Longitudinal Stability
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I think you are referring here to stick force stability, not aeroplane stability.
Negative .. refer, for instance, to FAR25.173 and FAR 25.175.
All the usual stories about balls in teacups and such are fine but, at the end of the day, it comes back to off trim speed stick forces. Indeed, I always find the ball and teacup confusing .. unless we are talking about HOLDING the ball somewheres up the side of the teacup ... but that might just be me, I guess.
Negative .. refer, for instance, to FAR25.173 and FAR 25.175.
All the usual stories about balls in teacups and such are fine but, at the end of the day, it comes back to off trim speed stick forces. Indeed, I always find the ball and teacup confusing .. unless we are talking about HOLDING the ball somewheres up the side of the teacup ... but that might just be me, I guess.
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Hi Darth, you say...
'From what I've read so far: when we hit a gust (up, increasing angle of attack) the 'plane will naturally want to pitch back nose down - due to the longitudinal stability. But my (rather limited) experience seems to suggest that the aircraft will want to pitch up and climb whenever we hit a gust.'
I think what it really means is that it will want to pitch back down -AFTER THE GUST- has occurred.
If you have gained height in the gust, you will have also lost speed, so the plane will be flying slower and producing less lift... hence it descends to increase speed, back to the previous level, and so the start of a phugoid oscillation... see chapter 6.1.14
For my Cessna 172 a stick induced pull up of 200ft resulted in a loss of 20 knots. Then when I released the stick the plane dived 300ft in 18s and regained 30 knots, then it climbed 200ft and lost 20kts in the next 18s, then dropped about 100ft and was just about back where we started from.
The relationship between speed and height is mentioned in chapter 1.2.1... Try to remember the conversion formula. It is very near 10ft/kt/100kts i.e. one division of the altimeter is equal to one division of the ASI at 100kts... but only at 100kts (it is half that at 50kts.). As one goes up the other goes down.
'From what I've read so far: when we hit a gust (up, increasing angle of attack) the 'plane will naturally want to pitch back nose down - due to the longitudinal stability. But my (rather limited) experience seems to suggest that the aircraft will want to pitch up and climb whenever we hit a gust.'
I think what it really means is that it will want to pitch back down -AFTER THE GUST- has occurred.
If you have gained height in the gust, you will have also lost speed, so the plane will be flying slower and producing less lift... hence it descends to increase speed, back to the previous level, and so the start of a phugoid oscillation... see chapter 6.1.14
For my Cessna 172 a stick induced pull up of 200ft resulted in a loss of 20 knots. Then when I released the stick the plane dived 300ft in 18s and regained 30 knots, then it climbed 200ft and lost 20kts in the next 18s, then dropped about 100ft and was just about back where we started from.
The relationship between speed and height is mentioned in chapter 1.2.1... Try to remember the conversion formula. It is very near 10ft/kt/100kts i.e. one division of the altimeter is equal to one division of the ASI at 100kts... but only at 100kts (it is half that at 50kts.). As one goes up the other goes down.
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phiggsbroadband,
I think you're comparing apples and oranges.
A stick-induced pull-up results in an exchange of kinetic and potential energy at more-or-less constant total energy.
An upward gust adds energy, increasing height at constant speed.
I think you're comparing apples and oranges.
A stick-induced pull-up results in an exchange of kinetic and potential energy at more-or-less constant total energy.
An upward gust adds energy, increasing height at constant speed.
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Hi John, you quote...
Negative .. refer, for instance, to FAR25.173 and FAR 25.175.
If you notice these refer to Transport Category Aircraft. The stick only controls a piston in the Hydraulic Servos, which move the flying surfaces. The stick forces are all engineered feedback, much the same as power steering on modern cars... Its all artificial feedback, and on some cars can be switched high or low, or even a different sensitivity at different speeds.
Negative .. refer, for instance, to FAR25.173 and FAR 25.175.
If you notice these refer to Transport Category Aircraft. The stick only controls a piston in the Hydraulic Servos, which move the flying surfaces. The stick forces are all engineered feedback, much the same as power steering on modern cars... Its all artificial feedback, and on some cars can be switched high or low, or even a different sensitivity at different speeds.
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Hi,
I understand if we lose speed we loose lift from L = Cl.1/2.rho.V2.S
That would result in pitch down due to main wing loosing lift. What if the gust means that airspeed is constant (maybe highly theoretical...)?
The restoring force is only coming from the turning moment of the tail in that case? In that case lift from both wings (main and tail) is only modified by the relative change in AoA of both.
Maybe I'm getting out of my depth by getting too technical. I guess my question now is this:
If an upward gust causes a aeroplane to pitch up (consider speed to be constant), will the restoring effect of stability cause it to pitch back toward level, by pitching nose down slightly, even before the gust is removed? Or does the restoring pitch down only happen when the gust is removed?
My instinct leans toward the latter, which means the extra down pitch moment of the tailplane will not be present (gust removed at this stage, so the larger proportion of AoA increase on the tail, and hence larger turning moment, is removed). So the balance of forces will return to the pre-gust situation.
Is the tendency during the gust (constant speed) to start to recover to level, but this will be achieved fully once gust is removed? Maybe there is no such thing as a gust where speed remains constant??
I understand if we lose speed we loose lift from L = Cl.1/2.rho.V2.S
That would result in pitch down due to main wing loosing lift. What if the gust means that airspeed is constant (maybe highly theoretical...)?
The restoring force is only coming from the turning moment of the tail in that case? In that case lift from both wings (main and tail) is only modified by the relative change in AoA of both.
Maybe I'm getting out of my depth by getting too technical. I guess my question now is this:
If an upward gust causes a aeroplane to pitch up (consider speed to be constant), will the restoring effect of stability cause it to pitch back toward level, by pitching nose down slightly, even before the gust is removed? Or does the restoring pitch down only happen when the gust is removed?
My instinct leans toward the latter, which means the extra down pitch moment of the tailplane will not be present (gust removed at this stage, so the larger proportion of AoA increase on the tail, and hence larger turning moment, is removed). So the balance of forces will return to the pre-gust situation.
Is the tendency during the gust (constant speed) to start to recover to level, but this will be achieved fully once gust is removed? Maybe there is no such thing as a gust where speed remains constant??
gusts
But my (rather limited) experience seems to suggest that the aircraft will want to pitch up and climb whenever we hit a gust.
Anyway:
The CoG ahead of the CoP is the first thing to think about. In this sense the aircraft is like a dart. There is nothing else to worry about in the first instance. Get some darts and throw then backwards, sideways, everyway you like. Watch the flight.
Don't worry about the direction of the gusts or anything else.
The aircraft is flying along in a certain direction and a perturbation in the air changes its attitude. It is STILL moving through the air in the same direction. In the case of pitch and yaw, the 'dart' stability restores the aircraft so that it regains its orientation pointing in the direction of travel. Roll stability is different.
Now there is a lot more to aircraft stability as understood by test pilots but I think that this is the place to start and indeed may be sufficient for a beginning pilot. As far as I recall it was for me and I have the advantage of never having progressed beyond being a beginning pilot:-)
I did not know for instance that the tailplane produced downward thrust in normal flight or indeed that a canard produces upward thrust and is therefore more efficient (uses less fuel).
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Hi Hazelnuts.. you quote..
'An upward gust adds energy, increasing height at constant speed. '
I think you are 30% right. But that ignores the pitch up of the nose caused by the gust, which goes into adding the further 70% of the climb.
The nose of the plane then has to pitch down.. It does slightly too much, so it has to pitch up again, and again overshoots.. etc. ad infinitum.
'An upward gust adds energy, increasing height at constant speed. '
I think you are 30% right. But that ignores the pitch up of the nose caused by the gust, which goes into adding the further 70% of the climb.
The nose of the plane then has to pitch down.. It does slightly too much, so it has to pitch up again, and again overshoots.. etc. ad infinitum.
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Is the tendency during the gust (constant speed) to start to recover to level, but this will be achieved fully once gust is removed? Maybe there is no such thing as a gust where speed remains constant??
Let's assume a sharp-edged gust of constant velocity. When an airplane in steady level flight encounters an upward gust, the angle of attack increases. As the angle of attack increases, the lift increases, and when the lift is greater than the weight the airplane will be accelerated upwards at initially the same speed and pitch attitude. A stable airplane tends to return to the angle of attack it is trimmed at, so the airplane will tend to pitch down to regain the initial angle of attack. However, that tendency is not very strong but the angle of attack decreases mainly because the vertical speed increases. When the airplane vertical speed is equal to the gust velocity, the airplane continues to climb, at the trimmed angle of attack and speed, at the same pitch attitude as it had initially in level flight. When leaving the gust the airplane goes through a similar, but reversed sequence but its final altitude will be greater than where it started, by an amount of approximately the gust velocity times the time to traverse it.
Last edited by HazelNuts39; 25th Nov 2013 at 15:15.
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Doesn't angle of attack stability really mean maneuver stability (no matter how you spell it).
Static stability is the tendency of the aircraft to return to the original trim condition when an disturbance has been encountered and then removed.
Stick force and stick position are two different measures of this. With an irreversible flight control system (i.e. all hydraulically boosted) there is no difference between the two stabilities as the forces on the cockpit control must be artificially generated. With a reversible flight control system, stick free stability is what you get when you release the stick forces (as the friction in the system may prevent the stick from returning to the original trim condition). When you place the stick back at the original condition, that is stick fixed stability.
Confused?? Try teaching this to test pilots and flight test engineers!!!
Static stability is the tendency of the aircraft to return to the original trim condition when an disturbance has been encountered and then removed.
Stick force and stick position are two different measures of this. With an irreversible flight control system (i.e. all hydraulically boosted) there is no difference between the two stabilities as the forces on the cockpit control must be artificially generated. With a reversible flight control system, stick free stability is what you get when you release the stick forces (as the friction in the system may prevent the stick from returning to the original trim condition). When you place the stick back at the original condition, that is stick fixed stability.
Confused?? Try teaching this to test pilots and flight test engineers!!!
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If you are in a J3 cub with no gyros turn to 180 and trim,reduce power and you can safely descend with no gyros through a cloud deck as long as you hold your heading in an emergency. It works for jets too.
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If you notice these refer to Transport Category Aircraft
Indeed.
However, were one to rub out "25" and pencil in "23" one gets much the same story for bugsmashers .. as in FAR 23.173 and FAR 23.175.
The interest is in what the pilot "feels" at the stick, not what wizardy fools him/her into experiencing that perception. You might read up on elevator down springs and bobweights to see the simple ways of making reality morph this way and that ...
A lot of discussion in this thread confuses the two basic stability animals ... "static" (when one applies a load to the stick) and "dynamic" (when one releases it and sees what happens next).
Indeed.
However, were one to rub out "25" and pencil in "23" one gets much the same story for bugsmashers .. as in FAR 23.173 and FAR 23.175.
The interest is in what the pilot "feels" at the stick, not what wizardy fools him/her into experiencing that perception. You might read up on elevator down springs and bobweights to see the simple ways of making reality morph this way and that ...
A lot of discussion in this thread confuses the two basic stability animals ... "static" (when one applies a load to the stick) and "dynamic" (when one releases it and sees what happens next).
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When a gust upsets the 'plane, upwards for example, the angle of attack is increased on both the wing and tail. Both will increase in lift assuming airspeed is constant. The longer moment arm of the tail means it has a greater turning moment than the wing
L=Cl*d*square V*A/2
A is the surface of the airfoil.the surface of the wing is much bigger than the horizontal stabilizer.
L=Cl*d*square V*A/2
A is the surface of the airfoil.the surface of the wing is much bigger than the horizontal stabilizer.
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The tail is not lift, it is a downward force to give the aircraft stability.
The wing is the only surface that counteracts gravity which is lift. The tail down force must be supported by the wing lift force and why some aircraft save fuel by keeping the CG back in cruise. Airbus is one and we figured that ourselves in the Lear Jet by transfering fuel back and being able to reduce cruise power.
The wing is the only surface that counteracts gravity which is lift. The tail down force must be supported by the wing lift force and why some aircraft save fuel by keeping the CG back in cruise. Airbus is one and we figured that ourselves in the Lear Jet by transfering fuel back and being able to reduce cruise power.
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some aircraft save fuel by keeping the CG back in cruise
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Indeed - I recall an RAeS lecture by a QF chap which detailed a study for the 744 on max range sectors.
By some judicious variation to the OEM standard fuel schedule, the airline was able to vary the range-payload characteristic and pick up, as I recall, an additional several bodies in the paying section for the sector ....
By some judicious variation to the OEM standard fuel schedule, the airline was able to vary the range-payload characteristic and pick up, as I recall, an additional several bodies in the paying section for the sector ....
