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_
(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
Dunlap, TN (TN89)
N245E - Flying
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