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Airspeed Gain from Wheel Pants

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  • Airspeed Gain from Wheel Pants

    During the past couple of years I have been building fiberglass fairings for both ends of the wing struts, the landing gear shock struts, and wheel pants. This covered all of the areas that budd davisson said could use some aerodynamic cleanup. Many of you saw the finished versions at Oshkosh 2013.

    Of course, the first question that everyone asks is "How much speed did you gain?"

    Being blessed or cursed to be an aeronautical engineer and a professional flight tester, I knew that most claims of airspeed gain are wildly exaggerated. I would love to tell you "Yep, I gained about 30 to 40 knots of free airspeed. In fact, it was so good I have to pull the throttle back to keep it out of the yellow arc." I'd love to tell you that, but it certainly wouldn't be true.

    Based on nothing more scientific than looking at the true airspeed on my Dynon display and trying to remember what the values used to be a couple of years earlier, I have told many of you "about 5 knots." Not a very scientific approach.

    The problem is that cruise speed is affected by so many things, such as pressure altitude, temperature, power setting, gross weight, turbulence, and pilot's shoe size. Okay, maybe not the last one. Additionally, since airspeed varies with the square root of the drag coefficient and the cube root of the power, most changes don't make a big difference. Statistically it is tough to determine a 5 knot change while watching airspeed vary either direction by 5 knots. Noise in the data of similar magnitude to the change makes it hard to find the magnitude of the change with any accuracy.

    I recorded a bunch of cruise data during the Oshkosh 2011 trip with no fairings, and more cruise data during the Oshkosh 2013 trip with the fairings. I finally finished converting my data reduction software that I had written on the work computer (in Matlab) so that it would run on my home computer (in Excel).

    I haven't done much analysis yet, but the first look shows a reduction in drag coefficient with the fairings of 35 drag counts (delta coefficient of 0.0035).

    I know what you're thinking...WOW!! That's great! I have no idea what that means!

    Well, for a flight condition of 7500 feet pressure altitude, standard day, 2440 lb gross weight, that equates to an airspeed increase of ...4.5 knots.

    4.5 knots. Not quite 30, but surprisingly close to my initial guess of 5 knots. Not a whole lot, but it's free! A little more weight, but it doesn't take any additional fuel flow. I think it also makes the airplane look better, or at least more complete.

    Keep in mind that a given reduction in drag coefficient has a bigger effect on airspeed as the original airspeed increases. Thus, I added wheel pants and gained about 4.5 knots. A friend added wheel pants to his Glasair II and gained 10 knots. The difference was that his initial airspeed was 160 knots, not 125 knots.

    There's a lot more flight testing and analysis yet to be done, but I knew many of you were sitting in front of the computer thinking "when is Erbman ever going to figure out how much speed he gained with his fairings." Well, now you know.
    Russ Erb
    Bearhawk #164 "Three Sigma" (flying), Rosamond CA
    Bearhawk Reference CD
    http://bhcd.erbman.org

  • #2
    Thanks for posting it Russ. I've considered the Maule style panties, but that's a long ways out yet.

    Must be getting close to CD 6.0 time???
    Mark J

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    • #3
      Very interesting post. I understand the one of the primary high drag areas on most aircraft is the wing root joint to the fuselage, particularly at the trailing edge. Have you considered making a tapered fairing for this area? I made fibreglass fairings for the top half, but have not yet made a longer tapered fairing with the aim of reducing drag. Here's a low-wing example in case my description is too crummy: http://www.ch601.org/resources/fiber...g/Dsc00439.JPG

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      • #4
        The problem you describe is an issue with low wing aircraft. Since the wing and fuselage are both turning away from the flow in the area you described, the flow is very prone to separation which causes the drag. The fairing fills in that area and reduces the adverse pressure gradient. This isn't an issue for high wing aircraft. For supporting evidence you will note that you have never seen such a fairing on a Cessna, high wing Piper, Beaver, Twin Otter, or pretty much any other high wing aircraft.
        Russ Erb
        Bearhawk #164 "Three Sigma" (flying), Rosamond CA
        Bearhawk Reference CD
        http://bhcd.erbman.org

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        • #5
          Just thinking out loud here - in the speed bracket of the aircraft listed above, most low wing designs (of similar speed) don't have much of a wing root fairing either?? One might assume there's a speed where it starts to become more significant? Interesting thought - when was the last time you saw a properly high-speed high-wing aircraft....? All the fast ones are low or mid-winged. I wonder whether that's a structural or aerodynamic consideration which drives the design down that route.

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          • #6
            Perhaps you have forgotten the C-130, C-133, C-141, C-5, C-17, or even the BAe 146? For that matter, how about the F-15? I think anyone would call Mach 2.5 "high speed".

            The low wing advantage for high speed aircraft has a lot to do with the convenience of mounting and retracting the landing gear.

            Actually the best place for a wing aerodynamically is a mid-wing position. The wing spar tends to get in the way of the payload.
            Russ Erb
            Bearhawk #164 "Three Sigma" (flying), Rosamond CA
            Bearhawk Reference CD
            http://bhcd.erbman.org

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            • #7
              On the glider side of things, I fly a Schweitzer 1-26 in LaBelle Fl. Pretty much a mid-wing setup. Maybe a little higher. Per the info at the 1-26 association, taping the wing root to fuse area will decrease your stall speed by 5 mph. We taped ours and the stall speed dropped exactly 5 mph. Thought this might interesting for this discussion.
              John, Naples FL
              Bearhawk 4-Place Plans #1316
              Patrol Plans #006
              Experience is something you get, right after you need it.

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              • #8
                I hadn't forgotten any of those aircraft - I was picturing 2 through 6 seaters with a high wing, and couldn't think of any common example which can compete with a Glasair, Lancair, etc. As you say, there are design issues like retracting main landing gear into the fuselage instead of the wings, structural parts required around the cockpit, etc.

                Here's an example of what I mean, was helping out a bit on a Cessna 195 the other weekend:
                Last edited by Battson; 11-14-2013, 08:38 PM. Reason: Add photo

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                • #9
                  Have you guys heard of anyone modifying Cub fairings to fit the Bearhawk? If so, can you point me in the right direction.

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                  • #10
                    Bob sells the strut fairings specifically for the Bearhawk. http://www.randbaircraft.com/fiberglass-parts

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                    • #11
                      Russ, thanks for the info. We plan on making wheel pants for our Bearhawk 4 place after we get it in the air. Brent has made nice wing root fairings for the trailing edge of the wing already so that will be in place from the start. I have no idea if they will add a bit of speed but they look cool! We will get to see what sort of improvement we get from the wheel pants once we add them.

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