Determinig Aircraft comfort

Full Flaps! · 05-31-2003, 12:29 PM

I once read that by knowing an aircraft’s wing loading, the total weight of the aircraft, and some other factor, you can determine how smooth a ride would be for that particular airplane; Also you can compare between two airplanes to see which would have the best, and smoothest flight.

Does any body knows that formula?

ZX1100F1 · 05-31-2003, 01:44 PM

I once read that avoiding rough air would allow for a smooth ride.

Scott · 05-31-2003, 06:32 PM

Keeping a safe distance behind "heavy" aircraft is a bonus. And stay away from the cumulogranite!

richie154 · 05-31-2003, 07:46 PM

Quote:

Originally posted by Scott
Keeping a safe distance behind "heavy" aircraft is a bonus. And stay away from the cumulogranite!

a safe distance behind any other plane is also good! i was on an AA 752, JFK-SJC. maybe 7-10 minutes after takeoff a loud thud went thru the cabin; my dad and i instantly looked outside the a/c to see if anything had happened. the Captain came over the intercom and said we flew through a 727s (i think...) jetwash. that scared the hell outta me!

Phugoid · 05-31-2003, 07:51 PM

This is the formula for a change in g loading caused by a vertical gust.

ZX1100F1 · 05-31-2003, 08:21 PM

Pardon me Phugoid,
But how does this elementary (and ambiguous) G-load formula help to answer the original question?
Which is in and of it's self an impossible question. There are so many other factors (size of aircraft, velocity of turbulence, direction of turbulence, angle at which it strikes it strikes aircraft, were it strikes aircraft, etc, etc, etc) to be taken into consideration, and then there’s the fact that; not all of the bumps you feel are transmitted into the passenger compartment via the wings.

Full Flaps! Is sitting back right now laughing his @ss off because some Pollock is trying to piss in the corner of a round room.

I for one am getting tired of seeing some hyper technical chart or explanation that is mostly irrelevant, if you have something to add, do so in a manner conducive to normal standard human comprehension and relevancy. Unless (of course) you were only humoring him with your formula, in which case, carry on.

Phugoid · 05-31-2003, 11:54 PM

For those who have not attended an aircraft structures class, allow me to briefly explain flight-vehicle imposed loads. Turbulence is felt as a change in the g-force directed on the aircraft and occupants, hence when delta G is large, the turbulence effects will be acutely felt, and by extension, when delta G is small a smooth ride is not only perceived, but achieved.

Plugging in the known parameters of the slope of the lift curve, the density ratio of the air (an expression of altitude), the equivalent air speed, the magnitude of the gust, divided by the wing loading one arrives at the change in g-loading imposed by the turbulence. In deference to the mathematically challenged, a bigger number on the bottom means a smaller number is the result. It is therefore seen, from the equation, that a high wing loading translates into a smoother flight.

KU is often determined as a single value from accelerometers in the aircraft. When K = 1 the gust is instantaneous. A nominal value for K is 0.6.

If one wishes to compare different aircraft in the same turbulence, simply keep KU, Ve, and sigma constant. You can then get a workable, though qualitative, sense of which aircraft give a smoother ride in turbulence – everything else being equal (which in reality never occurs). An aircraft with a shallow slope to the CsubL curve and a high wing loading will provide a smoother ride.

There is a major caveat to this, which is wing bending. Semi-elastic wings will absorb some of the turbulence experienced by the wing. Even fairly rigid wings, such as the 727, will flex and relieve g-loading on the fuselage. This equation then is the starting point in determining the VsubB (turbulent penetration) speed of an aircraft. Though it appears elementary, the physics behind the equation is quite solid and can be found in most upper level aircraft structures texts.

Full Flaps! · 06-01-2003, 07:07 AM

Phugoid

Thanks for that formula

Let me write this down, try to understand it; Because I would like to make you one more question about this subject.

Good to have you in the forum.

ZX1100F1 · 06-03-2003, 06:31 PM

Quote:

Originally posted by Phugoid
For those who have not attended an aircraft structures class, allow me to briefly explain flight-vehicle imposed loads. Turbulence is felt as a change in the g-force directed on the aircraft and occupants, hence when delta G is large, the turbulence effects will be acutely felt, and by extension, when delta G is small a smooth ride is not only perceived, but achieved.

Plugging in the known parameters of the slope of the lift curve, the density ratio of the air (an expression of altitude), the equivalent air speed, the magnitude of the gust, divided by the wing loading one arrives at the change in g-loading imposed by the turbulence. In deference to the mathematically challenged, a bigger number on the bottom means a smaller number is the result. It is therefore seen, from the equation, that a high wing loading translates into a smoother flight.

KU is often determined as a single value from accelerometers in the aircraft. When K = 1 the gust is instantaneous. A nominal value for K is 0.6.

If one wishes to compare different aircraft in the same turbulence, simply keep KU, Ve, and sigma constant. You can then get a workable, though qualitative, sense of which aircraft give a smoother ride in turbulence – everything else being equal (which in reality never occurs). An aircraft with a shallow slope to the CsubL curve and a high wing loading will provide a smoother ride.

There is a major caveat to this, which is wing bending. Semi-elastic wings will absorb some of the turbulence experienced by the wing. Even fairly rigid wings, such as the 727, will flex and relieve g-loading on the fuselage. This equation then is the starting point in determining the VsubB (turbulent penetration) speed of an aircraft. Though it appears elementary, the physics behind the equation is quite solid and can be found in most upper level aircraft structures texts.

Ahhh Shit! That clears things right up.

David Hingtgen · 06-04-2003, 04:15 AM

Well since wings can absorb some of the load, what about the fuselages? Convair 880's have incredibly thick and stiff skins, as do L-1011's along with a notably reduced number of longitudinal stringers---they'd certainly behave/flex differently than your typical Boeing narrowbody.

Phugoid · 06-04-2003, 11:39 PM

A vertical gust will impose a g force that is felt throughout the aircraft, whereas a turbulent eddy causing a change in pitch will be felt differently depending on where in the aircraft one is located. If you are at the center of gravity a pitch change is not felt as a g-force, but a rotation of axis. If you are on the flight deck a rotation about the lateral axis will be felt as an acceleration upward, and this is expressed in ‘g’. The bending of the fuselage can be accounted for in the value of K, which is commonly referred to as a gain. The value of K will change depending on where in the aircraft the accelerometer is located. A value of .8 may be realistic for a wing mounted accelerometer, while a value of .6 may be more realistic in the aircraft fuselage. K in this equation is known as the ‘gust alleviation factor’. When aircraft were of similar design, in the 1950’s, K was not a part of the simplified equation, however with the advent of high speed flight and the variance in size, K has become a necessity to account for the discrepancy in the equation versus flight test data. K is related to wing loading, lift curve slope, air density and wing chord. Algebraically, K = .88 u / 5.3 + u, where u = 2 (w/s) / gcpCsubL. K is an attempt to qualify gust gradient, airplane response, and lag in lift caused by a change in angle of attack. Note that CsubL in this case is the whole aircraft and not only the wing lift. This equation is valid for rigid aircraft and is adequate for preliminary loads in the design phase. Dynamic gust analysis of a flexible airframe requires a less direct approach and much greater sophistication. Finite element analysis and adaptive grid embedding are techniques that are much more accurate but give no insight into the basic principles.

As the original question requested an equation of ride quality as a function of wing loading, this model serves that purpose. Is it a good equation for ride quality? No, ride quality is more a subjective feel and a better approach for comparing different aircraft might be a questionnaire for flight attendants and then a Chi square analysis. As no two aircraft experience the same turbulence get a very large sample size, say n = 3,000. In closing, science has become very good at describing our world, there is however no substitute for experience. If the engineering says the ride is good and the ride sucks, then the ride sucks.

Phugoid · 06-04-2003, 11:44 PM

To answer the fuselage question – the make up of the structure does have an impact on ride quality and is best addressed by one of the sophisticated methods. Elastic aircraft involve a whole new aspect which is better suited to the engineering teams and super computers.

Full Flaps! · 06-10-2003, 05:23 PM

Phugoid,
This is the best information I have got from this forum.

There is no need for the second question, Thanks, I got it!

05-31-2003, 12:29 PM	#1
Full Flaps! Senior Collector Join Date: Dec 2000 Location: Miami, FL. Posts: 264	Determinig Aircraft comfort I once read that by knowing an aircraft’s wing loading, the total weight of the aircraft, and some other factor, you can determine how smooth a ride would be for that particular airplane; Also you can compare between two airplanes to see which would have the best, and smoothest flight. Does any body knows that formula? __________________ I love my 1/400s

05-31-2003, 01:44 PM	#2
ZX1100F1 Buford T. Justice Join Date: Sep 1999 Posts: 2,004	Maybe it's just me I once read that avoiding rough air would allow for a smooth ride.

05-31-2003, 06:32 PM	#3
Scott Insane Collector Join Date: Mar 2000 Location: Escondido, Ca. USA Age: 50 Posts: 2,763	Keeping a safe distance behind "heavy" aircraft is a bonus. And stay away from the cumulogranite! __________________ Fly the Friendly Skies of United uniTED takes wing Feb 12, 2004

05-31-2003, 11:54 PM	#7
Phugoid Collector Join Date: Aug 2002 Posts: 90	It appears there may be some confusion as to how the equation may be applied. For those who have not attended an aircraft structures class, allow me to briefly explain flight-vehicle imposed loads. Turbulence is felt as a change in the g-force directed on the aircraft and occupants, hence when delta G is large, the turbulence effects will be acutely felt, and by extension, when delta G is small a smooth ride is not only perceived, but achieved. Plugging in the known parameters of the slope of the lift curve, the density ratio of the air (an expression of altitude), the equivalent air speed, the magnitude of the gust, divided by the wing loading one arrives at the change in g-loading imposed by the turbulence. In deference to the mathematically challenged, a bigger number on the bottom means a smaller number is the result. It is therefore seen, from the equation, that a high wing loading translates into a smoother flight. KU is often determined as a single value from accelerometers in the aircraft. When K = 1 the gust is instantaneous. A nominal value for K is 0.6. If one wishes to compare different aircraft in the same turbulence, simply keep KU, Ve, and sigma constant. You can then get a workable, though qualitative, sense of which aircraft give a smoother ride in turbulence – everything else being equal (which in reality never occurs). An aircraft with a shallow slope to the CsubL curve and a high wing loading will provide a smoother ride. There is a major caveat to this, which is wing bending. Semi-elastic wings will absorb some of the turbulence experienced by the wing. Even fairly rigid wings, such as the 727, will flex and relieve g-loading on the fuselage. This equation then is the starting point in determining the VsubB (turbulent penetration) speed of an aircraft. Though it appears elementary, the physics behind the equation is quite solid and can be found in most upper level aircraft structures texts.

06-01-2003, 07:07 AM	#8
Full Flaps! Senior Collector Join Date: Dec 2000 Location: Miami, FL. Posts: 264	Phugoid Thanks for that formula Let me write this down, try to understand it; Because I would like to make you one more question about this subject. Good to have you in the forum. __________________ I love my 1/400s

05-31-2003, 07:51 PM	#5
Phugoid Collector Join Date: Aug 2002 Posts: 90	This is the formula for a change in g loading caused by a vertical gust.

05-31-2003, 08:21 PM	#6
ZX1100F1 Buford T. Justice Join Date: Sep 1999 Posts: 2,004	Pardon me Phugoid, But how does this elementary (and ambiguous) G-load formula help to answer the original question? Which is in and of it's self an impossible question. There are so many other factors (size of aircraft, velocity of turbulence, direction of turbulence, angle at which it strikes it strikes aircraft, were it strikes aircraft, etc, etc, etc) to be taken into consideration, and then there’s the fact that; not all of the bumps you feel are transmitted into the passenger compartment via the wings. Full Flaps! Is sitting back right now laughing his @ss off because some Pollock is trying to piss in the corner of a round room. I for one am getting tired of seeing some hyper technical chart or explanation that is mostly irrelevant, if you have something to add, do so in a manner conducive to normal standard human comprehension and relevancy. Unless (of course) you were only humoring him with your formula, in which case, carry on.

06-04-2003, 04:15 AM	#10
David Hingtgen Complete Wacko! Join Date: Sep 2001 Location: Iowa Posts: 4,055	Well since wings can absorb some of the load, what about the fuselages? Convair 880's have incredibly thick and stiff skins, as do L-1011's along with a notably reduced number of longitudinal stringers---they'd certainly behave/flex differently than your typical Boeing narrowbody. __________________ *We need more TWA twin-stripes!*

06-04-2003, 11:39 PM	#11
Phugoid Collector Join Date: Aug 2002 Posts: 90	A vertical gust will impose a g force that is felt throughout the aircraft, whereas a turbulent eddy causing a change in pitch will be felt differently depending on where in the aircraft one is located. If you are at the center of gravity a pitch change is not felt as a g-force, but a rotation of axis. If you are on the flight deck a rotation about the lateral axis will be felt as an acceleration upward, and this is expressed in ‘g’. The bending of the fuselage can be accounted for in the value of K, which is commonly referred to as a gain. The value of K will change depending on where in the aircraft the accelerometer is located. A value of .8 may be realistic for a wing mounted accelerometer, while a value of .6 may be more realistic in the aircraft fuselage. K in this equation is known as the ‘gust alleviation factor’. When aircraft were of similar design, in the 1950’s, K was not a part of the simplified equation, however with the advent of high speed flight and the variance in size, K has become a necessity to account for the discrepancy in the equation versus flight test data. K is related to wing loading, lift curve slope, air density and wing chord. Algebraically, K = .88 u / 5.3 + u, where u = 2 (w/s) / gcpCsubL. K is an attempt to qualify gust gradient, airplane response, and lag in lift caused by a change in angle of attack. Note that CsubL in this case is the whole aircraft and not only the wing lift. This equation is valid for rigid aircraft and is adequate for preliminary loads in the design phase. Dynamic gust analysis of a flexible airframe requires a less direct approach and much greater sophistication. Finite element analysis and adaptive grid embedding are techniques that are much more accurate but give no insight into the basic principles. As the original question requested an equation of ride quality as a function of wing loading, this model serves that purpose. Is it a good equation for ride quality? No, ride quality is more a subjective feel and a better approach for comparing different aircraft might be a questionnaire for flight attendants and then a Chi square analysis. As no two aircraft experience the same turbulence get a very large sample size, say n = 3,000. In closing, science has become very good at describing our world, there is however no substitute for experience. If the engineering says the ride is good and the ride sucks, then the ride sucks.

06-04-2003, 11:44 PM	#12
Phugoid Collector Join Date: Aug 2002 Posts: 90	To answer the fuselage question – the make up of the structure does have an impact on ride quality and is best addressed by one of the sophisticated methods. Elastic aircraft involve a whole new aspect which is better suited to the engineering teams and super computers.

06-10-2003, 05:23 PM	#13
Full Flaps! Senior Collector Join Date: Dec 2000 Location: Miami, FL. Posts: 264	Phugoid, This is the best information I have got from this forum. There is no need for the second question, Thanks, I got it! __________________ I love my 1/400s

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