SEN 866 - 31 Mar 2004
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SCAT Electronic News 31 March 2004 issue 866
Table of Contents
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Electric Rules - Electric Bob
FF Discus Launch Glider (DLG) - Bauer
old rubber - J O'D
Electric Rules
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Roger
The Crossing FF Club started flying electric about 10 years ago, using
AMA rules. We have lowered the motor run to 15 seconds on all flights as
we improved. Flight time control by motor run seems to work and might be
preferred over a very low resistance measurement to assure current(and
power) limiting of model performance.
Thanks to Charles Groth for a great article in the 34th annual Sympo"
Electric Duration by the Numbers".
Bob
FF Discus Launch Glider (DLG)
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Here's the details on my new FF DLG which I just flew at Lost Hills.
Since lots of people are asking, and in an effort to share and promote
this new concept, I'll share my story here.
About a year ago I got a "Photon" RC DLG from Vasily. This was great
for learning the discus throwing technique and is lots of fun to fly
almost anywhere. This is a polyhedral glider that is already fairly
close to FF stability, and is provided by a FF guy so I decided to use
this design as the basis for a FF version. I thought it best to learn
all I could using the simple, crude RC methods before taking the
discus launch to the highest level of modeling- free flight. :-) I
ordered raw wing panels from Vasily along with a nose pod and carbon
boom. I built the tail surfaces from scratch using light balsa (true
HLG style) and built a special version of my F1A electronic timer to
control servos for the stab and rudder. Compared to the RC version
the FF Photon has more dihedral in the wings, a slightly bigger one
piece stab which pops up for DT, a smaller fin/rudder, and a slightly
longer tail moment. Wingspan is 60 inches, 1.5m, and weight is 290g.
I'm not sure which specific airfoil this is but it looks like it could
work on an F1C - smooth philips entry for low drag with mild
undercamber in the rear.
The timer is started with a switch installed in the launching post on
the left wingtip. The glider is launched by wrapping a couple fingers
around this post which closes the switch. At this point the timer is
"armed" analogous to an electronic F1A just before launch when the
hook is unlatched. The force of swinging the glider around by the
wingtip keeps the switch closed until the moment of release when the
glider is launched, the switch opens, and the timing for the bunt
sequence and DT begins.
It was clear to me that the largest challenge to overcome in this
project would be the conflicting demands of extreme yaw stability for
the launch which demands a large rudder with a long tail moment and
prefers small dihedral in the wings, with the requirement of spiral
stability for the glide which wants large dihedral and a small rudder.
Experiments with flying the RC version indicated that spiral stability
in the glide was marginal because if I fixed the rudder for a nice
thermal turn it would occasionally start to spiral in if not
corrected, but a very wide turn seemed to be OK. I finally decided
that the strategy for the FF glider would be make the fin/rudder
smaller and compensate for the lost yaw margin on launch by using the
timer to preset a large rudder offset immediately after the launch.
When launching the glider with the right hand it is yawing violently
to the left at the moment of release. There are several short videos
of RC launches on the internet that illustrate this very well - just
search for discus launch gliders. The RC guys normally all use a
fixed right rudder offset that is held in for the entire launch to
help straighten out the glider. What I did instead was to use a large
rudder offset on my smaller rudder for just a very short time which
was accurately controlled by my FF timer. Right now that time is .3
seconds which is working pretty well. The whole launch is in 3 phases
just like an F1A: 1- "pitchup" .3 sec hard right rudder and stab
glide incidence, 2- "cruise" 1.0 sec rudder straight and stab near 0-0
incidence, 3- "bunt" 1.6 sec rudder left toward glide and stab +
incidence with TE lowered about 1cm. Note that I elected to use a
fairly "shallow" or smooth bunt manuever rather than trying to bunt
the model over very quickly.
First flights were like trimming a power model - I made it DT
immediately after the climb to save the model. Launches were all over
the place until I started getting the rudder in the right place.
Fortunately there was very tall grass on my small field this time of
year to catch the model. The fin/rudder was originally about 90% of
the RC version. The left glide looked rather fast and on the verge of
a spin, so I started cutting wood off the rudder. I didn't notice any
large change in the launch so I kept cutting the rudder down until it
was about 70%, still large by FF standards but the glide was starting
to look like a FF model. I later added about 1mm of wash-in on the
left panel to further stop any tendancies to spin in to the left and
the glide started floating even better. The main problem with the
glide now is that stall recovery seems poor. Several times at Lost
Hills it would start stalling and not want to recover as the wash-in
would make it go straight and the very slight left rudder was not
enough to keep it turning. CG is already fairly forward at about 43%
and reducing incidence seemed to help. At Lost Hills I managed
several flights in thermals that climbed very high and seemed pretty
stable.
The bottom line is that this thing is more critical to launch that I
had thought. Flying the RC version is easy - if the launch is
slightly left or right it is no big deal - just correct for it. The
FF launch has to be very good each time for the bunt recovery to work
and it changes with wind and thermal conditions. If you hold on
slightly too long the launch goes left and then the bunt kicks in it
dives to the ground. Slightly too early and the launch goes right.
It is spectacular when the launch is right on, but it is dreadful when
the launch is wrong.
So I'm ready to fly this in the next FF HLG competition in the area
which should be the San Valeers in April. Everyone agrees it is
perfectly legal because the rules don't limit model size. It is clear
however that the challenge with this glider is just the same as any
other HLG - a good launch into a thermal should result in a 2 min max.
Either a bad launch or bad air won't max.
Ken Bauer
[A number of people have 2 servo Blackmagic F1B timers just to
use as ken has .. looks like Ken is leading the pack - doing
speaks louder than talking .. not that thsi was sent
on March 31 not April 1 so it is for real!!
So this is a further incentive to go to the San Valeers
meetin - Pity I will be at Omarama]
old rubber
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Dear Mike
Perhaps I can help with "old rubber" as per your request in the latest
SEN.
I have had a quick look, and could send you around 130 g of 3/16 Grey
FAI , and/or double this of 1/4. The rubber is unused, and has lived in
my freezer for many years.
If of interest please let me know your postal address etc.
Somewhere I should have a number of used Open Rubber motors of 3/16 Grey
FAI. If I can find these then obviously I will let you know.
What I now need is some one who wants Pirelli,Sig,Dunlop,Chinese,and
even Lactron !!!
Best Wishes
John O'Donnell
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[Some of that old stuff is good for catapult glider...
even if they are not DLGs]
.................
Roger Morrell
