Marenostrum

Pitch when at cruising altitude

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Posted (edited)

I am flying at FL340....no alarm...all seems to be right...but why have I the ange of attak gauge at 2.2 (I see I am not leveled also in the PFD)?

Flaps are retract...spoilers off. 

Is it due to my unbalance load? (144 pax and 10 + 100 cargo load aft and fwd)

Edited by Marenostrum

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36 minutes ago, Marenostrum said:

I am flying at FL340....no alarm...all seems to be right...but why have I the ange of attak gauge at 2.2 (I see I am not leveled also in the PFD)?

It's natural to have a slight pitch up when at cruise. It's just how aircraft are, across the board. In fact, there was a 727 out there in private service with a 3 degree tilt on the bed to counter the natural nose up pitch at cruise.

37 minutes ago, Marenostrum said:

Is it due to my unbalance load? (144 pax and 10 + 100 cargo load aft and fwd)

Nope. Just how planes work.

 

 

You've been posting a lot of questions lately. While that's certainly helping you learn, I would suggest taking some time to do a bit of reading and learning on your own as well. Seems like you're operating on an understanding of aviation that doesn't have a solid base. That's okay, but at the same time, you'll find it incredibly difficult to understand the complexities of our products, when some of the basic points of flight aren't fully understood.

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Most if not almost all airplanes have their step pitch at around 2 degrees nose up which is the most economic attitude, lowest drag. That‘s fine.

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51 minutes ago, scandinavian13 said:

 

You've been posting a lot of questions lately. While that's certainly helping you learn, I would suggest taking some time to do a bit of reading and learning on your own as well. Seems like you're operating on an understanding of aviation that doesn't have a solid base. That's okay, but at the same time, you'll find it incredibly difficult to understand the complexities of our products, when some of the basic points of flight aren't fully understood.

Thank you,

I am in the initial learning curve of this aircraft...I am learning by reading a lot, looking a lot of tutorials on youtube, flying, and finally asking....I come from about 1000hrs flown over the Q400 by Majestic, another complex aicraft, and to "switch" into this one is more complicated than I expected (many many things are different in behaviour and setting).

During the flight, in the cruising phase,  I usually have some time to better explore instruments, panels, etc etc.

When I do not find an answer in the books (I have to say that the manuals are not so easy to explore) I ask here...continuing my a mix of reading, viewing, flying...and asking.

And I have to say that all of you are very helpful...so I will continue this way...but please Kyle feel free to not answer if you find it excessive!! 😉

btw you are one authors of the tutorials I see

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57 minutes ago, Marenostrum said:

When I do not find an answer in the books (I have to say that the manuals are not so easy to explore) I ask here...continuing my a mix of reading, viewing, flying...and asking

Ask away my friend, I think Kyle was simply trying to encourage you to explore wider.  My bible when I was a student pilot many decades ago, and is still a good source is the Airman's Information Manual (I think they changed Airman to something PC like Aeronautical but it is still the AIM): https://www.faa.gov/air_traffic/publications/

Your question was not answered above.  You asked about angle of attack and the answers were to attitude pitch.  While it is true that both AOA and pitch are usually positive during level flight, the reasons for each are different.  It's possible to have a nose low pitch and a positive AOA, this is like a B-52 climbing with nose down attitude.  Back to question:  Angle of attack directly influences lift and in level unaccelerated flight there is a need for lift to balance weight therefore the positive AOA.  As the air gets thinner (less dense) the AOA required will increase given the same true airspeed, because as lift is a function of airspeed, air density and angle of attack (plus other stuff like coefficient of lift yada yada).  Less dense air means either higher speeds or AOA.  This continues upward until you get to a point where you can't go faster due to Vmmo (maximum mach operating speed) and you can't go slower or you stall.  AOA is one of those parameters that is very important to understand to be a pilot... I encourage you to dig deeper on this subject.

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11 minutes ago, downscc said:

Ask away my friend, I think Kyle was simply trying to encourage you to explore wider.  My bible when I was a student pilot many decades ago, and is still a good source is the Airman's Information Manual (I think they changed Airman to something PC like Aeronautical but it is still the AIM): https://www.faa.gov/air_traffic/publications/

Your question was not answered above.  You asked about angle of attack and the answers were to attitude pitch.  While it is true that both AOA and pitch are usually positive during level flight, the reasons for each are different.  It's possible to have a nose low pitch and a positive AOA, this is like a B-52 climbing with nose down attitude.  Back to question:  Angle of attack directly influences lift and in level unaccelerated flight there is a need for lift to balance weight therefore the positive AOA.  As the air gets thinner (less dense) the AOA required will increase given the same true airspeed, because as lift is a function of airspeed, air density and angle of attack (plus other stuff like coefficient of lift yada yada).  Less dense air means either higher speeds or AOA.  This continues upward until you get to a point where you can't go faster due to Vmmo (maximum mach operating speed) and you can't go slower or you stall.  AOA is one of those parameters that is very important to understand to be a pilot... I encourage you to dig deeper on this subject.

Thank you downscc....very interesting and technical  explanation ...as engineer always curious  I will read more about it. It reminds me some  curiosities I read about the famous Blackbird. 

I downloaded the 744 pages bible....it will be my slow reading book... really thank you for the link.

about the wider exploration....I am simulating flights on the 737-800 after only 24 hours of reading, asking, viewing...so I am aware I need nothing  less than a very deep/wider exploration.....and  this "training  on the job" way to explore while I fly makes it easyer and I think more practical.

So really thank you Dan and Kyle

 

 

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15 minutes ago, Marenostrum said:

It reminds me some  curiosities I read about the famous Blackbird. 

Exactly, the SR71 operated in the corner of the flight envelope they call "coffin corner."  Up around 70,000 ft it had about a 10 kt window to operate within.... and the excitement began when an engine stalled.  Those guys earned their flight pay.

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Ciao Stefano,

(sorry for the underline but I can't find how to delete that selection, I've tried by pressing the U button but w/o success).

As you  know in an airplane (pitch) attitude, power (N1%) and speed are linked togheter so is paramount thatYou've to study and memorize ("by heart") the attitude and power needed for the various phases of flight by looking at the "performance inflight/flight with unreliable airspeed/turbulent air penetration speed" section of the FCOM1 (I can't remember exactly the page but "performance chart should be around page "700") or by looking at the same section in your QRH"Chapter PI-QRH General Section 10) now I've looking at a QRH of an airline an there is at page 279.

Looking at those figure you'd learn that i.e. at 60 tons ISA 35000 ft if you aim for a level flight at 0,76 Mach in i.e. manual flight you should set at 2 degrees pitch nose up and 86% (about) N1%.

But these figures are mandatory to learn about APPROACH settings: there are published for Vref+10 flaps 30 and 40  but also for Vref+5 flaps 30-40 are similar. By heart: when I'm in the "big" sim (I go often in a fixed base B738 sim near Bergamo) flaps 30 Vref+5 60-62 ton I know I've to fly with pitch 2-2,5 degrees nose up engine 50-55% N1 with flaps 40 pitch 0-0,5 nose up engine 60-65% n1, you should try w/o FD and memorize those settings because this can be very useful the day that you've to disengage your A/P early i.e. for a failure (happened in the big sim) and execute all the approach and landing "by hand".

Ciao, a presto

Andrea Buono

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Dan,

Assuming that the wing‘s profile is parallel to the fuselage isn‘t the AoA the same as the pitch in level flight? 

Or in other words, the difference between angle of attack and nose up attitude is always the same in level flight. 

Of course, when climbing the pitch is higher than the AOA as the pitch relies to the horizon and the AoA to the airstream. But assuming an economically ideal climb the difference between pitch and AOA is around the same 2-2,5 degrees for „typical“ aircraft.

Isn‘t that correct? Or am I off the track?

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51 minutes ago, Andrea1 said:

C

But these figures are mandatory to learn about APPROACH settings: there are published for Vref+10 flaps 30 and 40  but also for Vref+5 flaps 30-40 are similar. By heart: when I'm in the "big" sim (I go often in a fixed base B738 sim near Bergamo) flaps 30 Vref+5 60-62 ton I know I've to fly with pitch 2-2,5 degrees nose up engine 50-55% N1 with flaps 40 pitch 0-0,5 nose up engine 60-65% n1, you should try w/o FD and memorize those settings because this can be very useful the day that you've to disengage your A/P early i.e. for a failure (happened in the big sim) and execute all the approach and landing "by hand".

Ciao, a presto

Andrea Buono

Ciao Andrea,

Among the thousands pages this is for sure something I have to know as a feeling....I found the page, but (it is the only one I found...but possibly it is written in another page) it refers only to VREF+10 and it is in the section "30.43 Flight With Unreliable Airspeed TurbulentAirPenetration)...

Am I reading the correct section?

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45 minutes ago, Ephedrin said:

Dan,

Assuming that the wing‘s profile is parallel to the fuselage isn‘t the AoA the same as the pitch in level flight? 

Or in other words, the difference between angle of attack and nose up attitude is always the same in level flight. 

Of course, when climbing the pitch is higher than the AOA as the pitch relies to the horizon and the AoA to the airstream. But assuming an economically ideal climb the difference between pitch and AOA is around the same 2-2,5 degrees for „typical“ aircraft.

Isn‘t that correct? Or am I off the track?

After that Kyle and Dan pushed me to study it...my understanding is that, by definition they are two different angles.

Pitch  is the angle between the front/back axis of the wing and the horizontal "ground"

AOA  is the angle between the front/back axis of the wing and the direction of the aircraft motion

Yes they should be similar in many flight situation but I think they could be really different depending the aircraft design and scope (think the military aircraft)

Did I correctly understand?

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1 hour ago, Ephedrin said:

Or in other words, the difference between angle of attack and nose up attitude is always the same in level flight

 

13 minutes ago, Marenostrum said:

AOA  is the angle between the front/back axis of the wing and the direction of the aircraft motion

Good topic.... common misunderstanding played out here.  The AOA and pitch will differ significantly if the aircraft encounters rising or falling air.  The significance of AOA becomes paramount during approach, in fact many unstable early fighters flew their approach using AOA as primary and IAS secondary.  At a given air density, the wing will stall at the same AOA regardless of speed or pitch.  The direction of aircraft motion would be better replaced with the angle between chord and air motion in front of the leading edge.  Ever notice the GA stall sensor is simply a little tongue depressor sticking out in front of the leading edge?

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2 hours ago, Ephedrin said:

Of course, when climbing the pitch is higher than the AOA as the pitch relies to the horizon and the AoA to the airstream. But assuming an economically ideal climb the difference between pitch and AOA is around the same 2-2,5 degrees for „typical“ aircraft.

Pop quiz:

In a steady climb, angle of attack is:

a) Less than the AoA in level flight

b) The same as the AoA in level flight

c) Greater than the AoA in level flight

(Assume airspeed is unchanged).

What about for a steady descent?

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5 minutes ago, skelsey said:

Pop quiz:

In a steady climb, angle of attack is:

a) Less than the AoA in level flight

b) The same as the AoA in level flight

c) Greater than the AoA in level flight

(Assume airspeed is unchanged).

What about for a steady descent?

The AoA is always the same as long as you fly with the same IAS. A variing pitch for climb or descent simply changes the vertical speed. 

As your pitch increases, the lifting force increases too and so Fres (vector) points upwards. Pitching down Fres (vector) points downwards. 

If you now decrease your airspeed either you have to increase your AoA or you have a lower lifting force and so your Fres (vector) points down again.

 

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2 hours ago, Ephedrin said:

The AoA is always the same as long as you fly with the same IAS.

Not quite. 

Anyone else? Think about:

- The lines along which each of the four forces act and how they change in a climb or descent

- Which force controls the climb? (Hint: it's not lift).

I'll draw a diagram in the morning ☺️

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Excess thrust controls climb,  power plus attitude equals performance.  At a given speed, applying power and holding that speed with pitch, the angle of attack increases because pitch increased, the "relative wind" comes from an angle that acts to decrease the angle of attach but not enough to offset the increased pitch.  However, if you increase pitch beyond the point that maintains speed such that speed decays your climb increases in rate, AOA increases, until past the maximum L/D point from where climb will decrease until maximum available power is being used to maintain altitude.... now you are behind the power curve and AOA is maximum.  The only way out is to loose altitude or stall.

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13 hours ago, downscc said:

Exactly, the SR71 operated in the corner of the flight envelope they call "coffin corner."  Up around 70,000 ft it had about a 10 kt window to operate within.... and the excitement began when an engine stalled.  Those guys earned their flight pay.

Sure!

There are so many interesting things around this "aircraft" (probably it should be necessary to create a new name for this strange incredible bird) 

there are some interesting video on youtube and an a long interview to Richard Graham (a famousSR71 pilot).

sorry for the off topic...consider it a tribute to that incredible airplane.

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"Ciao Andrea,

Among the thousands pages this is for sure something I have to know as a feeling....I found the page, but (it is the only one I found...but possibly it is written in another page) it refers only to VREF+10 and it is in the section "30.43 Flight With Unreliable Airspeed TurbulentAirPenetration)...

Am I reading the correct section?"

Ciao Stefano,

Yes it's..

My advice is to study and practise by your ngx at the same time 'cause is easier to memorize and remember (learning by doing..as you can see the learning curve is quite steep bacause 737 ngx is very realistic! But it's very interesting if you love that liner and want fly it "like a pro")

Ciao

Andrea Buono

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As promised:

First, to establish some basic principles:

We know the lift equation (stick with me -- I know Stephen Hawking talked about his publisher telling him every equation he put in to A Brief History of Time would halve the sales -- like him I'm only going to put this one here and then it gets far simpler with some nice pictures!): L = CL1/2ρV2S (where CL is Coefficient of Lift, ρ is air density, V is velocity (airspeed) and S is wing area).

As pilots, however, there are only two variables here which are easily within our control: airspeed and the Coefficient of Lift (which is directly relatable to angle of attack).

We can, therefore simplify the equation down to: Lift = Angle of Attack x Airspeed.

Next -- we can all agree that in a steady climb or descent, just as in straight and level flight, the four forces -- lift, weight, drag and thrust -- are in equilibrium.

So, let's look at the forces in a climb. For simplicity, I've shown all the forces acting through one point, and exaggerated the climb angle for clarity.

We can start with weight and drag:

drag_weight_r1.JPG.6925582688967ee2f7d06ca1829ec1c5.JPG

Drag acts parallel and opposite to the flight path, whilst weight always acts vertically downward toward the centre of the Earth. As such, you can see that in a climb there is a resultant force (R1) which combines weight and drag.

Because the aeroplane is in equilibrium, there must be a force equal and opposite to R1. This force, R2, is made up of thrust and lift (which acts perpendicular to the flight path):

lift_thrust_r2.JPG.ed0aedda6607203e8ceea0c0405d5361.JPG

In straight and level flight, thrust = drag.

In the diagram above, however, thrust is greater than drag. So why does the aeroplane not accelerate?

final_resolution.JPG.dd33fa059b2ff0805c3c24dd1ef53576.JPG

As can be seen above, we have a rearward component of weight (RCW) which is added to the drag vector and opposes thrust. In a steady climb, therefore, thrust is equal to drag plus the rearward component of weight.

It should also be apparent from the diagram above that in a steady climb lift is less than weight! But in straight and level flight, lift is equal to weight.

Remember from the start (if you've managed to get down this far!) we said Lift = AoA x Airspeed

If airspeed remains constant, the only way to end up with less lift is to reduce the AoA!

In a steady climb, therefore, the AoA is less than it was in level flight!

Well done if you got this far!

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22 hours ago, Marenostrum said:

Thank you,

I am in the initial learning curve of this aircraft...I am learning by reading a lot, looking a lot of tutorials on youtube, flying, and finally asking....I come from about 1000hrs flown over the Q400 by Majestic, another complex aicraft, and to "switch" into this one is more complicated than I expected (many many things are different in behaviour and setting).

During the flight, in the cruising phase,  I usually have some time to better explore instruments, panels, etc etc.

When I do not find an answer in the books (I have to say that the manuals are not so easy to explore) I ask here...continuing my a mix of reading, viewing, flying...and asking.

And I have to say that all of you are very helpful...so I will continue this way...but please Kyle feel free to not answer if you find it excessive!! 😉

btw you are one authors of the tutorials I see

Ask as much as you like.

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3 hours ago, icaruss said:

Ask as much as you like.

Thank you Icarus,

and thank to all experts of this fantastic airplane: I am sure that after one year I still will be here to ask questions!! 

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You got your cart pushing the horse here, you started the analysis in a steady state climb which ignores the transition from unaccelerated level to unaccelerated climb.  There is a transition that includes acceleration and this will reveal that the lift vector is increasing during the acceleration.  Too many engineers around here and we love throwing rocks at ideas... nothing personal of course.  I find the analysis entertaining.

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Posted (edited)

Great explanation Simon: clear and simple!

now, back to the 737 pitch during the approach phase

Here is the table I have to consider (PI.30.43 in FCOMv1)

Here I have the pitch as reference and not the AOA.  How to relate them, if possible?

Eg in the flap40 pitch is negative..I expect AOA to be positive

More than that, reading on the PFD a difference pf pitch of 1 or 2 is not so easy: reading an angle of AOA is ,on the contrary, easy and more precise ( it is written with a .decimal precision)

So what have I to monitor? and if it is the AOA, where is a table like the one below?

Thank you

[MOD: Don't post pics of copyrighted docs, please.]

Edited by scandinavian13

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6 minutes ago, downscc said:

You got your cart pushing the horse here, you started the analysis in a steady state climb which ignores the transition from unaccelerated level to unaccelerated climb.  There is a transition that includes acceleration and this will reveal that the lift vector is increasing during the acceleration.  Too many engineers around here and we love throwing rocks at ideas... nothing personal of course.  I find the analysis entertaining.

During transition from level flight to the climb, the forces will not be in equilibrium which makes everything extremely complex but AoA will, certainly, momentarily increase.

However, that was why the premise of my question was comparing a steady climb/descent to steady straight and level flight, rather than the dynamic transition, which is well beyond my level of aerodynamics :). Most people are surprised to discover that the AoA in a climb is less than in level flight (assuming airspeed is unchanged, of course).

Naturally this reduced AoA is invisible to the pilot, who is only aware of the higher pitch attitude.

Resolving the force vectors for a steady descent yields a similar result: again, lift < weight and therefore AoA will be less than in straight and level flight for a given airspeed.

2 minutes ago, Marenostrum said:

Here I have the pitch as reference and not the AOA.  How to relate them, if possible?

The short answer is that there's no real need to. Power + attitude = performance.

In most civilian aircraft, there is no direct and obvious indication of AoA (and I have a feeling that even the AoA indicator that is optional in the 737 may be calibrated in arbitrary "units" rather than degrees but I stand to be corrected). However, in unaccelerated (or close to) flight the airspeed may be used as a guide to the aircraft's AoA (and this is why we talk about a 'stalling airspeed' when, strictly, the wing stalls at a defined AoA).

The PFD (in the real aircraft) is a very large and easy-to-ready instrument: you may wish to "pop out" the PFD from the virtual cockpit so you can see it more clearly. With a little practice it is quite easy to be very precise -- it is also easier to fly the aeroplane in terms of pitch (and thrust) datums as this is how instrument flying is typically taught, rather than chasing the AoA (military-trained pilots - and particularly those with a carrier background, as I believe Wilhelm is - may disagree). Remember the PFD pitch ladder is graduated in 2.5 degree steps; so you simply pop the aircraft symbol (the box in the centre that sits over the pitch lines) either on (so the line bisects the box) or if you just sit the top or bottom edge of the box on top of or below the line that is a pitch change of about one degree (if that description makes sense).

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And yet I just finished a climb in the 77L from KDFW up to FL350 and AOA in the climb was 2.2 and now in cruise it is 3.0.  Mea culpa.

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