Consequences of massive under-propping?
03-26-2011, 11:10 AM
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Consequences of massive under-propping?
Ok, so I'm scratch building a balsa plane with a 50 inch wing span. I have 4 Optima 300 Brushless Motors (2208-1380KV) I will be using on it. However, since I want to be somewhat close to scale the largest prop I can go with is a 5 inch (gws 5x.4.3). I'm guessing that by using this combination at full throttle it will burn up the motor or just waste a ton of energy. I'm not sure how much the plane will weigh yet since I'm still in the CAD stages. Any help would be appreciated!
03-26-2011, 12:24 PM
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RE: Consequences of massive under-propping?
Couple'a things. Underproping with an electric motor is not nearly as bad as with wet power. Once the motor reaches KV (rpm/volt) that's as fast as it will turn. Amp draw and power will be reduced. You can increase voltage (within the range of the motor's design) to regain some of the lost power. This is somewhat more difficult with LiPos as many motors and escs cannot handle the 4 volt jump when you add a cell.
Walt
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03-26-2011, 02:28 PM
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RE: Consequences of massive under-propping?
So in this case I may want to stick with a 2 cell battery rather than go with a 3 cell (which under normal circumstances the motor could handle) ?
03-26-2011, 04:41 PM
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RE: Consequences of massive under-propping?
Quite the contrary, if the motor will handle 3 cells, by all means do so. Also check out APC props for higher pitches in your diameter.
Walt
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03-27-2011, 07:57 AM
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RE: Consequences of massive under-propping?
If your ESC's can cope with it go to 4s... a tiny prop on 2s or 3s will simply draw a low current... as mentioned you cannot over-rev an electric motor just by using a small prop...
By going to 4s you'll get an amp draw for the motors which is still within their limits.... but you'll need to re-think the GWS 5x4.3 'cos they won't cope with the RPM... APC E or EMP will (5x5?)
By going to 4s you'll get an amp draw for the motors which is still within their limits.... but you'll need to re-think the GWS 5x4.3 'cos they won't cope with the RPM... APC E or EMP will (5x5?)
03-27-2011, 08:08 AM
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RE: Consequences of massive under-propping?
I haven't found anything higher than 5x4.3 but I will look some more. I don't think the motors can handle 4s. This is all great information!
03-27-2011, 09:13 AM
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RE: Consequences of massive under-propping?
The motors will certainly handle 4s as long as you keep the amps down... the ESC's need to be 4s capable though.
Even at 14v no-load, the motor RPM cannot exceed 19,000rpm... with a prop on you be getting no more than ~15500rpm.... easily within the motors capability.
The idiots who distribute these Optima motors need to be taken to task... in the description they suggest 85W max... in the performance chart they list 15.5A on 3s... then later 170W max...... which is it???? I'd expect a 2208 to cope with 120W (~3W/g)... so if you run 4s and keep the amp draw under 9A you should be fine.... since it is amps rather than volts which generate waste heat... you could probably draw 10A-12A on 4s and still be okay. [and I doubt a 5x5 on 4s at 1380Kv draws more than 7A-8A anyway].
APC E has 5x5, 5.25x4.75 and 5.25x6.25
Even at 14v no-load, the motor RPM cannot exceed 19,000rpm... with a prop on you be getting no more than ~15500rpm.... easily within the motors capability.
The idiots who distribute these Optima motors need to be taken to task... in the description they suggest 85W max... in the performance chart they list 15.5A on 3s... then later 170W max...... which is it???? I'd expect a 2208 to cope with 120W (~3W/g)... so if you run 4s and keep the amp draw under 9A you should be fine.... since it is amps rather than volts which generate waste heat... you could probably draw 10A-12A on 4s and still be okay. [and I doubt a 5x5 on 4s at 1380Kv draws more than 7A-8A anyway].
APC E has 5x5, 5.25x4.75 and 5.25x6.25
