Building a 4" racer from the GEP MX3 Sparrow

The GEP RC MX-3 Sparrow was an awesome 3" quadcopter that sadly had little uptake because of it's high price ($249 retail). As at 28 June 2018 Gearbest is clearing them for $149 here which represents great value for money. I've chosen to convert this to a competitive lightweight 4" racer.

I haven't done a formal review on the quad but key features of the MX-3 sparrow but key features were:

• HGLRC F428a stack with 28a ESC
• Genuine Runcam Sparrow Micro Camera
• GEPRC 1408 3500kV motors

This was more of a power 3" than a lightweight so needed a 4s 850mah or 1000mah to feed the motors and get it moving. No doubt it was fast but it did handle like a heavy 3" where the props felt overwhelmed at times, even with a switch to the excellent Gemfan 3052 props (reviewed here). Some like this style of flying but perfer something lighter and less 'over-motored'. Speaking of the motors, they are the nicest 14mm stator motor I've ever come across, not sure but looks like it could be a Tmotor?

GEPRC 1408 motors. Very nice.

The change was simple, I purchased a lightweight Toothfairy 4" frame from piroflip, some HQ 4x4.3x3 v1s props and transferred. While I was at it I also switched the camera for a Caddx F1 and the stock TX20 vtx for and AKK FX3 with smart audio and up to 600mW power. Since dipoles are frowned upon at my club I've also used a UXII mmcx RHCP antenna. A full list of all components and links are at the bottom of this page.

Assembly pictures are below. I'll mention here that it was not until I disassembled the MX-3 sparrow that I realised the design and finish the GEPRC put in are second to none right down to the smallest detail. I was thoroughly impressed and can now see where the cost started to come in. Moving on, build pictures below, this was pretty straightforward.

I was really pleased with the finished build but was what was even more pleasing was that the final weight (ready to fly except battery) was pretty much identical to the MX3 sparrow - 150g, even with heavier components like the new VTX and CP antenna. Given this is swinging an inch larger prop I'd expect (and got) much better performance and efficiency.

This is now one of my mainstay racers where I can be competive on all but the most open tracks. It can give me a distinct advantage on small to medium size tracks as well where I'm well over 100g lighter than even the lightest 5" racers. I feel the 1408 motors are a much better match for the 4" props and feel like they could manage even more pitch than the 4.3 I'm currently using - I believe HQ are due to release a 5.0 pitch model I'm looking forward to trying soon.

If you are like the look of this build you can either strip down the GEP Sparrow like I did (currently on special for $149 at Gearbest or try your luck at Banggood). All other parts below:

GEP-RC MX-3 Sparrow 3" racing quad with FRSKY R-XSR receiver

Hyperlite Tootfairy frame

HQ V1s 4x4.3x3 propellers. Check out KababFPV's review here

Caddx Micro F1 1200TVL CMOS camera

AKK FX3 20mm vtx with smart audio, microphone. Check out my review here

HGLRC F428 FC, ESC

GEP-RC 1408 3500kV motors

Real Acc UXII RHCP antenna. Such a tidy install with these and the AKK VTXs. Only $6 each as well.