Thrust and Speed
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Maintaining altidude or in an ILS I think we all keep path with pitch, speed with thrust, don't we?
The point is that any change in speed requires a pitch input to maintain the flightpath. If you're any good you progressively adjust attitude during the speed change rather than waiting for the flight path to deviate and then correcting it.
Most AP/FD modes are closed loop feedback, so they have to wait for an error before correcting it.
All manouevres are controlled with pitch and power. The fact that in some cases you end up with the thrust lever either fully closed or fully open doesn't mean that thrust isn't a control input, it just means that you've run out of control authority.
I don't see the need to over simplify this into sound bites, its not like it's that complicated in the first place.
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Hi Capt PB
Yes, I know both thrust and pitch have effect in both path and speed, but:
Isn't it easier to "divide" the tasks in our brain (or in the FCC or FGC or whatever) and assign flight path to the pitch input and speed to the thrust input?
It is like dutch roll. Automatic dampers use yaw. But damping it manually is better done with roll, they say. They are interrelated motions. Each variable affects the others. So all variables should be considered. Letting one constant helps.
Of course, if you want to recover the G/S from slightly above with correct speed, you know that you will need to reduce thrust in addition to the lower pitch command, so you do it before the speed increases.
I guess it is all about energy state, instrument scan, attitude and thrust changes coordination and experience (a "model" in computers). Computers have a "super" scan rate, and trend sensing, and gains, and many other things I have never heard about. I think that the path-pitch and speed-thrust "alocation" is good for both computers and humans.
In an ILS, I know that many use thrust for glidepath and pitch for speed. But in level flight, I think we all do it the other way round. Because it is easier, or more intuitive, if you like.
Yes, I know both thrust and pitch have effect in both path and speed, but:
Isn't it easier to "divide" the tasks in our brain (or in the FCC or FGC or whatever) and assign flight path to the pitch input and speed to the thrust input?
It is like dutch roll. Automatic dampers use yaw. But damping it manually is better done with roll, they say. They are interrelated motions. Each variable affects the others. So all variables should be considered. Letting one constant helps.
Of course, if you want to recover the G/S from slightly above with correct speed, you know that you will need to reduce thrust in addition to the lower pitch command, so you do it before the speed increases.
I guess it is all about energy state, instrument scan, attitude and thrust changes coordination and experience (a "model" in computers). Computers have a "super" scan rate, and trend sensing, and gains, and many other things I have never heard about. I think that the path-pitch and speed-thrust "alocation" is good for both computers and humans.
In an ILS, I know that many use thrust for glidepath and pitch for speed. But in level flight, I think we all do it the other way round. Because it is easier, or more intuitive, if you like.
I don't see the need to over simplify this into sound bites, its not like it's that complicated in the first place.
If the speed drops, you put the power up. If you then start going high on the GS, you push the nose down.
Simple!
In an ILS, I know that many use thrust for glidepath and pitch for speed.
Change either pitch or power, and the aircraft takes time before it settles down again into a steady state. Power primarily controls Rate of Climb / Rate of descent and Pitch primarily controls speed. Inclimb and cruise this is how we fly all aircraft.
On approach, in a light aircraft these changes settle down pretty quickly, and thus the pilot can still fly the aircraft as above. In an airliner, the time taken for the aircraft to settle to a steady state in too long for the approach phase, so it is better to think on the thrust controlling the speed, and the pitch the ROD.
On approach, in a light aircraft these changes settle down pretty quickly, and thus the pilot can still fly the aircraft as above. In an airliner, the time taken for the aircraft to settle to a steady state in too long for the approach phase, so it is better to think on the thrust controlling the speed, and the pitch the ROD.
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Teaching methods that become dogma
Captain PB is totally correct - it is about energy management.
Checkboard is also correct in that some changes appear to work more quickly than others.
The old "power + attitude = performance" is true and correctly reflects that thrust (power) and angle of attack (attitude) are inextricably linked. But empirically we know that we can place more emphasis on one or the other depending upon our flight path management requirement.
If we are in a fixed power climb or descent, then the only way we can affect our performance, whether it be airspeed or RoC/RoD, is with angle of attack (attitude). Since one of our two key parameters is fixed, we can't totally control our flight path to achieve a specific point in space and we accept that every day - our flight path is not constrained and we happily accept variations in the end point. In this case, we happily control airspeed with attitude (which is really drag with angle of attack) and allow whatever excess power that exists to determine where we end up. And generally, that is the most fuel efficient process.
If we can control both variables but still don't care too much about where we end up, then we tend to select a fixed power anyway.
But the difference from a teaching perspective arises when we wish to constrain the flight path to achieve a particular energy state at a particular point in space. This can arise on climb but is most commonly associated with approaches. The simplest thing to teach someone to achieve a point in space is to use the most effective pointing control, the aircraft attitude - "find your endpoint and fly the aircraft to that point". Clearly, as you push and pull (and, to a much lesser extent, roll and yaw) the changes in attitude (angle of attack) will significantly change the associated drag and hence the speed. As a generalisation, it is more important to be on path than on speed (provided that the speed is not dangerously low) and thrust changes on a constrained flight path have the most immediate effect on speed. Power and attitude remain inextricably linked, but we choose to emphasise them separately in terms of the most obvious outcome - when we fly to a point in space, "attitude controls flight path and thrust controls speed".
I have had enough red wine lubricated discussions with naval aviators to know that they don't share that view - my understanding is that they tend to fix the angle of attack (as directly indicated to them in the cockpit) and therefore their approach speed and use their very responsive engines to fly into the approach path for the wires. And even though I have spent 30 odd years very successfully teaching what I have outlined above, it remains a teaching method rather than some form of scientific absolute.
Having said all of the above, it has always worried me that we have a growing band of aviators who attempt to emulate autopilots in flying the aircraft. Autopilots and autothrottles are simple engineering devices of limited capability and deliberately limited control authority that lack our ability to analyse and predict certain future states. For example, an autothrottle sensing an underspeed cannot predict what you or the autopilot are about to do with the attitude and the resultant drag changes - it cannot decide to leave the power where it is because you happen to be high on path and are about to lower the nose and reduce the drag. Autopilots without autothrottle can only control pitch and roll in response to sensed path error - they can't tell if you are about to stall on the ILS or overspeed the flap. My point here is that they can be good but they can't be (by design) smarter than you. You must know how they operate before you can choose to take any lessons from them.
Just remember, we break things down to teach more effectively - it does not change the physics. When you are really good at flying, the breakdown essentially disappears as you smoothly coordinate all the controls to achieve a smooth flight path with the energy managed to achieve the required targets.
Stay Alive....
Checkboard is also correct in that some changes appear to work more quickly than others.
The old "power + attitude = performance" is true and correctly reflects that thrust (power) and angle of attack (attitude) are inextricably linked. But empirically we know that we can place more emphasis on one or the other depending upon our flight path management requirement.
If we are in a fixed power climb or descent, then the only way we can affect our performance, whether it be airspeed or RoC/RoD, is with angle of attack (attitude). Since one of our two key parameters is fixed, we can't totally control our flight path to achieve a specific point in space and we accept that every day - our flight path is not constrained and we happily accept variations in the end point. In this case, we happily control airspeed with attitude (which is really drag with angle of attack) and allow whatever excess power that exists to determine where we end up. And generally, that is the most fuel efficient process.
If we can control both variables but still don't care too much about where we end up, then we tend to select a fixed power anyway.
But the difference from a teaching perspective arises when we wish to constrain the flight path to achieve a particular energy state at a particular point in space. This can arise on climb but is most commonly associated with approaches. The simplest thing to teach someone to achieve a point in space is to use the most effective pointing control, the aircraft attitude - "find your endpoint and fly the aircraft to that point". Clearly, as you push and pull (and, to a much lesser extent, roll and yaw) the changes in attitude (angle of attack) will significantly change the associated drag and hence the speed. As a generalisation, it is more important to be on path than on speed (provided that the speed is not dangerously low) and thrust changes on a constrained flight path have the most immediate effect on speed. Power and attitude remain inextricably linked, but we choose to emphasise them separately in terms of the most obvious outcome - when we fly to a point in space, "attitude controls flight path and thrust controls speed".
I have had enough red wine lubricated discussions with naval aviators to know that they don't share that view - my understanding is that they tend to fix the angle of attack (as directly indicated to them in the cockpit) and therefore their approach speed and use their very responsive engines to fly into the approach path for the wires. And even though I have spent 30 odd years very successfully teaching what I have outlined above, it remains a teaching method rather than some form of scientific absolute.
Having said all of the above, it has always worried me that we have a growing band of aviators who attempt to emulate autopilots in flying the aircraft. Autopilots and autothrottles are simple engineering devices of limited capability and deliberately limited control authority that lack our ability to analyse and predict certain future states. For example, an autothrottle sensing an underspeed cannot predict what you or the autopilot are about to do with the attitude and the resultant drag changes - it cannot decide to leave the power where it is because you happen to be high on path and are about to lower the nose and reduce the drag. Autopilots without autothrottle can only control pitch and roll in response to sensed path error - they can't tell if you are about to stall on the ILS or overspeed the flap. My point here is that they can be good but they can't be (by design) smarter than you. You must know how they operate before you can choose to take any lessons from them.
Just remember, we break things down to teach more effectively - it does not change the physics. When you are really good at flying, the breakdown essentially disappears as you smoothly coordinate all the controls to achieve a smooth flight path with the energy managed to achieve the required targets.
Stay Alive....
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mb2002
4dogs has it nailed. I have no problem with, in a particular type, the flying technique being taught in a particular way. My issue is that people tend to take that specific technique and then assume dogmatically that it is a complete statement that applies to all aircraft.
pb
Isn't it easier to "divide" the tasks in our brain (or in the FCC or FGC or whatever) and assign flight path to the pitch input and speed to the thrust input?
Just remember, we break things down to teach more effectively - it does not change the physics. When you are really good at flying, the breakdown essentially disappears as you smoothly coordinate all the controls to achieve a smooth flight path with the energy managed to achieve the required targets.
pb
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agree w capt pb 100%. you wanna know how to fly A PARTICULAR acft, study its FCTM, but never assume those techniques apply wholesale to ALL acft. otherwise, they'd just print a general FCTM, and every manufacturer would just state "For flt techniques, refer to gen FCTM." 
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I agree with both Captain PB and 4dogs (excellent post, by the way).
Actually, when I hand fly an ILS (not very often, I'm afraid) I usually maintain flight path one way (like the AP/FD A/TRH) as well the other way, instinctively, depending on circumstances. My body and my brain know what is required and when, so I just coordinate the two inputs (power and attitude) to achieve the desired performance (on glidepath, on speed) as necessary.
I guess we all do that. It is difficult to explain, so we set a simplifying rule. Or two. And then, we have something to discuss about.
cheers
Actually, when I hand fly an ILS (not very often, I'm afraid) I usually maintain flight path one way (like the AP/FD A/TRH) as well the other way, instinctively, depending on circumstances. My body and my brain know what is required and when, so I just coordinate the two inputs (power and attitude) to achieve the desired performance (on glidepath, on speed) as necessary.
I guess we all do that. It is difficult to explain, so we set a simplifying rule. Or two. And then, we have something to discuss about.
cheers
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Old, old tale (forgive me if this is elsewhere in this thread):
A nasty FAA Inspector boarded the aircraft with the crew. As they were preflighting the fed asked the captain, "What controls airspeed and what controls attitude/altitude?
The captain answers, "Power controls airspeed and pitch controls attitude/altitude."
Fed, "That is SO WRONG! You have it backwards!!"
Nothing more is said. They taxi out, then are cleared in position.
The captain calmly tells the F/O, "I'll pump the Hell out of the control column and when we reach 130 knots you cob the power to it."
A nasty FAA Inspector boarded the aircraft with the crew. As they were preflighting the fed asked the captain, "What controls airspeed and what controls attitude/altitude?
The captain answers, "Power controls airspeed and pitch controls attitude/altitude."
Fed, "That is SO WRONG! You have it backwards!!"
Nothing more is said. They taxi out, then are cleared in position.
The captain calmly tells the F/O, "I'll pump the Hell out of the control column and when we reach 130 knots you cob the power to it."
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Well, considering that most of those posting here have many thousands of hours of successful flight experience behind them, one has to assume that there is more than one way to skin the cat, so-to-speak.
One difference that perhaps migrates into our collective experience lies with propeller slipstream effects, or lack thereof. I believe there was one poster who reflected this in his past experience; it is highlighted by both Webb and Davies in their books when addressing this topic. Abzug, in Airplane Stability and Control, explains that propeller slipstream, in addition to providing nearly instant lift increase with zero speed change, can also influence longitudinal stability.
Abzug also points out that low aspect ratio wings, such as those used by carrier borne aircraft, are notoriously difficult to manage when power is used to control airspeed. The problem lies with the drag curve and where one is on that curve during the approach. In carrier ops, the approach speed is below the minimum drag speed; in air carrier ops with a much larger aspect ratio, it is not. This is one of the reasons that naval aviation opts for a firm policy of using a constant angle of attack to establish airspeed, with power used to control altitude. In fact, the power control issue is so important that throttle control systems must meet specific requirements for carrier operations. Ultimately, the problem of carrier operations seems to have led Lockheed to develop direct lift control for the S3 Viking, which of course later appeared on the L-1011.
An aspect of this issue that I have not seen mentioned here is the problem of pilot gain, or ability to track and respond to deviations. Gain is influenced by a whole range of things, including instrument response, mass and momentum, the relationship of the thrust line to the center of gravity, and the different thrust responses from constant speed turboprops to pure jets to high bypass jets. All of these things are considered, in contemporary designs anyway, in order to keep the pilot “in the loop” and able to make small but precise changes.
Some have said that this is not that complicated; actually I believe that it is extremely complicated, but fortunately most of the complication is removed by the designer. The result of a lot of good engineering is that most of us never fly anything with nasty characteristics, allowing us to employ a range of strategies as we have discussed here.
That said, it would seem prudent to be cautious when comparing manual flight techniques to autoflight principles. Depending on aircraft and autoflight generation, the autoflight system is capable of a much higher gain than the pilot. It is not uncommon for the autoflight system on the 757/767 that I fly, when properly maintained, to detect and respond to a deviation before it can even be seen through the flight director system. This high gain allows the autoflight system to employ one strategy to keep the four forces in balance, while the pilot might get better results using a different strategy.
It truly is unfortunate that our primary education on this tends to be lacking, leading us to import one “ironclad” axiom from one class of aircraft into another class for which it might not be appropriate. I suspect that between the carrier pilots, the large turboprop pilots, the pilots who learned on pure jets and the high bypass crowd, we have an amalgam of ideas and techniques that get passed around without the necessary differentiation.
One difference that perhaps migrates into our collective experience lies with propeller slipstream effects, or lack thereof. I believe there was one poster who reflected this in his past experience; it is highlighted by both Webb and Davies in their books when addressing this topic. Abzug, in Airplane Stability and Control, explains that propeller slipstream, in addition to providing nearly instant lift increase with zero speed change, can also influence longitudinal stability.
Abzug also points out that low aspect ratio wings, such as those used by carrier borne aircraft, are notoriously difficult to manage when power is used to control airspeed. The problem lies with the drag curve and where one is on that curve during the approach. In carrier ops, the approach speed is below the minimum drag speed; in air carrier ops with a much larger aspect ratio, it is not. This is one of the reasons that naval aviation opts for a firm policy of using a constant angle of attack to establish airspeed, with power used to control altitude. In fact, the power control issue is so important that throttle control systems must meet specific requirements for carrier operations. Ultimately, the problem of carrier operations seems to have led Lockheed to develop direct lift control for the S3 Viking, which of course later appeared on the L-1011.
An aspect of this issue that I have not seen mentioned here is the problem of pilot gain, or ability to track and respond to deviations. Gain is influenced by a whole range of things, including instrument response, mass and momentum, the relationship of the thrust line to the center of gravity, and the different thrust responses from constant speed turboprops to pure jets to high bypass jets. All of these things are considered, in contemporary designs anyway, in order to keep the pilot “in the loop” and able to make small but precise changes.
Some have said that this is not that complicated; actually I believe that it is extremely complicated, but fortunately most of the complication is removed by the designer. The result of a lot of good engineering is that most of us never fly anything with nasty characteristics, allowing us to employ a range of strategies as we have discussed here.
That said, it would seem prudent to be cautious when comparing manual flight techniques to autoflight principles. Depending on aircraft and autoflight generation, the autoflight system is capable of a much higher gain than the pilot. It is not uncommon for the autoflight system on the 757/767 that I fly, when properly maintained, to detect and respond to a deviation before it can even be seen through the flight director system. This high gain allows the autoflight system to employ one strategy to keep the four forces in balance, while the pilot might get better results using a different strategy.
It truly is unfortunate that our primary education on this tends to be lacking, leading us to import one “ironclad” axiom from one class of aircraft into another class for which it might not be appropriate. I suspect that between the carrier pilots, the large turboprop pilots, the pilots who learned on pure jets and the high bypass crowd, we have an amalgam of ideas and techniques that get passed around without the necessary differentiation.
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We're getting pretty close to a group hug here !
Mansfield makes a lot of sense to me. I still maintain that the underlying principle is simple, but agree that this doesn't mean that the aircraft handling or the requisite control engineering is going to be simple.
("simple" is not a synonym for "easy". E.g. if you fall off a cliff the problem is simple... )
One of my favorite sayings (in response to "it isn't rocket science"):
"Rocket science is easy - its just Newton 3. Rocket engineering on the other hand... now, there's a challenge."
pb
Mansfield makes a lot of sense to me. I still maintain that the underlying principle is simple, but agree that this doesn't mean that the aircraft handling or the requisite control engineering is going to be simple.
("simple" is not a synonym for "easy". E.g. if you fall off a cliff the problem is simple... )
One of my favorite sayings (in response to "it isn't rocket science"):
"Rocket science is easy - its just Newton 3. Rocket engineering on the other hand... now, there's a challenge."
pb
