RABE RUDDERS AND REVERSE ROTATION
Recently, I’ve seen a number of comments by builders of airplanes with
reverse rotation electric motors to the effect that their airplanes trim best if
the “Rabe Rudder” is disabled. I think it is important to point out that a
“Rabe Rudder” works well to trim the gyroscopic precession as long as it is
used with normal rotating engines. Reverse rotating motors have the effect of
magnifying instead of minimizing the effect of gyroscopic precession on our
airplanes.
The Rabe Rudder is an old invention dating back nearly a half century, but
not in general use by most stunt flyers. I was glad to see that the 2012 and
2014 World Champion Igor Burger remark in his Max Bee article in the
Jan/Feb 2013 in Stunt News that he used a Rabe Rudder, tractor propellers
and incidence.
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In 1978 I competed in the World championship placing 2nd with a score of
2954 to Bob Hunt's 2963 using my Mustang, Snaggletooth. Snaggletooth used
dihedral, shock gears, reversed bellcrank, internal muffler, incidence, sliding
block adjustable leadouts and, of course, a Rabe Rudder. All of these features
were original and had been published many years earlier in various model
magazine or web forums. Snaggletooth also won Open Stunt at the NATs in
1977, and the Walker Trophy. In 1972 and 1973, my two Sea Furys, with all of
the listed features except incidence, also won Open Stunts and another
Walker Trophy.
STUNT SHIP PROPELLERS ARE GYROSCOPES!
If you are going to move your stunt ship through the air with a propeller you
will encounter gyroscopic yaw in any maneuver that pitches the airplane.
Because our propellers are gyroscopes, they react with gyroscopic effects any
time we precess (apply force to tilt) the prop disc. When the propeller disk is
precessed in a maneuver, there will be an accompanying yaw 90 degrees later
in the direction the prop is turning. Gyroscopic effects are proportional to the
mass (weight) of the propeller and how fast it is spinning. (Wood props tend
to yaw less than carbon props because they are lighter. Higher revolutions
also precess more.) Gyroscopic yaw is also proportional to pitch rate. (Square
maneuvers yaw more than round maneuvers.)
GYROSCOPIC EFFECTS WITH NORMAL PROPELLER ROTATION
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Our stunt ships use props weighng an ounce or two. In flight, we turn these
props around 10,000 RPM. The weight and rotation combine to make our
props effective gyroscopes . Here is a simple exercise for visualizing
gyroscopic precession (yaws) and corrections that may be applied to them.
First, stand up and visualize the propeller disk as being in front of you. Bend
forward a bit. Visualize that leaning forward is simulating a nose down pitch
with the top of the propeller disk being pressed forward as we lean forward.
Then, visualize that the propeller in front of us is turning in the normal
direction with the top of the disk moving from left to right. Follow the prop
disk direction of rotation by bending to the right from the waist. Now imagine
that your head is still pressing forward. When the propeller has rotated 90
degrees, and is pointing to our right, the head press forward becomes a head
press to the right side of the disk. This tilts the disk to the left which is
actually "inward" yaw.
"Inward" yaw happens whenever we use down elevators. It reduces line
tension, particularly in the top of the vertical eight and hourglass. If we want
to correct for the inward yaw, we would have to move a rudder to the right.
That sounds wrong, doesn't it? Actually, the right rudder is used to push the
rear end of our airplane to the left. When the tail moves left, our airplanes
are actually turning to the right. This right turn is a correction for left, or
inward turning yaw.
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The opposite is also true. Let's imagine again that we are looking at the
propeller and leaning backward to simulate a pull on the top of the propeller
disk. Again, we follow the rotation of the propeller by leaning to the right.
When the propeller has rotated 90 degrees, and is pointing to our right we
imagine our head pulling back, simulating a pull on the right side of the
propeller disk. This tilts the propeller disk to the right which is actually
"outward" yaw.
"Outward" yaw happens whenever we use up elevators. It increases line
tension, most notably, in “inside" maneuvers below 45 where it isn't needed
much. If we want to correct for outward yaw, we have to move the rudder to
the left. Left rudder is used to push the rear end of our airplanes to the right.
When the tail moves to the right, the airplane is actually turning to the left.
Left turns are correction for right, or outward turning yaw.
If we want to correct for both gyroscopic yaws, we need something which
applies right rudder on" down" and left rudder on "up".
Our models are particularly affected by these uncorrected gyroscopic effects
when we perform square maneuvers with high pitch rates. Most modelers
seldom notice gyroscopic effects in inside maneuvers where they aren't much
needed. They typically seem OK with the excess line tension in inside
maneuvers. Line tension in outside maneuvers is noticably less, but usually
not too troublesome except when performing vertical maneuvers above 45.
For example, the first corner of a hourglass is inside which begins the near
vertical climb with good tension. Much of that tension is lost in the climb as
the airplane slows and the first, very high pitch rate, outside corner uses much
of the remaining tension. Typically, the straight leg across the top of the
maneuver has very little tension with the airplane almost in free flight. What
tension remains is hardly enough for another high rate outside corner to begin
the descending leg. The top outside half of the square eight is similarly
affected with loss of tension after the hard outside corner in the middle of the
maneuver. The vertical eight is also affected, with loss of tension above 45 but,
being a round maneuver, it's less affected than the hourglass .
If we could somehow apply left rudder on down and right rudder on up, these
corrections would add nothing to the average line tension of our airplanes.
But, it would trade a bit of unnecessary inside line tension for a bit more
welcome outside line tension. This trade is possible with reverse rotation of
the propeller. These corrections would mainly served to trim out both inside
and outside gyroscopic yaws and "clean " up yaw effects in our maneuvers.
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Even this simple trade of yaws would make a noticable improvment in the
pattern maneuvers because part of the trade would removed some of the
gyroscopic inward yaw in outsides.
The gyroscopic loss of tension on outside maneuvers is troubling to many
stunt flyers. This prompts efforts to improve the capabilities of their
airplanes by trimming. This trimming is usually involves a combination of
rudder and engine offset, nose and tipweight, and possibly "tweeking" flaps a
bit to equalize inside and outside tension. These efforts usually help a bit but
are frequently less than fully successful.
The use of rudder offset seems to have little effect on gyroscopic yaws.
Inboard and outboard gyroscopic forces still seem to apply equally, but
centered on the offset rudder position.
Tipweight in particular has interesting properties. In general, tipweight
creates line tension by tilting the roll axis of the airplane outward when our
airplanes are both upright or inverted. Excess tipweight magnifies this effect
by causing large roll excursions in high pitch rate maneuvers. We call these
slap-like excursions "hinging." Hinging is caused by the inertia of tip weight.
Inertia causes the mass of the tip weight to resist changes in direction as the
airplane maneuvers. A square corner from level flight may be initially resisted
by the tip weight’s tendency to remain in level flight. While the tip weight is
catching up with the airplane’s new attitude, the inertial effects may cause the
bottom of the airplane to be briefly visible. Excess tipweight may also cause
the top the airplanes to be visible on outside corners. The inertial effects of
hinging also causes airplanes to yaw on both inside and outside corners, but in
these examples, the yawing is inertial, not gyroscopic.
Hinging is unsightly and can negatively affect the judge’s opinion of the
quality of square maneuvers. On the other hand, hinging may also be
beneficial in that the roll caused by an excess of tip weight is always in an
outward direction, upright or inverted, and always increases line tension.
This additional line tension from tipweight, even to the point of hinging, may
offer a measure of protection on initial flights of a new stunt ship which will
be progressively reduced as trimming progresses.
In short, hinging is an interesting effect but different from gyroscopic
precession in that it increases line tension in both inside and outside
maneuvers. Gyroscopic precessions will always have an opposite yawing
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effect on inside and outside maneuvers. These yawing effects will also be
present and equal for reverse propeller rotation.
This article reaffirms that with normal propeller rotation, outside line tension
is lost and inside tension is gained in our maneuvers from the effects of
gyroscopic precession.
Of course there is much more as this bit of information was taken from My Al's Models 4 DVD, book 2 and book 3, and continues with differences with left prop rotation. This is from the DVD at Book 3 page 127. In book 2 (same DVD) there is another aircraft trimming article which begins on page 593 which contains more trimming information including more on gyroscopic precession. The Al's Models DVDs are still available.
Al
alsf8f@yahoo.com
Topic: Rabe Rudder - necessity or just another gizmo from the past (Read 8602 times)
