Building a 4" racer from the GEP MX3 Sparrow

28/6/2018

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.

4 Comments

Andrew

1/7/2018 03:55:48 am

I want to copy your build! Would this be flyable on 3s 650mah? I'm not looking for all out max performance at the moment.

QuadifyRC

1/7/2018 07:19:44 pm

Hey Andrew

Thanks for the endorsement. You could fly but it would sag the batteries a bit. If you want 3s I'd recommend 850 - 1000mah. In saying that I'd say just go straight for 4s and limit the throttle through the transmitter if needed. Alternatively you can limit throttle in betaflight from 3.4.0 onwards. Let me know if you have any qs on the build... Good luck :)

8/9/2018 02:22:03 am

I built this quad and have been flying it around. I really love the size and power.

I fly it with 650 3s and 850 3s. It keeps the weight under 250g so I can fly in a greater variety of places in Canada. I'm just learning to fly and am mostly just buzzing around parks, not railing gates, it's tons of fun for right now. The 650 3s will sag quite a bit at full throttle but is fine if you are are just flying around and doing a bit of half-assed freestyle.

I do have one complaint about this build which is the frame. I've managed to break 3 arms now. I find it a bit too fragile for the beginner pilot. Yesterday I broke an arm by pulling out of a fIip too late and landing almost flat on a soft grass field. The arms seems to like to break right in the middle between the standoffs. The arms also like to de-laminate very easily around the motors from collisions.

I'm thinking of moving the components over to one of the avant quads 4" frames with a 4mm bottom plate. It's only a 5-10g weight penalty and I think I need the extra durability.

8/9/2018 07:27:38 am

Hi Andrew

I've been wondering how your build is going, glad to hear you enjoy it too. I think it is not a bad idea changing frames based on those issues and Avant Quads make a really nice 4" (Leggara?). Interestingly crashing on sft grass has more chance of braking arms as it tends to catch in mud rather than slide. Better for eveything else though.

Racing quad builds