تاپیک آموزش خلبانی

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Earlier you learned that you can stall an airplane at any airspeed and in any flight altitude. You can easily stall an airplane in a turn at a higher-than-normal airspeed. As the angle of bank increase in level turns, you must increase the angle of attack to maintain altitude. As you increase the angle of bank, the stall speed also increases




If you attempt to maintain altitude during a turn by increasing the angle of attack, the stall speed
increases as the angle of bank increases. The percent of increase in stall speed is fairly moderate
with shallow bank angles less than 45 degree . However,once you increase the bank angle
60 beyond 45 degree, the percent of increase in the stall speed rises rapidly. For example , in a
degree constant-altitude bank, the stall speed increases by 40% ; a 75 degree bank increases stal
l speed by 100%




Actually, stall speed increases in proporation to the square root of the load factor. If you are flying an airplane with a one-G stalling speed of 55 knots, you can stall it at twice that speed ( 110 knots ) with a load factor of four G's . Stalls that occur with G-forces on an airplane are called accelerated stall . An accelerated stall occurs at a speed higher than the normal one-G stall speed. These stalls demonstrate that the critical angle of attack , rather than speed, is the reason for a stall . Stalls also can occur at unusually high speeds in severe turbulance, or in low-level wind shear






LIMIT LOAD FACTOR



When the Federal Aviation administration certifies an airplane, one of the criteria they look at is how much stress the airplane can withstand. The limit load factor is the number of G's an airplane can sustain, on a continuing basis, without causing permanent deformation or structural damage. On ther words, the limit load factor is the amount of positive or negative G's an airframe is capable of supporting



Most small general aviation airplanes with a gross weight of 12.500 pounds or less, and nine passenger seats or less, are certified in either the normal , utility, or acrobatic categories. A normal category airplane is certified for nonacrobatic maneuvers. Training maneuvers and turns not exceeding 60 degree of bank are permitted in this category. The maximum limit load factor in the normal category is 3.8 positive G's, and 1.52 negative G's. In other words , the airplane's wings are designed to withstand 3.8 times the actual weight of the airplane and its contents during maneuvering flight. By following proper loading techniques and flying within the limits listed in the pilot's operating handbook, you will avoid excessive loads on the airplane, and possible structual damage


In addition to those maneuvers permitted in the normal category, an airplane certified in the utility category may be used for several maneuvers requiring additional stress on the airframe. A limit of 4.4 positive G's or 1.76 negative G's is permitted in the utility category. Some , but not all, utility category airplanes are also approved for spins. An acrobatic category airplane may be flown in any flight altitude as long as its limit load factor does not exceed six positive G's or three negative G's


A key point for you to remember is that it is possiible to exeed design limits for load factor during maneuvers. For example, if you roll into a steep , level turn 75 degree , you will put approximately four G's on the airplane. This is above the maximum limit of 3.8 G's for an airplane in the normal category. You also should be aware of the conditions specified for the maximum load limit. If flaps are extended , for instance, the maximum load limit normally is less. The POH for the airplane you are flying is your best source of load limit information

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MANEUVERING SPEED




An important airspeed related to load factors and stall speed is the design maneuvering speed (VA) . This limiting speed normally is not marked on the airspeed indicator, since it may vary with total weight. The POH is the best source for determining maneuvering speed, and it may also be displayed on a placard in the airplane. Although some handbooks may designate only one maneuvering speed, others may show several . When more than one is specified, you will notice that VA decreases as weight is subject to more rapid acceleration from gusts and turbulence than a more heavily loaded one



Any airspeed in excess of VA can overstress the airframe during abrupt maneuvers or turbulence. The higher the airspeed, the greater the amount of excess load imposed. VA represents the maximum speed at which you can use full, abrupt control movement without overstressing of pilotinduced control movement, or gust loads resulting from turbulence, should not cause an excessive load on the airplane. This is why you should always fly at or blow VA during turbulent conditions


The design maneuvering speed also is the maximum speed at which you can safely stall an airplane. If you stall the airplane above this speed, you will generate excessive G-loads. At or below this speed, the airplane will stall before excessive G-forces buld up. By staying at or below VA , you avoid the possibility of overstressing or even damaging the airplane

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LEFT-TURNING TENDENCIES

Propeller-driven airplanes are subject to several left-turning tendencies caused by a combination of physical and aerodynamic forces _ torque, gyroscopic precession, asymmetrical theust, and spiraling slipstream. You will need to compensate for these forces, especially when you are flying in high-power , low-airspeed flight conditions following takeoff or during the initial climb. If you know what is happening to the airplane, you will have a better idea of how to correct for these tendencies



TORQUE

In airplanes with a single engine, the propeller rotates clockwise when viewed from the pilot's seat. Touque can be understood most easily by remembering Newton's third law of motion
The clockwise action of a spinning propeller causes a torque reaction which tends to rotate the airplane counterclockwise about its longitudnial axis



In a single-engine airplane, you will experience the greatest effect of torque during takeoff or climb
when you are in high-power , low-airspeed flight condition

Generally, aircraft have design adjustments which compensate for torque while in cruising flight, but you will have to take corrective action during other phases of flight. Some airplanes have aileron trim tabs which you can use to correct for the effects of torque at various power settings

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GYROSCOPIC PRECESSION
The turning propeller of an airplane also exhibits characteristics of a gyroscope rigidity in space and precession. Gyroscopic precession is the resultant reaction of an object when force is applied. The reaction to a force applied to gyro acts in the direction of rotation and approximately 90 degrees ahead of the point where force is applied



If the pitch attitude of an airplane is changed from a nose-high to a nose level position, a force will be exerted near the top of the propller’s plane of rotation, A resultant force will exerted near the top 90 degree ahead in the direction of rotation, which will cause the nose of the airplane to yaw to the left. Thos typically happens when the tail of a conventional-gear airplane is raised on the takeoff roll


ASYMMETRICAL THRUST

When you are flying a single-engine airplane at a high angle of attack , the descending blade of the propeller takes a greater “bite” of air than the ascending blade on the other side . The greater bite is caused by higher angle of attack for the descending blade, compared to the ascending blade. This creates the uneven , or asymmetrical thrust, which is known as P-factor makes an airplane yaw about its vertical axis to the left.


You should remember that P-factor is most pronounced when the engine is operating at a high-power setting , and when the airplane is flown at a high angle of attack. In level cruising flight , P-factor is not apparent , since both ascending and descending propeller blades are at nearly the same angle of attack , and are creating approximately the same amount of thrust .

A good way to learn more about asymmetrical thrust is to take a look at a tailwheel-type airplane the next time you’re on an airport ramp. Its high angle of attack is quite apparent , and you can easily see the difference between the ascending and descending blade angles

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SPIRALING SLIPSTREAM

As the propller rotates, it produces a backward flow of air, or slpstream , which weaps around the airplnae. This spiraling slipstream causes a change in the airflow around the vertical stabilizer. Due to the direction of the propller rotation, the resultant slipstream strikes the left side of the vertical fin



As the slipstream produced by propller rotation wraps around the fuselage, it strikes the left side of the vertical
fin. A left-turning tendency is created as the air "pushes" the tail of the airplane to the right and yaws the nose
left






GROUND EFFECT

Another significant aerodynamic consideration is the phenomenon of ground effect. During takeoffs or landings, when you are flying very close to the surface, the ground alters the three-dimensional airflow pattern around the airplane. This causes a reduction in wingtip vortices and a decrease in upwash and downwash

Wingtip vortices are caused by the air beneath the wing rolling up and around the wingtip. This caused a spiral or vortex that trails behind each wingtip whenever lift is being produced. Wingtip vortices are another factor contribulting to induced drag. Upwash and downwash refer to the effect an airfoil exerts on the free airstream. Upwash is the deflection of the oncoming airstream upward and over the wing. Downwash is the downward deflection of the airstream as it passes over the wing and past the trailing edge

If you remember how angle of attack influences induced drag, it will help you understand ground effect. During flight, the downwash of the airstream causes the relative wind to be inclined downward in the vicinity of the wing. This is refferd to as the average relative wind. The angle between the free airstream relative wind and the average relative wind is the induced angle of attack. In effect , the greater the downward deflection of the airstream, The higher the induced angle of attack and the higher the induced drag. Since ground effect restricts the downward Deflection of the airstream, both the induced angle of attack and induced drag decrease. When the wing is at a height equal to its span, the decline in induced drag is only about 1.4%; when the wing is at a height equal to one-tenth is span, the loss of induced drag is about 48%

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Drag

نیروی مقاومت یا پسا به دو صورت خودش رو در پرواز نشون میده:
Parasite Drag- یا مقاومت مزاحم (روی اسم فارسیش اصلاً حساب نکنید!)
Induced Drag- یا مقاومت القایی (بر خلاف بالایی، اسم فارسیش بسیار کاربرد داره)

اما در تعریف:
Parasite Drag مقاومتی است که از حرکت اجسام در هوا بوجود می آید و البته همیشگی هم هست (عملاً از بین نخواهد رفت هر چند تلاش برای کم کردن آن همیشه وجود داشته)

این نوع مقاومت از ترکیب سه مقاومت کوچکتر بوجود می آید:
Form Drag: که همونطوری که از اسمش پیداست ناشی از شکل جسم متحرک در هواست

Skin Friction Drag: بر روی سطح خارجی هواپیما همواره آلودگیهایی وجود داره حتی اگر این سطح کاملاً صیقلی به نظر برسه، به عبارت دیگه این آلودگیها از بین نمیره هر چند ممکنه در ابعاد میکروسکوپیک باقی بمونند. در ایر برخورد الیاف هوا با این آلودگی ها مقاومتی در اطراف محل برخورد بوجود میاد که هر چند ناچیز به نظر می رسه اما مهم خواهد بود.

Interference Drag: مقاومت ناشی از برخورد هوای اطراف سطوح مختلف پروازیست، در واقع این مقاومت از رویارویی هوای عبوری از قسمتهای مختلف هواپیما در محل اتصال این سطوح بوجود میاد. مثل محل اتصال بال و بدنه.

نکته: به طور کلی Parasite Drag با سرعت هواپیما رابطه مستقیم داره.

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Induced Drag مقاومتی است که به خاطر تولید لیفت در هواپیما بوجود خواهد آمد (تعریف انگلیسی این مقاومت بارا و بارها در امتحانات تکرار شده و یکی از مهمترین مطالب در آیرودینامیک هست)
This type of Drag is a byproduct of Lift

این مقاومت زمانی افزایش پیدا می کند که ما نیاز به تولید لیفت بیشتر در سرعتهای پایین تر داریم و مجبور به استفاده از High Lift Devices مثل فلپ و اسلات هستیم.
پس؛
نکته: این نوع مقاومت با Angle of Attack رابطه مستقیم داشته و با سرعت رابطه معکوس دارد.
The greater A.O.A, the greater the Induced Drag


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Total Drag
از مجموع Parasite Drag و Induced Drag تشکیل شده و ما اون به نام Drag در جهار نیروی اصلی میشناسیم.
نمودار زیر رابطه بین انواع Drag رو با سرعت به ما نشون می ده،



به طور کلی مبحث Drag از مباحث مهم در آیرودینامیک می باشد.

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FORCES ACTING ON A CLIMBING AIRPLANE


When you transition from level flight into a climb,you must combine the change in pitch attitude with an increase in power. If you attempt to climb just by pulling back on the control wheel to raise the nose of the airplane, momentum will cause a brief increase in altitude, but airspeed will soon decrease.
The amount of thrust generated by the propeller for cruising flight at a given airspeed is not enough to maintain the same airspeed in a climb. Excess thrust , not excess lift , is necessary for a sustained climb . In fact during a true vertical climb, the wings supply no lift, and thrust is the only force opposing weight



FORCES ACTING ON A DESCENDING AIRPLANE


Let's continue our discussion by considering the forces of weight, lift , thrust , and drag as they affect a descending airplane. If you are using power during a stabilized descent, the four forcees are equilibrium . during the descent, a component of weight acts forward along the flight path. as speed increases , this force is balances by an increase in parasite drag


During a power-off glide , the throttle is places in an idle position so the engine and propller produce no thrust. In this situation, the source of the airplane's thrust is provided only by the component of weight acting forward along the flight path. In a steady, power-off glide, the forward component of weight is equal to and opposite drag





the forces of wight , lift ,thrust, and drag are in equilibrium during a stabilized
climb just as they are during straight-and-level flight. When an airplane is
climbing, though the force of weight consists of two components. The first
opposes lift and is perpendicular to the flight path. The second is called
the rearward component of weight and acts in the same direction as drag
opposite to the relative wind

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