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Bearhawk STOL mods

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  • Originally posted by Nev View Post
    That all makes sense.

    Out of interest, do you guys normally land with full available flap ? I'm wondering what effect it would have if you flew an approach a few kts faster, with full flap so that the drag took care of the extra speed on round-out. Clearly a work-around. But would the few extra kts lower the approach attitude and increase over-the-nose visibility ?
    Yes, and that's generally what you have to do. The faster you fly, the lower AoA you can fly for a given weight. But energy is mass * velocity^2.


    • Nev, in my case, it's tinkering with a well-established design because it's fun to try to eek out a little more. From every account I've read, the stock machine performs wonderfully. But, think of the change from Model A to Model B - reportedly a few knots faster speed in cruise (likely owing to the reduction in tail incidence), and a favorable change in already good stall characteristics. There's nothing wrong (and plenty good!) with the Model A, but the B is "slightly better." Why not try? It's Experimental, it's fun to try something new if it doesn't kill you in the process. But, if you have a flying plane and don't want the down time, or you just want to get your build in the air, keep with what's been proven.

      I think the improvement in stall speed with a slotted flap would be small, but noticeable, and would also help with the deck angle when flying very slow. It would also improve roll control slightly, but again, with the current slotted aileron design, there may little, if any, noticeable improvement. I estimate that the Bearhawk plain flap improves maximum section lift coefficient by about 20-25%; a good slotted flap design could probably double that. It may sound like a lot, but stall speed will go with the square root of the ratio of lift coefficients. So, if I WAG our unflapped airfoil as having a Cl of 1.7 (about what you'd see with a NACA 4412 at this Reynolds number range), a plain flap may be about 2.1, and a slotted flap would be about 2.5, giving the plain flap a stall speed improvement of sqrt(2.1/1.7) = 1.11 (~11% reduction in Vstall) for that section; the slotted flap is then sqrt(2.5/1.7) = 1.21 (~21% reduction). That sounds like a lot, but only about 55% of the Bearhawk span is flapped. So if the outer section stalls at a Cl of 1.7, and the flapped section at 2.1 or 2.5, then, from a 2D flow perspective, the plain flap reduces stall speed by (0.55*1.11 + 0.45*1 = 1.06) about 6%, and the slotted flap by about 12% (or, conversely, a 6% reduction in stall speed from the plain flap). If your unflapped stall speed is 50 knots, that logic says your plain flap stall speed is 47 knots, and your slotted flap stall speed is 44 knots. (All speeds are notional, I don't want to get into a stall speed war at this time. Point is, you're talking *at most* a 6% and 12% reduction in stall speed vs. unflapped for plain and slotted, respectively.)

      Reality won't even be that kind, because that assumes you are hitting maximum lift on both the unflapped and flapped portions of your wing at the same time, and have no 3D losses (vortex at the wingtips or from the flap tip, uneven downwash distribution, etc.). Since the slotted flap will hit maximum lift at an angle of attack that is lower than the unflapped portion with the aileron (which is also slotted, but it won't be deflected as much as the flap unless you're in quite a crosswind!), the aileron portion of the wing won't be at maximum lift when the flapped portion is. However, what you will notice is the potential for a lower deck angle and slightly better aileron effectiveness, since the aileron portion of the wing won't be as close to stall.

      I'm attaching the following picture (not copyrighted) from Aerodynamics for Naval Aviators; a great, practical reference for aerodynamics if you haven't seen it: I think it under-represents what you can get from a thicker basic section, but the trends are all good.

      Last edited by nborer; 10-13-2021, 10:18 AM.
      4-Place Model 'B' Serial 1529B (with many years to go...)


      • I enjoy nerding out on just about anything. Fun discussion. I basically agree with Nick on my reason to install slotted flaps. Tweaking things is a lot of fun.
        Scratch Built 4-place Bearhawk. Continental IO-360, 88” C203 McCauley prop.


        • I'm about ready to cover (which for me is the last part of the fuse), but it's getting to be winter, so I have to wait until spring. Now it's on to firewall forward or wings. I think wings because I don't want to do firewall forward then remove it for the rotisserie. Maybe in the next few weeks I can mock up some lowered flap hinges.


          • The B model still has a servoing trim system, there is no reason to believe it will act any different than an A model. Here is the low hanging fruit:
            Woah, that's brilliant ! Removing the servo action and adding electric trim all in one go. Very keen to see the flying result.
            Nev Bailey
            Christchurch, NZ


            • schu
              schu commented
              Editing a comment
              Not only that, I have a full autopilot setup which can control the trim servos. This means my AP can trim my airplane. Also, there is a bearhawk flying like this already, it works just fine.
              Last edited by schu; 10-14-2021, 12:11 PM.

          • I checked the flaps on a friends Husky yesterday, the hinge point is moved forward as well, which lowers the flap as it is deployed, to bring the airflow over top. I'm not sure if the Maule hinge point is moved forward or just down.


            • Battson
              Battson commented
              Editing a comment
              It's probably a trade-off, if the hinge is aft then the flap moves rearward, which is good. If the hinge if forward, then the flap moves downward, which is good. NACA determined something along the lines of, the best gap size to maximise lift is 2% cord. So that needs to be factored in as well.