Neutral Point and Static Margin PDF

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AERO309 Neut r alPoi ntandSt at i cMar gi n In this video we are going to discuss the neutral point and the static margin. Two very important aspects of longitudinal stability control. This is continuing discussion longitudinal stability. So, if you remember a wing or any aerodynamic surface as a Hydrodynamics Center. We also know the Air Force has an aerodynamic center in this at the. So, at the urging of center wing as it was or where moment, pitching moment does not change with the angle of attack or. Well an aircraft also has a point where the pitching moment does not change. For an airplane. We call this point, Neutral points. Just head off to constantly right adults use NP. And not just in terms of the equation that means the derivative of some pitching moment or gravity with respect to the angle attack what equals zero. That means there is no change of the center pitching moment or gravity with respect to the angle of attack. So, it is the same definition I would be for Aerodynamic center of a wing or airfoil or an airplane. We just call this aerodynamic center the neutral point. Just reiterate change of the pitching moment with at center gravity with respect to ankle tac zero at the neutral point. Now this neutral point is a physical location. The location of the neutral point which we are going to say the non-dimensional location given as each sub in is based on aerodynamics. It all has to do with the relative sizes of the wing and the tail and their location with respect to each other. So, base. Now, so the location point is some physical location long airplane. The relative location or, yeah between the center of gravity and the neutral point can tell us about stability. Let us go a little further into that. If ZG is ahead of the neutral point. In other words, more towards the nose of the airplane. The C.G. is closer to the nose and the neutral point or H which remember is the distance to the dimensional distance to the C.G. is that distance is less than the distance to the neutral point. Then we have, a stable airplane. In pitch, if the C.G. is behind, the neutral point, the distance to the center of gravity is greater than the distance to the neutral point. Then, Unstable, ditch now. If the C.G. is at the neutral point of the words which equals. H.A. Then, mutual stability, mutually stable. And we can see that in terms of the Pitching moment coefficient plotted against angle of attack. If the C.G. is ahead of the neutral point, we have the stable line here. So, a negative slope. If the pitching moment or if it is the center gravity is behind the neutral point. We have a positive slope. So unstable. And it is a center gravity is at the neutral point. We have a neutral though slope of zero. Now the relative location is important but also the distance. Between the two important and specifically the distance between the center gravity and Neutral point is called the static margin. And I will abbreviate that as sum. So, the static margin is the distance, nondimensional distance to the neutral point minus the non-dimensional distance to the center gravity. This is non mentionable. Well the words it is in fractions of callings. So, because of what we found up no up here. If the center of gravity is ahead of the Neutral point, then needs a smaller than Hn. That means if you subtract the distances and her gravity from the distance to the Neutral point you end up with this static margin greater than zero and therefore stable means C.G. ahead of mutual. Now if you have a static margin that is negative that is unstable. And that means exactly what appear. C.G. behind neutral point. And if c.m is zero that means we are neutral. And that met the C.G. is at the neutral point. So, for stability we need positive static margins which means we need center gravity ahead of the neutral one. So that is using the information we have on these neutral points, so it is location with respect to us in her gravity. If we know those two things we can determine if the aircraft will be stable in pitch....