Robert, have you heard of any of these high bypass jets being developed for models and UAV's etc? Will be interesting to see how Mikes combo goes, I would prefer a single engine solution though.
Great to see some meaningful discussion where you get to learn something! Nig Peter Creswick wrote: > A good "rule of thumb" for take off calculations is > > (0.7 x static thrust) / AUW, which gives acceleration in "g" > (which is a very small figure) > > The 0.7 is to average thrust during the ground roll, since increasing > ram drag reduces net thrust with increasing speed, making the > calculation easier. > > Then of course you have to factor in slope, rolling resistance, and > increasing aero drag and don't forget induced drag at rotation. The kicker is the static thrust - which is modified by the Froude efficiency. Basically, this is the ratio between the speed of the aircraft divided by the speed of the gas leaving the jet engine nozzle. So when the glider is stationary, the Froude efficiency is close to zero and so there is very little thrust for a standard (no or low bybass turbojet). This is why I (and many others) are hanging out for high bypass turbojets. Whilst these will be larger diameter, it is likely that two of these jets will not be needed for a 15m glider and a single high bypass turbojet will provide all the thrust needed for a reasonable, unassisted take off. [A high bypass turbojet passes only a relatively small amount of the air that is accelerated by the first stage compressor through the actual combustion chambers. The remaining air bypasses this route and is mixed with the hot, high velocity gas that comes from the combustion chambers, resulting a much larger mass of lower velocity air, so producing an engine with a higher Froude efficiency.] -- Robert Hart [EMAIL PROTECTED] +61 (0)438 385 533 http://www.hart.wattle.id.au _______________________________________________ Aus-soaring mailing list Aus-soaring@lists.internode.on.net To check or change subscription details, visit: http://lists.internode.on.net/mailman/listinfo/aus-soaring _______________________________________________ Aus-soaring mailing list Aus-soaring@lists.internode.on.net To check or change subscription details, visit: http://lists.internode.on.net/mailman/listinfo/aus-soaring
