Bill McCreadie Posted July 2, 2011 Share Posted July 2, 2011 Hypothetically, if a 150 watt motor is required to 'briskly' fly a 44 inch wingspan 2 lbs weight model what would the equivalent ic motor be?? Quote Link to comment Share on other sites More sharing options...
flytilbroke Posted July 2, 2011 Share Posted July 2, 2011 .25 2st. Quote Link to comment Share on other sites More sharing options...
Bob Cotsford Posted July 2, 2011 Share Posted July 2, 2011 or even .15 2st? Actually 150W sounds a bit low to fly a 2lb model 'briskly' - 75W/lb? Might be a bit sedate with that. Quote Link to comment Share on other sites More sharing options...
Bill McCreadie Posted July 2, 2011 Author Share Posted July 2, 2011 So I have an elderly but perfect OS 20 (I don't think they make them any more!) and it should do the job nicely. In order to retain the 'real' look it needs a 4 bladed prop. I got on to APC head office and they recommend their 10 x 6 - 4 blader -- I assume they know whereof they speak but does anyone have any comments? Quote Link to comment Share on other sites More sharing options...
ken anderson. Posted July 2, 2011 Share Posted July 2, 2011 hello bill-i think some one is having a laugh...your OS 20 would struggle to turn a normal(2 blade) 10+6...and would find it impossible to turn the 4 blade one...... ken anderson ne...1. Quote Link to comment Share on other sites More sharing options...
leccyflyer Posted July 2, 2011 Share Posted July 2, 2011 As Ken said, an OS ,20 would struggle to turn a 10x6 2 blader and would definitely struggle with a 4 blader. The better news, if you can live with it, is that with a suitable prop - say an 8x6 or 9x5 - the motor will fly a 44" span, 2lb model quite nicely. Going to a decent .25 would give excellent performance - that's the motor of choice of the Cambrian funfighters, of that size and a little more weight. That's also a lot more than 75watts/lb. You'd probably be disappointed with only 150w in a 32oz model, if expecting sprightly performance. Quote Link to comment Share on other sites More sharing options...
Tim Mackey Posted July 2, 2011 Share Posted July 2, 2011 I dont like trying to make direct replacements for IC when choosing leccy motors - theres simply too many potential variables. However if you insist Try this one.take the IC size EG: .15 in this case. Double the numbers = 30 Add zero = 300. Thats a rough guide to the number of watts you need. Quote Link to comment Share on other sites More sharing options...
Bill McCreadie Posted July 2, 2011 Author Share Posted July 2, 2011 Posted by leccyflyer on 02/07/2011 20:02:13: As Ken said, an OS ,20 would struggle to turn a 10x6 2 blader and would definitely struggle with a 4 blader. The better news, if you can live with it, is that with a suitable prop - say an 8x6 or 9x5 - the motor will fly a 44" span, 2lb model quite nicely. Going to a decent .25 would give excellent performance - that's the motor of choice of the Cambrian funfighters, of that size and a little more weight. That's also a lot more than 75watts/lb. You'd probably be disappointed with only 150w in a 32oz model, if expecting sprightly performance. Would an OS 25 turn a 4blade 10x6?? or how about an OS 40 Quote Link to comment Share on other sites More sharing options...
leccyflyer Posted July 2, 2011 Share Posted July 2, 2011 The largest prop I use on miy OS .25FPs were 9x6" props, but I did get one second hand with a Graupner 9x6" three blader. Took it off for a 2-blader without using it. The rule of thumb that I was given that going up to a multiblade prop is like adding an inch in diameter, per blade. So a 3 blade 9x6 would be similar to a 2 blade 10x6 and a 4 blade 10x6 would be like an 12x6 2 blader. Probably too much for a .40FP and defininetly for a .25FP. Quote Link to comment Share on other sites More sharing options...
Bill McCreadie Posted July 2, 2011 Author Share Posted July 2, 2011 Posted by leccyflyer on 02/07/2011 21:13:02: The largest prop I use on miy OS .25FPs were 9x6" props, but I did get one second hand with a Graupner 9x6" three blader. Took it off for a 2-blader without using it. The rule of thumb that I was given that going up to a multiblade prop is like adding an inch in diameter, per blade. So a 3 blade 9x6 would be similar to a 2 blade 10x6 and a 4 blade 10x6 would be like an 12x6 2 blader. Probably too much for a .40FP and defininetly for a .25FP. OK, I give up, unless anyone knows where to get smaller diameter 3 and 4 blade props. Quote Link to comment Share on other sites More sharing options...
Steve Hargreaves - Moderator Posted July 4, 2011 Share Posted July 4, 2011 Do I guess Bill that you are converting an EP model to IC? If so then this prop issue is a typical problem. An electric motor can be "tuned" (or wound) to produce power at various rpm where as with an IC motor you have to take what you are given & lump it... This means you could have a nice scale model fitted with a motor that is "tuned" to turn a large prop. Putting a suitable IC motor in it will mean you are stuck with the prop it will manage... In IC the motor defines the prop...in EP the prop defines the motor....sort of.. If I am right & you are attempting a conversion then might I also suggest you take a look at reinforcing the firewall. EP models are often built very light & won't take the vibration of an ic motor..... Hope this helps.... By the way I agree with the other posters regarding the actual prop size....a 20 would take an 8x6...possible a 9x4 two blade. Anything more than this & you'd be overloading it... Quote Link to comment Share on other sites More sharing options...
Cyclicscooby Posted July 4, 2011 Share Posted July 4, 2011 Posted by Tim Mackey - Administrator on 02/07/2011 20:06:10: Try this one. take the IC size EG: .15 in this case. Double the numbers = 30 Add zero = 300. Thats a rough guide to the number of watts you need. Nice.... Not technical, but it kinda works.. My Cessna shud have a .36, which using your 'equation' is 720W, and I have a 750-800W Power 32.. !! Magic. I'll remember that.. Luv Chrisie... xx Quote Link to comment Share on other sites More sharing options...
Keith Simmons Posted July 4, 2011 Share Posted July 4, 2011 Am I correct that you add an inch for the dia, but 'lose a inch for the pitch? ie 10x8 3 blader for 11x7 2 blades? Is it the same for larger dia props, for example 20x8 3 blader for 21x7 2 blades or would it be 22x7 2 blades? Quote Link to comment Share on other sites More sharing options...
Bill McCreadie Posted July 4, 2011 Author Share Posted July 4, 2011 Posted by Steve Hargreaves on 04/07/2011 12:29:18: Do I guess Bill that you are converting an EP model to IC? If so then this prop issue is a typical problem. An electric motor can be "tuned" (or wound) to produce power at various rpm where as with an IC motor you have to take what you are given & lump it... This means you could have a nice scale model fitted with a motor that is "tuned" to turn a large prop. Putting a suitable IC motor in it will mean you are stuck with the prop it will manage... In IC the motor defines the prop...in EP the prop defines the motor....sort of.. If I am right & you are attempting a conversion then might I also suggest you take a look at reinforcing the firewall. EP models are often built very light & won't take the vibration of an ic motor..... Hope this helps.... By the way I agree with the other posters regarding the actual prop size....a 20 would take an 8x6...possible a 9x4 two blade. Anything more than this & you'd be overloading it... You are absolutely right! I had thought of taking up electric flight and to that end -- without much research -- I got an ART P-51D Mustang kit (balsa and ply not a foamie.) That was the easy bit!!! I then realised just how much extra was going to be required and the problems and caveats associated with e.f. not to mention the expense of complex battery charging and maintenance, horror stories of combusting aircraft and batteries, short flight times etc. etc. All the while I have a selection of ic engines languishing on the shelf so I thought why not -- with appropriate reinforcing of firewalls and front end, I could have my very pretty model up and running with a system that I already understand and have used for years. The challenge of getting a ic motor to fit into a rather slim front end would be interesting even if I had to fit it inverted to get it in. I would be interested to hear from others out there who have tried and suceeded (or failed!!) to attempt what I'm doing and any hints and tips would be greatly appreciated. Bill. Quote Link to comment Share on other sites More sharing options...
Ben B Posted July 4, 2011 Share Posted July 4, 2011 Posted by Keith Simmons on 04/07/2011 12:39:45:Am I correct that you add an inch for the dia, but 'lose a inch for the pitch? ie 10x8 3 blader for 11x7 2 blades? Is it the same for larger dia props, for example 20x8 3 blader for 21x7 2 blades or would it be 22x7 2 blades? Wouldn't have thought so, that's what you do when you want to pitch up/down but want to keep the load constant with the same number of blades. When going to 3 blades you'ld need to reduce the load (ie perhaps even less pitch AND diameter). Quote Link to comment Share on other sites More sharing options...
Bill McCreadie Posted July 4, 2011 Author Share Posted July 4, 2011 Posted by Ben B on 04/07/2011 13:08:52: Posted by Keith Simmons on 04/07/2011 12:39:45: Am I correct that you add an inch for the dia, but 'lose a inch for the pitch? ie 10x8 3 blader for 11x7 2 blades? Is it the same for larger dia props, for example 20x8 3 blader for 21x7 2 blades or would it be 22x7 2 blades? Wouldn't have thought so, that's what you do when you want to pitch up/down but want to keep the load constant with the same number of blades. When going to 3 blades you'ld need to reduce the load (ie perhaps even less pitch AND diameter). In my travels through the 'which prop size for which engine' sites and forums I came across quite a few sites which were of the opinion that moving from a 2 blade to a 3 blade was not really so much of a problem i.e. reduce diameter a little but leave the pitch alone. Anyone else come across this?? Bill Quote Link to comment Share on other sites More sharing options...
Steve Hargreaves - Moderator Posted July 4, 2011 Share Posted July 4, 2011 I've heard the down an inch in diameter & up an inch in pitch when swapping to a 3 blader before but I've never put it into practice..... I can understand the reduced diameter....extra blade equals more load & all that but I never quite understood the increase in pitch.... Good luck with the conversion Bill....I hope it goes well......getting an ic motor into the front of a Merlin powered warbird is always going to be a challenge.....I have heard of people fitting 30 size 4 strokes into the Ripmax Spitfire but much beefing up of the front end was required....... Quote Link to comment Share on other sites More sharing options...
Tim Mackey Posted July 4, 2011 Share Posted July 4, 2011 Posted by Cyclicscooby on 04/07/2011 12:38:03:Posted by Tim Mackey - Administrator on 02/07/2011 20:06:10: Try this one. take the IC size EG: .15 in this case. Double the numbers = 30 Add zero = 300. Thats a rough guide to the number of watts you need. Nice.... Not technical, but it kinda works.. My Cessna shud have a .36, which using your 'equation' is 720W, and I have a 750-800W Power 32.. !! Magic. I'll remember that.. Luv Chrisie... xx Waddya mean not technical ! - it had more than two sums in it - to me, that's technical LOL Quote Link to comment Share on other sites More sharing options...
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