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Old Sep 21, 2012, 06:19 PM
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Keenan smith's Avatar
United Kingdom, London
Joined Dec 2010
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Question
Need to Swing a16X8 3 blade prop on 5s What KV ?

Hi All,
i Need to Swing a16X8 3 blade prop on 5s for Long Durations of time What KV ?
The Setup needs to be as Efficient as possible and i need at-least 5Kg of thrust... (11.02 lbs)

I need to know ASAP...
its for My 5th scale MQ-9 Reaper Build..
http://www.rcgroups.com/forums/showthread.php?t=1737151

K
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Old Sep 21, 2012, 06:39 PM
Space Coast USA
hoppy's Avatar
Space Coast
Joined Oct 2000
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I'd use a program like MotoCalc to get some general info.
http://www.motocalc.com/

Why do you need it ASAP for a 1 year project just started.?
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Old Sep 21, 2012, 07:43 PM
Jack
USA, ME, Ellsworth
Joined May 2008
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What is the maximum RPM you want to turn the prop at?

Motors running at their continuous rating (the throttle setting where the motor will run continuously without overheating) will typically be at 75% of their no load RPM.

The no load RPM is the battery voltage x the Kv. You can allow for a small drop but an unloaded motor will not draw much current. So for your 5S batttery you could use 4.0V or 4.1V for the calculation and the 5S voltage would be 20V or 20.5V.

As an example, if the maximum safe prop RPM is 2400 RPM you want a no load RPM of about 1/3rd more than that or 3200 RPM. And with a 20V battery that would be a 160 Kv motor.

Jack
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Old Sep 21, 2012, 08:05 PM
Straight up is all I ask
Fairfield Ct USA
Joined Apr 2002
2,317 Posts
a 400 KV Outrunner is the ticket and i would bump the pitch up to 10 inch for lower rpm at cruising speed . I ran the scorpion 4020-16 in ecalc . You could go a little larger 4025 ...... if you are worried about heat .
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Old Sep 22, 2012, 02:49 PM
ich bauen groß modell flugzeug
Keenan smith's Avatar
United Kingdom, London
Joined Dec 2010
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Quote:
Originally Posted by hoppy View Post
I'd use a program like MotoCalc to get some general info.
http://www.motocalc.com/

Why do you need it ASAP for a 1 year project just started.?
Because im buying the power system Very soon.... and Need to know For ESC and Lipos needed for 3-4hr loiter time
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Old Sep 22, 2012, 03:26 PM
Jack
USA, ME, Ellsworth
Joined May 2008
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Pretty hard to get it all figured out in advance.

Best thing is to estimate the total load (the motor's current draw at loiter/cruise throttle plus the needs of any other accessories that might draw on the battery) and buy one battery that might make it. Then prove it later from real inflight data before committing to more and larger packs.

Jack
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Old Sep 22, 2012, 04:44 PM
We want... Information!
Bruce Abbott's Avatar
Hastings, New Zealand
Joined Jan 2001
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Quote:
Originally Posted by 3DDFHobbies View Post
Because im buying the power system Very soon.... and Need to know For ESC and Lipos needed for 3-4hr loiter time
A 60A ESC would probably do, but I would go higher for safety. An ESC which uses 'active freewheeling' (synchronous rectification) can obtain ~5% better efficiency at part throttle. I suggest the Kontronik Koby 90LV.

You will need at least 20Ah to get 3 hours of loiter time in dead air.

Motocalc loiter time estimation (@67% throttle):-

Motor: Scorpion 4020-16 (est); 420rpm/V; 1.5A no-load; 0.07 Ohms.
Battery: Turnigy nano-tech 5000 (25C); 5 series x 4 parallel cells; 4500mAh @ 3.85V; 0.003 Ohms/cell.
Speed Control: Kontronik Koby 90LV; 0.001 Ohms; High rate.
Drive System: MAS GF 16x8x3 (est); 16x8 3-bladed (Pconst=1.1; Tconst=1) direct drive.
Airframe: MQ-9 Reaper 1/5th scale; 125.7dm²; 7000g; 55.7g/dm²; Cd=0.037; Cl=0.57; Clopt=0.79; Clmax=1.2.
Stats: 53 W/kg in; 42 W/kg out; 9m/s stall; 11.1m/s opt @ 62% (209:10); 13.1m/s level @ 72% (162:25); 0.57m/s @ 3°; -0.63m/s @ -3.2°.
Conditions: Sea Level, 101.3kPa, 20°C

AirSpd (m/s) = 12.0
Drag (g) = 384
Batt Amps = 6.17
Motor Amps = 9.21
Motor Volts = 12.9
Input (W) = 118.4
Loss (W) = 24.2
MGbOut (W) = 94.1
MotGb Ef(%) = 79.5
Prop RPM = 5128
Thrust (g) = 472
PSpd (m/s) = 5.37
Prop Ef(%) = 59.0
Total Ef(%) = 46.7
Time (m:s) = 175:03
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Old Sep 22, 2012, 06:55 PM
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Doug Bartley's Avatar
Canada, ON, Owen Sound
Joined Oct 2008
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I might suggest you use an EMP 16/8/3 blade, far more efficient as well as being substantially lighter (designed for electric). Check out SDS Hobby.net.
They had them in stock earlier today. Hobbyking also lists the 16/8/3, tho they are not stocking again it seems. They still had some this morning tho. Doug Bartley
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Old Sep 22, 2012, 08:37 PM
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yomgui's Avatar
France, RA, Cormaranche-en-Bugey
Joined Feb 2011
540 Posts
Or use a 15x10E, 2 blades, with a motor with kv a bit above 400 and weight around 300, maybe more 400g ? My bet is : almost same rate of climb, more speed, more efficient power system in probably all the flight.

Why?

because a 16x8x3 like the MAS will drop its thrust very quickly after take off. 3 blade + low pitch ---> not good for loiter time.
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Old Sep 22, 2012, 09:22 PM
Space Coast USA
hoppy's Avatar
Space Coast
Joined Oct 2000
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Quote:
Originally Posted by 3DDFHobbies View Post
Because im buying the power system Very soon.... and Need to know For ESC and Lipos needed for 3-4hr loiter time
If you have any doubts about the final weight/wing loading including the batteries and power system, you might want to wait. Based on the weight/etc, the power plant requirements might change....just a thought FWIW.
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Old Sep 23, 2012, 09:41 AM
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Keenan smith's Avatar
United Kingdom, London
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i need to shift a max of 8KG
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Old Sep 23, 2012, 09:42 AM
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Keenan smith's Avatar
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And wingloading at that wieght is that of a sport plane
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Old Sep 23, 2012, 09:44 AM
ich bauen groß modell flugzeug
Keenan smith's Avatar
United Kingdom, London
Joined Dec 2010
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Quote:
Originally Posted by yomgui View Post
Or use a 15x10E, 2 blades, with a motor with kv a bit above 400 and weight around 300, maybe more 400g ? My bet is : almost same rate of climb, more speed, more efficient power system in probably all the flight.

Why?

because a 16x8x3 like the MAS will drop its thrust very quickly after take off. 3 blade + low pitch ---> not good for loiter time.
i need it to Run queitly thats why i chose 3 blade..
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Old Sep 23, 2012, 09:46 AM
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Keenan smith's Avatar
United Kingdom, London
Joined Dec 2010
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is this the EMP 3 blader ? http://website.informer.com/visit?domain=sdshobby.net
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Last edited by Keenan smith; Sep 23, 2012 at 10:28 AM.
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Old Sep 24, 2012, 03:14 AM
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France, RA, Cormaranche-en-Bugey
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Quote:
Originally Posted by 3DDFHobbies View Post
i need it to Run queitly thats why i chose 3 blade..
Ok ! (I have no "noise prediction tool"!!)

Anyway, with the same motor and batteries, the 16x8x3 will probably spin faster in cruise than the 15x10x2.

I calculate 52 % flight efficiency (thrust*airspeed/input watts) in cruise at 50 km/h with the 15x10, only 41 % with the 16x8x3... 300 g is too light a motor for my taste... maybe a good advice to wait and see what the final weight will be !
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