In aircraft environmental control, a simple air (open) cooling system is best suited for which flight speed regime?

Introduction / Context:
Air-cycle refrigeration (simple, bootstrap, regenerative) is used in aircraft. The simple air system relies on ram air compression and turbine expansion, making its performance closely tied to flight speed and altitude conditions.

Given Data / Assumptions:

• System: simple (open) air-cycle cooler without elaborate regeneration or multiple expansion stages.
• Ram effect increases with flight speed.
• Ambient at altitude provides low temperatures aiding heat rejection.

Concept / Approach:
At high flight speeds, ram compression raises air pressure before the turbine, improving cycle effectiveness. The high-speed slipstream also enhances heat exchanger performance. At low speeds or on ground, there is minimal ram effect; simple systems deliver poor COP and limited capacity, prompting use of more complex cycles or ground air carts.

Step-by-Step Solution:

Verification / Alternative check:
Performance maps show increased net refrigeration at cruise; on-ground performance deteriorates without supplemental fans or bleed strategies.

Why Other Options Are Wrong:

• (a) Low speeds provide weak ram effect; system is inefficient.
• (c) Hovering is rotorcraft-specific and does not provide useful ram for fixed-wing air-cycle systems.
• (d) Taxiing is slow; again poor performance.
• (e) Stationary operation relies on auxiliary equipment; not the intended regime for a simple air cycle.

Common Pitfalls:
Assuming an air-cycle system performs equally well on ground; it is highly speed-dependent.

Final Answer:
High flight speeds