[Before I start,
you might like to look up “gyroscope”
in Wikipedia]
Test flying and trimming of the “Heart of Gold”
F2B flying wing stunter has advanced to the point where the
model could be flown in a contest. It has been flown with
ST35C, STG21/40, STG21/46, Enya 45 6001 and Enya 45 6002 engines.
The lightest engine, the ST35C was flown with the engine as
far forward as permitted by the engine mounts, but still tail
heavy. Turns were extremely tight!
A problem appeared during outside squares. The model wanted
to yaw in and float on the lines: not good! Peter White suggested
this may be a problem related to the gyroscopic effect of
the propeller, a problem previously corrected by Al Rabe,
using a movable rudder. Hmm, sounded good, but my model has
no fin or rudder!
I was puzzled that the gyroscopic yaw problem was so bad
on the “Heart of Gold”. The lack of a fuselage
and fin possibly meant that there was little in the way of
yaw damping: that, plus the low moment of inertia, seemed
to make the flying wing model more susceptible to this problem.
But what to do about it?
First I reviewed the little I know about gyroscopes. Back
in an early Zaic Yearbook (Model Aeronautic Year Book, by
Frank Zaic,1951-52: also Model Airplane News, April, 1950,
by Don Foote) there were diagrams of the effect of forces
on gyroscopes, so I drew on that for my own movable rudder
on my first “Rivets” F2B, back in 1968. The first
thing to realise is that gyroscopes are weird. When you push
on them, they don’t just tilt over in the direction
you push them! Rather they tilt over somewhere else, which
is rather unnerving! The reaction force takes place 90 degrees
later in the direction of rotation, which takes some getting
used to.
For example, on a model, down elevator produces a reaction
force that makes the model want to yaw in: up elevator makes
the model want to yaw out. High RPM and heavy props make the
reaction force stronger. This seemed to be what I observed
on the “Heart of Gold”, so it was apparent that
the elevator was causing the problem. Well, I could hardly
leave the elevator off: having no flaps or fins to start with,
this could become an uninspiring model!
Being a proof-of-concept model, I have no hesitation in cutting
pieces off it to correct a problem, so the logical thing to
attack with the knife was the trouble-causing elevator. My
first thought was to yaw the elevator hinge line. By moving
the port side of the elevator forward, that would induce a
yawing force, out on down and in on up. (you gotta fly stunt
to follow that mish-mash) (hey, my spell-checker thinks “gotta”
is a real word!).
I ran this idea past Peter. Hmm, could work. But a week later
I was in Sydney for the wedding of Stumax, with a chance to
drop in on mountain-man Andrew Heath. I ran the idea past
him, and was amazed to discover the Russian F2D guys were
already doing this: nothing new under the sun there. Then
I considered tilting the whole elevator: this would do the
job too. But both methods required rather more hacking than
my lazy mind could handle. I needed something easy, you now,
just one slash and the job done!
Then it came to me: bend the port elevator down. One slash,
some epoxy left over from making a prop and the job done!
I guessed the angle, slashed and glued, then next morning
off to the field. Not so much a “Rabe Rudder”,
more a “Supercool Slash”. Honour and Advancement,
here I come!
After a week of high heat, wind and humidity, the day dawned
cool, calm and overcast: no blinding sun. Perfect for testing.
The mighty Enya 45 6002 purred into life first flick, and
off we went, roo poo flying everywhere. Damn, wish there was
some wind; I’m flying into my own turbulence. Now for
the outside squares. Yes, yaw cured! I’m definitely
a genius! Think I’ll tell Ricky Ponting how to bat!
I even felt like hugging Shane Warne! How sick is that!
Now you will notice I have been circumspect in my language
re assigning this problem to gyroscopic precession. There
is another effect which behaves in the opposite way, the so-called
“P” effect. Up-elevator pitches the nose up, with
the result that the prop blade on the inner disc gets to a
higher angle of attack, while the inner blade gets an increased
angle of attack. The net result is yaw in on up, yaw out on
down. If you are lucky, the “P” effect can cancel
the gyroscopic precession.
I have noticed that during an outside square turn, the model
first yaws out, then back in rather more strongly. It is as
though the “P” effect operates first, then followed
by the reverse gyroscopic effect. More work is needed here.
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