Originally Posted by Red 61

I like how his GWS-38 performed, but I'm not willing to spend 30 amps on it! I need something more efficient...

With decent cooling a TP 2100 PL will give you 30A "all day long" so to speak.

Besides your not going to be at WOT for the entire flight... right?

Tommy D

Ok just did static watt and amp tests on both motors.
Park 370 outrunners(1080kv)

Motor 1 with MAS 9x7x3 tractor prop:
static WOT
13.8 amps
151 watts

3/4 throttle
7.9 amps
85 watts

1/2 throttle
3.6 amps
58 watts

Motor 2 with MAS 9x7x3 pusher prop:
static WOT
13.7 amps
149 watts

3/4 throttle
7.8 amps
86 watts

1/2 throttle
3.6 amps
58 watts

These will do nicely. Yes, at WOT, the amps and watts are a little high for the motors, but, normal flying will not be at full throttle and cruising speed 1/2-3/4 is well below the max specs of the motor....plus they will unload in the air some. Even at full throttle, total amps is 27.5...so no problems with a TP 2100 3s. However.....both motors were run separately for the tests, running them both together will probably give slightly different results....with the amp draw on each motor being a little lower. Best guess will be around 12 amps per motor at WOT...but will test this to be sure once plane is together. I base this off of tests done with other setups using twin motors I have(aka FSK B-25).
Anyhow...if get 150 watts per motor at WOT...this gives me 300 watts...enough for a 3 pound plane using 100 watts per pound. There's no way this plane will weight in at 48 oz though, so I will have better then scale speed capability...not that I am trying for this...but extra breathing room is good.

kahloq

During my initial testing of a GWS 2208 motor on 2 cells and 9 x 7.5 HD propellers, the motor drew 7.12 amps on a 1200 mAh battery and short leads.

When installed two motor in the GWS-38 on a 2 cell 1800 mAh battery and using 9 x 7.5 HD propellers they drew a combined current of 12 amps. I put this a lot of this amp draw reduction down to the fact that the leads from the ESCs to the motors are 12 inches long and their resistamnce is a big part of the cause of the drop in the current draw.

No matter where you mount your ESCs there will be a resitance in the long length of leads between the battery and motors so I would say that you will get a similar % drop in current draw.

Originally Posted by kensp

kahloq

When installed two motor in the GWS-38 on a 2 cell 1800 mAh battery and using 9 x 7.5 HD propellers they drew a combined current of 12 amps. I put this a lot of this amp draw reduction down to the fact that the leads from the ESCs to the motors are 12 inches long and their resistamnce is a big part of the cause of the drop in the current draw.
.

The highest voltage drop will be in the internal resistance of the battery being typically 0.06ohm to 0.1ohm for a two cell and 0.1ohm to 0.15ohm for three cell ...... eg:0.1ohm x 20amps = 2volts drop internlly in the battery

This is were the 20C+ discharge cells win, they have low internal resistance but are heavier

Did the maiden of mine today. Two 6 minute sorties, no disasters! No probs on the handlaunch, and with just a bit of up trim on the elevator, all was well. Flies a bit slower than I would have liked, but I knew I was giving up a fair bit by going with the GWS 9070 3 blade props. Wouldn't go so far as to call it a floater, but it slowed down a fair bit before stall. Two nice belly landings, and the 3 blade props are still 3 blade props. Battery not overly hot, quite acceptable. I put my cog at 80 mm. I'll leave it there, it seems about right for me.

One thing I wasn't expecting is orientation issues. My plane is all silver, and because of the shape of the plane, and being all one color.....wow, you could loose this one easily! I'm not sure I'd want to fly this one too far out down the field.

Anyway, it flies nice and it definitely is a nice addition to the squadron. I'll post some pics another time after I have put the decals on.
J

Originally Posted by Bthree

The highest voltage drop will be in the internal resistance of the battery being typically 0.06ohm to 0.1ohm for a two cell and 0.1ohm to 0.15ohm for three cell ...... eg:0.1ohm x 20amps = 2volts drop internlly in the battery

This is were the 20C+ discharge cells win, they have low internal resistance but are heavier

When a static test is carried out using a battery for a power supply and the current is determined to be X then internal resistance of the battery has been taken into account.

Using a larger capacity battery for two motors lowers the internal resistance of the battery used.

So in my case of the two motors mounted in the GWS-38 with a larger capacity battery, a large part of the 15% less current will be on account of the increased resistance of the longer connecting wire.

I was just giving kahloq an example of the changes that will occur in a real life GWS-38 power unit instalation.

Ken

Congrats J! I know of the orientation problem. Since my first flights were unpainted, the all white airframe made orientation interesting. Needless to say the plane always stayed in close.

Pat

Originally Posted by crxmanpat

Congrats J! I know of the orientation problem. Since my first flights were unpainted, the all white airframe made orientation interesting. Needless to say the plane always stayed in close.

Pat

Pat my next flight is gona be intresing.

Ken

Looks good Ken

Got the camera out for mine today. You'll have to pardon the two different spinners, the starboard side one is just on temp. for the photos. I'm still waiting on my mail order for my other three blade GWS spinner.
Hope you like it.
J

Finaly, the painting is finished and the marking are all put on the aeroplane. I am quite happy with it.

Ken

She turned out very nicely, Ken. Good choice on the scheme.
J

Topnotch Jag! Nice Pinup!!

Originally Posted by Tommy D

Topnotch Jag! Nice Pinup!!

speaking of nice pinup...where is the pic you wanted turned into a decal?

Here's my nose art... looking good guys like those schemes