MFS
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If you really do have zero lift alpha=0 deg (unlikely, too)
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As I said, the main reason I selected zero as the zero lift alpha was to simplify the arithmetic. I did not suggest that this is commonly the case.
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then if one aircraft is at twice the AoA of the other for level flight, then it weighs twice as much.
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This is true only if zero lift alpha is zero. Had I not specified the zero lift alpha you could not have worked out the relative weights.
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So your 6/4 factor and your 10/8 factor are simply an expression of the relative size of the delta lift due to the gust, and the weight of the aircraft.
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No it isn't. It is actually the relative size of the lift in the gust and the lift immediately prior to the gust. Assuming straight and level flight before the gust this ratio is equal to the new load factor, provided the zero lift alpha is zero.
If zero lift alpha was other than zero then we would need
(New alpha - zero lift alpha ) / (old alpha - zero lift alpha)
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Heavier aircraft, however, possess more inertia, and so respond slower, relieving less of the load.
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This is true but deserves closer scrutiny.
If a heavy aircraft and a lighter aircraft experience the same increase in lift due to a sudden gust then the vertical acceleration of the heavier aircraft will certainly be less than that of the lighter one. So the heavier one will experience a greater increase in alpha.
But if both aircraft experience the same increase in load factor then they will both experience the same vertical acceleration. Now before you say " yes but they won't experience the same increase in load factor in a gust", let me assure you that I am aware of that fact. But many readers of this forum and many more students of aerodynamics are not. So it is worth looking into it in more detail.
If we were to put together a lesson plan to cover the subject of gust response to students who had no prior knowldge of the subject it would probably include the following.
1. A brief review of CL/alpha curves, load factor, and the relationship between alpha, dynamic pressure, airspeed and lift.
2. An examination of vector diagrams to show how vertical gusts change angle of attack.
3. An examination of how these changes in angle of atack change both the Cl and the load factor. If we were doing it for the JAR ATPL exams we would need to include the fact that new load factor in the gust = new Cl / Old Cl, because there are a number of examination questions that are based on this.
4. An examination of how changing the gradient of the Cl/alpha curve would change gust response.
5. An examination of how differences in weight and airspeed would change the gust response.
In doing all of this we would find that factors such as trimmed alpha, zero lift alpha and delta alpha were very significant indeed
Last edited by Keith.Williams.; 3rd Nov 2008 at 20:12.