The Importance of Aerodynamics Research for Aviation

… From the old times when the mankind made first trials to formalize the Dream about Flight on the paper, it was noticed that the air flowing around any surface applies a force against it, this force in accordance with Newton’s law, may be separated into several parts and one of them is Lift: the component which is perpendicular to the oncoming flow direction and depends on the profile of the surface, its position to the horizon (later this was called the Angle of Attack), speed of the air flow, Lift is always accompanied by a drag force, which is the component of the surface force parallel to the flow direction – so, all these knowledge formed the “Basis” which once helped Wright brothers to launch their “ Flyer-1 ” to its short first flight in 1903. Later these considerations were systemized and formed the basic aircraft aerodynamics.

Generally, Aerodynamics is the science studying the air movement around objects. The rules of aircraft aerodynamics explain the ability of an airplane to fly. The four forces of flight are Lift, Gravity, Drag, and Thrust. These forces make an airplane move up or down, faster or slower.

The following picture may explain the physical nature of Lift, which is required for heavy-than-air flying:

To obtain the Lift, upper wing surface is designed with the greater area than the lower one in accordance with Fundamental Bernoulli’s Principle : When the velocity of a fluid increases, its pressure decreases by an equivalent amount to maintain the overall energy, - so, higher air pressure in lower wing surface “pushes” the airplane upper producing the Lift.

Generally, airplanes are kept from falling down by forward thrust of the wings or airfoils through the air. The thrust, which drives the wing forward, is provided by the external source, for example, by

propellers or jet engines. The result of the wing moves through stationary air is a lift force perpendicular to its motion, which is greater than the gravitational force acting downwards, and keeps the aircraft in the air. The Lift is accompanied by drag induced by the air resistance against the wing. The Drag depends on the wing effective area facing directly into the airflow as well as the airfoil shape. Lift and Drag magnitudes are dependent on the Angle of Attack between the motion direction of the wing through the air and the chord line of the wing.

After establishing these rules, it was noticed that flight performance of the aircraft critically depends on its aerodynamic layout and it laid the foundation of the Aircraft aerodynamics research as a Part of Aircraft Design .

Nowadays every new aircraft, starting from the preliminary design stage, is being researched on its aerodynamic layout: choice of the arrangement and geometric parameters of the aircraft main parts that are streamlined by air during the flight, their coordination with each other in order to obtain such aerodynamic characteristics that would allow the designed aircraft to perform its mission.

For example, an airplane with retractable landing gear will have less drag in the main flight modes than the one without this system. However, the mass of an airplane with a retractable landing gear, all other things being equal, will be greater than the mass of the one without the retraction system due to higher structural mass of fittings, retraction mechanisms, hydraulics, fuselage cutouts and reinforcements and in general, more complicated structural design. This will require an increase in engine power, respectively, an increase in fuel consumption, which will reduce the airplane operating range or require the additional volume for the fuel tank, which reduces the max payload.

All these requirements are to be put into a compromise between Aerodynamics, Structures and Systems groups playing the main roles in the Aircraft design process.

Aircraft configuration of a “classical” aerodynamic layout (most of the commercial planes has this layout) is the following:

Aircraft control aerodynamic surfaces, also called flight controls, are divided into primary flight controls and secondary or auxiliary flight controls. Primary flight controls serve to move the airplane about the pitch, roll, and yaw axes. They include the ailerons, elevator, and rudder. Secondary flight controls include tabs, leading-edge flaps, trailing edge flaps, spoilers, and slats.

Other Aerodynamic layout types are listed below. The Designers are selecting each type during Preliminary Design stage depending on mission and requirements to the designed airplane:

1 – “Classical” aerodynamic layout. The horizontal empennage is after the wing. Most commercial and military transport aircraft has this layout.

2 – “Tailless” layout. Some jet fighters (Mirage III, Typhoon) and Concorde aircraft has this aerodynamic layout.

3 – “Flying Wing” aerodynamic layout. Tailless fixed-wing aircraft that has no definite fuselage . Northrop B-2 Bomber, numerous experimental aircraft.

4 – Canard layout. Arrangement wherein a small forewing or foreplan is placed forward of the main wing. Wright Brothers “Flyer-1” used this layout, XB-70 Valkyrie, Beechcraft Starship.

5 – Tandem Wing. This type of aircraft has two wings : one located forward and the other to the aft. Both wings contribute to lift.

After selection of the aerodynamic layout and scheme, establishing the dimensions of Wing, Fuselage and Empennage, obtaining the first 3D-models from CAD systems, engineers perform the first analysis to get the aerodynamic performance data: lift, drag, moments, lift and drag coefficients, obtain and compare the Wing Polar Diagrams to optimize the design. Obtain the loads acting on the airplane aerodynamic parts (wing, empennage, fuselage, controls) in different flight modes to perform stress analysis and sizing:

This analysis is performed using the specialized Computational fluid dynamics (CFD) software from different developers: MSC Software , Stallion 3D , Simscale and others.

Finally, to clarify theoretically obtained loads, make sure the airplane design is not affected by such destructive physical phenomenon, like Flutter (dynamic instability of an elastic structure in air flow) of the Wing and/or horizontal empennage Aerodynamicists use wind tunnels to test models of proposed aircraft or its separate components.
Typical Wind Tunnel and aerodynamics model testing:

Wind tunnels are large tubes with air blowing through them. From front to back, there are 5 main parts of Wind Tunnel: Settling Chamber, Contraction Cone, Test Section, Diffuser, and Drive Section. Wind tunnels are classified by the Air speed they can produce: low subsonic, transonic, supersonic, and hypersonic.

After the performance of all aerodynamics theoretical analysis and tests, Structures group has all spectra of the loads necessary for stress analysis and detailed parts design for manufacturing.