Vmc ?

[flyboyzz1]

Is VMC calculated with Max Gross Weight? I've heard it both ways that it is and it isn't. FAR 23.149 http://www.flightsimaviation.com/data/FARS/part_23-149.html says the most unfavorable weight. That would be less than gross weight but I have many books that says it is based off of gross weight. Whats the deal.
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[Metro752]

the heavier you are, the more lift you need to generate which means your controls become more effective, which will lower Vmc

This is favorable, so a lighter/lightest weight possible with a pilot and minimum fuel to demonstrate Vmc is UNfavorable, because it increases your Vmc.

Aft CG is also unfavorable, because the arm between the aircrafts CG and the rudder is shorter, making the loss of directional (Vmc) control occur at a higher airspeed, than if the CG was closer to the aircraft datum. This is UNfavorable

It is also highly likely I butchered that, and I apologize.

I'm also a big flightsim guy, but I really don't like mixing any flightsim websites with the important stuff. If you need a copy of the FAR/AIM online, you can get it here http://rgl.faa.gov/ = Regulatory and Guidance Library

 

[minitour]

What metro said.

-mini

 

[flyboyzz1]

thanks... didn't see that it was from flight sim just saw the FAR part 23! Thanks though makes sense and just figured out that I was looking at an old copy of the airplane flying handbook that stated that max weight is used but the new copy clears it up...duh..

 

[avbug]

Advisory Circular 23-8B, the Flight Test Guide for Certification of Part 23 Airplanes, provides the following explaination with respecting to test and certification, and Vmca:

a. Background. Section 23.149 requires the minimum control speed to be determined. Section 23.1545(b)(6) requires the airspeed indicator to be marked with a red radial line showing the maximum value of one-engine-inoperative minimum control speed. Section 23.1583(a)(2) requires that VMC be furnished as an airspeed limitation in the AFM. These apply only to multiengine airplanes. A different VMC airspeed will normally result from each approved takeoff flap setting. There are variable factors affecting the minimum control speed. Because of this, VMC should represent the highest minimum airspeed normally expected in service. The variable factors affecting VMC testing include:

(1) Engine Power. VMC will increase as power is increased on the operating engine(s). Engine power characteristics should be known and engine power tolerances should be accounted for.

(2) Propeller of the Inoperative Engine. Windmilling propellers result in a higher VMC than if the propeller is feathered. VMC is normally measured with propeller windmilling unless the propeller is automatically feathered or otherwise driven to a minimum drag position (for example, NTS-System) without requiring pilot action.

(3) Control Position. The value of VMC is directly related to the control surface travel available. Normally, VMC is based on available rudder travel but may, for some airplanes, be based on lateral control travel. For these reasons, VMC tests should be conducted with rudder and lateral (if applicable) controls set at minimum travel. In addition, rudder and lateral control deflections should be adjusted to the minimum production tolerances. If, during VMC tests, control force limits would be exceeded at full deflection, then a lesser deflection should be used so as not to exceed § 23.143 force limits.

(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.

(5) Lateral Loading. The maximum allowable adverse lateral imbalance (fuel, baggage, and so forth) should be maintained.

 

[SPilot]

I remember reading somewhere that they would load the aircraft like this: find the lightest test pilot around, load it up with just enough fuel to fly the test + reserves, and then load up the most aft cargo station with enough weight to move the CG to aft limit.