This is a Pistachio for the International Indoor Fly In 2008. It is a Koolhoven F.K.46, a cute bipe designed as a trainer.
Here is the fuse on the building board, made of 1x1.5 mm balsa. Weight of the shown side is 0.11 gram. I made 2 sides before, but they were twice as heavy as this one and won't be used. Here is also the set of wings ready for covering. Weight is 0.7 grams. Third pic is a close up of the wing tips, bottom side and upper side.
I have covered the wings and stab of the Koolhoven bipe doublesided with metalized mylar (1/4 mil, 6.8 gms per sq. metre) and have some data on the weight. The 4 wings weighed 0.7 grams before covering. After covering they are 1.01 gram, so the glue and covering added about 0.3 gram. The area is 2 x 2 x 4 cm x 20 cm = 3.2 dm2; so the mylar and glue adds 9.4 grams per square meter. The mylar itself should be 6.8 gms per sq. metre according to the supplier, the difference is caused by the glue as well as the overlaps of the mylar.
The stab weight increased by 0.05 grams, area is just a bit less than 1 of the four wings so this agrees with the weight increase of the wings.
- the mylar is definitely lighter than unfinished esaki (said to be 12 grams per square meter) and it is waterproof.
- A wing covered double sided in mylar is about the same weight as a wing covered single sided in Esaki and doped, which has been said to be 18 grams per square meter.
Wat gegevens over het model:
- spanwijdte: 20 cm
- gewicht: 4,2 gram, zonder rubber
- rubbermotor: 1 lus van 1,25 mm (0,05"), gewicht 0,85 gram
- propellor: 4" Peck
I finished the model on friday at 24.00, a few hours before the International Indoor Fly In 2008. On Saturday morning it placed second in static judging (first place went to Div Masters), afterwards I did some flights. It flew well with minor adjustments: a bit of up elevator and a trimtab on the left wing to keep the model from banking to the left to steeply. On 1250 turns on a loop of 0.75 grams 0.050" rubber it flew 27 - 28 second flights. A 0.85 gram loop of fresh rubber gave me flights of about 35 seconds. Enough to win me 2 prizes, 1st in pistachio and the "Flying Dutchman Award" for the best model of a Dutch designer!
The model flew well again in 2009, but due to the improved standards of the other competitors it "only" reached fourth place. The longest flight was 43 seconds. During the contest the model got damaged and it was stored in a shoe box waiting for repairs.
To prepare the model for Interscale 2010 I reduced the tailplanes in size to get the model more in scale. I also made the elevator ajustable which should be rewardeded with a higher static score.
For he wings I decided to use very thin balsa, single sided covered with mylar. This proved to be rather tricky. The wings were a bit wavy after the mylar was shrunk, and the single sided mylar covered tailplanes also warped. The model didn't get finished in time for the contest and returned to the cardboard box.
Time for a new attempt at improving the Koolhoven! I decided to use silver Esaki as covering. This is almost completely water resistant, which can be a good thing for a pistachio. If the tissue is attached to the balsa substructure wrinkle free it probably will stay that way.
The wingtips and tailplane outlines are laminated fom 3 layers of 0.35 mm balsa. The ribs are cut from light 0.6 mm balsa. The structure now weighs only 0.5 gram.
To get the silver Esaki free of wrinkles I decided to try preshrinking. I have a
spruce frame which takes 1/4 sheet of Esaki. After attaching the tissue to the
frame I used an atomizer to moisten the tissue. At first it looked like a newly
waxed car, the water formed nice droplets on the surface. But after a long time
it started to absorb part of the water. The next day it was tight as a drum,
proof that water shrinking silver Esaki is possible.
The tail structure is glued to the tissue while it is still on the frame. As glue I used shellac, because it dries relatively slow and contains no water. The structure is weighted during the drying of the shellac to prevent warps. The stab is now cut out with an extra margin around the structure. The extra tissue is folded around the edges and secured with shellac.
The fin and rudder are to be covered double sided, so they had to be turned over on the tissue.