Motor Selection

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Motor Selection

New postby Beavertail on Mon Sep 24, 2018 2:44 pm

Hello Friends!
Once again I am working on my plane for competition this year in Texas. Dramatic rule changes this year! One of which (thank god it has finally happened) is that it has gone electric! However, wow have they limited us. Long story short, I need a motor which runs on a 6s 3000mah (Min 25c) and will have a 750W limiter on it. Plane will be estimated at 55lbs (LOL I know) Open to suggestions for MAX thrust, any prop size and any esc size.

I have been looking at eflite Power 46/60, Align 460MX, and maybe scorpion equivalent.

Open to suggestion!

Thanks
Gary
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Re: Motor Selection

New postby retiredVTT on Mon Sep 24, 2018 6:35 pm

Hi Gary,
....these "parameters" should result in a real "lead sled" ...resulting in around 13 Watts per pound if my calculations are correct. Good luck getting this bird off the ground!
Bill
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Re: Motor Selection

New postby JohnOSullivan on Mon Sep 24, 2018 7:42 pm

I agree with Bill. At 13.5 watts/lb (750w/55lb) the model would not be capable of sustained flight and most certainly would not be able to take off.
Typical low powered Electric models require about 50 watts/lb, moderate power 100 watts/lb and aerobatics 150 watts/lb. My sailplanes are running about 250 watts/lb.
As you were running .45 size internal combustion engines, just converting to the so-called .45 commercial Electric motors is not a fair comparison.
At a guess at 55 lbs you would need an absolute minimum of 1500 watts.
It looks like the organizers have little knowledge of Electrical systems.
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Re: Motor Selection

New postby steve on Mon Sep 24, 2018 9:08 pm

Sounds interesting Gary !

Given what Bill and John said, I think you need to find ways to cut weight or consider a non- conventional airplane design ? Sounds easy ;) LOL
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Re: Motor Selection

New postby JohnOSullivan on Mon Sep 24, 2018 9:18 pm

I've had very little to do with I.C motors for several years but I managed to dig up performance figures for a couple of .46 size motors

1)
NX .46 Nitro (7,7cc)
NX 7.7 is the first engine of this size designed specially for use in a truck, not a converted helicopter engine. This rear exhaust engine features A.B.C (Aluminum, Brass, and Chrome) consturction for longer engine life and higher power output. NX 7.7 comes standard with large cooling head and 7.5mm, twin needle, sliding carburetor for easy engine tuning. Engine produces 5.0HP at 23,000RPM. This is equivalent to 3700Watts.
...........................................................................................

2)

OS .46AX Nitro

Displacement:
.44 cu in (7.5cc)
Bore:
.866” (22mm)
Stroke:
.772” (20mm)
Output:
1.65bhp @ 16K RPM
RPM Range:
2K-17K
Weight w/ muff:
17.2 oz (489g)
Sugg Props:
10.5x6, 11x6-8, 12x6-7


Manufacturer:
OS Engines
This is equivalent to (1.65x740) 1221 watts.

It looks like the OS 46 or equivalent is similar to the contest size and performance. ie. about 1200+ watts. This would be marginal with 55 pounds but should work.
Steve: the goal is to carry a payload at 55 pounds, so a lighter total weight is not on.

...........................................................................................
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Re: Motor Selection

New postby Beavertail on Tue Sep 25, 2018 8:31 am

Thanks for the replies fellas! :)
55 pounds is the absolute MAX weight, ideally we will be aiming for lower... also figured the wingspan limit is a huge 12 feet which should help get this bird off the ground. We are discussing with SAE currently as to if we have to drop all carry/ drop all three payloads AND the autonomous glider in each flight or can they be separate. if they can be separate then our weight should go down by alot. I guess for now i am just looking for a setup with maximum thrust with 6s and 750W lol The competition has increased in difficulty dramatically, unfortunate for smaller teams like Western however they are trying to differentiate the advanced and regular class i believe.

Thanks!
Gary

P.S Payloads this year includes Static, Water, "Habitat" (Foam nerf toy thingy), and an autonomous glider carrying as much pingpong balls as possible to land in a 100ft circle.
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Re: Motor Selection

New postby retiredVTT on Wed Sep 26, 2018 1:47 am

Happy Birthday Gary ..!
Enjoy being 21..it only happens once..!
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Re: Motor Selection

New postby Beavertail on Thu Sep 27, 2018 12:24 pm

Thanks Bill!
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