Setup to replace a Cox Reedie

[Tim Wescott]

This thread: http://stunthanger.com/smf/index.php?topic=16910.0 (Getting Started in CL for Less) has spun off into a discussion of small electric planes.

Before I pull it even more off-topic, I think I'll ask here:  What's the right combination of stuff to use to convert a plane designed for 1/2A sport to electric?  Is there a package that one can just buy?  What are the pieces if one wants to raid one's stash of RC stuff?

[Dennis Adamisin]

Try the Brodak 1/2A package #BH-1832

http://www.brodak.com/shop_productdetail.php?ProductID=8866

Here it is in action:

[CircuitFlyer]

Tim,

I use a Rimfire 300 (100W, 1380kv) motor with a 480mAh (20C, 3S) battery a 10A ESC and a JMP timer, total weight - 95g (3.35oz), flight time - 3 1/2 minutes.  The biggest variable is the prop.  I currently use a APC 7x4.  I found the out-runner manufacturer's recommended prop sizes to large in diameter.  So, unless you want to fly with extra long gear legs, keep the kv high and/or the voltage high to get the RPM up.

When I started this project nobody had a 1/2a electric setup published, let alone readily available.  So, when in doubt - test.  I built a test stand to measure static thrust, voltage and current just to make sure I was in the ball park and not going to melt things.  I used static thrust as a benchmark.  I figure if you want a sport model it should be able to accelerate vertically; it's thrust should exceed it's weight.  My best reedie gave me 330g (11.63oz) thrust on the old Cox 6x3 prop.  Here's my electric test results (all @11.1v):

GWS 6x3 - 230g (8.10oz) @ 3A
APC 6x4 - 245g (8.63oz) @ 4.4A
APC 7x4 - 335g (11.80oz) @ 6A
APC 7x5 - 340g (11.98oz) @ 6.7A
APC 7x6 - 300g (10.57oz) @ 7.1A
APC 8x3.8SF - 380g (13.39oz) @ 7.4A (gets hot on the test bench maybe ok for flight)

The GWS prop results were interesting.  I have some more sizes on order, 2 and 3 blade, and will test them when they arrive.

If you want to put this on an existing 1/2a model you will find that the battery has to be mounted right up front to get the C of G correct.

[Dennis Adamisin]

Paul:
That is great info on the props & thrust.  I am using the APC 6x5.5 on 2S and it works great.  Gonna also try the APC 5.25 x 4.75 on 3S as I think this will pull hard and keep the current draw down.

Uh, that is I will try it if it EVER stops raining on weekends around here... 

[Jim Moffatt]

Paul
Can you post the rpms that resulted in your thrust tests?

I have a hunch that the GWS props are more efficient since they have much thinner blades.
The only ones meant to run at high rpms are the direct drive or hyper drive.
Everything is give and take however. No argument that the APC props were made for higher rpms.

[CircuitFlyer]

Sorry Jim, I don't have a tach yet.  The GWS prop I did test definitely ran at a noticeably higher RPM, the exact number I don't have.   I'll see if I can get a tach when my new props arrive and I'll post the results.

[CircuitFlyer]

I finally got the static thrust test runs completed.  I compiled the results in the spreadsheet attached.  I used a stock Rimfire 300 (1380 kv) and one that I had smoked and rewound with a higher kv (1820 kv).  The GWS props definitely run at a higher RPM and lower amp draw.  Does this just mean that GWS props are more efficient at zero forward velocity?  I guess that's why we will always do the prop dance at the flying field to find the prop that works the best.  Hope these results are of benefit if you would like to electrify a Cox reddie.