FPV (Birdy)
 

Trying to explain flight path vector (FPV) to a friend, one thing is puzzling me.

We all know we can use the FPV to determine AOA (pitch angle minus FP angle). But is it correct? What about the angle of incidence? Am I missing something?

I think your suspicion is well-founded. The angle of attack will be found by subtracting the FPV from the aircraft pitch attitude and then adding the angle of incidence.

However, given that we don’t (normally) know what the angle of incidence is, and given that it is small and probably varies along the span, a good approximation of alpha may be assumed by ignoring incidence.

eckhard.
Would be really interesting to know about angle of incidence of various airliners.

It actually does not matter what the angle of incidence is - you can use the airplane longitudinal axis as a datum for the AoA and for all practical purposes, the difference between the pitch and FPV is your AoA.

This will be somewhat altered by headwind/tailwind - AoA is measured w.r.t to airspeed, whereas FPV uses ground speed for calculation

Well, given that most airliners seem to cruise with about 2 or 3 degrees nose up attitude, and given that the angle of attack for LRC is about the same, I would say that incidence is close to zero.

As I recall from an old high speed aerodynamics book written back in 80-s, the typical angle of incidence for a jet is about 3*. Can’t be zero as the supercritical swept wing needs more of the AoA to generate enough lift. It’s also being said that the optimum AoA at which you wold have your L/D max is about 6-7*.

Well then that also would make sense of the 3* nose up attitude in the cruise.

So, 3* angle of incidence + 3* attitude = 6* AoA.

Sorry to disagree " L/D max" in cruise?

You’re right of course. L/D max would equate to maximum endurance, but max range (which is very close to LRC) would be faster.

Here’s a quote from NAVWEPS by HH Hurt:

“The maximum range is obtained at the aerodynamic condition which produces a maximum proportion between the square root of the lift coefficient (CL) and the drag coefficient (CD). In subsonic performance, this occurs at a particular value angle of attack and lift coefficient and is unaffected by weight or altitude (within compressibility limits).”

Unfortunately, he doesn’t mention the actual angle of attack. I could look further into that very good book but I’m a bit busy right now.....

From what I recall an "ECON" angle for a jet is around 4°.

I’m actually ‘flying’ in a 787 FBS.
Here are some figures from the Sim Maintenance pages:

Weight 200 Tonnes (typical mid-cruise)
FL370 ISA (close to Optimum of FL366)
CG 25% MAC
LRC M0.853 / 279kts AoA 2.4 pitch att 2.4
Best Hold Speed 262kts AoA 3.2 pitch att 3.2
OEI drift-down speed 252 AoA 3.8 pitch att 3.0 (descending at 600fpm at an angle of -0.8)

So these figures would suggest that the wing incidence is zero, since for level flight the AoA and pitch attitude are the same.
Of course, this may only be a convenience adopted by the sim manufacturers and may not reflect the situation on the actual aircraft.

I-PIERLU: your 4 degrees is close to these figures.

Eckhard,

As far as I know, in 3D aerodynamics, ie. when we are discussing entire airplane , rather than an airfoil, the convention is to reference the AoA from the longitudinal axis of the plane rather than the chord of the wing.

Therefore in level flight AoA=pitch attitude.

If correct AOI on B727 is 2.0 deg and on B757 3.2 deg.

The Boeing 727 & 757

Some more interesting figures from the 787 sim at LRC, FL370, ISA, 200T:

N1 86%
FF 2700kg/hr (x2) = 5,400kg = 11,900lbs
Weight 440,873lbs
Total Thrust 21,900lbs
Therefore L/D=20/1

Thanks Sidestick and Gearlever for your input. Very interesting.
Yes, it makes sense to reference AoA to the whole aircraft and not just the wing.

Are we sure that the symbol we call attitude is really attitude reference to the aircraft fuselage longitudinal datum or is it attitude reference main plane incidence, or some average of the two?

Why would you fix your wing such that in the cruise the fuselage was not level? I could save you some fuel by re-rigging your wing!

A good question which I’ve often pondered myself.
There’s no doubt that the floor is inclined in the cruise. Just try pushing and pulling a heavy trolley!

The fuselage is not entirely symmetrical, so perhaps best L/D for fuselage is at ~2 deg?

BTW, thanks for the aerodynamic data for the 787. I would have actually expected it to operate at L/D slightly higher, than 20. Then again, it’s at high altitude and high Mach No, so the aerodynamics must be even better at low altitude. It would be great to compare it to other airplanes, eg. A330 and A359

It really doesn't matter...

The incidence angle of the wing chord is not constant along the span, so there can be no single value for AoA or incidence. Further, the extension of leading- and trailing-edge slats and/or flaps change the effective AoA at any point along the span, and accentuate the washout (higher AoA inboard, lower AoA outboard).

Just use the info given (pitch and FPV), and assume the engineers picked a reasonable datum.

Intruder, I fully agree.
I've stumbled over angle of incident to explain a student AOA versus flight path and didn't want to miss a point. As we all know there are many planes around where the angle of incident IS constant along the span (GA).

There’s a lot more factors at play here - like local upwash/downwash, aerodynamic and geometric twist, wing sweep etc.

I’d argue that even on a plain, straight wing with constant angle of incidence, each section operates at a different local AoA, because of 3D airflow. Therefore it’s sensible to pick up a different reference.

Also, in any case, the reference does not matter, as long as it’s consistent and data is appropriately adjusted for it. For example, I seem to remember from reading in one incident report (probably the Qantas A330 upset), that A330’s AoA vanes ‘zero’ reading was something like -45degrees down from the fuselage axis :8