55mph is pretty SLOW for cross country, even in the old days, from what I hear. I've heard tales of people chasing wooden airplanes at 70 or 80mph. Remember that if you go over a bump while chasing you may hit full up elevator while going at high speed, so if the winch is wimpy launching may not be the highest stress. I don't know what "ca" is, but it sounds like Cl (lift coefficient), so I'll assume that's what you meant. 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! 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 -- Lincoln Ross NOTE ADDRESS CHANGE: [EMAIL PROTECTED] RCSE-List facilities provided by Model Airplane News. Send "subscribe" and "unsubscribe" requests to [EMAIL PROTECTED]
