Unit 4: Fabrication of non-powered Gliders - Subjective Questions

A non-powered glider is an aircraft that flies without an engine or motor. It gains initial speed from a hand launch, tow, or release from a height.

The four principal forces are:

• Weight: Acts vertically downward through the centre of gravity.
• Lift: Acts approximately perpendicular to the airflow and supports the glider.
• Drag: Opposes the forward motion of the glider.
• Thrust: There is no continuous powered thrust. After launch, a component of the glider's weight along its descending flight path maintains forward motion.

During a steady glide, lift balances most of the weight, while the forward component of weight balances drag. Therefore, the glider gradually loses height as it moves forward.

The main parts of a non-powered glider are:

• Fuselage: Forms the main body and connects all other components.
• Main wing: Produces most of the lift required for flight.
• Horizontal stabilizer: Provides pitch stability and helps prevent the nose from moving excessively up or down.
• Vertical stabilizer or fin: Provides directional stability and reduces unwanted yaw.
• Nose: Often carries ballast and protects the front of the fuselage.
• Control surfaces or adjustable tabs: If provided, they allow small trimming corrections.

Correct alignment and proportioning of these parts are essential for stable and efficient gliding.

Typical materials and tools include:

• Lightweight plastic sheet or foam-plastic sheet: Used to make the wing and tail surfaces.
• Plastic strip, straw, or lightweight rod: Used as the fuselage.
• Adhesive tape or plastic-compatible glue: Joins the components.
• Modelling clay or reusable putty: Provides adjustable nose ballast.
• Paper template: Ensures accurate and symmetrical component shapes.
• Ruler and marker: Used for measurement and layout.
• Scissors or a craft knife: Used for cutting the plastic.
• Cutting mat: Protects the working surface.
• Sandpaper: Smooths rough edges when the type of plastic permits sanding.

All tools should be used safely, and cutting tools should be handled under appropriate supervision.

A plastic glider can be fabricated as follows:

1. Prepare a design: Draw or obtain templates for the wing, horizontal stabilizer, fin, and fuselage.
2. Transfer the shapes: Mark the outlines accurately on a lightweight plastic sheet.
3. Cut the parts: Cut carefully and keep the left and right sides symmetrical.
4. Smooth the edges: Remove sharp projections or uneven edges.
5. Form the wing: Add a small, equal dihedral angle to both wing halves if required by the design.
6. Attach the wing: Fix it to the fuselage at the marked position and keep it square to the body.
7. Attach the tail: Fit the horizontal stabilizer level and the fin vertically.
8. Check alignment: View the glider from the front, top, and rear to detect twists or tilts.
9. Balance the glider: Add a small amount of nose ballast until the intended centre of gravity is obtained.
10. Test and trim: Perform gentle test glides and make one small adjustment at a time.

Symmetry ensures that both sides of the glider produce similar aerodynamic effects. Alignment ensures that the wing and tail guide the glider in the intended direction.

Poor symmetry or alignment may cause:

• Unwanted turning or spiralling
• Rolling to one side
• Pitching up or diving
• Increased drag and reduced glide distance

They can be checked by:

• Folding a paper template to verify equal left and right halves
• Measuring from each wingtip to the tail
• Viewing the glider from directly above and behind
• Confirming that the horizontal stabilizer is level with the wing
• Using a set square to check that the fin is vertical
• Placing the glider on a flat surface to identify warping or twisting

Dihedral is the upward angle of the wings when the glider is viewed from the front. It improves lateral stability.

When the glider rolls to one side, the lower wing generally develops a greater restoring effect than the raised wing. This tends to return the glider toward level flight.

In a plastic wing, dihedral may be formed by:

• Dividing the wing at its centre line
• Raising both tips equally according to the design
• Making a gentle, controlled bend if the plastic is flexible
• Joining two wing panels at an angle with tape or suitable adhesive
• Reinforcing the centre joint with a lightweight strip

The two sides must have equal angles. Excessive dihedral can increase drag or produce undesirable handling.

The centre of gravity, or CG, is the point through which the glider's total weight effectively acts.

To locate it:

1. Mark the CG position recommended by the design.
2. Place a fingertip under each wing at equal distances from the fuselage.
3. Support the glider gently at the marked position.
4. Observe whether the nose, tail, or neither side drops excessively.

To adjust it:

• If the glider is tail-heavy, add a small amount of ballast to the nose or move an existing mass forward.
• If it is excessively nose-heavy, remove some nose ballast or shift an adjustable mass rearward.

The glider should usually balance with a slight nose-down tendency at the specified CG. Large or sudden ballast changes should be avoided because small changes can significantly affect flight.

The CG strongly influences pitch stability:

• A slightly forward CG generally improves stability but may make the glider descend steeply and reduce glide distance.
• An excessively forward CG can cause persistent diving and require excessive tail correction.
• A rearward CG reduces pitch stability and may cause repeated stalls, oscillations, or unpredictable flight.
• An excessively rearward CG can make the glider unsafe and difficult to trim.

For stable flight, the CG is normally placed ahead of the aircraft's neutral point. The exact safe location depends on the design. Balance should therefore be established from the plan and refined through gentle test glides using very small adjustments.

A safe test flight should be conducted as follows:

1. Select a large, clear area with calm air or only a very light breeze.
2. Inspect the glider for loose joints, sharp edges, warping, and correct balance.
3. Face into the light breeze, if present.
4. Hold the glider near its CG without squeezing or bending it.
5. Keep the wings level and point the nose slightly downward or nearly level, according to the design.
6. Launch it smoothly with moderate force; do not throw it upward like a ball.
7. Observe its pitch, roll, yaw, glide distance, and landing.
8. Retrieve it only after checking that the flight path is clear.
9. Make one small trimming adjustment and repeat the test.

A consistent launch is important because variations in launch force can be mistaken for design faults.

A stalling pattern often appears as a climb or nose rise followed by a sudden drop.

Likely causes and corrections include:

• CG too far rearward: Add a small amount of nose ballast.
• Elevator or tail tab angled too far upward: Reduce the upward angle slightly.
• Excessive wing incidence: Check the wing mounting and reduce the angle if it differs from the design.
• Warped wing or tail: Straighten or replace the distorted component.
• Launch directed upward: Use a smooth, nearly level launch.
• Launch speed too low: Apply a slightly firmer but controlled launch.

Only one variable should be changed at a time. Repeated trials under similar conditions help identify the true cause.

Common requirements are:

• Balsa sheets: Used for wings and tail surfaces because they are light and easy to shape.
• Balsa strips or a shaped balsa blank: Used for the fuselage.
• Wood-compatible modelling adhesive: Joins the wooden components.
• Templates or plans: Provide dimensions and component positions.
• Ruler, pencil, and set square: Support accurate marking and alignment.
• Sharp modelling knife: Cuts balsa cleanly along the grain or outline.
• Cutting mat: Protects the table and supports safe cutting.
• Fine sandpaper and sanding block: Shape airfoil sections and smooth edges.
• Pins and a flat building board: Hold parts in alignment while adhesive cures.
• Modelling clay or small ballast: Adjusts the CG.

Dust from sanding should be controlled, and blades should always be moved away from the body.

The fabrication procedure is:

1. Study the plan: Identify the dimensions, grain direction, CG, and positions of all components.
2. Select balsa: Use straight, undamaged pieces of suitable thickness and density.
3. Mark the parts: Trace the wing, stabilizer, fin, and fuselage accurately.
4. Cut safely: Make several light knife passes instead of one deep cut.
5. Shape the wing: Sand the leading edge round and taper the trailing edge according to the plan.
6. Create dihedral: Bevel the wing-root faces, raise both tips equally, and glue the centre joint.
7. Prepare the fuselage: Cut and sand it to shape while preserving sufficient strength.
8. Assemble: Attach the wing and tail with suitable adhesive on a flat building board.
9. Align: Keep the wing and stabilizer square to the fuselage and the fin vertical.
10. Allow curing: Do not move the assembly until the adhesive has set.
11. Finish lightly: Remove roughness without adding unnecessary coating or mass.
12. Balance and test: Set the CG with small ballast changes and conduct controlled test glides.

Balsa is anisotropic, meaning its properties differ with grain direction.

• Grain direction: A part is generally strongest and stiffest along the grain. Long components should be oriented so that the grain supports the main load direction.
• Cross-grain weakness: Thin parts may split easily if loads act across the grain.
• Density: Low-density balsa reduces mass but may be too soft for highly loaded or easily damaged parts.
• Higher-density balsa: Provides greater strength and stiffness but increases weight.
• Selection: Lighter wood is often suitable for tail surfaces, while a somewhat stronger piece may be chosen for the fuselage or wing centre.

Proper selection gives a useful balance between low weight, adequate rigidity, and durability.

A balsa wing may be shaped by:

1. Marking reference lines to prevent excessive sanding.
2. Using a sanding block to round the leading edge smoothly.
3. Tapering the trailing edge carefully while retaining enough material for strength.
4. Creating a smooth upper contour if the plan specifies a cambered section.
5. Keeping both wing panels equal in shape, thickness, and mass.
6. Finishing with fine sandpaper to remove scratches and sudden surface changes.

The shaping helps airflow pass more smoothly around the wing, reduces unnecessary drag, and allows the wing to produce lift efficiently. Over-sanding can weaken the structure or make the two panels unequal, resulting in poor stability.

Plastic is useful for quick, durable models, whereas balsa is especially suitable for lightweight structures and accurately shaped components.

The glide angle, , is the angle between the glider's descending flight path and the horizontal. The glide ratio is the horizontal distance travelled divided by the vertical height lost.

For a steady glide at constant speed, resolving the weight relative to the flight path gives:

Dividing the first equation by the second:

If the horizontal distance is and the height lost is , then:

Therefore:

Thus, under steady conditions, the glide ratio equals the lift-to-drag ratio. A higher allows the glider to travel farther for the same loss of height.

For rotational equilibrium about the CG, the sum of pitching moments must be zero:

A simplified balance may be expressed as:

Because the weight acts through the CG, its direct moment about the CG is zero in the idealized case. Wing aerodynamic forces and the wing's pitching moment may tend to rotate the nose. The horizontal tail produces an aerodynamic force at a distance from the CG, creating a balancing moment:

where is the tail force and is its moment arm. Since the tail is far from the CG, a relatively small force can produce a useful correcting moment. Correct CG position, tail size, and tail incidence together provide pitch equilibrium and stability.

Possible faults include:

• Unequal wing panels: Measure both panels and correct their shape or mass.
• Unequal dihedral: Reset the panels so both tips have the designed height.
• Wing warp: Straighten the affected plastic panel or gently correct the balsa panel.
• Tilted wing: Reattach it level and square to the fuselage.
• Misaligned fin: Set the fin vertical and parallel to the fuselage centre line.
• Twisted horizontal stabilizer: Remove the twist or replace the part.
• Unequal surface roughness or damage: Repair dents, rough joints, and protruding tape.
• Unbalanced lateral mass: Check whether one wing is heavier and correct the imbalance carefully.
• Poor launch technique: Launch with level wings and without sideways wrist motion.

The glider should first be inspected for structural faults. Trimming tabs should be used only for small corrections, and each change should be tested separately.

A fair comparison should use a controlled test procedure.

Variables:

• Independent variable: Glider material—plastic or balsa wood.
• Dependent variables: Glide distance, flight time, landing deviation, or glide ratio.
• Controlled variables: Launch height, launch angle, launch force, wing area, target mass where practical, test location, and wind conditions.

Method:

1. Inspect and trim both gliders for stable flight.
2. Mark a fixed launch point and height.
3. Use the same launcher or trained operator for every trial.
4. Conduct at least five trials per glider in calm indoor conditions or consistent outdoor air.
5. Measure horizontal distance and flight time for each trial.
6. Record abnormal launches separately rather than selectively discarding results.
7. Calculate the mean performance, for example:

8. Compare consistency using the spread of results and discuss material-related reasons for any difference.

Multiple trials reduce the influence of random launch errors.

Fabrication safety:

• Cut on a mat and always direct the blade away from fingers and the body.
• Use sharp tools only with appropriate supervision and store them safely afterward.
• Wear eye protection when there is a risk of fragments or snapped parts.
• Avoid inhaling balsa dust; sand gently in a ventilated area and clean dust safely.
• Follow adhesive instructions and avoid skin, eye, or flame exposure.
• Remove sharp plastic corners, splinters, and projecting pins.

Quality checks:

• Confirm that all joints are secure.
• Check wing and tail symmetry, dihedral, and alignment.
• Look for warps, cracks, loose ballast, and excessive adhesive.
• Verify the CG before every series of flights.

Flying safety:

• Use a clear area away from roads, windows, electrical lines, animals, and people.
• Never launch toward another person.
• Avoid strong or gusty winds.
• Retrieve the glider only when the area is safe.