Unit 5: Fabrication of powered Gliders - Subjective Questions

A powered RC glider is a remotely controlled aircraft that combines efficient gliding characteristics with an electric motor and propeller.

Differences from an unpowered glider:

• A powered glider uses a motor and propeller to gain altitude, while an unpowered glider needs a hand launch, winch, slope lift, or tow.
• It carries a battery and electronic speed controller (ESC).
• Its radio system includes a throttle channel.
• It is generally heavier because of its power system.
• After climbing, its motor can be switched off so that it flies like a conventional glider.

The four main forces are:

• Lift: The upward aerodynamic force generated mainly by the wings.
• Weight: The downward force caused by gravity and acting through the center of gravity.
• Thrust: The forward force produced by the motor-driven propeller.
• Drag: The aerodynamic resistance that acts opposite to the direction of motion.

During straight and level powered flight, lift approximately balances weight and thrust approximately balances drag. During a motor-off glide, the glider descends slightly so that a component of its weight provides the forward force needed to overcome drag.

A wing produces lift by moving through the air at a suitable angle of attack. Its airfoil shape and inclination create a pressure difference between the upper and lower surfaces and turn airflow downward, producing an upward reaction force.

The lift equation is:

where:

• is lift,
• is air density,
• is airspeed,
• is wing area, and
• is the coefficient of lift.

Lift increases with wing area, air density, lift coefficient, and particularly airspeed because it is proportional to .

• Ailerons: Control roll about the longitudinal axis. Opposite movement of the left and right ailerons banks the airplane.
• Elevator: Controls pitch about the lateral axis. It raises or lowers the nose.
• Rudder: Controls yaw about the vertical axis. It turns the nose left or right and helps coordinate turns.
• Throttle: Controls motor power and propeller thrust. Increasing throttle generally improves climb capability, while reducing it allows slower powered flight or gliding.

These controls must be coordinated to obtain smooth, stable, and efficient flight.

An RC flight simulator allows a learner to practice aircraft control without risking damage to a real model. It helps develop orientation awareness, control coordination, and emergency-response skills.

A suitable practice sequence is:

1. Learn transmitter stick functions and control directions.
2. Practice straight and level flight at a safe virtual altitude.
3. Perform gentle left and right turns.
4. Practice climbing, descending, and throttle control.
5. Learn to recover from stalls and unusual attitudes.
6. Practice rectangular landing circuits.
7. Perform repeated approaches and landings.
8. Add wind and equipment-failure scenarios after mastering basic flight.

When the airplane flies toward the pilot, its left and right appear reversed from the pilot's viewpoint. For example, the model's right wing is seen on the pilot's left. This can cause incorrect aileron or rudder commands.

Simulation helps by allowing repeated practice of:

• Flying toward and away from the pilot.
• Keeping attention on the aircraft's own orientation.
• Making small corrections instead of abrupt inputs.
• Recovering safely after an incorrect command.
• Practicing the rule of moving the control stick toward the visibly low wing during a head-on approach.

Repeated practice builds the automatic reactions required for safe real-world flying.

The electric power system contains:

• Brushless motor: Converts electrical energy into rotational mechanical energy.
• Propeller: Converts motor rotation into thrust.
• Electronic speed controller (ESC): Regulates motor speed according to the throttle signal and may supply receiver power through a battery eliminator circuit.
• Battery: Stores electrical energy; lithium-polymer batteries are commonly used.
• Connectors and wiring: Carry current between the battery, ESC, and motor.
• Receiver: Receives the pilot's radio commands and sends the throttle signal to the ESC.

All parts must have compatible voltage, current, power, and connector ratings.

Power-system selection should follow these steps:

1. Estimate the airplane's all-up mass and required performance.
2. Choose a motor whose voltage and power ratings suit the intended battery and aircraft.
3. Select a propeller recommended for the motor. Its diameter and pitch must provide sufficient thrust without overloading the motor.
4. Measure or estimate the maximum current drawn by the motor-propeller combination.
5. Select an ESC with a current rating safely above the maximum expected current.
6. Choose a battery with suitable cell count, capacity, mass, and discharge rating.

If battery capacity is ampere-hours and discharge rating is , the approximate maximum rated current is:

A wattmeter should be used to verify current and power before flight.

Balsa wood:

• Has an excellent strength-to-weight ratio.
• Is easy to cut, sand, shape, and glue.
• Produces a light and rigid airframe when properly designed.
• Can absorb moisture and may split along its grain.
• Usually needs covering or sealing.

Plastic:

• Is resistant to moisture and can form complex shapes.
• May provide good impact resistance, depending on the type.
• Is suitable for molded components and protective skins.
• Can be heavier and less stiff than balsa for the same structural role.
• May require special adhesives and can soften or deform under heat.

A practical design may combine a balsa structure with selected plastic components such as fairings, control horns, or protective panels.

A typical fabrication procedure is:

1. Prepare a full-size plan and protect it with a transparent sheet.
2. Select straight, lightweight balsa of suitable density.
3. Cut the ribs accurately from templates.
4. Pin the lower spar and trailing-edge pieces over the plan.
5. Position the ribs vertically and at the correct spacing.
6. Install the upper spar, leading edge, and shear webs.
7. Add wing joiners or dihedral braces at the center section.
8. Glue the joints and allow them to cure without distortion.
9. Remove the structure, sand it smoothly, and check symmetry.
10. Install control surfaces and servo fittings, then apply a lightweight covering.

The completed wing must be straight, rigid, symmetrical, and free from unwanted twist.

Plastic may be used for fuselage shells, wing skins, control horns, servo trays, fairings, and protective parts.

A general method is:

• Prepare a dimensioned template from the aircraft plan.
• Mark the plastic while accounting for folds and joints.
• Cut it with a suitable knife, saw, or shears.
• Score fold lines lightly where appropriate.
• Form curved parts carefully, using controlled heat only if the plastic permits it.
• Roughen and clean bonding surfaces.
• Join parts using an adhesive compatible with the plastic.
• Reinforce highly loaded points such as the motor mount, landing gear, wing attachment, and control horns.

Excessive adhesive or heat should be avoided because it can add weight or deform the material.

The center of gravity (CG) is the point through which the airplane's total weight effectively acts. Its position strongly affects longitudinal stability and control.

• A CG that is too far forward makes the airplane stable but nose-heavy, increases required elevator force, and can make landing difficult.
• A CG that is too far rearward reduces stability and may cause uncontrollable pitching or stalls.

The CG is checked at the location specified by the design, often near the forward part of the wing chord. The fully assembled airplane is supported under both wings at the marked points. Equipment, especially the battery, is moved until the model balances correctly. Any final ballast should be firmly secured.

The weight is:

Wing loading based on weight is:

Mass per unit wing area is:

Therefore, the wing loading is , or when expressed as mass per area.

Lower wing loading generally permits slower flight and gentler landings. Higher wing loading normally increases stall speed and landing speed, although it can improve wind penetration.

Servos should be mounted securely on rigid trays or brackets while remaining accessible for maintenance. Before fitting the servo arm, the transmitter and receiver should be switched on and the servo electronically centered.

For reliable linkages:

• Use stiff, lightweight pushrods.
• Keep pushrods as straight and short as practical.
• Align control horns with the hinge lines.
• Prevent binding throughout the complete range of movement.
• Use secure clevises or connectors with retainers.
• Minimize free play because it reduces control accuracy and may cause flutter.
• Set the required control throws and verify the direction of every control surface.

Wires and linkages must not touch the motor, propeller, or other moving components.

• Dihedral is the upward angle of the wings when viewed from the front. It contributes to roll stability by helping a disturbed, sideslipping aircraft return toward level flight.
• Angle of attack is the angle between an airfoil's chord line and the relative airflow. Increasing it raises lift only up to the critical angle, beyond which the wing stalls.
• Wing incidence is the fixed angle between the wing chord line and a reference line of the fuselage.

Angle of attack changes continuously during flight, while incidence is established during construction. Incorrect incidence or unequal angles between wing panels can produce poor trim, excessive drag, or persistent turning tendencies.

A complete preflight inspection should include:

• Check the wing, fuselage, tail, hinges, and adhesive joints for damage or looseness.
• Confirm that the wing and battery are securely retained.
• Verify that the CG is within the specified range.
• Inspect the propeller for cracks and confirm correct orientation and tightness.
• Check motor mounting, wiring, connectors, and ESC ventilation.
• Confirm adequate transmitter and flight-battery charge.
• Perform a radio range test according to the manufacturer's instructions.
• Check that ailerons, elevator, and rudder move in the correct directions.
• Verify control throws, neutral positions, and freedom from binding.
• Ensure the throttle starts at minimum and failsafe is correctly set.
• Inspect the flying area, wind, people, obstacles, and local operating rules.

• Launching: Point the model into the wind, keep the wings level, apply suitable power, and use a firm but smooth launch.
• Climbing: Maintain safe airspeed and a moderate climb angle. A steep climb may lead to a stall.
• Cruising: Reduce power, establish level flight, and use small coordinated control inputs.
• Gliding: Switch off or reduce the motor and maintain the best glide attitude without allowing airspeed to become too low.
• Approaching: Fly a planned circuit, descend gradually, and align into the wind.
• Landing: Maintain control of airspeed, reduce power, and gently raise the nose close to the ground to flare.

If the approach is unsafe, the pilot should apply power smoothly and perform a go-around rather than force the landing.

A stall occurs when the wing exceeds its critical angle of attack, causing a major loss of lift. It can occur at any flight attitude or speed if the critical angle is exceeded.

Causes:

• Excessive elevator input.
• Flying too slowly.
• Turning steeply at low speed.
• Climbing too steeply.

Warning signs:

• Reduced control response.
• Nose-high attitude.
• Wing rocking or an unexpected wing drop.

Recovery:

1. Reduce the angle of attack by easing forward elevator.
2. Level the wings with coordinated controls.
3. Apply power smoothly if available.
4. Regain safe airspeed before climbing.

Prevention requires maintaining airspeed, using moderate bank angles near the ground, keeping the correct CG, and avoiding abrupt controls.

Alignment and symmetry are maintained by:

• Building directly over accurate full-size plans.
• Using a flat building board, straightedges, squares, jigs, and centerlines.
• Measuring both wing panels and fuselage sides from common reference points.
• Pinning or clamping parts until adhesive has fully cured.
• Checking equal wing incidence and correct dihedral.
• Aligning the horizontal stabilizer with the wing.
• Ensuring that the fin is vertical and centered.
• Preventing heat-shrink covering from twisting light structures.

Poor alignment can cause unwanted rolling, yawing, excessive control trim, higher drag, and unstable behavior. A severely warped wing may stall asymmetrically and make the model unsafe.

A systematic project plan is:

1. Define requirements: Decide the size, mass target, flight duration, control arrangement, material, and skill level.
2. Prepare the design: Select an airfoil, wing area, tail sizes, fuselage layout, power system, and preliminary CG.
3. Produce drawings: Create full-size plans, templates, and a component list.
4. Select materials: Use suitable grades of balsa, plastic, reinforcements, hinges, and compatible adhesives.
5. Fabricate structures: Build the wing, fuselage, and tail accurately while checking alignment at every stage.
6. Install equipment: Fit the motor, ESC, battery, receiver, servos, and linkages with proper cooling and accessibility.
7. Finish and balance: Cover or seal the airframe, measure control throws, and set the CG.
8. Ground-test: Check radio range, failsafe, controls, motor current, thrust, vibration, and structural security.
9. Use simulation: Rehearse the launch, circuit, landing, stall recovery, and emergency procedures.
10. Conduct flight tests: Begin in calm weather with conservative control settings and a safe launch.
11. Record observations: Note trim, climb, glide, stall, motor temperature, current consumption, and landing behavior.
12. Improve safely: Make one controlled change at a time, such as adjusting trim, CG, control throw, propeller, or structural reinforcement, and test again.