Drag Performance
Introduction
Drag is the resistance to motion of an aircraft flying through the air generated by a combination of pressure and friction. This resistance is overcome by the engines.
Modern civil aircraft are designed to minimise drag in order to reduce the environmental and economic costs of flying. By reducing drag, the amount of power needed from the engines is lessened and as a result the amount of fuel required is also reduced. Less fuel means a lower environmental impact and lower costs for the airline.
Aircraft manufacturers can reduce drag in a number of ways – be
it with a subtle change in geometry like adding a winglet or a more
substantial change of configuration like changing the shape of the
wings or repositioning the engines.
The Role of Wind Tunnels
Wind tunnels play a crucial role in the design of modern, greener aircraft. Improvements to aircraft designs are initially made using computer models. But even the most powerful of today’s computers cannot generate a perfect simulation of the air flowing around a complete aircraft. So when a design is sufficiently advanced, it is tested in a wind tunnel to check that the computer predictions are correct and that the designers have correctly understood what is happening to the air as it flows around the aircraft model.
Measuring Drag Performance
Because different design options – and the resulting differences in drag – are often very subtle, the measurement of drag must be very accurate. For example, at a cruise Mach number of 0.8 (about 980 kph or 612 mph), the drag on each wind tunnel model must be measured reliably to within about 1 Newton (the weight of an apple). When scaled up to a full sized aircraft crossing the Atlantic ocean, a reduction in drag of this size can save around a ton of aviation fuel.
Drag is calculated from the aerodynamic loads on the model measured by the internal balance, the model attitude relative to the air flow measured by the inclinometer and the model cavity or “base” pressure measured by pressure transducers. To learn more about wind tunnel balances, inclinometers and pressure transducers, follow the link to our Test Technologies and Services section below or use the navigation panel on your left.
Flow Investigation
Standard flow visualisation techniques such as oil flow are available to our customers to help provide an improved understanding of the airflow over the model surface. More advanced techniques such as PSP (pressure sensitive paint) and PIV (particle image velocimetry – used to visualise the flow field away from the model surface) are under active development and have already been used in the Transonic Wind Tunnel with considerable success.
To learn more about the flow investigation techniques available, follow the link to our Test Technologies and Services section below or use the navigation panel on your left.
The Latest Balance Technology
The greatest challenge in measuring drag accurately comes from the temperature changes experienced by the balance during a test. In older balances, these temperature changes may introduce inaccuracies into the drag measurements which require special test techniques to overcome. However, these test techniques add time and cost to a test.
With decades of experience in developing wind tunnel balances,
ARA has always been at the forefront of balance technology. The
latest balance developed by ARA is thermally stable so that it
doesn’t require any special test techniques to compensate for any
temperature changes.
The latest balance is now available in a 57.15mm (2.25in) diameter
size for use by our customers.
Technical Information
Drag repeatability for performance testing with a 57.15mm (2.25in) balance is as follows:
- Within a test series, drag repeatability is within half a drag count (∆CD = ±0.00005)
- Between test series, drag repeatability is within a drag count (∆CD = ±0.0001)
Standards of drag repeatability in the Transonic Wind Tunnel are carefully monitored with special tests of the in-house reference model performed on a regular basis.
