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Bearhawk STOL mods

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  • #91
    Great to see the flap Mods work as well as I would have expected.

    The first thing I would do though is to instal VG's on the leading edge wing and on the tail surfaces as well as gap seals to maintain controllability at the slower speed.

    One of the most important and usually overlooked benefits of VG's is how they help to maintain control at the slower speeds not just the reduction in stall speed.

    One of the negative side effects of lowered stall speeds usually is reduced control at that slower speed, only VG's enhance control while reducing stall speed.

    We had the same issues with Robertson stol equipped 180and 185s and the addition of VG's made the aircraft very controllable at the slower stall speed.

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    • #92
      Some have been asking for an update.

      The update is, heavy snow continues to prevent the aircraft from test flying. Expect more in spring

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      • #93
        Just a bump as it looks like spring.

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        • Battson
          Battson commented
          Editing a comment
          Optimistic given the current conditions! There is no news. I can probably share some extra detailed information not previously posted, though.

        • Westward_Flyer
          Westward_Flyer commented
          Editing a comment
          Battson extra detailed information sounds great!

      • #94
        I'm eager for some news as well, this is one of the coolest BH mods I've ever seen.

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        • #95
          VGs on the leading edge of semi fowler flaps work extremely well but you can do either that or the VGs on the leading edge not both.

          The leading edge VGs give the benefit of better control while the VGs on the flaps just lower stall speed with a reduction in slow flight control.

          The reduction in stall speed is very close to the same in both instances, if in doubt go for improved controllability.

          I believe the Cessna Caravan comes with VGs on the flaps stock.

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          • Bcone1381
            Bcone1381 commented
            Editing a comment
            Please clarify something for me. Are you saying VG's can be done either on the flap or the wing, but not both?

          • Gerd Mannsperger
            Gerd Mannsperger commented
            Editing a comment
            That is correct

          • FastLaneAK
            FastLaneAK commented
            Editing a comment
            I have a Lake Buccaneer. The VG’s are on both the leading edge and the huge slotted flaps as per STC. I’m not sure who said that they don’t work on both as they do. Also used on Shane Madison’s cub slotted flap hangers with great success.

        • #96
          This is the latest I heard about the Fowler flaps modification:

          Final approach - estimated prevailing deck angle was in fact about 7 degrees. Ample visibility over the nose till the last couple of seconds and the round out.
          This compares to about 14 degrees in a Bearhawk 4 place A model flown at the same approach speed / profile. This is a big deal, and the main benefit which we would hope for - as well as the speed reduction. We are talking flying at the slow speed limit here, certainly nothing like a 50kt approach.

          That touchdown actually was concurrent with a full stall just as the stick hit the stops. It will take more time to really master it, results are still preliminary.
          The roll-outs are now very benign from the reduced energy touchdowns, the pilot has said it makes the roll-out a very relaxing experience.

          He is trialling some crosswind landings, noting the icy condition at the time this was reported. The pilot's view is that with the slotted flaps there is much less weather-cocking than the 52 degrees of barn door flaps. Crosswind control appears to have improved compared to the normal flaps.

          I've touched base for a further update.

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          • jetdriver
            jetdriver commented
            Editing a comment
            Hi Battson, thanks for the information you shared about these flaps, it's been helpful. Any update on the testing from the owner? Has he made further modifications and do you have video you can share? Thanks.

        • #97
          One bit of caution on fowler and semi fowler flaps. They generate a lot of force into the back spar and attachments. The early Cessna 206 aircraft used to bend the back spar , never a issue with the less powerful 185 flaps, later versions of the 206 had spar doublers. On the Found Bush hawk we used to have Barn-door flaps that were activated by a simple lever--when they build the semi fowler flap the stall speed dropped by 7 knots and the approach was nice and flat, but it was impossible to activate by hand and a 2 tone electric flap jack( a bit of overkill) was installed.
          My point is better make sure the engineering on the attache structure is up to the task. Extra weight (Attach structure and the flap) as well as added complexity are the negative side effects of fowler flaps.

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          • Battson
            Battson commented
            Editing a comment
            The mod has been fully tested using CAD processes with stress analysis, or such is my understanding. The torque on the rear spar and stresses transferred to the wing skin are known problems.

        • #98
          As long as there are known upgrades and fixes for known problems you are good togo.
          Fatigue is a tough target to design for when dealing with aircraft that need to be light.
          Just keep that in mind when you build, take extra care when you build the attach structure to save yourself a repair later.
          This is just a heads up for anyone upgrading to bigger more powerful flaps.

          Also watch your flap extension speeds especially in rough air, a few MPH lower Flap extension speed dramatically drops the induced forces on the flap,
          These Forces on your flaps increase exponentially with speed they go down the same when you slow down.

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          • svyolo
            svyolo commented
            Editing a comment
            Putting the flaps down with powerful flaps (fowler or semi-fowler) also causes a big change in center of pressure - i.e. big trim change. Doing it at a slower speed makes it a lot less unpleasant for the passengers.

        • #99
          Originally posted by Gerd Mannsperger View Post
          VGs on the leading edge of semi fowler flaps work extremely well but you can do either that or the VGs on the leading edge not both.

          The leading edge VGs give the benefit of better control while the VGs on the flaps just lower stall speed with a reduction in slow flight control.

          The reduction in stall speed is very close to the same in both instances, if in doubt go for improved controllability.

          I believe the Cessna Caravan comes with VGs on the flaps stock.
          That is also what MicroAeroDynamics told me. A long time, 20 years, user of their VG's on two different Rans S-7's, on the wings and the bottom of the horizontal stab., I had an idea to use them to "enhance" my flaps, and was told in no uncertain terms that it was not a good idea, and was not needed as the LE VG's were already "getting the job done."

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          • Gerd Mannsperger
            Gerd Mannsperger commented
            Editing a comment
            Yes that was my information as well

        • 92 VG's on leading edge and 58 VG's on leading edge of flap. Slotted flaps they work fine and help alot.
          You do not have permission to view this gallery.
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          Last edited by FastLaneAK; 05-02-2020, 11:58 AM.

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          • I've not installed VG's nor test flown various settings on the Ribblett airfoil of the Bearhawk. But regarding VG's in general, Jerry Burr is pretty much the authority on them, having spent likely hundreds of hours testing, learning and understanding what worked the best. I believe the graph below from him was on a SuperCub wing. The most common installation error in my opinion is simply that they are mounted too far back on the chord. The idea is to keep the airflow attached at high(er) AOA, and as his graph shows, it's almost impossible to put them too close the leading edge.

            You do not have permission to view this gallery.
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            • Battson
              Battson commented
              Editing a comment
              Mine are exactly 10" forward of the rivet line running down the main spar. I found this location gives the VG the approximately full exposure to the airflow at the stall AOA.

          • The most effective of all STOL mods is also the cheapest one.

            Weight -- less is more.

            If you want STOL capability exercise discipline build simple and light.

            Just got back from a Photo / training flight on some local sandbars with light aircraft and it is so easy.

            Usually I am always at gross weight ( for work) and it is a whole different world --weight should always be the first consideration for a STOL aircraft.










            Last edited by Gerd Mannsperger; 05-27-2020, 01:59 AM.

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            • Russellmn
              Russellmn commented
              Editing a comment
              Building simple and light is easy on the face of it, until you start really chasing grams, then it gets REALLY expensive!

          • Yes been there done that.

            But I pulled 160 pounds of misguided perceived needs out of my last Cessna 206 and 120 pounds out of my last M7 all in Vacuum systems Avionics auto pilots and heavy interiors.

            The Airframe and powerplant are still all stock.

            Paint is a big one too, more so on fabric aircraft.

            On the Maule I saved 25 pounds alone on the paint when I painted it and did the fabric.

            Both Aircraft already had lightweight Starters and Alternators but still have stock Batteries. If this is on the table on certified aircraft imagine what is possible to the builder of homebuilt aircraft.

            If STOL is your game watch the Weight





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            • kestrel
              kestrel commented
              Editing a comment
              Generally true. However, Frank Knapp (a _BIG_ fan of light) ended up adding slats, complex flaps and a more powerful motor to beat the big guys. Weight is _almost_ everything, but not absolutely everything.

          • You are correct Kestrel weight is not everything, I would not want to weaken the airframe or landing gear for example.

            However if you add more weight you will need to go to heavy duty landing gear and brakes for the higher weight - adding more weight - eventually you will need more HP = Bigger engine More weight now your stall speed goes up and you need to extend your wings to make up for it and on it goes. I hope you get my point.

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            • Kitplanes has been running an excellent series of articles on aircraft design. It has given me a lot of insight into the compromises and trade-offs made during that process. More fuel -> more weight -> stronger wing structures -> more weight -> more power needed -> more fuel, repeat ad nauseum... Understanding those trade-offs a little better have given me even more respect for Bob and his designs... No design is perfect, but Bob's designs produce some pretty incredible airplanes!
              Jim Parker
              Farmersville, TX (NE of Dallas)
              RANS S-6ES (E-LSA) with Rotax 912ULS (100 HP)

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