Performance - Delta State University

Chapter 5
Lecture 10
• 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
• 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
• 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
• 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
• 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
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
• 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
• 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
• 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?

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