Cube FPV

Crux 35 Frame Review

The Crux 35 frame from HappyModel is a 3.5 inch freestyle frame. It’s sold in analog and digital bind and fly versions, but in this review I’ll be focussing on the airframe only. Specifically, I will be assessing this frame for its potential to be used for custom builds. The information presented here is from my own experience building and flying this airframe in various configurations. In summary, the frame very well designed and quite rugged. It comes with a vast array of fittings and TPU mounts making it a versatile and complete frame kit. For freestyle, the main advantage of this frame is the centralised mass design a little like a racing frame. Having the mass cantered makes for very crisp and snappy flight manoeuvres. On the downside this compactness of the space in the frame makes for a challenging build. Please also refer to my build guides for more details on fitting out this frame.

YouTube Review

Main Points

The frame features a 3mm Base plate and 2mm top and side plates making it exceptionally strong. You certainly can’t complain about the finish either, sporting a silky satin feel with rounded edges on all the carbon parts. Looking at the intricate cut-outs you can see how much time goes into the cutting of this frame and that all pays off in weight reduction. These cut-outs have been well integrated into the frame design in a way which preserves the frame strength. The frame weighs in at just 26 grams assembled including the alloy standoffs, M2 bolts and a set of stack mount bolts with M2 nuts.

The kit includes ample bolts, spacers and dampeners to complete almost any build. Whatever way you choose to fit this frame out you’ll have spares left over. For some reason however the frame kit only includes three of each type of flight controller dampener which makes no sense at all. Fortunately, this is of little consequence as you will be using the dampeners which came with your flight controller anyway. Just add them to your dampener collection as spares.

Another strange choice is the inclusion of stack mounts which have countersunk heads. Cap head bolts are needed for stack mounting and I would really suggest that you use your own bolts for stack mounting instead of the ones included in the kit.

The top plate has 20 x 20 mount holes which actually align with the ones in the base plate. You can use these to mount an analog VTX or a vista unit directly to the top plate. I tend to avoid doing this as you will end up with wires from the main frame connecting to electronics mounted on the top plate. Keeping these wires short will make removing the top plate to work on the quad difficult. And longer wires will need to be ticked in and secured out of the way of props.

There is also an option to mount the stack on the base plate as normal and have the stack bolts go through the top plate and then cap them off with nuts above the top plate. This is my preferred method and doing it this way also adds strength to the frame. Particularly important with this frame if you are screwing bolts down on the TPU mounts. For this however you will need 30mm M2 bolts and these are not included with the frame.

The internal space of the frame is the gap between the top plate and the base plate – literally the height of the frame stand-offs. In this frame the height is 22mm instead of 25mm which is more of the standard for most frames.

The reason for this is because the BNF versions come with the HappyModel AOI flight controller and for this flight controller even with a vista mounted above 22mm gives ample clearance. This 22mm spacing however is just too tight for an ESC / FC stack even if using a naked vista on top. You could fit an analog VTX though. For this reason, you’re going to be limited to using an AIO flight controller.

For a choice of flight controller, the frame has both 20 x 20mm and 25 x 25mm mounting patterns. However, the 25 x 25mm mounting holes are straight on instead of being rotated 45 degrees. This means that when using a 25 x 25 AIO flight controller you will need to route motor wires to the front and rear of the board as well as both sides. It’s not impossible but definitely less than ideal. Add to this that most 25 x 25 (whoop style) AIO boards will have amp ratings less than 30 Amps it may be advisable no to go down this path.

For an AIO flight controller with a 20 x 20mm mount pattern there are many with 35 to 40 amp ratings. However, there is actually only one choice for this frame – the JHEMCU GHF420AIO-35A F4. The reason is because of the tight spacing between the frame standoffs. Note that the above mentioned AIO flight controller also needs to be mounted backward to actually fit in the frame. This means that you will need to reflect these settings in BetaFlight and also do motor resource remapping for the motors.

The 25 x 25mm flight controller mount holes may also cause a weak point in the frame. I have had a leg on one frame break at this point after a hard crash on concrete. However having said that I can’t put this damage down to the weakness of the frame. My second frame has been going strong now for months with no sigh of weakness at that point.

The side plate spacing is 14mm and has slots to adjust the camera position. However, camera choice needs some attention. A shorter camera is needed to ensure that the camera does not protrude past the front of the side plates. For example, a good choice is the Caddx Polar Vista or the Caddx Nebula Nano Pro and both of these are shorter camera. Whereas for example, the Nebula Nano is a much longer camera and will protrude out the front of the side plates too far.

Unfortunately, the motor mount holes are 9mm spacing only. Many 1404 motors have 12mm spaced motor mount holes, so you will need to check this when choosing a motor for your build.

The frame is an “extended true-x frame” This means that the angle between the adjacent arms is 90 degrees but the front and rear arm sets are moved slightly further apart (about 10mm). A bit like stretching the frame in the middle. This combined with the very short and compact frame which pulls the camera further back, means that you will have a perspective with props in view much like a racing drone.

The compactness of the frame gives a centralised centre of mass. I feel that this centralised mass makes the frame a bit more agile in that especially in the pitch axis the rotations come on faster. The downside of this however is that the reduced space in the frame makes for a very challenging build – especially of you want to mount a Caddx Vista instead of an analog VTX.

The kit Includes a variety of TPU mounts – two antenna mounts and a camera mount suitable for a SMO 4K or Insta 360 Go. These TPU mounts are secured in place with the frame stand-off mount bolts. It’s not an ideal situation because tightening these bolts down will just pull the bolt head through the TPU.  I recommend that you put an M2 washer on the bolt to spread that force over a greater area. Although not a perfect solution it will allow you to tighten down the bolts more.

You can mount the battery top or bottom but you will need to mount it on the bottom if you intending using the included camera mount. With the battery on bottom, flip after crash is not reliable and sometimes just impossible without a horn up top like many racing drones have. Arming angle also needs to be increased because the quad might be tilted after crash when it is resting on an underslung battery.

Finally, the battery strap supplied with the kit is much too long. I am running this drone with a 750mah 4S battery and the included battery strap needs to be cut down to be used. I would suggest going with a 19cm battery strap instead.

My Preferred Setup

(upgrade ESC Firmware with JESC or BlueJay to set up Bidirectional DShot and tune accordingly)
VTX: Naked Vista
Camera: Caddx Polar Nano or Nebula Pro Nano
Motors: iFlight Xing 1504 3100KV
Propellers: HQ Prop 3520 (T3.5 x 2 x 3)
Battery: Tattu R-Line 95C 750mah 4S (or similar hight C rated). 650mah is also good.
Battery Strap: 19cm Strap