Vmca

[maximus]

I understand that VMCA is calculated with no flaps. I assume it figured with the gear down. Is this corect? also, how much of an increase in Vmc would you have if you deploy the flaps.

Max

 

[Ed Guthrie]

I understand that VMCA is calculated with no flaps. I assume it figured with the gear down. Is this corect?

Part 23 Vmc for 6000# or less aircraft is with flaps at take-off setting. This might be deployed or retracted, depending on the aircraft. Within this category Vmc is determined with gear up.

also, how much of an increase in Vmc would you have if you deploy the flaps

Probably decrease Vmc, actually, as the flaps extended would add drag behind the CG/center of thrust, thus adding directional stability.

The elements within the Vmc requirement of Part 23 are not all destabilizing. Gear up is de-stabilizing. Flaps down is stabilizing. Aft CG is de-stabilizing. Etc.

 

[Richard]

As Ed mentioned, all the criteria used to establish Vmc are not negative. In terms of understanding Vmc or engine out operations it is best to think of it in terms of arms and moments and aerodynamic "wt", or lift.

 

[Ryan Ferguson]

[/size]
The elements within the Vmc requirement of Part 23 are not all destabilizing. Gear up is de-stabilizing. Flaps down is stabilizing. Aft CG is de-stabilizing. Etc.

I have two nits to pick with this statement.

There is nothing conclusive about extended gear being a 'stabilizing' force in OEI ops. Depending on whether the gear extends forward/back, inboard/outboard, the effect on longitudinal/lateral CG, gear/gear door design, aerodynamic effects on control surfaces, etc., extended gear could have a zero, or even negligable effect on stability; fact is, this is simply not an evaluation factor during aircraft certification. We just don't know.

See the April 2003 Designee Update for more information about this. This was a hot topic among DPEs a few years ago. http://av-info.faa.gov/data/designeeupdate/du_apr2003.pdf

Second, aft CG is generally, but not always, de-stabilizing, depending on the aircraft in question. Part 23.149 (Minimum Control Speed) is actually written to read:

23.149(b): "... VMC must be determined with the most unfavorable weight and center of gravity position... "

Which for many light (less than 6,000 lbs.) aircraft is, of course, aft limit CG and max gross weight. But not all.

-Ryan

 

[Ed Guthrie]

I have two nits to pick with this statement.

There is nothing conclusive about extended gear being a 'stabilizing' force in OEI ops.

Ryan has a somewhat valid point in that I assumed tricycle gear with more aerodynamic drag added behind CG versus ahead of CG. As an example, I believe that a Beech 18 might not fit that assumption.

We just don't know.

See the April 2003 Designee Update for more information about this. This was a hot topic among DPEs a few years ago. http://av-info.faa.gov/data/designeeupdate/du_apr2003.pdf

Well, if you read the article you referenced you'll find that the article takes exception to the blanket statement (and the underlying assumption I noted), but it really doesn't say "we don't know" (or more important--we can't predict based on aerodynamic principles). What the article does say is that the blanket statement is not universally true, which I agree is correct--see my Beech 18 comment), and more important, that gear down is not really a viable option due to the performance penalty, which, I believe, is the greater issue.

Second, aft CG is generally, but not always, de-stabilizing, depending on the aircraft in question.

Not saying it isn't phsyically possible that an exception exists, but you'd need to cite the exception before I'll buy this one. CG moving aft will shorten arm to whatever sum horizontal control (rudder) components exist aft of CG, and the sum of those components is aft of CG on every multi-engine airplane I can remember/imagine. As I said, there might be an exception, but I can't picture it right now.