Performance Chapter 5 Lecture 10 Performance • What does performance mean? • What determines performance? – How fast will it climb, how quickly will it take off, land, or how far will it go. Level of Performance • Airframe- in-flight performance depends on its drag characteristics. • Remember power is required to move something a specific distance per unit or time. • Power is a force times velocity • Power required at any given velocity is determined by multiplying the drag times velocity. Power Curve • Plotting power& drag a curve appears • This is the power required curve or the power curve • The power curve has nothing to do with the engine • The curve is simply the drag curve replotted in terms of drag • The term power required refers to the power required for level flight Power Curve • Figure 5-1 p. 116 • A power required curve for an airplane that indicates 200kts to fly, with a 160hp required • Remember that the rating is given in the brake horsepower output of the engine • The actual power available for thrust is determined by multiplying bhp x prop. efficiency Figure 5-2 p. 117 • The dotted line 200 horsepower, the max. bph of the 200 horsepower engine • Not all 200 horsepower overcame drag • The amount available is the thp, which is the bhp x prop efficiency • Prop efficiency is always less than one, so thp is always less than bhp Efficiency • The efficiency is usually greatest in cruise & decreases at lower speeds • Where the power available and the power required curves cross is the max level flight speed • Above that speed more power is required than is available Figure 5-3 p. 118 • Power curves showing max power available & 75% power • Sustaining the speed demands the required amount of power being delivered • Below that speed, increasingly more power is required to sustain level flight Back side of the power curve • The curve changes direction and curls up on the low speed side of the speed range • This effect is due to increased induced drag in this region & results in a reversed trend in power requirement with airspeed Back side of the power curve • The back side of the power curve is usually very short due to minimum power speed is low • Many airplanes stall before reaching that slow speed Climb Performance • The airplane’s ability to climb is also determined by the power curve • Excess power is not used in level flight but is used in a climb • Excess power is also called power differential Excess Power • The rate of climb at any speed is proportional to the amount of excess power • This is inversely proportional to weight • Figure 5-4 shows the power required curve & the maximum power available curve with max power Power Curves • • • • Figure 5-4 p. 120 Figure 5-5 p. 121 Figure 5-6 p. 122 Figure 5-7 p. 122 Vx & Vy • When do you use Vx & Vy? • How are Vx & Vy different? • Figure 5-8 p. 123 Terms • Calibrated Airspeed • Ceiling – No more excess power climbing ability is zero • Absolute Ceiling – Point where the curves touch & is required for level flight • Service Ceiling – Rate of climb is only 100fpm Power Curves • Figure 5-9 p. 124 • Figure 5-10 p. 125 • Figure 5-11 p. 125 Twin Engine Climb Performance • The power required is divided into two engines • Figure 5-13 p. 127 • Twin that looses 50% looses up to 80% of performance and up to 90% of climb rate • This is due to loss of power, asymmetric thrust adds to drag; climbing even harder Descent & Glide Performance • When there is a negative difference in power we get a negative climb rate • Sinking and the resulting vertical velocity is called rate of sink • Minimum rate of sink does not necessarily mean the best overall glide performance. – At a very low airspeed, the aircraft is moving slowly at minimum sink speed Glide • The maximum glide distance would be obtained at the minimum ratio of rate of sink to forward speed • This speed corresponds to the point were power & velocity touch • This is best glide speed – Figure 5-16 p. 130 Glide Ratio • The ratio of horizontal to vertical distance for a given amount of altitude • Glide distance= altitude x L/D Time & Distance • Range • Endurance • Cruise Efficiency Quiz on Lecture 10 Chapter 5 Please take out a sheet of paper Include today’s date & your name Quiz on Lecture 10 Chapter 5 • Describe the back side of the power curve. • Compare and contrast absolute ceiling and service ceiling. • How do you determine glide distance?