Pitch when at cruising altitude

[downscc 1,327]

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.

[Marenostrum 4]

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.

[Andrea1 6]

"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

[skelsey 765]

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 = C_L¹/₂ρV²S (where C_L 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 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):

In straight and level flight, thrust = drag.

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

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!

[icaruss 18]

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.

[Marenostrum 4]

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!! 

[downscc 1,327]

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.

[Marenostrum 4]

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 May 26, 2018 by scandinavian13

[skelsey 765]

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).

[downscc 1,327]

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.