Charles,
Please forgive me for taking apart your post piece-by-piece and pointing out your errors.
I'm sorry formulaben, but I don't agree
Formulaben's post is, on the whole, correct. The only bit of his post I disagree with is where he says: "
actual Vmca is constantly changing". I agree with his sentiment in this statement, but his terminology is very slightly wrong. What he means is "
Vmc is constantly changing".
However, if you are going to criticise someone for a minor terminology error, you'd better understand the terminology. You say:
VMCA is a MINIMUM controll speed, i.e. the speed above which you are certain during flight that you can maintain directional controll during an engine failure.To establish this minimum number, the manufacturer takes a worst case scenario, i.e. configuration just after t/o
That is not correct. It is true that Vmca relates to configuration just after take-off, but there is no assumption anywhere (except by you) that this is the worst case scenario. There are sure to be worse cases, and it is not true that you are guaranteed to be able to maintain directional control during an engine failure above Vmca if you are configured in one of these worse cases. I'm being quite picky here, more so than I would normally, but only because your criticism of Formulaben's post was even more picky than I am being now. And a small error in terminology such as Formulaben has made is unlikely to have any impact on his ability to fly a multi-engine aircraft safely, whereas a lack of understanding of what a number given in a POH actually means just might impact safety, albeit it's very unlikely in this case.
the wing with the working engine and the flaps down, would give you more lift than without the flaps. Therefore the speed to maintain direction would theoretically be higher with flaps than without
So what about selecting slightly more than take-off flap? Let's not use landing flap, because landing flap often has a high element of drag - but say 20 degrees of flap? On a type like the Duchess, where no flap is used for take-off (and therefore Vmca is measured with flaps fully up), using 20 degrees of flap would, by your own argument, give more lift on the wing with the working engine, increase the effect of the asymmetry, and increase Vmc to something
above Vmca - hence we have a possible situation (a go-around, maybe?) where a speed
higher than Vmca is needed to maintain control.
To repeat - nowhere is it stated that Vmca is a "worst case" figure, or a figure above which there is a guarantee that you will be able to maintain control. It is a
specific case relating to the take-off configuration, but that's about all.
Therefore the speed to maintain direction would theoretically be higher with flaps than without, therefore the flaps in t/o position requirement.
Although your argument about increased lift and increased Vmc sounds good, and I'm sure it's correct, it's not the full story. Zakka has posted, just a couple of posts above, a quote from the POH of an aircraft which states the oposite - for the type he's discussing, Vmc actually goes
down when flaps are lowered. So a blanket statement about Vmc increasing with flaps is not true in every case. That's why myself, Zakka and others have specifically avoided giving a blanket answer.
I hope you can all live with this explanation, if not, just fire at will
I think I just did!
FFF
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