Aerospace_Powered

aerospace Feb 23, 2026

Objectives

To build a stable powered airplane that will fly the farthest using a rubber-band propulsion system.

Guiding questions

How does the position of the Center of Gravity (CG) shift when a propeller and rubber band motor are added to the nose?
How does the propeller diameter and pitch affect the thrust-to-drag ratio during the powered phase?
What effect does the vertical stabilizer have on counteracting propeller torque roll?
How do adjustments to the horizontal stabilizer angle compensate for the nose-up pitching moment induced by thrust?
What modifications can be made to the airplane to minimize drag during the unpowered glide phase after the rubber band unwinds?

Materials

Balsa Sheet (1.5x75mm x 915mm) (1 pc)
Balsa Strut (5x5x50mm) (1 pc)
Plastic Propeller (approx. 15 cm diameter) (1 pc)
Rubber Band Motor (3/32” x 12” FAI Tan rubber, 2 strands) (1 set)
Thrust Bearing / Plastic Washer (1 pc)
Propeller Shaft Hook (bent paperclip or music wire) (1 pc)
Rear Hook (bent paperclip or music wire, secured to fuselage tail) (1 pc)
Hot Glue or Standard Wood Glue
Cutting knife
Ruler
Cutting board
Rubber bands for wing attachment (2 pcs)

Balsa wood is selected as the primary construction material due to its exceptionally high strength-to-weight ratio. Despite having a density as low as 100–200 kg/m³, balsa maintains sufficient structural rigidity to withstand aerodynamic loads during flight. This allows the aircraft to achieve minimal wing loading while preserving structural integrity. Additionally, balsa is way easier to cut than other materials, which enables precise configuration design during construction.

The rubber band motor is selected as the propulsion mechanism because it provides a simple, lightweight, and repeatable energy source. Compared to electric motors, rubber band motors introduce no additional battery weight and offer a smooth thrust decay profile, which naturally transitions the aircraft from a powered climb phase into an unpowered glide phase.

How does the grain affect the airplane building?

When working with a sheet of balsa wood, cuts made parallel to the grain tend to proceed smoothly with less resistance. In contrast, cuts made perpendicular to the grain generally require substantially greater effort. These findings indicate that the compressive strength of balsa wood is significantly greater along the longitudinal grain axis. During flight, the main wing structure is subjected to distributed aerodynamic lift forces acting along its span, inducing bending moments about the wing root. When the wood grain is oriented parallel to the spanwise direction, that is, perpendicular to the fuselage centerline, the structure demonstrates enhanced resistance to bending deformation. Conversely, when the grain orientation is perpendicular to the primary loading direction, the interlaminar shear strength is reduced, rendering the structure more susceptible to flexural deformation and potential failure.

For the fuselage of a powered aircraft specifically, the grain must run along the length of the stick to resist the tensile load imposed by the wound rubber band motor. A cross-grain fuselage under rubber tension would snap almost immediately.

Building process

First, I cut out 3 balsa pieces, prepared a wooden stick, and assembled the propeller shaft hook at the nose end, as shown in Figures 1 and 2.

Figure 1: Raw Materials (Section) with Propeller and Rubber Motor

Figure 2: Vertical Stabilizer Assembled with Rear Hook Installed

2 rubber bands were used to fasten the main wing and the horizontal stabilizer on the fuselage. The propeller shaft was mounted at the nose with a thrust bearing washer to reduce friction during unwinding.

Figure 3: Wing and Stabilizer Assembled with Motor Hook Visible

Figure 4: Final Step Before Adding Blu-Tack and Winding Motor

Final Product

Figure 5: Airplane Top View 1

Figure 6: Airplane Top View 2

Size Data

Wing: $20 cm \times 4 cm$
Horizontal Stabilizer: $8 cm \times 2.5 cm$
Vertical Stabilizer: $4 cm \times 2.5 cm$
Propeller Diameter: $15 cm$
Rubber Motor Length (loop): $24 cm$

Balancing

An additional balsa wood stick was utilized to determine the center of mass of the aircraft, ensuring equilibrium along both the fuselage axis and the spanwise direction of the main wing. The center of mass was found to be positioned along the longitudinal centerline of the aircraft, slightly aft of the main wing. With the rubber motor installed but unwound, the CG shifted noticeably forward due to the concentrated mass of the propeller assembly and rubber at the nose.

Figure 7: Center of Mass Measurement

Figure 8: Center of Mass Along Fuselage Measurement

Initial structure

The aircraft is expected to undergo a brief powered climb phase under rubber motor thrust, followed by a transition into an unpowered glide. If the CG and thrust line are not properly aligned, the aircraft may exhibit severe pitch oscillations or an uncontrollable climb leading to a power stall.

Observations:

Trail #1

Upon release with approximately 200 winds, the aircraft exhibits a pronounced nose-up pitching moment under full thrust, climbing at an angle of about 60 degrees above the ground level, leading to a power stall immediately following the initial climb phase. After the rubber motor unwound, the aircraft entered an unrecoverable dive.

Analysis

Compared to physics, where objects are often idealized as point masses, engineering requires us to account for the effects of torque due to the actual geometry and mass distribution of components to make sure we are aware of the kinematics. In a powered aircraft, the propeller introduces an additional thrust vector and a reactive torque that must be considered in the force and moment balance.

Figure 9: Free Body Diagram of Powered Airplane

Center of Gravity (mass)

The center of mass depends solely on how mass is distributed within an object. It is an intrinsic property of the body itself, independent of external forces or reference frames. For gravitational analysis, the entire weight of an object can be treated as acting through this single point.

In a powered configuration, the rubber band motor and propeller assembly add significant nose-weight. As the rubber unwinds during flight, the effective mass distribution shifts slightly, causing a minor aft CG migration. This dynamic CG shift must be anticipated during the initial trim setup.

Center of lift

The center of lift is an aerodynamic property that depends on multiple dynamic factors

Wing geometry: the shape, planform, and airfoil profile of the wing
Airflow conditions: changes in airspeed, air density, and flow characteristics
Flight variables: the center of lift moves in response to changes in angle of attack, airspeed, flap deployment, and wing configuration.

Because these factors are all variable during flight, the center of lift is inherently dynamic.

Thrust Line and Its Effect on Pitch

In addition to the CG-lift relationship, the thrust line introduces a new pitching moment. If the thrust line passes above the CG, the propeller force creates a nose-down moment. If it passes below the CG, it creates a nose-up moment. A slight downthrust angle (tilting the propeller shaft 2–3 degrees downward relative to the fuselage) is commonly applied to counteract the natural nose-up tendency during the high-thrust phase.

Center of Lift vs. Center of Mass

When the center of lift is behind the center of mass, the aircraft tends to pitch down, since the lift force acts behind the pivot point, pulling the tail up.

When the center of lift is ahead of the center of mass, the aircraft pitches up, as lift now pushes the front end upward.

Propeller Torque Effect

When the propeller rotates (e.g., clockwise as viewed from the front), Newton’s third law dictates that the fuselage experiences an equal and opposite reaction torque, tending to roll the aircraft in the opposite direction (counterclockwise). This torque-induced roll is most pronounced during the initial high-power phase when the rubber band is tightly wound.

To counteract this, a small amount of right-thrust (offsetting the propeller shaft laterally by 1–2 degrees) and slight asymmetric trim on the vertical stabilizer can be applied.

Practical optimal structure

In a conventionally stable powered aircraft, the center of gravity (CG) is positioned forward, with the main wing’s center of lift located slightly aft of the CG. The horizontal stabilizer at the tail generates a downward balancing force to maintain trimmed flight.

This arrangement ensures longitudinal stability, so that if the aircraft experiences an up-pitching perturbation, the resulting aerodynamic moment produces a restoring pitch-down tendency, naturally correcting the deviation rather than allowing divergence.

For the powered phase specifically, a 2–3 degree downthrust angle and 1–2 degree right-thrust offset are incorporated into the propeller mount to counteract thrust-induced pitch-up and torque roll.

Trail #2

In this configuration, the main wing was adjusted aft to reposition the center of lift further behind the CG, additional weight (Blu-Tack) was applied to the nose to move the CG forward, and a 2-degree downthrust shim was inserted beneath the rear of the propeller shaft mount, enhancing longitudinal stability during the powered phase.

Figure 10: #2 Configuration

Following this modification, flight performance improved significantly, and the aircraft achieved a stable powered climb at approximately 20 degrees, followed by a smooth transition into a glide phase with negligible pitching. However, the aircraft exhibited a noticeable left turn during the powered phase due to uncompensated propeller torque, so further tuning of the structure was conducted to achieve a more precise trim.

Trail #3

Figure 10: #3 Configuration CG locating

I moved the center of mass to a position around 35% back from the leading edge of the main wing. This is to make sure that the center of mass and the center of lift are distributed precisely. Additionally, I added a 1.5-degree right-thrust offset to the propeller shaft and applied a small trim tab deflection on the vertical stabilizer trailing edge to the right to counteract the propeller torque roll observed in Trail #2.

Enhanced Balsa

Dihedral

Figure 11: Schematic Diagram of Wing Configurations

Figure 12: Schematic Diagram of Dihedral

When the aircraft sideslips toward the dropped wing, dihedral causes the lower wing to meet the airflow at a higher angle of attack as well as produce more lift, and the upper wing at a lower angle of attack as well as produce less lift. This lift asymmetry rolls the aircraft back to level.

Then the airplane rolls to the right

The right wing becomes lower
The left wing becomes higher

Because of changes in the airflow direction and the wing geometry, the lower wing usually gains a greater effective angle of attack and therefore more lift. As a result, the airplane is pushed back toward level flight.

The function of dihedral is to increase an aircraft’s lateral static stability in sideslip. In a powered configuration, dihedral is especially critical because propeller torque constantly induces a rolling moment that must be passively corrected.

Sideslip & Dihedral Effect

The most important function of dihedral is to increase the aircraft’s rolling tendency to recover during a sideslip.

For example, when the airplane is pushed off course by the wind or by propeller torque and enters a sideslip

The relative airflow approaches from the side
The dihedral causes the left and right wings to respond differently to this sideways airflow
This then creates a rolling moment that tends to roll the airplane back

Advanced Design

Figure 13: Enhanced Powered Balsa Airplane Design

Dimensions

Main Wing

Wing total span: 36 cm
big enough for a good lift
not so large that it becomes floppy or twisty

Wing chord

6.0 cm center chord

Center panel: 14 cm span × 6.0 cm chord
Left tip panel: 11 cm span × 6.0 cm root chord tapering to 4.2 cm tip chord
Right tip panel: same as left

This gives a good aspect ratio, better lateral stability, and lower tip drag than a pure rectangular wing.

Wing area estimate

center panel area = 14 × 6.0 = 84 cm²
each tip panel area ≈ 11 × (5.2 + 4.2)/2 = 51.7 cm²
total wing area ≈ 187 cm²

Horizontal Stabilizer

Span: 14.0 cm
Chord

3.2 cm root
2.6 cm tip

Area

About 40–42 cm²

Vertical Stabilizer

Height above fuselage: 5.0 cm
Root length along fuselage: 4.5 cm
Tip chord: 1.5 cm

Area roughly 13–15 cm²

Fuselage

26.7 cm wooden stick as the main fuselage boom

Fuselage layout along the stick

Measured from the nose
0.0–2.5 cm: nose/propeller mount and thrust bearing zone
7.5–8.5 cm: wing mount center
22.8–23.5 cm: horizontal tail leading edge
22.8–23.5 cm: vertical tail root front
26.7 cm: rear rubber hook anchor point

Propulsion System

Propeller: 15 cm diameter, moderate pitch plastic propeller
Rubber Motor: 2 strands of 3/32” FAI Tan rubber, loop length 24 cm
Thrust Bearing: plastic washer between propeller shaft and nose block
Downthrust Angle: 2 degrees below fuselage datum
Right-Thrust Offset: 1.5 degrees right of fuselage centerline

Material Allocation

500 cm² total sheet area

Main wing

187 cm²
Horizontal stabilizer
about 42 cm²
Vertical stabilizer
about 15 cm²
Nose block / propeller mount reinforcement
about 8 cm²
Others
about 20–30 cm²
Total
Roughly 272–282 cm²

Reinforcement pieces

2 wing joint gussets: 2.5 cm × 1.5 cm triangles or diamonds
1 center wing doubler: 5.0 cm × 2.0 cm
2 tail mount tabs: 1.5 cm × 0.8 cm
1 nose block reinforcement: 3.0 cm × 2.0 cm (to resist rubber tension and propeller vibration)
several trim tabs: 0.8 cm × 0.4 cm small strips

Airfoil

Since the balsa sheet is a thin sheet of wood, it is not able to build a ribbed airfoil.