The Embryo Experience
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By Thomas Lee and Joshua Finn
December 25th, 2004
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Embryo Endurance is a fun competition class for small
rubber FF models of 50 sq. in. or less. With no minimum
weight, and very few rules, the design possibilities are
endless. Perhaps the most familiar embryo design is the
Peck Polymers
Prairie Bird, which is still a competitive
model after over 20 years. |
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In this article we'll describe how we design, build,
and fly our embryos. Some of the techniques we both
use are similar, and some are different. Most of
our designs are fairly conventional, while a few
are unorthodox. |
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Before getting started, let’s review the rules:
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EMBRYO
ENDURANCE RULES
- Monoplanes not over 50 sq.in wing area* (70
sq.in. biplanes with 45 sq.in maximum for largest
wing)
- Stabiliser area not to exceed 50% wing area.
- Fuselage volume to include minimum space 1.25"
x 1.5" x 3"
- Wing and tail surfaces built up, covered both
sides with Japanese tissue or equivalent
- Landing gear in conventional configuration
with ¾" diameter wheels, or larger,
which must turn on axle
- Unassisted ROG from a card table top
Bonus points awarded as follows:
- 5 pts for raised cabin (minimum 30 degree
windshield slant) or windscreen with open cockpit
and headrest.
- 3 pts for 3-dimensional wheel pants
- 1 pt. for 3-dimensional exhaust pipes
Highest flight total plus bonus points wins.
Three flights to 120 second maximum, plus one
unlimited flyoff flight if required.
** wing area measured with wing flat, NOT projected
area.
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DESIGNING |
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Thomas-
I try to stick with a cabin type fuselage, as they are
usually lighter than open cockpit types. The less side
area in front of the CG it has the smaller the rudder
can be, and thus the lighter the plane. A short tail moment
enables the plane to recover from a stall faster and lose
less altitude in the process, whereas a plane with a longer
tail moment has better overall performance, but tends
to be heavier and weaker.
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I like to design wings with an aspect ratio of 8.0
to 8.5, though I have used aspect ratios up to 9.7
with success. I usually make wings in the shape
of a simple rectangle, then add triangle wingtips
with tip dihedral. I find that this works quite
well and eliminates some of the vortex in front
of the wingtips. I've also had success with wings
that are tapered or double tapered, using polyhedral
or just plain V-dihedral. I use less dihedral than
most other modelers - about 1/16th of wingspan at
each tip is usually sufficient. |
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Open rib construction
is my preferred method of building wings, but some of
the leading embryo designs have geodetic wing-structures.
Also, I've found more success in semi-laminar airfoils
than typical airfoils with highpoints at 25-30 % of chord.
Rudders and fins are the most fragile part of the whole
plane. If the fin sits on top of the stab (as on most
embryos), I'll build the whole fin using 1/20th wood with
all the edges sanded to an airfoil shape - i.e. only sand
the left side for an enhanced right turn. I use a warren
girder structure if I plan on shrinking the tissue.
If not, I will have only one stick (or possibly none)
inside the outline. This structure is always gusseted.
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For stabs, I generally choose 1/20th or 1/16th (usually
1/20th) square balsa. I use warren girder
construction if the tissue is to be shrunk, or traditional
construction of not. I sand a round profile to the
leading edge and trailing edge of the stab only
on the top. |
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Propeller is usually Peck-Polymers’
7 inch silver prop, but I have had success with balsa
props 7 to 8.5 inches long. Rubber is 1 loop of 1/8th
20-25 inches long usually, or if I'm feeling good that
day I might put in 3 loops of 1/16th rubber 20 inches-25
inches long and watch it climb…fast!
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Joshua-
Like Thomas I stick to cabin models almost entirely for
their simplicity, stability, and ruggedness. I’ll
address the design of wing, fuselage/empennage and prop/motor
separately:
Wing
There's an old saying, “the wing is the thing.”
While the prop is very important in free flight, I tend
to design the whole model around the wing because most
of the flight is spent gliding. For flying in calm conditions
I use a wing that is about 2.5” x 20.” For
for windy conditions I prefer a 2.75” x 18”
wing. All are usually tapered at the tips. I’ve
had good success with thin undercambered airfoils, so
most of my designs use these - despite a little loss in
maximum altitude. Since I sometimes fly in windy conditions
I strive to build very strong, but lightweight wings.
This is done by using relatively heavy spars, leading
edges, and trailing edges, while keeping the ribs thin
and light. Most of my models incorporate wing tip plates
and very low dihedral angles with tip dihedral setup.
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Fuselage/Empennage
The fuselage has to contain the required volume, support
the stresses of the motor, and hold everything together.
I keep it as skinny as possible to minimize drag, and
long for plenty of tail moment. Most of my fulelages are
20” long, but for long motor models they may be
over 25”. With only 2 to 2.5 degrees decalage between
the stab and wing, these models are very stall resistant.
This setup is quite useful for high power/weight ratios.
For windy weather models I use short coupling to allow
fast recovery from upsets. The fuselage usually incorporates
retractable landing gear and sometimes a lightweight silly
putty DT timer. The stab is usually about 40% of the wing
area and of high aspect ratio (usually about 10”
wide). Tip rudders are used to get them out of the propwash
and the wake from the fuse, and to minimize vortex effects
at stab tips.
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Prop/motor
I typically use small, low pitch props of 6”
to 6.5” diameter, with very thin blades. The
small size minimizes drag, while the low pitch allows
for maximum climb performance, especially useful
for windy days. A spinner is often incorporated.
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Power is usually a loop of 1/8” Tan, 20" to
22” long, or an 18” loop of 3/32 for ultralight
models. The motor run is usually 60-80 seconds, producing
a very fast, high climb and a long glide. For models over
14 grams, I use a 7” prop and two loops of 3/32”
about 18” long.
Overall I shoot for 10-12 grams for long-winged models,
and 6-8 grams for windy weather short wing models. This
may seem like the reverse of the way it should be, but
the longer winged models require more structure to hold
together and can thereby support thicker motors.
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