RC Groups

RC Groups
    Power Systems
        Question Reality check before I buy please

#1 xcr600 Jan 04, 2010 04:05 PM

Reality check before I buy please
 
I am new to electrics so my power system knowledge is non existent!

I am building a 50" jim young plans dh88 and want to run my power system plan by the 'experts'.

I am running (2) scorpion 2208-30 brushless motors (only parts I already bought) and plan on using (2) HK / volcano / cheap 20amp speed controls off a 3s 4000 lipo. I understand the best way is to link both ESC to one channel with a Y cable? Will the BEC still work like this? My radio will support mixing 2 throttle channels if i need to go that direction instead.

Scorpion spec sheet shows a max draw of 15amp so the 20esc should be enough. I gather the cheap esc are working for people?

Finish weight is unknown as it is not yet done, i suspect about 35oz though from other builders results. I dont need rocket performance just good run times.

thanks all!

#2 Dr Kiwi Jan 04, 2010 05:12 PM

If they are cheap ESCs you may be safer to disable one of the BECs by "severing" the red wire [actually don't physically sever it.. use a short 3" servo extension and cut the red wire on that... then both your ESCs remain pristine for future projects]

#3 spastic Jan 04, 2010 08:51 PM

you can just use a blade to lift the plastic tab and pull the red wire pin out of one of the ESC plugs. put some heat shrink on it or tape..

#4 marc1 Jan 05, 2010 01:13 AM

That's a pretty ambitious build as a first electric model.

There is a DH-88 thread, you may be able to entice more responses specific to that airframe there:

http://www.rcgroups.com/forums/showthread.php?t=85770

I may be wrong but I suspect your Lipo will be much too heavy for that airframe. Maybe a 3s 1800mah, or 2s 3000 mah would be more appropriate.

========================
Rough estimates

Even with a 300 W system (150W / per side), drawing a max of about 14A per side on a 3s Lipo, you would get about 7-8 minutes out of a single 3s 2100mah ... but I believe the recommended is more around 200-250W total.

Suggested AUW = 32 oz = 2 lbs
2 lbs x 100W/lb = 200W

So 100W per side should be more than sufficient as a starting point.

Working with 200W on 3s, we get about 8 minutes out of a 3s 1800 mah.

====================
Working with your selected Scorpion motors ... scorpion 2208-30

Considering you can buy 7x5E normal and pusher prop (ie: rig for counter-rotating), you could look at 3s power which would give you about 9 a side, 18A total under full throtle giving you 27oz of thrust. That's roughly 8-10 A average for the entire flight.

http://www.scorpionsystem.com/files/i389_data_chart.htm

- 1800 mAh x 80% capacity = 1440 mah usable
- 1.440 Ah / 9A x 60 min/h = 9.5 min

So a 3s 1800mAh pack would give you about 9.5 minutes of sport flying discharging a single pack down to 80% of capacity.

You are expecting roughly 35oz, so 193 Watts / 2.19 lbs = 88W / lb

Personally, I would like increase the thrust a little bit, pitch speed and total power up a little bit from that number. What is the next usable counter rotating prop size that you could use?

100W / lb is a decent setup for sport flying. (if you can keep the efficiency of the power system above 70%) ... see Scorpion Calc for that.

Also, another good rule of thumb is that the power system (motor, esc, lipo) should be about 1/4 to 1/3 of the total AUW... so in a 35 oz plane, expect the power system to weight about 9-12 oz.

=============

Regarding cheap ESC ... you can have failures with cheap & expensive ESCs, but chances are you will have fewer failures with brand names. Is all the time you are going to put in this plane (and the good brushless motors) really worth saving $50 on ESCs?

CC Phoenix 25A weights about 17 grams a piece (or Thunderbird 18A), otherwise the Turnigy Plush (HobbyWing Pentium) have good reviews, but are a bit heavier at 19 grams for 18A or 22 grams for 25A.

One advantage of the CC Phoenix ESC is you can run them in parallel and not have to disable one of the BEC (red wire), because their BECs are all calibrated to the same voltage. This is usually not the case with cheaper ESCs.

#5 xcr600 Jan 05, 2010 06:52 AM

Thank you all for the thoughtful replies.

marc1, you gave me a lot to digest! I appreciate it very much.

#6 KenSt Jan 05, 2010 08:44 AM

What should he do to prevent one motor from shutting off before the other?

#7 Alan Hahn Jan 05, 2010 08:53 AM

Quote:

Originally Posted by KenSt (Post 13986224)
What should he do to prevent one motor from shutting off before the other?

Land before that happens!:D

#8 marc1 Jan 05, 2010 09:32 AM

Disable LVC, use a timer and a flight data recorder every now and then.


All times are GMT -5. The time now is 03:37 AM.