Factors Effecting Vmc - One big headache

[oktex88]

So I have a "mock" oral for my multi-checkride tonight.

I am confused on the issues of how Vmca is effected by Ground effect, trim and cowl flaps.

Ground Effect...I hear from one instructor that ground effect is nil on Vmca. From another instructor when we are in ground effect Vmca is raised because there is less induced drag being produced by the wing and less drag = more excess thrust which causes a loss of directional control sooner

Trims...from one I hear it has a nil effect. The other I hear it can help decrease Vmca by using rudder trim to get more rudder authority to counter act the yawing.

Cowl Flaps...with the cowl flaps open it provides a stabilizing effect the same as the flaps?

 

[tgrayson]

I am confused on the issues of how Vmca is affected by Ground effect, trim and cowl flaps.

Nobody really knows; if they say they do, they're lying. Ask for evidence.

 

[BajtheJino]

So I have a "mock" oral for my multi-checkride tonight.

I am confused on the issues of how Vmca is effected by Ground effect, trim and cowl flaps.

Ground Effect...I hear from one instructor that ground effect is nil on Vmca. From another instructor when we are in ground effect Vmca is raised because there is less induced drag being produced by the wing and less drag = more excess thrust which causes a loss of directional control sooner

Trims...from one I hear it has a nil effect. The other I hear it can help decrease Vmca by using rudder trim to get more rudder authority to counter act the yawing.

Cowl Flaps...with the cowl flaps open it provides a stabilizing effect the same as the flaps?

Doesn't the manufacturers handbook tell you how they calculated Vmc for whatever aircraft you're flying?

 

[oktex88]

Doesn't the manufacturers handbook tell you how they calculated Vmc for whatever aircraft you're flying?

I know under what configuration the aircraft was tested under to determine *Vmc* from FAR 23.149 but I'm wondering how these factors I listed will effect my *Vmca*

Vmc different from Vmca. Vmc is a published speed as tested by the manufacturer under FAR 23.149......Vmca is not published and varies depending on several factors.

 

[BajtheJino]

I know under what configuration the aircraft was tested under to determine *Vmc* from FAR 23.149 but I'm wondering how these factors I listed will effect my *Vmca*

Vmc different from Vmca. Vmc is a published speed as tested by the manufacturer under FAR 23.149......Vmca is not published and varies depending on several factors.

No , Sherlock. I've got a Seneca I manual that lists everything that they did to determine Vmc (sea level, 12,000 ft) and then they explain why they did it that way and how it affects Vmca.

 

[oktex88]

In the POH for the Seminole I have it only mentions that it was determined in accordance with Federal Aviation Regulations (I assume FAR 23.149). It makes no mention on how Vmca would change if the conditions were different.

The only publication I could find from the stance of aircraft certification is AC 23-8B at http://rgl.faa.gov/Regulatory_and_G...4955f4e86256da60060c156/$FILE/Final-Part1.pdf

It does explain how some factors could effect Vmc.

It seems that "back in the day" determining Vmca was equivalent to what is determining Vmc for a new aircraft going through certification today.

Vmca is no longer defined as the speed determined by the manufacturer during certification. It is now called just "Vmc" as shown in FAR 23.149 and AC 23-8B. Vmca is now NOT a published speed. You can compare this to slow flight where during slow flight your not aiming for "Vs or Vso" (which is published) your aiming for the "minimum controllable airspeed" (which is not published and changes on conditions).

 

[tgrayson]

Vmca is now NOT a published speed

Yes, it is. Vmca = Vmc. That's as opposed to Vmcg.

 

[ryan1234]

Ground Effect...I hear from one instructor that ground effect is nil on Vmca. From another instructor when we are in ground effect Vmca is raised because there is less induced drag being produced by the wing and less drag = more excess thrust which causes a loss of directional control sooner

Has anyone actually demostrated Vmc in the ground effect? (and lived to tell about it)

If an examiner asks what happens to Vmc in the ground effect - ask him why it's relevant.

For one - if you're there (in the ground effect) at full power on take off at Vmc and you do have an engine failure - so many other factors take precendent to determine Vmc - either way... if you're in that position, you should probably pull the power rather than ponder what exactly effects Vmc - because if you're wrong the stakes are pretty high.

There are some STOL twins that will take off well below Vmc - in which case you have to carefully plan your take-off and either accept the risks or mitigate it a little by staying in the ground effect to accelerate to Vyse or Vxse for a normal both engine take-off. Either way, you can feel the sink/yaw/roll rate to know when it's time to pull the power accordingly and/or lower the nose, etc if you do have an engine failure.

If you were actually at Vmc and you had a really serious and sudden engine failure in the ground effect (enough to cause a Vmc situation/roll-over) the aircraft would probably yaw/roll faster than any minute details you could think about other than immediately pulling the power on both engines. It would also mean that you took off prior to Vmc - because the acceleration in the ground effect would generally push you through - and at that time, it's time to make the split second decision of what to do.

 

[oktex88]

Thanks guys... I just had a "mock" stagecheck with another Instructor (I'm in a part 141 so we don't have checkrides just stagechecks) and I said the effects of Vmc on cowl flaps and ground effect is nil. I didn't even bring up trim. Everything else like systems went great...now off to study some more and pass the real thing!

 

[waco1990]

Heres a chart I made to help me learn Vmc characteristics.


Standard day at sea level (0’ DA) At sea level the engine is a maximum performance at 180 HP with a strong asymmetrical thrust affect, increasing Vmc. In reality the maneuver will be done at approximately 5,000 DA and the aircraft engine will not put out the full 180 HP resulting in a decrease in the amount of asymmetrical thrust. Vmc will decrease.


Max power from operating engine (180 HP) Vmc increases as power increases on the operating engine. With normally aspirated engines, Vmc is highest at takeoff power and at sea level, and decrease with altitude. In reality the Maneuver will be done at approximately 5,000 DA and the aircraft engine will not put out the full 180 HP resulting in a decrease in the amount of asymmetrical thrust. Vmc will decrease.


Windmilling propeller When the propeller is in a low pitch (unfeathered), it presents a large area of resistance to the relative wind. This resistance causes the engine to “windmill”. The windmilling creates a large amount of drag and results in a yawing moment into the dead engine. When the propeller is “feathered,” the blades are in a high pitch position, which aligns them with the relative wind, minimizing drag. This will result in Vmc increasing. In reality we will feather the prop in the event of an engine failure. A feathered prop will decrease drag and lower Vmc.


Most unfavorable weight (2730 lbs) When the aircraft is lightly loaded Vmc increases because in a given angle of bank, the aircraft is producing a lesser horizontal component of lift compared to a heavier aircraft. During local training flight the aircraft weighs approximately 3600 lbs which is 870 lbs heavier then what piper used to determine Vmc. In a given angle of bank, the heavier the aircraft, the greater the horizontal component of lift that adds to the rudder force, so at an aircraft's maximum gross weight, the possible horizontal component is greatest and Vmc is decreased.


Aft most center of gravity Vmc increases as the center of gravity is moved aft. The moment arm of the rudder is reduced as the center of gravity is moved aft at the same time the moment arm of the propeller blade is increasing, aggravating asymmetrical thrust. During local training flight the aircraft with two pilots with minimal baggage results in a forward center of gravity causing Vmc to decrease because the moment arm of the rudder is increased while the moment arm of the propeller blade is decreasing causing less of an effect form asymmetrical thrust with the large amount of leverage the rudder now has.

Maximum 5° bank into the operating engine In order to counteract the yawing motion with an inoperative engine, bank must be added toward the operative engine to aid the rudder force. As the bank angle increases, the horizontal component of lift increases, lowering the yawing motion further and decreasing Vmc. Vmc increases significantly with decreases in bank angle. Tests have shown that Vmc may increase more than 3 knots for each degree of bank angle less than 5°. Vmc will be experienced at speeds almost 20 knots higher then Vmc at wings level because of all the drag in the sideslip condition.

Landing gear, flaps, trim for takeoff

Out of ground effect

150lbs max rudder pressure

0 sideslip

20 degrees heading change max

Slow deceleration 1 knot per second

 

[dlcmdrx]

Heres a chart I made to help me learn Vmc characteristics.



Maximum 5° bank into the operating engine In order to counteract the yawing motion with an inoperative engine, bank must be added toward the operative engine to aid the rudder force. As the bank angle increases, the horizontal component of lift increases, lowering the yawing motion further and

why???

 

[tgrayson]

why???

"Why" what? If you're asking why the increased bank lowers Vmc, it's because for every degree you bank, you're sideslipping less into the bad engine, and by the time you reach 5 degrees, you're sideslipping into the good engine. This provides extra authority on the vertical stabilizer.

 

[CoffeeIcePapers]

why???

You are asking about the underlined part, right?

PHAK:

At a given airspeed, the rate at which an airplane turns depends upon the magnitude of the horizontal component of lift. It will be found that the horizontal component of lift is proportional to the angle of bank; that is, it increases or decreases respectively as the angle of bank increases or decreases. It logically follows then, that as the angle of bank is increased the horizontal component of lift increases, thereby increasing the rate of turn. Consequently, at any given airspeed the rate of turn can be controlled by adjusting the angle of bank.

 

[stratoduck]

I am confused on the issues of how Vmca is effected by Ground effect, trim and cowl flaps.

ok, i'm going to take a stab at this.

open cowl flaps will lower Vmc. since the cowl flaps are symmetrical, there is no effect when both engines are running. but when one engine is running, the cowl flap on the running engine is creating far more drag than the one on the windmilling engine. this drag effectively reduces the trust of the engine, lowering Vmc. basically an effective speed brake on the running engine holding it back, and an ineffective speed brake on the windmilling engine.

trim: if the rudder trim is 'out of trim', it will lower Vmc. the deflected trim tab will augment the rudder. just the same, trim the rudder to reduce pedal force, and the tab deflecting opposite of desired rudder travel will reduce the effective area of the rudder.

if the horizontal stabilizer or tailplane is trimable (like on the fabric pipers and the jets), an out of trim condition will lower Vmc. in this case, the horizontal stabilizer is fighting the elevators and creating excess drag. drag behind the CG will help reduce the yawing.

if the elevator trim is a tab on the elevator or stabilator, then there really isn't a change in drag (maybe tiny amount), so with most light twins, an out of trim elevator would have no effect.



i would think ground effect would lower Vmc. a descending wing will generate additional lift as it approaches the runway, especially if the aircraft has dihedral. there would be a strong natural tendency to reduce rolling, more so than when airborne.

and if ground effect gives the ailerons more bite, then it would also lower Vmc by allowing the pilot to hold the dead engine up longer.

am i full of crap on the ground effect ideas? i'm open to opinions!

 

[shdw]

and if ground effect gives the ailerons more bite, then it would also lower Vmc by allowing the pilot to hold the dead engine up longer.

The limiting factor for Vmc on most light twins is the rudder. So, whether this is true or not doesn't really matter since the effect of the rudder is still the same. For aircraft other than light twins, I have no idea.



On an aside to this entire discussion of ground effect and Vmc, I never understood why anyone would even care whether it raises or lowers Vmc. As far as I'm concerned, the only time my butt will be in ground effect during OEI is when I'm landing. There might be a few seconds spent in ground effect if I lose an engine on takeoff, but I surely won't alter my flying habits for those 3 or 4 seconds. Clean it up, fly for best performance until safe altitude, and then get her back on the ground.

 

[tgrayson]

m i full of crap on the ground effect ideas? i'm open to opinions!

Your speculation on the cowl flaps is reasonable and, if I had to wager money, it would be in lines with your conclusions. Note, however, that cowl flap position is not mentioned in the definition of Vmc.

But I think all bets are off with trim and ground effect. The airflow around the airplane is more complex than our simplistic assumptions when we take various pieces of the airplane in isolation. Ground effect not only affects the air flow around the main wing, but also the horizontal stabilizer. And the airflow around these surfaces will have some effect of the flow around the vertical stabilizer. I think predicting the net effect on Vmc would be a roll of the dice.

And the trim has implications other than purely aerodynamic. The regulations limit rudder force to 150 pounds, so the trim position will affect the amount of rudder that the pilot is allowed to apply.

While it's interesting to discuss some of these issues, I don't think they should be taught as fact by MEIs to students, since they are only speculation and undocumented by any authoritative source. I see nothing wrong with a CFI saying "I don't know." Some of these requirements are in the regulations merely for standardization and it would be wrong to draw any conclusions from their existence.

 

[stratoduck]

i stand by the trim idea. a rudder trim tab can either augment or reduce the effective rudder area. this obviously does not take into considerations of force limitations of the pilot or aircraft.

and an out of (elevator) trim aircraft with a moveable horizontal stabilizer can generate considerable drag, especially on the transport categories. a speed brake in the form of a split tailcone (like on the bae-146 avro) or a drag chute would accomplish a similar effect.

as soon as the aircraft becomes uncoordinated, the drag will create a moment resisting further yawing.

and you are correct that talking about these things are a bit absurd, and all we would need is an "advanced thinking" CFI teaching students to trim IN more rudder pressure as it will make the rudder more effective. not to mention that there might be a one knot effect from these things on Vmc, nor are they considered in Vmc calculations and measurements.

to me, these absurdities an intricacies stimulate my curiosity and thus learning process too. that's it.

 

[shdw]

and an out of (elevator) trim aircraft with a moveable horizontal stabilizer can generate considerable drag

Why would you think this? An out of trim elevator, or any surface for that matter, will change the required pressures by the pilot(s) to hold a given attitude. If the same attitude is held, trimmed versus out of trimmed, the aerodynamic differences are negligible.

 

[stratoduck]

assuming a rudder with a trim tab: if the rudder trim is used to reduce pedal pressure, the trim tab is deflecting opposite of rudder travel. this reduces the frontal area of the deflected rudder. just the same, if the trim is used to increase pedal pressure (wrongly trimmed), the trim tab is further deflected into the airflow, increasing the rudder frontal area.

but like tgrayson was implying, there is a danger to this. incorrectly trimming the rudder would give it a tiny bit more bite, but it is likely cause a mechanical failure (rudder system) or a physiological failure (pilot's leg muscles). unless there is plenty of altitude, airspeed, and quick thinking, this kind of failure could likely be fatal.

as for the elevator trim, again, this would only be applicable to aircraft with moveable horizontal surfaces (like the old ragwing pipers and most jets). if the aircraft is out of trim, the pilot would be required to hold the elevator in a position that is not aerodynamic with the horizontal stabilizer. say the aircraft is trimmed full nose down. the leading edge of the horizontal stabilizer is full up. the downward pitching of the aircraft will be counteracted by the pilot holding up elevator. looking at the tail from the side, it would have a slight V shape causing drag. when this trim system is properly trimmed, the horizontal stabilizer and elevator would look like -- from the side; aerodynamic. i'd say a full out of trim condition would give the tail an equivalent frontal surface area similar to half deflection spoilers.

this drag can be more than insignificant. an improperly trimmed old piper knock can knock 200 fpm off the climb. i've also noted that in transport aircraft, adding some trim in the flair greatly increases the tendency of the aircraft to float, whereas a flare done entirely with the elevators tends to make the aircraft settle right in.

it is drag, and it can be notable.

 

[shdw]

an improperly trimmed old piper knock can knock 200 fpm off the climb.

I've only flown 100ish hours in pipers, I've never experienced this phenomena. We had electric trim, so part of my training was a simulated takeoff with runaway trim to both stops. I saw no recognizable difference in our rate of climb. It was long ago though, so I certainly could have missed it.

Where me and tgray run into a problem with this sort of presentation is that it is based on biased observations and lacks empirical proof. I say biased not to be condescending, but because you have formed an opinion and are looking for proof.

At a cursory glance (I'm on vacation without my books!) I notice one thing missing from your analysis: you don't seem to consider that an out of trim aircraft will require a different control deflection angle to achieve the same attitude as a trimmed aircraft. A concept that, I believe, would result in both actions canceling each other out, thus no change in drag other than negligible changes in parasitic drag.