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jhurditch
9th Mar 2010, 14:27
Heard something interesting in my A/C engineering lecture today that was
"If you wish to increase speed of an aircraft you do not increase thrust, you must move the stick instead, aircraft speed is controlled by the stick due to angle of incidence". this hardly made sense to me as i would consider a stall to be a situation in which most thrust is required as a/c speed is lowest and during landing you would not require full power to descend. can someone explain this to me more clearly.

Cheers

bfisk
9th Mar 2010, 16:02
This doesn't make any sense to me either. For one; the flight regime is not specified, secondly, the angle of incidence is (normally) fixed.

He might have meant something like this (my italic inserts): "If you wish to increase speed of an aircraft andyou do not want to, or are able to increase thrust, you must move the stick forward instead.

That last portion doesn't make any sense to me. The angle of incidence is the angle between the chord line of the wing and the fuselage/aircraft longitudinal axis, and has nothing to do with speed. He may be confusing this with angle of attack, though; since drag varies with angle of attack, so does the speed (or, more correctly, longitudinal acceleration). Also, with a change of angle of attack, everything else being the same, a change in load factor will occur. This will normally lend itself to the exchange between potential and kinetic energy, ie altitude and speed.

Some prefer to control the aircraft speed with pitch rather than power (thrust) at low speed (the "back side of the power curve") due to the natural speed instability at these speeds. That might also be where he's going.


edit: reading your question again, I see that you might be asking about power and pitch in a stall. If that is the case, your statement makes more sense. When an aircraft stalls, yes, it is important to set full thrust as soon as possible, but very few aircraft will be able to "power" themselves out of the stall. The first reaction, and the most vital action, is to lower the nose (really, lower the angle of attack) to regain lift. Power application is only vital to recover from a low-speed (and possibly low-altitude) regime to normal flight, but power in itself does not get you out of the stall (unless you're flying some kind of rocket!)

Microburst2002
9th Mar 2010, 17:31
If that is said during a stall, ok. Although all you need is reducing angle of attack (not incidence, probably that is what he wanted to say).

Stall occurs at an AoA. Push the stick, you unstall the wings. Of course you will loose altitude. The time to add power is usually after you lower the nose, so the effects of the propeller and others do not put you in trouble before you manage to unstall the wings.

If it is the old wrong assumption that speed is controlled by pitch, and not power, well... It is wrong except during climb or descents at fixed power, where it is totally correct. In level flight, or in an ILS, speed is controlled with power and flight path with pitch.

For instance. To maintain altitude you pull or push the stick as necessary, while maintaining speed advancing or retarding the power levers as required. Altitude increasing? push the nose a bit. Speed decreasing? add some power.

If you are studiying aircraft engineering and are interested in how pilots fly an airplane there are very nice and simple books for private pilots.

FoxHunter
9th Mar 2010, 17:51
Suggest the next time you take off just push and pull on the elevator and as soon as you get the speed then add power to climb.

Capn Bloggs
9th Mar 2010, 23:11
Is this a pilot conversion course? Why didn't the whole class tell the instructor that he/she was talking rubbish? Ask your instructor the fundamental principle behind an autothrottle system - if the speed's low, it pushes the power up. It doesn't push the nose down.

If you're an effo:

"Always remember and forever take heed; right hand for glidepath and left hand for speed!".

One of the most fundamental rules of flying.

galaxy flyer
10th Mar 2010, 01:05
Attitude + Power = Performance

GF

thepotato232
10th Mar 2010, 02:03
I've been hearing this old chestnut since Day One of groundschool. "Pitch controlls airspeed, power controls altitude." Never understood why people keep saying that. It's the coordinated application of both. Is that concept so hard?

Dan Winterland
10th Mar 2010, 02:30
Except in a glider!

Mansfield
10th Mar 2010, 03:03
Bfisk hits the nail on the head...it depends on where you are with respect to the power curve. This argument is almost older than the Wright Brothers...years ago, I had a running feud with the chief at the college program I instructed at over it. "Aerodynamics for Naval Aviators" is adamant that pitch controls airspeed, and that makes sense when you are operating close to, or behind, the power curve (say during a carrier approach). But when I get slow in my 757 I sure ain't thinkin' of pitch, because I am well ahead of the power curve.

The problem arises most apparently in light aircraft, because many of them have little power margin. The power-control-airspeed approach works, but only within a small band of the approach speed range. If you get just a bit slow, there isn't enough power to get you back as quickly as your instructor might like. Naturally, this class of airplane is the one we are using to teach primary students, who have only an early comprehension of the issue. Thus, my preference was to use pitch, because it was more consistent, and it was guaranteed to be primary when the engine quit. But this should be accompanied by a thorough discussion of the relationships and an understanding that primary speed control would change from pitch to power as soon as you matriculate into a larger airplane.

PantLoad
10th Mar 2010, 04:33
GF is one million percent correct. All information contrary to that is total
BULLXXXX.

It's that simple....

There are a number of fine publications....many come from the U.S. Navy...that will confirm Galaxy Flyer's post.


Fly safe,

PantLoad

Capn Bloggs
10th Mar 2010, 04:37
P + A does indeed = P, Pantload, but that is irrelevant to the thread. The thread is about changing the speed, not steady-state performance.

Intruder
10th Mar 2010, 05:10
It depends quite a bit on the airplane design...

Many airplanes are speed-stable. If you increase thrust without changing trim, it will likely settle in a climb at or near the trimmed speed. This is especially true with underslung engines, where the thrust vector is well below the pitch axis.

bbrunton
10th Mar 2010, 05:13
Which came first the chicken or the egg?

Pitch Power Altitude Speed.

What controls what?


Well - think of these things...


Some military jet fighters can accelerate to a speed in excess of Mach 1 in a vertical climb...


As a flight instructor I teach my primary students the following. (in low power general aviation training aircraft)

Cruise flight - Pitch controls Altitude. You select your cruise speed with power. (small changes in pitch produce a large change in lift and very little change in drag at high speed)

Slow flight - Pitch controls airspeed, power controls altitude. ( a small change in pitch makes a large change in drag and very little change in lift therefore making the speed change)


They learn it that way and can succesfully operate an aircraft usssing that information....


If I am flying with an experienced pilot, and I ask them to do slow flight during recurrent training or transition to a new aircraft, and usually in a higher performance aircraft, I remind them to adjust pitch and power as necesssary to maintain airspeed and altitude. What a cop out!! I do not tell them how to accomplish slow flight unless they are having trouble. It is surprising how many pilots forget how to precisely control airpeed and altitude especially in higher performance aircraft. If they can make it work acceptably, I do not say anything... Many pilots get into the bad habit of using flaps and gear to control their speed and forget how it really works.


In reality.... Airspeed and altitude, power and pitch are all interrelated.


If we assume that airspeed is controled by power, and altitude is controled by pitch, then what happens in slow flight? Try getting up to lift off speed without adding full power on the runway, and try lifting off without increasing angle of attack or pitching up...

In slow flight, if we want to increase speed, we can lower the nose. (Pitch controls airspeed) When we lower the nose and decrease the drag, of the wing and airframe, the speed begins to increase but at the same time the aircraft begins to descend as there was a decreasse in lift due to the angle of attach change. We notice the descent and add power to compensate. (Power controls altitude).

Again in the low flight senario above, if we want to increase speed and we add power, if we have enough available power, the speed will begin to increase. As the speed increases more lift is produced causing a climb. We notice the climb and slightly lower the nose to compenate. (Pitch controls altitude).. The problem is in a small aircraft the available power may not be sufficient to accelerate the aircraft very much so lowering the nose and decreasing the drag has a more immediate effect.

So in conclusion..

I recommend that flight intructors teach their primary students using small low power aircraft how to control airspeed and altitude as I stated at the top of this post... Cruise and slow flight so not operate the same.

I recommend that flight instructors teach and permit their students to discover that airspeed is controlled by power and altitude is controled by pitch at all times when tranitioning to higher power high performance aircraft. That is one of the most important concepts to learn when transitioning form simple training aircraft into a high performance aircraft.

However, if a pilot can control airspeed and altitude succesfully there is no real reason to force them to adopt any particular method nor does their method need to be changed.

My 2 cents worth


Bill

Capn Bloggs
10th Mar 2010, 05:49
As a flight instructor I teach my primary students the following. (in low power general aviation training aircraft)

Cruise flight - Pitch controls Altitude. You select your cruise speed with power. (small changes in pitch produce a large change in lift and very little change in drag at high speed)

Slow flight - Pitch controls airspeed, power controls altitude. ( a small change in pitch makes a large change in drag and very little change in lift therefore making the speed change)


They learn it that way and can succesfully operate an aircraft usssing that information....

Sorry Bill, I can't agree (and welcome to Prune! :ok:).

While it may work on a C150 because lighties are so speed-stable, it absolutely DOES NOT work on a jet on final, the most critical time of the flight where changes are required almost instantly.

Low-time pilots should not be taught to control the aeroplane via the secondary effects of controls, if for no other reason that when they fly something bigger, the technique doesn't work. While they are coupling effects so that the nose may come up when the thrust is increased, the primary method of changing the speed must be with the throttles (or speedbrake). Likewise, flight path change must be made with the elevator. Any speed changes are then sorted by changing thrust.

There is a case where, in the flare, the angle of attack/nose attitude is so high that jamming the thrust on may have a cushioning effect, but that is not the norm down final.

My analogy is flying down an ILS (or letting the AP do it). If you get low on the slope, you pull the nose up, just as the AP does. You don't put up the thrust and wait for the secondary effect of controls to get you back up onto the GS. Carrier pilots maybe, but not normal aircraft.

Similarly, speed. If you get slow, you definitely do not lower the nose! The speed will never increase satisfactorily; all that will happen is your descent rate will increase.

PantLoad
10th Mar 2010, 06:43
Carrier aircraft are definitely not real aircraft. They're top secret and fly
quite differently from conventional aircraft.

I stand corrected....


Fly safe,


PantLoad


Aerodynamics for Naval Aviators

(If you really want to know how an airplane flies.....)

FE Hoppy
10th Mar 2010, 10:12
Bernuli vs Newton

Pitch vs thrust

It's a wonder we fly at all!!!

There is only one way to accelerate allong the current flight path and that is to increase thrust/drag ratio. :confused:

cosmo kramer
10th Mar 2010, 12:02
If you are too high on the G/S (or want to descend etc), you reduce thrust.
If the plane is in trim, it will keep the speed

Vice versa if you are too low / want to climb.

Hence pitch controls speed and thrust altitude :ok:

These are all constant speed scenarios. If you want to go down you need to remove energy (thrust) from the equation - it's as simple as that.


However, if you want to change the speed you use thrust to change speed and pitch / trim to maintain altitude. But this is not a constant system or stabile environment (and not the way an e.g. an ILS is supposed to be flow).

Capn Bloggs
10th Mar 2010, 13:46
Pantload,
I wasn't trying to be smart. The reason I said
Carrier pilots maybe, but not normal aircraft.
is because my understanding is that carrier technique is (was?) to do what the original poster mentioned: low on slope, push up the power to get back on it ie hold a constant AOA and control slope via the secondary effect of power; maybe has a more useful effect because of the body angle/thrust vector.

Kramer,
These are all constant speed scenarios. If you want to go down you need to remove energy (thrust) from the equation - it's as simple as that.
You are flying the aeroplane via the secondary effects of controls. Works well in a bugsmasher, not well in a jet in my experience because the effect doesn't happen fast enough to correct the error that prompted the correction. Each to his own, tho.

p51guy
10th Mar 2010, 16:06
Flying opposite of how an autopilot/autothrottle works doesn't seem logical. Speed gets low the autothrottle increases power. Autopilot gets high it lowers the nose. It doesn't matter if it is in altitude hold or following a glide slope.

cosmo kramer
10th Mar 2010, 22:33
Capn Bloggs
You are flying the aeroplane via the secondary effects of controls.
Only the rudder and aileron have secondary (or adverse) effects. The rudder that it causes roll and vice versa with the aileron. In other words a motion in another axis than the primary.
Elevator on the other hand have no secondary effect (not causing any motion around other axis).

Capn Bloggs
10th Mar 2010, 23:16
Kramer,

The secondary effect on the aircraft from use of the elevator (up) is that the speed reduces. The secondary effect of putting the power up is that the nose rises. It works "well" in a bugsmasher because they are so stable, but try it next time you're handflying down the ILS at 140kts in your jet.

FE Hoppy
10th Mar 2010, 23:19
Elevator on the other hand have no secondary effect

Pull back and wait. If you touch nothing else you will soon discover a secondary effect.

Primary - pitch
Secondary - speed.

Unless you can turn off gravity of course!

cosmo kramer
10th Mar 2010, 23:57
Pitch for speed and thrust for altitude works extremely well in a 737.

There is no such thing as secondary effect of the elevator for the reason stated in my previous post. To write it again would only be repetition.

What you mention are not effects. Those are consequences of moving the elevator. I could add to FE Hoppy's list of "effects":
3rd - Stall
4th - spill coffee
5th - wife complains about stain on your shirt

The only controls that have secondary effects are:
Yaw - induced roll
Pitch - adverse yaw

Show me one quote from a book or one link to a webpage that mentions secondary effects of the elevator.


I'll leave the discussion here, Thank you. :hmm:

TheWanderer
11th Mar 2010, 00:48
I recommend reading this: http://www.faa.gov/library/manuals/aviation/instrument_flying_handbook/media/FAA-H-8083-15A%20-%20Chapter%2002.pdf

Remember aerodynamics and the four forces:


lift
!
!
!
thrust <----+----> drag
!
!
!
weight
When the elevator control is pushed or pulled, the angle of attack is changed.
The angle of attack has a direct influence on lift, the aircraft starts decending or climbing.

When thrust is changed, thrust becomes greater or smaller than current drag, the aircraft accelerates or decelerates.

Lift is a function of airspeed as well, in result a change in speed is followed by a change of lift, creating a requirement to correct the angle of attack by using an elevator input in order to maintain level flight.
To maintain lift the same when accelerating, a decrease in angle of attack is required. The opposite applies to decreasing speeds, an increase of angle of attack is required.

The same applies to climb or decent, the difference is that in climb excessive thrust is used to add potential energy (altitude), while in decent potential energy (altitude) is used to overcome drag.

StallBoy
11th Mar 2010, 00:52
What about the HANDBRAKE :ok:????????

TheWanderer
11th Mar 2010, 01:01
What about the HANDBRAKE http://images.ibsrv.net/ibsrv/res/src:www.pprune.org/get/images/smilies/thumbs.gif????????

The handbrake is linked to the toilet and will result to an inflight toilet dump when pulled while not on ground. All the bull$&%t will go overboard in this case.

Capn Bloggs
11th Mar 2010, 01:17
Kramer,

It took me 5 seconds to find this:

how to control an aircraft (http://www.esparacing.com/sport_pilot/how%20to%20control%20an%20aircraft.htm)

What is stated there, of course, is dead right, regardless of what your theoretical definition of "effect" may be.

You would be well advised to also have a read of Handling The Big Jets, in particular the section on takeoff and landing. One bit says:
"a high sink rate must be countered by increased incidence coincidentally protected by an increase in thrust to counter the extra drag."

And a quote from "Fly The Wing":
It is dangerous to believe that rate of climb or descent is strictly a function of power, that an airplane goes up or down as a function of applying power of pulling it off. A very basic understanding of aerodynamics proves that a wing in flight climbs or descends as a result of angle of attack and lift variations at various angles of attack. Power or thrust merely pushes or pulls the plane through the air fast enough for the wing to become aerodynamic, to furnish lift. From then on, climb and descent are functions of pitch attitude (lift coefficient) and are controlled by the elevators. The power controls the speed-period!. This is the whole essence of flight, particularly jet flight"

Practical and safe aviation verses blinkered theory...

Beg Tibs
11th Mar 2010, 03:45
Pitch for speed and thrust for altitude works extremely well in a 737.

Good luck with that..!! :ugh:

Intruder
11th Mar 2010, 04:17
While it may work on a C150 because lighties are so speed-stable, it absolutely DOES NOT work on a jet on final, the most critical time of the flight where changes are required almost instantly.

Low-time pilots should not be taught to control the aeroplane via the secondary effects of controls, if for no other reason that when they fly something bigger, the technique doesn't work. While they are coupling effects so that the nose may come up when the thrust is increased, the primary method of changing the speed must be with the throttles (or speedbrake). Likewise, flight path change must be made with the elevator. Any speed changes are then sorted by changing thrust.
Sorry, but it's not that simple.

Both the A-6 Intruder and the 747 were/are relatively speed-stable in the landing configuration. In both airplanes, thrust is primarily used to control rate of descent, and pitch trim is used to control airspeed.

While all the performance inputs and parameters are interrelated, there is a primary or leading control input for change of each parameter. To attempt to control speed PRIMARILY with thrust in the landing configuration in either airplane will likely result in a very unstable approach.

This discussion could expand to roll/yaw response for turning in different airplanes, but that would even further complexify the conversation...

Intruder
11th Mar 2010, 04:22
Flying opposite of how an autopilot/autothrottle works doesn't seem logical. Speed gets low the autothrottle increases power. Autopilot gets high it lowers the nose. It doesn't matter if it is in altitude hold or following a glide slope.
Just as with manual control, it's not that simple...

Autopilots and autothrottles work together in autoflight to make very small incremental changes. Each change in one system will trigger a change in the other. Note that in VNAV climbs and descents, the announced pitch mode is often VNAV SPEED, where the elevators and stab control the speed...

Bullethead
11th Mar 2010, 04:30
I was taught pitch for speed and power for flight path control when flying a prop and pitch for flight path and power for speed when I transitioned to jets.

Personally I reckon it's a bit of both on finals for either type and after a while it becomes second nature and you don't consciously have to think about it.

Regards,
BH.

p51guy
11th Mar 2010, 05:12
It is that simple. That is why I left VNAV climb out because obviously with a climb power setting only pitch could control airspeed. Alt hold or established on a glide slope the autothrottle only controls speed and the pitch axis of the autopilot only controls flight path. Most pilots flew this way before their first automated aircraft. Thrust vs drag = speed. Lift vs gravity = flight path. Any correction with either will eventually require an adjustment in the other axis to balance the other vector. When I was instructing I always showed my students how changing thrust could either increase speed or vertical rate depending on what you did with the elevator. The primary change with thrust is in the horizontal plane with speed. The primary change with the elevator is in the vertical plane with lift.

Microburst2002
11th Mar 2010, 12:08
To maintain a flight path: pitch
To change or maintain speed in a constant flight path: power or thrust.

In every airplane. In Trevor Thom's IFR instrument flying books is explained like that, for light airplanes.

However, what would happen if an AP in a 737 had the A/THR coupled with the altimeter to maintain altitude and the pitch with the ASI to maintain airspeed? This egg and chicken thing can be very puzzling, indeed.

I think that, for a human brain at least, it is easier to divide the tasks conventionally, pitch for path, power for speed.

The control and performance instruments concept is quite clear and it has been stablished long time ago.

rudderrudderrat
11th Mar 2010, 12:30
Hi MB2002,

what would happen if an AP in a 737 had ...... the pitch (coupled) with the ASI to maintain airspeed?

Respectfully, doesn't it depends on the particular flight regime? With fixed power set e.g. During Climb & Descent the autopilot controls speed with pitch.

Also in a visual circuit whilst descending or when hand flying a NPA - pitch controls speed, and power ROD. When there is a profile to fly e.g. ALT, VS, FPA, GS etc. then it's reversed.

FE Hoppy
11th Mar 2010, 13:22
I think what is being lost here is that in all the examples of pitch for speed what ia actually being described is trading flight path angle for speed while maintaining a fixed thrust. This is not the same as controling flight path angle with pitch and controling speed with thrust.


So the example I like is fixed flight path angle. now tell me how i control speed using pitch?

Correct. I cant. I must change the thrust.

Whenever a specific vertical path is required we have to change the thrust drag ratio to change speed and that requires a change in thrust.

Capt Pit Bull
11th Mar 2010, 13:28
Either can be used to control either.

Its really not that complicated, but sadly this industry has a developed a compulsion to dumb things down to the extent that they can be rote learnt. Then people start getting dogmatic about it.

Personally, I'd rather take an energy based approach. If Thurst > Drag then KE + GPE is increasing. KE could be going up or down, GPE could be going up or down. But the sum of them is going up.

pb

Capt Pit Bull
11th Mar 2010, 13:33
Oh, and in passing:

The secondary effect of putting the power up is that the nose rises

Type dependent.

TheWanderer
19th Mar 2010, 09:21
A simple experiment:

Steady level flight at FL100, speed 250 KIAS.
Increase speed to 300 KIAS while maintaining level, 0 fpm rate of climb.
=> Thrust needs to be increased to increase speed. If you change pitch, you start descending or climbing.
Steady stable climb with 250 KIAS and 500 fpm rate of climb.
Increase speed to 300 KIAS while maintaining 500 fpm rate of climb.
=> thrust needs to be increased to increase speed. If you change pitch, the the rate of climb changes.
Steady stable climb with 300 KIAS and 500 fpm rate of climb.
Reduce speed to 250 KIAS while maintaining 500 fpm rate of climb.
=> Thrust needs to be reduced to decelerate. If you change pitch, rate of climb changes.
Steady level flight with 300 KIAS. Reduce speed to 250 KIAS, keep 0 ft rate of climb.
=> Thrust needs to be reduced to reduce speed while maintaining level flight.
Steady decent with 250 KIAS, 500 fpm rate of decent.
Increase speed to 300 KIAS.
=> Thrust needs to be increased to accelerate. Change of pitch changes rate of decent.
Steady descent with 300 KIAS, rate of decent 500 fpm.
Slow down to 250 KIAS while maintaining 500 fpm rate of decent.
=> Thrust needs to be decreased to slow down. Change of pitch changes rate of decent.Conclusion: Thrust controls speed and pitch controls rate of climb or decent.


Special cases

Transition from level flight to climb:
Potential energy is added, additional thrust is required to deliver this additional energy requirement.
Transition from level flight to decent:
Potential energy is used up to overcome drag, less thrust is required to maintain constant speed.

Capt Pit Bull
19th Mar 2010, 10:58
A simple experiment:
1.Steady level flight at FL100, speed 250 KIAS.
Increase speed to 300 KIAS while maintaining level, 0 fpm rate of climb.
=> Thrust needs to be increased to increase speed. If you change pitch, you start descending or climbing.

To maintain level, 0 fpm you will need to pitch down as you accelerate. If you maintain pitch you will start to climb. Eventually an equilibrium will be reached where (Thurst - Drag) / Weight = The Rate Of Climb.

Conclusion: Both pitch and thrust affect speed and the rate of climb.

2.Steady stable climb with 250 KIAS and 500 fpm rate of climb.
Increase speed to 300 KIAS while maintaining 500 fpm rate of climb.
=> thrust needs to be increased to increase speed. If you change pitch, the the rate of climb changes.

To maintain 500 fpm you will need to pitch down as you accelerate. If you maintain pitch you will increase the climb rate. Eventually an equilibrium will be reached where (Thurst - Drag) / Weight = The Rate Of Climb.

Conclusion: Both pitch and thrust affect speed and the rate of climb.


3.Steady stable climb with 300 KIAS and 500 fpm rate of climb.
Reduce speed to 250 KIAS while maintaining 500 fpm rate of climb.
=> Thrust needs to be reduced to decelerate. If you change pitch, rate of climb changes.

To maintain 500 fpm you will need to pitch up as you decelerate. If you maintain pitch you will decrease the climb rate. Eventually an equilibrium will be reached where (Thurst - Drag) / Weight = The Rate Of Climb.

Conclusion: Both pitch and thrust affect speed and the rate of climb.


4.Steady level flight with 300 KIAS. Reduce speed to 250 KIAS, keep 0 ft rate of climb.
=> Thrust needs to be reduced to reduce speed while maintaining level flight.

same again

5.Steady decent with 250 KIAS, 500 fpm rate of decent.
Increase speed to 300 KIAS.
=> Thrust needs to be increased to accelerate. Change of pitch changes rate of decent.

and again

6.Steady descent with 300 KIAS, rate of decent 500 fpm.
Slow down to 250 KIAS while maintaining 500 fpm rate of decent.
=> Thrust needs to be decreased to slow down. Change of pitch changes rate of decent.

and again

Conclusion: Thrust controls speed and pitch controls rate of climb or decent.

Conclusion: Where were you in "straight and level" part 2?


Special cases

Hardly special cases when aircraft do this day in day out. The fact that you invoke these as being exceptions to your conclusion simply demonstrates your conclusion is not a generalised solution.

Its about as valid as saying 'on Fridays its always Friday'.

pb

Microburst2002
20th Mar 2010, 10:04
The egg and the chicken...

But AP/FDs are all the same, aren't they?

To maintain a given flight path, they control speed with thrust, and path with pitch (as it taught to IFR pilots: pitch is primary for altitude(or G/s) and power is primary for speed)

To maintain a climb or a descent at fixed power, IFR pilots are taught that pitch is primary for speed, and AP/FD control speed with pitch, while A/THR controls thrust setting.

To maintain a given speed and a given rate of climb or descent, AP/FD use thrust for speed and pitch for rate. Probably the opposite is what is used in light piston airplanes. And probably in this case is when half of the pilots do it one way and the other half the other way.

Maintaining altidude or in an ILS I think we all keep path with pitch, speed with thrust, don't we?

Capt Pit Bull
20th Mar 2010, 11:24
Maintaining altidude or in an ILS I think we all keep path with pitch, speed with thrust, don't we?

Actually, no, we don't.

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.

Microburst2002
20th Mar 2010, 14:28
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.

Capn Bloggs
20th Mar 2010, 14:55
I don't see the need to over simplify this into sound bites, its not like it's that complicated in the first place.
That's right. If you're going low on the GS, you pull back on the stick. If the speed the changes, you put the power up.

If the speed drops, you put the power up. If you then start going high on the GS, you push the nose down.

Simple! :rolleyes:

In an ILS, I know that many use thrust for glidepath and pitch for speed.
Trained by C150 pilots, fly like C150 pilots. Wrong.

Checkboard
20th Mar 2010, 16:10
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.

4dogs
21st Mar 2010, 15:54
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.

:ok:

Stay Alive....

Capt Pit Bull
22nd Mar 2010, 10:50
mb2002

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.

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

capt. solipsist
22nd Mar 2010, 10:59
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." :bored:

Microburst2002
22nd Mar 2010, 11:30
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

aterpster
22nd Mar 2010, 14:13
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."

Mansfield
23rd Mar 2010, 16:19
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.

Capt Pit Bull
23rd Mar 2010, 17:07
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