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

[sixpack]

I think the argument in centered around slightly different questions.

Here's where I think we all agree.
1. Adding flaps increases drag.
2. Adding flaps increases lift.
3. Drag from flaps will stabilize yaw somewhat, thus reducing VMC speed.
4. VMC can happen with full flaps or with zero flaps. The FAA says VMC should be "demonstrated" with take-off flaps. A question about whether or not flaps stabilize an airplane is independent of how the FAA says we should set our flaps when doing the demo.

I think "say_speed" is trying to say that the increase in lift (from flaps) effects the controllability of the airplane. The accellerated slipstream over wing which causes extra lift on the good engine side, will be somewhat enhanced with the flaps down. This of course, would be a rolling tendency. The magnigude of the rolling force (increased roll due to flaps) is related to how much of the slipstream hits the flap area of with wing, and how far out this is from the centerline (moment arm for rolling). <<sayspeed... if there were yaw effects caused by the flaps that you were trying to expain, please chime in>>

As far as the 90/10 ratio goes, I'm not sure how one could measure that. Since bank and yaw are orthogonal, it would be tough to quantatively state how much each adds to the "uncontrollability"

[Alchemy]

Dunno if this has been brought up....


Extending flaps enables the airplane to fly at a lower angle of attack. P-Factor is reduced at a lower angle of attack. Therefore, extending flaps reduces Vmc.

[sixpack]

Hmmmm, interesting point.

So now we have three different effects on Vmc when flaps are extended:
1. Extending flaps reduces Vmc due to drag.
2. Extending flaps increases Vmc do to slipstream getting extra lift (roll).
3. Extending flaps reduces or increase Vmc because P-factor changes center of thrust (COT).
-----------a. reduces Vmc when operating on right-engine of conventional because p-factor moves COT inward
-----------b. increases Vmc when on counter-rotating twin because p-factor moves COT outward.

[say_speed]

Allright,
I am not saying that flaps decrease or increase vmc; this is airplane specific, and flap setting specific. Sure, flaps on take-off position add drag, therefore increase latteral stability, but lift is mostly created; I am trying to point out that lift being mostly added, we need to talk about the effect of lift on vmc. From my personal experience as an instructor, I found out that flaps set for take-off position reduce vmc, but more flaps increase vmc. The addition of drag (and latteral stabilizing effect) is overcome by lack of power and slower speed. I am talking about PA34-200 and BE76.
As for the 10% 90%, the answer lies in the equation of lift. But to make it simple, the reason why you use flap around 10 degres for take-off and not 30 or 40, is because 30 or 40 create more drag than lift, and would increase take-off roll.
Anyways, great discussion.

[sixpack]

[ QUOTE ]
From my personal experience as an instructor... ... but more flaps increase vmc. The addition of drag (and latteral stabilizing effect) is overcome by lack of power and slower speed. I am talking about PA34-200 and BE76.

[/ QUOTE ]
If I understand you correctly, you're saying that when you increased flaps (e.g. to full flaps) you've experienced a higher vmc (loss of control at a higher speed).

When the airplane finally could not be controlled, what was the limiting factor? No rudder authority? No aileron authority? No pitch control? Stall?

Which one of these was the first to reach its limit?

[say_speed]

I let my student fly the a/c, I would put my foot under the rudder to simulate runing out of it. The ailerons were used to maintain 5 degres of bank, so you don't run out of those; there was no stall, just the typical yaw.
I was also confirmed the same results by a local de... I recognize that my method isn't the most realistic one (since you don't run out of rudder authority, I keep always a safety margin...), but this is what we found. But my same foot was under the same rudder, so I would simulate the same rudder deflexion.

[sixpack]

[ QUOTE ]
...just the typical yaw...

[/ QUOTE ] Did the full-flaps cause your higher Vmc, or did the blocked rudder.

If you compare Vmc with (a) take-off flaps versus (b) full-flaps, and don't change any other parameter what happen?

If you're saying Vmc speed increases with full-flaps and reaches it limit due to yaw, then I just don't believe it. I've never observed this in an airplane, and it does not make sense from a technical standpoint.

Can you explain how this would be possible?

[Soonermurph]

People wiser than us have concluded that flaps up/gear up increases Vmc speed. Sixpack is right.