Flight 101

Lesson 6: Flow Effects and Flight

In this lesson, we will cover the aerodynamics of objects which move rapidly through the air without contact with the ground. We will also learn about types of flow and effects such as subsonic, transonic, supersonic, and hypersonic.

Section 1 - The Aerodynamics of Objects

CATEGORIES OF OBJECTS

There are many objects that move through the air by means of aerodynamics. This includes airplanes, helicopters, missiles and bullets, super rockets and even golf balls.

These objects can basically be put into two categories: “ballistic” and “aerodynamic.” The ballistic category includes bullets, shells and rockets. These objects are symmetrical in shape, and their motion is determined by their speed, drag and weight.

They travel in curved paths and usually at supersonic speeds which are not commonly found in nature, with the exception of meteors.

Some guided missiles have a more complex type of aerodynamics because they have wings to develop lift forces for maneuvering. They also have control surfaces to change the angle of the missile to the direction of the air.

In a manned airplane, the air not only provides the lifting force, but also serves as the source of oxygen for the propelling engines, and as cabin air for the occupants to breathe.

THE FLIGHT OF BIRDS AND INSECTS

Here it is relative to examine the flight of birds and insects. Bird flight provides an example of laminar flow and high lift slots and flaps for landing.

Birds in flight are able to achieve speeds of up to 100 mph, altitudes up to nearly 30,000 ft, and ranges of 3000 miles without stopping in some species.

There have been amazing studies conducted on the flight of locust. Scientists Weis-Fogh and Jensen harnessed living locusts to a measuring balance in a wind-tunnel and measured lift and drag forces at different wind speeds and incidence. They did this while filming the wind motion.

Thorax temperatures were measured and recorded as an index of muscular work done. Further experiments yielded that the locust can detect the speed and direction of the relative wind by means of sensitive hairs on its head.

AERODYNAMICS AND MAN

We can also relate aerodynamics to man concerning air entering nasal, throat and lung passages during breathing, sneezing and coughing, and the cooling of wind on the skin and the perspiration process.

The air drag of a human body is important to consider in the riding of bicycles and parachuting. Wind tunnels measuring this have indicated typical values at 100 ft/sec to be 110 lb when facing the wind and 15 lb when lying down. The ejection of pilots from aircraft at very high speed exposes them to an air blast capable of tearing face tissue and breaking bones by the shock force. This has been countered in supersonic aircraft by encasing the pilot in an airtight capsule.

HOVERCRAFT

A Hovercraft is interesting to study although no longer commonly used as a method of transport. Also called a Cushioncraft or Ground Effect Machine, the Hovercraft rides on a cushion of air with pressure raised above normal by fan-supplied energy.

Horizonatal motion is provided in the Hovercraft by an air propeller, jet or air directed from the fan. In a Hovercraft, aerodynamic problems include the efficiency of the side ‘curtain’ or air which must be ducted carefully around the edge, to trap the higher pressure air cushion underneath.

AERODYNAMICS OF THE AIRPLANE

In discussing aerodynamics and the airplane, there are seven basic features which are determined largely by aerodynamic considerations.

These are:

1.Provision of sufficient lift. The lift is produced by the wings, downward – deflected jets or rotor blades. The lift on a fixed-wing aircraft results from its speed as well as a circulation flow induced by viscosity and an asymmetry of shape. The lift equals the weight in steady level flight. However, much more is required for climbing, maneuvering and landing.

2.Efficiency of lift in relation to drag. Since lift is generated by air flowing over wing surfaces, it cannot exist without drag. The correct flight speed is only achieved when the propulsion engine thrust is at least equal to the drag. Effective wing design, therefore, involves a knowledge of both lift and drag. It is also necessary to know how they change with shape, incidence and moving flaps and slots.

The lift-drag ratio or L/D is a very important factor of flight efficiency.

3.Efficiency of propulsion propellers or jets. The propulsion system provides the forward force needed to overcome drag and inertia. This enables the desired flight speeds.

4.Stability. Stable flight becomes possible if two conditions are satisfied. The first is that the forces and moments should all balance. The second is that if the airplane is disturbed from a position of equilibrium by a gust of wind, the changed aerodynamic forces should automatically act in order to return the airplane to its previous steady state. To enable this, the tail surfaces provide most of the stability and the forces on the tail, depending on how the air flow is bent passing over the wing and bodies.

5.Control. An airplane frequently changes speed and direction. Aerodynamic control is exercised by the pilot to place the airplane at the correct angle to the oncoming airflow in order to create the right lift, drag and side-forces, which will then move the airplane in the desired flight path.

Control is usually arranged by movable surfaces which change the airflow as well as the forces and moments. They must not vibrate or become stable and be light to operate.

6.Air-conditioning. Special inlets are provided on airplanes to collect air. This air is then heated or cooled, pressurized, humidified, and finally passed to the occupants conditioned air also filters to the electronic equipment.

7.Determination of local air pressures and heating over the surface for stressing purposes. The overall shape determines lift, drag and stability. However, the structure must be strong enough to withstand the air pressure on the surface. It must not deflect and disturb the smooth airflow. It must be able to expand with heating, and without failing.

The shape and speed of an airplane determine how the air is deflected by its passage. Any changes in air velocity lead to the aerodynamic pressures, forces and moments which contribute to the seven features outlined.