Transcript Document

Sailplane climb performance and
airfoil characteristics
L.M.M. Boermans
TU Delft, The Netherlands
Kernploeg dag
24 Januari 2009
July 7, 2015
Ad
, Faculty of Aerospace Engineering.
Vermelding onderdeel organisatie
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Presentation Layout:
•
•
•
•
•
•
Velocity distributions in thermals
Characteristics of typical airfoils
Dynamic behavior
Results in measured gusts
Conclusions
How to fly existing sailplanes in turbulent thermals
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Vertical velocity record of a thermal
traverse
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Measurements of gusts in thermals
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Wind tunnel model with wind vane
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Angle of attack and vane angle
Due to curvature of the flow in front of
the airfoil the angle indicated by the
vane differs from the angle of attack.
This has been calibrated in the
windtunnel.
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Conversion from vane angle to verical gust
speed
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Characteristics of typical airfoil
For explanation see
next sheet.
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Characteristics of typical airfoil
Consider the lift and drag curves at 20 degrees flap deflection in the
previous sheet.
When the angle of attack increases from 0 to 2.5 degrees, the flow starts
to separate from the end of the flap and the drag starts to increase.
When the angle of attack increases from 2.5 to 5.5 degrees, transition
moves from 65% chord to 20% chord and separation moves forward to
the beginning of the flap (85% chord). The loss of lift at the back is compensated by the gain in lift on the forward part of the airfoil (design goal).
When the angle of attack increases from 5.5 to 20 degrees, transition
moves forward from 20% chord to the leading edge and separation moves
forward from 85% chord to 20% chord. At the same time the lift first
increases up to the maximum lift coefficient and then decreases.
A loss of lift instead of a plateau, shown in the next sheet, causes bad
handling characteristics in thermals.
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Dip in lift curve
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Gust load factor n
av
1/ 2  ρ  U  V
n  1  K  C Lα
 1
W/S
g
av
K = gust alleviation factor (ca. 0.7)
C Lα = lift curve slope
ρ
= air density
U = gust velocity
V = flight speed
W/S = wing loading
a v = vertical acceleration
g = gravitational constant
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If the lift curve slope (change
of lift with angle of attack) is
zero (in plateau), there is no
vertical acceleration i.e. no
change in vertical movement.
This relation is derived for
horizontal flight, i.e. no sink
rate.
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Equation of motion
This is the general expression for
the vertical acceleration. The
change of angle of attack is equal
to the difference between the gust
speed and the sink rate, divided
by the flight speed.
The first term at the right hand
side is due to the change in lift
(essential) and the 3 remaining
terms are due to the change in
drag (negligible).
CL
ΔC DP
dVs
ΔC
1
 g  α  Δα  g 
 Δα  2g  L  Δα  g 
 Δα
L / Do
dt
C Lo
πA
C Lo
where
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Δα 
w g  Vs
The first term at the right hand
side is equal to the acceleration on
the previous sheet when the sink
rate is zero.
Vo
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Airfoil characteristics
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Measured gust distribution, T=30s
Typical airfoil, flight speed at CL=1.2
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Measured gust distribution, T=30s
Typical airfoil, flight speed at CL=1.3
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Measured gust distribution, T=30s
Typical airfoil, flight speed at CL=1.4
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Loss of altitude after T=30s
Typical airfoil, CL=1.2 – 1.4
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Measured gust distribution, T=30s
New airfoil, flight speed at CL=1.4
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Measured gust distribution, T=30s
Typical and new airfoil
Trajectory
Loss of altitude at 30s
Trajectory in 30s of typical and new airfoil Altitude loss after 30s depends on Cl (alfaflight speed at Cl=1.4
start) i.e. flight speed.
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Measured gust distribution, T=60s
Typical and new airfoil
Trajectory
Loss of altitude at 60s
Trajectory in 60s of typical and new airfoil Altitude loss after 60s depends on Cl (alfaflight speed at Cl=1.4
start) i.e. flight speed.
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Measured gust distribution, T=120s
Typical and new airfoil
Trajectory
Loss of altitude at 120s
Trajectory in 120s of typical and new airfoil, flight speed at Cl=1.4
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Altitude loss after 120s depends on Cl
(alfa-start) i.e. flight speed.
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Sink rate of due to dynamic effects
Alfa (deg)
0
0.5
1
1.5
2
2.5
-0.1
new airfoil
-0.2
60 s
120s
30s
sink rate (m/s)
-0.3
-0.4
-0.5
typical airfoil
-0.6
-0.7
-0.8
30s
60 s
120s
-0.9
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Improvement of climb rate with new
airfoil
Improvement of climb rate (m/s)
0.6
0.5
0.4
120 s
0.3
60 s
30 s
0.2
0.1
0
0.5
1
1.5
2
2.5
Alfa (deg)
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Comparison of performance
• At low speed, profile drag contributes 25% to total drag, hence a
profile drag reduction of 10% means a total drag reduction of
2.5%. At a sink speed of 0.8m/s in a 35 degr. turn this corresponds to a reduction in sink speed of only 0.02 m/s.
• A slight gradient instead of a plateau in the lift curve reduces the
sink speed (i.e. increases the climb speed) in the measured 30s,
60s and 120s vertical velocity distributions up to about 0.5 m/s.
• This shows that in dynamic thermal flight conditions the lift
properties are far more important than the drag properties.
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Conclusions
•
Current sailplane airfoils with flaps have a plateau in the lift
curve.
•
Flying in this plateau generates no lifting-up effect in
upgusts, and flying below this plateau generates a pushingdown effect in downgusts; in total the climb performance
deteriorate.
•
New airfoils have been designed that have a slight positive
gradient instead of a plateau in the lift curve.
•
This slight positive lift gradient appreciably improves the
climb performance.
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How to fly existing sailplanes in turbulent
thermals ?
•
The mean vertical air velocity in a thermal determines the
steady climb rate of the sailplane.
•
The variations of the vertical air velocity and shape of the lift
curve determine the dynamic behavior of the sailplane.
•
If the angle of attack increases due to an upgust and the lift
increases, the sailplane is lifted up.
•
If the angle of attack decreases due to a downgust and the lift
decreases, the sailplane is pushed down.
•
If the angle of attack changes due to up- or downgusts and the
lift does not change, the sailplane persists in its movement.
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ASW-15, ASW-19
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No problem.
27
St. Cirrus
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No problem.
28
St. Libelle. Similar: LS-1. No problem.
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DG-100.
Similar: Astir, ASW-19X
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Problems !
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ASW-24
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Problems !
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ASW-22, ASH-25
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No problem.
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Mini-Nimbus, Mosquito
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No problem.
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LS-3, DG-200,
DG-400, PIK-20
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No problem.
34
LS-8, ASW-28
No problem.
Discus 1 and 2
Airfoils unknown.
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DG-800, ASH-26, ASW-27, ASG-29,
Ventus-2
Problems.
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Fig. 19 Measured gust distribution, T=30s
Typical and new airfoil
Trajectory
Loss of altitude at 30s
Trajectory in 30s of typical and new airfoil Altitude loss after 30s depends on Cl (alfaflight speed at Cl=1.4
start) i.e. flight speed.
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Trajectory at measured gust distribution, T=30s
Typical airfoil is lifted up in upgusts and not pushed down in
downgusts. New airfoil is lifted up
in upgusts but less than pushed
down in downgusts.
1.8
1.6
1.4
1.2
1
CL
Typical airfoil is not lifted up in
upgusts and pushed down in
downgusts. New airfoil is lifted up
in upgusts but less than pushed
down in downgusts.
0.8
0.6
0.4
0.2
0
-10
0
10
20
alfa
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Typical airfoil and new airfoil (30s case)
1.8
3
4
1.6
2
1.4
3
1.2
1
1
CL
1
4
0.8
2
0.6
0.4
95
95
90
90
86
86
2
84
86
0.2
82
86
81.5
83.5
0
-10
0
10
20
alfa
Altitude loss after 30s depends on Cl (alfa-nul) i.e. flight
speed.
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Estimation of climb rate, typical airfoil
-
Alfa = 2.5 degr. (start of plateau)
Thermal air velocity
3.8 m/s
Steady rate of sink
- 0.8 m/s
Dynamic rate of sink 19m in 30 s
- 0.63 m/s
Climb rate
-
(ASW-19 climbed 3m/s in
tests, rate of sink -0.8m/s)
2.37 m/s
Alfa = 7.5 degr. (end of plateau)
Thermal air velocity
3.8 m/s
Steady rate of sink (estimated)
Dynamic rate of climb 12m in 30 s
Climb rate
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- 1.6 m/s
0.4 m/s
2.6 m/s
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How to fly existing sailplanes in turbulent
thermals ?
•
That depends primarily on the shape of the lift curve.
•
There is no problem for older sailplanes without and with
flaps, except in those cases that the maximum lift is
constant over several degrees (DG-100, Astir, ASW-19X,
ASW-24). Stay away from that maximum lift plateau by
flying at a lower lift coefficient i.e. faster.
•
There is no problem for recent Standard Class sailplanes
LS-8 and ASW-28. Discus 1 and 2 airfoils are unknown.
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•
All recent sailplanes with flaps (ASW-22, ASH-25, ASH-26,
ASW-27, ASG-29, Ventus-2, DG-800) have a plateau in the
lift curve at flap deflections for climbing. For Nimbus versions
the airfoils are unknown but probably similar. Stay away from
this plateau by flying at a lower lift coefficient i.e. faster.
•
What about trying to fly slower, at the end of the plateau?
Since Cl at the worst situation (start of plateau) and best
situation (end of plateau) is the same, the flight speed is the
same, see speed scale for typical airfoil in fig. 39. The speed
margin for improved climb rate at the end of plateau is
extremely small and close to stall, hence probably impossible
to apply in turbulent thermals. Try it yourself, carefully!
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Thank you
Fly safe
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