OKDECS

I'd like to see if anyone can give me some clarification on some items that impact Vmc

1) When flaps are extended, Vmc drops. This is usually explained in some simplistic terms like "the flaps have a stabilizing effect" which to me is a little too simplistic. My assumption is that there are two things going on. First, the increased lift from the flaps at a given airspeed is contributing to the horizontal component of lift, thus freeing up some more rudder authority. Second, the accelerated slipstream from the operating engine is creating more asymmetric drag and thus countering the yaw towards the dead engine. One problem I see with the latter part is that the accelerated airflow also increases asymmetric lift on the operating engine side, which is increasing the tendency to bank towards the dead engine...so can anyone clear this up for me?

2) When the gear is extended, Vmc drops (usually). There are two explanations - one is pretty straight forward (CG). I understand that if the CG is farther forward with the gear down (mostly depends on which direction the nosegear extends) there's a longer arm from the CG to the rudder, therefore increasing effectiveness. However, the whole "keel effect" explanation confuses me...again because of the "stabilizing effect" simplification. I again default back to the accelerated slipstream in explaining this; if the mains are behind the engine, then the accelerated airflow from the operating engine increases drag on that side, which will aid in directional control. Any insight on whether this is right/wrong?

That's it for now...thanks!

Cubdriver

Flaps serve to degrade the slipstream quality behind the operating engine, thus and reducing its efficiency and thrust. This in turn allows a lower VMC number. If the air behind an engine is clean, it generates more thrust. Both "keel effect" and slipstream quality are factors in VMC performance. Yes there is some additional lift on the operating engine side from flap deployment, but obviously the other factors outweigh and when you think about it that additional, asymmetrical lift on the operating engine side is also there with the flaps up. In addition, higher slipstream speeds means more induced drag on the flap behind the operating engine, so there are a bunch of things to consider.

All that is correct but you are missing the reduction of thrust explained above. The gear hangs down in the slipstream and this reduces the quality of the air there in terms of using it to generate thrust. It is more turbulent. In turn, less thrust means a lower VMC. I will try and come up with a common sense analogy for this and for the keel effect, but I have to run out for a bit.

clipperskipper

Sure and a higher gross weight and gear extended will also decrease Vmc, the gear giving
you the 'keel effect' you touched upon. Good explanation btw.

rickair7777

The keel effect of the flaps is real but is not a steady-state effect. It will help to slightly reduce the yaw rate while yaw is occurring. Gives slightly more time to react.

Cubdriver

I got to talking this over with Herb today and he was curious about the difference having flaps or gear down has on VMC so we flew some test flights in his Apache. We got an 8 knot decrease in VMC with flaps down, a pretty good bit. We found about a 4 knot decrease in VMC with the gear down only. This for an Apache but you can pretty well extend this situation to any propeller airplane in a similar configuration. Some exceptions might be if the gear is not located behind the operating engine, and in that case the difference would be negligible. I can't think of any twin trainer props like that but there are a few military props that house the wheels inside of fuselage blisters that might qualify. Also, any flap setup not located within the propeller slipstream would of course not offer as much additional drag and turbulence as one where the flaps are located behind the operating engine. I can't think of a single airplane that fits that description right off hand. Extra points if you can.

AKASHA

1) Flaps Extended - The simple reason Vmc decreases with flaps extended is because a greater component of horizontal lift to counteract yaw. There may be contributing factors, but they are secondary.

2) Gear - " Vmc does not necessarily decrease, and control does not necessarily improve with the landing gear in the extended position. The ‘stabilizing effect’ of landing gear on individual aircraft is not tested. Vmca is determined with the gear in the retracted position, and therefore the extended gear’s effect on Vmca is unknown and would vary between different airplanes and gear systems." - From a Multi Engine Instructor Document

plasticpi

I'm not sure I follow your flaps argument, maybe I'm just misunderstanding. Assuming level, unaccelerated flight, there is no more lift created with flaps than without, in both situations, lift equals weight. That point aside, I still don't see how increased lift (either vertical or horizontal components) would have anything to do with controllability with asymmetrical thrust. It seems to me the only effect flaps would have on Vmc is due to decreasing the effective thrust on the operating engine.

If lift had anything to do with it, you would be able to say that pulling up would also decrease Vmc, and I can't come up with anything to support that.

AKASHA

Don't think in terms of level flight. Its true Vmc is not affected by weight in level flight. Vmc is only affected by the weight of an airplane in a bank... in this case 3-5 degrees. Try this....

"When an airplane is banked, a component of the aircraft weight acts along with the horizontal component of lift to create a more effective sideslip towards the operating engine. So for a given bank, the heavier the airplane, the lower the aircraft's Vmc."

Cubdriver

Just as the above explanations has it, my understanding is that dropping flaps adds to the available lift that can be tapped for adding to the rudder just as a higher gross weight does this as well. The idea some people miss is that slipstream quality behind the operating engine is also affected which reduces the thrust on the operating engine and helps reduce VMC that way. I would say the latter factor is the stronger but I doubt anyone knows for sure. You would have to rig out the airplane with a ton of test equipment to really nail it down and no one cares that much.

The OP seemed to be saying in his post that flaps may add to the problem rather than helping it, which I toyed with for a while but dismissed. Some theory I can cook up if it were true, and I do not think it is true, is that having flaps down produces an unequal extra lift factor on the operating side. The problem with this idea is that while some extra lift does happen on the operating side, the induced drag from the flap also increases exponentially with the extra lift, and I think this would be a negating effect.

Singlecoil

Adding flaps and lowering the gear both increase drag, which acts opposite to thrust. We all know that reducing thrust on the operating engine decreases Vmc, adding drag is essentially the same thing as reducing thrust.