Aerodynamics/Stability question

[fredgrav]

Hello Everyone,
1. Neutral point is defined as the CG position when the sum of the changes in the tail moment and wing moment caused by a gust is zero.
2. Manouevre point is defined instead as the CG position when the tail moment equals the wing moment while manoeuvring.

I guess the main difference stays in the fact that the first one does relate to 1g flight, the second doesn't ... is that right ?

Thanks in advance,
fredgrav

[Microburst2002]

Sort of.

I think it has to do with aerodynamic damping, which gives an "extra" stability margin to the neutral point.

[fredgrav]

Thanks for answering Microburst,
surely it has to do with aerodynamic damping, but couldn't quite understand the why of this further margin for manoeuvre stability ...

[Microburst2002]

I am not very sure, but the I think it is because the stability study for the neutral point considers only the forces and moments that appear instantaneously after a change in angle of attack. The subsequent trend determines it stability or unstability.

But when an airplane manoeuvers (pitches up, for instance) the angular rotation about the lateral axis makes the tail have a vertical downwards velocity. This velocity plus the forward TAS induces an extra angle of attack increment in the tail, which is always stabilizing. Therefore an airplane with its CG at the neutral point still has some stability when manoeuvring.
this damping is less effective as TAS increases, so for the same IAS it is reduced with incresing altitude.

Or something like that...

The experts are around here, they will explain much better when they spot this post

[fredgrav]

Hi guys, finally found an answer !

The neutral-point (NP) is the point around which all the aerodynamic forces are balanced. The stick free neutral point for an aeroplane is the aerodynamic center that the aeroplane would have with no pilot control force if the elevator and its mechanical linkage to the cockpit control had no mass and no friction.

hn = h0 + hs * Vs ( as/aw) [1 - (de / da )]

hn = neutral-point
h0 = aerodynamic center of the wing [typically 0.25]
hs = stabilizer efficiency [typically 0.6] - [0.9 for a T-tail]
Vs = stabilizer volume coefficient
as = lift curve slope of the stabilizer
aw = lift curve slope of the wing
de /da = change in stabilizer downwash angle versus change in wing angle-of-attack (AOA) [typically 0.5 to 0.33]

Beyond NP, there are two types of maneuvering points, the stick-fixed maneuver point and the stick-free maneuver point:
- The stick-fixed maneuver point is defined as the center of gravity position for which no change in the elevator angle (stick travel) is required for a normal acceleration (ng);
- The stick free maneuver point is defined as the center of gravity position for which no change in the hinge moment or stick force is required to maintain a normal acceleration (ng).

Everyone who wants to shed some light on this topic is widely appreciated ...

A pleasant evening,
fredgrav