Glider Stall Diagnosis Under Time Pressure
A cadet team builds a hand-launched glider for a squadron competition. The glider has a mass of 180 g and a wing area of 0.12 m². On its first three launches it climbs steeply, slows, and stalls, landing within 12 m. The team's launch speed is measured at 8 m/s, and the throw is consistently 25° nose-up. The rules require a minimum straight-line flight of 20 m. A teammate suggests adding 40 g of ballast to the nose and launching harder. Another suggests reducing the wing incidence and launching at a shallower angle. The team has one afternoon and two spare foam wings to test. The same thrower will launch every trial, and a fixed rail can reproduce the 8 m/s launch speed.
1. Calculate the glider's wing loading and explain, in physical terms, why a steep nose-up launch leads to a stall.
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Wing loading = weight ÷ wing area = (0.180 × 9.81) ÷ 0.12 ≈ 14.7 N/m², or 1.5 kg/m².
A 25° nose-up launch sets a high angle of attack. If that angle exceeds the critical angle, the airflow separates from the wing and lift drops sharply - a stall. The steep climb also converts kinetic energy into height, so airspeed falls. At lower speed the wing must produce a higher lift coefficient to support the same weight, which pushes the angle of attack even closer to critical. The result is a short, steep flight ending in a stall within 12 m, well short of the 20 m requirement.
2. Evaluate the two suggestions using stability and trim reasoning. Which is more likely to work, and why?
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Nose ballast raises wing loading to (0.220 × 9.81) ÷ 0.12 ≈ 18.0 N/m². Stall speed rises by about √(18.0/14.7) ≈ 1.11, roughly 11%. Nose ballast may improve pitch stability, but it makes the glider heavier and demands a faster launch; with the same 8 m/s throw, it is likely to stall sooner, not later. It treats a symptom.
Reducing wing incidence and launching shallower lowers the angle of attack at release. That keeps more of the launch speed as airspeed and delays the critical angle. The risk is that too shallow an angle reduces lift and climb. The second suggestion is more likely to work, provided the incidence change is small and tested; a launch angle near 10–15° is a reasonable starting point.
3. Design a quick afternoon test plan that isolates one variable. State what you would measure and what would make you reject a change.
Read the worked answer
Use the fixed rail and the same thrower. Run a baseline of three launches at 25° with the standard wing. Then change only the wing incidence by 1° and run three launches at the same 25° setting; then change incidence by 2° and repeat. Record distance, whether the glider stalls, and time to stall. Do not change ballast and incidence in the same trial.
After that, test launch angle at the best incidence: 10°, 15°, 20°. Reject any change that reduces the median distance or makes the stall sharper, even if one lucky launch exceeds 20 m. Keep the two spare wings for the two incidence settings so the baseline wing is not damaged.