My 4-Inch Kestrel - Can I Keep It Under 250 Grams?

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My open-source Kestrel frame is easily configurable. For my first build, I filled the frame with components that I already had on hand—the guts from a Leader 3 bind-n-fly quad. I knew that I wanted to try to fit a 4” HD quad in under the 250-gram limit, but I wanted to work out as many design bugs as I could first.

The design work on the Kestrel is pretty much complete. I found a few bugs when assembling my 4” build that didn’t show up on the 3”, but they were all easy to correct, and the changes didn’t introduce any incompatibilities with my official 1.0 release. I’m so glad I don’t have to increment to version 2.0 already!

I don’t have any sort of legal requirement to stay under 250 grams, so I’m just using that figure as a guideline. I can get away with using components that aren’t quite suited to the lightest build possible. The FC, ESC, and VTX modules are overweight, but they really aren’t all that heavy. Using lighter, less-capable electronics might have saved me 10 to 15 grams.

That’s not a lot of weight, but my build comes in at 270 grams with a 650 mAh 4S battery. My build does manage to come in at 244 grams with a 450 mAh 4S battery, but that battery is on the small side.

I’m right on the edge here. If I could save 10 grams on my components, and then save another 10 grams by using a 550 or 600 mAh battery, that would be awesome!

My goals for this build

I’m writing this section of the blog before having a chance to actually fly my 4” Kestrel build. It is fully assembled and configured, and it survived a test hover in the front yard last night. It has been raining all day, so I won’t be testing it for a few days.

I want to be honest. I want to tell you what I’m hoping to get out of this build before I can actually verify my hopes and hypotheses!

I used to fly 6” and 5.5” props on my freestyle miniquads. I didn’t fly 6” for long, but I flew 5.5” props for the better part of a year. On the same quad with the same motors, 5” props are more responsive and have a higher top speed compared to 6” props. 6” props have more bottom end that you’ll really notice when pulling out of a dive, and they are significantly more efficient.

5.5” props land somewhere in the middle, but I used to get about 20% more flight time compared to 5” props. Will this relationship between 3” and 4” be similar to the relationship between 5” and 6”? That’s my hope!

I’m a little concerned, though. I once tried 6” props on my BFight 210 with it’s 2204 motors. I didn’t lose any flight time, but I didn’t gain any, either. The 6” props were just too heavy for those motors. Will the 1606 motors be beefy enough for 4” props?

I’m expecting significantly longer flights out of the 4” props, assuming the motors I chose don’t wind up being ridiculously underpowered. I owned a 4” FPV miniquad a long time ago. It was big, heavy, and used 2204 motors and 1,300 mAh 4S batteries.

I expect the 4” setup to be smoother than a 3”, and I’m hoping it doesn’t feel too sluggish when doing snappy freestyle. I feel like my 220-gram 3” Kestrel might be a bit too snappy, so I’m thinking I might enjoy the 4” build!

What if it is terrible at freestyle?! I decided to use a big, heavy 1,000 mW VTX in my build. Assuming that I’m correct about this being more efficient, I would enjoy the idea of having a sub-250 gram medium-to-long-range HD quad!

I don’t trust static thrust tests—especially from manufacturers! A quick look at the numbers tells me that the 4” props on the 1606 motors should be able to cruise along at 30 to 40 MPH while consuming about 40% less power. They’re also capable of producing nearly double the amount of thrust with 4” props, but at the cost of about 30% higher amps. How is that going to work out in the real world?

If things work out in my favor, I will be able to cruise longer and meet or exceed my 3” Kestrel’s flight times when doing aggressive freestyle.

The parts list

My old 4” Holybro Shuriken 180 Pro was a scaled-down 5” racing quad. My new build is attacking this problem from the opposite direction. I’m scaling up a 3” micro, so instead of using 2204 2750 kv motors, I’m using EMAX 1606 3300 kv motors. This is why I’m referring to my build as a light 4” quad.

  • 4” Kestrel frame
  • EMAX 1606 3300 kv motors (4)
  • Aikon 20x20 AK32PIN 35a blheli32 4-in-1 ESC
  • Aikon 20x20 F4 flight controller
  • RaceDayQuads Mach 3 1000 mW VTX (mmcx)
  • Luminier AXII antenna (mmcx)
  • Caddx Turtle V2
  • TBS Crossfire Nano

The Aikon stack is total overkill for this build. That little 20x20 stack can easily power a 5” 6S racing quad. It would have absolutely no trouble driving my 680-gram 5” freestyle quads.

I’m certain that there is a cheaper, smaller, lighter FC and ESC combo that could drive these motors and props. This is going to be my test platform, though. I have a plethora of interesting motors and props lying around. I have some efficient 2205 motors that I’d like to try with 5” props. I have all sorts of 2306 and 2207 motors, too. Maybe I’ll try a ridiculous 6” Kestrel. It is easy to cut arms and stick them on this fuselage!

My 4-inch Kestrel

The VTX and antenna are on the heavy side, too. The VTX is only 6.5 grams, and the AXII only adds another 2 or 3 grams. The little 200 mW VTX and tiny whip antenna from my Leader 3 weighs less than 4 grams.

These heavy components are buying me versatility for testing and some extra VTX juice for long range. I guess my light 4” build is a bit on the tubby side!

Minor problems with my Kestrel design!

I was worried that the Aikon AK32PIN would be too long to comfortably fit on my Kestrel’s bottom plate. I didn’t want to alter the spacing between the mounting points of the side plates, but I could see that there was some wiggle room to push the front and rear stacks closer to the edges.

I was only able to move each stack forward or backward by 3 mm, and that made the build a lot more comfortable. Unfortunately, this also pushed my Caddx Turtle’s 20x20 board too close to the camera!

I raised the top plate a bit to make more room for the stack. The combination of the bottom plate, side plates, and the bushings that hold everything together is rigid enough, but it will flex quite a bit in a crash. This will make it harder to damage your flight controller in a crash, and give you a bit more room to build.

The center stack uses M3 holes as of version 1.0. I have grommets to bring those holes down to size for M2 stacks while providing a bit of vibration isolation for your flight controller. The grommets work great, and the M2 screws are no problem.

I didn’t remember to leave clearance for the head of an M3 screw! There’s a cutout in each arm that leaves enough room, but the arm bracing plate won’t let an M3 screw head pass. This was an easy problem to correct, and I was able to work around the issue when assembling my 4” Kestrel with an M3 center stack.

How does the 4” Kestrel fly?

Other than a few mistakes that I’ve made, the 4” Kestrel is flying great! A lot of what I suspected is true. Even though it is only 20 or 30 grams heavier than most 3” builds, it feels bigger. It reminds me more of flying a 5.5” or 6” quad. It still gets up to top speed quickly, and it catches itself from a fall with barely any throttle.

What kind of mistakes did I make? My side plates are rather floppy, and I’m only certain about what caused one of the problems that has these things flopping around!

When I moved the front and rear stacks outward, that created an opportunity to carve some material out of the bottom plate. This was a mistake, because my bottom plate is now quite flexible!

Also, a pair of my bushings are fitting loosely, but only on the left side! These aren’t fresh bushings, but they don’t have a lot of mileage on them, either. I’ve adjusted the bushing holes and the tabs with every prototype. I think I’ve just pushed things too far. I’ve added 0.5 mm back into the tabs in the model. We’ll see if that fits better.

My side plates are floppy enough that the camera shakes around. It looks terrible!

I also made a mistake regarding the propellers. I had a few pairs of HQ 4x4.3x3 v1s props in my drawer. I shopped around a bit, and that was the gentlest prop I found, so I ordered a few more.

GetFPV is one of the few stores that stocks HQ 4x3x3 v1s props. I haven’t flown them yet, but I’m excited to try them. They should be arriving before the end of the week.

I don’t think 4” props are ridiculous for a 1606 motor, but I’ve been worried that the HQ 4x4.3x3 would be too aggressive for such a small motor. It flies well, but it isn’t as efficient as my 3” 1306 Kestrel with HQ 3x3x3 props.

The 4.3 pitch props cruise at around 35 MPH with 22% throttle. That’s at least 10% less throttle than my 3” Kestrel. I spent an entire 650 mAh pack flying around like that, and couldn’t fly any longer than 6 minutes. My 3” Kestrel usually hits 5 minutes doing pretty aggressive, heavy-throttle freestyle. I’ve never tried for endurance on the 3”, but I know I’ve accidentally flown longer than 6 minutes on it before!


So far, I like the 4” Kestrel, and it is showing some promise. If all the HQ 4x3x3 props wind up doing is bringing the efficiency in line with my 3” build, I’ll be extremely pleased. I think I’m going to enjoy the 4” build more than my 3”, even if it doesn’t offer me longer flights. It just suits my flying style a bit more!

I believe there’s a good chance I’ll see significantly longer flight times out of the gentler pitch 4” props. It would be nice to be able to put that heavy 1000 mW VTX to good use. I’ll know in just a few days, but I’m not patient enough to wait until then to publish this blog!

Do you think I’ll get an extra minute or two out of the HQ 4x3x3 props? Do you think a similar build could be squeezed in under 250 grams with a big battery if I used a lighter FC, ESC, and VTX? Or do you think I should stick to 3”? Do you have any questions? Let me know in the comments, or stop by our Discord server to have a chat!