## Aeroplanes and gliders

Aeroplanes or gliders for the Martian atmosphere have reached a significant level of technical maturity, in that proposals sufficiently well-developed to be seriously considered in competitive NASA mission selections have been made. The Martian atmosphere is very thin - comparable with the Earth's high stratosphere -which makes aviation difficult, but not impossible. Rather high flight speeds are required to achieve sufficient lift.

Key aerodynamic parameters are the wing loading (i.e. the vehicle weight per unit wing area) and the lift-to-drag ratio (L/D). The lift L on a wing is expressed as

where V is the flight speed, CL is the lift coefficient, S the reference area (usually the wing area) and p the air density. In level flight, this expression must equal the vehicle weight W. High wing loading (W/S) therefore requires high dynamic pressure (0.5pV ), thus a high flight speed, or a very high flight speed if the air is thin (low p). The drag is similarly written

In steady flight, this must be balanced by a forward component of weight (in gliding, the glide slope will equal L/D or CL/CD) or by thrust from some sort of propulsion. The drag power (and thus an absolute minimum propulsive power requirement - propulsive efficiencies of propellers, etc. may in fact be quite low, especially in thin atmospheres) equals the drag multiplied by the forward speed. In level flight, this is easily calculated knowing W = L and D = L/(L/D) - thus the lift-to-drag ratio is a key determinant of performance. The drag power is therefore P = VW/(L/D) - of course more power is needed if the vehicle is to climb.

The considerations above show how it is energetically favourable to fly a given mass with a large wing, and thus to fly slowly. Extremely power-limited aircraft on Earth, notably human-powered aircraft, have low flight speeds and very large (and light and therefore flimsy) wings. There are structural limits to the size of a practicably rigid wing (particularly for vehicles delivered to other planets where the wing must be folded inside a launch vehicle and/or entry shield) and some minimum flight speed may be required for traversing a given range in a fixed time.

For terrestrial subsonic propeller-driven aircraft at least, an empirical relation of required flight power is

Two important dimensionless numbers apply to flight - the Mach number and the Reynolds number. The achievable L/D scales strongly with Mach number and Reynolds number; it is impossible to develop high lift-to-drag ratios at high supersonic or hypersonic speeds (the Mach number being the ratio of the flight speed to the local sound speed). The Reynolds number is a measure of the relative importance of pressure (or inertial) forces in the fluid to the viscous forces (Re = vlpl„, where v and l are the characteristic velocity and dimension, „ is the dynamic viscosity). At low Reynolds numbers, viscous effects predominate. The important point for prospective aeronauts is that the lifting performance of a wing or a propeller declines significantly at low Reynolds number. This is a significant degradation for high-altitude flight on Earth, and particularly for flight on Mars.

The conventional aircraft that have been proposed for Mars include very light long-endurance planes (which would have to fly in the polar summer to continuously capture enough solar power for flight), gliders, and hydrazine-powered propeller planes - the latter of these would last only a few hours.

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