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The simulation to the left shows how the lift force is inclined in a turn. You can roll the airplane left or right using the left and right arrow keys. The thrust vector can be imagined as going directly into your computer monitor, and the drag vector is coming directly out toward you. As a result these forces cannot be seen in this diagram. To keep things simple assume that T = D, therefore the airplane is flying level. However, it does not really matter. If T > D the airplane is making a climbing turn and if T < D the airplane is making a descending turn. The gray vector represents the weight, which is 1860 lb. In all the simulations we have looked at so far the hollow black vector has been the sum L + T + W, and it has been equal and opposite to weight. But, in this case you can see that there is an additional force. This is called the force of centripetal acceleration(Fac.) It is shown as bright green. It is the horizontal component of lift (L.) As the bank increases Fac increases. Fac is the force that makes the airplane turn. We will return to a complete discussion of rate and radius of turn in the performance chapter.
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The important thing to note at this time is that Lift must increase as bank increases. If this did not happen there would be a force acting downward which would cause the airplane to accelerate downward. It is crucial to realize that even if the airplane is descending when turning there is NO FORCE acting downward. As you saw on the previous page. Therefore, at all times the vertical component of lift, which is the hollow black vector in the simulation, must be equal and opposite to weight.
Load Factor (LF) is defined as Lift/Weight. 99.9% of the time we fly at an LF or 1.0. As we saw on the previous page L=W in straight and level flight. So, for that portion of a flight LF = 1.0. When we climb or descend you saw that L is slightly less than W so technically LF is a bit less than 1 (but it is very close to 1 even in a climb or descent, unless you are going straight up in an F16.)
If we assume no aerobatics, the only time the average pilot will experience anything significantly more than LF = 1 is when doing a steep turn. The simulation displays the calculation of LF as you change bank angle. LF has no units, but despite that fact pilots like to have "names" for things, so we call it "g" [pronounced "gees."] Pilots' are fond of saying they are "pulling g," which you should understand as simply meaning that LF is greater than 1.
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Every airplane has a maximum LF that it is designed for. The CARS and FARs specify the value. The airplane in the simulation has a limit of 3.8g. This is reached between 74 and 75 degrees of bank. If you roll beyond that you will over stress the airplane. Go too far beyond the limit and the wings would come off. (Of course, in the real world you might pass out instead. Passing out due to LF is called G-LOC (g induced loss of consciousness.)) To calculate LF you don't need to know the weight of the airplane. As you can see in the diagram to the left there is a simple trigonometric relationship between lift and weight. Specifically the cosine of the bank (b) equals W/L. This is the inverse of LF so we can say that in a turn: LF = 1/cos(b) The two vertical lines surrounding weight in the diagram mean that we must use the absolute value of weight. It is worth noting that 2.0g is reached at 60 degrees of bank and that 45 degrees of bank only creates 1.41g. If you keep your flying down to less than 60 all the time, and less than 45 most of the time you won't need to develop high-g tolerance the way a fighter pilot must. Perhaps the most important thing for you to note on this page is that every airplane you will ever fly will experience the same g-force at a given angle of bank. It doesn't matter how big the airplane is, or how fast it flies. At 60 degrees of bank all airplanes experience 2.0 gs, etc. |
You may have done the aerobatic maneuver called a chandelle. Depending on how "aerobatic" you were willing to get you might have flown close to 90 degrees of bank while doing a chandelle. And you are thinking - "I don't remember a high-g force." Your memory is correct.
When you perform a chandelle you don't pull back to create the g-force called for in the simulation above. As a consequence the hollow black vector is much shorter than weight. In other words there is a downward force on the airplane and the plane literally "falls" toward the center of the earth. As a result the relative wind is from below and this causes the airplane to yaw and pitch toward the earth. The result is a thrilling sensation, but one you can only experience for a few seconds. Once the nose drops you must reduce the angle of bank and bring the g-force up to match the value in the simulation.
Hopefully this explanation makes it clear that a chandelle, or any other aerobatic mane over is not a violation of the laws of physics.
Be definition of LF given above LF = 0 when Lift = 0. You can cause the wing on your airplane to stop producing lift by reducing angle of attack to the value at which no lift is produced. We will discuss this angle of attack in the chapter on lift. Once you push the nose forward to this AOA there is no lift and the airplane begins to "fall" like any other object (e.g. like a parachutist jumping out of the airplane.) Everything in the cockpit will begin to float around and you will experience the same sensations as an astronaut in orbit.
Of course the airplane falls toward the center of the earth, so the relative wind comes from below. Consequently the airplane quickly rotates until it is pointing straight down. So, you can only sustain zero g for a few seconds.
NASA has an airplane that they use to provide zero g training to astronauts. To maximize time at zero g the airplane must be flown as fast as possible and pitched to the most nose up attitude possible before pushing to zero g. The airplane then begins to fall and the astronauts can practice in zero g. The whole time the nose of the airplane pitches earthward. Once it gets down to a safe limit the pilot announces the run is over and returns to normal g force. Each such run will last only one or two minutes.
In a light aircraft with much less speed you will only be able to experience zero g for a few seconds. And you really have no need outside aerobatic flying to do so.
By definition negative g occurs when lift is negative. Lift will become negative if you pitch to an even lower AOA than the zero lift AOA. If you do so everything in the cockpit will be thrown to the roof (including your head; so have your seatbelt tight.)
Normal airplanes are designed for almost no negative gs. Therefore, you should totally avoid negative g flying. If you want to experience negative gs take up aerobatics in an airplane designed for it.
Generally negative gs would be experienced when flying upside down. If you "push for negative g" with the wings upright the airplane will do an outside loop. Such a maneuver in a normal airplane can only be done once (i.e. the wings will collapse, so don't try it.)
When rolling into a turn a pilot generally increases the AOA thus increasing lift by the required amount, as specified above. But what if the pilot doesn't pull back. Perhaps s/he is not paying attention and the airplane simply rolled into the bank.
Clearly the hollow black vector will not be long enough. As a result the airplane will "fall." Here is the sequence of actions, much like the one provided on the previous page:
The above sequence leads quite rapidly to the airplane being quite nose down and the airspeed rising rapidly. If the pilot does not intervene the airspeed could rapidly exceed design limits.
The spiral dive is only a problem if the airplane is unstable laterally. I.E. tends to roll further into the bank. Any airplane that is laterally unstable will spiral because the speed and g-force are the natural response to the increasing bank as described in the steps above.
In the stability chapter we will discuss what makes airplanes stable or unstable. For now simply realize that the typical light airplane is relative unstable in the lateral axis, therefore most light airplanes will tend to spiral if you don't pay attention.