Thread: Discussion Figuring out L/D ratio and drag coefficients View Single Post
 Jan 19, 2013, 07:41 PM Registered User France, RA, Cormaranche-en-Bugey Joined Feb 2011 633 Posts I'm also trying to make some kind of in flight measurement of drag. Not easy ! Here's what I do : I have the eagle tree data logger, OSD, FVP,... so I can have readings in flight of RPM, speed by the pitot tube, speed by the GPS, speed by simply measuring the time to travel a known distance. RPM can give you an idea of the thrust using the UIUC propeller database. But it needs to know airspeed and air density. Speed is uneasy to know, pitot tube isn't accurate at low speeds and is influenced by air density, GPS speed can't be very accurate too because of the wind (even only 5 km/h of wind, when a slow flying speed can be down to 30 km/h), same for direct measurement. BTW, in flight thrust can be only 30 to 50 % of the static thrust at the same throttle level, just to give an idea, so your L/D = 2 can if fact be L/D = 6 ! Making things carefully, you can still have an idea of the drag coefficient, +/- 0.005, something like that. I have a 1/12 Yak-3, with a dirty finish... I estimate it's drag coefficient around 0.04, when the full scale Yak-3 should be around 0.026. My FPV plane with camera, antennas, various things almost everywhere should have a drag coefficient in the 0.05 / 0.055 range, low climb rate suggest high induced drag, max L/D might be a bit more than 6. Here is Mathieu Scherrer's Jibe 2 wind tunnel test results : http://sailplane-matscherrer.blogspo...nd-tunnel.html Less than 0.015 for the drag coefficient of this skinny sailplane ! Here, if I recall correctly, the Ultra Stick 120 has a minimum drag coefficient close to 0.03 : http://www.uav.aem.umn.edu/index.php...d=49&Itemid=80