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Plastic Model Conversion
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Plastic Model Conversion combines the challenge of using craftmanship to build a nice looking Plastic Model kit that is not intended to fly, but that is converted to fly on rocket power and land safely. This is quite a challenge to do well. The Plastic Model must be of a rocket, space vehicle, or a jet whose engines are in or spaced apart to the rear of the fuselage. No cars, no fire engines, no ships, etc. The model has to essentially be based on one Plastic Model kit, not a conglomeration of kits (no Saturn-V with shuttle SRB's). Plastic Model entries cannot exceed 1500 grams (approximately 3.3 pounds) mass at liftoff. In this event, it is rare that anyone has a model close to the mass limit. The model must make at least one safe and stable flight (two flights are allowed), otherwise the model is disqualified and the static score is useless. The model can be clustered or staged (stages need to recover in a safe manner: a Saturn-V first stage tumbling to the ground "like the real thing" would be disqualifed). Clustering or staging is not recommended though for inexperienced fliers. Entering for judging: The model needs to be entered in the same condition it will be for flight. No parts added or removed, other than internally (recovery system can be added/changed later). Your NAR number should be on the model, many people use decals or dry transfers. A display stand of some type is useful, but not required. The stand is not judged but is nice to keep the model from falling over as well as holding certain model types at a desired viewing angle. Scoring - For Plastic Model Conversion, the model is judged in
advance for a static score (turn-in deadline will be Sunday night), based on
Craftsmanship and Difficulty. When flown, it is also judged for General
flight points (including lack of flight damage), and " Engines - You can use any NAR Contest Certified engine that is suitable, considering the weight and drag of your model and the time delay to ejection. However, the maximum engine total impulse allowed is 160 Newton-Seconds (Full G power). Multiple motor combinations (clustered or staged) N-Sec totals are added together towards the 160 limit. That limit is usually not an issue for Plastic Models. For the full rules for this event, please see the Plastic Model Conversion Rules on the NAR web page. |
In addition to the article below, check out the Plastic Model Conversion article written by Jeff Vincent, in the July/August 1999 issue of ASTRE's "Stardust" newsletter (downloadable PDF issue, 3.6 megs).
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Stability: Just
about every plastic model needs some help to be stable. The easy way is to add
noseweight (such as clay, or small lead fishing weights embedded in clay),
which is fine for some models. For other models, the engine may not be able to
lift any additional weight, or there may not be room to add noseweight; or the
fins may be too small, or non-existent, so the fin area must be increased. You
can get sheet plastic at most hobby shops; 0.04" styrene is a typical
thickness to use. Models such as the Saturn-V can benefit from having the fins
made somewhat larger; the old Estes kit used fins that were larger than scale.
Since the contest event is "plastic conversion", and not
"plastic scale conversion," you do have the leeway to make fins
oversized even if it's out of scale. The "cardboard cutout" method
can be useful in determining the Center of Pressure (CP), indicating whether
the fins are large enough or if noseweight may be needed after all.
Some models that have no fins can use clear plastic
fins; or you could choose to use normal opaque painted plastic. Clear fins are
notoriously difficult to glue on neatly and mask off for painting. But if you
choose to use clear fins, you can get the 0.04" clear plastic made by
Plastruct. That clear plastic is a bit flexible; this is not a problem for
small fins but can be for large fins. If flexing is a problem, try to find
cheap clear plastic boxes to cannibalize for stiff clear fin material. However,
one of the most ideal clear fin materials is Lexan (aka Polycarbonate). You can
find 1/32" and 1/16" thicknesses of it sold by Micro-Mark. Do not use
Cyanoacrylate to attach clear fins; not only does it not work well, but it
often frosts up the plastic. Use a liquid plastic cement for bonding clear fins
to the plastic kit.
If you have the skill to do it neatly, and if the
model is suitable, you can use the slot and tab method with the clear fins. Cut
the fins with a tab in the root meant to fit into a slot in he plastic model.
And use great care in cutting out slots that are just exactly the width of the
fin thickness, or maybe a hair undersize then file slightly wider as needed. Be
careful about the use of liquid cement to glue the fin in place. Mask off the
fins very carefully so that paint won't creep onto the clear fin.
Also keep in mind that a model has to be stable in
both pitch and yaw. If one were to convert a Klingon Battle Cruiser (something
best left to experts) it might be stable in pitch but not in yaw, so
rudder-like fins may be needed.
Some models with wings can be technically
"stable" but pitch up (or down) due to a different incidence angle
between the wings and tail surfaces. Check that the angle of attack of the wing
and the horizontal tail surface (stabilizer) are the same. If the tail is at a
different angle, the model is likely to loop into the ground due to the
aerodynamic lift generated (the 1/32 Revell X-1 has this problem). It will be
necessary to attach the tail surfaces at a different angle of attack so that
they are parallel with the wing angle of attack.
If you still have doubts about whether the model may
pitch due to thrust or aerodynamics, you can make the model roll to help even
out the pitch, in a "barrel roll" boost. This can be done by adding
spin tabs to fins, or cutting & gluing ailerons to make the model roll.
click drawing to see at twice size
[ QCR has
special kits
for the F-104 and F-100 jets, which come with everything needed for
conversion (parts) as well as instructions. ]
Ejection:
When the ejection charge fires, where it is going and where will the chutes be
pushed out? In a simple conversion this is no problem; the engine mount tube
can run all the way through the model, or it can be glued with centering rings
into a larger tube for the parachute compartment. But, if the recovery system
is going to come out at a different angle than the engine thrustline, the
ejection charge needs to be ducted in some way. For a model like the F-16, it
is possible to do a "bent tube" ejection by having the front of the
engine mount tube join the rear of the recovery system's tube with one
centering ring, glued at an angle instead of straight. For others, it could be
more complex, using a tube that is ducted or even a series of tubes that do a
zig-zag or a 180 degree turnaround to eject out of the rear.
No matter where the ejection charge is routed, the
ejection heat does tend to damage the first 2-3 inches of tube ahead of the
engine (with composite motors there is some significant smoldering flame after
ejection). The exposed areas can be protected by adding in a tube coupler or a
layer of epoxy, or using thin CA soaked into the paper.
Some models may be suited for rear-ejection. An
ejecting engine mount tube with centering rings glued to it, freely slides out
of a larger tube plugged at the forward end. The recovery system is stored in
the space between the outside of the engine tube and the inside of the larger
tube. The ejection speed is high; sometimes shock cords snap trying to stop the
ejecting mount tube. It is recommended to use separate chutes and shock cords,
so that problem can be avoided (see the shuttle orbiter conversion).
Some models have no obvious or easy location for the
separation joint which allows the parachute to eject; you may need to make your
own nose section separation point. Wrap masking tape around the body as a guide
and use a razor saw to carefully cut the body apart. This can be particularly
useful for some jets (see the F-105), allowing complete cockpit detail to be
used.
click drawing to see at twice size
Recovery system: It
is important to have a good strong reliable shock cord and parachute system for
the model. Plastic models are heavy and tend to stress the recovery system a
lot at deployment. To keep the parachute compartment tube clear, try to mount
the shock cord to the outside of the tube, but still inside the plastic model.
One approach is to use flexible wire cable, secured deep in the model, with a
looped end coming out at the front to tie the real shock cord to (see the
F-105). Or, secure 50-100 pound test Kevlar line inside the tube, extending it
out a few inches to allow shock cord attachment (Kevlar line is heat-resistant,
but beware that for example D12 type ejection charges will burn thru 100 pound
Kevlar). When the shock cord is mounted outside of the tube like this, it is
necessary to cut a slot into the shoulder of any nose block that may be used,
so that when packed the shock cord will not jam between the inside of the tube
and shoulder.
Not surprisingly, the shock cord should be able to
absorb shock. Elastic is good for this. For models of 4-6 ounces, 1/8"
elastic is adequate, for heavier models 3/16-1/4" elastic is better. The
more length, the better the elastic can absorb the shock when the chute
deploys; but do not use so much that the recovery compartment tube is crammed.
Parachutes need to be able to be stored small, deploy
well, and hold up without ripping or losing shroud lines. Cloth chutes are
great for strength, but take far too much room. Mylar chutes can be packed very
small and tight, but they can rip easily and sometimes do not deploy reliably.
Most commercial plastic chutes are suitable, but need to be reinforced. For a
6-line chute, replace the 6 lines with 3 very long lines, and run them across
the top of the chute. Use extra tabs to secure the lines across the top near
the center. This ensures that no lines will pull out.
click drawing to see at twice size
Chute size is a judgment call. The larger the chute,
the softer the landing...unless it's so big that it gets jammed inside and the
model crashes! Smaller chutes cause harder landings, but are more reliable.
Experiment with packing chutes in the tube you're using; if you are not able to
blow the chute out using lung power, go to a smaller chute or try a different
way to pack the chute. And remember this when it comes time to fly, to pack the
chute in the manner it requires. Consider using more than one chute; it may be
easier to fit in 2 smaller chutes than one larger one, and if only one opens
the model won't crash.
Given the above considerations, you've chosen a model
and are ready to put those ideas and your plans into action by building; but
will the conversion really work? Would you like to know for sure it will work
before building a detailed pretty model? It's highly recommended to build a
"boilerplate" model first and find out. This means you simply do just
the flight conversion work to build a model; don't paint it or worry about
filling seams and so forth. Then fly it, and see if it works. If it crashes,
you saved a lot of time and effort, and might be able to determine how to solve
the problem if you want to try again. If it works, great, now you have the
confidence to invest that time and effort.
Building Tips:
Plan out what order of assembly is needed, you don't want to glue two body
halves together and then realize that you need to do something inside, like
mounting a shock cord.
The engine mount tubes and other internal body tubes
need to be secured into the model. For a cylindrical plastic model, custom
centering rings can be made up, or if the fit is close simply wraps of masking
tape, then glue the tubes to the model using the rings or gluing to the tape. For
other types, spacer blocks from balsa or spruce are needed to hold the tubes in
the proper position. Plastic cement does not work well with wood or paper, so
use cyanoacrylate to bond the spacer blocks (careful not to let any run to the
outside of the model).
Remove as much excess material inside as practical to
save weight, especially near the tail.
Tools: A
razor saw is very useful for cutting separation joints for models. A Dremel
moto-tool can be used with a cutting disc to remove large areas, where the disc
can fit into. As well, a grinding bit can be used on the moto-tool for removing
material more carefully and in smaller spaces, and to remove internal
obstructions such as bulkheads and landing gear bays to allow a tube to pass
through. A small number of file shapes can also be handy for shaping, rounding
out smaller holes, or "deburring". For some detail work, small drill
bits turned by hand can be useful; a pin vise can be used, or you can simply
glue small drill bits into 3" long 1/4" dowels, rounded at the top
end for low friction when pressing down.
Finishing:
This article is mainly about how to flight convert models; whole magazines are
devoted on how to actually build and finish plastic models. Some keys to a good
finish include neat glue joints, and the use of wet or dry type sandpaper of
increasingly finer grits to wet-sand the seams and putty (Flex-I-grit sandpaper
by K&S is best). Plastic putty can be used not only for filling seams, but
to blend and smooth trouble spots, particularly around engine mount tubes if
the body had to be cut to make it fit.
Use paints intended for plastic models; do not use
dope! Spray cans are typically used. An airbrush is nice but overkill for many
basic model's needs. If you want a flat finish but will be using decals, use
gloss paint(s) first, apply the decals, then apply a flat clear coat.
Otherwise, decals applied over flat paint look "silvery" in the clear
areas of the decal. Anytime different paint types are used, do a test first on
some scrap plastic or other material. Some paints are not compatible with other
paints. Some clear coats are not compatible and can even attack decals and/or
dry-transfers. If you plan to mask off colors, paint the lighter color first.
Let the paint dry and harden for a coupe of days, then mask. Carefully applied
scotch "magic" tape can work well for some masking jobs. After
applying the 2nd color, remove the tape mask about 30 minutes later, before the
new paint gets "hard", so the tape will come off without bringing
chipped flakes of the dried 2nd layer of paint with it. Decals are best applied
using a decal setting solution; this not only can make the decal conform well
over any surface detail, but allows the decal to "slide" easily into
place (push the decal into place with a small model paint brush).
This article includes several conversion examples
which are known to work. Pay particular heed to the listed distance from the
nose for the lengthwise CG locations. The CG can be forward of the location
shown, but not further towards the rear. If you build a model of a different
scale, be sure to convert the CG location accurately.
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Model Kits & Article |
Designer |
Notes |
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Ken Brown of Qualified Competition Rockets |
"All kits include the plastic model, tubes, shock cords, weights for nose, launch lugs, parachute kit, instructions, and plastic model conversion literature." |
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Plastic Model Conversion article from ASTRE's "Stardust" newsletter, July/August 1999 |
Jeff Vincent, article author |
Downloadable PDF issue, 3.6 megs. |
Last
Updated 4/21/2004