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B Helicopter Duration
For NARAM-50, the Helicopter Duration event is for B 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.
A very good compilation
of information on helicopter models , including designs, how they work,
blade angles, and so forth is in a pdf presentation created by Trip Barber.
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- 18mm
Plan. 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, but those are 13mm A sized plans. 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.
There is now a Rose-A-Roc
KIT by FlisKits! Sized for 18mm engines. There will probably be a lot
of these kits competing at NARAM.
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.
Here is a pdf file of the R&D project that Pavel Pinkas of the
Chddar-1++ Team wrote about Design of a
Competitive Heliroc. The design was an internal bladed model.
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"
(This is an A sized plan, needs to be scaled up for B power). It 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.
I always liked using Klett hinges, because they had hinge pins that
could be easily removed for assembly. Unfortunately, Klett stopped making
R/C plane hinges some years ago. The most suitable non-Klett replacements
are the small hinges made by Du-Bro. However for models like Rotarocs, it
is still very useful to be able to remove the hinge pins for some assembly steps,
and the Du-Bro hinge pins are not meant to be removed. The hinge pins can
be modified though, either using vice-grips to un-crimp the crimped end, or
using a Dremel tool with cut-off wheel to remove the flattened crimped end
of the hinge pin.
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Attaching the hinges to the body tube is often the hardest part for
some people to do. Fortunately, it is possible to make jigs that greatly
assist in attaching the hinges, and attaching them at the same angle
every time. Click on the thumbnail at right to see how you can make a
single hinge jig, or better yet a triple hinge jig that allows all three
hinges to be attached at the same time. The jig requires the use of Klett
type hinges which have removable hinge pins.
The 4 degree hinge skew angle shown is suitable for most helicopter
models, unless the plans specify different.
Rotor Blades - Take note of which direction that the model will
be rotating, so that you build the rotor blades to rotate the same way!
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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. It is an "A" sized
model, but the info on making blades is useful regardless of size.
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 warping 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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Engine recommendation for B
Helicopter:
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B6-2 (Typical helicopter
models, including Rotaroc types)
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or
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B6-4 (light and low-drag
models such as Rose-a-Roc types)
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B4-2 (lower boost stress -
Typical light weight helicopter models, including Rotaroc types)
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or
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B4-4 (lower boost stress
- light and low-drag models such as Rose-a-Roc types)
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