This is only a preview of the June 1998 issue of Silicon Chip. You can view 32 of the 96 pages in the full issue, including the advertisments. For full access, purchase the issue for $10.00 or subscribe for access to the latest issues. Articles in this series:
Items relevant to "Universal High-Energy Ignition System":
Articles in this series:
Articles in this series:
Articles in this series:
Items relevant to "Universal Stepper Motor Controller":
Items relevant to "Command Control For Model Railways; Pt.5":
Purchase a printed copy of this issue for $10.00. |
RADIO CONTROL
BY BOB YOUNG
Radio-controlled gliders: Pt.2
This month we will look at some of the
factors to be taken into account when
designing a 2-metre glider and see how these
were applied in the Silvertone Stingray, an
unconventional 2-metre design.
The concept of the MAAA sanctioned 2-metre class was to provide
a simple entry level model on which
to learn the craft of R/C glider flying.
This model was to place few demands
on the radio equipment and the model
builder’s skills.
The main parameters call for rudder
and elevator only (no ailerons, camber
changing preset flaps or releasable
tow hooks) and a span not exceeding
two metres. “V” tails are allowed.
Wing loading is to be in the range of
12-75g/dm2. For those interested in
the complete rules, see the MAAA
Official Rules and Instructions Handbook (Chapter 3, Provisional Rules.
pp2-41).
As a result, the typical 2-metre glider
has evolved along rather old fashioned,
conventional lines with a polyhedral
wing, a simple (lightweight) structure,
and rudder and elevator controls. This
is typified by the yellow and red glider
This is a typical 2-metre glider showing the polyhedral wing and a simple
structure. It has just two controls, elevator and rudder.
pictured in this article.
Sometimes the designs include a
butterfly (“V”) tail with mixing on
the rudder/elevators. In the thinking
of most design
ers, the rudder-only
design dictates that large amounts of
dihedral (ie, wings sloping upwards)
are required in order to induce the
model to turn. To my mind, this is
wrong as the dihedral can fight the
rudder.
True, dihedral is required to initiate the turn but it then tends to pull
the model out of the turn and the net
result is a model that is difficult to
hold in a constant rate turn, a most
important point in thermal soaring.
But 2-metre gliders do not have to
look like models out of the 1930s.
The design we will be discussing this
month does not follow the current
trend and had its genesis during the
1970s when I was producing models
for the military.
While I had often visited glider
fields in the past and flown the odd
glider, I had never been interested
enough to undertake a glider design of
my own and fly seriously in competi
tions. In the good old days, if models
did not make a noise and go fast they
held no interest for me. Nowadays,
if they make a noise I cannot hear
them and if they go fast I cannot see
them. Much has changed since I was
30 years old.
During the early 1980s, Harold
Stephenson, a very keen glider flyer,
became a regular customer and finally
convinced me to design a model for
the new 2-metre class just gaining
popularity at the time. He even offered
to help me build it, an offer too good
to refuse.
I finally relented and drew up the
plans on a strip of brown paper from
my roll in the shop. Fig.1 shows the
finished design, redrawn recently on
June 1998 53
In contrast with conventional 2-metre gliders, the Silvertone Stingray has
swept-back wings, a “V” rudder and most important, a blended wing/fuselage
junction to keep turbulence to a minimum.
a computer using a CAD program.
Harold built the wing and I built the
fuselage and there it sat for the next
15 years or so (in the tradition of all
good models). That is until another
friend, Barry Ming, incensed that
such an interesting model should sit
unfinished for so long, offered to take
it and finish it. So in 1996 a finished
model, painted all over in black,
rolled into my workshop. Barry then
informed me that the original plan
had disintegrated due to age and my
only record of the design was gone.
It took me another 12 months to apply the colour trim and plug in the radio (one cannot hurry these things) and
finally, in late 1997, the model turned
out for its test flight. This I might add
was on the day of the contest. Why
54 Silicon Chip
is it that I sense a lack of surprise at
this last statement? Old hands know
exactly what I mean. The model still
looks quite modern 16 years on and is
quite eye-catching in style.
Since then the model has flown in
four contests, winning several rounds
and maxing in several others and has
attracted a good deal of interest, largely as result of its excellent flying characteristics and pleasing appearance.
The weak link is my piloting, for I
simply do not have the finesse necessary to read the subtle signs required
for good thermal flying and my spot
landings are appalling; with no throt
tle to adjust the final approach I tend
to undershoot all the time. Thus I
would be very interested to see this
design in the hands of a good flyer,
part of my reason for publishing it
here.
With the addition of ailerons the
model would make a great slope soarer
and the addition of flaps and ailerons
would convert it into an interesting
open class sailplane. I am currently
working on an F3B version which is
scaled up approximately 1.5 times
with flaps, ailerons and 2.5° of dihedral and a much better wing section.
I must point out that this is not intended as a full construction article as
it is a difficult model to build and only
for experienced modellers. The design
lends itself well to fibreglass but the
original is all wood with a built-up
wing using 1/2" x 1/8" spruce spars
in a “H” girder arrangement enclosed
in a 1/16" “D” box leading edge. The
plug-in wing dowels for the wings
are 1/4" steel rods in brass tubes. The
wing stubs are laminated out of 1/2"
sheet balsa and hand shaped. Note
that the large American influence in
modelling tends to favour the Imperi
al system of measurements in some
components.
Finished weight is 1.05kg, quite
heavy by 2-metre standards, whereas
a very simple lightweight can come
in at 0.5kg, ready to fly. Even so, the
wing loading is still only 33.8g/dm2
(7.9oz/sq ft) due to the large wing.
There is some evidence to suggest
that this loading is too light for the
Eppler 205 section used on this model
and the next round of trimming will
concentrate on the effects of ballast
and elevator trim on performance.
The model certainly likes to fly fast
and I feel that I have been flying it too
slowly in the last two contests.
The original model pictured has
several shortcomings. Firstly, the
nose is too short and this has been
corrected on the drawing presented
in Fig.1. The plastic film was also a
mistake as it goes slack in the heat.
A better approach is a fully sheeted
wing covered with silk or Oz Cover
and painted all over. Finally, the wing
section is over 20 years old and now
completely outclassed by the modern
thinner sections. However the overall
design shows promise and I believe
it could be developed into a potent
performer.
Design fundamentals
There is a fundamental rule in glider design that all gliders eventually
come down and that little gliders
June 1998 55
Fig.1: this 17-year
old design has
recently been
redrawn with a CAD
program. The nose
has been lengthened
slightly to correct an
original design
shortcoming.
The plug-in wing dowels for the wings are 1/4" steel rods in brass tubes. The
wing stubs are laminated out of 1/2" sheet balsa and hand shaped.
This photo shows the high degree of blending between the wing and fuselage.
The fuselage height has been kept to a minimum by laying the servos on their
sides.
come down more quickly than big
ones. Which is just a cute way of
saying that one of the key factors in
glider design is Reynolds numbers.
We examined Reynolds numbers in
the recent articles on jet turbines and
concluded then that the bigger the
chord (width) of the wing, the more
efficient it will be.
Now there is a fundamental conflict
in glider design that arises out of this
simple statement. One major source of
losses in the wing is the induced drag
which arises at the wing tips. Allied
to this is the problem of interference
drag which arises at the junction of
the wing and fuselage.
Thus the turbulence from the induced drag extends inwards along
the wing panel from the tips and the
turbulence from the interference drag
56 Silicon Chip
extends outwards from the wing/
fuselage junction. The traditional answer to this problem in sailplanes is
to increase the aspect ratio of the wing
(ratio of wingspan to wing chord or
width), thus increasing the clear span
(free of turbulence) panel size on each
wing half. A good example of this is
the 3-metre F3B glider featured in one
of the photos in this article.
Unfortunately, in doing this we
immediately reduce wing chord and
thus the Reynolds numbers on the
wing and to some extent defeat the
purpose of improving the overall efficiency. On full size sailplanes, this
is not quite as important as on small
models, for there is strong evidence
to suggest that a wing section with a
chord of less than 200mm falls into
the very low Reynolds numbers and
ceases to work effectively as an airfoil
section at model speeds.
A quick glance at the data panel in
Fig.1 will show that the mean aerodynamic chord on the Stingray-2M
is only 184.5mm, a figure somewhat
short of that minimum, so the overall wing efficiency is not going to be
anywhere near as high as on the F3B
version or larger models in general.
This applies to all 2-metre gliders and
small models.
So what to do? I have kept the aspect
ratio as low as I could on the Stingray
to keep the spar depth and Reynolds
numbers as high as possible and yet
I have still fallen below the recom
mended minimum chord. There is
little we can do on tip drag (winglets
on the tips may offer some help here)
but we can do something about wing/
fuselage interference drag.
There was a lot of work done during
the 1930s and 1940s on wing junction
drag and the Vought Corsair F4U was
one result. This work showed that
wing/fuselage junction angles of 90°
or less gave rise to a marked increase
in interference drag. The crank
ed
wing of the Corsair was one method of
increasing the wing/fuselage junction
angle to above 90°. The results were
spectacular and the Corsair was one
of the fastest piston engine aeroplanes
of WWII.
The McDonnell XP-67 experimental
twin-engined fighter in 1942 was an
even more interesting example and
the fuselage, nacelles and wing in this
design were an almost seamless blend
of aerodynamic styling.
Thus, by blending the wing/fuselage junction and increasing the
junction angles to above 90°, we can
substantially minimise the junction
turbulence and thereby increase the
clear span panel size without reducing
the chord.
The cross-section at BB on Fig.1
shows just how close the junction
angle approaches 180° on the Stingray-2M. One of the photos shows
even more detail of the blending. To
achieve these angles, the height of the
fuselage has been reduced by laying
the servos on their side.
If we could completely eliminate
the wing/fuselage junction we could
almost double the effective aspect
ratio of the wing without a reduction
in chord. Flying wings do just this
and the result is a very efficient flying
machine indeed.
ELECTRONIC
COMPONENTS &
ACCESSORIES
•
RESELLER FOR MAJOR KIT
RETAILERS
•
•
PROTOTYPING EQUIPMENT
•
FULL ON-SITE SERVICE AND
REPAIR FACILITIES
•
LARGE RANGE OF
ELECTRONIC DISPOSALS
(COME IN AND BROWSE)
CB RADIO SALES AND
ACCESSORIES
M
W OR A
EL D IL
C ER
O
M
E
Croydon
Ph (03) 9723 3860
Fax (03) 9725 9443
Mildura
Ph (03) 5023 8138
Fax (03) 5023 8511
Truscott’s
ELECTRONIC WORLD Pty Ltd
ACN 069 935 397
30 Lacey St
Croydon Vic 3136
24 Langtree Ave
Mildura Vic 3500
P.C.B. Makers !
•
•
•
•
•
Bruce Curl with “Calypso” a 3-metre F3B glider. Note the high aspect ratio of
the wing, the traditional answer to minimising induced drag and interference
drag.
So the essence of the Silvertone
Stingray-2M is the blended wing/
fuselage. But the design is more
complex than this for there are many
other factors which can be incorporated into this blended junction. The
strakes down the fuselage sides serve
a dual purpose. At low or zero angles
of attack they serve merely as flow
separators, inducing the airflow into
a smooth separation at the wing junction. At high angles of attack, when
combined with the swept-back wing,
they serve as turbulators, inducing the
wing to stall at the centre section, well
before the tips begin to stall.
With the centre of gravity (CG)
well back from this point, the nose
begins to settle first during a stall, a
very handy outcome. The net result
is to reduce the need for washout on
the wing tips, further increasing the
efficiency of the wing overall.
An additional minor benefit of the
blended fuselage is an improvement
in fuselage lift which can be quite
significant in some aircraft. The
Grumman Panther, another blended
fuselage aircraft, produced 30% of its
SC
overall lift from the fuselage.
•
•
•
•
If you need:
P.C.B. High Speed Drill
P.C.B. Guillotine
P.C.B. Material – Negative or
Positive acting
Light Box – Single or Double
Sided – Large or Small
Etch Tank – Bubble or Circulating
– Large or Small
U.V. Sensitive film for Negatives
Electronic Components and
Equipment for
TAFEs, Colleges and Schools
FREE ADVICE ON ANY OF
OUR PRODUCTS FROM
DEDICATED PEOPLE WITH
HANDS-ON EXPERIENCE
Prompt and Economical Delivery
KALEX
40 Wallis Ave E. Ivanhoe 3079
Ph (03) 9497 3422
FAX (03) 9499 2381
• ALL MAJOR CREDIT
CARDS ACCEPTED
June 1998 57
|