Vy & Vmc questions

[pilotdude]

1)*Why does Vy decrease with altitude?

Since a lighter plane produces a smaller horizontal component of lift when banked several degrees into the good engine, requiring more rudder travel to correct for sideslip—resulting in a higher Vmc than in a more heavily loaded plane, thus Vmc is tested on lightly loaded planes.

FAR, Sec. 23.149 says that “VMC must be determined with the most unfavorable weight.”*

2)*How, practically, does the FAA determine and load the “most unfavorable weight” for airplane it tests for Vmc? Do they use midget pilots, remote controls, partial fuel, or ghosts?

3)*Is there a quantifiable definition the FAA uses for "most unfavorable weight" among all planes it tests? All the other “specific circumstances” the FAA uses to test Vmc are specific; weight seems to be an ambiguous anomaly.

[airyana]

1)*Why does Vy decrease with altitude?


Become a Pilot With Rod Machado - Learn to Fly Page (http://rodmachado.com/Articles/Why%20Vx%20and%20Vy%20Converge.html)

[Jumbo Driver]

... or here (http://www.rodmachado.com/Articles/Article%20of%20Month/Articles-of-Month/Vx%20and%20Vy/Vx%20&%20Vy.html), where the same question is answered slightly differently ...


JD
:)

[john_tullamarine]

“VMC must be determined with the most unfavorable weight.”
How, practically, does the FAA determine and load the “most unfavorable weight” for airplane it tests for Vmc?

From AC 23-8 (http://www.faa.gov/regulations_policies/advisory_circulars/index.cfm/go/document.information/documentID/74377) 48 (a) (4)

"Weight and C.G

For rudder limited airplanes with constant aft c.g. limits, the critical loading for VMC testing is most aft c.g. and minimum weight.

Aft c.g. provides the shortest moment arm relative to the rudder thus the least restoring moments with regard to maintaining directional control.

VMC should be determined at the most adverse weight.


Minimum practical test weight is usually the most critical because the beneficial effect of banking into the operating engine is minimized. Light weight is also desirable for VMC testing because the stall speed is reduced."
Do they use midget pilots, remote controls, partial fuel, or ghosts?

Loading will be whatever is reasonably achievable. If the weight is determined to be limiting, there will be a minimum weight declared, usually in the permissible CG envelope. Vmc determination is not a straightforward exercise and usually involves a progressive workup to establish a static Vmc (rather akin to the usual pilot endorsement demonstration exercise) with subsequent dynamic Vmc checks.