A researcher at the University of Southern California Viterbi School of Engineering has found that aircraft wings at smaller scales perform in a way counter to the standard relationship between lift and the angle of attack.
Wings used on aircraft have a particular cross-section design, known as an airfoil, which is designed according to the textbook standard relationship between lift and the angle of attack. This standard says that as the angle between the wing and the oncoming air (the angle of attack) increases, the lift also increases.
Thus, in standard models, as the aircraft tilts upwards, it generates more lift, making it go higher. This continues until a critical angle, after which the airplane will stall and the lift drops, causing the plane to lose altitude.
However, in experiments, Professor Spedding from the School’s Aerospace and Mechanical Engienering Department found very different performance when the wings are at a smaller scale.
The airfoil design that Professor Spedding tested with his student has a wingspan of just over 45 cm, a size which is often used in sail planes, and unmanned surveillance aircraft. They tested it in a wind tunnel.
“We’ve been finding these curious little known or unknown things in regular fixed wings just because they’re small, but not too small,” Professor Spedding explained.
“So, it’s the ‘Goldilocks Wing’ – just the right size where the aerodynamics becomes extraordinarily complicated.”
In their tests, instead of lift going up as the angle of attack increased, it dropped into the negative region. Then it went way up resulting in a 'super-lift' effect before eventually tapering off in a gradual stall.
The team repeated the experiment to eliminate the possibility of errors in the test setup. They spent months retesting, recalibrating, using new instrumentation and a new airfoil model, but got the same results.
Their experimental findings led to a surprisingly simple, but somewhat tricky design consideration.
“When a symmetric airfoil is at zero degrees, the flow always goes over it in a nice, symmetric way. But, because this airfoil is a certain size, the flow is always separated before the trailing edge,” explained Spedding.
“So, the effective shape of the airfoil is not just the solid body, but the solid body plus this separation region, and it changes as you change the angle of attack.”
Despite this difference, plane with this wing size and design are still able to fly because in the real world, they are subject to unsettled or turbulent conditions which shift the separation points. It is only in perfect experimental conditions in labs where the effect can be isolated.
Professor Spedding says the results highlight the disparity between experiments, computations and aerodynamic models and the need to work towards greater understanding of the performance of airfoils, particularly at smaller scales, such as those that the next generation of drones would fall into.
[Image: Airfoil designs for testing in the USC Dryden wind tunnel. Photo/Valentina Suarez.]