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View Full Version : 80'' CL-415 canadair Prop problem


pappy_69
01-12-2009, 11:23 AM
H guyes!!

A couple of days ago i adressed a question to another section of the forum but i realised that here i could get an answer from an appropriate guru of sea planes. SO....


I am building a CL-415 Canadair (waterbomber) and i decided to go Electric.
To power up the system i got 2 Dymond 4260 (600kVa) and the Dymond 60 Plus ESC's.Because of the scale look i would like to use 4 blade props so i got 2 10 x 6 x 4blades.
I installed on one motor and did a test with an analyzer (watt's up from lindinger.au).
(Every reading supposed to be multiplied by 2 since its a 2 engine model).
To my surprise the analyzer showed me only 29Amps and 620Watts.
The final models weight will be 7,5kgr or 16,5lb.
I know a rule of thumps that says that to fly scale and safe in the same time you need something like
200 to 240Watts/kgr or 90W/lb.
The Batteries are 2x3 cell in series of 5000mAh each.

I have a question:

Does anybody knows if 11x6 will do the work based on real experience and NOT a power calculator program?
Calculator showed me 38amps but the real readings are obviously far out!!

????????

Larry3215
01-13-2009, 05:42 AM
4 blade props are a bear to predict. On top of that, outrunners dont work that well in the software either, so your well advised to ask for first hand experience.

The problem is, there aren't that many guys with a lot of 4 blade experience and the odds of finding one who has used your exact motor/battery/prop combo make things more problematic yet. Of course, you might just get lucky :)

I doubt going to 11" will make all that much difference as your not even really begining to load up that motor yet. I suspect your kV is too low for that size range of prop.

Can you go to a higher cell count or a higher kV motor without too much $$?

pappy_69
01-13-2009, 11:08 AM
For this size and weight the prices for more than 600kva are around 90 to 100$ per motor.
I tested on the computer the 4120-14 AXI and showed me adequate power for that weight.

I tested also the power and counted 7 cells instead of .
My problem is:
I already have 2pacs of 3 cells in series. If i add one more in series would that be a problem? I suspect difference in discharge rate although i am not sure.

Larry3215
01-14-2009, 02:46 AM
Adding another cell in series will effect several things.

The voltage will be higher so you need to be sure your ESC can handle 7S voltage. Most normal ESC's cant. You may need a hi voltage controller.

You of course need to be sure the extra cell matches the other cells closely as to capacity and 'C' rating.

Adding the extra cell will increase the RPM and thus the amp draw but how much is hard to say. I'd guess 3 or 4 amps maybe at about 3.5 more volts so that should put you into the 750 watt range - maybe?

pappy_69
01-14-2009, 07:05 AM
Hi Larry!!

More the days pass more i dived into serious calculations and drastic measures i have to take and decisions as well.

So here are decisions : (please tell me your opinion)

1st
My plane will be in the heavy scale planes side! That means that i can do 2 things. I ordered already a 11x6x4 prop. If this prop gives me more then 37Amps with the setup as is then i can go fly the plane. 37amp x 22V = 814W
that gives 1628W in total. That makes 197W per kgr. or 85W/lb.
In case this prop doesnt gives me what i expect then i go 2 blades.
I tested a 13x6 APC prop and as i mentioned in an earlier post, it gives me 920W while the voltage drawn is 22.4 Volts.

2nd
I know that i have to adopt a more scale flight concerning this project which means longer taxi on the runway before taking off, in order to gain speed. That speed rule i believe applies in landing as well.

In calculations i made to measure the thrust if i use the 13x6 prop gives me 3850Kgr/motor which is almost 1:1 thrust to fling weight ratio.

if you'd like to take a closer look on my project here is the URL:

http://www.aeromodelismos.gr/index.php/topic,1925.0.html

(it's in english as well)

Pappy

Larry3215
01-14-2009, 07:21 AM
That seems like sound reasoning to me. Larger diameter props are always more efficient especially if thrust is important.

The only concern I have is the total power. I am more comfortable with 100 watts per pound but on this size model I think 85 watts/pound would be sufficient. Water takeoff may be more dificult but dry land should be no problems.

Keep us posted on the progress!

pappy_69
01-15-2009, 05:55 PM
News flash!!

I just got my 11x9 props (in a previous post i mentioned 11x6 but i meant 11x9) and tested them.
According to Analyzer i got 960W at foul throttle.(1920W from both motors!!!)
The batteries weren't foully charged and they showed 22,73V before i give foul throttle.
At foul throttle the motor drown 46,4Amps.

i believe that if the batteries will be fully charged i will have more than 2kW total (both motors!!!!!!)

:tc::tc::tc::tc:

Larry3215
01-16-2009, 09:01 PM
Thats much better! Should be plenty of power now :D