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

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.?

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

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 .

Originally Posted by hoppy

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

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

Originally Posted by 3DDFHobbies

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

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

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.

Originally Posted by 3DDFHobbies

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.

i need to shift a max of 8KG

And wingloading at that wieght is that of a sport plane

Originally Posted by yomgui

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..

is this the EMP 3 blader ? http://website.informer.com/visit?domain=sdshobby.net

Originally Posted by 3DDFHobbies

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 !