mjderstine

Hey there,

I have gotten some help from you guys before and it was great. I have another question.

I have a Cobalt 40 motor. its a brushed electric motor. I will be running a 4 cell 22.2 volts 4250 miah for power. It has a 1.63:1 gear reduction. The motor is rated to 24 volts. The motor is rated at 35 amps and will have 3.25 oz per amp.
It also get 418rpm per volt.

my max rpm will be 9280 with 22.2 volts.

What do you guys think would be the best prop for my application? i know 10x6 is a good place to start. But the gear reduction is what was confusing me. i could up to a 11x7 with no problem correct?

thanks.
-Michael

here are some motor and batt specs
Cobalt 40
Model No. p/n 640G
Name 40 Geared
Gear Ratio 1.63 to 1
Armature Winding 8 turns
Armature Resistance 0.121 ohms
Magnet Type Sm Cobalt
Bearings Ball Bearings
Motor Speed 682 rpm/volt
Geared Motor Speed 418 rpm/volt
Motor Torque/amp 1.99 in-oz /amp
Geared Torque 3.25 in-oz /amp
Voltage Range 12 to 24 volts
No Load Currrent 2.5 amps
Maximum Continuous Current 35 amps
Maximum Continuous Power 750 watts
Gear Motor Length 3.8 inches
Motor Diameter 1.7 inches
Motor Shaft Diameter 0.25 inches
Prop Shaft Diameter 5/16 inch
Gear Motor Weight 17 oz

hyperion battery
Hyperion 4250 LCX 18C-27C 77Amp Continuous, 117amp Burst Li-poly packs

Matt Kirsch

My Feedback: (21)

Okay, the battery is either 22.2V or a 4S LiPoly. A 4S LiPoly is 14.8V and a 22.2V LiPoly is a 6S pack.

That'll make a huge difference in the prop size.

Just off the cuff, I think you'll be able to spin a much larger prop than 10". This is a big motor with a gear reduction. I'm thinking minimum 12x6, maybe as much as a 15x6. Do you have a Whattmeter? You can keep trying props until you hit the 35A limit.

mjderstine

Thanks for the reply man.

I will be running two 11.1 volt packs. They will be wired in series so I will be getting a total of 22.2 volts. My ESC is rated to 30 Nimh cells, so the Volts on the ESC is not a problem. I have a wattmetter. I have have to get the motor mount built up and i can start testing. My overall goad is speed, which usually means a high pitch prop if I'm not mistaken.

The reason I brought up the 14.4 volt pack was that I have one of those sitting around already, but I am in process of procuring the 2 11.1 volt packs. More volts will definitely be the end goal. So i will stick with testing to the 22.2V range.

I can get the testing done sometime this weekend and put some numbers up here for you to look at as well.

I assume the closest i can get to 35A without going over the better?

thank,
-Mike

mjderstine

anyone know guys?

Dr Kiwi

I can't guarantee Motocalc, but here is what it says with a 14x8 on 6s.


Motor: Astro Cobalt 40 8T20 640 682rpm/V 2A no-load 0.121 Ohms.
Battery: Polyquest PQ-3300XP (20C) 6 cells 3300mAh @ 3.7V 0.0051 Ohms/cell.
Speed Control: Castle Creations Griffin 55 0.001 Ohms High rate.
Drive System: 14x8 (Pconst=1.17 Tconst=1) geared 1.63:1 (Eff=93%).
Airframe: Anything you want.
Conditions: 600ft above Sea Level, 29.92inHg, 68°F

Motor Amps = 33.1
Motor Volts = 21.2
Input (W) = 699.2
Loss (W) = 203.8
MGbOut (W) = 495.4
MotGb Ef(%) = 70.9
Shaft Ef(%) = 67.5
Motor RPM = 11701
Prop RPM = 7178
Thrust (oz) = 99.7
PSpd (mph) = 54.4


And a 13x10 on 6s:

Motor: Astro Cobalt 40 8T20 640 682rpm/V 2A no-load 0.121 Ohms.
Battery: Polyquest PQ-3300XP (20C) 6 cells 3300mAh @ 3.7V 0.0051 Ohms/cell.
Speed Control: Castle Creations Griffin 55 0.001 Ohms High rate.
Drive System: 13x10 (Pconst=1.17 Tconst=1) geared 1.63:1 (Eff=93%).
Airframe: Anything you want.

Motor Amps = 31.6
Motor Volts = 21.2
Input (W) = 670.0
Loss (W) = 191.6
MGbOut (W) = 478.4
MotGb Ef(%) = 71.4
Shaft Ef(%) = 68.2
Motor RPM = 11851
Prop RPM = 7271
Thrust (oz) = 82.7
PSpd (mph) = 68.9

mjderstine

thanks, i think i can swing a 13 or 14 on there. i can also take the gear drive off and that opens up some other possibilities.

should i be using electric APC propellers? or can i get away with regular APC props

Dr Kiwi

APC E are a bit more efficient, but at these power levels a glow prop would not be as big a disadvantage as it would be at only 100W!

mjderstine

i thought so. i never used elec props with my larger 1.20 size elec aircraft. the regular wood props worked just fine.

i have APC E props right now for this setup, but may experiment with regular APC props as they are more easily aquired at the LHS.

Alan Hahn

Just a comment. When you give prop rpm in your first post, you mention an rpm of 9280. This will be the rpm with no prop (unloaded), so a real prop will be at a lower rpm, say ~80% of 9280, or ~7400 rpm as a typically ballpark loading with reasonable motor efficiency..

A 10" prop isn't going to buy you much at even 9000 rpm (it is a light load), so you definitely want a larger diameter prop, say 12" or so, with some pitch (I don't see what kind of plane it is going on). Maybe even look into mutiblade props if clearance is a factor.

mjderstine

its actually not going on an airplane but on a dumas swamp buggy.

quick pic. thats a 12x8E apc prop

weezyrelick

looks fun good concept.