Apropos the recent thread on this topic, I found an old posting by
John Lawton which may be enlightening:
> In a message dated 5/9/2007 2:59:30 AM Eastern Daylight Time,
> europa-list@matronics.com
> writes:
> My understanding is that preventing the air going over the flap, in
> cruise, reduces drag, resulting in higher cruise speed, which I have
> experienced. I don't have the engineering background to completely
> understand what is happening to the wing, in flight. That's why I
> depend on the guys on the list to keep me straight. I'm more of a
> "try
> it and see" kinda guy... :) John's advice is based on the success
> that
> the glider guys have had, sealing the gaps. It seems to have worked
> on
> Baby Blue, as well. Continued testing will tell to what degree of
> advantage the seals make. It may very well prove that the extra few
> knots is not worth the float upon landing. I won't know until I
> satisfy
> myself that I've explored all the angles (and strip widths)... ;)
>
> Hey Jeff, et al,
>
> The concept behind seals is pretty simple. In flight the wing
> creates a low pressure area on the top and high pressure on the
> bottom. (Bernoulli at it again) This pressure differential will try
> to equalize through the gap in the flap area and the hinge line of
> the ailerons. This is also true to some extent on the rudder hinge
> line and the trim tabs when they are deflected. This equalization
> causes drag, a lot of drag. So, the idea behind seals, both flap gap
> and positive seals on ailerons (and rudder and trim tabs), is to
> prevent this pressure equalization.
>
> On N245E I used PVC weather-strip to create the flap gap seal. The
> size will depend on how big your gap is between the leading edge of
> the flap and the flap closeout when retracted. Choose a foam that is
> slightly thicker than the widest spot in the gap you have. The XS
> flap closeout looks as if it were designed to be sealed, given the
> flat area at the bottom rear of the closeout. If you apply the foam
> strip to the area where the flap "noses" into the closeout when up
> it will seal this area for cruise flight, but still allow air to
> flow over the flap when extended. Use a foam that crushes easily
> when the flap comes in contact with it. You don't want to use a foam
> that is too stiff or too thick or else your flaps won't fully
> retract. PVC weather-strip is soft and pliable and works quite well.
> It is also fairly UV resistant and is very cheap. It's available at
> any hardware store in various thicknesses. We've found that open
> cell foams tend to not last very long due to UV and they don't seal
> as well as closed cell foams like PVC. I also used weather-strip on
> the ends of my flaps so that they seal against the side of the
> fuselage when they are in the up position. According to Bruce
> Carmichael, renowned Aerodynamicist who wrote a book on drag
> reduction in homebuilts, the wing root/fuselage junction is another
> area of high drag on most airplanes. Sealing it up helps, too.
>
> Aileron seals are a bit more tricky to employ. Some folks apply
> Mylar strips over the hinge lines and think they've sealed their
> ailerons. This is not true. Mylar is not a seal, but rather is
> applied to help the boundary flow stay attached over the hinge line.
> Mylar is also somewhat cosmetic in that it hides the hinge line. At
> high speeds the Mylar will lift up off the wing due to the pressure
> equalization, thus negating the effects of any sealing the Mylar
> might be offering. In some cases the Mylar will "buzz" at high speeds.
>
> For sealing ailerons you need to install what are known as positive
> seals. These seals go from the leading edge of the aileron to the
> rear of the closeout. There are commercially available materials for
> this, including various types of cloth and Teflon tapes. Some folks
> even use cloth tape along the length of the aileron on the
> undersurface. This is fine on low speed gliders, but I think at the
> speeds we fly this method likely creates more drag. Personally, I
> like parachute cloth for making positive seals. Whatever you use it
> needs to be zero porosity, i.e., allowing no airflow through the
> material. Parachute cloth is thin, flexible, zero porosity and fits
> the bill here perfectly. Here at the 'Possum Werks we use Parachute
> cloth almost exclusively to fabricate positive seals. (it also helps
> that we have a parachute manufacturer a few miles down the road that
> gives us their scraps)
>
> First remove the aileron. Then, cut a strip of parachute cloth about
> 3" wide exactly the length of the aileron. The width you use might
> take some trial and error to get it right. You don't want it so wide
> the the material bunches up when the aileron is deflected. 2.5' to
> 3" width should work on most Europas. Then, yellow contact cement is
> applied to the leading edge of the aileron, the rear of the closeout
> and the mating surfaces of the cloth. We use a brand of glue called
> Plio-bond. It's handy in that it comes in a small bottle with a
> brush applicator. 3M also makes good contact cement if you can find
> it. Avoid the water based stuff. It is crap.
>
> Scuff the area to be glued with 180 to enhance the adhesion. Apply
> the glue in a swath about 3/8" wide to both the seal, the aileron
> and closeout. Use masking tape to keep your glue line to the
> prescribed width and remove the tape once the glue is dry. You can
> also use the masking tape to provide a guide when you scuff the area
> to be glued, but wipe it with acetone to remove dust before putting
> the glue on. Allow the glue to dry, usually about 20 minutes, remove
> the masking tape, then stick the seal to the leading edge of the
> aileron first, preferably while on your work bench. If you screw
> something up Xylene will remove the glue easily without harming your
> paint. The next part takes an extra set of hands.
>
> With one person holding the aileron close to the wing, attach the
> other half of the cloth to the closeout forming an "S" pattern.
> Avoid wrinkles. See this drawing for details:
> http://wingsandwheels.com/page28.htm
>
> Then, reattach the aileron as you would normally do. Check the throw
> and make sure you are not binding! If you apply the seal correctly
> you will not even know it is there. I have also sealed my rudder in
> a similar fashion as well as the trim tabs. They create similar drag
> when deflected.
>
> I can't say specifically what improvements have occurred in N245E as
> a result of the seals since I've not ever flown it without them. I
> can tell you that I exceed Europas published numbers with a fixed
> pitch prop. I can also tell you that on gliders we generally see 2
> to 4 points increase in glide and a noticeable decrease in sink
> rate, even on the old "woodies", after the application of seals. In
> power planes this translates to faster cruise, better climb rates
> and better fuel economy.
>
> Anyway, hope it helps!
>
> Regards,
>
> John Lawton
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