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The other point of importance on the Vg diagram is the intersection of the negative limit load factor and line of maximum negative lift capability. Any airspeed greater than this provides a negative lift capability sufficient to damage the aircraft; any airspeed less than this does not provide negative lift capability sufficient to damage the aircraft from excessive flight loads.
The limit airspeed (or redline speed) is a design reference point for the aircraft—this aircraft is limited to 225 mph. If flight is attempted beyond the limit airspeed, structural damage or structural failure may result from a variety of phenomena.
The aircraft in flight is limited to a regime of airspeeds and Gs which do not exceed the limit (or redline) speed, do not exceed the limit load factor, and cannot exceed the maximum lift capability. The aircraft must be operated within this “envelope” to prevent structural damage and ensure the anticipated service lift of the aircraft is obtained. The pilot must appreciate the Vg diagram as describing the allowable combination of airspeeds and load factors for safe operation. Any maneuver, gust, or gust plus maneuver outside the structural envelope can cause structural damage and effectively shorten the service life of the aircraft.Rate of Turn
The rate of turn (ROT) is the number of degrees (expressed in degrees per second) of heading change that an aircraft makes. The ROT can be determined by taking the constant of 1,091, multiplying it by the tangent of any bank angle and dividing that product by a given airspeed in knots as illustrated in Figure 4-48. If the airspeed is increased and the ROT desired is to be constant, the angle of bank must be increased, otherwise, the ROT decreases. Likewise, if the airspeed is held constant, an aircraft’s ROT increases if the bank angle is increased. The formula in Figures 4-48 through 4-50 depicts the relationship between bank angle and airspeed as they affect the ROT.
NOTE: All airspeed discussed in this section is true airspeed (TAS).
4-34
Rate of turn f
or a given airspeed (knots, TAS) and bank angle 1,091 x tangent of the bank angleairspeed (in knots)ROT = 1,091 x tangent of 30°120 knotsROT = 1,091 x 0.5773 (tangent of 30°)120 knotsROT =ROT = 5.25 degrees per secondExample The rate of turn for an aircraft in a coordinated turn of 30° and traveling at 120 knots would have a ROT as follows.
Figure 4-48. Rate of turn for a given airspeed (knots, TAS) and bank angle.
Increase in the speed
1,091 x tangent of 30°240 knotsROT =ROT = 2.62 degrees per secondAn increase in speed causes a decrease in the rate of turn when using the same bank angle.Example Suppose we were to increase the speed to 240 knots, what is the rate of turn? Using the same formula from above we see that:
Figure 4-49. Rate of turn when increasing speed.
To maintain the same Rate of Turn of an aircraft
traveling at 125 knots (approximately 5.25° per second using a 30° bank) but using an airspeed of 240 knots requires an increased bank angle. 1,091 x tangent of X240 knotsROT (5.25) =240 x 5.25 = 1,091 x tangent of X240 x 5.25 = tangent of X 1,0911.1549 = tangent of X49° = XExample Suppose we wanted to know what bank angle would give us a rate of turn of 5.25° per second at 240 knots. A slight rearrangement of the formula would indicate it will take a 49° angle of bank to achieve the same ROT used at the lower airspeed of 120 knots.
Figure 4-50. To achieve the same rate of turn of an aircraft traveling at 120 knots, an increase of bank angle is required.
120 knots
11.26 x tangent of bank angleR = 120211.26 x tangent of 30°R =V2R =11.26 x 0.577314,400R = 2,215 feetThe radius of a turn required by an aircraft traveling at 120 knots and using a bank angle of 30° is 2,215 feet.
Figure 4-51. Radius at 120 knots with bank angle of 30°.
Airspeed significantly effects an aircraft’s ROT. If airspeed is increased, the ROT is reduced if using the same angle of bank used at the lower speed. Therefore, if airspeed is increased as illustrated in Figure 4-49, it can be inferred that the angle of bank must be increased in order to achieve the same ROT achieved in Figure 4-50.
What does this mean on a practicable side? If a given airspeed and bank angle produces a specific ROT, additional conclusions can be made. Knowing the ROT is a given number of degrees of change per second, the number of seconds it takes to travel 360° (a circle) can be determined by simple division. For example, if moving at 120 knots with a 30° bank angle, the ROT is 5.25° per second and it takes 68.6 seconds (360° divided by 5.25 = 68.6 seconds) to make a complete circle. Likewise, if flying at 240 knots TAS and using a 30° angle of bank, the ROT is only about 2.63° per second and it takes about 137 seconds to complete a 360° circle. Looking at the formula, any increase in airspeed is directly proportional to the time the aircraft takes to travel an arc.
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Pilot's Handbook of Aeronautical Knowledge飞行员航空知识手册(66)
