Hello!For quite a while now, I've been trying to create a formula for fuel flow. However, no matter what formulas I use it just won't work.I've searched the Avsim forums, FSDeveloper forums, all sorts of websites and also gone through the documents at http://www.aero.sors.fr/afsd.html.The formulas I have for now, are:TBL1506 = (q * S * CD) / (F_STATIC * delta2 * F_SCALAR)CD = CD0 + ((CL^2) / (pi * e* (b^2 / S)))CL = W / (q * S)FG = TBL1506 * F_STATIC * delta2 * F_SCALARFR = TBL1507* A_INLET * delta2 / (theta2^0.5) * v_TAS * 1.68781 / gFF = TSFC*(FG-FR)Could someone tell me what I'm doing wrong here?

While I'm not the worlds for most expert on this don't you need to be compensating for altitude somewhere in there? Density?Why not take the Fuel Flow figures given by FS and try to "reverse engineer" them in a sense?

Hello!For quite a while now, I've been trying to create a formula for fuel flow. However, no matter what formulas I use it just won't work.I've searched the Avsim forums, FSDeveloper forums, all sorts of websites and also gone through the documents at http://www.aero.sors.fr/afsd.html.The formulas I have for now, are:TBL1506 = (q * S * CD) / (F_STATIC * delta2 * F_SCALAR)CD = CD0 + ((CL^2) / (pi * e* (b^2 / S)))CL = W / (q * S)FG = TBL1506 * F_STATIC * delta2 * F_SCALARFR = TBL1507* A_INLET * delta2 / (theta2^0.5) * v_TAS * 1.68781 / gFF = TSFC*(FG-FR)Could someone tell me what I'm doing wrong here?

I don't recognize this formula: TBL1506 = (q * S * CD) / (F_STATIC * delta2 * F_SCALAR) and I really wouldn't know what it represents. TBL1506 is a data set that defines the thrust curve of the engines based on CN1 and speed in mach.Formula for Cd and Cl both are incorrect (for FS). I say this because those formulae have never resulted in matching values with Herve's AFSD gauge values.Where:W = weight in poundsa = acceleration (g-forces)A = wing areap = ambient densityV = velocity, feet per secondCl = ((W*a)*2)/(A*p*V²)For Cd you need to know the Cl-v-AoA slope and calculate the value from the Cl-v-AoA table itself and combine that with parasitic and mach drag.

The purpose of the TBL1506 equation, was that I was going to try to remove CN1 as a variable by making the two unknown functions f(M, TBL1506) = g(M, TBL1507). However, I never got that far.The fuel flow values that I wish to calculate are during cruise conditions, as I wish to produce fuel flow tables for flight planning purposes.I don't know much about aerodynamics, so if anyone could provide me with a formula that will produce fuel flow values, it would be very helpful. The reason why I'm not simply using Hervé's tool for this is because I want to be able to do many calculations in just a few clicks. I've made several attempts over the years, so it would be cool to finally see what the answer was, so to speak. :-).

You can't remove CN1... it's what determines how much thrust is being generated. Hence thrust and fuel flow are connected to the value of CN1 in the sim.FG = TBL1506 * (F_STATIC * delta2 * F_SCALAR)andTBL1506 = (q * S * CD) / (F_STATIC * delta2 * F_SCALAR)are the same equation... where (q*S*CD)==FG.It's not gonna work.

The total thrust of an aircraft during level flight is equal to drag. In other words, Net thrust = Drag.This means that FF =Drag *SFC.Table 1506 has nothing to do with fuel consumption, since it only affects the output thrust due to CN1 and Mach.Am I correct about this?EDIT: What is the complete equation for total CL?

Edited February 17, 201214 yr by johnor999

Forget Tables 1506 and 1507. Starting from there will lead you nowhere..these are only for defining relationships between engine parameters (CN1) and thrust/ram drag factors Basically the only thing you have to do is to solve the steady level flight aerodynamical equations:Weight = Lift + Thrust*Sin(AoA)Drag=Thrust*Cos(AoA)that can be combined in(Weight/(CD.q.S)) - (CL/CD) - Tg(AoA) = 0 [q: dynamic pressure, S: Wing area]However, it is a rather complicated processFor calculating total CL and CD, you have to take into account all relevant Lift and Drag coefficients that apply during steady cruise flight that areFor Lift:- CLw (from Tables 1545 or 404 as appropriate) as a function of AoA, also correcting for cruise_lift_scalar (if not 1) and dCl/dAoA mach factor (table 401) and also in some models (for ex some PMDGs) for relative height (table 400) as far as a greater than 1 factor is defined even at high altitudes.. - CLhs (fixed lift from horizontal stabilizer) from CL_dh (1101), delta CL_dh (from table 413) and htail_incidence (will be 0 anyway if htail_incidence is 0)- And in some models (for ex some PMDGs),For Drag:- CDo from #1101 or #1540 and parasite_drag_scalar (if not 1)- CDm from table 430- CDi that can be approximated from CLw (or AoA) and CDk (depending on wing span, wing area and oswald), although as Ed pointed out it will not fit for some models and we still don't know how FS calculate induced drag coefficient..but minor errors will not significantly affect the final results as far as CDi is usually low at cruise speeds The difficulty is that it is impossible to directly solve the equation as far as AoA is also involved in lift calculation. Consequently, you have to iterate on AoA until you find an acceptable fitFinally total thrust will then be calculated fromThrust = CD * q * S/Cos(AoA)or in case some pitch and/or lateral offsets are defined for the engine thrust vector (FSX)Thrust = CD * q * S / Cos(AoA+AvgThrustPitchAngle) / AvgLateralThrustScalarandFF = Thrust * TSFCDynamic pressure (q) is easily calculable from TAS and current density (from altitude and temperature)In FS, TSFC is 0.5 * fuel_flow_scalar This kind of calculation can hardly be performed from a basic Excel sheet as far as many parameters from air file and aircraft configuration file are involved..exemple of such calculation is provided in my AFSJCC software you may have a look at Hervé

Edited February 17, 201214 yr by hervesors

Thank you for your help Hervé! I'll try to make a VBA function that is able to calculate the fuel flow.Just one question, what function does Tg(AoA) refer to?Also, what tests would it take to retrieve the MSFS equation of CDi?

Been fighting with these equations for some time now and... I just can't get the equation for total CL right.CL= TBL400 * CL0 * CLw + CL_ih * cl_dh(1101) + htail_incidenceIs this correct for level flight?

Have you even tried the formula I offered?

I was under the impression that CL was calculated by combining the different CL's. However, I tried your formula vs FSX and the calculated CL was 0.31566275 while the actual CL from the model was 0.31449419. I assume it's pretty accurate then, thank you! :-)

The formulae you were wanting to use are fine for a spreadsheet... but in actual 'flight'... they are serious overwork.The formula I provided is essentially a 'summation' of the value of CL based on what it all boils down to: LIFT == WEIGHT. Adding G-forces to the equation adjusts the actual gross weight to account for banks, climbs, descents (assuming FS calculates g-forces correctly).

Well, I intend to use them for flight planning and not in flight, so these equations will do fine. Thanks again! :-)

Okay, after further testing, the formula gives rather drastic deviations. (Around 2000-6000 lbs +- real fuel flow). Hence, either the CDi formula or the CL formula or both formulas are incorrect or incomplete.What I can't understand, is why MS would deviate from real equations... could it be that something simply is missing from the CL and CD equations?

Edited February 21, 201214 yr by johnor999

Hi JohanEd formula will give you a pretty good approximation. However more accurate results (probably not achievable with a simple Excel sheet unless you are an Excel guru and know how to include and exploit such complicated things as air file tables data and perform associated calculations) may benefit from taking into account ALL relevant aerodynamical parameters and tables as they are defined in the aircraft definition files. For that, you may have a try to my updated AFSJCC software (2.0) that performs such calculations and is now also able to build cruise performance tables (fuel flow, nam, N1%) without making any assumption and needing to launch the simulator (you just have to define the folder containing the aircraft.cfg and associated air file)http://www.aero.sors.fr/afsd.htmlHope it will help you getting the data you need RegardsHervé