Introduction
The Diatone 2019 GT R249+ is one of the newest series of micro quadctopers from Diatone that is built around the Mamba mini f4 stack.

The full range from smallest to largest includes:

Overall look and feel
The first thing I'll say is that this is a very polished looking quadcopter. The build is well put together and includes some thoughtful parts like buzzer, spare screws, cable ties etc. although sadly only one set of propellers are included. Most impressive is perhaps the canopy that ties everything together. This is not a 3d printed canopy (like on the Leader 2.5) but an injection moulded one meaning a tidier finish and lighter weight as well as providing much more torsional strength. The design is clever and ingregrates with the quad well and includes well though-out features like the camera protection moulded in. The canopy is common across all of their 2019 GT range and is available in clear, white or black. No doubt other colours will be available later too. A quick shout out that the kit also includes 2 battery straps - these are my new favourites - lightest I've come across at a hair over 2g and 150mm long.

A closer look at the specs (my comments in red)
Wheel base: 115mm A good tight frame for 2.5" props. carbon fibre thickness is 3.0mm
Propeller: 2.5 inch. These are gemfan 2540 props, simply the best 2.5" prop available
Color: lucency / black / white
Weight: 77g This is the weight without receiver or props. Weight with receiver, props, battery strap is just under 83g.
Lipo battery: support 4S (not included) Can support 4s but I feel more comfortable with 3s on 5500kV
Flight controller: Mamba F405 mini; MPU6000; AT7456 OSD; 16M flash; 5V 1A BEC. Very pleased for this to have F4 and blackbox flash for tuning. 1 spare UART available outside those used for receiver and vtx control
ESC: Mamba F25 / 25A 4S ESC Dshot600 2-4s capable. Pleasantly suprised that this has a current sensor - my first 20 x 20mm ESC to have one.
Motor: Mamba Racing MB1105 5500KV Good kV choice for 3s. These are made by BBB (3BR) - a well recognised motor manufacturer 
Camera: RunCam Micro Swift. The original micro CCD cam and still very capable. A testament to reliability 
VTX: RunCam TX200U 48CH 25 / 200mW - Piggback VTX that runs off 5v. Has OSD control. Needs unlocking - more below.

Setup prior to flying
Before anything else I'd recommend unlocking the video transmitter which is a Runcam TX200U. By default this is locked to 25mW transmission power and many of the channels cannot be accessed. By holding down the button for 10 seconds you can unlock the vtx meaning all available channels plus 200mW transmission power will be available via betaflight OSD (tramp protocol). 

Next you'll need to add your own receiver and I use the XM+ (FRSKY) fairly exclusively. I don't use telemetry because the XM+ has a great feature that sends RSSI over an auxilliary channel so that it can be displayed in betaflight OSD (once firmware is updated). It's cheap too - Hobbycool.com have this permanently on sale at less than $10 here. Its very easy to wire up with the flight controller already pre-wired. The receiver can then easily be stuck to the top of the flight stack thanks to the plastic cover that is included.

Lastly I've captured the default betaflight settings below both in screenshot form and as a diff file but spoiler: This tune is awful, at least the version I have. Further down in the flight section I'll show updated screens and diff file.

First flight impressions?
Awful! After getting excited that there was a custom tune I was very disappointed to find out this tune was just rubbish. I switched to stock PIDs in betaflight OSD but this was even worse. I think someone has much about with the filters meaning it oscillates like crazy, even with extremely low (single digit!) P and D values on pitch and roll.

Resetting betaflight and next flight
Before taking to the sky again I updated to betaflight 3.5.3 but more importantly, reset all the custom settings. I then followed the betaflight 3.5 tuning guide here to enable all of my favourite settings like I-term relax and RC interpolation. Screenshots on the tune below and a diff file as well.

After tuning (using Albert Kim's betaflight tuning guide) I managed to get rid of the oscillations but still have some work to do in tightening it up. All of my tuning was done with a GNB 3s 450mah battery which I feel is a good match although anything up to about 650mah 3s would offer a little more flight time without compromising performance too much. Update: I now have my final tune in the gallery above. I'm pretty happy with it on 3s. It's conservative but not osciallation or Jello. Please make sure you run PID loop at no more the 4kHz.

The first thing I noticed about this quad it how quiet it is. I think this is because it is lighter than most and props aren't spinning as fast running 3s on a mid kV motor. Although not as quick as the Leader 2.5 (reviewed here) it was a lot more gentle on the battery meaning I could get a fairly comfortable 2.5-3 minutes from the 3s 450mah battery with low and fast flying which is reasonably intensive for a battery (vs. say floaty freestyle). That said I found throttle on this very linear with plenty of thrust up high in the throttle range where I'm used to it levelling off and just making more noise. Grip, cornering is all getting familiar now and assuming nothing strikingly wrong with the quad is more down to the software setting and tune than anything else. In terms of feel thought with was really good and given I have a preference for lower kV and lighter weight this suited my tastes well.

4s
Although I prefer this quad with 3s it can most definitely handle 4s. I used my China Hobby Line Ministar 4s 650mah battery and dropped Ps and Ds a little to compensate. It is noticeably faster but I'm not a fan of this flying style where it is hard to scrub off momentum in a corner - it essentially becomes a flying bring (albeit a very fast flying brick). I'd still recommend 4s for wider open spaces if this is your only quad and yes it will perform well but I personally think wide open spaces are more suitable for larger quads. Again this is my bias showing through but I like this R249+ best on 3s and threading through smaller and more technical gaps. If you are looking for a 4s battery for this though I'd recommend a light 4s 450mah like this one from china hobby line. Update: PIDs finalised for 4s. I did quite a bit of work with D to make sure that motors were only slightly warm with a full, hard run on 4s 650mah.

FPV performance
Since I unlocked the VTX above, all of my flights have been on 200mW. Performance is good but not great. At first I was really impressed by the runcam VTX but after more high throttle runs I found out that there was noise coming through at high throttle. Given the runcam TX200u runs off the 5v supplied by the flight controller, the flight controllers' BEC may well be the culprit which is typical - this is what I found with the Leader 120 when I ran the vtx off the 5v line compared to battery voltage. Unfortunately though the TX200u only runs on 5v. Don't get me wrong though, it was fine, just not as clean as the Leader 2.5 or babyhawk R

Comparison to other 2.5" micros
The 2.5" brushless micro market is now relatively crowded so it's hard to stand out. I've been lucky enough to review a number of these so feel like I am in a good position to make these comparisons. There is enough differences between them in weight and motor spec to suggest some maybe more suitable than others (particularly if you have existing batteries you wish to use) but for a large part they are interchangeable.

So you can see by comparison this is relatively light for a modern 2.5" micro, only the Leader 120 is lighter. The components on the Leader 120 are now getting rather dated (although it can be greatly improved) so will take no further part in this comparison. The 1105 5500kV Motors for the 249+ are slightly smaller and lower kV than the other modern 2.5" quads and personally I find them more suitable for a light-ish 2.5 inch propeller. I also like the fact that I can get the bulk of the performance out of it on 3s unlike the skystars bolt that really needs a 4s, although it is nice that the option is available. Although the babyhawk r pro and leader 2.5 have electronics capable of managing 4s, their motors are too high in kV to be practical here.

​For me the R249+ hits a nice balance on price, weight and power meaning it's not the fastest, lightest or most powerful but it is a good balance of these which makes for respectable battery life, good performance less noise during flight. In the end you'll probably be driven more on promotional prices than any specific features here and unless you are very passionate about something specific it would be hard to argue with that reasoning. I think the last word on this would be that the quality of Diatone, FullSpeed, and EMax are somewhat on par with the Skystars bolt somewhere behind. Same goes for availability of spare parts - Fullspeed, Diatone and Emax support their products well.

2019 GT R249+ Conclusion
Diatone have done a Diatone and released a very nice quad on paper but this time at a much more competitive price than the previous 2018 GTR90. The R249+ reviewed here shares the same electronics and motors as the 1.9 inch R239 and 2 inch R249 but the 1105 5500kV motors suit this size best rather than adding unneccessary weight with the smaller-propped versions. It's a very predictible flyer that is far from slow on 3s which I this is where the sweet spot is on this quad although you can run on 4s for more speed but also more weight.

Would I recommend it? Sure it's a good quad but no better or worse that the Leader 2.5 or Babyhawk R Pro. Diatone, Fullspeed and Emax all have a strong brushless micro pedigree and resultant support for these models so that is a wash too. Really it will come to price and that will change from time to time depending on promotions so at the prices listed above. For now, the Diatone GT R249+ and Leader 2.5 look to have the best offering and for me personally the Diatone edges the Fullspeed Leader 2.5 with a faster F4 processor and motors that better suit my flying style. See my leader 2.5 review here to compare. For me the Babyhawk R pro is just too expensive AND heavy.

The Diatone GT 90 (Rabbit) R249+ is available from Banggood here as a plug and fly model

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T-mount propellers are those that are based on a 1.5mm diameter centre shaft secured by a 2mm screw either side. They have been the only mounting option for 2-2.5 inch props but are now becoming more popular for 3" due to the weight they save over a 5mm single shaft prop mount. There are only 4 different types of t-mount 3" prop currently available so I've compared them all plus the new HQ 2 blade which is due for imminent release plus a darkhorse prop in a smaller size to make up the numbers. Please note this is a subjective comparison based on my opinion but my help as a startpoint for your decision. This is based on real world feel, light time and bitter experience with motors I've burnt out.

Emax Avan 3 inch tri-blade
This is the first modern design 3 inch prop designed for a t-mount which was released with the 3" Babyhawk R (reviewed here).

• Weight and Response: 1.27g This is very light for  tri-blade and mass is concentrated near the hub meaning response is very good for a triblade of this pitch.
• Pitch: Mid-range. This is well suited to 3s on an 1106 4500kV and will manage a 4s as well but is reasonably amp-hungry. Make sure your motors are not hot.
• Balance: Very good, typical of modern tri-blade
• Durability: Low-medium. Less material on the outer part of the blades means they bend/break easily, especially towards the edge of the blade
• Other notes: warning: given the unique design of the blade near the hub, this is unlikely to fit any motor with a stator size larger than 11mm. Update: confirmed these fit successfully without modification on the RCX 1304. New RCX 1306 unconfirmed.
• Recommendations: Excellent on 3s but be cautious on 4s considering this is limited to 13mm stator size
• Available from: Banggood, Hobbycool

Dark horse to consider...

Others:
Please note the King Kong 2840 propeller was not included here because I found the Gemfan 2540 superior in every way. Also I have not include the Rotor X 3044 because it is difficult to source where I am based. The feedback that I have had is that it is more aggressive in pitch than any of the above props and probably only suitable for for stator sizes 13xx and up.​

Ever since I saw the first image of the first FlexRC Ascent I've been impressed. It is a simple micro quad frame that accepts 2" propellers, is super lightweight and has a cage that is designed around a micro CCD camera (runcam micro, foxeer micro, caddx micro, furibee micro etc.) This was one of the first frames designed around these superior cameras and in my opinion still one of the best.

Dmitry from FlexRC has since expanded on this winning formula and this has given birth to the 2.5" Ascent, 3" ascent desinged around the runcam split and most recently the Komori 3". The Komori is essentially based on the original 2" with a baseplate embiggened to take 3 inch props and extra torsion supports between front and rear arms to allow for minimal flex whilst stick with thin (but lightweight) baseplate. All up weight of the frame is 15g with included hardware. Shape is a squashed x - front and rear motors are closer together than left/right motors. *Theoretically* this should mean more stability but slower movement on roll and faster but less stable in pitch.

After spending a lot of time with the Leader 120, I was looking forward to a frame that better fits the micro CCD camera and can take a genuine 3" prop rather than 2.8".

Build
For build components I essentially transferred my gear across from the Furibee X140 (review here) where I felt the heavy frame (although durable) was letting the performance down.