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Stolspeed VG's

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  • #31
    I've installed mine at 5%, but having given it some thought now I don't think we'll see a lot of variation in the results as long as they're not too far aft. I found it interesting that my power on stall speeds remained very similar with or without VG's, seeming to indicate that most of the results are there once the prop is firing that airflow over the wing. The noticeable change was in the handling, and the ability to descent in a controlled manner (fully stalled), and recover by applying power.

    A789A002-AC52-491A-95E5-ECA6648B358A.jpg
    Last edited by Nev; 12-27-2021, 03:59 AM.
    Nev Bailey
    Christchurch, NZ
    Builders-log
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    • #32
      Originally posted by Nev View Post
      I've installed mine at 5%, but having given it some thought now I don't think we'll see a lot of variation in the results as long as they're not too far aft. I found it interesting that my power on stall speeds remained very similar with or without VG's, seeming to indicate that most of the results are there once the prop is firing that airflow over the wing. The noticeable change was in the handling, and the ability to descent in a controlled manner (fully stalled), and recover by applying power.

      A789A002-AC52-491A-95E5-ECA6648B358A.jpg
      I think the propwash also makes a lot of induced airflow over the tail, which can keep the airflow from separating there. This also allows for slower stall speeds in certain configurations. If the flow separates at the elevator, the nose will drop.


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      • zkelley2
        zkelley2 commented
        Editing a comment
        Ya, I'm not really stalled at roughly 40kts power off. I'm just out of elevator.

        Power on it actually stalls around 37-38.

    • #33
      No doubt I will be installing these, 5-6mph slower is a big difference
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      • #34
        Great to see Nev's data on this. Nev, maybe you've posted it elsewhere but I haven't seen it... how is your airspeed calibration at these speeds? I'm not questioning the effectiveness of the VGs, just curious about the observed whole-aircraft lift coefficient at stall. For our club's 172N, indicated airspeed at stall with full flaps is 7 knots low, per the flight manual. Also, what was your CG? Stall behavior at forward vs. aft CG can be vastly different, and can be the difference between a tail stall vs. a wing stall. Can you comment on the differences in elevator? Are you full aft stick with one vs. finding you still have a little left over with another?

        The difference in power-on vs. power-off could partially be related to differences in airspeed calibration at low speed (particularly if your static port may be effected by prop slipstream), but I wouldn't think that explains the bulk of it. A strong downward pitch at stall can be indicative of the tail letting go first before the wing stalls; a mush (particularly if you can still have some pitch authority) *can* (but not necessary *does*) indicate your tail is still working and your wing is indeed stalled. Power-on will probably have more effect on the tail. High-wing, low-tail airplanes have strong responses to change in downwash from the wing, and the power-on blowing effect could lead to locally higher wing lift and locally stronger downwash at the tail. There are probably some prop slipstream effects on the tail as well, but I'd think the wing and induced downwash would be a stronger effect. If only we could resurrect the NACA 30x60 tunnel...
        4-Place Model 'B' Serial 1529B (with many years to go...)

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        • #35
          These stalls were a basic look at mid weight, mid CG, and I've since done some at aft CG max weight to ensure they "pass" for the flight testing, but not enough to gather data as such. I've been pretty focused in other areas prior to finishing the test phase. My engineer used the word "pedantic" so I've probably done enough

          In a couple of weeks I'll do a few hours testing the VG's in more detail to get some usable data. I suspect that at this stage what I was seeing was the elevators "letting go" without power on, and the elevators "holding on" with power. I didn't check for position error yet but there will be some for sure. Sorry I can't be more help yet but I'll report back once I've done more.
          Nev Bailey
          Christchurch, NZ
          Builders-log
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          • Redneckmech
            Redneckmech commented
            Editing a comment
            Perhaps your engineer considers it pedantic but everyone here appreciates your methodical approach!
            Thank you for taking the time and effort to gather data before and after the installation and share it with the community.

          • nborer
            nborer commented
            Editing a comment
            Agreed, I love seeing this data, and appreciate that you've taken the time to control the experiments, gather the data, and provide to the community.

        • #36
          The shape of the aerofoil in a Bearhawk 4-place is different to a Bravo, because the curve of the wing is different - particularly in that area that VGs attach. This is a considerable variable, in terms of a wing - the shape is everything. I suggest people installing VGs consider which aerofoil they have, rather than going straight for a generic location - especially from the manufacturer, who offers one number to cater to all the common wing designs...

          Aside from the above, I thought the location of the VGs generally was interesting to discuss. There has been a huge body of study done on this, and I am not an authority... maybe someone here is?

          So far as I can tell, the best results are achieved in a relatively narrow range, a few percent of the chord. Lots of evidence to support that.

          Too far forward:
          The available data seems to show that vortex is strongest immediately behind the VG and deteriorates as it passes over the wing, losing it's effectiveness. One could assume that placing the VG forward moves the effective separation point forward, away from the part of the wing which generates the lift and where the separation occurs. This has been modelled in 2D cad and tested on various aerofoils, but it's hard to draw a concrete conclusion for the Bearhawk - although you can make good assumptions.

          Too far back:
          VGs need attached airflow across them to generate an attached vortex (VGs behind the separation point actually reduced lift compared to a bare wing, in some experiments). If the VGs are masked by curve of the wing, as viewed by the on-coming free stream airflow, they are not going to work as effectively. I assume this is because the airflow begins to separate before it reaches the VG. Some designs prevent this by making the VG taller, but that's another thread.

          Considering the latter point, to me it seems clear that the shape of the aerofoil is likely to be important when selecting a suitable VG location.
          Last edited by Battson; Yesterday, 11:17 PM. Reason: Spelling fix

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          • #37
            With any luck by the end of the year someone will have a definitive placement location on where to put the Stolspeed VG’s by the end of the year.

            I am interested specifically for the Companion wing
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