Turbulence Risk versus a/c weight
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Turbulence Risk versus a/c weight
I was under the impression that although turbulence feels worse at light weights it was potentially less damaging to the airframe and vice versa.
Is this true and can anyone supply the aerodynamical/physics reason(s) why turbulence is more hazardous at higher weights?
Note:- Am not referring to buffet boundaries with respect to weight versus altitude.
Thanks for any help
Is this true and can anyone supply the aerodynamical/physics reason(s) why turbulence is more hazardous at higher weights?
Note:- Am not referring to buffet boundaries with respect to weight versus altitude.
Thanks for any help
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At a high gross weight the overall accelerations on the airframe will be lower for a given amount of lift (due to hamfistedness, turbulence or both), meaning there will be less strain on bits and pieces holding heavy items such as engines, batteries and pax destined for cruise ships.
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There are two competing effects.
If you consider entry to the kind of "design gust" called for by design standards, then you are considering a rapidly changing vertical component of aircraft velocity with respect to the air mass. if for now we assume the aircraft velocity with respect the the ground to be constant, we get a rapid change in angle of attack, and hence lift. Since a given amount of lift will accelerate a lighter aircraft more, it follows that the lighter aircraft sees more "g" and thus the components also see more accelerations. Since the components are of fixed weight, it follows that their fittings see higher loads at higher "g" - as mentioned by ft.
But consider those components which are varying with weight, and those fittings which take loads associated with the aircraft mass, not component masses - things like wing-to-fuse fittings. In this case, although the "g" has gone up the associated weight is lower, and these cancel. Which would tend to indicate that weight is not a factor in loading of major airframe components in turbulence.
but
We made an assumption that "the aircraft velocity with respect the the ground [will] be constant". The reality is that this is not the case. Gusts aren't instantaneous, they take time to build to peak. So the aircraft has time to respond a bit to the gust, which tends to reduce the effect of the peak gust disturbance. Heavier aircraft, however, possess more inertia, and so respond slower, relieving less of the load. Thus the actual peak load on a heavy aircraft is higher, and thus the airframe level loads are actually higher than for a light aircraft, although the peak 'g' is still less.
Therefore, the effect of turbulence is:
* better ride quality for havier aircraft (lower peak 'g')
* less component loading for a heavy aircraft (lower peak 'g')
* worse airframe loads for a heavy aircraft ('g' times mass is higher)
With all the usual caveats about specific aircraft characteristics, of course ....
If you consider entry to the kind of "design gust" called for by design standards, then you are considering a rapidly changing vertical component of aircraft velocity with respect to the air mass. if for now we assume the aircraft velocity with respect the the ground to be constant, we get a rapid change in angle of attack, and hence lift. Since a given amount of lift will accelerate a lighter aircraft more, it follows that the lighter aircraft sees more "g" and thus the components also see more accelerations. Since the components are of fixed weight, it follows that their fittings see higher loads at higher "g" - as mentioned by ft.
But consider those components which are varying with weight, and those fittings which take loads associated with the aircraft mass, not component masses - things like wing-to-fuse fittings. In this case, although the "g" has gone up the associated weight is lower, and these cancel. Which would tend to indicate that weight is not a factor in loading of major airframe components in turbulence.
but
We made an assumption that "the aircraft velocity with respect the the ground [will] be constant". The reality is that this is not the case. Gusts aren't instantaneous, they take time to build to peak. So the aircraft has time to respond a bit to the gust, which tends to reduce the effect of the peak gust disturbance. Heavier aircraft, however, possess more inertia, and so respond slower, relieving less of the load. Thus the actual peak load on a heavy aircraft is higher, and thus the airframe level loads are actually higher than for a light aircraft, although the peak 'g' is still less.
Therefore, the effect of turbulence is:
* better ride quality for havier aircraft (lower peak 'g')
* less component loading for a heavy aircraft (lower peak 'g')
* worse airframe loads for a heavy aircraft ('g' times mass is higher)
With all the usual caveats about specific aircraft characteristics, of course ....
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Isn't the relationship between weight and angle of attack at any given EAS also a factor?
Let's suppose the lighter aircraft has 4 degees of alpha and a heavier one has 8 degrees. If they both hit an instantaneous upgust that increases alpha by 2 gegrees, the light aircraft experiences 1.5 g (due to 6 deg / 4 deg) while the heavy one only experiences 1.25 (due to 10 deg / 8 deg).
All of the above ignores the effects of different inertias of course and assumes that zero lift alpha = zero degrees (to keep the arithmetic simple).
Let's suppose the lighter aircraft has 4 degees of alpha and a heavier one has 8 degrees. If they both hit an instantaneous upgust that increases alpha by 2 gegrees, the light aircraft experiences 1.5 g (due to 6 deg / 4 deg) while the heavy one only experiences 1.25 (due to 10 deg / 8 deg).
All of the above ignores the effects of different inertias of course and assumes that zero lift alpha = zero degrees (to keep the arithmetic simple).
Last edited by Keith.Williams.; 2nd Nov 2008 at 18:47.
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Isn't the relationship between weight and angle of attack at any given EAS also a factor?
Let's suppose the lighter aircraft has 4 degees of alpha and a heavier one has 8 degrees. If they both hit an instantaneous upgust that increases alpha by 2 gegrees, the light aircraft experiences 1.5 g (due to 6 deg / 4 deg) while the heavy one only experiences 1.25 (due to 10 deg / 8 deg).
All of the above ignores the effects of different inertias of course and assumes that zero lift alpha = zero degrees (to keep the arithmetic simple).
Let's suppose the lighter aircraft has 4 degees of alpha and a heavier one has 8 degrees. If they both hit an instantaneous upgust that increases alpha by 2 gegrees, the light aircraft experiences 1.5 g (due to 6 deg / 4 deg) while the heavy one only experiences 1.25 (due to 10 deg / 8 deg).
All of the above ignores the effects of different inertias of course and assumes that zero lift alpha = zero degrees (to keep the arithmetic simple).
If you really do have zero lift alpha=0 deg (unlikely, too) then if one aircraft is at twice the AoA of the other for level flight, then it weighs twice as much. 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. And that factor can be expressed independent of the trimmed alpha or the zero lift alpha; indeed, all that matters is that the delta alpha due to the gust remains on the linear lift0curve slope for both the heavy and light aircraft.
That is, if you assume that the gust creates a delta alpha on both a/c, creating a delta lift due to gust of G for both, then the total lift on the light aircraft is w+G and on the heavy is W+G where w and W are the light and heavy weights. Diving by the weight of each, we get accelerations of 1+G/w or 1+G/W respectively. It doesn't matter what the trimmed alpha or zero lift alpha is.
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Originally Posted by Mad(Flt)Scientist
less component loading for a heavy aircraft
thanks
bf
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The effect of gust on an airframe isn't just a straightforward question of weight and aerodynamics. The mass distribution in the airframe is also important. The airframe structural response to a given gust is a function of the strutural mode shapes, which change depending on how the mass is distributed. When loads analysis is done for a new aircraft type, different combinations of weight distributions are analyzed with different gust intensities and wave lengths. The most critical loads for a piece of a structure will occur at a combination where the gust frequency is most exciting the structural mode shape. Not all parts of the aircraft will have critical loads at the same combinations of weight and gust. So, for example, you can imagine two aircraft with same total weight, however one has a lot of cargo and little fuel in the wings, the other little cargo, but a lot of fuel in the wings. Even though the total aircraft weight is the same, the loads produced on the wings will be very different due to the mass of the fuel.
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MFS
Quote
If you really do have zero lift alpha=0 deg (unlikely, too)
Unquote
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.
Quote
then if one aircraft is at twice the AoA of the other for level flight, then it weighs twice as much.
Unquote
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.
Quote
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.
Unqoute
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)
Quote
Heavier aircraft, however, possess more inertia, and so respond slower, relieving less of the load.
Unquote.
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
Quote
If you really do have zero lift alpha=0 deg (unlikely, too)
Unquote
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.
Quote
then if one aircraft is at twice the AoA of the other for level flight, then it weighs twice as much.
Unquote
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.
Quote
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.
Unqoute
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)
Quote
Heavier aircraft, however, possess more inertia, and so respond slower, relieving less of the load.
Unquote.
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.
Psychophysiological entity
So, for example, you can imagine two aircraft with same total weight, however one has a lot of cargo and little fuel in the wings, the other little cargo, but a lot of fuel in the wings. Even though the total aircraft weight is the same, the loads produced on the wings will be very different due to the mass of the fuel.
As a driver, this is what I was intending to say. When looking at the suggested Mmo in rough air, the ratio of the mass in the wings compared to the payload was about all we had to consider.
A higher fuel : payload ratio, gave us that little bit more protection.
