Yes, but an electric tug doesn’t have to carry the equivalent of 100L of avgas like a “conventional” electric aircraft would for cross-country flying. If you allowed regenerative battery charging, the tug could completely exhaust its battery getting to 3000ft and then recharge just enough on the way down to allow for a go-around or hold. The battery + motor should weigh much less than a 6-cylinder Lycoming.
Taras Kiceniuk gave a presentation to the Experimental Sailplane Association in California in September where he outlined his vision for “Perpetual Soaring.” His concept is for an electric-launch glider that recharges for tomorrows launch on the ground, using either solar cells (which don’t work too well at night and require prohibitive area and weight) or using the glider’s propeller as a windmill. “Perpetual” means it never needs to be plugged in or refuelled. Taras has built a scale model prototype and measured in-flight recharging with it. He claims to get 40% regenerative efficiency. His model takes 75W electric power to fly level and he showed graphs which appeared to indicate he has recharged the batteries at up to 50W. (The graph was not intended to show recharge efficiency so this is my own interpretation of his figures.) The airfoil section on the prop must be symmetrical to work as both a windmill and a propeller. (The next time you meet an aerobatic pilot droning on about symmetric airfoils, ask him what airfoil is on his prop.) For the best efficiency as a windmill, the prop needs to have a large diameter and the delta-V (change in velocity along a streamline) needs to be minimised, indicating that a descent at close to minimum-sink speed would be required for maximum energy recovery. On the other side of the efficiency curve is the high-altitude windmills which work like box kites with propellers on them. The prop needs to work just well enough for the kite to self-launch with power supplied from the ground but as soon as the kite is established in the high-speed wind at altitude, the prop goes into windmill mode. - Morgan Sandercock (no relation) From: aus-soaring-boun...@lists.internode.on.net [mailto:aus-soaring-boun...@lists.internode.on.net] On Behalf Of Todd Sandercock Sent: Friday, 30 October 2009 11:44 AM To: Discussion of issues relating to Soaring in Australia. Subject: Re: [Aus-soaring] Glider pilots applying their skills to another activity Ha! yeah right. This is the completely wrong thing to be putting the electric motor in. The tug is the problem not the solution. Its the GLIDER that needs to be electric!! Anyway here is some calculation that may or may not be correct: Taking 100l of avgas at about 44.65 MJ/kg and .69kg/l @ 15 degrees would give you 3080 MJ of energy. Taking an IC motor efficiency of about 15% or most likely less gives about 462MJ going to the prop. Now going backwards from here because the prop would be common between the aircraft: The prop requires 462MJ through a electric motor which would be getting about 80% efficiency at best = 577.5MJ Changing this to common power terms gives us about 160kWh of power required. Now this is where electric always dies: a reasonable LiPo battery will give about 150Wh/kg so you can quickly see that providing enough battery power is going to get really weighty 160/.15 = 1066kg of battery! Now I am not going to estimate the recovery of energy through a wind-milling prop but I would suspect that a variable pitch prop designed to efficiently pull the aircraft through the sky would be an extremely inefficient windmill. I would guess that the energy regeneration would be extremely low. Todd _____ From: Mike Borgelt <mborg...@borgeltinstruments.com> To: Discussion of issues relating to Soaring in Australia. <aus-soaring@lists.internode.on.net> Sent: Fri, 30 October, 2009 8:20:40 AM Subject: Re: [Aus-soaring] Glider pilots applying their skills to another activity At 07:25 AM 30/10/2009, you wrote: > Mike Borgelt wrote: >> >> >> Somebody really ought to look at electric towplanes too. It might just be >> the first real practical use for an electric aircraft and the economics >> compared to an I.C. engined towplane just might even make some sense. > I would expect the viability to be significantly improved if regenerative air > braking in a steep descent was included by having the propeller spinning the > generator. Very rapid descents (decreasing tug turn round) that also return > energy to the battery would be possible. > > -- > Robert Hart <mailto:ha...@interweft.com.au>ha...@interweft.com.au > +61 (0)438 385 533 http://www.hart.wattle.id.au > _______________________________________________ Excellent point, Robert. Now would some bright young mechanical engineer care to run some numbers and publish them here? I'm looking at you ,Todd. Mike Borgelt Instruments - manufacturers of quality soaring instruments since 1978 phone Int'l + 61 746 355784 fax Int'l + 61 746 358796 cellphone Int'l + 61 428 355784 email: mborg...@borgeltinstruments.com website: www.borgeltinstruments.com _______________________________________________ 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 _____ Get more done like never before with Yahoo!7 Mail. Learn more.
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