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Aerodynamic Free Lunch: VGs increase L/D ratio up to 880%. Does it work for flaps too?

I understand this research was for a wing without flaps.

I understand this reduction in drag with VGs works only for the alpha range past the max Cl point of the wing without VGs, eg. 8-18 deg, increasing the L/D by 880% at 18 deg AOA as shown in the table and graph below.

Questions:

  1. Would a wing with 20% chord NON-SLOTTED flaps see a similar 84% reduction in drag for each flap setting eg 10,20,30 deg?

  2. Would a wing with 20% chord SLOTTED flaps see a similar 84% reduction in drag for each flap setting eg 10,20,30 deg?

  3. Why does the drag almost stay the same even thought the lift goes up? My intuition is that the VG wing drag value stays close to the 8 deg drag value w/o VGs, as that is the local AOA the wing is seeing due to the VGs, instead of the drag at 18 deg, even though the lift is increasing. Is that correct?

Article:

Experimental and Numerical Analysis of the Effect of Vortex Generator Height on Vortex Characteristics and Airfoil Aerodynamic Performance Xinkai Li 1,2,3,* , Ke Yang 1,2,3,4 and Xiaodong Wang 5 1 2 3 4 5 102206, China; wangxd@ncepu.edu.cn * Correspondence: lixinkai@iet.cn; Tel.: +86-010-8254-3038 Received: 29 January 2019; Accepted: 7 March 2019; Published: 12 March 2019 Institute of Engineering Thermophysics, Chinese Academy of Sciences, Beijing 100190, China; yangke@iet.cn Key Laboratory of Wind Energy Utilization, Chinese Academy of Sciences, Beijing 100190, China Dalian National Laboratory for Clean Energy, CAS, Beijing 100190, China Graduate School, University of Chinese Academy of Sciences, Beijing 100049, China College of Energy, Power and Mechanical Engineering, North China Electric Power University, Beijing Abstract: To explore the effect of the height of vortex generators (VGs) on the control effect of boundary-layer flow, the vortex characteristics of a plate and the aerodynamic characteristics of an airfoil for VGs were studied by both wind tunnel experiments and numerical methods. Firstly, the ratio of VG height (H) to boundary layer thickness (δ) was studied on a flat plate boundary layer; the values of H are 0.1δ, 0.2δ, 0.5δ, 1.0δ, 1.5δ, and 2.0δ. Results show that the concentrated vortex intensity and VG height present a logarithmic relationship, and vortex intensity is proportional to the average kinetic energy of the fluid in the height range of the VG. Secondly, the effects of height on the aerodynamic performance of airfoils were studied in a wind tunnel using three VGs with H = 0.66δ, 1.0δ, and 1.33δ. The stall angle of the airfoil with and without VGs is 18◦ and 8◦, respectively, so the VGs increase the stall angle by 10◦. The maximum lift coefficient of the airfoil with VGs increases by 48.7% compared with the airfoil without VGs, and the drag coefficient of the airfoil with VGs is 84.9% lower than that of the airfoil without VGs at an angle of attack of 18◦. The maximum lift–drag ratio of the airfoil with VGs is lower than that of the airfoil without VGs, so the VGs do not affect the maximum lift–drag ratio of the airfoil. However, a VG does increase the angle of attack of the best lift–drag ratio.

Source: www.researchgate.net › publication › 331712096

VG's

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  • $\begingroup$ You might want to change the case use of Vg to use all upper case. Or, explicitly spell out Vortex Generator. I came here looking to find out how best glide speed gave you a better L/D ratio. $\endgroup$ – Dean F. Jan 31 at 19:25
  • $\begingroup$ It might not even work for a different wing. This airfoil is a thick, well cambered airfoil intended for wind turbine blade roots, which are prone to stalling. Impossible to know without doing the analysis and testing. $\endgroup$ – Pilothead Jan 31 at 23:18
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    $\begingroup$ You do realize that there is no "free lunch" here, right? The only thing the vortex generators are doing is delaying flow separation at the expense of reduced efficiency across the board, as clearly shown in graph (c). $\endgroup$ – AEhere supports Monica Feb 1 at 2:05
  • $\begingroup$ Also, could you link this account to (what I am pretty sure is) your old one? $\endgroup$ – AEhere supports Monica Feb 1 at 2:09
  • $\begingroup$ yes, I accidentally started a new account when I was trying to access the old one, as I think it was only a guest account. How do I get back to my old account, or better yet, merge the two? $\endgroup$ – Fred Feb 1 at 16:25
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Graph C tells the story. Better to go with slats and flaps. Vortex Generators remain valuable to alter the airflow characteristics of a section of the airfoil or control surface.

Drag producing and lift destroying chaotic turbulence on the upper rear of the wing appears to be reduced by the Vortex Generators at higher Angle of Attack. Notice turbulence starts to form before the wing is fully stalled on a "clean" wing, resulting a peak Lift/Drag of around 6 degrees. By energizing the air flow and keeping it attached, more lift can be created at higher AOA as seen in Graph A.

However, anything that cannot be retracted will carry a drag penalty at optimum L/D, but, as with many larger radius leading edges, Vortex Generators, or VGs, can make the L/D curve much smoother and predictable rather than a sharp break from a very narrow peak.

VGs have found application in elevators, but a flap beyond a given angle of deflection is actually more useful stalled and draggy. Lower degrees of flap deflection, along with slats, serve to increase camber of the wing to create more lift at lower speeds, also with a drag penalty compared with "clean".

Note the airfoil used in this study is cambered. It starts to generate lift a -5 degrees AOA, and drag with VGs is actually greater than "clean" at lower AOA typically found in cruise (Graph B), with little difference in lift produced.

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  • $\begingroup$ VGs are pretty popular with the STOL ultralight crowd. In fact, Savanahs come factory equipped with VGs instead of slats over the entire wing span, unlike the Zenith 701. $\endgroup$ – Fred Feb 1 at 20:18
  • $\begingroup$ I'm not sure you answered my question. 1. Would VGs reduce drag about 80+% with flaps? 2. Is the low drag due to local AOA being 8 deg, or the energizing of the boundary layer, or a bit of both? $\endgroup$ – Fred Feb 1 at 20:20
  • $\begingroup$ @Fred right! One prominent writer here calls VGs "poor mans slats", and, as the graphs suggest, certainly would be helpful for STOL. I would want slats and flaps on a thinner wing for cruise (if I cruised alot). $\endgroup$ – Robert DiGiovanni Feb 1 at 20:40
  • $\begingroup$ @Fred drag reduction depends on what AOA you are at. Notice delaying the onset of "flow separation" with VGs or (or dimples) allows much higher L/D at a higher AOA (and more lift). This might be useful for take-off/climb. Judging from deltas (are VGs mini deltas?) it may be "energizing" the boundary layer. The back of the wing (or golf ball) is curving away from the airflow (I'm not seeing "local AOA at 8 deg"). $\endgroup$ – Robert DiGiovanni Feb 1 at 20:48

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