Unit 4: Fabrication of non-powered Gliders

I. Principles of Non-Powered Gliding

A non-powered glider is an aircraft that flies without an engine, propeller, or onboard thrust source. After a hand launch, gravity supplies the energy for forward motion: the glider descends gradually while its wings generate lift. Successful fabrication therefore depends on low mass, accurate alignment, adequate stiffness, smooth surfaces, and stable balance.

• Four forces: A glider experiences weight, lift, drag, and temporary launch force.
  • Weight (W): Acts vertically downward through the centre of gravity (CG).
  • Lift (L): Acts approximately perpendicular to the flight path.
  • Drag (D): Acts opposite the glider’s motion through the air.
  • Launch force: Gives the glider its initial speed but disappears after release.
• Lift relationship: Wing area and flight speed strongly influence lift.

L = ½ρV²SCL

• L = lift in newtons (N)
• ρ = air density in kilograms per cubic metre (kg/m³)
• V = airspeed in metres per second (m/s)
• S = wing planform area in square metres (m²)
• CL = dimensionless lift coefficient
  • Glide ratio: In still air, glide ratio is approximately horizontal distance divided by height lost.

Glide ratio = horizontal distance travelled ÷ vertical height lost

A glider covering 20 m while descending 2 m has an approximate glide ratio of 10:1.

• Wing loading: The ratio W/S compares aircraft weight with wing area. A light glider with a large wing generally flies more slowly than a heavy glider of equal area.
• Longitudinal stability: The CG must lie ahead of the neutral point. For a simple straight-wing model, 25–30% of the mean wing chord behind the leading edge is a useful initial CG range, followed by test-flight adjustment.
• Directional and lateral stability: The vertical fin resists unwanted yaw, while wing dihedral—the upward angle of the wings—helps the model recover from a bank.
• Construction conventions: Both wings should have equal span, incidence, mass, and dihedral. The tailplane should be square to the fuselage and the fin should be vertical.
• Safe operation: Testing requires an open indoor hall or calm outdoor field, eye protection during cutting, and launches directed away from people, roads, animals, and electrical lines.

II. Plastic Glider — Durable Sheet-and-Beam Construction

A. Purpose and Principle

A plastic glider uses lightweight plastic components to create a moisture-resistant, impact-tolerant airframe whose stiffness is sufficient to preserve aerodynamic alignment.

• Suitable materials: Thin polypropylene sheet, corrugated plastic, or closed-cell plastic foam can form the flying surfaces; a straight drinking straw or lightweight plastic channel can serve as the fuselage.
• Material selection: Flexible film alone is unsuitable because it changes shape under airflow. A wing requires a stiff sheet, a folded section, corrugations, or a spar to prevent twisting.
• Typical components: The airframe consists of a main wing, fuselage, horizontal stabilizer, vertical fin, nose ballast, and adhesive joints.
• Planform choice: A rectangular wing is easy to measure and balance. For example, a 300 mm × 60 mm wing has an area of 18,000 mm², or 0.018 m².
• Adhesive compatibility: Low-temperature hot glue, double-sided tape, or plastic-compatible contact adhesive may be used in small quantities. Excess glue adds weight and can distort thin foam.

B. Fabrication of non-powered glider and flying using plastic

Fabrication requires transferring an accurate pattern to plastic, assembling the parts symmetrically, balancing the model, and improving its flight through controlled tests.

• Pattern preparation: Mark the wing, tailplane, and fin with a ruler and fine marker before cutting.
  • A representative classroom model may use a 300 mm wing span, 60 mm wing chord, 140 mm × 40 mm tailplane, and 55 mm-high fin.
  • Rounded corners reduce sharp edges, but left and right outlines must remain identical.
• Cutting procedure: Place the sheet on a cutting mat and use scissors or a craft knife with a straightedge.
  • Make several light knife passes instead of one forceful cut.
  • Keep fingers behind the cutting direction and obtain supervision when using blades or hot glue.
• Wing formation: Keep the wing flat or add a shallow camber by gently curving the sheet over a cylindrical object.
  • Camber must be smooth and equal on both sides; a crease can create excessive drag.
  • Add dihedral by raising each wingtip about 10–15 mm while securing the centre joint or fuselage attachment.
• Fuselage assembly: Use a straight plastic straw, tube, or narrow folded channel as the longitudinal beam.
  • Attach the main wing near the forward-middle portion.
  • Position the tailplane at the rear with its leading and trailing edges perpendicular to the fuselage centreline.
• Tail installation: Fix the vertical fin along the fuselage centreline and at 90° to the tailplane.
  • A tilted fin produces continuous yaw.
  • A twisted tailplane creates unequal lift and may cause a spiral turn.
• Joint control: Apply only small adhesive dots or narrow tape strips. Check alignment before the adhesive hardens because a misplaced joint is more damaging than a minor surface imperfection.
• Balancing: Mark a preliminary CG at approximately one-quarter to one-third of the wing chord behind the leading edge.
  • For a 60 mm chord, begin around 15–18 mm behind the leading edge.
  • Support the glider under both wing roots at this line; add modelling clay to the nose until it balances level or slightly nose-down.
• Initial flying test: Hold the model near its CG, point it level, and release it with a gentle forward push from shoulder height.
  • Launching upward converts speed into height and often produces a stall.
  • Throwing hard can conceal trim faults and damage the model.
• Flight correction:
  1. Stall or repeated climb-and-drop: Move the CG forward by adding a very small amount of nose clay or reduce excessive upward tail deflection.
  2. Steep dive: Remove some nose ballast or add a slight upward adjustment to the trailing edge of the tailplane.
  3. Persistent turn: Check wing symmetry first; then make a tiny rudder correction opposite the turn.
  4. Spiral descent: Inspect dihedral, wing twist, fin alignment, and left-right mass balance.
• Controlled adjustment: Change only one feature per flight and use increments of approximately 1 mm for control-surface bends or small clay pieces for ballast. This links each change to its observed effect.

C. Applications and Limitations

Plastic gliders are especially useful where repeated handling and rapid prototyping are more important than minimum structural mass.

• Advantages: Plastic resists moisture, survives minor impacts, and can be cleaned. Reusable templates also make multiple models reasonably consistent.
• Limitations: Dense sheet raises wing loading, while thin sheet may flutter or warp. Some plastics are difficult to glue because of their low-energy surfaces.
• Environmental consideration: Offcuts should be collected for reuse or appropriate recycling; small plastic scraps must not be left at the flying site.
• Performance emphasis: A plastic design generally benefits from generous wing area, minimal adhesive, and reinforced high-stress points rather than uniformly thick material.

III. Balsa-Wood Glider — Lightweight Grain-Oriented Construction

A. Purpose and Principle

A balsa glider uses the wood’s low density and favourable stiffness-to-mass ratio to produce a light, efficient aircraft, but its performance depends on correct grain direction and careful handling.

• Material character: Balsa is a soft, porous hardwood available in sheets and strips of different densities. Two pieces of equal dimensions may therefore have noticeably different masses.
• Grain orientation: Wood is strongest and stiffest along the grain.
  • Wing grain should normally run from wingtip to wingtip.
  • Fuselage-strip grain should run from nose to tail.
  • Cross-grain parts are more likely to split during launch or landing.
• Component matching: Similar-density wood should be selected for the left and right wing panels so that one side is not heavier.
• Surface finish: Sanding removes rough fibres and improves shape, but excessive sanding weakens thin parts and can create asymmetry.

B. Fabrication of non-powered glider and flying using balsa wood

A balsa glider is fabricated by cutting or sanding accurately oriented wooden parts, joining them without excess adhesive, and trimming the completed model through gentle glide tests.

• Template transfer: Trace the design lightly onto balsa sheet, clearly marking the centreline, CG reference, and grain direction.
  • A simple model may use a 350 mm × 55 mm wing, a 160 mm × 35 mm tailplane, and a strip fuselage approximately 300 mm long.
  • Exact dimensions may vary, but the tail must be smaller than the main wing and mounted well behind it to provide stabilizing leverage.
• Cutting and shaping: Cut slightly outside the marked line with a sharp modelling knife, then sand to the final outline using a block.
  • Support thin balsa close to the cut to prevent splitting.
  • Sand both wing halves together when possible so their outlines match.
• Airfoil preparation: Round the wing’s leading edge and taper the trailing edge gently.
  • The leading edge should remain thicker and smoothly curved.
  • The trailing edge should be thin but not fragile; abrupt or unequal sanding can cause unwanted roll.
• Dihedral construction: If the wing has two panels, sand the root faces to the required angle and glue them while the tips are supported equally.
  • With the centre resting flat, equal supports—for example 20 mm beneath each tip—produce symmetrical dihedral.
  • Reinforce only the centre joint if necessary; heavy reinforcement across the wing reduces the weight advantage.
• Fuselage and tail assembly: Glue the wing and stabilizers to a straight balsa strip or shaped fuselage.
  • Use a set square to place the tailplane perpendicular to the fuselage.
  • View the model from the front and rear to detect wing or tail tilt before the glue sets.
• Adhesive discipline: Use a thin film of wood glue or a small amount of suitable model adhesive. A large glue bead adds mass far from the intended design and can alter balance.
• Balancing procedure: Begin with the CG near 25–30% of the wing chord behind the leading edge.
  • For a 55 mm chord, this corresponds to roughly 14–17 mm.
  • Add a small clay weight, metal washer, or shaped nose block rather than shortening the tail without redesigning the model.
• Hand-launch technique: Grip the fuselage close to the CG and release the glider horizontally into still air with a smooth, gentle push.
  • Conduct initial tests over grass or indoors to reduce impact damage.
  • Repeat launches from the same height and with similar force so flight changes reflect trimming rather than inconsistent throwing.
• Trimming sequence:
  1. Pitch first: Correct stalls or dives with minute ballast changes and very small elevator adjustments.
  2. Turn second: Remove warps and check lateral balance before bending or offsetting the rudder.
  3. Glide refinement: Once stable, smooth rough surfaces and reduce unnecessary mass without shifting the CG.
• Warp correction: Mild warps may sometimes be corrected by carefully holding the part straight while applying controlled moisture or warmth, followed by drying under restraint. The part must be fully dry before rechecking alignment.
• Damage inspection: After a hard landing, examine the wing centre joint, fuselage grain, nose, and tail attachment. Even a fine crack can change stiffness and produce inconsistent flight.

C. Applications and Limitations

Balsa construction supports high-performing educational gliders but demands greater precision and care than plastic construction.

• Advantages: Low mass reduces wing loading, while sandable surfaces permit accurate aerodynamic shaping. Wood glue also forms reliable joints on clean balsa.
• Limitations: Balsa dents, splits, absorbs moisture, and can warp. Thin wings may require a spar or careful density selection for adequate rigidity.
• Material efficiency: Parts should be arranged on the sheet to preserve correct grain direction while minimizing waste; structural offcuts can become gussets or nose blocks.
• Performance emphasis: The best balsa glider is not simply the lightest one—it must remain stiff, aligned, correctly balanced, and strong enough to tolerate launch and landing loads.