Aircraft for Amateurs

[المنتصر]

Turn Lift Requirements

As the bank angle increases, the amount of pull required to maintain level flight increases rapidly. It is not possible to maintain level flight beyond a given bank angle because the wings cannot produce enough lift. An attempt to fly beyond this point will result in either a stall or a descent.

Physiologically speaking, the most important part of a turn is the necessity to pull "Gs". As the back pressure is increased to maintain level flight, the increased force is felt as an increase in "G" level. In a 30 degree bank, 1.2 G is required to maintain level flight. The G level increases rapidly with an increase in bank; at 60 degrees, it goes to 2.0 G, and it takes 9.0 G to fly a level 84 degree bank turn. As long as there is enough airspeed, the G level can be increased in any bank angle by pulling back on the stick.

Finishing the turn, a simple matter of leveling the wings by using the ailerons and coordinated rudder, takes time; the airplane continues turning until the wings are level, so the roll-out must be started a little prior to reaching the desired heading. Back-stick pressure must also be released as bank decreases or the aircraft will climb.

Maneuverability

Airplanes are not limited to being a relatively fast means of getting somewhere. Long ago thrill-seeking pilots discovered that aircraft have the potential for providing loads of fun while getting nowhere fast. Aerobatics are an essential skill for fighter pilots; and the training that it gives to pilots in position orientation and judgment is considered so vital that a great deal of time is spent teaching these maneuvers. Maneuverability is defined as the ability to change the speed and flight direction of an airplane. A highly maneuverable airplane, such as a fighter, has a capability to accelerate or slow down very quickly, and also to turn sharply. Quick turns with short turn radii place high loads on the wings as well as the pilot. These loads are referred to as "g forces" and the ability to "pull g's" is considered one measure of maneuverability. One g is the force acting on the airplane in level flight imposed by the gravitational pull of the earth. Five g in a maneuver exerts 5 times the gravitational force of the earth.

Maneuverability

Aileron Roll The aileron roll is simply a 360 degree roll accomplished by putting in and maintaining coordinated aileron pressure. The maneuver is started slightly nose high because, as the airplane rolls, its lift vector is no longer countering its weight, so the nose of the airplane drops significantly during the maneuver. Back stick pressure is maintained throughout so that even when upside down, positive seat pressure (about 1 G) will be felt. As the airplane approaches wings-level at the end of the maneuver, aileron pressure is removed and the roll stops.

[المنتصر]

Aileron Roll

Loop

Loop A loop is simply a 360 degree change in pitch. Because the airplane will climb several thousand feet during the maneuver, it is started at a relatively high airspeed and power setting (if these are too low, the airspeed will decay excessively in the climb and the maneuver will have to be discontinued.) The pilot, once satisfied with the airspeed and throttle setting, will pull back on the stick until about three Gs are felt. The nose of the airplane will go up and a steadily increasing climb will be established. As the maneuver continues, positive G is maintained by continuing to pull. The airplane continues to increase its pitch until it has pitched through a full circle. When the world is right-side-up again, the pilot releases the back stick pressure and returns the aircraft to level flight.

MISTAKES

Any time you place yourself in a several thousand pound machine and force it to travel through the air at high speeds and altitudes, there is going to be some risk. Many think that the primary risk in flying is mechanical failure or weather. Contrary to this belief, most airplanes (even those made of cloth and wood) that crash do so as a result of pilot error --frequently from attempting to fly too slow!

Stall

The stall is the initial result of letting the airspeed decay below what is required for the wings to produce sufficient lift. With insufficient lift to counteract aircraft weight, the airplane is not being "held up" by the wings any more and it accelerates toward the ground. At low altitude, the stall can be immediately disastrous but with enough altitude below, the pilot can take action to recover.

Recovery from the stall is accomplished by correcting the condition that led to it. Since the stall is caused by attempting to fly at too high an AOA, the pilot must immediately reduce the AOA by moving the stick forward. At the same time, the throttle is advanced to full power to rapidly increase the airspeed needed for a return to level flight or climb.

Aircraft are almost always designed to give some warning prior to a stall. In very large aircraft, special sensors detect the impending stall and physically shake the control stick. Cessna uses a buzzer located in the wing root for its light aircraft. High-performance aircraft have a horizontal stabilizer placed so that, as a stall is approached, the turbulent air coming off the top of the wing hits the horizontal stabilizer and shakes the flight controls. In extreme conditions, the whole airplane will shake. These warnings are difficult to ignore; they give the pilot sufficient time to act to prevent the stall.

Spin

If a stall is maintained and yaw is somehow induced, a spin can result. Spins can be recognized by high descent and roll rates, and a flight path that is straight down. Clearly, this is a situation to be entered with some forethought. Harder to recover from than a stall, and much more dangerous in terms of altitude loss, the spin is an extremely complex maneuver and beyond the scope of this ****. The good news is that if you do not stall, you cannot spin.

[المنتصر]

Landing

All good things must come to an end," and most flights end with a landing. The relative difficulty of this maneuver is often expressed by a student pilot after the first solo flight: "The first thought that came to mind after I took off was `Oh boy, now I've gotta land this thing!'"

After lining the airplane up with the runway and configuring it properly (landing gear, proper flap setting, speedbrake out), the pilot uses the throttle setting to maintain the proper airspeed (100 knots) and uses the elevators and ailerons to keep the airplane headed for the runway. The airplane is set up in a shallow descent (about three degrees) aimed at the near end of the runway. If this part of the landing, the "final approach" is flown correctly, it will look like the jet is headed for a collision with the approach end of the runway.

As the airplane closes in on the approach end, the pilot begins to ease the stick back to level off the airplane several feet above the runway and slows to landing speed by reducing the power to idle. As the airplane levels off just above the ground in idle power, it will lose speed rapidly because there is little or no thrust to counter the drag. The pilot continues to move the stick back to increase the AOA and keep the airplane flying for just a little while longer. In a well-flown landing, the airplane will settle to the ground just before the stall AOA is reached.

Now a land-based vehicle, the airplane is controlled with the brakes and slowed to taxi speed.

The Axis System

A good understanding of the basic axis system used to describe aircraft motion is necessary to appreciate flight data. Aircraft translational motion is described in terms of motion in three different directions, each direction being perpendicular to the other two (orthogonal). Motion in the X direction is forward and aft velocity. The Y direction produces sideways motion to the left and right, and up and down motion is in the Z direction.

Rotational Axes

The rotational motion of an aircraft can be described as rotation about the same three axes; pitch rotation (nose up or nose down) is about the y axes, lateral or roll rotation (one wing up or down) is about the x axis, and yaw rotation (nose right or left) is about the z axis.

There are several slightly different versions of the basic axis system just described. They differ primarily in the exact placement of the zero reference lines, but are generally similar in their directions. (For example, the body-axis system uses the fuselage center line as the x axis, while a wind-axis system uses the direction that the aircraft is moving through the air as the x axis.)