University of Queensland researchers have found a way to make the business of flight more efficient by significantly reducing drag, work which has implications for vehicles from aeroplanes to space craft.
Australia's first professor of space engineering Emeritus Professor Ray Stalker of UQ's Centre for Hypersonics led the project, which offers potential multi-billion dollar savings to the aviation and space industries.
The research, which builds on the work of colleagues and former students Dr Chris Goyne, Dr Craig Briesciani and Dr Allan Paull, was recently reported in the Journal of Spacecraft and Rockets. It uses a technique which has been patented by UniQuest Pty Ltd, the University's technology transfer company.
The work involves injecting hydrogen into the turbulent boundary layer over the body of a craft under conditions when the hydrogen burns in the layer. As a result, turbulent skin friction drag can be reduced to one quarter of its original value, increasing net thrust.
The new theory has been confirmed with a new computer code written by Dr Briesciani and ground testing over 18 months in the University's T4 shock tunnel at speeds of 3km/second and at high densities to combust the hydrogen.
"Up to 40 percent of the drag in an aircraft like a 747 is due to skin friction," Professor Stalker said. "This technique could allow smaller engines to go from A to B using less engine thrust, achieving quite a saving.
"These economies are extremely important in the hypersonics game where we're developing models for space craft to operate at earth orbital speeds. We've found skin friction drag on models in hypersonic aircraft to be at least half the total drag.
"By using this technique we can get away with less thrust from the engine. Since hypersonic aircraft comprise nearly all engine, it means we can get away with making smaller engines and smaller aircraft to do the same job."
Professor Stalker said the method potentially could be used in supersonic fighters needing extra acceleration to get away from hostile missiles.
"They could have a system inject hydrogen over the aircraft body, effectively improving thrust by up to 30 percent."
Professor Stalker said scientists had examined the concept of skin friction drag for decades and wondered how it could be further reduced. He developed the new theory after worrying about "unusual results" achieved during PhD studies four years ago by Dr Goyne, now based at the University of Virginia at Charlottesville.
"Chris focused on methods for measuring and predicting the amount of air resistance encountered by aircraft flying at speeds of up to 15 times the speed of sound, using the University's T4 shock tunnel," Professor Stalker said.
"He measured skin friction in hypersonic flows and among other things, looked at a particular effect associated with the air layer area close to the model in the tunnel.
"Air flowing over an aircraft starts flowing smoothly, an effect known as laminar flow. This soon breaks down and becomes turbulent flow, or the turbulent boundary layer.
"The phenomenon was first observed in the 1880s by Osborne Reynolds, the first engineering professor at Manchester University, known for his work on the flow of fluids like air, water, oil and steam. The Reynolds Number is a ratio showing the effect of viscosity in a flow.
"People have studied this effect for more than a century because when flow becomes turbulent, the friction on the surface, or skin friction, increases. Scientists have known that if you inject a gas like helium or cold air into the boundary layer you can cut down skin friction for a short distance but it did not seem to achieve enough savings to make the effort worthwhile.
"In a side project to his PhD, Chris achieved some unusual findings, which we more or less gave up on. After he left I couldn't put them out of my mind and spent months looking at the data until I twigged to a way of analysing the data that showed the effect. Scientific curiosity made me keep going but I didn't expect it to amount to very much."
Professor Stalker, who does not use a computer, and writes in longhand, asked Dr Bresciani to develop a computer code to deal with skin friction when he started to get results from Dr Goyne's work. The results indicated that overall skin friction drag could be halved by boundary layer combustion.
Professor Stalker has kept quiet about the theory for the past three years waiting until it could be tested. Although the project potentially could save the aviation and space industries billions of dollars, it now needs at least $250,000 to advance to the next stages.
The Centre for Hypersonics has applied to the Australian Research Council to fund two projects. One will be conducted by Centre director Associate Professor Richard Morgan in the world's fastest shock tunnel, the X3 - also at The University of Queensland - to test the upper limits when the effect cuts out. The second project by Dr David Mee in the T4 shock tunnel, will find the lower limits.
Professor Stalker initiated the biggest and most significant program in hypersonics and space propulsion in the Southern Hemisphere after arriving at UQ. With his Centre for Hypersonics colleagues, he has since developed several generations of free piston-driven shock tunnels with Earth orbital velocity simulation capability. His revolutionary inventions, which became known internationally as "Stalker tubes", enabled the low cost ground-based research of space vehicle re-entry problems impossible by any other means.
In 1993 Professor Stalker and colleague Dr Allan Paull provided the first proof that air-breathing engines, scramjets, could produce more thrust than drag, the essence of flight. (Dr Paull is leading an international project in Australia in July in a race with NASA to conduct the world's first flight tests of a scramjet using supersonic combustion).
If the current project pans out, he sees it as potentially the most important of his life.
Media: For further information, contact Jan King at UQ Communications (telephone 07 3365 1120, email: communications@mailbox.uq.edu.au)
