I responded to the first post on this topic with the comment of 'low drag turbulator'. I'll amend that comment slightly with 'low drag way of avoiding separation bubbles'.
Having scanned a couple of technical papers this evening, I'll see if I can translate it into everyday English. I'll apologise for any technical inaccuracy that I will introduce for the sake of everyone's general understanding. (Hopefully I'll have got this right.) The way a 'conventional' turbulator works is to generate small vortices within the boundary layer to cause mixing of the airflow and cause it the boundary layer to become turbulent. By doing this just ahead of a separation bubble you get a net reduction in drag. Normally the drag of a separation bubble is huge compared to the additional drag by adding more turbulent flow to your aircraft. Drag reduction = drag of separation bubble MINUS extra drag due to turbulent flow from turbulator strip. What they are trying to do with the Cirrus is to cause a transition from laminar to turbulent flow just ahead of a separation bubble without introducing turbulence to the airflow. Dr Sinha is trying to set up a condition not dissimilar to mountain wave in the boundary layer just ahead of the separation bubble. He is trying to set up a wave pattern in the very bottom of the boundary layer next to the skin. This wave pattern causes the flow to transition to turbulent flow a couple of wavelengths behind the Sinha strip. The wave pattern is damped out after a few oscillations just like in the mountains. The big difference to a conventional turbulator is that the wave pattern is laminar hence you don't have the drag from the additional turbulent flow. How he is achieving this is his big secret. Drag reduction = drag of separation bubble You have to read the Oxford Aero article with a little bit of knowledge. They have only measured the drag reduction on the lower surface of the wing. As he says in the second last paragraph of the test results article, the upper surface contributes far more drag than the lower. He also hints that the upper surface either doesn't have a separation bubble or it has a very small one. So as an example, assuming no separation bubble on the upper surface: Clean Cirrus at 50 knots: Drag from lower surface = 1 unit, drag from upper surface = 2 units. Total profile drag = 3 units. Sinha'd Cirrus at 50 knots: Drag from lower surface = 0.8 (claimed 20% reduction on the website at this speed), Drag from upper surface = 2 (unchanged as we assumed there isn't a separation bubble). Total profile drag = 2.8. % drag reduction = 100 x .2 / 3 = 6.7% So the performance improvement numbers don't look so good after all. If the wing makes up 40% of your total drag at this speed, then your total drag reduction is only 2.68%. Enough to take your 38:1 glider up to 39:1 So is it snake oil? In theory no it is not. It is possible that Sinha's strip will be better than the turbulators we have now. -----Original Message----- From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED] On Behalf Of Mike Borgelt Sent: Tuesday, 21 October 2003 3:46 PM To: [EMAIL PROTECTED] Subject: RE: [aus-soaring] Sinha Flexible Composite Surface Deturb At 11:58 AM 21/10/03 +0930, you wrote: >Read SNAKE OIL. Lets see what happens when some more samples of this stuff get out to test. Maybe it's bad experimental design, equipment or technique or just maybe it's real. Maybe the Cirrus has a lower surface laminar bubble that this thing is preventing by acting as a turbulator strip? 20+ years ago nobody used turbulators on high performance gliders either, now they are common. Winglets were tested by Centrair at the same time and dismissed as not being much good but they didn't test small ones which appear to be beneficial. Unless you have some special knowledge of boundary layer behaviour it might be smart to withold judgement. Mike Borgelt Instruments - manufacturers of quality soaring instruments phone Int'l + 61 746 355784 fax Int'l + 61 746 358796 cellphone Int'l + 61 428 355784 Int'l + 61 429 355784 email: [EMAIL PROTECTED] website: www.borgeltinstruments.com -- * You are subscribed to the aus-soaring mailing list. * To Unsubscribe: send email to [EMAIL PROTECTED] * with "unsubscribe aus-soaring" in the body of the message * or with "help" in the body of the message for more information. -- * You are subscribed to the aus-soaring mailing list. * To Unsubscribe: send email to [EMAIL PROTECTED] * with "unsubscribe aus-soaring" in the body of the message * or with "help" in the body of the message for more information.
