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FSX Table 404 speed

D

Daniele

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unitedkingdom
Hello,
I am tweaking the a320 air file with real data and I have a question regarding table 404 (Cl vs AOA): what's the speed at which I have to define the lift coefficient with respect to the angle of attack? Or, can it be anything because I'll set the offset in the table Cl vs Mach?
 
Table 404 or alternatively table 1545 gives CL versus AOA in radians. Few points about the tables:
AOA is fuselage AOA. The program does not take wing incidence into play, so you need to allow for wing incidence when entering CL data against AOA.
The program calculates CL by interpolating between AOA values. Ideally you need to have a data point for the AOA where CL is zero and another at least 10 degrees AOA above that value.
The slope of the straight line CL vs AOA part of the curve is output as A:LINEAR CL ALPHA, number and it is in CL per radian.
The AOA where CL is zero is output as A:ZERO LIFT ALPHA, degrees (or radians, your choice)

In FSX there is only one table for CL versus AOA, whereas in real airplanes the relationship will vary throughout the envelope.
Flaps add lift and have the effect of lifting the curve upwards, but the slope is not affected.
Leading edge flaps, which in real life extend the length of the curve to higher AOA, just add lift same as trailing edge flaps in the sim.
Table 401 gives a scalar on CL according to Mach, and it has the effect of locally increasing lift in the same way as flaps, ie no slope change.
Flaps in real life change the stalling AOA, but not so in the sim. Stall AOA is the highest positive value anywhere on the curve and is output as A:STALL ALPHA, degrees (or radians)

So given all that, there is no "speed at which I have to define the lift coefficient". You need to know the wing incidence, the wing zero lift AOA and the clean wing lift slope.
Roy
 
There is always a speed Roy ;)

Standard is Mach 0.2 at 1g for CL(airplane) on airliners. Your FS tables already match this. The measurements are always taken at a specific speed.


"In FSX there is only one table for CL versus AOA, whereas in real airplanes the relationship will vary throughout the envelope."

Not exactly. FS uses the same system that is used real world. This is CL(airplane) which is different from CL(wing). CL(wing) is somewhat useless after the wing is installed. An installed wing behaves much differently. CL(wing) is only seen in a wind tunnel. All the variations in real airplanes are charted separately, the same as in FS, but FS is crude and has a few errors, some of which you have pointed out above.
 
Joe,
As usual my ignorance about airliners showed through. Airliners are inconvenient means to transporting me from place to place, gin and tonic in hand.
Real airplanes have one seat (maybe two).
Real airplanes measure AOA in milliradians and /or units. They do the mils between about 350 and 550 KCAS because mils are part of the manual bomb sight setting calculation.
Units are measured for all speeds above the stall (if it stalls) and throughout the whole altitude, Mach and g envelope.
Given AOA you can calculate airplane CL.
But, there is still only one CL/AOA table in the sim, which was my point. Have a look at :
http://www.fsdeveloper.com/forum/th...trol-derivatives-table-404-and-others.435689/
Roy
 
Roy,

Yes airliners are boring lol

...but the CL in wind tunnel and on airplane is charted at a set speed, depending on company. McD used Mach 0.2 trimmed to Vs 1.23 and plots CL by weight at max rear CG. As you know lift decreases with Mach, and there is no way to chart a lift table at zero mach; and on the real airplanes, elevator trim load changes the table as well (decreasing lift with more nose up trim.)

In the effort to create a well balanced airfile, these considerations need to be built into our table 404 and flight testing should be done at these speeds as well.
 
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