I suspect his error is in his drag calculations, which is a wee bit more complex.

Agree Ed..in particular regarding CDi calculation that in some models deviate from the classical formula based on CLw,Oswald, AR although it usually fits rather well at usual cruise wing AoA..MS has never been very clear on that..even in Zyskowski's paper (that seems to indicate it is calculated by FS from AoA-AoACL0 but I don't know precisely how..). Other CD components are not a problem..

Yes, I probably messed up the CDi calcs. However, I am able to do calcs that includes the tables. (Without having to manually calculate new values)Cool Hervé, I'll try it out!While running some tests (using AFSD) I tried calculating the lift, however, it seems like q * CL * S / cos(AoA), isn't enough to make Lift * cos(AoA) = Weight. (Note that I used the CL from model option)Since the AoA should also cause the Net Thrust to blast of at an angle, wouldn't this also add to the total lift? Now (sin(AoA) * Net Thrust) + (q * CL * S / cos(AoA)) = Lift still doesn't do it, but might this be somewhat in the right direction?

Edited February 27, 201214 yr by johnor999

it seems like q * CL * S / cos(AoA), isn't enough to make Lift * cos(AoA) = Weight. (Note that I used the CL from model option)

Lift * Cos(AoA) has no meaning at all..Cos(AoA) applies to thrust on the drag axeAlso CLw should be corrected for several modifiers, especially the dCL/dAoa mach table (401) that may have significant effects

Since the AoA should also cause the Net Thrust to blast of at an angle, wouldn't this also add to the total lift?

Evidently..see my first equation above! Weight = Lift + Thrust*Sin(AoA)..that is W= (q.CL.S) + Thrust*sin(AoA)..no Cos(AoA) here..May be you should first draw a vector representation of ALL involved forces on lift/drag axes (flight path axes) otherwise I'm affraid you will get it all mixed up ;-)

Sounds like a plan Hervé! A tired brain could be considered "Inop" when it comes to visualizing force vectors. xDBy the way, that performance table option is very neat! :-)

Alright, I've managed to write some VBA functions that retrieve correct values from the required tables.Iterating the AoA is rather simple programming, so I'll get that working too.Now, just to ensure that I'm using the best possible approximation without having to do any FS tests, could someone comfirm this equation:CDi = (CL ^ 2) / (pi * e * AR)Where CL = CL0 * CLa * cruise_lift_scalar + CL_dh (1101) * htail_incidence * CL_ih(413)

At last I made the equation work. However, after gaining more knowledge in this matter, I started an attempt at solving the CDi problem once and for all.I know that significantly more skilled people have attempted this in the past, however, I'm still gonna have a go at it.Since it is generally said that CDi = CL^2 / (PI * e *AR), and it gives a standard error, this means that MSFS must be compensating for something.Since L = CL * q * S * cos(AoA) = W, this means that either something is correcting the CDi equation or it uses a different CL.The experimentally calculated CDi that relies on y = kx + m also returns deviations, this suggests that a variable might be missing.I have collected very accurate data for FL410-FL240 @ M0.78 using FSX default B738 and ASFD, and by the look of this data, the error seems to be related to the altitude as the error get's greater and greater with decreasing altitude. Whether it is due to air density, temperature, pressure or gravity, I'm not sure about.The question I ask myself is "What would they have to correct to make it more realistic?".If anyone would be interested in some sort of cooperation please let me know.

Hi,All I know is that when I set No Weather (i.e. ISA conditions) the Indicated Altitude vs the True Altitude values in FS do not match those on the web...

After some further thinking, decreasing gravity acceleration can't be part of the problem as this would have a non-linear effect. Furthermore, the lift equation would return errors as well, should it be part of the problem.The only thing that would make at least some sort of sense, would be if the CL used by the CDi equation isn't the same as the CL used by the lift equation.When I tried calculating CDi using CLa * a without the CL0 mach factor, the error was smaller, but still there. I assume this is due to mach already being accounted for by CDp and that CDi = CDa * a.I did some tests with the default B744, and the test showed an even greater error. I'm thinking this might be related to the plane being bigger. (Not sure what parameter though)Maybe the elevator area is somehow related to the error?

For what it's worth, according to the Aircraft Airfile Manager comment on Fuselage AOA at Min Induced Darg and Table 1101

Important for correct Induced Drag!Set to (AoAw@Cl=0) - (AoI+Twist/2)[TBL1204] typCd=Cdo + [(AoA+AoAmd)*(dCl/dAoA)Cl=0]^2 * Induced_Drag_ConstantdCl/dAoA is 'average Cl vs AoA slope' in TBL 404. Gritsevsky/Pierson

For what it's worth, according to the Aircraft Airfile Manager comment on Fuselage AOA at Min Induced Darg and Table 1101

Yes I know about that equation, and I'll try working with it as I've found out that if MSFS relies on the common way to calculate CDi, then it doesn't use the same CL as the lift equation.

I've noticed that side force seem to be unaccounted for in the currently known equations. A plane that is side slipping, SHOULD have a greater drag than a plane that isn't side slipping. I'm thinking about the fact that if a plane is sideslipping, the fuselage should cause some drag.I know this is mentioned in the Aircraft Airfile Manager, however, I've not seen it mentioned anywhere.

FS does model sidefore using the Cy coefficients. They act at right angles to the direction of motion and so don't cause drag directly.I doubt that it is used in fuel flow calculations.I also doubt that going inio this level of detail with provide reliable estimates. My understanding is that FS calculates intsantaneous thrust and fuel flow based on throttle lever setting, aircraft speed and environmental conditions using tables in the .air file. If the instantaneous drag isn't equal to the thrust the aircraft will change speed until thrust and drag balance. In the process the environmental conditions may well also change.

Since it calculates fuel flow from thrust and the ammount of thrust needed to keep the plane at a constant speed has to deliver a composant that is equal to drag, it would be strange if side force wasn't part of the equation.

The following equation works for calculating the thrust at level flight:T = q*S*(((W / (q * S) - CL) / tan(AoA))) / cos(AoA)At least it works when using MSFS calculated AoA, Q and CL.Now, this doesn't solve climb thrust calculations, but it does solve cruise thrust calculations. (At least according to my data so far)