A question for all you aerodynamicists out there:

I'm working on a project which entails converting several aircraft performance manuals into Excel. The goal is to allow a user to enter the desired payload and range in a spreadsheet automatically calculates the appropriate takeoff weight, fuel consumption, and flight time. For takeoff, departure, climb, descent, and landing, this isn't a problem, as fuel consumption, time, and distance are presented graphically. I pick several representative points and have Excel give me a quadratic equation for the curve.

For the cruise segment, specific range curves relating specific range (nm/kg) and Mach number for a given cruising altitude are the only data presented. Time and distance can be calculated with the selected cruise Mach number, distance to top of descent, and ISA values for Mach 1. To calculate fuel consumption:

SR = Nm. / Gal. * [1/hr] / [1/hr] = [nm/hr] / [gal/hr]

Given the calculated true airspeed (assuming zero wind), rearranging to solve for gal/hr gives a point estimate of fuel flow at a particular weight. However, the aircraft's weight is changing constantly, so I can't just multiply by the hours spent at cruise.

The specific range curves are only given at discrete weights. I'd like to find a continuous function that describes fuel consumption as a function of weight, altitude, and Mach number. Is there a way to do this? Specifically, I'm trying to find a way to interpolate between the various SR curves, but am not having much luck. Any suggestions?

Another manual shows specific range versus aircraft weight at a given Mach number, which is much easier to work with!

Thanks,

--B2707SST

PS - This is not for a real airline, so absolute accuracy is not required.