Papa,
It's hard to answer the question without more information.
"According to local rules ..."
What are the local rules? What is the locale, in particular, what is the
ceiling height? The ceiling puts a limit on how much rubber can be used
because it limits the amount of energy that can be used. The torque curve has a
certain shape and the energy curve has a certain shape and it has to "fit under
the ceiling". A thicker motor will allow a faster but shorter climb. A
thinner motor will allow a slower but longer climb. Ideally the plane takes off
from the ground with a fully wound motor, climbs and levels off just under
the ceiling, descends with the prop turning and lands just as the last few
turns come out of the motor. If it does that, that is the best time you can get
with that plane. It takes several trials to find the best motor. The plan
shows 3/32" strip, a bit thicker than I would use in our low ceiling (24').
What is included in the kit? A quick guess for good starting motor cross
section: measure torque to fly level. On the midpoint of the torque curve,
calculate the cross section to meet that torque. Calculate length from cross
section and maximum weight. My guess is this will result in a motor longer than
12".
"...I have to use the prop block in the kit ..."
What kit? The one sold by Penn Valley Hobby Center? Is it required to
build everything from the kit? You should use the lightest wood you can get that
will do the job. If you must use kit wood, put the heavy wood in front and
the light wood in back. What are the dimensions of the prop block? Can you
determine the pitch? Using a prop block allows some choice concerning the
blade shape, airfoil used and even a bit of latitude on pitch. The plan shows
a prop made from sheet, scored and cemented. A terrible prop. It also shows
rubber rings being cut from a bicycle inner tube and linked together to make
the motor.
"...plane cannot weigh less than 5 grams with 12" of rubber."
I assume 12" is the motor length, since the hook distance is a bit more than
8" and the usual practice is to make the motor 1&1/2 times the hook
distance. So if you have a two strand motor, the rubber is 24&1/2" long. If there
is no limit on motor weight, build the plane as lightly as possible and cut
the motor to whatever width will get your total weight up to 5 grams. That
seems light for a kit Phantom Flash, so I assume you mean the plane must weigh
no less than 5 grams without the motor? Several things may be done to lighten
the structure without modifying the areas, if the rules allow. The wing
ribs don't have to be full depth. The tips can be laminated. The rudder can be
laminated and R-2 can be reduced to a couple pieces of 1/16" square and a
short trapezoidal gusset. Use only one central rib A. The wing mounts can be
simplified, but add cross pieces sticking out on either side, over which
small dental rubber bands are fit to hold the wing down. Not only lighter, but
easier to get on and off. Cover with lightest film. Use the best rubber you
can get for the motor.
"Some people are using penny plane thrust bearings."
Or put a tiny drop of oil on whatever bearing you are using.
Glide test the plane with the prop replaced with an equal weight and find
the wing position that gives minimum rate of sink. Fly in the largest circles
you can fit in the site safely. The rudder is offset for a left turn. On
the plan that I have, the right wing is a bit longer than the left. Some
people would reverse that, to counter the motor torque. Some would use a bit of
washin (raise leading edge, about 3/16") on the left tip to counter torque
roll, and this also develops an adverse yaw to the left. You might want a bit
of right thrust if the initial torque rolls it too much left. A banked plane
is a sideslipping plane is a plane falling down and wasting energy.
Gary
[Non-text portions of this message have been removed]
Received on Thu Nov 02 2006 - 22:06:54 CET
This archive was generated by Yannick on Sat Dec 14 2019 - 19:13:44 CET