I'm looking for a spreadsheet (or formulas) that would let me do a
"what if" calculation of aircraft performance when the current
performance, and specs, of an aircraft is known.

For example, if I had an airplane with a stall speed of 45mph, wing
span of 30 feet, cruise speed of 110 ... take off distance, rate of
climb, etc. What would happen if the wingspan were reduced or
increased, horse power reduced or increased and things like that.

Any ideas ???