I'm Getting Excited About 4-Inch Quads Again

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At first, I was excited about trying out 4” props. A few years ago, I experimented with 5.5” and 6” props on my 5” quads. On all but my most underpowered 5” quad, these larger props added several minutes of cruising flight time.

Not only that, but the larger props have a very different feel. You lose a bit of top speed, but you gain a bunch of low-end grunt. If you’re pulling out of a 200’ dive with 6” props, the quad just feels like it stops dead in its tracks when you add some throttle.

Kestrel with 4x2.5x3 HQ props

I was hoping that scaling a 3” freestyle quad up to 4” props would lead to some of the same improvements. Could I build an HD freestyle quad that is capable of long-range flights and still fit everything in under 250 grams?

When I started testing, the most efficient 4” props available were the HQ 4x4.3x3 and HQ 4x3x3. On my 4” Kestrel with 1606 3300 kv motors, neither of these props led to the efficiency gains I was hoping to see. I prefer the flight characteristics of these 4” props for freestyle, but not by a large enough margin that I would encourage anyone else to build a light 4” freestyle quad.

WARNING: I don’t have much data yet!

I have a few problems. I have a dozen 450 mAh and 650 mAh 4S batteries for my Kestrels. They’re all in rough shape. I tend to be mean to micro quad batteries. I accidentally bring them in at 3.3 volts or less more often than I would care to admit. I charge a bunch of them, but don’t get around to flying them, so they’re always dying a slow death, because they’re not at storage voltage.

This means I don’t currently have any batteries that make it easy to compare directly with data in my old blog posts. I have a post that says my 4” can cruise for more than 5 minutes, and the 3” can manage 5 minutes of aggressive freestyle. I rarely get a 4-minute flight out of the 3” these days, because the batteries are old.

3” Kestrel on 4S

One day when I was doing GPS speed runs on the 4” with the new Avan props, I borrowed a fresh 550 mAh RaceDayQuads battery from my friend Tanner. On my 650 mAh pack, my top speed was 76 MPH. With Tanner’s 550 mAh pack, I hit 86 MPH on my first try. That was when I knew for certain that my batteries were getting tired.

Not only that, but the long-range flight testing takes time. I have to fly at about 15 MPH for nearly 20 minutes for those tests. Then I have to charge the 3250 mAh pack at about 1C. Even if I wanted to run tests back to back, I’ll be waiting an hour for the battery to charge, then I have to fly for 20 minutes again.

That said, I’m excited. I want to tell you what I’ve learned so far, and I want to tell you where I think this is going.

There are some new props

Emax has come out with three new props since I last wrote about 4” props: the Avan 4x2.8x3, the Avan 4x2.4x3, and the Avan 3.5x2.8x3. I’ve logged a good bit of flight time on the Avan 4x2.4x3 on my 1606 quad, and I like them a lot. They make the quad feel more like a 5” than any 3” prop I’ve tried, they’re pretty efficient, and they handle propwash a lot better than the HQ props.

I have a set of the Avan 3.5” props on there now. I’ve only flown one pack, but they’re promising. I’m hopeful that 3.5” is a sweet spot between 3” and 4” props. Before these were available, I cut down a set of HQ 4x3x3 props to about 3.5”. The 3.5” Avan props are definitely better than my homemade bullnose 3.5” props.

NOTE: In this snippet with the 4x2.5x2 props on 4S, there’s definitely more propwash than on 3x3x3 props. However, I like the way the quad feels. I’m going to be experimenting more.

I’m even more excited about HQ’s new 4x2.5x2 T-mount prop. It is a thin, gentle prop designed for light motors. Since it is a T-mount prop, I can only use it on my lighter 3” Kestrel with 1306 4000 kv motors. This is exciting because that is my lighter build by somewhere around 40 grams. It is disappointing because I have no way to directly compare this biblade prop to the other 4” props.

Zoe FPV may have solved my long-range problem

I am excited about the idea of my Kestrel freestyle quads being capable of long-range flight. I haven’t quite figured out what I would personally do with my long-range setup, but I’m tickled by the idea of having a $250 HD build that I can send two miles away. I’m even more excited about that build if it is also an awesome little freestyle quad.

My hopes of the 4” props increasing my flight time by 3 or 4 minutes didn’t pan out. If you’re cruising in the 15 to 20 MPH range, that would be an extra mile of range.

Then I saw Zoe FPV talking about her 2S 3250 mAh Lithium Ion pack from Outcast Droneworks. She has a 2” build that has managed to hover for 26 minutes with this pack. Lithium ion has more energy density than our usual lithium polymer batteries, but they can’t deliver as much amps. Her pack can deliver up to 40 amps, and that should be more than enough for my purposes.

Will all my hardware power up with only a 2S pack? Will my 3” Kestrel even be able to get off the ground? Will it still fit in under 250 grams?!

Tests with the Outcast Droneworks pack have been quite successful!

As soon as the battery arrived, I plugged it into one of my Kestrels. The VTX was transmitting, the Caddx Turtle was recording HD video, and I was able to do a test hover. Not only that, but my 3” Kestrel comes in at 248 grams with the 2S pack!

I also managed to fit the HQ 4x2.5x2 props onto my 3” Kestrel frame, but only just barely. Some of the props were rubbing the frame, but I was able to rotate the arms ever so slightly out of place to buy myself a fraction of a millimeter of clearance for each prop.

My 3” Kestrel does not yet have a GPS module, so I’m not doing good science here. I got the feel for about how fast 15 to 20 MPH would feel using the GPS on my 4” Kestrel. I’m probably flying in the right ballpark, at least.

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This is the last 30 seconds or so of my 19 minute test flight of my 3" Kestrel with HQ 4x2.5x2 props shoehorned on while using @outcastdroneworks 3250 mAh 2S pack. I don't have GPS on this build, but I do have GPS on the other Kestrel, so I did a sort of calibration run to get the feel for 15 to 20 mph cruising. This was 19 minutes of cruising. No hovering or any nonsense like that! With this 2S pack the build comes in at 248 grams. It has a Caddx Turtle, Crossfire, and a 1,000 mW VTX on 1306 4000 kv motors. The last 3 minutes of the flight were spent cruising circles around the dry parking lot, because I didn't know what voltage things would stop working at, and I didn't want to fall into a puddle. I chose to land when I started seeing less than 3v per cell. For the next test flight, I swapped on a set of HQ 3x2.5x3 props. On that pack, I started dropping under 3v at 16 minutes. I pushed it another 30 seconds, and quickly got to the point where I didn't have enough thrust at 90% throttle to hover. For this quad, 3v per cell is the end of the pack, for sure! #fpv #fpvracer #drone #drones #droneracing #droneracer #fpvdrones #fpvrace #multirotor #quadcopter #fpvfreestyle

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I was amazed by my first test flight. I managed to cruise around our local abandoned golf course for 19 minutes! Math says that may have been enough time to cover nearly 5 miles. I’m going to need to add a GPS module to this little guy to verify this.

The tiny 5-volt GPS module from Banggood only weighs about 6 grams with the full wiring harness, and it seems to work great. Installing it will push me up to at least 254 grams. That’s fine for testing, but a bummer if 250 grams is your legal limit. I could definitely save 4 grams by using a different VTX and antenna, so I’m not too worried.

This was a boring flight. I started by making big, slow trips around the nearby fairways. I wasn’t sure what I’d see in the OSD when the battery started getting low. There were a lot of wet spots out there, so when the voltage got down under 3.3 volts per cell, I started making short laps around the dry parking lot instead.

When I started the trip, it took nearly 50% throttle to maintain my cruising speed. Once it was down under 3.3 volts, I had to increase to around 60% throttle. I landed when my readings started to drop below 6 volts.

After landing, the battery recovered to 3.5 volts per cell, so it should be in quite good shape still. These lithium-ion packs can be safely discharged below 3.3 volts per cell.


This is more than I ever hoped for. I did another test with HQ 3x2.5x3 props. On that flight, I was dipping just under 6 volts right at the 16-minute mark. I was forced to land 30 seconds later. I didn’t quite make it back to home base, because at this point it was requiring more than 80% throttle just to stay in the air.

I made a good call when I stopped at nineteen minutes on that first flight.

I started a test flight on my 1606 Kestrel, but I didn’t get to finish it. I crashed into a railing at around six minutes. Then the dopey pilot managed to trip and faceplant into a short concrete wall. Maybe I’ll post some video of that!

That test got cut short when I crashed into a railing that popped up out of nowhere, but I don’t think it was going to make it to even 16 minutes. This was on the Avan 3.5x2.8x3 props.

Can you see the bobbling in the HD video?

I haven’t attempted tuning anything as of yet. The bobbling is big enough that I can plainly see it in the goggles while flying. I figured I should fly 3” and 4” props on the 1306 motors with the 2S battery for a comparison before I started messing around with things.

My Kestrel quads are tuned for 4S. Flying in that 13.5 to 16.8 volt range provides a lot more power to the motors than the 6 to 8 volt range I’m seeing on the Outcast Droneworks battery battery.

I’m confident that bumping up the PID values will make an improvement, but I am a little worried that I won’t ever see perfectly smooth video out of this setup. Tuning is the next step, though, so we’ll find out soon enough!

I’m sorry, but I had to do a little tuning

As soon as I finished typing that last paragraph, I knew I had to go straight outside and attempt to tune that bobble out of my Kestrel. I popped the Outcast Droneworks pack on the charger, pushed 500 mAh into it, and went outside. I didn’t want a full battery, but I didn’t want to tune on a dead pack, either.

I bumped up P and D on pitch and roll a few times. I bumped up my d_min quite a bit. I have a feeling d_min is the real answer here. I believe I increased all these numbers twice. By the third time, I couldn’t see the bobble in my goggles at all!

I did this tuning on the HQ 3x2.5x3 props, because that’s what I was flying last. The HD footage isn’t quite smooth, but it is getting close. This is where the tuning process gets difficult, though. To get any farther, I need to pull footage off the SD card every time I change settings, and I may even need to pull blackbox logs. That’s a lot more work than just peering through the goggles and pushing values higher in the OSD!

At least progress has been made, and it was made quickly. Next time I fly, I’ll push those numbers by another 30% or so. Maybe I’ll luck out!

What is the goal here?!

First and foremost, my Kestrels are freestyle quads. I wanted a freestyle frame with the Acrobrat’s suspension, but the Acrobrat just isn’t for me. I didn’t want to carry as much weight in the frame, and I wanted individual, replaceable, configurable arms. An extra ten grams of battery is worth more to me than ten grams of carbon, assuming I can still keep things sturdy enough that I’m not breaking an arm every week!

With the Kestrel, I wanted to design a frame that would allow me to build a 3” or 4” freestyle quad with long arms for extra stability, and I wanted to get as close to 250 grams as I could without going over. My 1306 Kestrel is 223 grams with a 650 mAh 4S, and my 1606 Kestrel is 273 grams with the same battery.

Both of my Kestrels have TBS Crossfire receivers and 800 or 1,000 mW VTX modules. One of them has a tiny GPS module, and I have a spare GPS module that’s ready to be installed on the 1306 Kestrel.

It seems like these two quads are just begging to fly long range.

Jack of all trades, master of one

Maybe. I enjoy flying freestyle with both my Kestrels, but I am never pleased with the HD footage from the Caddx Turtle. I’m also disappointed that the FPV feed is so much worse than my Runcam Eagle. Upgrading to a Runcam Hybrid would improve the HD situation a little, and nearly match the Runcam Eagle’s FPV feed.

Let’s just assume that the HD video situation will fix itself over time, either by upgrading to a Runcam Hybrid, or upgrading to whatever comes out 6 months from now. I’m quite pleased with how these quads fly, and next year, I might even be happy enough with the video they record.

My Kestrels are both excellent little freestyle quads. They record HD video. The parts cost about $250. That’s less than most GoPro cameras, and I think that’s neat. What else can they do?

As long as you can tolerate Caddx Turtle footage, my own Kestrel builds are good HD freestyle quads. I could use them in a race in a pinch. It is starting to look like they’ll be useful long-range quads, too.

I can leave my awesome, giant ThinkTank backpack at home. I can pack my Kestrel, Taranis, Fat Shark HDO2 goggles, a 19-minute long-range battery, enough 4S batteries to fly freestyle for an hour, and a chair into my small AmazonBasics DSLR backpack. This comes in at around 10 pounds, and that’s a lot less less than half the weight of my fully loaded big bag!

There’s room in my Kestrel frame to use a battery strap to mount a GoPro. Both my builds fly well enough while carrying a GoPro, and I even managed to hover with the 2S pack and GoPro. I don’t currently have any sort of prop guards, but I’m absolutely certain that my Kestrel builds would do a fine job doing the duties of a Cinewhoop.

Zoe and I are working on very different things

Zoe’s MicroHawk is a tiny quad running 2” props. It is built with autonomy in mind, and I believe her build comes in at around 180 grams with the 3250 mAh 2S pack. That’s about 70 grams lighter than my current long range setup! She’s managed a 26-minute hover test, and 20 minutes of cruising.

I guess we’re attacking a similar problem here, but from different directions. I’m absolutely certain her autonomous, long-range MicroHawk build that carries a Raspberry Pi can fly freestyle. That said, I’m willing to bet that my Kestrels will capture cleaner, smoother HD freestyle footage than Zoe’s MicroHawk. Not only that, my comparatively beefy Kestrels are made to take quite a beating!

The reverse is also true. Sure, I can shoehorn a set of 4” biblade props onto my Kestrel and throw an Outcast Droneworks 3250 mAh 2S pack on there. Sure, it’ll manage to cruise for 19 minutes, but that’s a total accident. I’m lucky that this worked out at all!

Everything on my Kestrel is scaled for 4S freestyle. The motors and props are both scaled to be driven by a 16-volt LiPo battery. It is severely underpowered when using a 7-volt lithium-ion battery.. I can probably take it on a cruise up and back down the side of a mountain, but I shouldn’t be doing powerloops and S-turns on the way up. My Kestrel on 2S feels like an overgrown whoop. It can manage to do a powerloop, but is it a good idea?!


I’m excited again. Since I’m always unhappy with the Turtle’s HD footage, my Kestrels have spent more time in my bag than in the air. I really only fly them just to make sure they still fly. I have a good reason to be flying, testing, and tuning my Kestrels again! Woo!

What’s next? I think it is time to try out Betaflight’s RPM filter on the blheli_s ESCs on my Diatone Mamba stacks. I don’t know if it will clear out the last of the vibrations, but it is something I’ve wanted to try on my Kestrels anyway. If that doesn’t work, maybe it will be time to do some blackbox logging and manual filter tuning!

I know I stated that I’m rarely happy with my Turtle footage, but I was just looking for flight snippets to include in this blog, and I realized that I rarely even post short flight snippets to Twitter or Instagram. Just about the only recent footage I’ve posted is the 2S testing. I’m going to have to correct that!

You’ll probably notice that I rarely post YouTube videos with Kestrel HD footage. Scaling the footage down to Instagram or Twitter covers up a lot of what I don’t like about the footage!

What do you think? Are you wanting to do some long-range flying, but you’re living somewhere with a 250-gram weight restriction? Are you building a customized long-range setup, or are you trying to squeeze long-range flights out of a freestyle build like me? Do you think I’m on the right track with these HQ 4x2.5x2 props and the Outcast Droneworks 3250 mAh battery? Let me know in the comments, or stop by the Butter, What?! Discord server to chat with me about it!