PPRuNe Forums - View Single Post - CG location effect on asymmetric yaw
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Old 29th Dec 2020, 18:49
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john_tullamarine
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It seems to me that fwd lateral CG should reduce Vmcg coz of the rudder's longer effective lever arm.

Absolutely, but this was excluded by the OP in the question. Additionally, we need to keep in mind that, while the effective Vmcg on the day will vary, the certification Vmcg, which is the basis for the AFM stuff, is a single number representing a reasonably worst case scenario.

Think engine trim, density altitude inaccuracies, tight wheel bearings, slight brake drag, slightly underinflated tyres.

Concur, but these aren't really relevant in a simple comparison as one wouldn't expect any of them to change, say, from one takeoff to the next in a short time frame ?

On the ground more weight on the nose wheel will reduce the swing of an engine out, is that what they meant?

Indeed, which led to my including that caveat.. As safetypee observes, there is a disjoint between the typical real world day (which should be conservative to some extent) and certification practice.

However, for real world situations, the load on the nosewheel is a small fraction of the aircraft's gross weight so a small change in load should result in a much smaller change in tyre/surface coefficient which, I suspect, will tend to get lost a bit in the face of other, larger forces. Safetypee, being of the FT persuasion, probably is better placed than I to speak to that consideration.

Caveat - the nosewheel load (and reject braking) goes up significantly in the case of nosewheel brakes (thinking B727).

I was thinking surely if the CG was say over the wings (very close to underslung engines) and if it was way AFT by the exit door (for argument's sake) that these 2 varying distances from the engine would have some difference in the resulting 'swing' upon failure.

A common misconception. We are thinking about moments and the consideration is the perpendicular distance to the force being considered in respect of the moment. The picture in the following link may help a little Moment of a Force | Engineering Mechanics Review at MATHalino As you observe, you found a reference which said the same thing. Probably a bit simplistic to suggest that the thrust offset is the only consideration but, certainly, it is the main driver, I suggest.
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