AoA instrumentation
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USN Carrier Approach Culture
PBL,
A few coments on the subject:
The LSOs have an extremely accurate means of determining AOA of an approaching aircraft. It is familiarly known as the Mk 1 Mod 0 eyeball. The LSOs stand abeam the touchdown area on the carrier and observe aircraft attitude using key landmarks on each aircraft's structure. Things like how much of the rudder is visible or the position of the horizontal stabilizer relative to the wing are used. The aircraft also illuminates its approach light system which mirrors the pilot's AOA indications. Green light=slow, Amber=onspeed, Red=fast. There is/was a doppler system aboard ship for measuring closing velocity. In my day, an enlisted man pointed it at the aircraft and a readout was available at the LSO platform, on the PLAT (TV) and in other locations around the ship. But the key indicator of approaching aircraft AOA was the visual "picture".
One of the key reasons for the USN method is the strange concept
that an aircraft should be fully trimmed up to maintain stable flight. Once an aircraft is trimmed up to approach speed, even an approach speed well on the backside of the power curve, the aircraft will try to hold that speed without much intervention. Now if you add sufficient power to hold a 3.5 or 4 degree glideslope, you will have a stabilized glidepath. If the pilot sees that the aircraft is trending down on the glidepath, he adds sufficient power to reverse the trend for a few seconds and then resets the original power plus a small amount (I used to be able to do this by ear). The pilot could then fly an almost completely heads up approach scanning the 3 key factors-meatball, lineup, AOA without fear of falling out of the sky approaching the spud locker. His stick inputs are done with three fingers and are directed at damping the phugoid and controlling lineup.
Navy carrier aircraft are speed stable on approach. The F-4 for example, although having an artificial feel control system, used a bellows balanced against downsprings for pitch feel control. The bellows took a pitot air signal and the force it generated was used in a balance beam system to control airspeed. As you trimmed the aircraft, you adjusted the fulcrum of the balance beam.
Personally I don't see any other way to think about the power-airspeed-climb/descent relationship. If aircraft were not naturally speed stable (at least as they are now designed) then perhaps power would equal speed and the attitude/ nose position would equal climb, but to me this is bass ackwards.
In airline type operations where you aren't really flying the aircraft (the autopilot is) then of course using more power will make you go faster, but that isn't the way the aircraft actually natively acts.
A few coments on the subject:
The LSOs have an extremely accurate means of determining AOA of an approaching aircraft. It is familiarly known as the Mk 1 Mod 0 eyeball. The LSOs stand abeam the touchdown area on the carrier and observe aircraft attitude using key landmarks on each aircraft's structure. Things like how much of the rudder is visible or the position of the horizontal stabilizer relative to the wing are used. The aircraft also illuminates its approach light system which mirrors the pilot's AOA indications. Green light=slow, Amber=onspeed, Red=fast. There is/was a doppler system aboard ship for measuring closing velocity. In my day, an enlisted man pointed it at the aircraft and a readout was available at the LSO platform, on the PLAT (TV) and in other locations around the ship. But the key indicator of approaching aircraft AOA was the visual "picture".
One of the key reasons for the USN method is the strange concept
that an aircraft should be fully trimmed up to maintain stable flight. Once an aircraft is trimmed up to approach speed, even an approach speed well on the backside of the power curve, the aircraft will try to hold that speed without much intervention. Now if you add sufficient power to hold a 3.5 or 4 degree glideslope, you will have a stabilized glidepath. If the pilot sees that the aircraft is trending down on the glidepath, he adds sufficient power to reverse the trend for a few seconds and then resets the original power plus a small amount (I used to be able to do this by ear). The pilot could then fly an almost completely heads up approach scanning the 3 key factors-meatball, lineup, AOA without fear of falling out of the sky approaching the spud locker. His stick inputs are done with three fingers and are directed at damping the phugoid and controlling lineup.Navy carrier aircraft are speed stable on approach. The F-4 for example, although having an artificial feel control system, used a bellows balanced against downsprings for pitch feel control. The bellows took a pitot air signal and the force it generated was used in a balance beam system to control airspeed. As you trimmed the aircraft, you adjusted the fulcrum of the balance beam.
Personally I don't see any other way to think about the power-airspeed-climb/descent relationship. If aircraft were not naturally speed stable (at least as they are now designed) then perhaps power would equal speed and the attitude/ nose position would equal climb, but to me this is bass ackwards.
In airline type operations where you aren't really flying the aircraft (the autopilot is) then of course using more power will make you go faster, but that isn't the way the aircraft actually natively acts.
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Machinbird,
you say that because of the speed stability of certain aircraft, things work best with using pitch to affect airspeed and power to affect flight path. Well, yes, that is also the way I learnt to think in my instrument training in the U.S.; as JF says, it is a culture thing.
Here is the other point of view, the aerodynamic one
Pitch affects AoA most directly, AoA means coefficient of lift, and coefficient of lift means lift which means, balancing against weight, flight path.
Pitch also affects thrust vector, and flattening that is also going to take something out of total lift.
Speed stable or not, aerodynamics says you're going down, and directly.
Isn't that just as true? Indeed, truer! Fly flatter with the same thrust, you're going down, instantaneously.
Why is that aspect somehow suppressed?
There has to be some reason why the one way of looking things is more compelling to a large proportion of pilots. I don't know what it is at this point, likely because I have not thought it through enough. I am not yet convinced that speed stability is the explanation.
PBL
you say that because of the speed stability of certain aircraft, things work best with using pitch to affect airspeed and power to affect flight path. Well, yes, that is also the way I learnt to think in my instrument training in the U.S.; as JF says, it is a culture thing.
Here is the other point of view, the aerodynamic one
Pitch affects AoA most directly, AoA means coefficient of lift, and coefficient of lift means lift which means, balancing against weight, flight path.
Pitch also affects thrust vector, and flattening that is also going to take something out of total lift.
Speed stable or not, aerodynamics says you're going down, and directly.
Isn't that just as true? Indeed, truer! Fly flatter with the same thrust, you're going down, instantaneously.
Why is that aspect somehow suppressed?
There has to be some reason why the one way of looking things is more compelling to a large proportion of pilots. I don't know what it is at this point, likely because I have not thought it through enough. I am not yet convinced that speed stability is the explanation.
PBL
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PBL
Seems that the difference is flying in stabilized flight versus unstabilized flight.
If I wanted to intercept something 45 degrees above me, I'd plug in burners and haul the nose up to put it where I wanted it-but the aircraft's kinetic energy would be changing (decreasing) as the potential energy would be increasing. That is the "aerodynamic approach to flying. When you are trading potential and kinetic energy back and forth, why bother to trim?
On approach to the ship at 1.1 Vs, if you see you are going to be a little short and inch the nose up, you had better match that with a power change or in two seconds you will begin to devellop a dangerous rate of sink due to airspeed loss. Typically there is only 11-15 feet of tail hook to ramp clearance for on glide slope operations so unstabilized flight on approach to the boat is a no-no.
And if you want to decel from 300 knots to 250 knots as you go below 10,000 ft on an approach, you would trim to keep your speed stable and adjust rate of altitude change with your power, wouldn't you?
Each technique has its place depending on what you are doing with the aircraft.
Seems that the difference is flying in stabilized flight versus unstabilized flight.
If I wanted to intercept something 45 degrees above me, I'd plug in burners and haul the nose up to put it where I wanted it-but the aircraft's kinetic energy would be changing (decreasing) as the potential energy would be increasing. That is the "aerodynamic approach to flying. When you are trading potential and kinetic energy back and forth, why bother to trim?
On approach to the ship at 1.1 Vs, if you see you are going to be a little short and inch the nose up, you had better match that with a power change or in two seconds you will begin to devellop a dangerous rate of sink due to airspeed loss. Typically there is only 11-15 feet of tail hook to ramp clearance for on glide slope operations so unstabilized flight on approach to the boat is a no-no.
And if you want to decel from 300 knots to 250 knots as you go below 10,000 ft on an approach, you would trim to keep your speed stable and adjust rate of altitude change with your power, wouldn't you?
Each technique has its place depending on what you are doing with the aircraft.
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My thoughts and experience
Regarding these quotes...
I agree. I'm a glider pilot myself and if you want to speed up you pitch down, to slow down you pitch up. Every glider pilot should understand that.
I agree that in the majority of flight "situations" and the majority of aircraft, the concept that speed is controlled by AoA (pitch) and climb/descent controlled by thrust (throttle), will provide the best control and handling of the aircraft. Only in some flight situations is it reversed; one I can think of is slow flight recovery... If you break it down into two separate actions you'll see that if you first pitch forward, you will lose altitude. So by adding thrust first you will prevent altitude loss, and then pitching forward will build airspeed. That is what I teach in those situations. Both actions should happen very close together.
Advocates of pitching down first then adding throttle use the argument that when you pitch forward you are decreasing the AoA (correct), and therefore you decrease induced drag and allow the aircraft to accelerate by adding throttle. To do it this way, you will lose altitude the moment you pitch forward. In slow flight you have not stalled and so stall recovery is not beneficial.
This is something like the recovery from slow flight. You're on approach and 10 kts slow and you don't want to lose altitude on your recovery. Applying full throttle in this case would be the thing to do.
This is leading to my last point. Although these are treated as two separate inputs that affect two separate outputs, if you change one you have to change the other! If you are at your correct speed on your approach and are descending too fast, if you add throttle to compensate, you will have to pitch up slightly to maintain your speed. (Most aircraft have a couple that will pitch the nose up when thrust is increased so you might not need to adjust pitch much.) Pitch is the fastest/most efficient way to control airspeed in most flight situations. Throttle is the fastest/most efficient way to control altitude in most flight situations. However, pitch still can affect altitude and throttle still can affect speed.
Going back to the glider pilot stuff.... it illustrates it very well because your "thrust" is controlled by moving the stick forward or back and speed is controlled the same way. If you pitch down you increase speed and increase rate of descent. If you pitch up you decrease speed and increase rate of climb. They are tied together so a good glider pilot has a very good understanding of conservation of energy and how to most efficiently use the energy.
I agree with this.
Originally Posted by Hurt, AfNA, p27
Note that for the conditions of steady flight, each airspeed requires a specific angle of attack and lift coefficient. This fact provides a fundamental concept of flying technique: Angle of attack is the primary control of airspeed in steady flight. In the same sense, the throttle controls the output of the powerplant and allows the pilot to control rate of climb and descent at various airspeeds.
The real believers of these concepts are professional instrument pilots, LSO's, and glider pilots.......
Note that for the conditions of steady flight, each airspeed requires a specific angle of attack and lift coefficient. This fact provides a fundamental concept of flying technique: Angle of attack is the primary control of airspeed in steady flight. In the same sense, the throttle controls the output of the powerplant and allows the pilot to control rate of climb and descent at various airspeeds.
The real believers of these concepts are professional instrument pilots, LSO's, and glider pilots.......
I agree that in the majority of flight "situations" and the majority of aircraft, the concept that speed is controlled by AoA (pitch) and climb/descent controlled by thrust (throttle), will provide the best control and handling of the aircraft. Only in some flight situations is it reversed; one I can think of is slow flight recovery... If you break it down into two separate actions you'll see that if you first pitch forward, you will lose altitude. So by adding thrust first you will prevent altitude loss, and then pitching forward will build airspeed. That is what I teach in those situations. Both actions should happen very close together.
Advocates of pitching down first then adding throttle use the argument that when you pitch forward you are decreasing the AoA (correct), and therefore you decrease induced drag and allow the aircraft to accelerate by adding throttle. To do it this way, you will lose altitude the moment you pitch forward. In slow flight you have not stalled and so stall recovery is not beneficial.
One day when I was playing at being a thick student I put us 10 kts slow over the lead in lights and said "you have control Sir please show me again how you lower the nose to make the speed increase" Of course he slammed the throttle forward, refused to speak to me again and I was given a new instructor.
This is leading to my last point. Although these are treated as two separate inputs that affect two separate outputs, if you change one you have to change the other! If you are at your correct speed on your approach and are descending too fast, if you add throttle to compensate, you will have to pitch up slightly to maintain your speed. (Most aircraft have a couple that will pitch the nose up when thrust is increased so you might not need to adjust pitch much.) Pitch is the fastest/most efficient way to control airspeed in most flight situations. Throttle is the fastest/most efficient way to control altitude in most flight situations. However, pitch still can affect altitude and throttle still can affect speed.
Going back to the glider pilot stuff.... it illustrates it very well because your "thrust" is controlled by moving the stick forward or back and speed is controlled the same way. If you pitch down you increase speed and increase rate of descent. If you pitch up you decrease speed and increase rate of climb. They are tied together so a good glider pilot has a very good understanding of conservation of energy and how to most efficiently use the energy.
On approach to the ship at 1.1 Vs, if you see you are going to be a little short and inch the nose up, you had better match that with a power change or in two seconds you will begin to devellop a dangerous rate of sink due to airspeed loss. Typically there is only 11-15 feet of tail hook to ramp clearance for on glide slope operations so unstabilized flight on approach to the boat is a no-no.
And if you want to decel from 300 knots to 250 knots as you go below 10,000 ft on an approach, you would trim to keep your speed stable and adjust rate of altitude change with your power, wouldn't you?
Each technique has its place depending on what you are doing with the aircraft.
And if you want to decel from 300 knots to 250 knots as you go below 10,000 ft on an approach, you would trim to keep your speed stable and adjust rate of altitude change with your power, wouldn't you?
Each technique has its place depending on what you are doing with the aircraft.
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Machinbird,
We are talking here about longitudinal static stability, that is, what happens in the aircraft x-z plane, which in the case of approach and landing is also the earth x-z plane.
I have no experience with, and thus little intuition about, military kit. My interest in pursuing this question is as follows. We're discussing "pitch is primary for AS; thrust is primary for VS" versus "pitch is primary for VS, thrust is primary for VS". Let me introduce the acronyms p/as,t/vs versus p/vs,t/as for these “rules”. My question is whether one can make objective sense of the attribute "primary for".
I can see three possible reasons.
First is cultural: one learns to drive on the left; one learns to drive on the right. There are habits and reactions appropriate to how one learnt, the habits and reactions of the "other-siders" are at first "foreign", but one can learn. JF suggested the p/as,t/vs versus p/vs,t/as question is cultural.
Second is handling qualities. The airplane reacts "more appropriately", whatever we can construe that to mean, if a pilot uses, say, p/as,t/vs. Or the other way around.
Third is aerodynamics.
Pitch control and thrust control influence a vector in the x-z plane, namely the velocity of the aircraft. This velocity has a magnitude, airspeed, AS, and its projection onto the vertical (earth-z axis) is vertical speed, VS. AS and VS are not independent unless the wing is flying at AoA for zero lift (usually slightly negative). Let's assume low-speed (incompressible) flow.
AoA = Pitch + FPA. Since we are talking about pitch and FPA, It's more convenient for me to use aerodynamic normal and axial force (the aerodynamic forces on the wing normal to and parallel to the chord) than about lift and drag.
The forces are (1) thrust, (2) normal force, a function of dynamic pressure and AoA, proportional to dynamic pressure and (in the range we care about) AoA; (3) weight; (4) axial force, also a function of dynamic pressure and AoA proportional to dynamic pressure, but roughly to the square of AoA. There is also a moment, say about the quarter chord as usual, proportional to dynamic pressure and AoA. The dynamic pressure is proportional to the square of AS.
Start from equilibrium (stabilised). If you are on approach, FPA is 3° say. Say you want to speed up.
Equilibrium parallel to the chord: Thrust= AxialForce + Weight x sin(Pitch). Thrust is probably less than 0.5 MaxThrust. Let's do it for AoA = 10° and for AoA = 15°; so Pitch = 7°, resp. 12°.
So, let's do it for a Cherokee. Say 0.5 x MaxThrust = AxialForce + 2500 lbs x 0.12. If you put in full thrust, which you get quickly, then the force accelerating you along the chordline is 0.5 x MaxThrust (namely, the reserve thrust).
Now say, instead of putting in thrust, you reduce AoA. Then thrust remains at 0.5 x MaxThrust, but AxialForce is reduced and the contribution of weight = 2500lbs x 0.12 is reduced to 2500 x 0.(something less). So you get a net force in the direction of aircraft-positive-x, but it is rather less than 0.5 x MaxThrust.
Conclusion: if you look at the magnitudes, and want to speed up, then putting in thrust is the most effective way, aerodynamically.
This is for a Cherokee, but I bet the same reasoning works for many airplanes, including your Navy combat aircraft on approach.
So it doesn't look as if the aerodynamical viewpoint (your suggestion it was entailed by speed stability, then the second suggestion that it was entailed by stabilised versus unstabilised flight) will give us the p/as,t/vs conclusion.
Here is another way of looking at it aerodynamically.
If you want to go vertical, you have to use thrust for AS. If you want to go horizontal, you also have to use thrust for AS (otherwise drag will slow you down to zero AS, and we are back to the vertical :-). What happens in between pitch=90° and pitch=0°? Whatever it is, it is going to be continuous (all the physics in sight is continuous functions), so, for some region around pitch=90°, AS is mostly going to be affected by thrust. Similarly, for some ways around pitch=0°, AS is mostly going to be affected by thrust.
You want to say that somewhere in between 0° and 90°, it is the case that p/as,t/vs is "best". If the reason is to be aerodynamic, that must mean (continuous physics) that there is some point between 0° and 90° when t/as changes to p/as, and then some higher point at which it changes back. Can we say anything qualitative and useful about those points? I doubt it. But it does mean that, for any aircraft whose thrust capability is at least equal to its weight, there are going to be those two “switchover” points at which thrust being “primary for” AS change to pitch being “primary for” AS, and then back. It cannot be the case that pitch is “primary for” AS over the entire pitch range 0° to 90°.
So I am going for either pure cultural, or handling qualities. But then, if handling qualities, how would that be explained?
PBL
Originally Posted by Machinbird
Seems that the difference is flying in stabilized flight versus unstabilized flight
I have no experience with, and thus little intuition about, military kit. My interest in pursuing this question is as follows. We're discussing "pitch is primary for AS; thrust is primary for VS" versus "pitch is primary for VS, thrust is primary for VS". Let me introduce the acronyms p/as,t/vs versus p/vs,t/as for these “rules”. My question is whether one can make objective sense of the attribute "primary for".
I can see three possible reasons.
First is cultural: one learns to drive on the left; one learns to drive on the right. There are habits and reactions appropriate to how one learnt, the habits and reactions of the "other-siders" are at first "foreign", but one can learn. JF suggested the p/as,t/vs versus p/vs,t/as question is cultural.
Second is handling qualities. The airplane reacts "more appropriately", whatever we can construe that to mean, if a pilot uses, say, p/as,t/vs. Or the other way around.
Third is aerodynamics.
Pitch control and thrust control influence a vector in the x-z plane, namely the velocity of the aircraft. This velocity has a magnitude, airspeed, AS, and its projection onto the vertical (earth-z axis) is vertical speed, VS. AS and VS are not independent unless the wing is flying at AoA for zero lift (usually slightly negative). Let's assume low-speed (incompressible) flow.
AoA = Pitch + FPA. Since we are talking about pitch and FPA, It's more convenient for me to use aerodynamic normal and axial force (the aerodynamic forces on the wing normal to and parallel to the chord) than about lift and drag.
The forces are (1) thrust, (2) normal force, a function of dynamic pressure and AoA, proportional to dynamic pressure and (in the range we care about) AoA; (3) weight; (4) axial force, also a function of dynamic pressure and AoA proportional to dynamic pressure, but roughly to the square of AoA. There is also a moment, say about the quarter chord as usual, proportional to dynamic pressure and AoA. The dynamic pressure is proportional to the square of AS.
Start from equilibrium (stabilised). If you are on approach, FPA is 3° say. Say you want to speed up.
Equilibrium parallel to the chord: Thrust= AxialForce + Weight x sin(Pitch). Thrust is probably less than 0.5 MaxThrust. Let's do it for AoA = 10° and for AoA = 15°; so Pitch = 7°, resp. 12°.
So, let's do it for a Cherokee. Say 0.5 x MaxThrust = AxialForce + 2500 lbs x 0.12. If you put in full thrust, which you get quickly, then the force accelerating you along the chordline is 0.5 x MaxThrust (namely, the reserve thrust).
Now say, instead of putting in thrust, you reduce AoA. Then thrust remains at 0.5 x MaxThrust, but AxialForce is reduced and the contribution of weight = 2500lbs x 0.12 is reduced to 2500 x 0.(something less). So you get a net force in the direction of aircraft-positive-x, but it is rather less than 0.5 x MaxThrust.
Conclusion: if you look at the magnitudes, and want to speed up, then putting in thrust is the most effective way, aerodynamically.
This is for a Cherokee, but I bet the same reasoning works for many airplanes, including your Navy combat aircraft on approach.
So it doesn't look as if the aerodynamical viewpoint (your suggestion it was entailed by speed stability, then the second suggestion that it was entailed by stabilised versus unstabilised flight) will give us the p/as,t/vs conclusion.
Here is another way of looking at it aerodynamically.
If you want to go vertical, you have to use thrust for AS. If you want to go horizontal, you also have to use thrust for AS (otherwise drag will slow you down to zero AS, and we are back to the vertical :-). What happens in between pitch=90° and pitch=0°? Whatever it is, it is going to be continuous (all the physics in sight is continuous functions), so, for some region around pitch=90°, AS is mostly going to be affected by thrust. Similarly, for some ways around pitch=0°, AS is mostly going to be affected by thrust.
You want to say that somewhere in between 0° and 90°, it is the case that p/as,t/vs is "best". If the reason is to be aerodynamic, that must mean (continuous physics) that there is some point between 0° and 90° when t/as changes to p/as, and then some higher point at which it changes back. Can we say anything qualitative and useful about those points? I doubt it. But it does mean that, for any aircraft whose thrust capability is at least equal to its weight, there are going to be those two “switchover” points at which thrust being “primary for” AS change to pitch being “primary for” AS, and then back. It cannot be the case that pitch is “primary for” AS over the entire pitch range 0° to 90°.
So I am going for either pure cultural, or handling qualities. But then, if handling qualities, how would that be explained?
PBL
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pitch and thrust
The pilot uses elevator to control pitch, and power lever for thrust. The latter control loop is usually less responsive than the former, particularly for jet engines at low power.
regards,
HN39
regards,
HN39
Last edited by HazelNuts39; 20th Jul 2010 at 11:58.
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Originally Posted by hn39
The pilot uses elevator to control pitch, and power lever for thrust.
Originally Posted by hn39
The latter control loop is less responsive than the former, particularly for jet engines at low power
Besides, if thrust has higher latency, it's going to have that whether you are using it to control airspeed, or using it to control rate of descent.
PBL
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Article
A good article about the Boeing vision on this can be found in the Boeing Aero magazine, issue 12.
It explains well why the world of large airliners does not use AoA as indicator on approach, such as fighter jets do. Shape of lift curve has to do with it, civvies with long slender wings, fighters with short, stubby wings being the big difference.
Still, having AoA as backup instrument for case of pitot static problems would be a nice thing to have.
In terms of equipment, most fighter jets will have AoA indication available to the pilot. As does the US Navy, all F-16's are flown on approach using AoA instead of speed for approach. Limit for landing an F-16 is not (low) speed, but body angle on touchdown (you will scrape airplane parts if angle is too large).
It explains well why the world of large airliners does not use AoA as indicator on approach, such as fighter jets do. Shape of lift curve has to do with it, civvies with long slender wings, fighters with short, stubby wings being the big difference.
Still, having AoA as backup instrument for case of pitot static problems would be a nice thing to have.
In terms of equipment, most fighter jets will have AoA indication available to the pilot. As does the US Navy, all F-16's are flown on approach using AoA instead of speed for approach. Limit for landing an F-16 is not (low) speed, but body angle on touchdown (you will scrape airplane parts if angle is too large).
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