One of the unconventional aircraft configurations NASA is researching is a cruise-efficient short take-off and landing (CESTOL) airliner that would be able to make steep approaches to and departures from short, underused runways, increasing airport capacity while keeping more of the noise within the boundary fence.
CESTOL is a powered-lift configuration, with a circulation-control wing that uses leading- and trailing-edge blowing to increase lift for take-off and landing. Most STOL aircraft are slow, but CESTOL is intended to cruise at jet speeds by also using wing blowing to reduce drag.
Not much experimental data exists to help design a circulation-control aircraft, so California Polytechnic State University San Louis Obispo built a large windtunnel model for NASA to collect both aerodynamic and acoustic data with which to validate the computational tools.Called AMELIA* - Advanced Model for Extreme Lift and Improved Aeroacoustics - it is a 10ft-span, 1/11th-scale powered model that was tested (below) in the 40 x 80ft National Full-Scale Aerodynamics Complex (NFAC), located at NASA Ames and managed by the US Air Force's Arnold Engineering Development Center.
Preliminary results from the tests were presented in Cleveland last week, at NASA's Fundamental Aeronautics program annual meeting, by Cal Poly assistant professor Tina Jameson (aerodynamics) and NASA Ames engineer Clif Horne (acoustics).
First of all, the idea of circulation control is to use blowing from thin slots in the leading and trailing edges to increase the velocity of air flowing over the wing and keep the airflow attached, increasing lift and allowing slower, steeper approaches and departures.
The diagram below shows how blowing can keep flow attached to the entire deflected high-lift flap. Although AMELIA has only 10ft span, the large NFAC tunnel was chosen to avoid interference from the walls because the circulation-conrol downwash can extend a wingspan or more below the model.
AMELIA is equipped with a pair of turbine propulsion simulators mounted over the wing so that their jet noise is shielded from the ground, and so that their jet exhaust blowing over the flaps helps increase lift at low speed. The model was tested with a clean wing, with and without blowing, and with the engine simulators mounted on low and high pylons.
The model is instrumented to measure aerodynamic forces and moments, wing surface pressure and skin friction, and flow visualization. The sequence of smoke-wand photos below shows air flowing over the wing at 30kt as blowing is increased - Cµ is the momentum coefficient. The higher the blowing, the greater the angle through which the airflow is turned.The oil-flow visualization photos below show how leading- and trailing-edge blowing stops the airflow separating and keeps it flowing smooth and straight across the wing and down the flaps.
As for the preliminary results, they look much as predicted. Blowing substantially increases lift. With trailing-edge blowing alone, that extra lift falls off as angle of attack increases because flow begins to separate from the leading edge. With leading- and trailing-edge blowing, lift increases with angle of attack because the flow stays attached. Adding the engines increases lift at high angle of attack, but their height makes only a small difference to performance.
Simultaneously, noise measurements were being taken using seven sideline microphones, a 48-element flyover acoustic array and five model-mounted sensors. The sequence of photos below shows array beamform maps overlaid on the model, showing noise sources and intensities at different frequencies with circulation-control wing blowing only (engine simulators turned off).Overall, says Jameson, the AMELIA windtunnel tests show leading-edge blowing is critical to the performance of a circulation-control wing. On the acoustic side, Horne says the tests show the noise and aerodynamic trends are consistent for a circulation-control wing.
* The acronym AMELIA was chosen to commemorate Amelia Erhart's stop-over at the Cal Poly Aeronautical Engineering Department campus in 1935 for structural repairs to her aircraft.