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I
decided for last winters building project to get away from the
complexities and frustrations of designing my own aircraft and
instead to build from a tried and tested plan. I came
across an article on the DH 108 Swallow by Vic Steel in the
1997 June/July issue of QFI and was impressed with the
beautiful shape of this aircraft. Vic has produced a
very detailed and easy to follow plan. There is also a
vac-formed cockpit canopy available, suppliers name on the
plan.
It
was a pleasure to start working from a plan, and construction
was fairly straight forward. The huge size of the
cockpit area started me thinking about some sort of parachute
ejector seat mechanism, more of this
to
follow. The fuselage was built in two identical halves,
but instead of skinning with panels of 1/64" ply as per
the plan I planked using 3/16" medium weight balsa
strips. A tedious method of construction, but one which
allows greater accuracy and shape during the sanding
process. 14mm joining and 10 mm incidence tubes were
built in prior to joining the two fuselage sides
together. The fin was then built into the fuselage.
Each
wing panel was cut from three sections of foam, butt joined
and ply mounting boxes let into each wing. The wings
were then obechi covered. The under surfaces of the
model were covered with medium weight glass cloth, upper
surfaces light weight
glass
cloth. Two coats of epoxy were applied and rubbed down
followed by two coats of cellulose primer filler. The
final silver finish would be applied after the ejector seat
and sliding canopy were installed, and what a problem that
was.
The
front and rear sections of the canopy were cut off and built
into the fuselage. I then attached two plastic tubes to
the lower edges of the central section of the canopy, these
were made to slide quite freely into two larger opened plastic
tubes built into the fuselage. A pivot arm connected to
a standard servo and to the inner top of the canopy gave a
maximum of 5" of movement which is sufficient to allow
the ejection seat to clear the canopy. A micro switch
was installed to switch on the ejector seat servo when the
cockpit canopy was fully open. This was to stop
accidental firing when the canopy was closed.
The
release mechanism was converted from a child's toy that used
to be supplied by Mothercare, but they considered it to be
dangerous, and stopped producing it many years ago. Some
of you may remember buying this toy for your children.
The base was dome shaped 5" diameter with a central
spring loaded plunger onto which a number of hoops were
placed. These were then fired into the air (whoopee)
with considerable force. I found this toy in my loft
(I'm a great hoarder of all things useful ). I then
asked my twenty one year old son if he had finished with it
and he said, in that tone of voice which is sometimes reserved
for fathers in mental decline "Yes dad you can have that
toy I used to play with nineteen years ago, after all I have
better things to play with " as he returned to his
computer.
I
removed much of the outer casing, replaced the original spring
with a stronger one and fitted a mini servo to the
trigger. A ply plate was then epoxied
to the top of the plunger. I purchased an ultra
lightweight ejection seat and pilot plus a couple of forty
pence parachutes from a toy store. A recess was cut into
the back of the ejection seat into which the folded parachute
was fitted. A button magnet holds the ejection seat
quite firmIy onto the ply plate. The device was lifted
into the aircraft and alter lots of trial and error it now fires
and clears the high fin in static testing, but not high enough
to unfold the parachute. The moment of truth would be
out on the slopes.
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The
ejector seat mechanism, removed from the cockpit. |
That day
arrived with a 12 mph wind. The model was launched and
after a few hair raising moments until the trim was sorted
out, the model flew very well. At a height of
approximately 75 feet above the slope, I wound back
the
canopy and fired the ejection
seat,
this exited the aeroplane vertically and, deploying the
parachute, floated gracefully down. The vision of the
parachute wrapping around the fin and possibly causing the
aircraft to crash still haunts me. The ejector seat has
continued to work with 100% success at each outing. The
only slight problem is if I release the ejector seat when the
aircraft is very high it's a long walk to recover the pilot!
RC
Model World Plans Service - MW2598
DH 108 Swallow by Vic Steel
AUW - 8.25lbs
Wing Area - 7.5 sq.ft. approx.
Wine Loading - 18ozs. per sq.ft.
Wing Span - 86"
Section - MH-45
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