its a drag, hawk
Yeah, total drag. Calculate the surface area of the whole aircraft and say it comes to 21000sq ft. Now paint it so that it has a surface roughness coefficient of say 0.0036. Then its flat plate area is meant to be 0.0036 x 21000 = 76 sq ft. Your inspectors look at the paint finish through a microscope and say “Yes that matches the definition of a 0.0036 finish!” Now measure the drag coefficient cdo of the whole plane for real and it comes in at mebbe 15% more!! That is the fiddle factor because aerials stick out, windows and doors aren’t flush and there are gaps here and there. You can now apply the fiddle factor and say that the effective wetted area is 1.15 x 21000 = 24150 sq ft and the effective flat plate area is 1.15 x 76= 87 sq ft. Let’s say your aircraft is a 744 for which Boeing tell you the wing reference area is 5825 sq ft. What is the real drag coefficient cdo? It is 87/5825 = 0.015! That's a bit rushed but okay for the back of a fag packet. And now a last word on the 777 where aspect ratio is 8.4, “e” is 0.89 wing area is 4605sq ft, parasite drag is 0.013 and compressibility drag is say 7%. You are at FL330 in still air on a standard day clocking 488kts and weigh 550000lbs. I make my lift coefft 0.442 therefore my induced coefft is 0.0083 and my compressible coefft is 0.0015. So L/D is 0.442/(0.013+0.0083+0.0015) =19.4 and my drag is 550000/19.4 = 28350lb. Fuel flow is sfc x drag = 0.57 x 28350 outta the service shop = 16160lb/hr or 7.33 tonnes per hour. Is it? Yes. I’m a happy bunny. Isn’t it? I find out why. Currently I estimate my sfc at 0.587 and the paint job is nearer 0.0042 than 0.0036 in finish coefft. Not useless knowledge, just the tools to give the Enicalyth the edge and the courage to say this baby needs a service and a scrub up.