Sumon Sinha and Jim Hendrix

Sumon Sinha is a name that World Gilding is about to hear a whole lot more of!

The gliding world has only heard vague rumours about the deturbulator tape and its ability to give new life to old ships and additional performance to new ones. Many don't believe what they hear, but what follows is for the first time, the story about the development of the concept, the people involved, and the ultimate possible results. This story is another first for New Zealand's Gliding Kiwi.

EVER HEARD OF ACOUSTICS?

Sumon Sinha (of Indian extraction) is a mechanical engineering professor at the University of Mississippi, Oxford, U.S.A. His PhD is in fluid dynamics. Born in Calcutta (now called Kolkata), India, in 1956, he developed an interest and intuitive understanding of aerodynamics by building and flying bamboo and cardboard model gliders as a child. He subsequently obtained his B.Tech (Hons) in Mechanical Engineering from the prestigious Indian Institute of Technology, Kharagpur in 1978 and was recruited by the graduate school of the University of Miami.

His Ph.D. and M.S. are from the University of Miami, specializing in Fluid Mechanics. His intuitive feel for fluid flow resulted in the realization that separated flows, if properly contained, could reduce drag enormously. This culminated in the development of his patented electrically operated active flexible wall transducer. This was further refined and simplified into the Sinha Flexible Composite Surface Deturbulator (or FCSD).

He was led to this research following a suggestion from Dr. F. D. Shields, an experimental physicist at the National Centre for Physical Acoustics. Dr Shields believed that acoustical waves might be able to control boundary layer flow. That was almost 15 years ago.

Initial research showed that acoustical waves have no effect on boundary flow, but it was possible that compliant surfaces could be made to interact with boundary layer flows in beneficial ways.

Sumon’s first work was with active surfaces that were vibrated electrically. Although not dramatic, the results were encouraging. When Sumon began seeing drag reductions with the active surface turned off, he shifted gears and started working with passive surfaces. Bingo! They yielded much more impressive results.

Drag probe measurements 9/17/03 and 10/18/03

This immediately raised the obvious question – why? Why had others failed to make compliant surfaces work well when they studied them years ago? Sumon was well aware of the history of the technology. Previous researchers projected their boundary flows over flat plates in their wind tunnels. Sumon found that a pressure gradient is required to make surfaces work, and so his work began with round and with airfoil shapes.

Way back in the late 1950s, the ability of compliant surface treatments to reduce drag of cylindrical objects like torpedoes in water flows was reported on by Kramer. According to Dr Sinha, Kramer attributed his results to transition suppression, which ushered in a new area of research. The direction of this research was determined largely by theoreticians who simplified the mathematics to transition control on a zero pressure gradient flat plate as the basis of explaining Kramer's results. However, experiments aimed at exploiting this effect have never yielded significant drag reduction in a consistent manner, and the results degraded severely at higher Reynolds number turbulent conditions typical of prototypical real life situations. The availability of a non zero-pressure gradient in real flows over wings changes the approach completely. Most likely the successful drag reduction results by Kramer embodied the Sinha FCSD principle. In these experiments, the role of the compliant wall is in dissipating turbulence in the separated shear layer (not present in a zero pressure gradient flow) and not delaying transition to turbulence in an attached boundary layer flow by mitigating Tollmien Schlichting waves as postulated by Kramer.

Dr. Sinha's first flight tests were on a power aircraft using an NLF 0414F laminar airfoil, and the drag reductions were essentially the same as was seen on Jim Hendrix's 35-year-old Standard Cirrus in his test on September 17, 2003.

But in respect of the newer 21st century laminar airfoils, Sumon sees the same magnitude of improvement on them as being equal to what the deturbulator tape did for Jim Hendrix's Standard Cirrus with its Wortmann airfoil.

The theoretically most efficient wing is not a fully laminar flow wing – certainly not as much as people tend to think. True, laminar flow reduces turbulent energy loss to a minimum, but the skin friction drag is still there. You can do much better by lifting the high velocity gradient bottom of the boundary flow off the surface a little. This gives dramatic reductions in skin friction drag. Even a turbulent boundary layer can be more efficient than a laminar boundary layer in this case.

Impromptu parallel flight vs ASW-28 at 80 kts

Sumon says "there is a subtle, but important difference in philosophy between our approach and the long standing tradition in wing design. Rather than strive for the most laminar airfoil possible and getting a wing that must be perfectly clean and requires well behaved air to fly in, we have accepted the reality of turbulence and sought instead to manage it to achieve very high efficiencies from less sensitive airfoils. This has been our much different approach to the concept."

"It should be born in mind" says Sumon "that we can achieve similar efficiencies without resorting to expensive structural complications like blown turbulators. Our method is cheaper on new aircraft and also can be installed as a retrofit on existing aircraft".

Dr. Sinha describes the operation of his deturbulator as follows: The deturbulator, in the form it is currently used for sailplanes, is a multilayer tape about 50 mm wide and 80 microns thick. The principal components include an aluminium substrate with micro structured ridges and a thin flexible aluminised Mylar membrane. The membrane is vibrated by the flow velocity fluctuations (turbulence) when exposed to a boundary layer at the proper location. The geometry of the microstructure enhances certain modes of oscillation of the membrane (typical amplitudes in the order of 10 100 nm) and helps dissipate most of the fluctuations through viscous damping in the substrate. The deturbulator damps out turbulence in separated shear layers, thereby attenuating their growth. This promotes a long thin separation bubble-like structure without the rapid breakdown and thickening caused by transition to turbulence. This negates the wall shear stress and speeds up the free steam flow. It also helps pressure recovery on the lower surface.

THE PARTNERSHIP

Gliding Kiwi reported in our last issue the results of the first real airborne tests on a sailplane –a Standard Cirrus owned by Jim Hendrix. Through his company, Oxford Aero Equipment (ww.oxaero.com) Jim provides flight testing and other services to Dr. Sinha's company, SinhaTech (www. sinhatech.com). They are not business partners, but have a relationship that has worked well for two years.

Writing to Gliding Kiwi, Jim reported that before the deturbulator tape can be sold to the potentially thousands of likely buyers, representatives will have to be schooled on installation and maintenance procedures. "For my part" says Jim, " I expect to be busy making drag probes and pressure sensors for customer use to check the operation of deturbulator installations".

L/D with upper inboard 60% of wings deturbulated

"We are anxious to go to market as soon as possible, but we are still tweaking the configuration on my glider and learning things by running simulations in XFOIL and testing in Sumon's wind tunnel. We plan to get the top surface right before going back to the lower surface."

Referring to the odd but persistent 50kt hump in the crude polar data, Jim says this comes from not getting the outer panel right. It did not appear when we were treating only the inboard 60% of the wing. Sumon's assistant's theoretical analyses suggests that we may get substantial improvement by changing the outer panel configuration. (Dr. Sinha gives considerable credit to software developed under his guidance by his employee and graduate student, Sundeep Ravande, for estimating induced drag).

But reverting to Gliding Kiwi's earlier article (from Jim's web site) about the parallel flight with an ASW 28, the reader must appreciate that something amazing was happening to that old Wortmann wing. "We have a major breakthrough," says Jim. "The Standard Cirrus was a completely different sailplane – it went through the air so well".

It is even more impressive when you consider that the deturbulator is applied in segments of about 18" in length with a taped up space of about two inches between each segment. It was really a "make do" generating quite a lot of drag. There were 12 or 13 of these joints on each wing. That's about two feet of messed up span on each wing. Still, the improvement is enough to overcome that as well as the excessive drag from my old fuselage to produce some really good performance at high speeds. We can reduce the taped joint to 1cm for yet another noticeable improvement. At some point, not too far out, we will just say that that's good enough and go to market with it".

"Meanwhile" says Jim, "we will finalise the configuration on my Standard Cirrus so as to get good polar data, and then take it to Dick Johnson for independent testing. At that point we should be on our way. In between all this, I'm using it for normal soaring flights to see how the deturbulator stands up".

Jim has told Gliding Kiwi that the quality of the initial product is not up to planned production standards. Setting up a professional manufacturing facility will be expensive and this is going to take time. Initial buyers will receive the beta versions that work but are not very pretty. Dr. Sinha plans to offer free upgrades to buyers of beta versions.

Oil flow of deturbulated wing surface

To follow the progress of the Sinha Deturbulator project, take your browser to www.sinhatech.com and click the Progress link. This will take you to an index of articles tracking the progress. Also, you may subscribe to an email list that will let you know when additional progress occurs.

Gliding Kiwi will continue to keep readers up to date on the progress of the deturbulator tape. Testing is continuing, but we suspect the time is not too far away from when the first commercial sales of the tape will be made. The big trick though must be the positioning of the tape and as yet, the researchers are saying little on that vital aspect of the project.