Pink Panther
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Why aft CG increases TAS
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Cardinal
Of The Kremlin
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The horizontal stabilizer usually generates downforce in conventional aircraft, to balance the moment caused by the CG being ahead of the wing's center of lift. Moving the CG aft reduces that nose-down moment, thus the horizontal stab doesn't have to work as hard, and it will generate less induced drag.
But since the horizontal generates downforce or negative lift, the wing has to lift the weight of the aircraft and cancel that negative lift. Thus the aft CG will mean less induced drag from the wing and the horizontal stab.
Application can be seen in the 747, MD11, and A330/340. They all have a tank in the vertical stab to which fuel can be transferred, providing not only extra capacity but helpfully moving the CG aft during cruise.
But since the horizontal generates downforce or negative lift, the wing has to lift the weight of the aircraft and cancel that negative lift. Thus the aft CG will mean less induced drag from the wing and the horizontal stab.
Application can be seen in the 747, MD11, and A330/340. They all have a tank in the vertical stab to which fuel can be transferred, providing not only extra capacity but helpfully moving the CG aft during cruise.
banned username 2
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.....
The previous response was good, but I'd like to add a little. As the CG moves aft the aircraft generally goes faster, but it becomes less stable (the phugoid damping decreases). The fly-by-wire systems on the newer aircraft automatically stabilize the aircraft and allow it to maintain good flying qualities with the more efficient aft CG.
The fly-by-wire fighters (e.g. F-16, F-117) fly with the CG so far aft that the aircraft would simply not be controlable without the computers.
The previous response was good, but I'd like to add a little. As the CG moves aft the aircraft generally goes faster, but it becomes less stable (the phugoid damping decreases). The fly-by-wire systems on the newer aircraft automatically stabilize the aircraft and allow it to maintain good flying qualities with the more efficient aft CG.
The fly-by-wire fighters (e.g. F-16, F-117) fly with the CG so far aft that the aircraft would simply not be controlable without the computers.
InHot,
"Phugoid" damping refers to the pitch oscillations which slowly decrease in amplitude over time. When an aircraft is designed, two main types of stability are considered. First, static stability. This is the tendency for the aircraft to initially return to it's original attitide after being disturbed. For example, if turbulence pitches the nose up after being trimmed for staight and level, it is desirable for the nose to initially pitch down in an attempt to return to straight and level. This is called positive static stability. Is the nose stays where it is, it exhibits neutral static stability. If it keeps rising after the upset, then negative static stability exists (not very desirable in conventional aircraft).
Once positive static stability is attained, half the battle is won. Now we have to make sure that the aircraft returns to straight and level. As you know, we need postitive static stability to even consider the next type, which is dynamic stability. In our example situation, after the nose initially pitches down, it will overshoot the level flight attitude and pitch below until the airspeed increases enough to increase the tail-down force to bring the nose back up again. The nose will then overshoot straight and level again but the nose will not rise to the original position after the disturbance. This cycle repeats over and over with smaller and smaller overshoots until they are barely noticable and the aircraft is back in the original position. This is called positive dynamic stability. Neutral dynamic stability is when the nose rises and falls the same amount with each oscillation. Negative dynamic stability is when the nose rises and falls a greater amount with each oscillation (once again, not very desirable).
If our aircraft which is dynamically stable were to turn some smoke on, the path it traces would resemble a sine wave with decreasing amplitude but roughly constant frequency. The frequency would depend on the design of the aircraft (I.e. size of the horizontal stabilizer) and the loading of the aircraft (CG location), which brings us back to the original discussion. I'm sure everybody knows most of this stuff, so sorry to ramble. Hope this helps.
SuperD
"Phugoid" damping refers to the pitch oscillations which slowly decrease in amplitude over time. When an aircraft is designed, two main types of stability are considered. First, static stability. This is the tendency for the aircraft to initially return to it's original attitide after being disturbed. For example, if turbulence pitches the nose up after being trimmed for staight and level, it is desirable for the nose to initially pitch down in an attempt to return to straight and level. This is called positive static stability. Is the nose stays where it is, it exhibits neutral static stability. If it keeps rising after the upset, then negative static stability exists (not very desirable in conventional aircraft).
Once positive static stability is attained, half the battle is won. Now we have to make sure that the aircraft returns to straight and level. As you know, we need postitive static stability to even consider the next type, which is dynamic stability. In our example situation, after the nose initially pitches down, it will overshoot the level flight attitude and pitch below until the airspeed increases enough to increase the tail-down force to bring the nose back up again. The nose will then overshoot straight and level again but the nose will not rise to the original position after the disturbance. This cycle repeats over and over with smaller and smaller overshoots until they are barely noticable and the aircraft is back in the original position. This is called positive dynamic stability. Neutral dynamic stability is when the nose rises and falls the same amount with each oscillation. Negative dynamic stability is when the nose rises and falls a greater amount with each oscillation (once again, not very desirable).
If our aircraft which is dynamically stable were to turn some smoke on, the path it traces would resemble a sine wave with decreasing amplitude but roughly constant frequency. The frequency would depend on the design of the aircraft (I.e. size of the horizontal stabilizer) and the loading of the aircraft (CG location), which brings us back to the original discussion. I'm sure everybody knows most of this stuff, so sorry to ramble. Hope this helps.
SuperD
.....
To answer the 'what is phugoid damping' question...
The phugoid 'mode' was described in the previous post. Phugoid damping is a measure of how much the amplitude decays from peak to peak. As the CG move aft the damping decreases so the aircraft will need more occillations to return to straight and level flight. The phugoid mode has a period (time from peak to peak) of about a minute, so it is actually possible to fly and airplane with an unstable phugoid (negative damping). The aircraft will require a fair amount of pilot effort to fly with an unstable phugoid, but the benifit is a very responsive airplane that just goes where you point it. Most competition aerobatic aircraft are flown this way as well as some of the pre-fly-by-wire jets like the F-4.
Scott
To answer the 'what is phugoid damping' question...
The phugoid 'mode' was described in the previous post. Phugoid damping is a measure of how much the amplitude decays from peak to peak. As the CG move aft the damping decreases so the aircraft will need more occillations to return to straight and level flight. The phugoid mode has a period (time from peak to peak) of about a minute, so it is actually possible to fly and airplane with an unstable phugoid (negative damping). The aircraft will require a fair amount of pilot effort to fly with an unstable phugoid, but the benifit is a very responsive airplane that just goes where you point it. Most competition aerobatic aircraft are flown this way as well as some of the pre-fly-by-wire jets like the F-4.
Scott
starvingcfi
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AlbieF15
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FYI...
F16...true fly by wire. Aircraft would be unstable without computer interface. Same for Stealth F117.
F15...fly by wire system augments hydraulic system (CAS). Part of TR-1 flight for new Eagle guys is diabling CAS so they can see jet is quite comforable to fly. CAS acts as "power steering" and masks effects of stores...but otherwise not required for flight. It does, of course, improve roll and pitch authority.
F4...never drove it, but I think the Phantom guys will tell you it has NARY A BIT of Fly by wire. It is all hydromechanical.
Fly safe.
F16...true fly by wire. Aircraft would be unstable without computer interface. Same for Stealth F117.
F15...fly by wire system augments hydraulic system (CAS). Part of TR-1 flight for new Eagle guys is diabling CAS so they can see jet is quite comforable to fly. CAS acts as "power steering" and masks effects of stores...but otherwise not required for flight. It does, of course, improve roll and pitch authority.
F4...never drove it, but I think the Phantom guys will tell you it has NARY A BIT of Fly by wire. It is all hydromechanical.
Fly safe.
B-J-J Fighter
Royce Gracie in Action
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Excellent Post
Great post guys, Im really learning a lot. Aerodynamics is always been hard for me to understand because its kind of hard to me to picture what is going on. Your doing a great job putting this stuff into laymens terns.
How about explaing the reason some a/c pitch up when flaps are applied and explain the different kinds of drag.
Great post guys, Im really learning a lot. Aerodynamics is always been hard for me to understand because its kind of hard to me to picture what is going on. Your doing a great job putting this stuff into laymens terns.
How about explaing the reason some a/c pitch up when flaps are applied and explain the different kinds of drag.
B-J-J,
I'll try and tackle your drag question. Basically, there are two main types of drag on an aircraft. They are parasite drag and induced drag.
Induced drag is that drag caused by the production of lift. We all know about the pressure differences between the upper and lower surfaces of the wing. The air "spills" over the tips to fill in the void caused by the lower pressure above, thereby creating wingtip vortices. Looking at the wing spanwise (at the tip looking towards the fuselage), as the flow comes up and over the tips, it imparts a downward velocity on the flow leaving the trailing edge. This downward velocity component has the effect of tilting the lift vector rearwards (since lift is perpendicular to the relative wind). So, the lift from the wing now has a vertical component (to offset weight) and a horizontal component opposite the direction of flight. This horizontal component is drag. This drag force is not constant though. It depends on the lift coefficient (angle of attack), the planform shape of the wing, and the dynamic pressure (density and speed) of the airflow. Remember the FAA's "heavy, clean, and slow"? Well, the heavier the aircraft and the slower the airspeed, the higher the AOA will have to be to sustain the necessary lift. Flaps produce vortices of their own that tend to dissipate the main vorticies, so a clean wing will usually produce greater wake turbulence, but it varies with each wing and flap structure. "Ground effect" is something that we're all familiar with. As the aircraft approaches the pavement, the ground tends to dissipate the vortices, causing the lift vector to tilt forward. This reduces the drag and increases the lift at the same time. Some people will tell you that it is a "cushion of air" that causes ground effect. No such thing.
Basically, parasite drag is everything that is not induced drag. One exception is wave drag, but that's for another time. Parasite drag consists of skin friction drag (air viscosity), form drag, and several other types of less-significant drag. Form drag is the result of the shape of the aircraft and the size of the wake it produces. Flat plates held perpendicular to the airflow will exhibit lots of form drag, but very little skin friction drag. The opposite is true if you align the plate with the airflow. The classic example is sticking your hand out the window in a moving car. If your palm is forward, you will have much more drag (mostly form drag) than if your palm is down (mostly skin friction, but a much lesser value).
As a pilot, it's important to know when the aircraft will have the most drag. Parasite drag increases with airspeed (exponentially I might add!) and induced drag will decrease (mostly due to the lower angles of attack found at higher speeds). But as the aircraft slows, the parasite drag will decrease but the induced will increase. When the induced drag far overshadows the parasite drag, this is called the "backside of the power curve", meaning that more power will be required to sustain level flight as the aircraft slows.
Anyway, enough for now. This is a good discussion, let's have some more answers!
SuperD
I'll try and tackle your drag question. Basically, there are two main types of drag on an aircraft. They are parasite drag and induced drag.
Induced drag is that drag caused by the production of lift. We all know about the pressure differences between the upper and lower surfaces of the wing. The air "spills" over the tips to fill in the void caused by the lower pressure above, thereby creating wingtip vortices. Looking at the wing spanwise (at the tip looking towards the fuselage), as the flow comes up and over the tips, it imparts a downward velocity on the flow leaving the trailing edge. This downward velocity component has the effect of tilting the lift vector rearwards (since lift is perpendicular to the relative wind). So, the lift from the wing now has a vertical component (to offset weight) and a horizontal component opposite the direction of flight. This horizontal component is drag. This drag force is not constant though. It depends on the lift coefficient (angle of attack), the planform shape of the wing, and the dynamic pressure (density and speed) of the airflow. Remember the FAA's "heavy, clean, and slow"? Well, the heavier the aircraft and the slower the airspeed, the higher the AOA will have to be to sustain the necessary lift. Flaps produce vortices of their own that tend to dissipate the main vorticies, so a clean wing will usually produce greater wake turbulence, but it varies with each wing and flap structure. "Ground effect" is something that we're all familiar with. As the aircraft approaches the pavement, the ground tends to dissipate the vortices, causing the lift vector to tilt forward. This reduces the drag and increases the lift at the same time. Some people will tell you that it is a "cushion of air" that causes ground effect. No such thing.
Basically, parasite drag is everything that is not induced drag. One exception is wave drag, but that's for another time. Parasite drag consists of skin friction drag (air viscosity), form drag, and several other types of less-significant drag. Form drag is the result of the shape of the aircraft and the size of the wake it produces. Flat plates held perpendicular to the airflow will exhibit lots of form drag, but very little skin friction drag. The opposite is true if you align the plate with the airflow. The classic example is sticking your hand out the window in a moving car. If your palm is forward, you will have much more drag (mostly form drag) than if your palm is down (mostly skin friction, but a much lesser value).
As a pilot, it's important to know when the aircraft will have the most drag. Parasite drag increases with airspeed (exponentially I might add!) and induced drag will decrease (mostly due to the lower angles of attack found at higher speeds). But as the aircraft slows, the parasite drag will decrease but the induced will increase. When the induced drag far overshadows the parasite drag, this is called the "backside of the power curve", meaning that more power will be required to sustain level flight as the aircraft slows.
Anyway, enough for now. This is a good discussion, let's have some more answers!
SuperD
Flaps
Flaps generally cause the nose to rise because of an increase in downwash on the tail. It's not that the wing is making any more lift with the flaps down, but that the lift is redistributed so more of the lift (and thus downwash) is made inboard right in front of the tail and less outboard. The wing itself actually wants to pitch nose down with flap extension, but the effects of downwash are usually stronger. There are aircraft that pitch nose down with flaps, but I can't think of any right now.
Scott
Flaps generally cause the nose to rise because of an increase in downwash on the tail. It's not that the wing is making any more lift with the flaps down, but that the lift is redistributed so more of the lift (and thus downwash) is made inboard right in front of the tail and less outboard. The wing itself actually wants to pitch nose down with flap extension, but the effects of downwash are usually stronger. There are aircraft that pitch nose down with flaps, but I can't think of any right now.
Scott
MetroSheriff
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Huh? Redistributed lift and downwash???
I wont swear to it, but....I vaguely recall something about flaps increasing the camber, AOA, and on some types, (i.e. Fowler) wing area resulting in an increse in total lift produced by the airfoil. The resultant shift of the center of pressure in relation to the CG of the aircraft causing a pitch-up or pitch-down tendancy, but maybe I'm wrong.
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Re: Flaps
Huh? The flaps do increase lift by increasing the angle of attack. They also add drag.
Not sure about the tail theory.
My understanding is that the force of drag upon the flaps acts above (or below) the CG of the airplane, thus rotating the airplane around that axis, causing a pitch up (or down).
sstearns2 said:
Huh? The flaps do increase lift by increasing the angle of attack. They also add drag.
Not sure about the tail theory.
My understanding is that the force of drag upon the flaps acts above (or below) the CG of the airplane, thus rotating the airplane around that axis, causing a pitch up (or down).
Drag
The previous post on drag was good, but here's a little different way of looking at it (the way I think of it anyway).
The way I think of it, there are 3 kinds of drag...
1- Parasite or 'skin friction' drag.
2 - Induced drag or the drag caused by making lift in whatever direction.
3 - Wave drag or the drag cause by air decellerating thru the speed of sound (shock waves).
They are all really the same basic thing, the acceleration of the air in some direction. Skin friction drag is caused by the entrainment of air in the boundary layer (the layer of air right next to the skin of the airplane). Basically skin friction drag is caused by the air your just dragging along with you as you fly along. Induced drag is caused by the acceleration of the air in some direction to make lift in the opposite direction. The wing accelerates the air down to make lift up.
Imagine there is a box of still air, then a glider flies thru the box of air. Now the box of air has a velocity component in the direction of the flight of the glider caused by the glider dragging (acclerating) air along with itself and there is a vertical velocity component caused by the need of the glider to accelerate air down in order to make the needed lift. The air will also be rotating some amount because of 'wing tip vorticies'. (I hate the word wing tip vortex and disagree with the book explainations, but that's another tirade.)
Wave drag is caused by the sudden deceleration of air thru a shock wave. Air cannot decelerate thru mach 1 smoothly, it will be going Mach 1.5 for example and the hit the shock wave and slow to Mach 0.75 (or so, I don't have a mach table in front of me) in a fraction of an inch. Basically there is a big wall of air being dragged along behind the shock wave that causes a huge amount of drag.
I hope this make some amount of sense.
Scott
The previous post on drag was good, but here's a little different way of looking at it (the way I think of it anyway).
The way I think of it, there are 3 kinds of drag...
1- Parasite or 'skin friction' drag.
2 - Induced drag or the drag caused by making lift in whatever direction.
3 - Wave drag or the drag cause by air decellerating thru the speed of sound (shock waves).
They are all really the same basic thing, the acceleration of the air in some direction. Skin friction drag is caused by the entrainment of air in the boundary layer (the layer of air right next to the skin of the airplane). Basically skin friction drag is caused by the air your just dragging along with you as you fly along. Induced drag is caused by the acceleration of the air in some direction to make lift in the opposite direction. The wing accelerates the air down to make lift up.
Imagine there is a box of still air, then a glider flies thru the box of air. Now the box of air has a velocity component in the direction of the flight of the glider caused by the glider dragging (acclerating) air along with itself and there is a vertical velocity component caused by the need of the glider to accelerate air down in order to make the needed lift. The air will also be rotating some amount because of 'wing tip vorticies'. (I hate the word wing tip vortex and disagree with the book explainations, but that's another tirade.)
Wave drag is caused by the sudden deceleration of air thru a shock wave. Air cannot decelerate thru mach 1 smoothly, it will be going Mach 1.5 for example and the hit the shock wave and slow to Mach 0.75 (or so, I don't have a mach table in front of me) in a fraction of an inch. Basically there is a big wall of air being dragged along behind the shock wave that causes a huge amount of drag.
I hope this make some amount of sense.
Scott
bobbysamd
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Flaps
I cast my vote with Metro Sheriff and IFly4Food. Flaps indeed add lift. Isn't that why so many aircraft require extended flaps for takeoff (and landings, of course)? When flaps are extended, camber changes, the center of pressure moves and, hence, the pitch change. Bernoulli lift. Relative wind hits the extended flaps and causes a pitching moment. Newtonian lift.
Very stimulating discussion, folks. Better that taking a ground school refresher or digging out the books. Great posts, too, on drag and aircraft stability (one of my favorite aerodynamic subjects
).
I cast my vote with Metro Sheriff and IFly4Food. Flaps indeed add lift. Isn't that why so many aircraft require extended flaps for takeoff (and landings, of course)? When flaps are extended, camber changes, the center of pressure moves and, hence, the pitch change. Bernoulli lift. Relative wind hits the extended flaps and causes a pitching moment. Newtonian lift.
Very stimulating discussion, folks. Better that taking a ground school refresher or digging out the books. Great posts, too, on drag and aircraft stability (one of my favorite aerodynamic subjects
).
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Re: Drag
Dude! I think you need to lay of aerodynamic theory for a while.
Parasite drag is direct contact of leading edge surfaces with the air and friction. Did you know that there are over 200lbs/in2 on every surface at 250KIAS?
Induced drag is a by product of lift. Wake vortices are a great example (whether you believe in them or not... but I hope you do). It is what you described as parasite... dragging air behind you.
Mach drag? I won't worry about that until my airline buys a Concorde!
Who do you fly the Brakillya for?
sstearns2 said:
Dude! I think you need to lay of aerodynamic theory for a while.
Parasite drag is direct contact of leading edge surfaces with the air and friction. Did you know that there are over 200lbs/in2 on every surface at 250KIAS?
Induced drag is a by product of lift. Wake vortices are a great example (whether you believe in them or not... but I hope you do). It is what you described as parasite... dragging air behind you.
Mach drag? I won't worry about that until my airline buys a Concorde!
Who do you fly the Brakillya for?
....
Previous post....
>The flaps do increase lift by increasing the angle of attack. They >also add drag.
Flaps increase the ability of the wing to make lift. In order to fly staight and level lift has to equal wieght. The weight of the aircraft doesn't increase with flap extension, so the amount of lift made in straight and level flight is the same with the flaps up or down.
Scott
Previous post....
>The flaps do increase lift by increasing the angle of attack. They >also add drag.
Flaps increase the ability of the wing to make lift. In order to fly staight and level lift has to equal wieght. The weight of the aircraft doesn't increase with flap extension, so the amount of lift made in straight and level flight is the same with the flaps up or down.
Scott
Re: ....
Your logic is flawed. It has nothing to do with weight.
Lift is a combination of speed, angle of attack, and the wings lift coefficient. Flaps allow you to maintain a high value of lift at a lower than otherwise airspeed. In essence, they change the coefficient of lift of the wing (by increasing camber) and increase the angle of attack (by lowering the trailing edge of the wing).
The total lift doesn't change in relation to the weight; you just get that value at a lower airspeed with flaps.
I recommend the book Aerodynamics for Naval Aviators where you can research these issues. It's also a great cure for insomnia
sstearns2 said:
Your logic is flawed. It has nothing to do with weight.
Lift is a combination of speed, angle of attack, and the wings lift coefficient. Flaps allow you to maintain a high value of lift at a lower than otherwise airspeed. In essence, they change the coefficient of lift of the wing (by increasing camber) and increase the angle of attack (by lowering the trailing edge of the wing).
The total lift doesn't change in relation to the weight; you just get that value at a lower airspeed with flaps.
I recommend the book Aerodynamics for Naval Aviators where you can research these issues. It's also a great cure for insomnia
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I'm sorry everyone, I should have stopped with the flap post, but I have to respond to this.
The previous post was
*****
Did you know that there are over 200lbs/in2 on every surface at 250KIAS?
Induced drag is a by product of lift. Wake vortices are a great example (whether you believe in them or not... but I hope you do). It is what you described as parasite... dragging air behind you.
Mach drag? I won't worry about that until my airline buys a Concorde!
Who do you fly the Brakillya for?
******
Check your math, your answer is probably in lb/ft^2 rather than lb/in^2. The dynamic pressure is on the order of 3 or 4 psi at 250 KIAS.
Induced drag is a product of lift, I agree completely.
It's not a matter of me believing in wake vortices, they clearly exist in real life. I just disagree with the book explainations of where they come from and think the term 'wing tip vortex' is a misnomer.
I fly for Skywest.
Scott
The previous post was
*****
Did you know that there are over 200lbs/in2 on every surface at 250KIAS?
Induced drag is a by product of lift. Wake vortices are a great example (whether you believe in them or not... but I hope you do). It is what you described as parasite... dragging air behind you.
Mach drag? I won't worry about that until my airline buys a Concorde!
Who do you fly the Brakillya for?
******
Check your math, your answer is probably in lb/ft^2 rather than lb/in^2. The dynamic pressure is on the order of 3 or 4 psi at 250 KIAS.
Induced drag is a product of lift, I agree completely.
It's not a matter of me believing in wake vortices, they clearly exist in real life. I just disagree with the book explainations of where they come from and think the term 'wing tip vortex' is a misnomer.
I fly for Skywest.
Scott
MetroSheriff
Hittin' the road...
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With a constant power setting (thrust) that is completely untrue.
Hence the reason most aircraft will tend to balloon when flaps are applied. This is why the pilot, or autopilot,will have to pitch-down (on most a/c), in order to lower the AOA and thereby compensate for the increase in total lift resulting from flap deployment.
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sstearns2 said:
Well, I guess picking on me for typing the wrong units is the best you can do. I'm sure I meant lbs/ft2, but that's not really the issue.
I'd love to hear your explanation of wake vortices.
Most aerodynamicists (of which one I'm not) agree that they form because of lower pressure air above the wing relative to the air under the wing. Across the span of the wing, the wing itself separates these pressure fields except at the end of the wing. With no barrier, air flows from high to low pressure, so the relatively high pressure air below the wing flows up and around the wing to meet the lower pressure air. Because of the airplane's forward motion, a vortex, or a "horizontal tornado" forms at each wing tip.
Ok, straight from Aerodynamics For Naval Aviators :
With regards to the Flap/C.G. stuff.
"Positive Camber produces a nose down twisting moment - especially when large camber is used well aft on the chord (an obvious implication is that flaps are not practical on a flying wing or tailless airplane). The deflection of a flap causes large nose down moments which create important twisting loads on the structure and pitching moments that must be controlled with the horizontal tail.....The fowler flap causes the greatest change in twisting moment while the split flap causes the least."
So, as several have stated before, it is the change in the center of pressure that creates the pitching moment (yeah that quote doesn't say it specifically but a few pages ahead it explains pitching moments).
My question is... why do some aircraft pitch up and others down? My aerodynamics teacher in college would kill me if he found out that I forgot why but...
Seems to me that most high wing aircraft pitch up, while the low wing aircraft pitch down. For the high wing Cessnas I am willing to bet that part of the reason is increased downwash on the tail. But I know of high wing T-tail aircraft that pitch up as well. I suppose if I took the time to do some force vector analysis on a high wing aircraft I could figure it out. I will work on it, but if someone can figure it out before me, by all means do!
Skeezer
With regards to the Flap/C.G. stuff.
"Positive Camber produces a nose down twisting moment - especially when large camber is used well aft on the chord (an obvious implication is that flaps are not practical on a flying wing or tailless airplane). The deflection of a flap causes large nose down moments which create important twisting loads on the structure and pitching moments that must be controlled with the horizontal tail.....The fowler flap causes the greatest change in twisting moment while the split flap causes the least."
So, as several have stated before, it is the change in the center of pressure that creates the pitching moment (yeah that quote doesn't say it specifically but a few pages ahead it explains pitching moments).
My question is... why do some aircraft pitch up and others down? My aerodynamics teacher in college would kill me if he found out that I forgot why but...
Seems to me that most high wing aircraft pitch up, while the low wing aircraft pitch down. For the high wing Cessnas I am willing to bet that part of the reason is increased downwash on the tail. But I know of high wing T-tail aircraft that pitch up as well. I suppose if I took the time to do some force vector analysis on a high wing aircraft I could figure it out. I will work on it, but if someone can figure it out before me, by all means do!
Skeezer
Metrosheriff,
You explaination of why the aircrafts pitches with flap extension is correct. It has much to do with the change of pressure distribution on the wing as flaps are deployed. Whoever said that the pitching depends on the position of the wing with respect to the CG is only partly right. For instance, the BE1900D has a noticable pitch-up tendency when flaps are expended and it is clearly a low-wing aircraft (wing is below CG).
For some reason many people buy into the FAA's written test answers and continue to believe that the purpose of flaps is to increase lift. NOT TRUE! You only need a certain amount of lift for a given flight condition. One of the main purposes of flaps is to acheive the same amount of lift at a slower speed. It is true that the aircraft balloons when flaps are initially extended, but it soon returns to equilibrium with only the required lift.
SuperD
You explaination of why the aircrafts pitches with flap extension is correct. It has much to do with the change of pressure distribution on the wing as flaps are deployed. Whoever said that the pitching depends on the position of the wing with respect to the CG is only partly right. For instance, the BE1900D has a noticable pitch-up tendency when flaps are expended and it is clearly a low-wing aircraft (wing is below CG).
For some reason many people buy into the FAA's written test answers and continue to believe that the purpose of flaps is to increase lift. NOT TRUE! You only need a certain amount of lift for a given flight condition. One of the main purposes of flaps is to acheive the same amount of lift at a slower speed. It is true that the aircraft balloons when flaps are initially extended, but it soon returns to equilibrium with only the required lift.
SuperD
MetroSheriff
Hittin' the road...
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SuperD,
Well put. Also the resultant lower angle of attack required to maintain level flight at a lower airspeed keeps the a/c from having to maintain excessively high deck angles on approach. Not to mention lowering the stall speed and better low airspeed handling characteristics.
BTW, great thread everyone. Quite a refreshing break from the usual intra-union and inter-carrier bickering...
MetroSheriff said:
I will have to disagree with you. In straight and level flight, you are producing the same amount of lift with flaps, 10, 20, 30, 45, 70, whatever. The key is "Level flight."
It is true that when you add flaps the aircraft will create more lift INITIALLY at a given airspeed till you lower the nose to reduce the AOA. Once that is done, and you are once again straight and level, the lift equals the weight. The AOA may change, but the lift will remain the same.
For an aircraft to be in level flight the vertical component of lift must equal the weight of the aircraft. If the vertical component is greater than the weight, the aircraft will accelerate upwards. Simple physics. An imbalance of forces will cause the object to accelerate in one direction.
Peace out
Skeezer
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MetroSheriff
Hittin' the road...
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Skeezer,
I agree. That was the point I was trying to make. Most aircraft will tend to balloon (climb) with a constant power setting, UNLESS the nose is lowered (reduce the AOA) or change the power setting (reduced thrust). That is why I would contend that there is an increase in total lift, AOA or thrust MUST be reduced in order to maintain level flight.
B-J-J Fighter
Royce Gracie in Action
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MetroSheriff said:
Ok, Cool deal. The reason that I disagreed was in response to your disagreement with SStearns2 on his post about flaps and lift. You stated that he was wrong when I think he was trying to say the same thing that you and I agree on.
Anyway, I love this thread as well. I just glanced back and saw how we progressed from CG and TAS, to Phugoid dampning, to pitching moments with flaps, to types of drag, etc. Good stuff!
Skeezer
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