Vmca and flaps

[italia458]

I'm surprised at the lack of knowledge here and the ridiculous quotes people are using.

Granted I'm bringing the thread back to life years later but the correct Vmc testing standards were published in 1996, years before 2007, so why are incorrect versions of the FARs being quoted?

The current version is: § 23.149 Minimum control speed.

(a) VMCis the calibrated airspeed at which, when the critical engine is suddenly made inoperative, it is possible to maintain control of the airplane with that engine still inoperative, and thereafter maintain straight flight at the same speed with an angle of bank of not more than 5 degrees. The method used to simulate critical engine failure must represent the most critical mode of powerplant failure expected in service with respect to controllability.

(b) VMCfor takeoff must not exceed 1.2 VS1, where VS1is determined at the maximum takeoff weight. VMCmust be determined with the most unfavorable weight and center of gravity position and with the airplane airborne and the ground effect negligible, for the takeoff configuration(s) with—

(1) Maximum available takeoff power initially on each engine;

(2) The airplane trimmed for takeoff;

(3) Flaps in the takeoff position(s);

(4) Landing gear retracted; and

(5) All propeller controls in the recommended takeoff position throughout.

Now, when aircraft were certified they most likely were using previous versions of 23.149 and possibly where it said that it was to be completed at MTOW, among other things. However, that's not the current version and I would be teaching my students the current version as well as letting them know that aircraft that have been certified before the current version date would have been tested differently.

Touching on the point of being the "worst case"... Vmc is determined for a set configuration. Nowhere does it say that it is required to be the worst case for the aircraft. You'd probably be able to pick that up just by looking at the previous versions where they say that it should be at MTOW and the first versions don't even mention weight and then the current one says most unfavourable weight. They can't all be right kids!

As FFF has stated, there are times where actual Vmc would be higher than what the POH states.

The reason I came here was to find some solid evidence about what happens with regard to Vmc when flaps are either retracted or extended. I personally think that there are too many variables to state what will happen as a rule because each airplane is so different. It makes sense that fowler flaps would increase the lift and drag symmetrically along the wings. Both the lift and drag would increase from 0 degrees to full deflection. The lift part coupled with the blown side would create more lift, banking the aircraft towards the dead engine. The opposite aileron used to counteract this would produce drag on the dead wing side which would increase Vmc.

As surprising as this might be, wings produce induced drag and parasite drag. The flaps produce the same. At small deflections the induced drag is in the majority and when increasing towards 40 degrees or even 60 degrees, parasite drag is now a big player. What's the difference between flap at 60 degrees at the trailing edge of the wing or a flap opening up to 60 degrees at the middle bottom of the wing? Both will slow the airplane down due to parasite drag.

Induced drag will decrease with an increase in speed so the induced drag on the blown wing will be lower than the dead wing, increasing Vmc. However, the parasite drag at large flap settings will produce more drag on the blown side than the dead side because of the higher velocity on the blown side. The question is where is the equal point and how much do these factors actually affect the Vmc speeds of the aircraft? That depends on a whole bunch of things. That's my proposition.

It would be good to note that there are multiple factors with regard to single engine flight. Three basic ones I can think of are stability, control and performance. Vmc is only talking about performance. The reason the 5 degree limit is there is so that manufacturers don't "abuse" the rule and state an airplane has a very low Vmc by using 10 degrees of bank into the live engine. The 5 degrees has nothing to do with performance. If you want performance you want zero sideslip which is different between aircraft but as a rule of thumb it's ~2 degrees for non critical engines and ~3 degrees for non critical engines. So, decreasing the bank from 5 degrees will increase the Vmc but will increase the performance, i.e.: climb rate. And what about weight? It's said to decrease Vmc. That's true if the angle of bank remains the same, the side component of lift will be greater which helps oppose the asymmetric thrust. But what if you decreased the bank so as to maintain the same side component of lift? What happens to the zero sideslip? Well, if your Vmc is calculated at 5 degrees of bank, it would go towards the zero sideslip, improving performance. But the extra weight would be a penalty in your climb performance. I think the overriding factor would be the weight, therefore reducing your climb performance but like above, it really depends on the aircraft.

When you're thinking about Vmc, think about the "control" of the airplane. Disregard performance factors such as zero sideslip until you're ready to start thinking about the performance of the airplane.

EDIT: When I'm reviewing material or studying I always try to ask "why" something is the way it is. Or why the regulation states something. Going back to the point of this not being the "worst case" scenario... when is the airplane at full takeoff power, gear retracted and flaps in the takeoff position? Right after takeoff, which is also where you're passing through Vmc at full power. I can't think of any other time in normal flight that you'd be passing Vmc at a high power setting. It's also where an engine failure has a significant chance of happening since you're working the engine quite hard. And "significant" meaning more so than the other phases of flight. I didn't talk to the person who made the regulation but it makes sense to test Vmc in this configuration here.