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Daniel Johnston Aerospace engineering — guidance, navigation and control

Aircraft design and manufacture

VTX-1 Pathfinder

A fixed-wing surveillance UAV, sized from first principles and built from a parametric twin.

The aircraft is the deliverable, but the pipeline is the point: a sizing calculation that feeds a parameter file, that drives a generator, that rebuilds all 34 bodies of the airframe on one command. Change the battery and the centre of gravity re-solves itself.

A plan-view drawing of the VTX-1 Pathfinder, nose up, generated from its parametric twin: a straight, constant-chord wing spanning the full width, with ailerons along its outer panels; two motors and propeller discs just ahead of the leading edge; the camera, video transmitter and flight controller on the centre line ahead of the wing; the two batteries, dashed, under the wing, with the centre of gravity marked between them; and a carbon boom running back to the V-tail.
1500 mm span — Clark Y, AR 8.0
Fig. 1 A twin-motor V-tail aircraft for autonomous area survey, sized in MATLAB, generated as a fully parametric digital twin, and printed and assembled from that twin.
All-up weight
1368 g

Against a 1500 g cap, from 13 of 13 measured component masses

Endurance at cruise
34.7 min

Computed on measured masses and a modelled drag polar

Centre of gravity
33.00 % MAC

Inside a 32 to 37.5 % window, solved rather than trimmed

Regenerated on one command
34 bodies

The digital twin rebuilds from the parameter file with zero clashes

Measured against limits

All-up weight 1368 g
0 1600 g

132 g of margin against a hard cap, on physically measured masses.

Centre of gravity 33.00 % MAC
25 50 % MAC

The window was re-cut once the neutral point was found at 47.5 % MAC; the original 25 to 30 % target was arithmetically incompatible with the static margin it was meant to deliver.

Sizing before geometry

Nothing was drawn until the numbers closed. A MATLAB sizing model takes the mission — a lawnmower pattern over a paddock, camera on the ground, return on low battery or lost link — and the hardware already owned, and solves for the wing that makes it work.

The planform that came out is deliberately dull: 1500 mm span, constant 187.5 mm chord, aspect ratio 8, Clark Y section. A constant chord means one rib design instead of thirteen, which matters a great deal when every rib is printed.

A MATLAB plot of the aircraft drag polar and power required against airspeed, with the cruise point marked.
Fig. 2 Power required against airspeed. Cruise sits at 10.8 m/s and 61 W, of which 23 W is avionics.

The parametric twin

The sizing model writes a parameter file; a Python generator reads it and rebuilds the entire Fusion model from scratch. One command wipes and regenerates 34 bodies across 20 components, checks for clashes, and cross-checks its own mass manifest against what Fusion reports.

Numbers flow downstream only, and that rule is the whole value of the pipeline. Editing a dimension in the CAD is not a shortcut — it is a change that the next regeneration silently discards. If a number is wrong it gets fixed in MATLAB and the aircraft is rebuilt.

  • The manifest and the Fusion model agree to 0.01 g and 0.02 mm.
  • The battery sits exactly on the centre of gravity, so adding the second pack is trim-neutral and takes computed endurance from 17.4 to 34.7 minutes.
  • Zero clashes across the regenerated assembly.
The Fusion 360 digital twin of the airframe, showing printed ribs, carbon spars, the electronics bay and the tail boom.
Fig. 3 The generated twin. Every body in this model came from the parameter file.

A problem the twin caught

The aileron servos sit in the wing, on the rib that already carries the motor, laid flat so the output shaft drives the hinge line directly — no bellcrank. An upright servo cannot fit the 19.9 mm deep section; the twin's section-fit check caught that, after an earlier model had left the servos floating 50 mm below the wing.

Moving them there cost nothing. Solved on the same clearance budget, the wing needs 0 mm of fuselage stretch; a fuselage bay needed 45.2 mm plus 10 g of linkage.

Where it stands

The airframe is generated and the component masses are measured. What remains is manufacture, then the flight test and autonomy programme that the whole thing was sized for.

  • Done — requirements and sizing, component measurement, the parametric twin.
  • In progress — the detail CAD needed to close the external surface for flow simulation.
  • Not started — printed structure, wiring loom, flight test.