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incorrect pitch attitude during turns(737NG)

I

IMC?

Messages
12
Hi guys,

sorry for the late reply, I was busy in the past weeks.Just came back to check on the thread and wow it has developed into a major discussion of aerodynamic forces.

Just to get back to original topic:

@Roy:
Roy ,if you have the time could you kindly reply to these questions regarding the test results which I posted earlier as I still don't fully understand your statements.
if I may quote myself:
--------------------------------------------------------------------------
"However I have two more questions regarding
case 4&5 of the test results.
You stated for case 4:

"AOA is hardly increased because the stall is commencing and the autopilot is most likely trimming nose down. Look at the pitch angle, it is 12.6and your wing stalls at 12 AOA."

What makes you say stall is commencing? While it is true that the pitch is at 12.6, however the critical stall angle is 12degree AOA not 12 degree pitch.
As you can see in my results, in this case(4)even in the turn, the AOA is only at 8.8 degrees here.
So we are still 3.2 degrees away from the AOA stall angle.

Same goes for case 5:
You say the aircraft is stalled now.
Again, even in the turn we are at only 9.9 degrees AOA, still 2.1 degrees away from stall AOA.So the aircraft is not stalled in my opinion and neither did the stall warning come on during the testing.

Maybe I understood something wrong, but as far as I am aware stall is referenced to a certain limit AOA angle (12degrees here in my example) and not to a certain pitch angle.Neither in case 4 nor 5 is the critical AOA stall angle actually reached/exceeded.There is even some buffer remaining.

So I have some trouble following your statements about stall commencing (case 4) and actual stall (case 5)."
-------------------------------------------------------------------------

@MGH:

thanks for your furhter inputs, I checked the test regarding the speeds and whether they may have influence on the issue, but the Autothrottle kept the speed stable to +-1kt during the entire testing so speed was constant.


@JX
since I'm really new to flight dynamics design, in what way do you suppose to modify CLadot under table 1011 to fix the behaviour?
If you can, please explain what to do in simple terms.Thanks for your help in the matter as well!

Regards,

Oliver
 
J

jx_

Messages
555
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unitedstates
I was speaking from memory and I misspoke. It's cn_beta not cl_beta.

Increase cn_beta (weathervane stab) by about 200 or so until you start to see the results you're looking for in the turn. Remember the total value is combined with (scaled by) table 460, so you could have a valid value in 1101 but it is scaled down to nothing useful somewhere else. Check both.


I also recommend setting CL_adot to zero or a very small value. If your wing and geometry are correct you normally wouldn't need it.
 
J

jx_

Messages
555
Country
unitedstates
Oliver,

I'm posting my response to your PM here just in case others have similar issues or other suggestions.

-----------------


First, the basics.

The entries in 1101 (FS9+) or the corresponding entries for FSX+ are a base value. For example, let's take CL_adot.

There is the CL_adot entry in 1101 that is summed with a table called Delta CL_adot(M) table 4xx, I believe it is around 410 (FS9+), and a matching 15xx (FSX+), if I remember right it is around 1550. These tables add or subtract to the base value of a coefficient. The base value is a static number that never changes in 1101 (or the AIR_80 tables for FSX+.)

The numbers we enter into a table are the coefficient * 2048. So if you want a coefficient of 0.02, you will need to enter 0.02*2048 into the table.

Some of the Mach tables are positive addition only, and some are half addition and half subtraction, but most are addition only. The way they work is, as mach increases, the Delta XX_xxxx (M) tables can add to the base value. This gives the affect of changes in handling due to velocity. As your mach changes, MSFS interpolates an output value from the CL_adot (M) table, adds it to CL_adot from record 1101, then divides the sum by 2048 to get the coefficient it needs.

That is how the mach tables work, but there are also AOA tables. These are simply scalar maps.

For example, if x (AOA) = 10 degrees (some are degrees, others are radians...they are all labeled), then multiply base value in 1101 by y value at x.

Take a look at the weathervane stab 460 table for example. It multiplies the cn_beta number in 1101 by the y value looked up.

The CM vs AOA table and all the tables between 400-500 (and even a few in the 1500's) that say "vs Body AOA" work this way.


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


What does CL_adot mean? adot represents the letter "a" with a small dot over top of it similar to mdot. adot means alpha rate (mdot means mass flow rate). Alpha means angle of attack "in the pitch axis". Likewise, beta means angle of attack "in the yaw axis".

CL_adot means coefficient of lift rate of change in response to changes in angle of attack. It signifies the affect of momentum on lift. When you slam on the brakes in a car, the tires take some amount of time to reach 100% grip. This is different in each car based on tire model, rubber stiffness, tire size, tire pressure...etc. If we wanted to make a car simulator we would need to tweak a coefficient of grip with an additional coefficient modifier to change the rate of lag change from 0%-100% grip. This is similar in concept to CL_adot.

CL means Coefficient of Lift force.
CY means side force and affects strafing side to side.
Cn affects yaw force, which is different from strafing, it is rotational like a flat spin.
Cl means Roll force. Ailerons can increase it and wings stabilize it.
Cm refers to pitching moments.

These are standard and can all be googled.


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

1101:Cn_beta & table 460

Weathervane stability affects the likeliness you will fly through air sideways. If you decrease this value to zero, you won't be able to maintain controlled flight. You'll spin randomly. This is what would happen to a fighter jet that has it's tail blown off in combat, or the TWA flight that crashed when it's vertical tail broke off.

When you bank in a turn, you are adding more negative side force due to gravity. Instead of pulling your weight downward against the bottom of the wing, some of the aircraft's weight is now pulled sideways through the low side wingtip (looking out the green windows that can only see the ground, you are being pulled toward the ground!) While this helps you turn some, it also requires a good amount of positive side force to keep you from sliding sideways toward the ground. If the side force isn't enough, the airplane would compensate by pitching up to produce more lift and maintain altitude (sounds like your problem to me...)

If the side force in the sim is too much (not possible in real world), the angle of attack will actually decrease in a bank because the side force is pushing you upward through your high side wing (the all blue windows). You wouldn't notice in level flight because side force is usually dormant in straight flight.


To complicate things further, your table 1101 could be set right, but the problem could be triggered by a bad setting in table 460 that is triggered by the normal (and correct) increase of angle of attack in a turn.

I would recommend setting 460 to 1.0 across, then tweaking 1101 cn_beta until you get good normal turn AOA. Then fly some tests to set 460.


=====


Here's how that works:

Anytime you make a turn you are pulling a G-Load. The G-load is a weight scalar.

G-load is n. Weight is W and Lift is L.

L = W * n

cn_beta needs to be set to increase AOA by W*n.

Step 1.

Set ISA conditions and turn off weather.

Load a flight in FS at some middle weight. At a high indicated airspeed at 10,000 altitude, start a level turn with autopilot and record the g-load and angle of attack. You can see the g-load with a gauge or by hitting Shift-Z.

Step 2.

Let's say the g-load you recorded in step one is 1.2. Add additional weight for your next tes flight to equal weight * 1.2. Be sure you maintain the same CG%. Fly at the same speed, and record the level flight AOA. The turn AOA in step one should be close to or just above the straight and level AOA at the higher weight. Adjust the curve of 460 as needed.


The angle of attack the airplane flies always equals Weight * n. When n is not 1.0, the airplane flies "as if" the actual weight loaded on board was Weight * n.
 
I

IMC?

Messages
12
Hi JX,

thank you very much for your extensive answer !Very informative .
I will try to modify it in the next days and report back, whether it
worked successfully.

Thanks so much again!

Kind regards,

Oliver
 
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