Illustrations by TC and Hammer
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Your plane has four basic forces working on it during flight –Thrust, Gravity,
Drag, and Lift.
Lift and Thrust are your friends while Gravity and Drag usually work against you
but can help in some situations.
Thrust is produced by your engine. It is directed directly back from the
propeller. In a propeller driven plane, it is created by the propeller pushing
air to the rear of the plane. In a jet, it is created by accelerating the
exhaust created by burning the fuel and discharging it to the rear. Thrust
pushes (or pulls) your plane forward and thereby creates lift for the wing by
generating airflow over it. The amount of thrust produced by your engine can be
controlled by the manifold pressure (throttle) or by adjusting the RPMs.
Aircraft engines produce different amount of thrust at different altitudes, and
some planes’ engines are optimized at different altitudes than others.
Gravity (weight) is the pull of the earth on objects. It is the weight of the
plane and is always directed towards the center of the earth. Don’t confuse this
force with centrifugal force, which is what causes you to “pull G’s” during a
maneuver. Gravity acts on all planes equally at all times.
Drag is the resistance of air against the surfaces of your plane. It will always
be directed opposite the direction of travel. Because air is less dense at
higher altitude, drag decreases at altitude. The force of drag on your plane
increases with speed until it cancels out your plane’s thrust. When this
happens, you have reached your maximum speed. Some planes are much more
aerodynamic than others, meaning they have less drag. This can help them go
faster and hold energy more efficiently, but it can sometimes cause problems
trying to reduce speed if you need to do so in a hurry.
Lift is generated by the wing as it moves through the air. It will always be
directed perpendicular to the direction of travel when looking from the side and
perpendicular to the leading edge of the wing when looking at the plane from the
front. The faster a wing is moving through the air, the more lift is generated
by that wing. The second major factor for producing lift is the “Angle of
Attack” which is discussed in more detail below.
In discussions about maneuvering your plane, you will often hear the term
“vector” and in particular “lift vector”. A vector is a depiction of the
direction a force is acting on something. The blue arrows in the picture at the
top of this page depict the force vector of each of the forces acting on the
plane while it is in flight. For the purpose of almost all discussions of the
forces acting on your plane, think of the force vector as acting on the plane’s
center of gravity.
As discussed above, all of the force vectors act relative to a part of the
plane, the direction of travel, or to the center of the earth. The most basic
concept in understanding force vectors is they must cancel each other out in
order to maintain constant speed level flight. In the picture at the top of the
page, Thrust is equal to Drag and Lift is equal to Gravity (weight). In the
picture below, however, it is not as simple.
Imagine this plane flying at extreme low speed but maintaining level flight at
constant speed. As the speed lowered, the pilot was forced to put his nose up in
order to maintain level flight. This creates more lift by increasing the Angle
of Attack of the wing (discussed in more detail below). In this nose-up
attitude, Gravity is still pulling the plane straight down towards the center of
the earth, Drag is still working opposite of the direction of flight, and Lift
is still being generated perpendicular to the relative wind. Thrust, however, is
now working in a different direction.
As discussed above, the forces acting on a plane must cancel each other out in
order for the plane to fly at a constant speed and altitude. Since Thrust is no
longer acting exactly opposite to drag, it is useful to break it into
components. In this case, Thrust can be broken into a horizontal component (the
green line) and a vertical component (the red line). In level, constant-speed
flight, the horizontal component of thrust is equal in magnitude to drag. Lift +
the vertical component of thrust are equal in magnitude to Gravity.
Angle of Attack, Indicated Air Speed, and Lift
Besides giving a vertical component to thrust, lifting the nose of the plane
increases the Angle of Attack of the wing which increases the lift produced by
the wing. The Angle of Attack is the angle at which the chord of the wing meets
the relative wind. The chord is the line between the leading edge and the
trailing edge of the wing. As mentioned above, the relative wind is opposite
your direction of flight and equal in force to your indicated air speed.
It is important to note the relative wind does not have to be level with the
ground. In the pictures below, the wing on the left is in level flight while the
wing on the right is climbing at a constant rate and speed. Both wings have the
same Angle of Attack.
The two main factors affecting how much lift any given wing produces are
indicated airspeed and the Angle of Attack of the wing. Indicated airspeed is
important because it takes into account the density of the air as it changes
with altitude. The faster the wing moves through the air, the more lift it
produces. However, the thinner air at high altitudes produces less lift for a
given “true” airspeed than the thicker air at sea level. This difference is
accounted for in the indicated airspeed.
an aircraft’s speed decreases, Lift decreases unless the Angle of Attack is
increased. The Angle of Attack can be increased until the wing reaches its
“critical angle”. This is the Angle of Attack at which airflow over the wing is
disrupted to the point that lift is no longer produced. At this point, the wing
stalls. The critical angle varies with speed, weight of the plane, and wing
design. The Angle of Attack is increased by using the elevator to increase the
pitch of the aircraft.
In order to maintain level flight, you must increase the AoA as speed decreases
and vice versa. This is why you must raise your nose as you slow down and lower
your nose as you speed up if you want to maintain the same altitude.
Lift Vector, Angle of Attack, and Maneuvering
vector is the force vector you will discuss the most when talking about
maneuvers. That is because almost all maneuvers are done by manipulating your
lift vector and increasing the wing’s Angle of Attack. To turn your plane, you
first roll your wings so the lift vector is pointed towards the direction you
want to go as in the picture to the right.
By rolling your wings, you change the direction of your lift vector. You can now
divide this vector into a horizontal component and a vertical component as shown
in the image above. In order to maintain a level turn, the vertical component
must equal the weight of the plane. The horizontal component causes the plane to
turn. Both components can be increased by applying up elevator to increase your
wing’s Angle of Attack. This increases the lift vector which in turn increases
both the horizontal and vertical components. It also exposes more of the wing to
the virtual wind which increases drag and causes you to slow. The higher the
angle of attack, the more drag must be overcome because more of the wing (and
the other surfaces of the plane) is exposed to the virtual wind.
Besides the four forces always experienced by a plane in flight, a turning plane
experiences a virtual force know as Centrifugal Force. Centrifugal force does
not actually exist, but objects moving in a circle act as thought it does. This
is the force that causes you to “Pull G’s”.
While gravity is a constant force acting on your plane at all times, maneuvering
your plane often causes you to “pull G’s”. As noted above, pulling G’s is the
result of Centrifugal Force. The University of Virginia’s “Phun Physics” website
describes Centrifugal Force like this:
An object traveling in a circle behaves as if it is experiencing an outward
force. This force is known as the centrifugal force. It is important to note
that the centrifugal force does not actually exist. Nevertheless, it appears
quite real to the object being rotated.
In level flight, you are at 1 G. When you pull back on your stick, you pull
positive G’s. At 6 G’s, you black out. This blackout is preceded by a “grayout”,
which is a gradual narrowing of your field of view.
Pushing forward on your stick causes negative G’s. At about 1 negative G, you
will red out. Holding at 0 G’s will give your plane its best acceleration.
A stall occurs when the airflow over the wing is disrupted to the point that the
wing no longer produces enough lift for controlled flight. Stalls are most often
associated with getting too slow but may actually occur at any speed.
Technically, a stall occurs when the wing exceeds its critical angle of attack.
This can be caused by raising the nose in an attempt to maintain level flight at
low speeds or by an excessively abrupt maneuver at higher speeds. Any stall
caused by a maneuver when flying above stall speed is sometimes called an
accelerated stall. Most stalls can be recovered by lowering the nose or
increasing throttle. Failure to recover from a stall quickly can result in a
The last flight dynamic we will discuss is compression. Compression occurs when
the air moving over your control surfaces “locks” them so they do not respond.
This phenomenon happens at different speeds in different planes. Altitude also
affects the speed at which compression occurs with compression setting in sooner
at higher altitudes.
In some planes in Aces High, the Combat Trim function can make it seem like you
are experiencing compression even if you are not. This is because combat trim
tries adjusts your trim for level flight at whatever speed you are going. In
some planes, most notably the Bf 109 series, the combat trim will be full down
at high speeds. While this keeps you in level flight, it also keeps you from
being able to pull out of a dive. To counter this, you must trim up to at least
center and preferably trim up. Adjusting trim also helps when you are
experiencing true compression.
If you find yourself in a steep dive and your controls won’t respond, reduce
throttle, trim up, and use your rudder to skid and hopefully slow you down
enough that you regain control.