Aerodynamics and G Forces


Aerodynamics refers to the physics of flight: the forces that produce, affect and control flight. The most important forces for pilots are thrust, lift and drag.




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 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 cross section 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 pressure 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 maneuverability. 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 corresponding 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." Because the wave is always moving away from the aircraft, the pilot never hears the sound of sonic boom, even when crossing the sound barrier.


Yaw, pitch and roll

Click on Picture to enlarge

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 reference as pilot. Pitch is movement around the horizontal axis. 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 horizon 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.



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 encounter when engaged in high-speed dog fighting and BFM. There are both positive and negative G forces; both can be dangerous to a fighter pilot. 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 finally 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 ordinary 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 lessening. 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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