Project-X8: The Joys of AUW, Maths, and a Lucky Motor

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Every RC plane build eventually runs into the AUW question:

“How heavy is this thing actually going to be… and will the motor laugh at me, or cry?”

For Project-X8, that meant sitting down with a notepad, a scale, and just enough optimism to believe the numbers would work out.


AUW = All-Up Weight = everything that actually leaves the ground.

Not just the bare airframe. Not just the shiny motor. We’re talking the whole shebang:

  • Airframe
  • Wings (plus extensions)
  • FPV gear
  • Autopilot + sensors
  • Batteries (the big one!)
  • Reinforcements (carbon spars, fiberglass)
  • Random extras we’ll swear are “essential” later

It’s the difference between a plane that floats gracefully and one that lands like a sack of wet cement.


Here’s how the numbers break down:

ComponentWeight (g) estimatesNotes
Skywalker X8 bare airframe (fuselage + stock wings)1,200 gbaseline
Wing extensions (2 × 80 mm at root)200 gfoam inserts
Fiberglass cloth + polyurethane varnish400 gthin but tough skin
Carbon spars (2 × 4 mm square × 800 mm)80 grigidity boost
Servos (5 × 30 g each)150 gelevons + rudder/stab
Autopilot (BeagleBone Blue + sensors, GPS, etc.)250 gincludes wiring
Secondary GPS + backup sensors100 gredundancy FTW
RX5808 diversity receivers + LCD150 gground-based gear mounted onboard
FPV gear (camera, VTX, antenna)200 g5.8 GHz setup
LEDs, buzzer, odds & ends100 ghot glue not included… yet
Battery pack (6S 10,000 mAh LiPo)1,500 gthe big weight
Mounting hardware, tape, velcro100 gnever underestimate velcro

Subtotal: 4,430 g

Add in ~5% “builder’s margin” for wires, connectors, and over-enthusiastic glue = ≈4,650 g AUW


Our motor: SunnySky X4110S (because, honestly, we already had one sitting on the shelf).

  • On a 6S with a 15×8 prop: ~2.8 kg thrust
  • Thrust-to-weight ratio = 2.8 ÷ 4.65 = 0.6 : 1

That might not sound like fighter-jet territory, but remember:

  • The X8 is a cruiser, not a drag racer.
  • A T:W of ~0.5–0.7 is perfect for efficient, long-endurance flight.
  • The wide wing gives plenty of lift to carry the load.

In other words, by sheer luck (and garage inventory), the motor fits the bill almost perfectly.


AUW isn’t just a scary number. It tells us:

  • Whether the motor/prop combo is enough
  • Whether we can realistically expect 30–60 minutes of flight
  • Whether the wings can take it (thanks carbon spars)

Without it, you’re basically throwing a foam brick and hoping physics feels generous that day.


Project-X8’s AUW came together with a mix of careful weighing, rough maths, and good fortune. And the motor? Well, sometimes the best engineering decision is just already owning the right part.

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