VG stall angle improvement

Discussion in 'Aircraft Design / Aerodynamics / New Technology' started by Biplane Ranch, Apr 25, 2015.

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  1. Apr 25, 2015 #1

    Biplane Ranch

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    As I understand it, VG's will increase the stall angle of a wing. For a rectangular plan form Cub type wing, by how many degrees can one expect the stall angle to increase? Say a wing normally stalls at 15 degrees, what is the new angle after adding VG's, 17? 21? I have no idea. Thanks.
     
  2. Apr 25, 2015 #2

    harrisonaero

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    You'll typically see about a 10% reduction in stall speed so you can determine your alpha from that based on your airfoil curves. But the most important benefit isn't stall reduction, it's improved control authority at low speeds. VGs are like power steering for slow flight. [sold over 2000 sets of experimental VGs over the last decade and we designed and currently manufacture them for a certified aircraft, the Quest Kodiak]
     
  3. Dec 9, 2015 #3

    Aviator168

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    This is kind of important if you are building a part 103. A 10% reduction in stall speed comes to over 10% increase in stall speed without VGs and that ends up wing area (weight of the wing) reduction of 20%. That's a lot if you ask me.

    Check this video. Stall speed from 60 down to 50 with VGs installed
     
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  4. Dec 9, 2015 #4

    Rockiedog2

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    Harrisonaero is right on. I flew my slick wing Legal Eagle for some time then installed a homemade set of VGs...the paired AL type. On the first takeoff after the install I knew it was a different plane before getting outa ground effect. Rock solid; nice. It was a good flying plane...I was surprised how much better after the VGs. Yeh, the stall speed decreased on the LE 10%; if you're trying to make the 103 stall speed limit they may be the answer. I got curious and rigged up a crude AOA indicator that worked surprisingly well. The AOA at stall increased from about 15* to about 30*. This on a 4414 airfoil...flat bottom, fat LE kinda like a Cub. Couple pics of the AOA indicator below from June 2004 Sport Pilot article. The vgs in that pic made measurable drag on my install; I went to the single plastic vanes from Stolspeed and the speed increased while installed at the same position as the paired ones. A certain manufacturer sent me a set of paired AL ones and requested test results which I did. That set made drag too, about 2 mph. I looked at the Quest Kodiak images and it looked like Mr Harrison makes singles for that plane. Maybe he could talk to us a little more about what he knows about vgs

    IMG_4429.jpg IMG_4430.jpg
     
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  5. Dec 9, 2015 #5

    Aviator168

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    Yeah. If we can reduce the stall speed, we can reduce the size of wing which leads to reduction in weight. Every # counts in 103.
     
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  6. Dec 9, 2015 #6

    djschwartz

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    Often a quick search can turn up some real information. For example, a Google search on "airfoil polars with vortex generators" turned up this report from Ohio State University for the NREL titled "Effects of Surface Roughness and Vortex Generators on the NACA 4415 Airfoil":

    https://wind.nrel.gov/airfoils/OSU_data/reports/7x10/N4415_7x10.pdf

    and this AIAA report from Rolling Hills Research Center, "Enhanced Airfoil design Incorporating Boundary Layer Mixing Devices":

    http://www.rollinghillsresearch.com/Aero_Research/Files/AIAA-2003-0211_NLFairfoil.pdf
     
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  7. Dec 9, 2015 #7

    BJC

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    dj:

    I'm sure that you have seen Bud Granley fly his Yak 55 with VG's on the top and bottom. I don't know how much difference they make, but that airplane seems to fly off a vertical very well for a heavy airplane.


    BJC
     
  8. Dec 9, 2015 #8

    Rockiedog2

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    I clicked on that link and skimmed it...it got above my interest level pretty quick but I did notice a couple things. They said the stall with VGs was more abrupt and at lower AOA than w/o VGs. That's contrary to most every other reference I've ever seen as well as my experience. Also noticed they installed the VGs at 30% MAC. I'm surprised the vgs did anything there they're so far back as to be likely out of the flow when at increased AOA but maybe not. My experience is that around 8% MAC has been optimum. They call an increased Clmax from 1.45 or so to 1.85 or so with the vgs. All that seems contradictory
    But I may be misinterpreting something there...when they start talking about Reynolds Numbers and such I lose interest

    >>>The VGs were tested at the 30% chord upper surface
    station only<<<

    that from p6 I think it was
     
  9. Dec 9, 2015 #9

    dcstrng

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    Was wondering as well based on yours an others VG positioning – wonder if they weren’t that far aft to try to preserve as much as they could from the NLF or something (if I read that correctly; pure speculation from the peanut gallery… but I wondered too…). Maybe that is why the more abrupt stall as well, the VGs that far back are simply dropping out of the flow, with immediate effect (says he with zero aerodynamic credentials).
     
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  10. Dec 9, 2015 #10

    Aviator168

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    If I were installing them, would want their effect to kick in at about the AoA of stall (e.g. 12 degree), not before. So 8% MAC is about right.

    VG that far back will only make the stall more abrupt. Here is why. Stall starts from the TE of wing and gradually moves forward to the LE. VGs that far back prevents the stall from happening behind the VGs AS LONG AS the air stream that reaches the VGs is still attached to the wing. However, with VGs this far back, even with normal stall AoA, the air stream is already separated from wing before it reaches the VGs and at this point, the whole wing (from TE to LE) stalls.
     
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  11. Dec 9, 2015 #11

    djschwartz

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    I've actually not seen Bud fly the Yak. I have other aerobatic friends who have tried VGs with mixed results. This is not because VGs are bad in any way but because they were doing exactly what was intended, delaying stall. With the VGs the aircraft would pull around a corner at high angles better but had worse snap roll performance precisely because of the delayed stall.

    I have another friend with a YAK 55 and it is impressive in its ability to pull around a corner at low speed as is even without VGs. So how much the VGs would help in that regard is hard to say.

    Dave
     
  12. Dec 9, 2015 #12

    harrisonaero

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    Thanks for the kind words. My wife basically runs the business now, I'm pretty busy with aircraft engineering, but you can find more out on her website at Land Shorter! Home.
     
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  13. Dec 10, 2015 #13

    SVSUSteve

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    I'll definitely be talking to you in hopes of improving the aerodynamics of the Praetorian.

    Just out of curiosity, wouldn't those little zig-zag strips that are used on some gliders accomplish basically the same thing without making the wing look like a porcupine with a crew cut?
     
  14. Dec 10, 2015 #14

    harrisonaero

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    Different applications. Zig zag (turbulator) strips are to prevent laminar flow separation (bubbles) on laminar surfaces. VGs like ours energize the air on the non-laminar portion of the foil in order to keep it attached longer. One way to think about it is micro vs macro level of flow.

    And take it to the next level and you have vortilons, strakes, dorsal/ventral fins, stall strips, etc. All devices that make the air go where you want at different local angles of attack.

    Check out these articles for some good reading.

    http://cafefoundation.org/v2/pdf_cafe_reports/localflow1.pdf

    http://cafefoundation.org/v2/pdf_cafe_reports/localflow2.pdf

    http://cafefoundation.org/v2/pdf_cafe_reports/localflow3.pdf
     
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  15. Dec 10, 2015 #15

    Aviator168

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    Are you saying the function of these strips is to prolong laminar flow? If that is case, I am lost. I thought they turn laminar flows to turbulent flows. As long as I can remember, there isn't any passive device that can prolong laminar flow.

    BTW. Can't access those links.
     
  16. Dec 10, 2015 #16

    Kyle Boatright

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    I suspect it has to do with the depth of the boundary layer. The tape is applied where the flow is laminar, therefore the boundary layer is thin and a little protrusion (a piece of tape), can impact the flow and keep it attached for a longer distance. In non-laminar flow, you need a device that pokes all the way through the (mostly stagnant) boundary layer to reach and redirect a moving airstream.
     
  17. Dec 11, 2015 #17

    radfordc

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    https://en.wikipedia.org/wiki/Turbulator

    Airfoil turbulators

    When air flows over the wing (an airfoil) of an aircraft, there is a layer of air called the boundary layer between the wing's surface and where the air is undisturbed. Depending on the profile of the wing, the air will often flow smoothly in a thin boundary layer across much of the wing's surface. The boundary layer will be laminar near the leading edge and will become turbulent a certain distance from the leading edge depending on surface roughness and Reynolds Number (speed). However there comes a point, the separation point, in which the boundary layer breaks away from the surface of the wing due to the magnitude of the positive pressure gradient. Beneath the separated layer, bubbles of stagnant air form, creating additional drag because of the lower pressure in the wake behind the separation point.

    These bubbles can be reduced or even eliminated by shaping the airfoil to move the separation point downstream or by adding a device, a turbulator that trips the boundary layer into turbulence. The turbulent boundary layer contains more energy, so will delay separation until a greater magnitude of negative pressure gradient is reached, effectively moving the separation point further aft on the airfoil and possibly eliminating separation completely. A consequence of the turbulent boundary layer is increased skin friction relative to a laminar boundary layer, but this is very small compared to the increase in drag associated with separation.

    In gliders the turbulator is often a thin zig-zag strip that is placed on the lower side of the wing and sometimes on the vertical stabilizer.[1] The DG 300 glider used small holes in the wing surface to blow air into the boundary layer, but there is a risk that these holes will become blocked by polish, dirt and moisture.

    For the aircraft with low Reynolds numbers (i.e. where minimizing turbulence and drag is a major concern) such as gliders, the small increase in drag from the turbulator at higher speeds is minor compared with the larger improvements at best glide speed, at which the glider can fly the farthest for a given height.
     
  18. Dec 11, 2015 #18

    Victor Bravo

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    The blow holes worked much better in the wind tunnel than they did in actual practice.

    I did have one or two flights in the DG-300 (didn't like it), but I had nothing to compare it to as far as blow holes vs. turbulators.

    BUT... I was truly fortunate enough to have owned and flown both the AS-W20 (now usually referred to as the "A" model) and the AS-W20B in soaring competitions. The "A" had Wortmann airfoils (62-K-131 and 60-126 if distant memory serves) without turbulators. The "B model" had a new HQ (Horstmann & Quast ??) airfoil with the blow holes.

    It is unfortunately impossible for me to quantify it, because the B had a different airfoil, and was heavier, and far less flexible than the magical A model (and the B could carry significant additional ballast which moved the glide polar around). I'm not a qualified test pilot either. But the overall subjective feeling that I got (as just another egomaniacal competition glider junkie kinda guy) was that the blow holes did not do any better than zig-zag turbulator strips.

    I recall that the whole idea was that the velocity of the air coming out of the holes would increase or decrease with airspeed, matching the airfoil's changing need for more or less turbulation. But the little hypodermic needles that they used were installed at a 90 degree angle to the lower wing surface. I'm GUESSING that they should have been installed at some kind of crossed angle to create swirling vortices, or something. I have ZERO technical expertise on this subject, and I'm sure the people who designed the blow holes were very highly qualified.

    The last time I checked several years ago, the sailplane manufacturers went back to zig-zag turbulators.
     

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