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 10Chapter 5

Please take out a sheet of paper

Include today’s date & your name

Quiz on Lecture 10Chapter 5

• Describe the back side of the power curve.

• Compare and contrast absolute ceiling and service ceiling.

• How do you determine glide distance?