dashvii(at)hotmail.com
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 Posted: Mon Jun 25, 2007 9:37 am Post subject: Gap Seal Application Advice |
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I will post my final findings online soon. Nick is going to provide me with
an airspeed calibration soon and then I'll get the rest of my results up for
performance changes.
Well, after talking to Earl F. this morning, we now have a second airworthy
Lightning with gap seals. Unlike my previous experiment, this one is a
light sport compliant Lightning aimed to improve efficiency. It is also
made to be a temporary installation. There were several lessons learned
during my experiment in the fabrication of the seals. I would make some
changes if I were going to make a permanent installation.
1. seal length - The gap seals were mostly good, but on the flaps
especially I would trim them to where they covered the majority of the hole,
but didn't overlap the flaps. This way the flaps could be extended without
the gap seals vibrating against the belly of the plane. I think that I'd do
the same to the underside of the ailerons, although we had no problems with
the ailerons.
2. cloth seals added to ailerons - I would put either an S-shaped or V
shaped cloth seal in the aileron before adding the mylar seals to cover the
hole. The idea is that the cloth seal would stop the airflow better and the
mylar would smooth the airflow over the void. This is commonly done on
gliders who need every bit of energy that they can recovered. There are two
methods. One is the S - seal which looks like an S from the side. You
attach it to the top of the wing, flex the aileron full travel and then
attach the rear part to the bottom of the aileron. The second method is
where you attach a seal to the upper and lower parts of the wing. You flex
the aileron full opposite and trim to length. You then create a crease in
the void area to where when the aileron is in the nuetral position it is
pretty much flush across the top of the wing, but as you deflect the aileron
you stretch the V shape open. I have included a graphic of this, but in my
pic the V shape is open at nuetral to illustrate the shape. I believe that
we would've had a much tighter seal in terms of airflow escaping from the
high pressure to the lower pressure side of the wing if we had've done this
one extra step.
3. Seal thickness - In our experiment we got some uneven "waiviness" in the
larger seals along the bottom of the wing. This had to have cost us a
little in terms of drag and if I had've had several more sets of hands and
sharper eyes and lots of time we probably could've gotten the wrinkles out.
The wrinkles were all spanwise and not chordwise though. I would probably
go to some thicker duralar, specifically on the flaps. The longer the seal,
the harder to minimize the ripples, but I believe a stiffer seal would've
worked better. I believe the seals were 7/1000th's of 1 inch thick for
reference. Again, Duralar is just a trademarked name for a higher tensile
strength version of mylar.
4. Prepare for the unexpected - For any aircraft this is a good rule, but
when modifying an experimental yourself, you're essentially retesting the
flying qualities of the aircraft. I was very careful to try to take this in
steps. First Nick taped up the flaps only to see what it felt like. No
real change. We then put on the seals for the ailerons and elevator. Nick
took another short flight and reported that it felt basically the same in
terms of control pressures and handling. With my method of application
though there was the possibility of one of the top aileron seals coming
loose from the double sided tape, but not the 2nd layer of tape (see
graphic) This has been reported on several gliders. What happens is the
the front of the seal is still attached, and being on the lower pressure
side of the wing it pulls up. This creates an effect like a spoiler and
aileron and possibly a little rudder has to be put in to counteract the
roll. I'm not sure of any accidents where this happened, but several
anxious pilots. The Lightning would have more than enough aileron to
counteract this problem, but it would have to be recognized and corrective
action taken. Another option is a permanent installation using glue, but
I'd still recommend keeping alert whenever flying.
5. Cost/Time analysis - The cost for manufacturing the seals was probably
less than $100, although I haven't went back to tally up the costs. This
could be manufactured in a precut and shaped kit from probably realistically
$500-$750. The benefits would be tried and proven materials, instructions &
pics for installation, and improved safety because of the proven
installation and material methods. Time, if you had an extra person to help
you would probably be around 2 hours for the install to make sure that you
have it on smoothly and that it looks good and is straight. For that you
get about 5 extra knots of cruise performance. If you kept your cruise
speed the same as before the mod, you should get extra fuel efficiency,
although I didn't test for that. It stands to reason that if your drag is
decreased though that you will be able to turn higher RPMS, run at higher
TAS's, and burn less fuel at a set RPM.
6. Looks - Didn't change much on the top side of the plane. Looked like a
piece of white tape along the top of each wing near the aileron. If you
glued it on you'd see a white rectangular strip of mylar covering up the
hole which was pretty close to the same color as the plane. On the bottom I
used clear gap seals across the wing and white across the tail. You could
use all white or all clear. On the ground, if you looked up under there you
could barely notice them, but standing out on the runway after Nick got off
the ground and looking up it appeared that the gap was gone from the ground.
So overall you would have negligible change in looks. Very similar to the
gap seals on the aileron of the Cirrus from the top side.
Overall I think that our experiment was a success. Hopefully Earl can
independantly verify the results. We also got a favorable result from the
experiment which means that it is potentially applicable. I did find it
interesting that there's a Penn State Aero professor that had calculated
that you get basically a certain percent increase in each performance field
for installation of gap seals on small GA aircraft. His research led me to
try out this experiment as he always meant to do a Phase II with composites
(metal airplanes in his original tests). What we've seen is a greater
increase in terms of performance percentages, than as calculated by his
results. I'm not sure whether to attribute that to the wing planform or to
that of composites (clean surface) versus a riveted metal surface.
Interesting however you look at it. Brian W.
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