Vmc: Flap and Gear Positions, 5 degrees of bank and Max Gross

Alchemy

Partner, Ally, Friend
I recently stumbled a bit on my multi-engine instrument commercial oral exam on this topic. I was asked by the DPE what critera manufacturers use to determine the value for Vmc in an airplane. I responded:

1. Standard Day Temperature and Pressure at Sea Level
2. Critical Engine at Zero Thrust
3. Critical Engine Propellor Windmilling (max drag)
4. Operating Engine at Full throttle
5. Flaps Up
6. Gear Up
7. Aft Legal Center of Gravity
8. Maximum Gross Weight
9. Up to 5 degrees of bank into the operating engine.

The Examiner then Asked me to categorize each factor as affecting:

A) Assymetrical Thrust
B) Assymetrical Drag
C) Controllability

I categorized them as follows

1) AT
2) C
3) AD
4) AT
5) Neither, Couldn't really categorize this one
6) Neither, "Keel Effect"
7) AT
8) C
9) None of the above

My examiner insisted that I explain why having the flaps retracted increased VMC. The best I could answer was that having flaps extended creates more drag on the operating engine side due to propwash and therefore decreased the overall power of the operating engine and decreased VMC. Anyone have a better explanation?

I got complteley ripped apart on "keel effect". I tried to explain that the lateral profile of the landing gear creates resistance to uncommanded yawing movements and therefore decreases VMC but the DPE wouldn't accept that answer. Can anyone explain to me simply why having the gear down decreases VMC?

Then on the 5 degrees of bank, I explained that holding 5 degrees helps maintain a zero sideslip condition which keeps the stall speed and VMC down. THe DPE didn't buy this either, and told me that 5 degrees of bank can help and hurt. By this time I was thoroughly confused, as the Examiner was contracdicting most of what I had read in the Airplane flying handbook and Multi Oral exam guide.


All in all, my DPE's main argument was that ALL the factors above are designed to increase VMC, whereas I was thinking that 5 degrees of bank and max gross weight decreased Vmc. Anyone have good explanation for how each of the factors listed above increases VMC?
 

ready2fly

Well-Known Member
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having flaps extended creates more drag on the operating engine side due to propwash and therefore decreased the overall power of the operating engine and decreased VMC.

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Hmmm - It's my understanding that having the flaps EXTENDED indeed causes more drag and INCREASES Vmc... so, as far as my knowledge exists, you were partly correct there. More drag = increases the speed at Vmc could occur.

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Can anyone explain to me simply why having the gear down decreases VMC?

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In short - having the gear down DECREASES Vmc because the landing gear act to help stablize the plane... so your "keel affect" answer would have worked for my DE.. in fact, it DID work for my DE.

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Then on the 5 degrees of bank, I explained that holding 5 degrees helps maintain a zero sideslip condition which keeps the stall speed and VMC down.

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As far as what I read and was taught - you are absolutely correct there too.

I haven't the slightest idea where your DE was going and/or why he contradicted your answers.

Perhaps someone with greater experience can assist you; but as far as MY knowledge of Vmc exists - your answers were correct.

R2F
 

mikek123

Well-Known Member
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I explained that holding 5 degrees helps maintain a zero sideslip condition which keeps the stall speed and VMC down

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In most light twins zero sideslip is actually attained at about 2-3 degrees of bank, which is why holding 5 degrees can hurt you. At 5 degrees bank you aren't getting the maximum performance out of the airplane because you are likely in a sideslip. Using 5 degrees of bank will decrease Vmc (3 knots decrease per degree) but will hurt your performance.
 

pscraig

Well-Known Member
We teach that deploying flaps increases stability, and decreases Vmc. Your description of keel effect is fine, and it does decrease Vmc. You are correct that use of max gross weight and up to 5 deg bank into the operating engine will lower Vmc. You are creating a horizontal component of lift by doing this, which reducees the amount of rudder pressure required to maintain directional control. The magnitude of this horizontal component is proportional to the weight of the aircraft since the heavier the aircraft, the greater the amount of total lift generated.

The remaining factors you listed do increase Vmc. I disagree with the examiner, and at that point would start pulling out the AFH (chapter 14) and stack of ACs.

The DPE didn't "buy" the zero sideslip concept??? That baffles me.

I do see some problems in comparing your answer list to the questions however, looks like they might have been typed incorrectly? (#4 should be AT, etc)
 

pilot602

If specified, this will replace the title that
[ QUOTE ]
My examiner insisted that I explain why having the flaps retracted increased VMC. The best I could answer was that having flaps extended creates more drag on the operating engine side due to propwash and therefore decreased the overall power of the operating engine and decreased VMC. Anyone have a better explanation?
I got complteley ripped apart on "keel effect". I tried to explain that the lateral profile of the landing gear creates resistance to uncommanded yawing movements and therefore decreases VMC but the DPE wouldn't accept that answer. Can anyone explain to me simply why having the gear down decreases VMC?

[/ QUOTE ]

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Hmmm - It's my understanding that having the flaps EXTENDED indeed causes more drag and INCREASES Vmc...

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Anything that is stuck into the flow of air in a symmetrical fashion (i.e. flaps, gear, even cowl flaps, etc.) will help to stablize the yawing moment of the asymmetrical thrust condition. Drag does increase ... but so long as the drag is symmetrical it helps to stabilze the aircraft thus lowering the speed at which directional control is lost. Which is why, if you look, to determine Vmc the aircraft is in a completely clean, high power setting and then the thrust on the critical engine (if one exists) is immediately reduced to a zero thrust/windmilling condition.

Also, the phrase "raise or lower 'Vmc'" is technically wrong. We all say it but Vmc is a fixed airspeed as represented on the ASI (the red radial line). It will never change. The speed at which directional control is lost (what we refer to inn slang terms as "Vmc") will change depending on atmospheric conditions, pilot technique, weight, etc.
 

ready2fly

Well-Known Member
[ QUOTE ]
Anything that is stuck into the flow of air in a symmetrical fashion (i.e. flaps, gear, even cowl flaps, etc.) will help to stablize the yawing moment of the asymmetrical thrust condition. Drag does increase ... but so long as the drag is symmetrical it helps to stabilze the aircraft thus lowering the speed at which directional control is lost.

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That makes sense. I guess I had the idea stuck in my head that having the flaps extended would creat TOO much drag... so, thanks for clearing that up.

That's one of the reasons I love this site, man, ya know??


Ya feelin' me?
 

pilot602

If specified, this will replace the title that
Heh.. yeah .. now comes the kicker ...

It's arguable as to whether or not it really helps. At the moment the engine is made inoperable it may help to reduce the yawing/rolling created by the asymmetrical thrust. However, once equilibrium has been found (everything settles down) the loss in performance (you've already taken an 80%+/- hit when the engine went) that the added drag from the gear/flaps/passenger hanging out the window trying to escape may very well not be worth the "stability" it/they provide.

It's much better to fly as clean a ship as possible in a zero-sideslip condition and eek out as much performance as possible. You can much more easily "replicate" the effect of the symmetrical drag by reducing power on the operating engine a touch.
 

dakovich

Well-Known Member
L : Landing gear (retracted)
C : CG (most unfavorable)
W : Weight (most unfavorable)
B : Bank (no more than 5 degrees)

F : Flaps (retracted)
A : Aircraft trimmed for takeoff
T : takeoff power

C : cowl flaps (i think it is closed, can't remember)
O : Out of ground effect
W : Windmilling prop
S : Standard conditions (temp and pressure)

also:
1. Airspeed(for Vmc) must be 1.2 Vs1 or less
2. no more than 150 pounds of rudder pressure applied
3. no more than 20 degrees of heading change

all of this is specified in FAR part 23. these are the rules that govern the certification of Vmc for manufactures of multi-engine aircraft (i.e. the conditions for testing and publishing what the a/c Vmc is)


also, if you have the Jepp. Multi Textbook ( i'd hope, its a great piece of reference) look in chapter 3 page 22-23 for explanations of Vmc and information related to it

as for the sideslip discussion: Jepp chap. 3 starting at page 25 and on for some great explanations and visuals
 

dakovich

Well-Known Member
as for the "keel effect" you explained: you mentioned to the DE that it had to do with the profile of the landing gear, actually it has more to do with the profile of the fuselage. the profile of the gear really isn't a factor in keel effect. again, good explanation in the Jepp. PPL and Comm/Inst. text.

also, as was said earlier the 5 degree bank thing is really just a criteria of "no more than 5 degree of bank into the good engine". most a/c find an optimal bank far less, such as the Piper Seminole which is 2.2 bank.

hope some of this helps, its been a long while since i delt with Multi stuff...sniffle
 

Baronman

Well-Known Member
My understanding is that extended flaps raise Vmc

In my mind it is less of a DRAG issue as it is a rolling issue. Unlike extended landing gear which lowers Vmc due to the keel effect, the two flaps are not being "treated" equally.

What do I mean? Each main wheel is in the same environment and acting like mini-vertical stabalizers.

There is a big difference between the left and right flap. The flap on the operative engine will have a MUCH higher induced airflow over it, (presumably we are at close to max-power) thus creating more lift over that wing.

The flap on the inop engine will have relatively less induced airflow over it thus making less lift on the inop wing.

At lower speeds we may be unable to effectively counteract this rolling/yawing force thus we need a higher Vmc.

IF the Flaps were UP, there would be less difference in the amount of lift being produced between the two wings however the operative engine side would still make more lift due to greater airflow. THe extended flap just exacerbates the situation.
 

ready2fly

Well-Known Member
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My understanding is that extended flaps raise Vmc.

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That was my understanding too.

As I stated earlier, I felt that the added drag produced by the flaps would significantly raise the speed at which Vmc could occur.

I understand the Vmc is a "fixed" value... but, that is in certain pre-determined configurations. At least, that's how I understood it.

Is that not the case?
 

Alchemy

Partner, Ally, Friend
The Jepp multi textbook looks like it would be a good item to pick up. I'll definitely look into it even though my checkride is already over (instructor checkride is in the distant future). The Airplane Flying Handbook is just too thin to clarify items in a discussion like this.

To quote my examiner on a few points during the oral:

Examiner: "I say all of these factors will raise the speed at which heading control is lost".

Me: "I have to say I disagree".

Examiner: "You'd be dead wrong."

Me: "The Airplane Flying Handbook, an FAA publication, states that holding up to 5 degrees of bank into the operating engine lowers the speed at which a loss of directional control occurs and improves stall characteristics"

INSERT AWKWARD SILENCE HERE

-------------------------------------------------------

Examiner: "I say 5 degrees of bank will hurt you, they say say 5 degrees of bank will help you. We're both right, now explain to me why."

Me: Uhhhhhh, uhhhhhhhhhhhh give me a second to think about that.....
 

pilot602

If specified, this will replace the title that
Baron -

Think of it this way ... with the flaps extended there is more junk the airplane has to pull through the air in order to yaw to the side of the dead engine. More drag does not necissarily mean the aircraft will loose directional control. It may stall, but dierectional control (one wing/rudder stall) will still be intact. Anything that ads symmetrical drag will help to stabilize (i.e. make it harder for the aircraft to depart its heading/pitch/roll) when one engine is made inoperative.

However, like I said before, once and equilibrium is found after the engine dies the added performance hit of the added drag is probably not worth the "stabilizing" affect it provides.
 

Alchemy

Partner, Ally, Friend
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Was this guy aslo an attorney?????

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Not that I know of, but it sure felt like I was being cross-examined!
 

aloft

New Member
[ QUOTE ]
L : Landing gear (retracted)
C : CG (most unfavorable)
W : Weight (most unfavorable)
B : Bank (no more than 5 degrees)

F : Flaps (retracted)
A : Aircraft trimmed for takeoff
T : takeoff power

C : cowl flaps (i think it is closed, can't remember)
O : Out of ground effect
W : Windmilling prop
S : Standard conditions (temp and pressure)


[/ QUOTE ]"LCWB FAT COWS"??? Sounds suspiciously like USAFA nonsense....
Speaking of which, check out www.eDoDo.org sometime when you're in need of a good laugh.
 

dakovich

Well-Known Member
nah, not USAF stuff. it was something we learned at Panam in Florida. of course i've heard it up north also while doing my CFI training, so i assumed it was a popular way to remember all the criteria. of course you know what making assumptions will do to you

also, i just want to know why some people keep refering to the gear as being part of Keel effect.am i missing something? from my studies and training i never heard or saw the two related. IMHO i believe the gear have nothing to do with keel effect as it is defined. anyone want to elaborate or give opinion on the fact?
 

pilot602

If specified, this will replace the title that
I think it was a slip of the tongue (or fingers).

The keel effect is really nothing more than the reisistance fo the fuselage to yaw (like a keel of a boat in water).

The gear stabilzes yaw through symmetrical drag.

The two are seperate but related in that they both help to dampen unwanted yaw when the critical engine is made inopeative.
 

Jason

Well-Known Member
I've been watching this thread with some interest as I'm preparing to take my MEI ride in a few days (hopefully) and I haven't posted because I wanted to see where it would it would go before I threw ya'll for a loop - I was on a trip in Scotland and wondered into a chain bookstore (Barnes and Noble I think) in downtown Glasgow -there was a book there by a British author about flying light twins with alot of aerodynamics in it. I don't know why I didn't buy it on the spot - I was going to go back and get it as it was based on the BE76 Duchess which is what I'm doing my training in. Anyway - as I was skimming thru it I noticed the aerodynamic explanations of the flaps and gear vs. VMC - I'm not saying this is absolutley correct but this is what it said - regarding flaps, flaps down RAISES VMC because it adds a bunch of drag on the wing with the inoperative engine and a bunch of lift on the wing of the operating engine (due to air flow from the prop) and both of those factors increase the rolling and yawing tendencies towards the inop engine. Regarding the gear - he ran some scenarios with both the gear down and gear up and concluded - thru the use of formulas that I couldn't even begin to understand - that it doesn't make a damn bit of difference. Basically he said that the gear does in fact add a small stabilizing force which helps but it also adds a small amount of drag (this is in the Duchess - depends on the gear size, gear doors etc) and the benefit/cost cancel each other out.

On the flaps I remember that they raise VMC this way - the FAR 23 certification standards states that VMC is determined with the flaps at takeoff position which in the Duchess is 0 degrees - since it has counterrotating props the Duchess has a ridiculously low VMC and if that list of standards (the 12 point laundry list) is met exactly the Duchess will stall before it VMCs about 99.9% of the time and when I was doing my initial multi training in the Duchess back in the day we had to put the flaps down to raise VMC high enough to even get it to lose control before it stalled.

Clear now? ha yeah right.

Jason
 

C650CPT

Well-Known Member
I believe the issue on flaps is not that it raises Vmca, allbeit very little but what it does for you is that it lowers the stall speed, allowing you to "see" Vmca. Remember it is better to run out of directional control (Vmca) and recognize it and recover than it is to stall the airplane with one engine out = single engine stalls BAD. When you stall the airplane with one turning you imediately introduce a yaw at the stall and are going to spin the airplane, of which you are now a test pilot. The manufacturer will want to talk to you if you live through it.

Remember this that during all Multi training we explore static Vmc not Dynamic Vmc, the difference will kill you. When doing a Vmca demo we allow airpspeed to degrade at a rate not to exceed one knot per second, this allows us to identify loss of directional control and still recover. If you recall stories of pilots ending up on thier backs or in the dirt, those were the results of dynamic Vmca where the loss of control exceeded the pilots ability to recognize and or recover from it.
 
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