment with Vmc. To the contrary, what was said is that there isn’t enough information in the Pilot’s Operating Handbook to work Vmc out and that Vmc and Vmca should be treated as the same and not experimented with. I think what the whole point is here, is that once you are below the committal height on one engine you aren’t limited to Vmca (in case of missed approach) if the field limiting.

As to the text I used when making these notes about 10 years ago, I think it was Aerodynamics For Naval Aviators, Advanced Pilots Flight Manual and also an Australian instructing publication of which I can’t remember the name.

[Lefthanded_Rock_Thrower]

VMCA as described would actually happen somewhere between 10 ft above the runway and 2-300 ft, ie between positive rate-gear up and flap up on the take off run, segment 2 or there abouts.

if one could not retract the landing gear or asymmetric flap condition the aircraft would more than likely lose directional control above the published VMCA.

[404 Titan]

Lefthanded_Rock_Thrower

What you are suggesting is real world scenario. What I was alluding to is the criteria specified by the Certifying Authority in determining Vmca during flight-testing. Two different things. I agree, if you do leave the gear down and flap at the take off setting, the other engine will probably take you to the crash sight.

[Woomera]

For FAR 23 types i.e. MTOW =<5700Kg

THIS DATA CURRENT AS OF THE FEDERAL REGISTER DATED MARCH 3, 2003



14 CFR - CHAPTER I - PART 23




View Part
§ 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.

(c) For all airplanes except reciprocating engine-powered airplanes of 6,000 pounds or less maximum weight, the conditions of paragraph (a) of this section must also be met for the landing configuration with --

(1) Maximum available takeoff power initially on each engine;

(2) The airplane trimmed for an approach, with all engines operating, at VREF, at an approach gradient equal to the steepest used in the landing distance demonstration of § 23.75;

(3) Flaps in the landing position;

(4) Landing gear extended; and

(5) All propeller controls in the position recommended for approach with all engines operating.

(d) A minimum speed to intentionally render the critical engine inoperative must be established and designated as the safe, intentional, one-engine-inoperative speed, VSSE.

(e) At VMC, the rudder pedal force required to maintain control must not exceed 150 pounds and it must not be necessary to reduce power of the operative engine(s). During the maneuver, the airplane must not assume any dangerous attitude and it must be possible to prevent a heading change of more than 20 degrees.

(f) At the option of the applicant, to comply with the requirements of § 23.51(c)(1), VMCG may be determined. VMCG is the minimum control speed on the ground, and is the calibrated airspeed during the takeoff run at which, when the critical engine is suddenly made inoperative, it is possible to maintain control of the airplane using the rudder control alone (without the use of nosewheel steering), as limited by 150 pounds of force, and using the lateral control to the extent of keeping the wings level to enable the takeoff to be safely continued. In the determination of VMCG, assuming that the path of the airplane accelerating with all engines operating is along the centerline of the runway, its path from the point at which the critical engine is made inoperative to the point at which recovery to a direction parallel to the centerline is completed may not deviate more than 30 feet laterally from the centerline at any point. VMCG must be established with --

(1) The airplane in each takeoff configuration or, at the option of the applicant, in the most critical takeoff configuration;

(2) Maximum available takeoff power on the operating engines;

(3) The most unfavorable center of gravity;

(4) The airplane trimmed for takeoff; and

(5) The most unfavorable weight in the range of takeoff weights.

[Doc. No. 27807, 61 FR 5189, Feb. 9, 1996]


Note: the treatment of VMCG as well, an issue not well understood or even known to most GA drivers.


FAA Reg 23 source here

Big kids aircraft =>5700 Kgs


THIS DATA CURRENT AS OF THE FEDERAL REGISTER DATED MARCH 3, 2003



14 CFR - CHAPTER I - PART 25




View Part
§ 25.149 Minimum control speed.

(a) In establishing the minimum control speeds required by this section, the method used to simulate critical engine failure must represent the most critical mode of powerplant failure with respect to controllability expected in service.

(b) 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 maintain straight flight with an angle of bank of not more than 5 degrees.

(c) VMC may not exceed 1.13 VSR with --

(1) Maximum available takeoff power or thrust on the engines;

(2) The most unfavorable center of gravity;

(3) The airplane trimmed for takeoff;

(4) The maximum sea level takeoff weight (or any lesser weight necessary to show VMC);

(5) The airplane in the most critical takeoff configuration existing along the flight path after the airplane becomes airborne, except with the landing gear retracted;

(6) The airplane airborne and the ground effect negligible; and

(7) If applicable, the propeller of the inoperative engine --

(i) Windmilling;

(ii) In the most probable position for the specific design of the propeller control; or

(iii) Feathered, if the airplane has an automatic feathering device acceptable for showing compliance with the climb requirements of § 25.121.

(d) The rudder forces required to maintain control at VMC may not exceed 150 pounds nor may it be necessary to reduce power or thrust of the operative engines. During recovery, the airplane may not assume any dangerous attitude or require exceptional piloting skill, alertness, or strength to prevent a heading change of more than 20 degrees.

(e) VMCG, the minimum control speed on the ground, is the calibrated airspeed during the takeoff run at which, when the critical engine is suddenly made inoperative, it is possible to maintain control of the airplane using the rudder control alone (without the use of nosewheel steering), as limited by 150 pounds of force, and the lateral control to the extent of keeping the wings level to enable the takeoff to be safely continued using normal piloting skill. In the determination of VMCG, assuming that the path of the airplane accelerating with all engines operating is along the centerline of the runway, its path from the point at which the critical engine is made inoperative to the point at which recovery to a direction parallel to the centerline is completed may not deviate more than 30 feet laterally from the centerline at any point. VMCG must be established with --

(1) The airplane in each takeoff configuration or, at the option of the applicant, in the most critical takeoff configuration;

(2) Maximum available takeoff power or thrust on the operating engines;

(3) The most unfavorable center of gravity;

(4) The airplane trimmed for takeoff; and

(5) The most unfavorable weight in the range of takeoff weights.

(f) VMCL, the minimum control speed during approach and landing with all engines operating, 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 maintain straight flight with an angle of bank of not more than 5 degrees. VMCL must be established with --

(1) The airplane in the most critical configuration (or, at the option of the applicant, each configuration) for approach and landing with all engines operating;

(2) The most unfavorable center of gravity;

(3) The airplane trimmed for approach with all engines operating;

(4) The most favorable weight, or, at the option of the applicant, as a function of weight;

(5) For propeller airplanes, the propeller of the inoperative engine in the position it achieves without pilot action, assuming the engine fails while at the power or thrust necessary to maintain a three degree approach path angle; and

(6) Go-around power or thrust setting on the operating engine(s).

(g) For airplanes with three or more engines, VMCL-2, the minimum control speed during approach and landing with one critical engine inoperative, is the calibrated airspeed at which, when a second critical engine is suddenly made inoperative, it is possible to maintain control of the airplane with both engines still inoperative, and maintain straight flight with an angle of bank of not more than 5 degrees. VMCL-2 must be established with --

(1) The airplane in the most critical configuration (or, at the option of the applicant, each configuration) for approach and landing with one critical engine inoperative;

(2) The most unfavorable center of gravity;

(3) The airplane trimmed for approach with one critical engine inoperative;

(4) The most unfavorable weight, or, at the option of the applicant, as a function of weight;

(5) For propeller airplanes, the propeller of the more critical inoperative engine in the position it achieves without pilot action, assuming the engine fails while at the power or thrust necessary to maintain a three degree approach path angle, and the propeller of the other inoperative engine feathered;

(6) The power or thrust on the operating engine(s) necessary to maintain an approach path angle of three degrees when one critical engine is inoperative; and

(7) The power or thrust on the operating engine(s) rapidly changed, immediately after the second critical engine is made inoperative, from the power or thrust prescribed in paragraph (g)(6) of this section to --

(i) Minimum power or thrust; and

(ii) Go-around power or thrust setting.

(h) In demonstrations of VMCL and VMCL-2 --

(1) The rudder force may not exceed 150 pounds;

(2) The airplane may not exhibit hazardous flight characteristics or require exceptional piloting skill, alertness, or strength;

(3) Lateral control must be sufficient to roll the airplane, from an initial condition of steady flight, through an angle of 20 degrees in the direction necessary to initiate a turn away from the inoperative engine(s), in not more than 5 seconds; and

(4) For propeller airplanes, hazardous flight characteristics must not be exhibited due to any propeller position achieved when the engine fails or during any likely subsequent movements of the engine or propeller controls.

[Doc. No. 5066, 29 FR 18291, Dec. 24, 1964, as amended by Amdt. 25-42, 43 FR 2321, Jan. 16, 1978; Amdt. 25-72, 55 FR 29774, July 20, 1990; 55 FR 37607, Sept. 12, 1990; Amdt. 25-84, 60 FR 30749, June 9, 1995; Amdt. 25-108, 67 FR 70827, Nov. 26, 2002]

FAA Reg 25 source here

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