David Ashby - Moderator Posted October 14, 2009 Share Posted October 14, 2009 OK, I've just about finished a big electric twin and while power should be adequate I've been told not to worry as the doubling up on power with a twin sort of over-compensates. There's a sort of twin 'rule' out there that helps? Now I'm probably articulating hat badly and I've tried a few searches but I can't find any written fact or name of the theory or phenomena that this is down to. I wondered if any of you bright spakrs have heard of this or could point me in the right direction? Edited By David Ashby - RCME moderator on 14/10/2009 19:54:23 Quote Link to comment Share on other sites More sharing options...
Dusty Posted October 14, 2009 Share Posted October 14, 2009 Power to weight ratio? if its electric i guess if you have faliure both pops are going to stop and the pilots going to sweat! If it's IC could it sustain altitude on one engine failing the other/with a lot of rudder etc 1. twin flying on 1 engine = brick that is flyable 2. twin on 2 engines working= flying machine 3.power/weight ratio Quote Link to comment Share on other sites More sharing options...
Martin Harris - Moderator Posted October 14, 2009 Share Posted October 14, 2009 The only rule on twins that I'm aware of is that the purpose of the second engine is to get you to the scene of the crash when the other one fails! I built a leccy Mossie to avoid engine out problems and one of the motor shafts sheared so don't get complacent...Edited By Martin Harris on 14/10/2009 23:50:59 Quote Link to comment Share on other sites More sharing options...
David Ashby - Moderator Posted October 15, 2009 Author Share Posted October 15, 2009 Thanks guys...er and Martin - it's twin electric so an engine-out wont be a problem. It's the ASM TIgercat - weighs 20lbs with 1600 watts on tap which is 80 per lb but a few learned sources have said that's ok because you get proportionately more bang for your buck with a twin. Sort of in the way both motors are sharing the airframes drag - as I say it's a theory thing that a few guys have touched on but have been unable to elaborate. Quote Link to comment Share on other sites More sharing options...
Peter Miller Posted October 15, 2009 Share Posted October 15, 2009 Twins do have more power than the equivelant single. Never seen the formula but I look at it this way. The engine rpm is roughly the same for two .15s as one .30 but the if you look at the area of the propeller discs the two .15s have far more . two 8" diameter props have an area of 100.54 sq.in while one 10" diameter prop only has an area of 78.53 so the the 10" is only 78.42 of the prop area. Quote Link to comment Share on other sites More sharing options...
Tom Foreman Posted October 15, 2009 Share Posted October 15, 2009 Depends how you wire it up with regards to losing a motor, if both are running off one battery and both ESC's are set up to reduce power at LVC in theory both motors should quit at the same time just like a single motored plane. If you are running independant batteries, 1 per motor then you could have one quit on you with the other still running. 80W per lb doesn't sound a lot but I guess their being a lot less airframe behind the prop should help as you and others have eluded to. Are you running one of the motors backwards with a pusher prop to be torque neutral? Tom Quote Link to comment Share on other sites More sharing options...
Martin Harris - Moderator Posted October 15, 2009 Share Posted October 15, 2009 Posted by David Ashby - RCME moderator on 15/10/2009 06:27:24:Thanks guys...er and Martin - it's twin electric so an engine-out wont be a problem. David - please read the second paragraph of my post again. Just a warning against being complacent because an electric "won't be a problem".... Quote Link to comment Share on other sites More sharing options...
David Ashby - Moderator Posted October 15, 2009 Author Share Posted October 15, 2009 Thanks Martin, yup sorry wasn't trying to be dismissive. Thanks guys. I was sitting there last night before the test flight today and starting to hyper-ventilate (worry a big bit) about the model, I'd done all the sums but you know what what it's like. First flights were today and they went very well - pretty awesome in fact. The AUW is 20lbs with 1600 watts on tap (2 x E-flite power 60's with 14x10s - can't bet any bigger props because of ground clearance) - but she tore round impressively at just half throttle. 2 x 6S Li-po, one in each nachelle. Daddy is very pleased. Quote Link to comment Share on other sites More sharing options...
TonyS Posted October 15, 2009 Share Posted October 15, 2009 David that is amazing. Did you build it from plans / kit or was it pre-built? Love the finish. Quote Link to comment Share on other sites More sharing options...
Tim Mackey Posted October 15, 2009 Share Posted October 15, 2009 Nice one David......bet it feels good to get that maiden over and done with eh! Quote Link to comment Share on other sites More sharing options...
Ben Mullins Posted October 15, 2009 Share Posted October 15, 2009 Sweet That is a very impressive model! Loving the Camel doing low inverted passes behind Edited By Ben Mullins on 15/10/2009 18:48:26 Quote Link to comment Share on other sites More sharing options...
David Ashby - Moderator Posted October 15, 2009 Author Share Posted October 15, 2009 Does indeed Tim, particularly as it went so well, she was smooth, fast and everything you'd want from a warbird. Just re-charged the 6S 3800mAH packs and put back just 1500mAH for a 6-min flight so they should be good for a comfortable 10 mins. Tony, it's the ASM TIgerCat distributed by Ripmax. An 'ARTF' although she needs a fair amount of work and I've got a little more to do - I left the steerable side of the nose leg and a few other details to try and minimise the number of things that might trip up proceedings. Quote Link to comment Share on other sites More sharing options...
Peter 'Ivanna Crashalot' Savage Posted October 15, 2009 Share Posted October 15, 2009 Lol Ben! My club mate had an electric twin mozzie (may even have been a plan in here) i think only 2 of the launches actually ended in flight! it was so hard to launch. Quote Link to comment Share on other sites More sharing options...
Simon Chaddock Posted October 15, 2009 Share Posted October 15, 2009 DavidI believe the ASM Tigercat is 82" span. If so then the scale diam prop would be 20"!So if you really can't get more than 14" it suggests either the ASM U/C is a bit short or perhaps the US Navy ran their props very close to the ground. Quote Link to comment Share on other sites More sharing options...
David Ashby - Moderator Posted October 15, 2009 Author Share Posted October 15, 2009 Didn't the Tigercats have three-bladers Simon? I was very concerned a month ago when the retracts arrived (late) to find that the 16" props I had earmarked for the Power 60s were too big and so the 600Kv motors would be doing a lot less with the 14x10s. The Power 60 is a bit of a beast and with a 16x8 I had measured 1kw per side! I had considered re-motoring the model but someone said fly her first and see and by today's showing the 14x10s are plenty sufficient. I would have liked 3-bladers but trying to find some isn't easy. I'm going to look up some master electric props next as they look much better than the APCs. Edited By David Ashby - RCME moderator on 15/10/2009 20:33:06 Quote Link to comment Share on other sites More sharing options...
David Ashby - Moderator Posted October 15, 2009 Author Share Posted October 15, 2009 Some impressive video of the full size...... Quote Link to comment Share on other sites More sharing options...
Kelly Posted October 15, 2009 Share Posted October 15, 2009 Very nice model David Have you considered a sound system ? It would make it the bees knees . Quote Link to comment Share on other sites More sharing options...
Stephen Grigg Posted October 15, 2009 Share Posted October 15, 2009 Just have that sound in the background David and that would be enough,twins sound really good dont they Quote Link to comment Share on other sites More sharing options...
Simon Chaddock Posted October 16, 2009 Share Posted October 16, 2009 I think the Grumman Tigercat and the DH Sea Hornet epitomises the difference between the US and GB approach to aircraft design. I love the opening of this sequence. I bet the driver of the "Follow Me" golf cart hoped the pilot kept his hand on the throttle! Note the props very neatly stop at exactly the same position. Quote Link to comment Share on other sites More sharing options...
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