Part 1: Development Log

Chapter 1: The Glider Concept

The Amihan began as a foot-launched glider — a 42-foot span sailplane light enough for a single pilot to carry to a hillside and launch under human power alone. The design target was 385 lb gross weight, with structural limits of +4G/-1.5G. The wing section chosen was the NACA 65(2)-415, a laminar-flow profile that rewards a clean build with low cruise drag. The span was set at 42 feet — long enough to give a glide ratio in the region of 30:1, and short enough to be trailerable in two panels. Chord is constant at 4 feet across the full span, keeping tooling and rib design simple and the build achievable outside a factory environment.

Chapter 2: The Spar Decision

The first major structural decision was the spar. The obvious choice for a long-span composite wing is a hollow box beam — efficient, light, well understood. But the Amihan has a specific problem at the root: the wing panels are removable, plugging into a centre-section socket for trailering. That socket joint is a stress riser. A hollow box beam, with its load concentrated in thin cap strips, is vulnerable there — a crack or delamination at the socket mouth can propagate quickly through the cap with little warning.

The decision was made to use a solid-core spar instead: a foam core wrapped in biaxial glass, with unidirectional carbon cap laminates building up the bending strength. The solid core distributes the socket bearing stress across the full spar cross-section rather than concentrating it in the caps. It is heavier than an optimised box beam, but it survives the stress riser gracefully and is far more forgiving of the load path disruption at the root joint.

To validate the design before committing carbon to a full wing, the 1000 Pound Beam Challenge was established: build a test beam to the same cross-section and layup as the Amihan spar, span it over 40 feet, and load it to 1000 lb at mid-span.

Chapter 3: The Slat Decision

Early in the design process the wing acquired automatic leading-edge slats — Helio Courier style, aerodynamically actuated with no pilot input required. At low angle of attack, aerodynamic pressure holds the slat closed against the leading edge. At high angle of attack, a suction shift pulls it forward and down on its tracks, opening a slot that re-energises the boundary layer and delays the stall.

The slats run the full span in two sections per wing, each section with its own spanwise idler torque tube connecting its two tracks. The idler tube keeps the slat from cocking or skewing as it deploys; it carries no aerodynamic load, that is handled by the tracks themselves. Because the two sections are independent, their deployment threshold can be tuned separately — the outboard section can be set to deploy at a slightly lower angle of attack than the inboard, so the tip keeps flying and aileron authority is retained while the root stalls first. This is the classic safe-stall progression.

The main wing leading edge under the slat was developed in foam. Once the slat is deployed and the new leading edge is exposed, the main element profile becomes much closer to a Clark Y — blunt, forgiving, good lift at low Reynolds numbers. The NACA 65(2)-415's fine laminar nose handles cruise; the Clark Y-like main element handles the low-speed end. Good lift at low Reynolds numbers.

Chapter 4: The One-Wing Realisation

At some point in the design process it became clear that the Amihan wing — 42 feet, constant 4-foot chord, NACA 65(2)-415, full-span slats — was capable of carrying far more than a foot-launched glider. The same geometry, with a stronger spar and a proper root fitting, could support a powered aircraft up to 2,000 lb gross weight. The wing did not need to change. Only the spar and the fuselage behind it needed to grow.

This led to the modular platform concept: one wing design, two aircraft. The Amihan Glider is the foot-launched sailplane the project started as. The Amihan STOL is a side-by-side two-seat powered aircraft in the Helio Courier tradition — short takeoff and landing, floats or wheels, a 200hp UL Power engine up front. Both aircraft share the same wing geometry, the same rib tooling, the same skin moulds, and — critically — the same root pin geometry, so the carry-through never changes and swapping between aircraft is a matter of changing the spar.

Chapter 5: The Amihan STOL

The powered aircraft target is a Helio Courier-style STOL machine: high cantilever wing, side-by-side seating, carry-through above the cabin, fixed or amphibious undercarriage, capable of operating from short grass strips or water. The gross weight target is approximately 1,200 lb, with the structure designed to the 2,000 lb case for margin and room to grow.

The Helio Courier's STOL performance comes not from brute power but from aerodynamics: full-span leading-edge slats, large Fowler flaps, and full-span drooping ailerons working together to give a stall speed in the low 30s at full gross weight. The Amihan STOL aims for the same approach — the slats are already designed in, the wing area is generous, and the UL Power 520iS gives 200hp at modest weight.

The 42-foot span exceeds the Helio's 39 feet. At 1,200 lb gross with 168 square feet of wing area, the wing loading is low enough that short-field performance should come naturally, without heroic flap deflections or dangerously low approach speeds.

Chapter 6: The Two-Spar Evolution

The glider spar and the STOL spar serve the same wing geometry but face very different loads. The glider case — 385 lb gross, +4G ultimate — produces a root bending moment in the region of 6,000–8,000 ft-lb. The STOL case — 2,000 lb gross, +3.8G ultimate, 1.5 safety factor — produces a root bending moment roughly eight times larger, in the region of 60,000 ft-lb.

A single spar sized for the STOL case would be far heavier than the glider needs. Two spars, sharing the same root pin geometry but differing in layup thickness, is the right answer.

The glider spar uses the solid-core construction established in the spar decision chapter — foam core, biaxial glass wrap, unidirectional carbon caps. The cap layup runs to approximately 8 plies at the root, tapering to 1 ply at the tip. The STOL spar uses a hollow box beam with carbon caps — more structurally efficient at the higher load case — with approximately 24 plies at the root tapering to 1 ply at the tip. Both spars use Divinycell H80 foam core in the web panels, with a solid carbon laminate root plug over the last 8–10 inches of each web panel where the root fitting bears.

Chapter 7: The Root Fitting

The root fitting went through several iterations before settling. The requirements are demanding: it must transfer bending, shear, drag, and torsion from a composite spar into a metal carry-through, survive a saltwater float environment, remain fully inspectable after assembly, and be common in pin geometry between the glider and STOL spars.

The settled design is an inverted-U aluminium fitting for the glider spar and a fabricated 4130 steel fitting for the STOL spar. Each fitting wraps over the top of the spar cap, with legs dropping down the web faces on each side. The attachment lug is machined into the web leg at the elevation that puts the pin centreline at cap mid-thickness — the cap's line of action — so there is no eccentricity in the load path.

The fitting is bonded to the cured spar with Plexus MA550, backed by AN5 cap-clamping bolts at 100–140 in-lb torque pulling the fitting flanges against both cap faces. The wing attachment pin is 5/8-inch 4130 steel, torqued as a friction-grip bolt at approximately 57 ft-lb. All root hardware is steel throughout.

Drag and torsional loads are carried by a separate leading-edge tang — an extension of the D-nose skin and stringer system that protrudes into the fuselage ahead of the main spar, engaging a pocket in the fuselage frame with its own pin. This separates the load paths explicitly: the main spar pins carry bending only, the tang pin carries drag and torsion only. Neither joint is asked to do everything at once.

The fuselage carry-through is a short aluminium beam spanning between two fuselage frames above the cabin, with steel lug ears at each end engaging the spar root lugs. Wing removal is straightforward: disconnect controls and wiring at the root, pull the tang pin, pull the upper and lower spar pins, and lift the wing clear. Swapping between aircraft is a matter of pulling three pins and lifting the wing clear.

Chapter 8: Where It Stands Now

The Amihan project is well-resolved on paper and in foam models. The spar layup schedules are worked out. The root fitting geometry is settled. The slat sections have been modelled in foam and the main-element profile confirmed. The carry-through concept is defined. The shared rib tooling, skin moulds, and root hardware between the glider and STOL give the modular platform concept real teeth — both aircraft are genuine builds from the same set of tools and fixtures, not a theoretical future option.

The build programme has not yet cut carbon. The plan is to build the glider spar first, validate it against the 1000 Pound Beam Challenge result, and work outward from there — ribs, D-nose, skin panels, root fittings — before the STOL spar is started. The glider will fly before the STOL is built. That is the right order: prove the wing on the simpler aircraft, then scale the structure up with confidence.

What remains open is mostly detail work: the exact rib spacing plan for the powered wing, the slat track geometry and deployment arc, the fuselage cross-section and carry-through sizing for the STOL, and the engine mount and systems layout. These are tractable problems. The hard decisions — spar type, root joint philosophy, high-lift system, modular platform architecture — are made.

PART 2 — SPECIFICATION SHEET: AMIHAN GLIDER

Airframe

Type: Foot-launched single-seat sailplane

Span: 42 ft (12.8 m)

Chord: 4 ft (1.22 m), constant

Aspect ratio: 10.5

Wing section: NACA 65(2)-415

Wing area: 168 sq ft (15.6 m²)

Weights

Gross weight: 385 lb

Empty weight: ~100 lb

Pilot weight range: 140–200 lb (design pilot 170 lb)

Structural

Load limits: +4G / -1.5G

Ultimate load factor: +6G / -2.25G

Spar: Solid foam core, UD carbon caps (8 plies/3.2 mm at root), biaxial glass web

Root fitting: Machined 7075-T6 aluminium, inverted-U, bonded with Plexus MA550

Wing attach pin: 5/8 in 4130 steel, friction-grip, ~57 ft-lb

Cap clamping bolts: AN5, 100–140 in-lb

High-lift system

Automatic leading-edge slats, full span, two sections per wing

Slat chord: ~10% of wing chord (~4.8 in)

Independent torque tube per section

Wing fence at mid-span section break

Main element profile under slat: Clark Y character

Performance (estimated)

Stall speed (clean): 31 mph

Glide ratio: ~30:1

Assembly: Two panels, root lug-and-pin, three pins per wing

---

PART 3 — SPECIFICATION SHEET: AMIHAN STOL

Airframe

Type: Side-by-side two-seat STOL aircraft

Span: 42 ft (12.8 m)

Chord: 4 ft (1.22 m), constant

Aspect ratio: 10.5

Wing section: NACA 65(2)-415 with full-span automatic slats

Wing area: 168 sq ft (15.6 m²)

Configuration: High-wing cantilever, carry-through above cabin

Weights

Design gross weight: 2,000 lb (structure designed to this case)

Target operating gross weight: ~1,200 lb

Load limits: +3.8G / -1.5G (utility category)

Structural

Spar: Hollow box beam, UD carbon caps (24 plies/9.6 mm at root), biaxial carbon web

Web: Six 0.5 in foam-core panels bonded in three pairs, solid carbon root plug over last 8–10 in

Root fitting: Fabricated 4130 steel, inverted-U, bonded with Plexus MA550

Wing attach pin: 5/8 in 4130 steel, friction-grip, ~57 ft-lb

Cap clamping bolts: AN5, 100–140 in-lb

Carry-through: Short aluminium box beam above cabin, steel lug ears

Powerplant

Engine: UL Power 520iS, 200 hp, fuel-injected, air-cooled

Dry weight: ~185 lb

High-lift system

Automatic leading-edge slats, full span, two sections per wing

Independent torque tube per section

Wing fence at mid-span section break

Performance (estimated)

Stall speed: Low 40s mph

Cruise speed: 150 knots

Undercarriage: Wheels standard, floats possible

Assembly: Root lug-and-pin, three pins per wing, shared pin geometry with glider spar