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View Full Version : AOA from 60' or 50' to T/D (b777)


agg_karan
9th Aug 2018, 04:54
Dear all,
The following doubt arises purely out of curiosity and is for academic interest only.
Typically in CRZ a glance at the AOA looks between 2.3-2.5 DEG (without getting into the CG, airspeed, etc etc details)

the question is - 1) from 60' or 50' to T/d what would be the AOA range typically (for flaps 30) if anyone has an idea or a snapshot etc
apparently Maintainance info display doesn't work on approach & beyond. Our a/c don't have a AOA display on PFD/ND - whereverr it's suppose to be)

Thankyou & regards

Pugilistic Animus
9th Aug 2018, 08:41
How are you determining AOA without an Angle of attack gauge? :confused:

eckhard
9th Aug 2018, 09:00
We can assume that AoA is the difference between the attitude and the Flight Path Angle (FPA). To be more accurate, we would have to know the rigger’s angle of incidence for the wing.

On a typical approach, attitude is about 2 or 3 degrees and FPA is -3 degrees, so AoA is about 5 or 6 degrees.
During the flare, attitude will increase to about 4 or 5 degrees as the FPA reduces towards zero at touchdown, so AoA will be pretty much the same as it was during the approach.

Wizofoz
9th Aug 2018, 09:01
Difference between attitude and flight path (as displayed by FPV) is pretty close- at the very least, the CHANGE in that value is the change in AOA.

agg_karan
9th Aug 2018, 19:50
#Pug - MAINTAINACE info display Flight controls page :)
# eck - I am aware of that FPA PITCH analogy but would love to have a digital readout than a visual observation :)
# wiz - would be happier to see a digital representation :)
​​​​​​​

Bluescan
10th Aug 2018, 11:45
On some aircrafts the AoA can be shown in the printed landing report. Have you had a look there?

8che
10th Aug 2018, 22:09
Some 777ís are fitted with an AOA gauge typically positioned on the top right of the PFD. Itís a customer selectable option. Otherwise it is the difference between FPV and Attitude as others have stated.

misd-agin
11th Aug 2018, 02:57
With landing flaps (20, 25, 30) a green band is displayed on the AOA gauge for 'on speed'. It's roughly 4-7 degrees.

Pugilistic Animus
13th Aug 2018, 02:21
Thanks Gentlemen

scifi
13th Aug 2018, 10:11
For nearly every wing aerofoil section the AoA on the Co-efficient of Lift Curve varies from about 4 degrees at cruise speed, and about 7 degrees at approach speeds, to 13 degrees at the stall.
Most small GA aircraft try to flair before landing, to land at the stall speed ( AoA = 13 deg.).
However larger aircraft do not often take it as far as the stall, as if overdone could cause a tail-strike. So they land positively at slightly higher than stall speed, with the AoA in the region of 8 to 10 degrees, and rely upon lift-dumpers to stop the excess energy being translated into a bounce.
.

FCeng84
13th Aug 2018, 17:13
I agree with eckhard's thinking about approach and flair pitch attitude, flight path angle, and resulting AOA. I question the data presented above by scifi. Pitch attitude during 777 landing rarely exceeds 5 degrees. Tail strike would occur at about 10 degrees depending on which minor model is involved. With flight path angle close to zero at touchdown, AOA is approximately equal to (no more than one degree higher than) touchdown pitch attitude. 777 touchdown AOA is much closer to 5 or 6 degrees than the 10+ degrees scifi suggests.

Another issue I take with scifi's data is stall AOA for 777 in landing configuration. With flaps out to 30 and slats fully gapped (position commanded with flap lever at 30) the stall AOA is much higher than 13 degrees. Stall AOA for this configuration is 20 degrees or higher.

Commercial aircraft are not and should not be operated near stall during any part of a normal mission. A key performance parameter is the ability to execute a prompt go-around if one is needed at any time during the landing sequence. This means having the ability to pull up with as much as 0.3g incremental (total of 1.3g) from the approach or even during flair. In order to maintain this performance margin, AOA cannot be allowed during normal operation to get so high as to not allow for a 30% increase in lift without risk of stalling. Because the zero lift AOA is actually quite a bit below zero with flaps extended, approach AOA cannot be allowed to get to much more than 2/3rd of stall AOA.

hans brinker
13th Aug 2018, 17:47
I always flair, but when I get to around 30ft RA, I transition to flare....

172_driver
13th Aug 2018, 17:55
During the flare, attitude will increase to about 4 or 5 degrees as the FPA reduces towards zero at touchdown, so AoA will be pretty much the same as it was during the approach.

Is it not the increase in AoA that arrests the descent then?

PEI_3721
13th Aug 2018, 19:30
I was going to ask about ground effect, but see para 6 in the reference below, which overall might clarify some aspects of the debate.
Note, it appears that the term ‘incidence’ (alpha) refers to AoA in the equations (pages 26-30).
Intuitively if Cl changes during the flare then AoA will change according to the Cl AoA lift curve?

http://naca.central.cranfield.ac.uk/reports/arc/rm/3602.pdf

FCeng84
13th Aug 2018, 21:51
Increasing pitch attitude to flare just prior to touchdown initially increases AOA by an equal amount, however as flight path angle increases from approximately -3 degrees during the approach to close to zero at touchdown AOA drops off some. At a given speed and airplane configuration, lift is roughly proportional to AOA (Cl vs. AOA). The lift when descending through 100 feet on a stable approach equals the weight. Similarly, when touching down after flaring the lift is back to just about matching the weight. In between the lift is higher during the flare when flight path angle is changing to reduce the sink rate. Final approach, flare, and touchdown are all rather dynamic, but if you think of it as sequencing through 1g on approach, approximately 1.2g during flare, and back to 1g at touchdown it is fairly straight forward to see that AOA must increase and then decrease back to close to the approach AOA as the wheels settle gently onto the runway.

Ground effect does provide slightly more lift at a given AOA thus allowing the touchdown AOA to be slightly lower than that during the final approach prior to getting to within a wing span of the ground where ground effect comes into play. Usually, thrust is retarded to idle during flare so the airplane is slowing as well as entering ground effect. These two changes work in opposite directions with regard to the AOA needed for 1g flight so the net effect is very little. AOA at touchdown is almost exactly the same as AOA during final approach.

Pugilistic Animus
17th Aug 2018, 08:59
Lift slope curve