Pitch power theories (hot stuff?)

[Tor]

After quite a debate in the questions forum, I would like to hear some opinions from a broader audience.

Do you support the "thrust controlles rate of decend, pitch controlles speed" theory? Why / Why not?

[Max Angle]

This one seems to run and run does'nt it. You can't split the two up. To reduce the rate of descent you raise the nose (change pitch) and having done that you add some power to maintain the speed. If you change one you have to change the other. Having said all that, for short term adjustments to ROD you have to use pitch. The a/c (airline jet at any rate) does'nt react quickly enough to thrust changes.

[DeltaTango]

exactly, max.
You have to think of it a bit backwards here....way lift the nose to reduce rod and then add power, then trim the nose down add correct power etc. etc., if reducing a bit of power gives you the same result with less workload....

Well, that's just my opinion.


DT

ps. Hvordan gaar det Tor?
Aergeligt turen blev aflyst.

[deltahotel]

How does the autopilot do it? VNAV descent, idle power, pitch controls flightpath and the FMC demands power or drag if the IAS changes too much. On final approach (ILS), power controls IAS, pitch follows G/S. In the climb, power is fixed at climb thrust EPR and pitch controls IAS. In the cruise, power=IAS, pitch=flightpath.

As Max said, any change in one will affect the other. And yes, you can fly final approach in a light ac using power=IAS, pitch=flightpath. It works well.

[Spooler]

Can imagine it has to do with the rate that the input affects the desired result. A change in pitch will (immediate term) result in an altitude descent alteration, to be backed up by the required throttle change to maintain, decrease or increase the speed. The heavier and larger the airframe, the longer a throttle change will normally take to affect a change in ROD. Approaches are a here and now situation to assure a stable approach and delays in a change of variable will only add to the workload. I'm only a PA28 flyer but I use pitch then throttle, I guess I just hate waiting...

Any more ides?

[CaptainSquelch]

To make the discussion a little more complex:

In the average jet we all fly Power-Speed and Pitch-V/S on final. Now the funny thing is that as soon as you add a little power on a jet with the engines under the wing the aircraft tends to pitch up, so reduce your V/S and thus reduce the speed-up-effect of the power-increase. Still, if you start to fly V/S with the power you will see an interesting flighpath.

Sq

[mitch3p]

Fellas,

I have to put in a word for AOA.

First what is the difference between hand flying an ILS and staight and level flight, in terms of the pitch/power discussion? For large altitude or glideslope changes power controls altitude. For small altitude or Glideslope changes, pitch controls altitude.

When one is flying a jet at cruise power the only real option to increase altitude is to pitch up. but what about to descend? Pitch works to Vmo/Mmo, but a reduction is power is going to get the AC down.

This is one reason why flying is an art and skill.


Here is a thought provoking scenario.
Think of a glider flying an ILS. (bear with me, for those prone to debating, lets pretend there is a adequate thermal along the inbound course).

How would the glider control ROD and speed?

All of the above scenarios are changes in AOA. AOA and the wing function strangly. With a constant power setting, pull back on the pitch, increase the AOA and the airplane climbs. A little more and the airplane stops climbing. Too much and the wing stalls.

AOA is really about aircraft control. (AOA controls lift, drag and airspeed.) Since there are rarely seen indicators in the cockpit it is never considered. I am still learning. The fact that we as pilots can't apply AOA makes its consideration impractical.

However, a change in pitch changes the AOA which changes airspeed and lift! So does pitch control airspeed or altitude?

How about both.

Comments?

Mitch

[Crossunder]

First of all I have to say that I have yet to see an aerodynamics textbook describe power as the primary means for airspeed control.
When discussing this subject we have to assume that we are in steady flight;
when on ground, pushing the stick forward will not accelerate the aeroplane and hauling back on the stick will not cause it to climb from standstill either...
To have a meaningful discussion, we must assume that the engine location does not affect pitching moment, and talk pure aerodynamics or else we must have type-specific discussion forums.
From straight and level flight; if you add power/thrust, the aeroplane will start to climb. In order to accelerate we must pitch down to decrease AoA, because each AoA equals a specific Cl, which in turn will give a certain airspeed.
People who really depend on making precision approaches are Navy pilots making CAT-traps. Try telling an F-18 pilot to adjust his glidepath by pitch control?!
If I am one dot high on GP I need to decrease my potential energy without increasing my kinetic energy. To do this I must reduce power. If the aeroplane is trimmed for the correct airspeed and I reduce power, it will pitch down to maintain AoA. Once back on GP I will add some power to stop the excessive ROD. I extreme cases, lift does not play an inportant part of climb angle or ROC, because I can go vertical in an F-16C without(preferrably) my wings producing any lift (which would cause my aeroplane to loop).
The main thing, however, is that all of us have different techniques and you can't do one thing without doing the other; we all adjust pitch & power simultaneously!
And; at high speeds, for example a jet aeroplane on cruise, pitching down ever so slighly can give a significant ROD without much increase in airspeed because ROD is a function of both TAS and descent angle. The secondary effects would then be greater than the primary effects of pitch. At low speeds (during approach) pitch will control airspeed and power will control ROD, regardless of engine type.
If you need a really thorough explanation on this subject, pick up an aeropdynamics textbook (no; not one of those Jeppesen PPL-things, but something for egineers / university level) and read for yourself.

Happy landings! (and remember; that hard landing can be prevented by adding some power because an increased AoA will give too much induced drag and may stall the aeroplane ;-))

[E120]

Pitch + thrust = ROD + speed. It works together. During an approach pitch normally controls ROD and thrust compensates for speed, however... That's why we're called aviators.

Greetz. E.

[Flanker]

Crossunder,one or two grey areas.

You have assumed 'that we are in steady flight;' also,
'....the engine location does not affect pitching moment'

'So from straight and level flight;if you add power/thrust, the aeroplane will start to climb'- Why? The pitch hasn't changed.So what changes? Airspeed perhaps?

'If the aeroplane is trimmed for the correct airspeed and I reduce power, it will pitch down to maintain AoA'- Will it? Why should it?

Actually if I want to understand how to safely fly an aeroplane I will ask a pilot, not a pure engineer.A proper Aerodynamics textbook is unreadable in any case unless you talk Maths.

This has certainly been a lively interesting topic.
Rgds

[Crossunder]

I assumed that engine location does not affect pitching moment, because otherwise we'd have to discuss a specific aeroplane type; I thought the discussion was about pure flight theory and not techniques & aeroplane types...

And yes, a properly "behahved" aeroplane will, if you add power, enter a climb:
If the elevator is trimmed to a fixed position (pitch control), this will make the aeroplane fly at a certain AoA. (Longitudinal stability also depends on this) To maintain AoA, the aeroplane must pitch up; the excess power will then make it climb. If we do not want it to climb we must decrease AoA to allow it to accelerate.
If you want to decelerate, you retard the throttles and in order to to maintain altitude, you pull back on the stick.
OK.
But if you do not re-trim (adjust the AoA to match the lower airspeed)and let go of the stick the aeroplane will pitch down and accelerate to the speed/AoA for which it was originally trimmed! (Then you will be in a descend because you retarded the throttles).

I guess all I want to say is that the elevator controls AOA and if the aeroplane is longitudinally stable it will always "seek" that AoA.
Books for pilots on this subject that are not too technical include the classic "stick and rudder" and "Flight theory and aerodynamics" by Charles E. Dole / James E. Lewis; those guys have taught this to US Navy,Air Force and Army pilots for almost 30 years.

But hey, I have my opinions and you have yours. I'm fine with that.

PS. If either Vmu or PAFB is reading this topic, please join in ;-)

[Tom Cat]

Hello! The Airbus principle when autopilot and flight directors are OFF:

Auto Thrust will maintain SPEED and the Sidestick is for pitch control!

[BobaFett]

Well god knows how big things work but I tend to select 2.5 degrees in the HUD and adjust the power for 10 AoA. After that pitch for glidepath adjustments and throttle for alpha (think speed).

Described by an incredibly irritating old groundschool instructor of mine as best imagined by tying the stick and throttle together with a bit of string looped through a nipple ring. One will always affect the other.

For those of you who may have known him - are you a catcher or a thrower? http://cwm.ragesofsanity.com/contrib/owen/fett.gif

[Propellerhead]

This is a copy of my post in Questions, with a bit added at the top here:

Captain Squelch - interesting comment about the underslung engines. I fly the 737, and if I get slightly low on the ILS, provided the speed is correct, I add a little bit of power (5-10% N1). As you say, this causes the nose to pitch up. Instead of resisting this with forward pressure and trim, I just let it. I get back on the profile, then reduce thrust back to the previous setting. Works a treat - no trim changes overall! SO, which method am I using here??!!

Also, I guess, if I am really low, I would bang on a load of power first, then raise the nose, as I don't want the speed to decay Pitch has more instantaneous results than power in terms of speed (eg it would decrease before the engines had time to spool up).

However, I agree with Max Angle, it's all about ENERGY - if slow and low, not enough energy so add power, but if fast and low, you have the right amount of energy, just in the wrong place, pull back and you get things back where they should be. I know this is basic, but I think worth saying.

COPY OF 'QUESTIONS' REPLY :
In the RAF UAS I was tought that pitch controls speed / power controls ROD on the d/wind to finals turn (oval circuit), then the opposite on finals! (eg point it at the threshold, keep the aimiming point stationary in the windscreen, and use power to control speed).
Then for my PPL / VFR at Oxford I did pitch controls speed all the time - it worked very well. However, leading up to and on the Seneca this was beaten out of us and told you MUST control speed with power.

The main advantage is that you can aim at the runway, keeping a stable, constant approach path, so important for an ILS.

Now I fly airliners, I can't imagine getting slow going into LHR, and dipping below the glideslope to pick up the speed, then increasing power to get back on the glide! (I know this is an exageration, and you could do it a bit more coordinated!)

If you think about it, autopilots and flight director systems use thrust to control speed, and as we folow the FD, this is the way we do it when hand flying. The only time pitch controls speed is during a FIXED thrust manoeuvre (full thrust / idle thrust).

[Tor]

<font face="Verdana, Arial, Helvetica" size="2">Spooler

Approaches are a here and now situation to assure a stable approach and delays in a change of variable will only add to the workload. I'm only a PA28 flyer but I use pitch then throttle, I guess I just hate waiting...</font>

As I see it there is a risk that you may end up chasing the G/S because of a constantly changing AoA and speed. Ideally AoA shouldn't change because you'll change lift, drag and speed. If you only change thrust and maintain a constant AoA (by changing pitch), you should maintain a constant speed, lift and drag. Thereby having an easier time maintaining a stabilized approach.

I have come to think it's very theoretical because belivers of both sides manipulates both the throttle and yoke simultaneously. Theoretically (aerodynamically and mathematically) I believe that the thrust for RoD/RoC is more correct, however, Flanker, I'm seriously considering both options for pratical use http://www.flightcrew.dk/ubb//wink.gif

[This message has been edited by Tor (edited 07 May 2001).]

[Sick Squid]

For my first 200 hours I flew..

Power = Rate of Descent

Pitch = Speed

For my subsequent x-thousand hours I have flown...

A combination of pitch attitude and power = speed

A combination of pitch attitude and power = Rate of Descent:

with the modifying factors;

Primary control of Speed on Power.
Primary control of Rate of Descent on Pitch attitude.

However change one, then you change the other, or appear on Teletext.

I hadn't realised it was this difficult until it was pointed out on this thread. You do realise, I have now had my bottom-lines thoroughly messed with, and need help? :)

£6

[This message has been edited by Sick Squid (edited 08 May 2001).]

[mitch3p]

'So from straight and level flight;if you add power/thrust, the aeroplane will start to climb'- Why? The pitch hasn't changed.So what changes? Airspeed perhaps?

No, lift over the wing. the more airflow the more lift.

'If the aeroplane is trimmed for the correct airspeed and I reduce power, it will pitch down to maintain AoA'- Will it? Why should it?

Becuase the AOA was never changed. Pitch (either yoke or trim changes) changes the AOA

[Flanker]

Mitch3p

Airflow over the wing can only increase if the airspeed increases!Anyway yes I take on board Crossunders post and am learning quite a bit by thinking about all this.

I am quite sure about how I fly the 737 on approach however and I also accept different people prefer certain techniques for different types.The original post (in questions)asked about 757 thrust lever movement on approach,and as this is something I'm familiar and current with, I have not changed my view for these types.

Tor thanks for the tip on the bold :)

[Tor]

No problem Flanker :)

Speedbird252's question wasn't 757 specific. "Im curious about one aspect and its not a 752 thing only im sure.." and My question is that how effective is this method when flying a fully manual approach in a Jet?.

Did you notice I said I was considering to switch to "pitch for G/S" (although I still believe "throttle for G/S" is theoretically more correct)? http://www.pprune.org/ubb/NonCGI/tongue.gif

[Tor]

Propellerhead
<font face="Verdana, Arial, Helvetica" size="2">
Now I fly airliners, I can't imagine getting slow going into LHR, and dipping below the glideslope to pick up the speed, then increasing power to get back on the glide! (I know this is an exageration, and you could do it a bit more coordinated!)</font>

Read your reply in Questions - LoL :)

What do you do? You increase thrust and lower the nose a bit to compensate for the increasing speed. What's the difference between increasing thrust and lowering the nose, or lowering the nose and increase thrust- if you do it simultaneously (and coordinated) anyway? :)

[Burger Thing]

... the difference is, that on a normal approach you correct the speed first and then trim out a slight AoA change. But you don't trim first and then change the thrust.... :)

[Propellerhead]

Tor, how do you do the bold type, and how do you do the quotation box from someone elses post. You seem a bit of a whizz with computers!

[Tor]

Follow this link: UBBCode Explained (http://www.pprune.org/ubb/NonCGI/ubbcode.html)

[Feather #3]

This chestnut was explained to me by a friend who instructs in C150's, etc.

Very light a/c don't have the power available to simply 'add power to increase speed', so students are taught to use pitch to increase the speed, and then control descent with power. As many have pointed out, it's a close run thing, but power in a lighty won't give you instant speed.

Switch to a jet with [some/lots] power to spare and we 'normalise' the practice we all use. As 'Prophead' said, if you get slow on the ILS, you don't dip below the g/p to increase speed. However, whichever way we do the inputs, coordination is required to maintain the desired flightpath.

It's the surplus power which dictates the ability to use power=speed, pitch=path.

BTW, that power is available in many larger piston a/c.

G'day

[edited for spelling]

[This message has been edited by Feather #3 (edited 08 May 2001).]

[Roc]

One aspect of this controversy is escaping alot of people. The US Navy teaches pitch controls airspeed, power controls attitude, but there is a very important reason for this. When an aircraft is approaching a carrier it must maintain a specific attitude, or "deck angle" so that the tailhook is the first part to hit the deck/wires. If this tail low attitude is not maintained the hook to deck angle is wrong and a trap will not occur. So the need to maintain a constant angle/attitude is paramount, hence one can't use pitch to control glidepath! Also, I fly C-141's and when we are on a run-in to drop paratroopers or cargo, we also have to maintain certain deck angles so we tend to use power to control ROD/ROC to maintain the constant deck angle. I also utilized this method flying the T-38, but it was equipped with an AOA guage and we flew approaches by slowing to a computed airspeed, then flying the "green doughnut" until landing. I suspect that the Navy Method is common to fighter type jets or even fast overpowered aircraft such as Lear Jets etc...any Lear pilots care to add in?

[E. MORSE]

Whenever POWER is AVAILABLE:
(e.g. level cruise / approach).
-------------&gt;Power controls speed !


Whenever power is NOT available:
(e.g. idle descent / full power climb
-------------&gt;Pitch controls speed !


=============================================

A few exemptions :- a vertical climb (power replaces lift / lift = zero).
- a carrier landing (requiring constant deck angle).

=============================================


If you like an example:

Can you increase to mach .82 ?
- Sorry sir , This will give me a 300 ft. descent..........NOT.

Can a glider change airspeed?
- Sorry sir , I need an engine first....NOT.

POWER AVAILABLE IS THE KEYWORD

cheers


:)


[This message has been edited by E. MORSE (edited 13 May 2001).]

[This message has been edited by E. MORSE (edited 13 May 2001).]

[E. MORSE]

bold test msg.
:rolleyes: