Description
This article is from the Aviation
Aerobatics FAQ, by Dr. Guenther Eichhorn with numerous
contributions by others.
05 Basic Aerobatics Figures
A description of a list of aerobatics figures is available at
http://acro.harvard.edu/IAC/acro_figures.html. Here is a summary that
tries to explain the difference between maneuvers that are often
misunderstood on the rec.aviation.* newsgroups.
Rolls
There are four different types of rolls:
* Aileron Roll
* Slow Roll
* Snap Roll or Flick Roll
* Barrel Roll
Aileron Rolls
Aileron rolls are flown with the rudder and elevator in the neutral
position during the roll. The aileron is fully deflected in the
direction of the roll.
This is the easiest of the rolls to fly.
The aileron roll is started by pulling the nose up to 20 - 30 degrees
above the horizon. The elevator is then neutralized and the aileron
fully deflected in the direction of the roll. The controls are
maintained in that position till the roll is completed. After the
roll is completed the nose is usually 20 - 30 degrees below the
horizon.
The aileron roll is not a competition maneuver.
Slow Rolls
Slow rolls have to be flown normally on a straight line (exception is
the avalanche). The roll rate has to be constant and the longitudinal
axis of the plane has to go straight. This requires constantly
changing rudder and elevator control inputs throughout the
roll. Hesitation or point rolls include stops at certain roll angles.
The number on the base of the roll symbol describes the number of
points the roll would have if it were a 360 degree roll. Allowed are
2 point, 4 point and 8 point rolls. The fraction on the arrow of the
roll symbol describes what fraction of a full roll is to be
executed. If no points are specified, rolling is done without
hesitations. If no fraction is specified, a roll symbol that starts
at the line specifies a half roll (see description of the Immelman).
A roll symbol that crosses the line specifies a full roll (first
figure). The second figure shows the symbol for 2 points of a 4 point
roll (adding up to half a roll) from upright to inverted flight.
Snap Rolls
Snap or flick rolls also have to be flown normally on a straight
line. A snap roll is similar to a horizontal spin. It is an
autorotation with one wing stalled. In the regular snap, the plane
has to be stalled by applying positive g forces. In an outside snap,
the plane is stalled by applying negative g. In both cases rudder is
then used to start autorotation just like in a spin.
Barrel Roll
The Barrel Roll is a not competition maneuver. I The barrel roll is a
combination between a loop and a roll. You complete one loop while
completing one roll at the same time. The flight path during a barrel
roll has the shape of a horizontal cork screw. Imagine a big barrel,
with the airplanes wheels rolling along the inside of the barrel in a
cork screw path. During a barrel roll, the pilot experiences always
positive G's. The maximum is about 2.5 to 3 G, the minimum about 0.5
G.
Turn-around maneuvers
There was a confusion about the difference between a wingover and a hammerhead
turn on the rec.aviation.* newsgroups a while ago. Here is a description of
the two maneuvers.
Wing Over
The Wing-Over is a competition maneuver in glider aerobatics. You
pull up and at the same time bank the plane. When the bank increases
past 45 degrees, the nose will start to drop while the bank keeps
increasing and the plane keeps turning. Halfway through the maneuver,
the plane has turned 90 degrees, the fuselage is level with the
horizon and the bank is 90 degrees. The plane is above the original
flight path. The nose then keeps dropping below the horizon and the
plane keeps turning, while the bank is shallowed. When the bank drops
below 45 degrees, the nose is pulled up towards the horizon and the
plane reaches horizontal flight with wings level after 180 degrees of
turn. At the completion of the maneuver, the plane is at the same
altitude as on entry and flying in the opposite direction.
Hammerhead
It starts with a quarter loop into a vertical climb. When the plane
stops climbing, it pivots around its vertical axis (which is now
horizontal).The nose moves in a vertical circle from pointing up
through the horizon to pointing down. After moving vertically down to
pick up speed again, the maneuver is finished with the last quarter of
a loop to horizontal flight. This figure can have optionally rolls on
both the up-line and the down-line.
The quarter loop is flown just like the first part of a loop. When
the plane is vertical, the elevator backpressure is released
completely. During the vertical line up, some right aileron and right
rudder is needed to maintain the vertical attitude because of the
engine torque and p-factor. When the plane has slowed enough, full
rudder initiates the turnaround. It is followed by right-forward
stick (right aileron and forward elevator) to keep the plane from
torquing off. The pivot is stopped with opposite rudder when the nose
points straight down. When the pivot is completed, the ailerons and
rudder are neutralized. Elevator and rudder are used to keep the nose
pointing straight down. Thee pivot must be completed within one
wingspan. Rolls on the downline require only aileron input if the
plane is trimmed correctly.
This maneuver is sometimes called a hammerhead stall. This is not an
accurate name because the airplane never stalls. The airspeed may be
very low, close to zero, but since there is now wingloading during the
turn-around, there is no stall (at zero g wing loading, a wing does
not stall). The plane is flying throughout the maneuver with all the
control surfaces effective (even sometimes only marginally so).
The previous paragraph is true even for gliders that don't have the
support of the propeller slip stream. The missing slip stream makes
it much more difficult to keep some flow over the control surfaces
during the turn-around in a glider.
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