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1/2 A Helicopter Duration For NARAM-46, the Helicopter Duration event is for 1/2A engine
class. Helicopter Duration combines the challenge of building and flying a
rocket which uses helicopter (autorotation) recovery, to stay in the air as
long as possible. In Helicopter Duration, the model must remain in one
piece for the whole flight, nothing can come off other than wadding. It
cannot be staged. The model must be returned for one of the two
flights allowed. For the full rules for this event, please see
the Helicopter Duration
Rules on the NAR web page. Scoring -
For Helicopter Duration, the scoring is the total of two flights allowed, but
at least one of those flights must have the helicopter model returned. Design considerations - There are many tradeoffs involving reliability,
weight, boost altitude, descent rates, and visibility. A list of plans and kits is included further down on this page. Designs -
The two most popular designs for Helicopter Duration are the Rotaroc and the
Rose-A-Roc
Rotaroc -
Designed by George Gassaway. For this event, the Rotaroc-A.
There have been variations by others, such as the QCR High Rotor kits,
and the Apogee Heli-Roc
kit. The basic design is a straight body tube with simple blades
folded flat to the tube on boost, the folded blades in a triangular
cross-section. The design is not too hard to build, though some people
dislike attaching the hinges. It's a reliable competitive design that usually
puts in solid flights. Rose-A-Roc
- Designed by Art Rose. There have been variations designed by others, such
as the Whirl-A-While
by John DeMar, and the Chicago
Chopper by Pavel Pinkas, plus the QCR Ultimate I
kit.. The basic design is to make the model as small in cross-section as
possible. So the blades split lengthwise, so the chord is half of normal,
then folds rearwards to be parallel to the body. Rather than a body tube,
most of the body is replaced by graphite tubing or a wood dowel, to give the
rotor blades clearance for their lengthwise folds and to have a smaller cross
section (frontal view). These models boost higher than Rotaroc type models.
In the hands of a good builder, they can out-perform Rotarocs. They are
harder to build than Rotaroc types. Internal blade
- There is another design which can be very competitive, which involves storing
the blades inside of the body tube. To do this effectively, the model needs
to have a diameter large enough to hold reasonable sized blades, without the
body weighing too much. In any case, the models are quite difficult, high
payoff if they work but at a higher risk than more conventional models. So,
this is not really recommended for newer competitors to try. But if you want
to know more about them, check out the Design
of a Competitive Heliroc R&D report by the CHEDAR 1++ Team ,
written by Pavel Pinkas. Qualified flights
- OK, sometimes performance does not matter. For various reasons, whether for
return or flight points, people just want to fly something that will qualify
as a helicopter flight. A sport design that some use is the "Tasmanian Devil",
which could be described as a pre-deployed helicopter, with small blades
permanently glued in the open position. It spins up very quickly on launch,
causing a lot of drag, making it boost low like a flying saucer type model,
and flying for only a few seconds. For that matter, a flying saucer type of
model, with properly added spin tabs, can make for a qualified flight
helicopter model (like a scaled-down Tri-F-O) Deployment magic
- The Rotaroc and Rose-a-roc type designs use the ejection charge only as a
starting point to cause the design to go from rocket boost mode to deploy for
helicopter descent. Rubber bands are used to make the blades deploy. What
keeps them from moving until ejection? Usually a burn-thread which is rigged
to hold the model n boost mode. At ejection, the thread burns, allowing the
rubber bands to do their thing. The burn thread needs to be thin. The button and
carpet type of thread that Estes uses for shroud lines is too thick for
anything short of a D12 ejection charge to burn. So use a finer thread. I
prefer to use white elastic thread. The Rotaroc plans show how to run the
thread to hold the blades. Some designs do not use burn-thread. Such as Pavel
Pinkas' Chicago Chopper, and internal bladed designs, where the ejection
causes a mechanical action that frees the blades to deploy. The rubber bands need to be selected carefully. Do
not just grab any rubber band and try to force it to stretch really far, as
the blades may bow outwards a lot when prepped for boost, or the hinges may
be ripped off from too much force at deployment. Look for rubber bands with
1/16" cross sections, in various lengths , at Office Supply type stores.
Look for the classic pale brown latex bands, colored bands tend to not
stretch as much. While the band should not be stretched so much that it
causes excessive force, it should not be relaxed at the end of travel so that
there is slack, as the blades might not deploy fully. This is not hard to get
right, just it can cause trouble if it's overlooked. Building Helicopter models - The two most critical parts of most helicopter
models are the rotor blades, and the hinges that connect the blades to the
main body.
Rotors are much like making wings for rocket boosted
gliders. Except the rotors are smaller in chord and tend to be more fragile. The instructions that Pavel Pinkas wrote for the Chicago
Chopper has good info on making rotor blades. Weight is a an important issue for helicopter
models, regardless of design. The lighter, the better, as long as the blades
(and the rest of the model) are physically strong enough. Try to find
reasonably light balsa for the rotor blades, but without being so weak that
the blades will bow outwards too easily when the blades are folded for boost. Long narrow blades can be hard to sand to shape, as
they are so relatively fragile. See the Rotaroc-A plans to see how the
trailing edge portions of the blades can be sanded to rough shape while still
attached to 3" wide balsa sheeting, then after cutting off the first
blade the other trailing edge can be shaped with the resulting 2" wide
sheet. Using that method, the bulk of the difficult shaping (trailing edge)
can be done to the blades before they are all cut to their final 1"
width. A VERY nice sanding block, useful for gliders,
helicopter, and any rocket really, is an all-metal 2-piece clamp-type sanding
block made by Red Devil, carried by most Ace Hardware stores (look near where
they stock sandpaper). It has a 1/8" foam rubber backing sheet, which
ought to be removed since it allows rounding things too easily in 3-D when
you usually want to shape in 2-D at a given time (as with a wing). So, remove
the rubberized portion. That sanding block holds a 4.5 x 5.5" sheet of
sandpaper (quarter of a 9 x 11" sheet), with a 3.5 x 4.5" sanding
surface area on the block. This works far better in most cases than a narrow
sanding block. Of course, the wings (or fins) should be shaped and finished
before they are glued to the fuselage or model. When doing very rough shaping for rotor blades, 80
to 120 grit sandpaper is good to grind off a lot of wood in a short time.
Don't over-do it though by sanding off too much. Then go to finer grit like
180 to 220 for finer shaping. Beyond that, 280/320 paper, is sort of a cross
between final shaping and setting up for a final finish. A final finish for
bare balsa is to use 400 grit paper. Get the "black" type wet or
dry sandpaper, it sands better and lasts longer than the reddish types. If
you use any clear dope, use 320 to 400 grit paper before and after. Ed LaCroix's instructions
for the Maxima-A Boost Glider has some good info on sanding wings to shape,
skills which are applicable to helicopter blades. Kevin Wickart wrote a nice short article on how to
do quick and easy airfoils, on the WOOSH section's website. Click
here to read it. Helicopter Model Finish - Never use paint. For newer fliers, no finish is
usually fine, better than weighting the model down too much and maybe having
warpage problems. If you want to improve the finish on the blades, use some
thinned clear dope in one or two light coats, sanding before and after with
240 and 400 grit sandpaper. The idea it not to weigh the model down too much,
or add so much that the wood parts might tend to warp too easily. Coloring -
OK, so bare balsa (even with clear dope) is not easy to see in the air or on
the ground. Use a large black magic marker to color the bottom of the rotor
blades black, as that shows up against the sky pretty well. But leave the
last 3-4" of the blades uncolored. Color those last 3-4", both top
and bottom, with a large red or orange magic marker. If you can find true
fluorescent markers (not to be confused with wimpy fluorescent highlighters),
an orange or red/magenta fluorescent color is highly recommended. The reason
for the bright color for the last 3-4" of the blades is that when the
helicopter model lands, it usually falls over on two blades, leaving the
other blade sticking upwards like a "flag". So, colors on the blade
tips can help you find the model Prepping -
it's usually easier to prep for flight by having the rubber bands unhooked,
when setting the blades or tying the blades for boost. Hooking up the rubber
bands afterwards. As long as you don't forget the hooking up the bands step
before launch! To prep helicopter models that use burn threads, it
can be useful to use something to act like a needle threader to carry the
thread thru the two small holes in the body. A simple tool is to take a
12" long piece of .015" music wire, and fold it sharply in half.
The resulting tool can be pressed thru the small holes easily, the thread
slipped inside the bent portion, then the thread pulled thru the holes in the
body so the thread is passed to the other side. Don't forget to hook up those rubber bands now.... Flying -
Regardless of design, try to fly it in a manner so it will not weathercock a
lot into the wind (angle a little bit downwind, relative to how windy it is).
For one, that will make the model eject higher up and therefore get a better
time (always a plus). But the other is that the model might never deploy at
all. If the model goes beyond apogee and is falling towards the ground at a
good speed, the rubber bands might not be able to overcome the high
aerodynamic forces, letting the model plummet to the ground. This is not too
likely for 1/2A, but it is worth mentioning. And remember....check the rubber bands! - Many people, even very experienced fliers, have
meant to hook up the rotor rubber bands at the pad, shortly before launch,
and forgotten to do so. Resulting in crashed flights. More Information
- Check out the "S9
Gyrocopter and helirocs" Yahoo group, which discusses NAR and
FAI (S9) Helicopter Duration models.
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Model Plans & Kits |
Designer |
Notes |
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Rotaroc-A
plan |
by George Gassaway |
Competitive reliable helicopter plan |
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Whirl-A-While
plan (NAR website) |
by John DeMar |
Competitive reliable Rose-a-roc type helicopter plan |
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Chicago
Chopper plan (WOOSH website) |
by Pavel Pinkas |
Competitive reliable Rose-a-roc type helicopter plan |
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QCR - High Rotor II
(Rotaroc) and Ultimate
I (Rose-a-Roc) Helicopter kits |
QCR
- Qualified Competition Rockets, by Ken Brown |
QCR's kits based on Rotarocs and |
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Apogee Heli-Roc kit |
Apogee Components |
Apogee's kit based on the Rotaroc |
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Design
of a Competitive Heliroc R&D Report & plans (WOOSH
website) |
By CHEDAR 1++ Team (Thomas Hanzlik-Pinkas, Pavel
Pinkas and Kevin P. Wickart) |
Winning R&D project which includes internal type
helicopter designs |
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Tasmanian Devil
sport helicopter plan |
by George Gassaway |
An easy to build, low-flying, sport design that is
sometimes used for "Qualified Flights" and sure return flights. |
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Last
Updated 5/18/04