2nd Nov 2002, 12:26
Just a quick question for those with an aerodynamic mind.
How does Vmca change with CofG position??
I realise that any change within certified Cof G limits will be very small, but someone asked me and I was'nt sure of the answer.
I think that Vmca would be lower with a fwd CofG, due to less yawing moment with an engine failure .
:confused: :confused: :confused: :confused:
Genghis the Engineer
2nd Nov 2002, 13:35
I would anticipate that Vmca would be lower at further fwd CG, since the directional stability should be slightly greater, aiding the pilots efforts to keep within sideslip and thus bank angle limits.
However, it will be certified at the least safe condition and I doubt that anybody would give different values for different CG positions.
2nd Nov 2002, 18:54
John Denker has an excellent section (http://www.monmouth.com/~jsd/how/htm/multi.html#sec-1out-cm-effect) on this, debunking some of the misconceptions about it.
Mad (Flt) Scientist
2nd Nov 2002, 19:16
However, it (Vmca) will be certified at the least safe condition and I doubt that anybody would give different values for different CG positions.
You're not permitted to quote VMCA as other than a single value or (optionally) as a dependency upon altitude and/or temperature (to incorporate reduced thrust effects).
2nd Nov 2002, 23:40
... but it would be unwise to fall into the trap of thinking that CG does not influence the "real" Vmca on the day .. which varies with a range of parameters, as opposed to the book value.
If the aircraft is at light weight and the takeoff speed schedules are, or are near, Vmca limited, then the effect of the CG's being on the forward or aft limit can be to alter the pilot's handling problems from being in charge of a sprited pony ... to blinking and finding himself rolling rapidly and being caught out by the bank-related increase in Vmca and ... away we go on a Vmca departure ... always opens new eyes in the simulator ...
12th Nov 2002, 20:39
Aft limit C of G = less rudder control, so higher speed required to maintain directional stability. I'm fairly certain that this is the worst case which would be the ONLY value given in the aircraft manual.
Generally all FAA references of VMCA stipulate 'most rearward c.of.g' for the highest speed limit.:eek:
Mad (Flt) Scientist
15th Nov 2002, 00:27
The actual wording is:
The most unfavorable center of gravity;
which, after making allowance for the usual spelling abominations, does not necessarily require the most aft c.g. position.
One example might be an aircraft with a relatively narrow c.g. range at light weights, and a more aft limit at heavier weights. In this case it is possible that the Vmca which would be determined at the most aft c.g. would not be as high as that for the lightest weight, and most aft c.g. for that weight.