On Mon, 29 Jul 2002, Lincoln Ross wrote: > You must live at a very LOW altitude, >as the air density you give is greater than that of water. Try >1.225kg/m^3, not 1,225!
That's just a difference in European notation. Commas and points are reversed between European and U.S. notation. So his numbers are (fortunately) not off by three orders of magnitude. -J > You also left out the velocity, plus your answer >will be in Newtons (force), not kg (mass). I get about 42 kg, but >judging by your answer you used the velocity in the actual calculation, >as well as the correct density, and just forgot to divide by 9.8 to get >kg. Of course you will have to multiply by 9.8 again in your structural >calculations. > >Also, watch out on your unit conversions. It's 12.9ft^2, not 129! > >Correct formula should be, if I'm not mistaken, L = Cl(1/2)rho(v^2)S >where S is wing area, rho is density, v is speed. > >You probably need to do some homework to define what a realistic maximum >airspeed for your XC plane is, and maybe you should get someone to help >you with the structural calculations or go over them a few times and >really make sure you understand them. It's very easy to mess up these >calculations if you just plug into them without being aware of sneaky >things like those Newtons. If it was me I would be tempted to design for >max Cl at max speed on the assumption that I might someday get a glitch >and experience full elevator throw at speed. Of course maybe the servo >isn't that strong, but that's not easy to define. > >BTW, I calculate a 200 foot vertical zoom (60m) with no losses (and >there are ALWAYS losses) would be 77mph or 34mps. I have seen models >zoom that high (as in 50% above release point), though I suppose not >many winches could do it to an XC plane. > >Good luck. > >snipanyway I creted a spreadsheet >> to calculate the forces for different ca's and different speeds.... I just >> took these values as an example... 55 mph seems to me as being rather high >> for such a ship while a ca of 0,9 seems rather normal although I don't know >> whether a real plane really reaches such a ca. The airfoil I want to use >> gave a Ca max of 1.4 in wind tunnel testing at Re = 200,000..... >> >> Trying to get a grip on things.... ;-)) >> >> Wolfram >> >> >> 417 kg, Ca of 0.9, speed 25 m/second, wing area of 1.2 sqm >> > >> > You mentioned during launching. I must ask: Is 25m/s and 0.9Ca happening at >> > the same time and DURING A LAUNCH realistic? >> >> > 417 kg with a ca of 0,9 ; speed 25 m/secand and a >> > wing area of 1,2 sqm >> > (919 lbs with a ca of 0,9 ; speed 56 mph and a wing >> > area of 129 sqfeet) >> > >> > That would be almost enough for a full scale >> > glider!! >> > >> > The equation I used is this: >> > >> > Lift = (ca * air density (1,225 kg/m^3) * wing >> > area)/2 > > RCSE-List facilities provided by Model Airplane News. Send "subscribe" and "unsubscribe" requests to [EMAIL PROTECTED]
