📘 Module 8 — Construction Techniques & Fabrication

8.1 Why Speed Control Matters

Scoring Rules and Target Times

The Science Olympiad Hovercraft event scoring formula typically rewards teams for completing a full-length track run and arriving at a target distance within a target time range. The exact rules vary by year, but the principle is consistent: too fast and you overshoot; too slow and you miss the window; perfect timing and distance yield maximum points.

Consider a hypothetical 10-meter track with a target completion time of 8–10 seconds (target speed range: 1.0–1.25 m/s). Teams are scored on:

• Distance traveled: 0–10 m, with scoring favorably toward the 10 m (full-track) completion
• Time to completion: Bonus for completing within the 8–10 second window; penalty for overshooting (>10 m) or undershooting (<8 m traveled in allowed time)

A craft that completes the full 10 m track in exactly 9 seconds scores maximum points. A craft that completes 10 m in 7 seconds (too fast) overshoots and is scored as traveling 10+ m — a penalty. A craft that completes only 8 m in 10 seconds undershoots — also penalized. The team whose craft stops at exactly 10 m after 9 seconds wins.

This scoring structure creates a direct incentive for speed control and calibration: the ability to dial in a specific velocity and maintain it throughout a run.

The Challenge of Precision Speed Control

Achieving a target speed within ±10% is straightforward; achieving it within ±2% (the margin between full points and partial) is difficult. Here's why:

1. Battery voltage variation: A fully charged 4-cell NiMH battery provides 10.8 V nominal; after 2–3 competition runs, that voltage may drop to 10.2–10.5 V due to internal resistance losses. This 2–5% voltage drop translates directly to a 2–5% reduction in motor speed (for DC brush motors, speed is roughly proportional to voltage).

2. Motor load variation: The lift fan motor's current draw depends on cushion pressure, which varies with track surface, skirt wear, and even temperature. Higher cushion pressure (rougher surface) means the motor works harder and draws more current, causing a voltage sag across the battery's internal resistance. The voltage seen by the thrust motor thus decreases, reducing its speed.

3. Temperature effects: Motor speed varies with winding temperature due to changes in winding resistance. A motor running hot (after 2–3 consecutive runs) may be 3–5% slower than one run cold.

4. Mechanical friction variation: The thrust motor's effective torque (and thus vehicle acceleration) depends on bearing friction, skirt contact friction, and air resistance. Worn bearings or skirt wear between runs can alter effective thrust and change the vehicle's speed for the same motor command.

5. Track surface variation: Different materials (vinyl vs. concrete, smooth vs. textured) impose different drag forces. A vehicle that achieves 1.2 m/s on a smooth gymnasium floor may only achieve 1.0 m/s on a rough outdoor surface due to increased skirt friction and air resistance.

The cumulative effect of these variations is that an uncalibrated hovercraft running at maximum throttle might achieve speeds anywhere from 0.9 m/s to 1.3 m/s depending on battery state, motor load, track surface, and thermal state. This range spans the difference between barely completing the track and overshooting by a meter.

Why Speed Matters More Than Distance

While the event rules reward distance, speed is often the controlling factor. A craft that can achieve a precisely calibrated 1.1 m/s on any surface will complete the track predictably. A craft with variable speed (ranging from 0.9 to 1.3 m/s depending on conditions) will sometimes complete successfully and sometimes fail, making it unreliable.

Competitive teams prioritize speed control over raw distance potential. A team that can consistently complete a 9 m run in 9 seconds (1.0 m/s, moderate speed) will score well and place reliably. A team that might complete 10.5 m in some runs (overshooting) and only 7 m in others (undershooting) will be inconsistent and place poorly.

Throttle Response and Droop

A key metric in speed control is voltage regulation and throttle response. When the thrust motor is commanded to full throttle, current demand spikes instantly. If the battery has internal resistance (typical for NiMH packs), the voltage sags momentarily:

V_motor = V_battery - (I_motor × R_internal)

For a 4-cell NiMH pack with 0.1 Ω internal resistance drawing 2 A current:

V_motor = 10.8 V - (2 A × 0.1 Ω) = 10.6 V (0.2 V sag)

When the motor current drops back to 0 A (coasting phase), voltage recovers to 10.8 V. The speed thus oscillates with the motor current cycle, causing the vehicle to "pulse" forward and backward if the pulsation is significant.

A well-designed power system minimizes voltage droop through:

• Large capacitors at the motor terminals (absorbing current spikes without voltage drop)
• Low-resistance wiring and connectors
• Fresh batteries at competition (fully charged, not partially discharged)

Multi-Run Strategy for Speed Control

Most competitions allow 2–4 runs within the 8-minute window. Teams use this to their advantage:

Run 1: Full throttle, collect data on speed and distance. If the craft completes the full track in less than the target time, the speed is too high. If it falls short, speed is too low.

Run 2: Adjusted throttle based on Run 1 data. If Run 1 was 7 seconds (too fast), reduce thrust fan speed by 10–15% and re-run. If Run 1 was 11 seconds (too slow), increase speed by 5–10%.

Runs 3–4 (if available): Fine-tune further based on Run 2 results, or attempt final scoring runs.

Teams that have thoroughly tested their vehicle before competition day come with a pre-calculated throttle setting that reliably produces the target speed. Teams that haven't tested rely on trial-and-error during the competition, wasting runs and time.

The Role of Calibration

Calibration is the process of measuring and recording the relationship between throttle setting and vehicle speed. A calibration curve might show:

• 30% throttle → 0.6 m/s
• 50% throttle → 1.0 m/s
• 70% throttle → 1.35 m/s
• 100% throttle → 1.5 m/s (overshoots)

With this curve, the team knows precisely which throttle setting produces the target 1.0 m/s. They can dial it in before the competition run and execute with confidence.

Without a calibration curve, teams guess. A guess that is 10% off in throttle setting might be 15–20% off in speed (depending on the throttle curve's slope), leading to overshoot or undershoot at competition.

Environmental Baseline vs. Competition Surface

Pre-competition testing typically occurs on a known practice surface (gymnasium floor, driveway, etc.). The competition surface may be different (outdoor concrete, textured vinyl, etc.). A hovercraft calibrated on a gymnasium floor will run 10–20% slower on rough concrete due to increased friction.

This is not a small effect. If the target speed is 1.0 m/s and the craft was calibrated on smooth floors to hit 1.0 m/s at 60% throttle, increasing to rough surfaces may require 70–75% throttle to maintain 1.0 m/s. Teams that don't anticipate this surface change will be underpowered at competition.

Best practice: If possible, conduct at least one test session on a surface type similar to the expected competition surface. If that's not possible, build a robust vehicle with ample speed margin — the ability to run 20–30% above target speed — so that friction increases don't prevent reaching the target.

Common Mistakes

• Testing only at maximum throttle: Teams that only practice "how fast can this go?" never determine the throttle setting for the target speed. They arrive at competition clueless about what throttle to use.

• Assuming battery voltage doesn't matter: A 10% voltage drop from a partially depleted battery causes a 10% speed reduction. Teams that use the same battery for practice (charged) and competition (possibly not fully charged) encounter unexpected speed changes.

• Not accounting for surface friction variation: A hovercraft optimized for smooth floors will underperform on rough surfaces. Teams competing outdoors should test on rough surfaces during development.

• Setting throttle arbitrarily at competition: Teams without pre-calculated throttle settings either run at maximum (likely overshooting) or guess conservatively (likely undershooting). Neither strategy is optimal.

8.2 Potentiometer-Based Speed Control

How Potentiometers Control Motor Speed

A potentiometer (variable resistor) placed in series with a DC motor acts as a throttle. By adjusting the resistance, you adjust the voltage delivered to the motor, which in turn adjusts the motor's speed. This is the most common speed control method in Science Olympiad hovercraft because it is simple, reliable, and requires minimal electronics knowledge.

The basic circuit is:

[Battery] → [Potentiometer] → [Motor] → [Ground]

The potentiometer has three terminals: two for the variable resistor (the knob), and one middle terminal (the wiper) that taps off the voltage divider created by the resistor. By rotating the knob, you change the voltage at the wiper terminal, and thus the voltage applied to the motor.

For a 4-cell NiMH battery (nominal 10.8 V) with a potentiometer adjusted to midway position, the motor might receive approximately 5.4 V (half the battery voltage). Rotating the knob fully one direction increases motor voltage to 10.8 V (full throttle); rotating it the other direction reduces voltage to 0 V (off).

Potentiometer Selection

Resistance range: Potentiometers for motor speed control typically range from 100 Ω to 10 kΩ. For a DC motor drawing 2–3 A of current, a 1–5 kΩ potentiometer is appropriate. A potentiometer that is too small in resistance (e.g., 100 Ω) wastes significant current as heat in the potentiometer rather than delivering it to the motor — inefficient and causes the potentiometer to heat up dangerously. A potentiometer that is too large (e.g., 10 kΩ) provides minimal voltage drop across the range of rotation, giving coarse speed control.

Power rating: The potentiometer must be rated for the power it will dissipate. For a 2 A motor on a 10.8 V battery with a 2 kΩ potentiometer adjusted to half throttle:

P_pot = I² × R_pot = (2 A)² × (2000 Ω × 0.5) = 4 W

Wait—this calculation is too high because the voltage divider doesn't work that simply. More precisely:

V_half = V_battery × (R_lower / (R_upper + R_lower))

With a 2 kΩ potentiometer (2 kΩ upper + 2 kΩ lower at midway), half of the battery voltage is dropped across the upper half of the potentiometer:

V_upper = 10.8 V × (2000 / (2000 + 2000)) = 5.4 V

Current at this point is limited by the motor's resistance. A typical DC motor has 3–5 Ω internal resistance, so current is:

I = V_motor / R_motor = 5.4 V / 4 Ω = 1.35 A

Power dissipated in the potentiometer at midway:

P_pot = V_upper × I = 5.4 V × 1.35 A = 7.3 W

This is significant! A potentiometer rated for only 1 W continuous power will overheat and may fail. Select a potentiometer rated for at least 10–15 W continuous power to safely handle the thermal load during extended runs.

Potentiometer Types

Linear taper: The resistance increases linearly with knob rotation angle. 0% rotation = 0 Ω, 50% rotation = 50% of max resistance, 100% rotation = full resistance. Linear tapers are straightforward and common.

Logarithmic taper: The resistance increases logarithmically, meaning small knob movements near the low-resistance end produce large resistance changes, while small knob movements near the high-resistance end produce small changes. Logarithmic tapers can provide more intuitive speed control (smaller throttle changes near maximum speed, larger changes at low speed).

For most Science Olympiad applications, linear taper is simpler and more predictable.

Mounting and Mechanical Linkage

Manual rotation: The simplest approach is to mount a potentiometer on the base of the craft and manually rotate the knob before launch. The team adjusts the potentiometer to the predetermined throttle setting (based on pre-competition testing), launches the craft, and lets it run.

This method is reliable and requires no additional electronics. Disadvantage: if the team miscalculates the throttle setting during a run (e.g., the track is rougher than expected), they cannot adjust speed mid-run. The craft must complete the run at the set speed, undershooting or overshooting the target.

Servo-controlled adjustment (advanced): Some teams use a servo motor to mechanically rotate the potentiometer during flight, allowing dynamic speed adjustment. However, the Science Olympiad rules typically prohibit servos or active control systems, restricting potentiometer control to manual pre-run adjustment.

Mounting position: Mount the potentiometer on an easily accessible part of the base where the team can reach it to adjust before launch. Avoid mounting it underneath the craft or in a location that requires turning the craft over to access. Mount it in a location that won't be damaged if the craft impacts the track edge.

Potentiometer Precision and Repeatability

Resolution: A mechanical potentiometer with a 300-degree rotation range provides roughly 0.3-degree resolution per 1% change in resistance. This is reasonable for speed control but not perfect — small thumb rotations can cause 5–10% changes in speed.

Repeatability: Can the team dial the knob back to the exact same position every run? Mechanical potentiometers often include a detent or notch at specific positions (e.g., center position), allowing repeatable adjustment. Marking the knob with a reference line and marking the base with target throttle positions (0%, 25%, 50%, 75%, 100%) greatly improves repeatability.

Better approach: multi-position switch. Instead of a continuous potentiometer, use a rotary selector switch (e.g., a 6-position rotary switch) with fixed resistor values at each position. Wiring is:

[Battery] → [Rotary Switch] → [Resistor for Position 1] → [Motor]

Each switch position selects a different resistor, providing discrete throttle levels (e.g., 0%, 25%, 50%, 75%, 100%). This is more repeatable than a continuous potentiometer and allows the team to quickly select the target throttle without fumbling with a knob.

Thermal Management

Potentiometers dissipate significant power and can heat up to 60–80°C during extended runs. Excessive heat can damage the potentiometer's internals or cause internal resistance to drift, changing the calibration mid-run.

Mitigation strategies:

• Use a potentiometer rated for higher power than calculated (15–20 W vs. calculated 7–10 W)
• Mount the potentiometer on the exterior of the base where it has access to ambient air for cooling
• Avoid running at intermediate throttle positions (50%) for extended periods; instead, run at high throttle (>80%) for shorter duration or low throttle (<30%) where power dissipation is lower
• Allow the potentiometer to cool between runs (1–2 minutes rest in-between)

Calibration Procedure

1. Charge the battery fully before beginning the calibration session.

2. Set up a calibrated track (measure the track length accurately) with distance markers (every 1–2 meters).

3. Set potentiometer to 30% throttle (based on visual estimate or use a multimeter to measure voltage if you have one). Launch the craft and measure:

   • Distance traveled
   • Time to travel that distance
   • Calculate speed = distance / time

4. Record the result: 30% throttle → X m/s

5. Repeat steps 3–4 for throttle positions 40%, 50%, 60%, 70%, 80%, 100%.

6. Plot a calibration curve: Throttle percentage (x-axis) vs. speed in m/s (y-axis). The curve should be roughly linear or slightly curved upward (nonlinear due to motor friction and load effects).

7. Identify the target throttle setting: Find the throttle percentage that produces your target speed. For example, if target speed is 1.0 m/s and your calibration curve shows 1.0 m/s at 65% throttle, the team knows to set the potentiometer to 65% before launch.

8. Verify repeatability: Run the craft 2–3 additional times at the target throttle, confirming that speed is consistent (±2–5% variation is normal).

Accounting for Battery Discharge

Calibration should ideally be performed with the same battery state (charge level) that will be used at competition. If calibration is done with a fully charged battery and the craft will run at competition with a partially charged battery, the speed will be lower at competition than expected.

Option 1: Calibrate at multiple battery charge levels (100%, 80%, 60%, 40%) and create separate curves for each. At competition, estimate battery charge level and use the appropriate curve.

Option 2: Calibrate with a partially depleted battery (e.g., after 2–3 test runs) to match the battery state expected at competition. This is conservative but realistic.

Option 3: Pre-charge the battery an hour before competition and verify the charge level using a cell counter or multimeter. Adjust the throttle setting if needed based on observed speed.

Common Mistakes

• Choosing a potentiometer with insufficient power rating: Causes overheating and drift in calibration during operation.
• Failing to account for voltage droop: When the motor is under load, voltage sags due to internal battery resistance. The actual motor voltage is lower than no-load voltage. Calibrate under actual operating conditions, not no-load.
• Not marking throttle positions: Without visible markers on the knob and base, repeatability is poor. Clearly label target throttle positions.
• Calibrating in a lab environment, then competing on a different surface: Friction varies with surface. Calibrate on a surface type similar to competition.

8.3 Tape & Physical Airflow Restriction

The Quick-and-Dirty Speed Control Method

When a potentiometer is not available or speed control needs fine-tuning mid-competition, teams can reduce thrust fan speed by physically restricting airflow with tape. This method is crude but effective: covering part of the thrust fan intake or exhaust with waterproof tape reduces the volume of air the fan can move, thus reducing thrust and speed.

While less precise than potentiometer control, tape restriction offers one major advantage: it can be applied or adjusted between runs in under 30 seconds, allowing rapid iteration during competition.

Restricting Intake vs. Exhaust

Intake restriction: Tape applied to the thrust fan's intake (the air inlet on the rear of the craft) blocks incoming air, forcing the fan to work harder to draw air. Result: lower volumetric flow rate through the fan, reduced thrust.

Exhaust restriction: Tape applied to the thrust fan's exhaust (the air outlet, usually at the rear) backs up the airflow, increasing back-pressure on the fan. Result: lower effective volumetric flow rate, reduced thrust.

Comparison:

• Intake restriction directly reduces air flow; it's more efficient and results in higher performance for a given restriction ratio.
• Exhaust restriction increases back-pressure but doesn't directly restrict inlet air; some of the fan's power is wasted compressing air against the back-pressure. Less efficient but easier to implement (tape is typically applied to the exterior of the craft at the exit point).

For most teams, exhaust restriction is easier: tape the rear of the craft's thrust fan exhaust opening and adjust how much of the opening is covered.

Quantifying the Effect

If the thrust fan normally produces flow rate Q at full throttle, and you cover X% of the exhaust area with tape, the effective flow rate is approximately:

Q_restricted ≈ Q_full × (1 - X%)

Since thrust is proportional to mass flow rate times exhaust velocity, and velocity increases as back-pressure increases (partially offsetting the flow reduction), the speed reduction is typically 70–90% of the area restriction:

Speed reduction ≈ 0.7–0.9 × Area restriction %

For example, if you cover 30% of the exhaust with tape:

• Theoretical flow reduction: 30%
• Actual speed reduction (accounting for back-pressure effects): 21–27%

This is a useful rule of thumb for initial estimates, though actual results depend on fan characteristics and motor response.

Application Technique

Step 1: Prepare tape. Use waterproof duct tape, gaffer tape, or Gorilla tape. Pre-cut strips to the appropriate size based on your estimate of required restriction.

Step 2: Identify the exhaust location. The thrust fan's exhaust jet exits from the rear of the craft, usually from an opening 15–30 cm tall and 5–10 cm wide.

Step 3: Apply tape carefully. Start from one edge and press the tape firmly onto the exhaust opening, smoothing out bubbles and wrinkles. Work slowly to ensure good adhesion; loose tape will billow out and tear during operation.

Step 4: Test. Launch the craft and observe speed. If too fast, apply additional tape to cover more of the opening. If too slow, peel back tape to open more exhaust area.

Step 5: Document the restriction level. Take a photo or sketch of how much of the opening is covered (e.g., "bottom 8 cm covered" or "left 60% covered") so you can replicate the same level for subsequent runs.

Advantages and Disadvantages

Advantages:

• Fast to implement (30 seconds application, immediate effect)
• Requires no electronics or precision components
• Can be adjusted mid-competition between runs
• Intuitive: more tape = slower; less tape = faster
• Low cost (tape costs pennies)

Disadvantages:

• Imprecise: it's difficult to cover exactly 25% or 50% of an opening consistently
• Thermal load: restricting the exhaust increases back-pressure on the motor, causing higher current draw and motor heating
• Durability: tape can come loose during high-speed runs, causing sudden speed changes or even tearing and becoming entangled in the propeller
• Aero effects: partially covering the exhaust creates asymmetric flow in some cases, potentially causing steering effects
• Not repeatable: two different teams (or the same team on a different day) will likely apply tape slightly differently, leading to different speeds

Combining with Potentiometer Control

Some advanced designs use both potentiometer control and tape restriction:

• Potentiometer sets the motor's base voltage (e.g., 80% of battery voltage)
• Tape restricts airflow to fine-tune speed below the base level

Example: A team calibrates the potentiometer to produce 1.2 m/s (target is 1.0 m/s). Before launch, they cover 20% of the exhaust with tape, reducing speed from 1.2 m/s to approximately 0.95–1.0 m/s. This two-stage approach provides coarse control (potentiometer) and fine-tuning (tape).

Thermal Consequences

Restricting exhaust increases back-pressure on the thrust motor. Increased back-pressure means increased current draw:

I = V / (R_motor + back-pressure effect)

As back-pressure rises, resistance increases, and current increases significantly. For a motor normally drawing 2 A, 30% exhaust restriction might increase current to 2.3–2.5 A. Over a 30-second run, this extra dissipation heats the motor:

Heat = (I²Δt) × R = (2.4)² × 30 × 4 ≈ 1000 Joules ≈ 240 calories

The motor temperature rises by approximately 5–10°C (depending on motor mass and thermal properties). This is tolerable for one run, but repeated runs without cooling between them cause cumulative heating and risk motor failure.

Mitigation: If using tape restriction, allow 2–3 minutes between runs for the motor to cool. Alternatively, use a lower potentiometer setting instead of tape restriction to achieve the target speed; this reduces current draw compared to tape-restricted, full-potentiometer operation.

Practical Scenarios

Scenario 1: Competition day surprise — unexpected surface. The team's hovercraft, calibrated on a gymnasium floor, arrives at competition to find a rough outdoor concrete track. The increased friction causes the craft to run 15% slower than expected. With 2 runs already completed, the team has 1–2 runs remaining to adjust. Solution: apply tape to cover ~15% of the exhaust, compensating for the friction increase and restoring target speed.

Scenario 2: Battery degradation. The team's battery was fully charged at competition but is now partially depleted after 2 runs. The craft is running 10% slower. Solution: remove any tape restriction from the previous run, allowing higher speed to partially compensate for battery droop. If the craft is now too fast, apply a small amount of tape.

Scenario 3: Fine-tuning between runs. The team has set the potentiometer to 70%, which produces 1.15 m/s (target is 1.0 m/s). Before launching, they want to test what 1.0 m/s feels like. Solution: cover approximately 15% of the exhaust (to reduce speed by ~12%), run the craft, and observe if the speed is closer to target. Use the result to inform the next run (apply more or less tape).

Alternative Restriction Methods

Beyond tape, other teams have experimented with:

Adjustable baffle plates: 3D-printed or wooden plates that can be repositioned at the exhaust to vary restriction. More repeatable than tape but requires more complex mechanical design.

Removable shroud sections: Building the thrust fan shroud in segments; removing segments increases exit area and speed. Quick to swap but requires advance planning in the design.

Throttleable motor: Using a motor controller with adjustable output rather than a simple potentiometer. More precise but adds electronics complexity.

For most teams, tape restriction is the optimal balance of simplicity and effectiveness.

Common Mistakes

• Applying tape unevenly or asymmetrically: Covering more of one side of the exhaust than the other creates asymmetric thrust, causing the craft to steer. Carefully center the tape application.
• Using non-waterproof tape: Standard duct tape loses adhesion when wet. Use waterproof (Gorilla tape) or gaffer tape.
• Covering the exhaust completely or nearly completely: A craft with >50% exhaust restriction often fails to accelerate or may not move at all due to excessive back-pressure. Stay below 40% coverage unless you're intentionally creating a speed limiter.
• Not checking tape adhesion before launch: Loose tape can tear and become entangled in the fan. Apply tape and gently tug it to confirm it's secure.
• Forgetting to document the tape coverage level: If the tape works well, the team wants to replicate it on the next run. Take a photo or write down exactly how much was covered.

8.4 Dual-Motor Speed Balancing

The Interdependence of Lift and Thrust

A hovercraft has two independently controlled motors: the lift fan (for hovering) and the thrust fan (for movement). While they serve different functions, they are not completely independent. The speed of each motor affects the craft's performance in complex ways:

Lift motor speed determines cushion pressure, which in turn determines hover height and skirt effectiveness. Higher lift speed → higher pressure → better hover, but also higher power consumption and potentially higher friction.

Thrust motor speed determines forward acceleration. Higher thrust speed → faster movement, but this interacts with cushion pressure. A thrust fan that accelerates the craft too quickly (relative to the lift cushion strength) may cause pitch instability or skirt collapse.

Optimal hovercraft performance requires balancing the two motors: neither should be set to maximum independently; instead, both should be tuned to work together effectively.

The Hover-First Principle

The fundamental rule of dual-motor balancing is: ensure the craft hovers first, then add thrust. A craft that cannot hover reliably will not move predictably. Setting both motors to maximum and hoping the craft hovers is a recipe for failure.

Procedure:

1. Set the lift motor to its operational point (the speed that produces the target cushion pressure for your vehicle)
2. Verify hovering is stable and repeatable
3. Gradually increase thrust motor speed until the craft moves at the desired pace
4. Balance between lift and thrust by observing behavior (see troubleshooting section below)

Motor Speed Ratios and Performance

For many hovercraft, the optimal ratio of lift motor to thrust motor speeds is approximately 1.5:1 to 2:1. That is, the lift fan runs 50–100% faster than the thrust fan. This ensures the lift motor can maintain cushion pressure even as the thrust motor draws significant power.

Example:

• Lift motor: 7.4 V, 18,000 RPM (high speed, high airflow)
• Thrust motor: 7.4 V, 12,000 RPM (lower speed, high torque)
• Ratio: 18,000 / 12,000 = 1.5:1

The reason this ratio works is that lift requires continuous, stable airflow to maintain cushion pressure, while thrust is typically cyclic (accelerate, coast, brake). The faster lift fan handles sustained pressure maintenance; the more torquey thrust fan handles transient loads.

Controlling Two Independent Motors

Option 1: Independent potentiometers. Each motor gets its own potentiometer on its own power path:

[Battery] → [Lift potentiometer] → [Lift motor] → [Ground] [Battery] → [Thrust potentiometer] → [Thrust motor] → [Ground]

This allows the team to adjust lift and thrust independently and is the most flexible approach. Disadvantage: requires careful calibration of both potentiometers before competition, and requires two separate potentiometer adjustments before each run.

Option 2: Shared battery with series-wired potentiometers. One potentiometer controls both motors proportionally:

[Battery] → [Potentiometer] → [Lift motor] → [Thrust motor] → [Ground]

Both motors see the same adjusted voltage. This is simpler (one control) but less flexible — if you want to increase lift without increasing thrust (or vice versa), this topology doesn't allow it.

Option 3: Tiered potentiometers. One potentiometer adjusts total voltage (affecting both motors), and a second potentiometer adjusts the voltage divider between the two motors (for fine-tuning the ratio).

Option 1 (independent potentiometers) is most common in competitive teams because it provides maximum control.

Tuning the Lift-to-Thrust Ratio

Once both motors are powered and the craft is levitating, the team should optimize their ratio:

Check 1: Pitch stability. With the craft hovering, increase thrust motor speed gradually and observe whether the craft tilts (pitches) forward or backward. If it pitches forward (nose down), the thrust fan is positioned too high relative to the center of mass, creating an undesired nose-down pitching moment. The lift fan is working against this moment and may not be generating sufficient pressure to sustain hover. Solution: reduce lift motor speed slightly, or reduce thrust until pitch is stable.

Check 2: Lateral stability. While moving, observe whether the craft maintains a straight path or drifts left or right. If it drifts, the CoM or thrust line is not aligned with the centerline. This is a structural issue (not a motor balancing issue), but the imbalance is amplified if lift pressure is low. Ensure the lift motor is generating sufficient pressure (verifiable by the craft hovering smoothly when thrust is off).

Check 3: Speed consistency. Accelerate from rest to full speed and observe whether the craft accelerates smoothly or in stutters/pulses. Smooth acceleration indicates good thrust fan torque and consistent hover. Pulsed acceleration suggests the lift cushion is collapsing intermittently due to insufficient lift motor power relative to thrust demand.

Check 4: Hover height during acceleration. With video recording (or careful observation), watch the craft's height above the ground as it accelerates. If hover height decreases noticeably during acceleration, the lift motor is struggling to maintain pressure as the thrust motor draws power from the battery. Solution: increase lift motor potentiometer setting to provide more cushion pressure reserve.

Battery Considerations

If both motors share the same battery, total current draw when both are at full throttle can be substantial. A typical hovercraft with two DC motors might draw:

• Lift motor: 1.5 A
• Thrust motor: 2.0 A
• Total: 3.5 A

A 4-cell NiMH battery (2000–2500 mAh typical) can sustain 3.5 A for approximately 30–45 seconds before voltage sags noticeably. For longer runs (60+ seconds), battery voltage will drop, reducing motor speed and craft performance.

Mitigation:

• Use a higher-capacity battery (3000–4000 mAh) if available
• Reduce thrust motor speed during longer runs to lower total current draw
• Use separate batteries for lift and thrust motors if regulation permits (some events allow this, some don't)

Synchronizing Motor Timing

In advanced designs, teams use electronic speed controllers (ESCs) or logic circuits to maintain a fixed speed ratio between lift and thrust even as battery voltage sags. For example, a circuit might be wired so that if lift motor voltage drops by 5%, thrust motor voltage also drops by 5%, maintaining the preset ratio.

For most Science Olympiad teams, this level of sophistication is not necessary. Manual potentiometer adjustment is sufficient if done correctly during pre-competition testing.

Pre-Competition Calibration for Dual Motors

1. Fully charge batteries and allow 10 minutes for voltage to stabilize.

2. Set lift motor potentiometer to 80% (conservative setting to ensure hover is never marginal).

3. Verify hovering is stable on a smooth, level surface. Allow the craft to hover for 30 seconds and observe for any drift, height changes, or instability. If any issues, increase lift motor speed to 90%.

4. Set thrust motor potentiometer to 40% (low speed, to measure the effect of thrust on lift balance).

5. Launch the craft and observe:

   • Does it accelerate smoothly?
   • Does hover height change as it accelerates?
   • Does it maintain straight-line motion?

6. If behavior is stable, gradually increase thrust motor speed (10% increments) until the craft reaches your target speed (based on pre-competition distance/time target). Record the thrust motor setting that produces target speed.

7. If behavior becomes unstable (pitch instability, height loss during acceleration), increase lift motor speed by 5–10% and repeat from step 5. The goal is to find the minimum lift motor setting that allows smooth operation at your target thrust speed.

8. Document both motor settings: Lift = X%, Thrust = Y%

9. Repeat the test 2–3 times with the same settings to verify consistency.

Common Mistakes

• Setting both motors to 100% and hoping for the best: Without tuning, the craft often lifts unevenly, drifts, or may fail to hover reliably.
• Prioritizing speed over stability: Running the thrust motor at 90% while the lift motor is at only 60% causes the lift cushion to collapse under the transient loads of acceleration. The craft may crash partway through a run.
• Not re-balancing when battery drains: A well-balanced craft on a fresh battery may become unbalanced mid-competition as battery voltage sags. Teams should either (a) pre-charge fresh batteries for each run, or (b) adjust motor settings based on battery voltage at the time of launch.
• Confusing potentiometer-based control with motor speed ratios: If lift is set to 80% of battery voltage and thrust to 60%, the ratio is 80/60 = 1.33:1. If you later change both by 10 percentage points (to 70% and 50%), the ratio changes to 70/50 = 1.4:1. Always verify absolute settings, not just relative changes.

8.5 Effect of Battery Voltage on Speed

The Linear Relationship (With Caveats)

For a DC motor operating at constant load, motor speed is approximately proportional to supply voltage:

ω = k × (V - I_a × R_a)

where ω is angular speed, k is a motor constant, V is supply voltage, I_a is armature current, and R_a is armature resistance. Rearranging:

ω ≈ k × V - k × I_a × R_a

The second term (voltage drop due to armature current) is typically small, so the relationship simplifies approximately to:

ω ≈ k × V

This means a 10% decrease in supply voltage should produce roughly a 10% decrease in motor speed. For a hovercraft running at 1.0 m/s on a 10.8 V battery:

• At 10.8 V (100%): speed = 1.0 m/s
• At 10.2 V (94%): speed ≈ 0.94 m/s
• At 9.4 V (87%): speed ≈ 0.87 m/s

This 13% speed reduction from fresh to moderately discharged battery is significant for precision speed control. A team expecting to run at 1.0 m/s will undershoot if the battery is partially depleted.

NiMH Battery Discharge Curve

NiMH batteries do not discharge linearly. The voltage remains relatively flat for most of the charge cycle, then drops steeply near the end. A typical 4-cell NiMH pack has:

• Fully charged (0% used): 10.8 V nominal (actually 11.1–11.4 V immediately after charging)
• 50% used: 10.8–10.6 V (plateau region)
• 80% used: 10.4–10.0 V (slight decline)
• 95% used: 9.6–9.0 V (steep drop begins)
• Dead (100% used): <8.4 V

The key implication: a team can run 2–3 competition runs without noticeable voltage drop because the battery remains on the plateau. The fourth run might see a 5–10% drop. By the fifth or sixth run, voltage is declining rapidly.

Run-by-Run Battery Degradation

For a hovercraft that runs approximately 30 seconds per run and draws an average current of 3 A:

Charge consumed per run = 3 A × 30 s / 3600 s/hr ≈ 0.025 Ah

For a 2.5 Ah battery pack:

Charge as % of total capacity = 0.025 / 2.5 ≈ 1%

Each 30-second run consumes approximately 1% of the battery's capacity. Three runs consume 3%, leaving the battery at 97% state-of-charge with almost imperceptible voltage drop. Four runs (4% consumed) still show minimal voltage change. By seven runs (7% consumed), voltage may be noticeably lower if the battery is at the steep part of the discharge curve.

However, the voltage sag during run (due to current draw and battery internal resistance) is larger than the cumulative discharge. During the run, when the motor draws 3 A, the voltage sags:

V_under_load = V_open_circuit - I × R_internal

For a NiMH pack with approximately 0.1 Ω internal resistance:

V_under_load = 10.8 V - 3 A × 0.1 Ω = 10.5 V

This 0.3 V sag (2.8% voltage drop) persists for the entire run, regardless of whether it's the first run or the fifth. When the motor is shut off and the battery rests, voltage recovers to the open-circuit value within seconds (due to the exponential decay of voltage sag recovery).

Calibration Implications

A hovercraft calibrated to run at a specific speed (say, 1.0 m/s at 65% throttle on a fully charged battery) will run slower on the same throttle setting if:

1. The battery is partially discharged
2. The battery is still being loaded (current is still being drawn)

Pre-competition strategy:

• Charge the battery fully 30–60 minutes before competition
• If the craft will be run multiple times in competition (as is typical), expect 3–5% speed reduction by run 3 or 4 compared to run 1
• Test during pre-competition training with the same battery state (or multiple battery states) to build intuition

Multi-Battery Strategy

Some teams carry multiple charged battery packs and swap them between runs to avoid voltage droop:

Advantage: Consistent voltage and thus consistent speed across all runs.

Disadvantage: Requires space for multiple battery packs, quick-disconnect wiring (crucial for fast swaps), and the cost of multiple batteries.

Practicality: A team with a single battery pack can run 3 competition runs without significant voltage issues (runs 1–3 occur while the battery is in the plateau region). Only if the team is attempting 5+ runs or needs perfect speed consistency for run 4 is a second battery pack necessary.

Temperature Effects

Battery internal resistance increases with temperature. A cold battery (0–5°C) has higher internal resistance than a warm battery (20–25°C), causing more voltage sag for the same current. A battery that was charged indoors and then brought outdoors to a cold competition venue may sag 5–10% more than expected.

Conversely, a battery heated by multiple rapid runs may have lower internal resistance, causing less voltage sag (but also degrading faster due to thermal stress).

Practical consideration: If competing in a cold environment, allow the battery to warm to room temperature indoors before running the craft outdoors.

Voltage Sensing and Dynamic Adjustment

Advanced teams with access to voltage sensors or multimeters can measure battery voltage before each run and adjust the throttle setting accordingly:

Pre-run procedure:

1. Measure open-circuit battery voltage with a multimeter
2. Compare to the fully-charged reference voltage
3. If voltage is 3–5% lower than reference, increase throttle setting by 3–5% to compensate

For example, if the calibration was done at 10.8 V and target speed is 1.0 m/s at 65% throttle:

• Fresh run at 10.8 V: 65% throttle → 1.0 m/s
• Third run, battery at 10.5 V (97% of 10.8 V): increase throttle to 67% to maintain 1.0 m/s

This compensation requires either quick mental math or a pre-calculated table:

Implications for Motor Selection

When choosing a motor for the thrust fan, consider the motor's efficiency and voltage sensitivity:

High-efficiency motors (brushless outrunners, or high-quality brush motors) maintain more consistent speed across voltage variations because they have lower armature resistance. A motor with 5 Ω armature resistance will show less voltage droop per amp than a motor with 10 Ω resistance.

Motor kV rating (RPM per volt) also matters. A motor rated at 1000 kV (1000 RPM per volt) will vary in speed more dramatically with voltage changes than a motor rated at 500 kV. For hovercraft, mid-range kV motors (600–1000 kV) provide a good balance.

Pre-Competition Checklist for Battery Management

1. Test with fresh, fully charged battery during pre-competition training. Record the throttle setting that produces your target speed.

2. Test with a partially discharged battery (after 2–3 practice runs). Verify that speed is still acceptable or only slightly reduced.

3. Charge the battery immediately before competition (within 30 minutes).

4. Verify battery voltage with a multimeter if available, immediately before running.

5. If multiple runs are planned, either (a) use fresh batteries for each run, or (b) increase throttle setting by 3–5% after each run to compensate for voltage droop.

6. Rest the battery for 10–15 minutes between runs to allow voltage to recover from sag.

Common Mistakes

• Calibrating with a fully charged battery, then competing with a partially discharged one: Results in 5–10% speed reduction and likely undershoot on the target distance.
• Using the same throttle setting for all competition runs without accounting for battery discharge: Runs 1–3 maintain consistent speed, but run 4 becomes noticeably slower.
• Not measuring actual battery voltage before competition: Assuming the battery is "charged" without verification risks bringing a partially depleted battery to competition.
• Letting the battery sit uncharged between practice sessions: A battery left for weeks gradually self-discharges. Recharge fully before competition day.
• Using cheap or low-quality batteries: These have higher internal resistance and thus more voltage droop. Invest in reputable NiMH packs (e.g., Tenergy, EBL, brand-name models).

8.6 Environmental Variables & Surface Adaptation

The Surface Is Your Opponent

A hovercraft's performance is exquisitely sensitive to track surface properties. Two identically built hovercraft running on different surfaces will achieve different speeds, hover heights, and stability characteristics. Teams that account for surface variation during design and pre-competition testing outperform teams that assume "a track is a track."

Key surface properties that affect hovercraft performance:

1. Friction coefficient (μ): Directly affects the friction force experienced by the skirt. Different materials have vastly different μ values.

2. Surface roughness (Ra, Rq): Measured in micrometers, roughness affects both skirt contact friction and air turbulence in the hover gap.

3. Surface compliance: Hard surfaces (concrete) vs. soft surfaces (artificial turf, rubber) affect how the craft sits and whether the skirt can conform to irregularities.

4. Temperature: Surfaces heat and cool, affecting their friction properties and the hovercraft's battery and motor temperatures.

5. Moisture: Wet surfaces increase friction, may cause tape seals to fail, and introduce hydrostatic pressure forces.

Common Competition Track Surfaces

Polished gymnasium floor (vinyl or polyurethane):

• Friction coefficient: μ = 0.08–0.15 (very low)
• Roughness: Ra < 0.5 μm (very smooth)
• Characteristics: Fastest surface for hovercraft; minimal friction; excellent for high-speed runs
• Hovercraft speed potential: 1.5–2.5 m/s depending on craft design
• Calibration strategy: Target the lowest throttle settings; fine control is achievable because small changes have clear effects

Painted concrete (outdoor or covered):

• Friction coefficient: μ = 0.20–0.35 (high)
• Roughness: Ra = 2–10 μm (moderately rough)
• Characteristics: Roughly twice the friction of gymnasium floors; more challenging for hovercraft; variable quality depending on paint type and condition
• Hovercraft speed potential: 0.8–1.5 m/s
• Calibration strategy: Higher throttle settings required to achieve target speeds; skirt design becomes more critical

Raw concrete (unsealed):

• Friction coefficient: μ = 0.30–0.50 (very high)
• Roughness: Ra = 5–50 μm (rough)
• Characteristics: Extremely challenging; hovercraft may struggle to reach high speeds; requires robust skirt design to avoid collapse
• Hovercraft speed potential: 0.5–1.2 m/s
• Calibration strategy: May require higher-powered fan systems or lower-friction skirt materials than expected

Plywood or hardwood:

• Friction coefficient: μ = 0.20–0.40 (high)
• Roughness: Ra = 1–5 μm (smooth to moderately rough depending on finish)
• Characteristics: Common in older Science Olympiad venues; often rougher than painted concrete despite appearance
• Hovercraft speed potential: 0.8–1.4 m/s
• Calibration strategy: Similar to painted concrete; high throttle required

Artificial turf or carpet:

• Friction coefficient: μ = 0.40–0.60 (very high)
• Roughness: Highly irregular due to fiber structure
• Characteristics: Extremely challenging; fibers can clog or degrade skirt materials; most hovercraft perform poorly here
• Hovercraft speed potential: 0.4–0.9 m/s
• Calibration strategy: Often not recommended for hovercraft unless the craft is specifically optimized for rough surfaces

Quantifying Surface Effects on Speed

The friction force during hover is:

F_friction = μ × W

where μ is friction coefficient and W is hovering weight. Available thrust is fixed by motor and propeller selection. Net acceleration is:

a = (F_thrust - F_friction) / m

For a 400 g craft with thrust capable of 0.4 N:

On gymnasium floor (μ = 0.10): F_friction = 0.10 × 3.92 = 0.39 N. Net force = 0.4 - 0.39 = 0.01 N. Acceleration is minimal; craft barely accelerates.

On painted concrete (μ = 0.28): F_friction = 0.28 × 3.92 = 1.10 N. Net force = 0.4 - 1.10 = -0.70 N. Craft accelerates backward (decelerates) or doesn't move.

Wait—this calculation shows the craft cannot move on painted concrete at low thrust! This is unrealistic. The issue is that the above calculation assumes static hover friction, but in practice:

1. The skirt in motion experiences lower friction than static skirt contact
2. The air bearing actively resists contact as it moves
3. Friction varies dynamically as the craft accelerates

A more realistic model accounts for kinetic friction being lower than static friction, and for air bearing effects:

F_friction_kinetic ≈ 0.5–0.8 × F_friction_static for many skirt materials on rough surfaces

With this correction: On painted concrete (μ_kinetic ≈ 0.15): F_friction = 0.15 × 3.92 = 0.59 N. Net force = 0.4 - 0.59 = -0.19 N. Still insufficient.

This suggests that a poorly designed hovercraft will not accelerate on rough surfaces without either (a) higher thrust, (b) lighter construction, (c) lower-friction skirt design, or (d) higher lift pressure (to reduce percent of weight carried by contact friction).

Pre-Competition Surface Testing

The best teams conduct at least one full test session on a surface type similar to the expected competition surface. If the competition is outdoors on painted concrete, the team tests on painted concrete. If the team cannot access the exact competition surface, they should test on the closest available alternative.

Testing protocol:

1. Establish a baseline on a known surface (e.g., gymnasium floor). Record throttle settings that produce target speeds and the achieved speeds.
2. Test on the target surface with identical throttle settings. Measure speed and distance.
3. Compare results and calculate the speed reduction ratio: speed_rough / speed_smooth.
4. Develop a correction factor. If speed is reduced by 30% on rough surfaces, multiply target throttle by 1.3 to compensate.
5. Prepare contingency throttle settings for different surface types in case the competition surface is different from expectations.

Acoustic and Temperature Effects

Noisy environments (outdoor competitions with wind, aircraft, traffic) can be disorienting, but they don't directly affect hovercraft performance. However, high ambient temperature does:

Motor winding resistance increases with temperature: R = R_0 × (1 + α × ΔT), where α is the temperature coefficient (~0.004 per °C for copper). A 20°C ambient temperature increase can increase motor resistance by 8%, causing an 8% speed reduction for the same voltage and similar load.

Battery performance degrades at temperature extremes: At high temperature (>35°C), battery internal resistance increases and voltage sag worsens. At low temperature (<5°C), battery capacity is effectively reduced.

Friction changes with temperature: Most surfaces become slightly more slippery at higher temperatures due to reduced adhesion. This partially offsets motor slowdown, but not completely.

Practical consideration: If testing on a cool morning and competing in the afternoon heat, expect modest performance changes. Use a slightly higher throttle setting at competition than was used during practice.

Adaptive Calibration Strategy

Top competitive teams use a tiered calibration approach:

1. Baseline calibration: Test on the assumed surface (gymnasium) and develop a throttle curve.
2. Contingency curves: Test on 2–3 alternative surfaces (painted concrete, plywood, etc.) and develop curves for each.
3. At competition: Upon arrival, inspect the actual competition surface. If it matches one of the tested surfaces, use the corresponding calibration curve. If it's novel, estimate which tested surface it most closely resembles and use that curve.

Example calibration table:

With this table, the team can quickly adapt to any surface.

Field Troubleshooting for Poor Performance

Symptom: Craft runs 20% slower than expected. Diagnosis: Likely higher surface friction than anticipated. Solution: Increase throttle setting by 20–25%. If still insufficient, surface is rougher than any tested surface; increase throttle further or accept lower speed.

Symptom: Craft accelerates inconsistently (speeds up, slows down). Diagnosis: Skirt is partially collapsing due to rough surface irregularities, or cushion pressure is marginal. Solution: Increase lift motor speed (if controllable) to build more pressure margin. Otherwise, reduce thrust speed slightly to allow the craft to hover more stably.

Symptom: Craft drifts or curves during acceleration. Diagnosis: Asymmetric friction on different parts of the skirt (due to skirt damage, uneven sealing, or asymmetric surface). Solution: Inspect skirt visually for tears or leaks. If found, repair with tape. If not visible, accept the steering bias and aim the craft at a slightly different angle to compensate.

Common Mistakes

• Testing only on gymnasium floors during development, then competing outdoors: A craft optimized for smooth surfaces often underperforms on rough surfaces. The team lacks contingency throttle settings.
• Assuming all "concrete" surfaces are identical: Concrete varies dramatically in texture and friction depending on age, weathering, and finish. Test on the specific surface if possible.
• Not accounting for skirt wear: A skirt that was low-friction when new may accumulate debris or wear thin, changing friction characteristics. Inspect and potentially replace the skirt before competition if heavy testing has occurred.
• Competing in a time zone or climate different from training: High altitude, high temperature, or high humidity all affect performance. Conduct at least one test session under competition conditions if possible.