2.5-inch toothpick: 118 grams of open-prop agility
A toothpick is what you get when you take the ducts off a whoop and let the props breathe. At 118 g on 3S it out-climbs a 3-inch cinewhoop that weighs three times as much, and it does it without the 250 g paperwork because it never gets close to the line.
Published By The Rotor Desk 120 mm · 118 g · 4.1 min
| Wheelbase | 2.5 in / 120 mm (120 mm) |
|---|---|
| Motors | 5000 KV |
| Props | 2.5x2.5x3 |
| ESC | Bluejay · DShot300 |
| VTX | 400 mW · 5.8 GHz |
| Link | ExpressLRS 2.4 GHz · 250 Hz |
| AUW | 118 g |
| Flight | 4.1 min |
| Thrust to weight | 13.56 : 1 |
| Rate limit | Actual 700 deg/s · expo 0.45 |
| Filters | PT1 180 Hz / PT1 170 Hz |
| Dynamic notch | Biquad 150-450 Hz |
| RPM filter | On (bidirectional DShot) |
Rates
- Max rate — Actual 700 deg/s · expo 0.45
- Expo — 0.45 · centre sensitivity 240
- Throttle mid — 0.52 · expo 0.05
- Filters — PT1 180 Hz / PT1 170 Hz / Biquad 150-450 Hz
Tuning pass
- Set Bluejay to 48 kHz with DShot300 and confirm all four motors spin up evenly.
- Enter roll P 66 / I 120 / D 30 and pitch P 70 / I 126 / D 32, then hover for 20 seconds.
- Set the gyro lowpass to 180 Hz and the dynamic notch to 150-450 Hz.
- Set 700 deg/s with 0.45 expo, a throttle mid of 0.52 and 0.05 throttle expo.
- Set failsafe to Stage 2 with a 0.25 second delay and a hard drop.
- Fly three packs and feel the motors after each; a warm motor at this weight means the filters are too open.
- Test a 650 mAh pack and compare the turn feel before deciding which pack to standardise on.
Assembly order
- Weigh the frame, motors and stack separately; at 118 g you need to know where every gram is.
- Solder the motors directly to the ESC without connectors.
- Mount the VTX on double-sided tape rather than a bracket to save 4 g.
- Fit the camera and check that the props are not visible at a 30 degree tilt.
- Set the board alignment and the motor direction before fitting the props.
- Set the battery profile to 3S 450 mAh and the voltage warning to 3.5 V per cell.
- Fly a range check at 100 m and confirm the link before flying near people.
Ducts off, airflow back
Removing the ducts costs about 30 g on a 2.5-inch frame and returns roughly 20 percent more thrust at forward speed. The trade is that nothing protects the props or the people around them, so this is an outdoor park aircraft, not a living room one. It also gets noticeably louder: 82 dB at 1 m against 71 dB for the ducted version.
1204 at 5000 KV on 3S
1204 at 5000 KV on 3S turns a 2.5x2.5x3 at about 31000 RPM at full throttle and draws 17 A for the aircraft. At a cruise of 45 percent throttle the current is 5.4 A, which gives 4.1 minutes from a 450 mAh pack. On 4S the same motor pulls 24 A and the flight drops to 2.9 minutes; toothpicks want 3S.
Tuning an airframe this light
At 118 g the aircraft has almost no rotational inertia, so P values that would be soft on a 5-inch are stiff here. Roll P 66 and pitch P 70 with D at 30 and 32 is the practical ceiling; go higher and every throttle change produces a visible twitch. The filters matter more than the PID: a 180 Hz gyro lowpass keeps the 2.5-inch prop noise out of the D term, which is what stops the motors from getting hot after three packs.
Rates and the throttle curve
700 deg/s with 0.45 expo suits the way a toothpick flies: quick direction changes with short throttle bursts. Set the throttle mid to 0.52 with only 0.05 expo so the hover point sits near the middle of the stick; a toothpick that hovers at 30 percent stick is hard to keep at a constant altitude in wind.
Battery life is the real cost
A 450 mAh 3S pack costs about 42 g and gives 4.1 minutes. Because the aircraft is so light, the pack is 36 percent of the all-up weight, which means pack choice drives the feel more than anything else. A 650 mAh pack adds 24 g and 70 seconds of flight but makes the aircraft noticeably more sluggish in a turn.
Build parts
- Frame - 120 mm toothpick, 3 mm arms, no ducts
- Motors - 4 x 1204 5000 KV
- ESC - 4-in-1 25 A, Bluejay 0.21
- Flight controller - F4 20x20 mm, 2.5 g
- Props - 2.5x2.5x3
- VTX - 5.8 GHz 400 mW, 20x20 mm
- Receiver - ExpressLRS 2.4 GHz, 250 Hz
- Battery - 3S 450 mAh 80C LiPo
- Camera - micro analogue, 1/3 in sensor, 2.1 mm lens
Common mistakes
- Running 4S on a 3S toothpick and cooking the motors in two packs.
- Flying an open-prop toothpick indoors or near bystanders; nothing protects the props.
- Raising P to sharpen the response and getting a twitch on every throttle change instead.
What matters
- Removing the ducts saves about 30 g and returns roughly 20 percent more thrust at speed.
- 1204 5000 KV on 3S cruises at 5.4 A for 4.1 minutes on a 450 mAh pack.
- Roll P 66 and pitch P 70 are the practical ceiling at 118 g.
- A 180 Hz gyro lowpass is what keeps the motors cool on a 2.5-inch build.