motor selection
05-08-2004, 06:05 AM
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motor selection
I'm currently building a Martin PBM Mariner flying boat, and have reached the point where I need to make a decision on what power I will use.
I am thinking of going electric, but know nothing about the motors currently available, so thought it a good idea to seek some advice here.
The aircraft is a twin, and will weigh in at about 13 - 13.5 (14 max) lbs with two YS 53's bolted on, so making an allowance for those, it should be about 11 - 12 lbs motorless. I'm actually thinking it will be less than that, and I'm on target for about 10.5 lbs without motors/engines, but I'm not finished yet, so it may go higher.
Can anybody point me in the general direction of what electric motors and batteries I should be looking at and what flight times could I expect from the best batteries now available.
I'm in touch with a guy building in electric, and will talk with him before making a final decision, but he's not quite at the motor stage yet, so I would appreciate any advice that is forthcoming.
I am thinking of going electric, but know nothing about the motors currently available, so thought it a good idea to seek some advice here.
The aircraft is a twin, and will weigh in at about 13 - 13.5 (14 max) lbs with two YS 53's bolted on, so making an allowance for those, it should be about 11 - 12 lbs motorless. I'm actually thinking it will be less than that, and I'm on target for about 10.5 lbs without motors/engines, but I'm not finished yet, so it may go higher.
Can anybody point me in the general direction of what electric motors and batteries I should be looking at and what flight times could I expect from the best batteries now available.
I'm in touch with a guy building in electric, and will talk with him before making a final decision, but he's not quite at the motor stage yet, so I would appreciate any advice that is forthcoming.
05-08-2004, 09:25 PM
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RE: motor selection
I'll move this to Glow to Electric Conversions for you. That's a more appropriate forum for this one.
With the best batteries available, flight times will depend on how much you want to spend. LiPoly batteries pack about 5 times as much energy into the same weight as NiCd. The plane will fly acceptably with heavy Sub C NiCds, and you can typically expect about 5 minutes duration from a conversion where no special efforts were made to lighten the airframe.
Going with the latest LiPolys, the yet-to-be-released 2100mAh Kokam cells, you could go capacity-for-capacity, and cut your battery weight by about 2/3, as an estimate (there is some overhead for the cells, wiring and whatnot; the "5 times as much capacity" is for the gooey nuggat core of the battery). On a plane that size, we're talking pounds of weight savings, and minutes of extra flight. Flight times can be extended by connecting additional packs in parallel, doubling or even tripling the capacity if you wish with small weight gains.
As far as the exact power system, you'll get several recommendations. They'll all be good, and will all work equally well. You'll have to make your decision based on less tangible factors such as philosophy (e.g. geared vs. direct drive), brand reputation, service, even price.
Unlike single-engine planes, twins often have prop diameter restrictions. Knowing just exactly how much prop you can get away with will help make deciding on a power system easier. With electrics, you almost always want to spin the largest diameter prop you can fit on the plane. Big diameter, high pitch prop spinning at low RPM is more efficient, and you can get away with signifigantly less pure horsepower as compared to a glow engine.
With the best batteries available, flight times will depend on how much you want to spend. LiPoly batteries pack about 5 times as much energy into the same weight as NiCd. The plane will fly acceptably with heavy Sub C NiCds, and you can typically expect about 5 minutes duration from a conversion where no special efforts were made to lighten the airframe.
Going with the latest LiPolys, the yet-to-be-released 2100mAh Kokam cells, you could go capacity-for-capacity, and cut your battery weight by about 2/3, as an estimate (there is some overhead for the cells, wiring and whatnot; the "5 times as much capacity" is for the gooey nuggat core of the battery). On a plane that size, we're talking pounds of weight savings, and minutes of extra flight. Flight times can be extended by connecting additional packs in parallel, doubling or even tripling the capacity if you wish with small weight gains.
As far as the exact power system, you'll get several recommendations. They'll all be good, and will all work equally well. You'll have to make your decision based on less tangible factors such as philosophy (e.g. geared vs. direct drive), brand reputation, service, even price.
Unlike single-engine planes, twins often have prop diameter restrictions. Knowing just exactly how much prop you can get away with will help make deciding on a power system easier. With electrics, you almost always want to spin the largest diameter prop you can fit on the plane. Big diameter, high pitch prop spinning at low RPM is more efficient, and you can get away with signifigantly less pure horsepower as compared to a glow engine.
