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Aerodynamics and G forces Aerodynamics refers to the physics of flight: the forces thet produce, affect and control flight. The most important forces for pilots are thrust, lift and drag. |
Thrust
Thrust is the force that moves an object, such as an aircraft,
along a specific path. It is the force that overcomes the inertia
of an object due to gravity and the resistance of an object moving
through air. In aircraft, thrust comes from propellers or jet
engines. The force of thrust of an aircraft will have a direct
bearing on its speed.
You may hear the phrase "thrust to weight ratio" in
discussions of jet fighter aircraft. This refers to a ratio between
the thrust of the jet engine and the weight of the aircraft.
All other things being equal, an aircraft with a thrust to weight
ratio greater than one /1/ can propel itself straight up against
the force of gravity. The F-16, with its 25,000 pounds of afterburner
thrust, has a thrust to weight ratio of 6.2 to 1.
Lift
Lift is the seemingly magical force that enables modern aircraft
to stay in the air. It is possible due to several interesting
principles of physics. When air moves quickly over an arched
surface, the air pressure above the surface drops. The wing on
an aircraft has a crossection shaped like the diagram below,
which causes air to speed up as it passes over. As the air speeds
up, the pressure drops above the wing. The air pressure under
the wing remains normal, which is now at a higher pessure than
the air above the wing. This difference in air pressure produces
a force in the direction of the low pressure area. This is the
force that creates lift in a wing.
The size, shape and thickness of a wing all determine
the amount of lift it will produce. Other factors that affect
lift are the velocity of the air moving past the wing and the
air pressure or density of the air.
Lift is also directly affected by the angle that the wing cuts
through the air. This angle is known as the angle of attack /AOA/.
As the AOA increases, so does lift. However, a high AOA may interrupt
the flow of air over the wing and cause a stall. This kind of
stall is almost impossible in an F-16 because the flight control
computer will never let the pilot fly with an AOA greater then
25 deg. , which is where a stall would occur in a stock F-16A.
While you don't have to worry about AOA and stalling,
your ability to control the AOA is particularly important when
landing an aircraft.
Every time we have movement through the air, we come upon the
problem of drag. Drag is the resistance of the movement of an
aircraft. While air is invisible, it is not without weight, mass
and inertia. An aircraft moving at Mach 2 is pushing aside an
enormous volume of air at a very high rate, and this air pushes
back in the form of drag.
There are three main types of drag that affect the performance
of an aircraft.
Induced drag
induced drag is the most important form of drag, if for no other
reason than it occurs as a result of the force of lift. Lift
is possible when a wing moves through the air at a positive AOA.
However, a wing at a positive AOA collides with the air it is
moving through, creating a backward force. This backward force
is called the induced drag force. Since it is a direct function
of lift, it is almost always present when flying an aircraft.
If you unload your aircraft by pushing the nose down, you will
counter the force of lift and, as such, induced drag will also
be gone. The rest of time, induced drag plays a part in the aerodynamics
of your craft.
Skin friction or parasitic
drag
Skin friction drag /also called parasitic drag/ is a simple kind
of drag that results from wind resistance to the rough surfaces,
bumps and protuberances of an aircraft. When you load up F-16
with weapons, jammers and fuel tanks, are complicating the aerodynamic
beauty of the basic F-16. This creates drags. In each type of
store has an associated drag factor. This drag will affect your
flight performance and may limit the number of Gs you can pull.
The drag factor for your plane is displayed on the armament screen,
and it changes as you add or remove stores. The more drag you
have, the more 'sluggish" the plane will feel. In addition,
greater drag increases fuel consumption, affects acceleration,
and degrades maneuverablilty. You may have to engage full afterburner
to take off with a full load due to weight and drag influences.
When fire weapons or jettison stores, you reduce the drag factor
and its correponding effects.
Wave drag
Wave drag is only found in jet fighters or supersonic aircraft.
When plane moves at supersonic speeds, it builds up a tremendous
shock wave in front of it. It takes enormous energy to move through
these waves, and this resistance is called wave drag. When the
shock wave reaches the ground, it is experienced in the rattling
form of a "sonic boom." Becouse tha wave is always
maving away from the aircraft, the pilot never hears the sound
of sonic boom, even when crossing the sound barrier.
Yaw, pitch and roll
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Thrust moves an aircraft through the air, and
there are three axes of movement that an aircraft can travel
through. The movements along the three axes are called yaw, pitch
and roll. Yaw is movement around the vertical axis of an aircraft. You experience it as the nose moving left and right from your point of refence as pilot. Pitch is movement around the horizontal aaxis. You experience it as the nose moving up and down. Roll is movement along the long axis of the aircraft. You experience a roll by seeing the horizont rotate in front of you. These points of reference are on the point of view of the pilot, regardless of his orientation in real space. As you crank around on the stick, you will be pulling your aircraft through all three axes in various combinations. By practicing basic fighter maneuvers, you will gain a detailed understanding of movement within the three axes. |
Stalling
In order to fly, an aircraft must have enough thrust to create
lift. This thrust translates into a forward velocity. If the
aircraft falls below a certain minimum velocity, it will not
be able to generate enough lift to stay airbone. In short, it
will stall.
Every aircraft has a minimum speed it needs to maintain flight.
This value is called the stall speed, because a stall will occur
if the plane's velocity falls below it. This value is usually
associated with takeoff and landing since you cross the stall
speed in both activities, but actually there are many stall speeds
for an individual aircraft.
The different stall speeds depend on the air pressure /also called
air density/. You will encounter different air densities according
to your altitude. The air pressure is the greatest on the surface
and diminishes as you get higher. A plane's landing and takeoff
stall speed is applicable near the surface of the earth, but
at 50,000 feet the same aircraft will stall at a much different
sppeed. For example, an aircraft with a stall speed of 125 knots
at ground level may have a stall speed of 165 knots at 10,000
feet, 220 knots at 25,000 feet, and 350 knots at 50,000 feet.
The stall speed increases as the aircraft goes higher because
the air is thinner. Thinner air creates less lift for tha same
amount of thrust.
The most common forms of stall are caused by insufficient velocity
or by exceeding the maximum AOA. There is another kind of stall
called a compressor stall. The maximum compressor blades in a
turbofan engine are designed as airfoils and, like the wing of
an aircraft, can be stalled if the airflow hits them above a
critical angle. This kind of stall is usually associated with
certain problems of the afterburner. Fortunately, this kind of
stall is very rare in the F-16.
How to recover from a stall
You can easily recover from a stall in the F-16. It almost does
it for you. If your airspeed drops below about 120 knots, the
nose of the plane will start to drop. In addition, you will see
the Stall light on the right upper section of the glare screen
illuminate and the stall horn will sound. This indicates that
you do not have enough airspeed to maintain flight. As the nose
drops, you begin to pick up speed - with more speed you regain
your ability to fly. If you find yourself in a stall situation,
simply drop the nose of aircraft to pick up speed. In case of
a severe stall, you may want to roll fighter 180 deg. Before
you head down so that you don't incur negative G forces.
In order to recover from stall, you need sufficient altitude
since you are trading altitude for airspeed. Don't put yourself
in a stall situation if you don't have sufficient altitude, or
you'll end up as a colorful spot on the landscape.
As the nose drops and you begin to gain speed, gently pull the
nose of F-16 back up toward the horizon. If you pull up quickly,
you may bleed off speed too rapidly and find yourself stalling
out again.
Fuel usage
The F-16A is powered by a single Pratt and Whitney F100-PW-200/3/
turbofan engine which generates approximately 25,000 pounds of
thrust with afterburner. This power plant is what keeps the F-16
airborne, but not without a price. The F-16 burns rather large
amounts of JP-5 fuel, particularly when you use the afterburner.
Monitoring your fuel usage is critical when you fly missions.
There are many factors that affect fuel consumption, including
engine RPM, altitude, aircraft weight, drag and damage. But by
far the most likely cause of running out of gas is the use of
the afterburner.
The afterburner can give you a great advantage in a dogfight
by keeping your energy level high, but beware - it burns fuel
at over three times the rate of full military thrust. When you
go to burner, you are burning fuel at rate of 860 pounds per
minute! Since your internal tank only holds about 7,000 pounds
of fuel, you can drastically reduce your linger time by overusing
the burner.
Use your burner when you need it, but don't overdo it. There
is nothing more humiliating than to hit your targets and then
have to punch out because you didn't manage your fuel properly.
The Air Force frowns on losing planes this way.
G forces
You can't fly combat aircraft without considering G forces. G
forces are the forces of acceleration that pull on you when you
change your plane of motion. They are the forces that pilots
encouter when engaged in high-speed dogfighting and BFM. There
are both ppositive and negative G forces; both can be dangerous
to a fighter ppilot. The force of gravity on Earth is used as
a baseline for measuring these forces of acceleration.
The force of gravity when you sit, stand or lie down is considered
1 G. In normal activity, we rarely experience anything other
than 1 G. But flying a combat aircraft such as the F-16 is not
exactly normal. The F-16 is capable of pulling 9 Gs without even
trying. But the effect of 9 Gs on your body will be significant.
As you pull more Gs, your weight increases correspondingly. Your
10-pound head will weigh 90 pounds when you pull 9 Gs!
If you continue to pull high Gs, the G force will push the blood
in your body towards your feet and resist your heart's attempts
to pump it back up to your brain. You will begin to get tunnel
vision, then things will lose color and turn white, and finnaly
everything will go black. You've just experienced the onset of
Gravity Induced Loss of Consciousness /GLOC/.
The modern fighter pilot has some aids in helping him overcome
the forces of gravity he experiences from combat. The most obvious
is the G suit. The G suit uses the principle of pushing the blood
back up toward the head during high G maneuvers. The British
first used water bladders placed around the legs to help fight
against Gs. As the pilot was pressed into his seat from high
G forces, the incompressible water would push against his legs
and keep the blood from pooling there. Modern G suits use compressed
air to force the blood back up towards the pilot's head.
The G force from such maneuvers as pulling out of a dive or banking
sharply are called positive Gs because they increase our ordinaly
sense of gravity. It is also possible to maneuver in a way that
produces negative forces of gravity. These are called negative
Gs, and they have a very different effect on you.
If
you are flying straight and level and push the nose of the plane
down, you will experience your weight lessning. The harder you
push the nose down, the more "weightless" you will
feel. You are experiencing negative Gs. The effect of negative
Gs is to push the blood up into the head, just the opposite of
positive Gs. However, while the body can stand up to 9 positive
Gs without severe consequences, blood vessels in your eyes will
start to rupture when you apply as little as 2 to 3 negative
Gs. This is known as redout.. A pilot who pushes too many negative
Gs will be seeing the world through bloodshot eyes.
There is a simple way to avoid negative Gs that also gives you
much better maneuverability. Instead of pushing forward on the
stick to dive /which creates negative Gs/ , roll your aircraft
180 deg. And pull back on the stick. If you roll so that your
cockpit is facing toward the ground and then pull back on the
stick, you will still be diving toward the ground but will be
experiencing positive Gs instead. Your tolerance is much greater
to positive Gs.
Corner velocity
Corner velocity /also called corner speed or maneuvering speed/
is an important value for each aircraft. It is determined by
plotting the structural limitations /in G forces/ against airspeed.
The corner velocity is the minimum speed at which an aircraft
can pull its maximum rated Gs. An aircraft at corner velocity
attains maximum instantaneous turn performance.
The corner velocity for the F-16A in a stock configuration is
450 knots. This means that at 450 knots the F-16 has its best
turn performance. At speeds above the corner speed, turn performance
drops off.
Corner speed also affects the minimum turn radius. The size of
the turn radius of an aircraft depends on the speed it is traveling.
A faster aircraft requires a larger circle to turn in than a
slower one. However, the turn redius isn't only a function of
speed. It also depends on the number of Gs a pilot pulls during
the turn. An aircraft at a constant speed will make a relatively
wide circle at 1 G but will turn in a very tight circle at 7
or 8 Gs. The corner velocity is the speed that gives the optimum
balance between turn rate and turn radius.
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