hello hawk - enicalyth here
I guessed that at about FL350 the A340 would have 7% compressibity i.e. cd tot = cdo + cdi +cdcc = 1.07(cdo + cdi). cdtot is the sum of all the wetted areas plus a fiddle factor for protuberances times a finishing coefficient, usually 0.0036. This area is then divided by the wing area to give cdo. I usually add the trim drag coefft into cdo, for example Boeing quote 0.0144 for their 744 cdo but I take 0.015 which is spookily accurate when it come to fuel consumption. cdi the induced drag is the square of the lift coefft divided by pi x aspect ratio x Oswald's "e". For the DC10 "e" was 0.83, the 744 "e" is 0.86 and the 777 "e" is 0.89 about as high as you can go. cdo for a DC9 is 0.02, a 737 0.0176, a 777 0.013. In cruise when lift coeffts go above 0.55 the aircraft tends to have little buffet margin left so the highest first cruise altitude is usually the density altitude where cl = 0.55 for the chosen speed. likewise when weight falls off in cruise you will step up when the new cl is 0.55. eventually you cannot step up any more and cl rolls off. when it gets down to about 0.4 compressibilty has dropped from about 7-7.5% to 5-6%, speed and weight and altitude dependent. 737NGs, 777s, A330/340s can run at L/D of 19-ish when cl is 0.5-0.55; 744s 17.5. specific fuel consumption is 0.65lb/lb-hr for 737NG at FL350 cl 0.5-0.55, 0.59 for a 744 and 0.57 for a 777. the distance you travel in cruise is (v knots/sfc) x L/D x natural logarithm(start weight/finish weight). So your A340-500 at 480 kts FL350 will travel 20.5 nm per 1000lb from the equation distance = (480/.587)*19.55*0.001283 assuming I am right in guessing compressibility at 7%. I made cl 0.522 and cdtot 0.0267 which is the 19.55 term in above equation. It feels right to me but I can only speak with authority on the 777 and the 744.
NB I forgot to say what weight didn't I ?...780000lb!!! When the weight falls to 480000lb expect 28.2nm/1000
Best regards to hawk from the enicalyth
Last edited by enicalyth; 17th Jul 2004 at 09:46.