Summer Operation in a 85 HP J3 Cub 

1. Consult the Aircraft’s Pilot Operating Handbook (POH): The POH for the Piper J3 Cub with a C-85 engine provides performance charts that show the rate of climb at different altitudes and temperatures. However, the J3 Cub is a simple aircraft, and detailed POH documents may not be readily available. In such cases, we rely on general data and rules of thumb.

2. Determine the Standard Rate of Climb at Sea Level: For a Piper J3 Cub with a C-85 engine, the rate of climb at sea level under standard conditions (15°C, 59°F) is approximately 500 feet per minute (fpm).

3. Adjust for Density Altitude:

   • Temperature and Altitude Effects: As altitude increases, the air becomes less dense, resulting in reduced engine performance and aerodynamic efficiency. At a density altitude of 7000 feet, the engine produces less power, and the propeller and wings are less efficient.
   • Performance Charts or Estimation: Use performance charts if available. If not, a general estimation can be made using rules of thumb.

4. Estimate Rate of Climb at 7000 feet Density Altitude:

   • Rule of Thumb for Climb Rate Reduction: For every 1000 feet increase in density altitude, the rate of climb typically decreases by about 8-10% for small piston-engine aircraft.
   • Calculating the Estimated Climb Rate:
     • Sea level rate of climb: 500 fpm.
     • Density altitude increase: 7000 feet.
     • Percentage reduction: (7000 feet / 1000 feet) * 9% (average) = 63%.
     • Estimated reduction in climb rate: 500 fpm * 63% = 315 fpm.
     • Estimated rate of climb at 7000 feet density altitude: 500 fpm – 315 fpm = 185 fpm.

Conclusion

The estimated climb performance of a Piper J3 Cub with a C-85 engine at a field elevation of 4700 feet on a density altitude day of 7000 feet is approximately 185 feet per minute. This estimate assumes standard conditions and typical aircraft weight. For precise performance data, always refer to the specific aircraft’s POH and performance charts if available.