Vmc and Flaps?

[Kenny G]

Does anyone have a really good explanation for WHY, when flaps go down Vmc also goes down?

Any explanation would be great.

[FlyYukon]

Because flaps increase lift? Although, dont forget, once flaps also increase drag, so at a point, you'll loose the lift the flaps created and down you go!

Did I make sence? lol

[captainc]

When you extend the flaps you increase the parasite drag and the induced drag, making it more difficult for the operative engine to yaw the aircraft into the failed engine due to the extra drag. You have essentially created a heavier aircraft as far as the operative engine is concerned.

Hope that helped. Are you currently doing your Multi rating Kenny G?

Later

Captainc

[FlyYukon]

Thanks captainc ..

I'm still learning as well, no multi here. I have a question on your answer however, wouldnt the drag be equal on each side, negating each other, causing the aircraft to still yaw with an inop. engine regardless of flap setting? If you 'make the aircraft heavier' wouldnt that increase Vmc?

I was just talking to a pilot, and he said that for Vmc, when you lower the flaps, it moves the centre of gravity forward making it earier to control the aircraft.. essentialy as I sais, increasing the lift. That goes the same for the stall speed, deploying the flaps increases the camber of the wing and produces more lift.. as said to aq piont where drag exceeds the lift.

Cheers!

[Blakey]

On the C130, when you select flap, the rudder boost pressure increases. This allows for more rudder deflection given the same pedal pressure. Not the same on all aircraft for sure but it is for all large aircraft that have boosted control systems. In truth, the reason for this is the reverse to your VMCA question. The rudder pressure is reduced when the flaps are retracted in order to prevent overstressing the aircraft in cruise with too large a rudder deflection. Does have the effect you mention on VMCA though.

[Spokes]

When you extend the flaps you increase the parasite drag and the induced drag, making it more difficult for the operative engine to yaw the aircraft into the failed engine due to the extra drag. You have essentially created a heavier aircraft as far as the operative engine is concerned.

Hope that helped. Are you currently doing your Multi rating Kenny G?

Later

Captainc

Not quite. Extending flaps reduces induced drag. You fly a lower angle of attack to maintain the same amount of lift. This moves the stagnation point more forward on the airfoil, which reduce the intensity (is that the right word?) of the vortecies. This means less downwash, and consequantly less induced drag. All very simple.

[Spokes]

I'll take a stab at this. I am assuming you are talking about the ability of the rudder to maintain directional control of a single engined aircraft at low speed - i.e high nose attitudes.

One of the sources of YAW in this situation is the asymetric thrust of the propeller which at high nose attitudes is turned up from the direction the aircraft is going. At high nose attitudes, the prop produces more thrust from one side, normally the right, than the other. this creates left YAW, which must be overcome with rudder. At too low of a speed, there may not be enough rudder authority to control this (i.e not enough airflow over the rudder). Assuming you are still above stall speed.

When flaps are extended, we can maintain S&L with the nose attitude alot lower. I suppose this would reduce asymetric thrust of the prop, and as a result put less demand on the rudder to maintain directional control. So we can fly slower than without flaps and still go in a straight line.

My guess anyways. This does not take slipstream into account. With flaps the extra drag will require extra thrust. This will counter directional control gains made by extending flaps. (Unless in both cases we are near full power I suppose).

[captainc]

Well, Fly Yukon and others when you increase the weight of the aircraft Vmc becomes lower. Why is this you say? because with a heavier aircraft the operating engine has to work harder to yaw the aircraft into the inop. engine. Therefore requiring less rudder authority to counteract the yaw, meaning you can fly a slower speed and still maintain control of the aircraft.

When establishing Vmc for a particular aircraft the manufacturers have certain guidelines that need to be followed to allow the aircraft to be certified aircraft. These include...

8. Minimum Control Speed (Vmc) - the minimum flight speed at which it is possible to retain control of the aeroplane and maintain straight flight, with maximum rudder deflection and not more than 5 degrees of bank, following sudden failure of the critical engine.

NOTE: (Vmc) for an aeroplane type is generally determined under the following conditions:

1. all engines developing maximum rated power at the time of critical engine failure;

2. the aeroplane is at a minimum practical test weight with a rearmost centre of gravity;

3. landing gear retracted, flaps in take-off position and the propeller of the failed critical engine windmilling.

Now these conditions are set out to determine Vmc for the aircraft under the worst possible conditions. If for example you moved the centre of gravity forward Vmc would decrease and so on with all the things listed above. There are many factors that affect Vmc.

Anyway, just my two cents and hope this clears it up a bit. If anyone else can contribute by all means.

Lets kick the tires and light the fires!!!

Captainc

[mellow_pilot]

I'll take a stab at this. I am assuming you are talking about the ability of the rudder to maintain directional control of a single engined aircraft at low speed - i.e high nose attitudes.

One of the sources of YAW in this situation is the asymetric thrust of the propeller which at high nose attitudes is turned up from the direction the aircraft is going. At high nose attitudes, the prop produces more thrust from one side, normally the right, than the other. this creates left YAW, which must be overcome with rudder. At too low of a speed, there may not be enough rudder authority to control this (i.e not enough airflow over the rudder). Assuming you are still above stall speed.

When flaps are extended, we can maintain S&L with the nose attitude alot lower. I suppose this would reduce asymetric thrust of the prop, and as a result put less demand on the rudder to maintain directional control. So we can fly slower than without flaps and still go in a straight line.

My guess anyways. This does not take slipstream into account. With flaps the extra drag will require extra thrust. This will counter directional control gains made by extending flaps. (Unless in both cases we are near full power I suppose).

VMCA is the minimum control speed for an engine failure in a multi, so no, it has nothing to do with high AOA in a single.

Here's something I found after a quick search. I haven't really had alot of time to go over it, but it seems pretty good so far.

fergworld.com/cfi/pdf/Multi-engine_Instructor_Quick_Reference.doc

I think this is american too, so just note that....

[mcrit]

There are a few factors at play here. The first is that Vmc is established using 5 degrees of bank into the good engine. This bank produces a horizontal component of the lift vector that helps counter act the turning moment of the operating engine. The second factor is that flaps increase lift for any given airspeed; or you need less airspeed to generate the same amount of lift when the flaps are down. As the overall lift increases so does the horizontal componenent; or by putting flaps down you can get the required lift at a lower airspeed. This is why Vcm goes down as flaps go down.

[Spokes]

VMCA is the minimum control speed for an engine failure in a multi, so no, it has nothing to do with high AOA in a single.

Here's something I found after a quick search. I haven't really had alot of time to go over it, but it seems pretty good so far.

fergworld.com/cfi/pdf/Multi-engine_Instructor_Quick_Reference.doc

I think this is american too, so just note that....

Yes, I understand Vmc for a multi engined aircraft. I have also heard it used with reference to single engined aircraft. In theory it is possible for a single to have a Vmc because there are asymetric thrust situations a single can be in.

Since this guy is doing a PPL, I assumed (silly me) that he was asking about Vmc as it relates to singles. In a multi it is kinda a no brainer.

[Kenny G]

Sorry, my bad, I agree with Spokes there is a Vmc for single.... but I was looking for Vmc on a multi engine aircraft.

mcrit I think you nailed it... I like your logic

Notice how many different views we had in this form?

See the problem I had when trying to explain it?

And no I am not a multi student. I am a copilot with an unfinished argument against my captain.

Thanks for your help

[Spokes]

If you are in level flight, isn't your total lift the same regardless of flap settings- i.e your weight?

[mellow_pilot]

Wing loading might change depending on the type of flaps, no?

[ahramin]

If you are in level flight, isn't your total lift the same regardless of flap settings- i.e your weight?

Yes but the speed is different for that same amount of lift.

[FlyYukon]

And the stall spedd is lower with flaps extended.

[co-joe]

Yes Vso is lower than Vs. This is kind of a toughie. In the plane I fly, Vmc is 94 with flaps up and 93 with flaps apch. I never thought to ask why so I'd have to guess. Here goes;

Mellow's comment about wing loading could be a factor since the flaps on the a/c I fly are slotted fowler which in addition to increasing the camber of the wing, also increase its surface area. Less poundage per square foot would be less lift required but Vmc is more of a factor of rudder affectiveness than wing lift required.

Mcrit's explanation makes a lot of sense to me so give it some thought.

Spokes idea about P factor also makes sense if you really think about it. Less AOA is required with flap down, so in effect there would be less of a turning moment, and consequently less rudder deflection for a given airspeed.

The discussion about C of G moving is completely false. Fly Yukon your friend lied to you. The formula for C of G is Weight times Arm is moment. No mention of flap setting. But what they might have been after is "centre of pressure".

Someone else alluded to this calling it "the stagnation point" I think. I can't seem to wrap my noodle around how that would effect Vmc but the C of P does move when flaps are selected. Flap down, moves if forward I think but don't quote me. But rudder leverage is Force times Distance (to the C of G, which hasn't moved).

The key is to look at the definition of Vmc. It's the lowest airspeed at which the a/c is controllable (with the use of rudder) in the even the critical engine fails and auto feathers (if installed), and the good engine is at take off power, gear and flaps in t/o position, aft C of G, gross weight, ISA. Aaaaahhhhhh

Now say that without a breath.

So the answer to your question is no I don't have a really good answer. I don't even have moderately good one. But the clues are here somewhere. Good luck man, and your textbooks are a lot newer than mine so please get back to me with the correct answer.

[FlyYukon]

No mention of flap setting. But what they might have been after is "centre of pressure".

Yeah, that makes more sence.. thanks! Once I start my Multi trainining, I will try to remembe to ask about this!

FY

[Spokes]

Could it be something as simple as no flaps = highr nose attitude so there is less airflow over the rudder because the bulk of the fuselage is blocking the airflow getting to the rudder? Just a thought, but don't really know.

[mellow_pilot]

It is truly a complex question. Nose attitude, lift, drag, power required, wing loading (I mentioned it because it is related to induced drag), increased downwash, p-factor, center of pressure...

Good god! I'm more confused than when we started.

The answer is, "because I said so, that's why!"

[Kenny G]

I am happy there is so much disscusion.

I have looked in text books and could not find anything.

mcrit, yours is the answer I am going to run with... it seems that makes more sense especialy if you are in the 5 degree bank...

Thanks for your input guys...