Brushless Zero - Mounting an outrunner on a GWS Warbird

I've finally gotten round to installing the brushless engine on the zero, and here is a bit of a post about how it was done.

The Options

There were about three different brushless upgrades paths available (or so I speculated) for my little GWS Zero (which is proving to be such an awesome plane). Here's how I saw them breaking down:

1. Stick mount a geared brushless engine (or bolt a brushless engine into the existing gearbox if one could be found).
2. Use one of the GWS Brushless stick mounts.
3. Remove the engine stick and put a firewall in place to mount the outrunner against.

Option 1 had a heck of a lot going for it. By simply replacing the engine in a gearbox, or replacing the entire gearbox unit you kept your thrustline exactly the same. I even bought a little 4000kv Venom motor to do it. However, when I went to put it in the gearbox I realised a couple of things - the recommended prop was a 10x8 - a 10x8! The Zero will be making three point landings with one of those points being the prop with a 10 inch prop. Further, I didn't have a 350 gearbox I had a 400, s0 the gear ratio would be way too low, and I didn't want to buy a $26 gearbox just to fit a brushless motor too (if I could even find one in the correct gear ratio).

Option 2, using the GWS Mount, seemed like the next logical step. A little bit of measurement suggested that by laying a piece of engine stick width balsa on top of the original engine stick I would get the thrust line just about right. Add some CA and a screw perhaps, bolt in the outrunner and away you go... if only it had been that simple. When I actually measured the distance the prop stuck out the front of the zero (about 5.5 cms) and looked at the brushless mount it was very quickly apparent that I would have to be running the mounting stick into the battery bay, and doing some pretty serious foam cutting so the prop almost lined up. Cutting foam didn't bother me, but I wouldn't be able to get the battery as far forward once the change was made. The Zero already builds a fraction tail heavy, and I was about to put a lighter motor into her. Scrapped that idea - too much work and no certainty of outcome, and the CoG will be very hard to get right afterwards.

So as you can guess dear reader, that lead me to option 3. If you still had your stock 350 gearbox then I would suggest the fastest way to go is get a gearbox mounted brushless. If not, the rest of this post is about how to mount a firewall and outrunner to your GWS warbird (or at least this is how I did it).

The Concept

Because I was worried about getting the thrustline right (slight down and to the right on the Zero) I decided to build a two firewall mount. Firewall one would mount to the fuselage, firewall two would screw into firewall one (allowing me to space and adjust the thrustline using scraps of balsa).

The Job

Here's the Zero with the cowling off. The fibreglass tape is a "tie it up with wire" cleanup from the last serious nose in the Zero had (some time ago now). The photo perhaps starts to give an idea why option two wouldn't work. When you consider the size of the outrunner, the engine mount, the outrunner engine mount stick (which needs to be glued on top of the original engine mount stick) you can see why you run out of space.


So, took off the gearbox (which I'll salvage for getting my Tigermoth back in the air one of these days), and started hacking at the engine stick (which needed to be cut of flush with the front of the fuse). Before removing the gearbox I made careful note of where the prop shaft ran in relation to the fuselage.

Cutting through the engine stick with a hobby knife took longer than you might expect. Let's just say the first photo was bascially natural light, whilst this one was under flash.

The engine mount was cut back so that it was below the level of the fuselage around it. But I didn't want it back too far, as I was planning to secure one of the firewalls into the engine mount stick.

The engine mounts were cut from 3 layer plywood, which was massively overkill for the job, but is what I had hanging about in the garage.

A hole was drilled through the centre of both bits of ply (to assist in centreing on the thrustline). The outrunner's base was mounted to the outer firewall. Aligning holes were drilled through the inner and outer firewalls to allow them to be screwed together later.

Although not obvious in this photo, holes consistent with the postions of the screws holding the outrunner base in place were drilled - to a larger diameter - on the innner firewall. Whether you need to do this will depend on whether the screws securing your outrunner stick through the outer firewall or not. I needed them, otherwise the firewalls wouldn't sit flush.

Once this was done a hole was drilled on the inner firewall above the central hole, and a corresponding hole (taking care to get the position of the central hole correct relative to the original prop shaft correct) was drilled into the engine stick. The inner firewall was then mounted against the fuselage using 5 minute epoxy, and also screwing into the original engine stick.

Looking at the outer firewall in this photo (lieing on the cutting board) you will note that a slot has been cut from the central hole almost to the edge. This will allow the outer firewall to sit flush against the inner firewall without being pushed out of position by the screw holding the inner firewall to the engine stick.

With this done the outrunner was mounted on the outer firewall and the connections made through the hole in the fuselage (after the usual amount of soldering and burnt fingers) to the avionics compartment.

Also in this photo on the top right corner you can see a small scrap of balsa I've put in place with some tape to give the motor down and to the right thrust angle.


The outer firewall was then screwed into place.










Here she is with her cowling back on and a new APC 7x5. I checked out the amp pull with the clamp metre and she came up to about 12A at WOT. Bit dissapointed by that - might need to experiment with some different props later (power plant can do 16A).

About the Motor

The powerplant is an emax 2212 1500kv 16A 60s teamed with 18A speedie. I got them as a package from my lhs.

The 7x5 prop combo may not be the best, as the amp draw wasn't really what I had hoped. On the other hand, a smaller prop with a nice aggressive pitch will tend to suit a warbird. I'm running the Zero on a 3s 800mAh pack that fits nicely into the pre chiselled space for the battery.

Flight Testing

Keen to see how she flew I planned to get down to Curtin with her this morning. Before heading off I took her weight using the kitchen scales - 310g without the battery, so she shed about 40g by getting rid of the brushed 400 gearbox. She was almost certainly going to be tail heavy.

When I got to the field I checked the CoG. CoG maybe just a fraction too far back. If I had tried flying her before, and knew how she handled I might have accepted the balance, but didn't want tail heavy for a test flight. Taped two big washers to the bottom of the cowling. CoG came forward (later I checked the weight - she was 340g - 10g shy of her original no battery weight) and was fine for flying.

Her take off rollout was almost non-existent. As soon as I knew she wasn't going to nose over (a continual problem with GWS warbirds) I opened the throttle and she lept into the air climbing very very quickly.

The performance increase was noticeable, but not out of this world by any means. The Zero was already fairly fast on the 3s 800mAh and the 400 brushed. Overall the performance improvements could probably be summed up thus - about 30% faster, and about another 2s of vertical performance (so a pretty solid 5 seconds vertical from level flight at WOT).

Apart from climbing with the extra power, she seemed in good trim at all throttle positions so I guess I must have got the thrust line about right - woohoo!

Her scale speed is impressive. Whether she will be fast enough to keep up with some of the more serious Eflite kits she tries to mix it up with remains to be seen.

I think that's it for Zero upgrades for the moment. I need to get onto my Tucano and P51.