I am surprised that with all the recent developments in rubber

							I am surprised that with all the recent developments in rubber flying (auto surfaces,
variable pitch props, delayed start, etc,) there is still interest in this plain Jane
model that was in its heyday before the Russian revolution.
The model, retired since 1996, was rarely flown in any open rubber events other
than the one flight at dawn competition. The ship being of a somewhat flimsy nature
would not have survived the rigors of an all day event without some sort of damage.
One of the exceptions was a record trial, where the ship set a Cat III record on an
absolutely perfect day with no wind at all. On the relatively small Galeville field, a
day with no wind is a must in order to have any chance at all of setting a record. On
the record trial day, everything but the weather seemed against me. I got a late start
leaving the house for the field. On the drive to the field my constant companion, a
Yellow Labrador, saw another dog through the window, got excited and jumped in
the back of the van, crushing the rudders on two airplanes that were in a cardboard
florist box. Another hour was wasted in repairs and retrimming flights before any
serious attempt at setting a record could be started. Anyway, long story short, if I
could have gotten an earlier start the ship might have done a few more maxes before
the wind picked up and the air deteriorated.

In retrospect, I think the 1/32" motor tube was overkill; 1/16” would be more
appropriate. The weight of the balsa is not that big an issue compared to the dope
and tissue necessary to keep it stiff enough. Although thin, I keep them straight by
applying tissue and dope to the inner surface before rolling. The outer surface is
spiral wrapped with tissue. Several coats of dope are applied to the outside, and
then left on the form for about a week or more to gas out. In the past, to speed the
process I left tubes, still on the form in the car with the windows closed on a hot day.
After spending a few hours in unbearable heat, installing an exhaust fan in my attic,
I'm thinking that the attic is the ideal place to cure a tube. It reaches 120 degrees or
more up there at mid-day in the summer!

Obsession development started out as a 100 gram Coupe. When the 100-gram rule
was instituted in the early 70’s, it was obvious that the 80-gram Coupe I was
currently flying couldn’t carry the extra weight well. The “Paper Doll” Coupe was
designed with increased wing area. It was a very successful model, and with it’s long
motor run (60 sec,) maxes were fairly easy to come by. Of course it wouldn’t be
competitive against today’s Coupes, but then the current ships don’t have to cope
with 100 grams and a crossection rule. Its airfoil was the result of a good deal of
experimenting with turbulators and other modifications to a basic B-6405-B. The
nose of the airfoil was extended by 1/10” and sanded to an almost sharp nose radius.
External turbulators seemed to be more degrading to performance than beneficial. I
couldn’t improve over the sharp leading edge and the multispar wing’s inherent
turbulator effect.

Later, the basic “Paper Doll” plan form was used to create a Coupe sized Unlimited.
A minimum of changes (listed below) was made, hopefully to maintain the same
characteristics as the “Paper Doll” Coupe.
The Pylon was shortened, since the crossection was not needed.

The fuselage was lengthened to accommodate a modest 30 grams of rubber.

A 2” longer propeller with a higher pitch was used.

A slightly larger stab was used to reduce any oscillations caused by the increase in
rubber mass.

The center of the rubber motor was kept as close as possible to the CG.

Particular care was made to keep the tail moment essentially the same, in an
attempt to preserve whatever magic the “Paper Doll” Coupe had.

When the “one flight at dawn” event became popular, the Coupe sized unlimited
was scaled up to the maximum allowed for the Mulvihill event. The “Obsession”
was designed to be able to compete in both Mulvihill and the one flight event, so the
wing area was kept to less than 300 sq in. The only major changes from the smaller
model other than scaling was the use of an airfoil (BO-56-26) that had excellent
performance on my Wakefields. The airfoil was modified for a slightly thicker
trailing edge. The Carrol Allen propeller was adapted because it did so well on his
Wakefields. Carrol’s propeller theory (Constant Reynolds Number) was well
documented in one of the model magazines in the 70’s and also in the NFFS digest,
February 1975.