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Day 139 - Roll Correction, Axion G4, foam flights
Location:Doonside, NSW, Australia
Conditions:Clear skies, 5km/h
early, 15km/h later,
Team Members at Event: PK, GK,
Paul K, and John K.
It's only been a week
since the last launch so we didn't quite finish preparing
the Gravity Mechanisms Part #2 experiment. It was
also the last
Doonside launch for the year, so we brought a few of the smaller
rockets to the launch. We wanted to fly some casual rockets and
also try to reduce the roll of one of the rockets as much as we
could to get more stable video.
We configured the Axion G4 for flight with the 15mm nozzle
using 22mm full-bore tornado tubes to join the segments. Launched
at 210psi the rocket accelerated very nicely and went up to 657
feet (200 m). Unfortunately it landed fins first on hard
ground and broke off a fin at the glue joint. This is easily
Norm getting ready to launch
Photo: David Bell
Because it was such a beautiful day we launched a couple of
foam flights. The foam flights are always fun to see against the
blue sky. Both flights went went well and recovered safely.
Photo: David Bell
Dad getting ready with the
next foam refill.
Here is a highlights video from the day:
During last week's launch we saw that the new fin cans reduced
the roll of the rocket, but not quite enough as the rocket still
went through ~250 degrees from launch to apogee. Although the
fins may be aligned correctly, it is the total sum of all the
protrusions on the outside of the rocket such as the camera,
parachute cord, deployment mechanism etc. that contribute to the
overall aerodynamically induced roll.
The plan was
simply to put little flaps on the fins to try to trim out the roll.
We made the flaps out of small PET wedges about 2cm wide 5mm
tall and taped them to the
outside edge of the fins to make sure they didn't change their shape
during flight. If they were just flat flaps on the trailing
edges they could easily bend back at speed and it would be difficult
to adjust them accurately on subsequent flights. This way we could change the height
of each wedge to change the trim.
PET wedge taped to the side of
to induce roll.
Just after air pulse
... and landing
We first flew the rocket and reviewed the video to see which
way it was spinning so we could put the wedges on the correct side
of the fins. Well we flew it again and reviewed the video only to
realize that we must have put them on the wrong side and the rocket spun
twice as much! Doh! We swapped the wedges over onto the other
side of the fins and flew the rocket for a third time. This time
the roll was reversed correctly but a little too much in the
other direction. We think that halving the height of the wedges
should be just about right to reduce the roll right down.
Unfortunately we ran out of time for another flight, but we'll just
continue the trim procedure at the next launch to see if we can get it
down to less than 90 degrees. We've only had a couple of
good flights a few years ago where the
rocket had virtually no roll.
We haven't been able to re-create that since. Jamie from JSB
Rocketry also recently
posted a video of some of his flights also with virtually no
But wait there's more ...
Lesson in Aerodynamics #23
So let's roll forward a few days ... while again reviewing the
onboard videos for this update, we noticed that the wedges
had been on the correct side in the first place as intended. The
rocket was rotating in the direction of the wedges not in the
opposite direction as expected. What the heck was going on?
After a bit of head scratching we realized that we must have
created an aerofoil on the side of the fin with the wedge which
would have generated a low pressure behind it causing the
fin to be pulled in that direction.
So we set up a simple experiment to see if that was actually
happening. We made the tail end of a rocket with cardboard fins
and suspended it on a piece of wire to let it rotate freely. We
then added flaps to the trailing edges of the fins and used an
electric air mattress inflator as the wind source. Sure
enough the rocket spun in the right direction as expected. We then moved
those flaps forward on the fin angled about the same amount emulating the wedges we used on the real
rocket, and what do you
know, the fins spun in the other direction! ..... Lesson
Hmmm... this implies that somewhere between the two positions,
you should be able to position a specifically shaped protrusion, say a camera, and
not induce rotation?
Testing with tabs on trailing
Testing with tabs on fin
Airflow over trailing edge
with induced force
Airflow over flap on fin
with induced lift
Here is a
video of the tests:
Paul also managed to launch 3 of his pyro
rockets all with good
recovery. That made a total of 9 good
flights for the day.
Packing up the car with rocket
Paul getting ready to
launch his rocket.
Photo by: David Bell
Prepping Paul's rocket
Paul wiring up his "Black
John launching a big
Photo by: David Bell
Sometimes life can be a drag.
John giving David Bell's
parachute a trial run.