Scratch 51" B-17 4 engine electric
04-17-2006, 09:44 PM
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Scratch 51" B-17 4 engine electric
I have just begun scratch build from Cleveland Airline's fortress 1/24th scale (51"W.S.) Any comments, tips or knowledge greatly appreciated.
I have ordered electri-calc program (waiting) to try and solve for a target of 4X speed 300 in series-parallel operation. Gear reduction and prop calculations from program will post when solved but I am having to estimate aircraft weight. Fortunately I have (I design for a living) analytical electronic scales and I'm weighing everything I assemble to hone in on a projected final weight.
I am very concerned about weight, with 4 motors, batt ESC etc. this will qickly get heavy all this on 285 SQ/IN wing area!
Am I crazy to try sheeting in 1/32" balsa?
I will gladly share any thing I can during the development.
Did I mention this is my first scratch?
Air Bob
I have ordered electri-calc program (waiting) to try and solve for a target of 4X speed 300 in series-parallel operation. Gear reduction and prop calculations from program will post when solved but I am having to estimate aircraft weight. Fortunately I have (I design for a living) analytical electronic scales and I'm weighing everything I assemble to hone in on a projected final weight.
I am very concerned about weight, with 4 motors, batt ESC etc. this will qickly get heavy all this on 285 SQ/IN wing area!
Am I crazy to try sheeting in 1/32" balsa?
I will gladly share any thing I can during the development.
Did I mention this is my first scratch?
Air Bob
04-18-2006, 10:03 AM
#2
RE: Scratch 51" B-17 4 engine electric
I STRONGLY suggest using brushless motors and LIPO batteries. The motors will provide more power and the battery weight will be substantially lighter. With brushed motors you MAY be able to get away with one ESC per 2 motors, with brushless you will need 1 ESC per motor.
You may also want to D/L Motocalc http://www.motocalc.com/ You can try it for 30 days for free. I use Motocalc, and while not 100% accurate (none of them are ), it is USUALLY pretty close. I can play "What If" with the props, batteries, gearboxes, etc to get what I need.
I have done several planes from Cleveland plans. I find the plans to be quite good. Of course, since they are designed for free flight rubber, you will need to do some modifications, but for electric, the mods will be minimal.
You may also want to D/L Motocalc http://www.motocalc.com/ You can try it for 30 days for free. I use Motocalc, and while not 100% accurate (none of them are ), it is USUALLY pretty close. I can play "What If" with the props, batteries, gearboxes, etc to get what I need.
I have done several planes from Cleveland plans. I find the plans to be quite good. Of course, since they are designed for free flight rubber, you will need to do some modifications, but for electric, the mods will be minimal.
04-18-2006, 02:35 PM
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RE: Scratch 51" B-17 4 engine electric
Run 5series LiPo at 18-20 amp
Then series 2 of them geared 300/350 to get a decent usable voltage : 5/2 Lipo. Should prop/gear out to 9-10amps
Then just Y 2 of them pairs off the 25-30amp ESC and Presto!
2s2p geared 300/350 motors off a 5s 20amp LiPo pack and a 30amp ECS.... may look at 5s2p for flight time, but once up, throttl back... Bombers aren't known for their Unlimited Vertical or Hovering. Bombers are known for carrying a pound or two payload, so you may be able to get by on 16 nicads or nihms depending on empty model weight & loading.
Plenty of options at this point, and remember, bombers aren't acrobats, they're sky barges. Don't expect a 45* climbout from a 15' Takeoff Roll.
Then series 2 of them geared 300/350 to get a decent usable voltage : 5/2 Lipo. Should prop/gear out to 9-10amps
Then just Y 2 of them pairs off the 25-30amp ESC and Presto!
2s2p geared 300/350 motors off a 5s 20amp LiPo pack and a 30amp ECS.... may look at 5s2p for flight time, but once up, throttl back... Bombers aren't known for their Unlimited Vertical or Hovering. Bombers are known for carrying a pound or two payload, so you may be able to get by on 16 nicads or nihms depending on empty model weight & loading.
Plenty of options at this point, and remember, bombers aren't acrobats, they're sky barges. Don't expect a 45* climbout from a 15' Takeoff Roll.
