FLSC 2006 Safety Seminar
IMPACT OF WIND AND SINK
ON GLIDER PERFORMANCE*
Doug Cline
*Based on topic suggested by Tom Roberts
Polars for 1-26 and ASK 21
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Figure 1 shows the polars for the Schweizer 1-26 and ASK21 taken from the evaluations by Dick Johnson.
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The 1-26 assumes the minimum mass pilot leading to a
wing loading of 3.06 lbs/sq ft.
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The ASK-21 shows 574KS at minimum pilot weight and
maximum weight with corresponding wing loadings of
5.39 lbs/sqft and 6.84 lbs/sqft respectively.
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Glider wing loadings range from 3 lbs/sq ft to 11 lbs/sqft
for racing glider.
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The 1-26 has a maximum L/D of 21.6 at 37kt
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The ASK21 has a best L/D of 31.7 at 48kts (light) and 56kts
(heavy)
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The ASK21 best performance occurs at higher speeds due
to the higher wing loading since the polar plot, of forward
speed versus vertical speed, scales as the square root of
the wing loading. Jets have wing loadings around
100lbs/sq ft shifting the polar by a factor of about 4 to the
right and downward.
•
The ASK21 has about a 50% better L/D compared to a 1-26
GLIDING PERFORMANCE FOR 2KT THERMALS
FOR ZERO WIND AND SINK
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The average gliding speed with
respect to the parcel of air is given by
the intercept on the x-axis of the
tangent to the polar from the 2kt lift
point
2kt overall climb rate corresponds to
instantaneous climb rates of up to
4kt.
The average cross country speed of
the 1-26 is 20kt if it cruises at 46kts
The average cross country speed of
the ASK-12 is 30kt and 32kt flying at
the two wing loadings cruising
speeds of 60 (68) kts.
Like the polars, the average cruise
speed of the ASK-21 is roughly 50%
better than the 1-26.
GLIDING PERFORMANCE FOR 2KT THERMALS
WITH 20kt HEADWIND, ZERO SINK
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The average gliding speed with respect to
the ground is obtained by subtracting
20kts from the airspeed shown on the x
axis
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The 1-26 makes zero headway against a
20kt headwind for 2 kt lift whereas the
ASK21 averages 10kts ground speed
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The best L/D for the 1-26 drops to 10.8 at
46 kts
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The best L/D for the ASK 21 drops to 18
(21) at 55 (59) kts.
•
The headwind accentuates the difference
in performance, that is the ASK21 now is
100% better than the 1-26
GLIDING PERFORMANCE WITH 40kt
HEADWIND, ZERO SINK
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The average gliding speed with respect to
the ground is obtained by subtracting
40kts from the x axis.
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The best L/D for the 1-26 drops to 4.6 at 65
kts
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The best L/D for the ASK 21 drops to 10
(12) at 74 (76) kts.
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Note the best L/D speed to fly is increased
by roughly half the wind speed
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CONCLUSION:
A headwind greatly accentuates the
difference in performance, that is the
penetration of a ASK21 now is 160% better
than the low wing-loading 1-26
GLIDING PERFORMANCE WITH 10kt SINK,
NO WIND
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With a 10kt sink (1000ft/min) the L/D of
the 1-26 drops to 4.2 at 67 kts.
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The best L/D for the ASK 21 drops to
5.8 (6.3) at 90 (97) kts. This is
equivalent to using full dive brakes.
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CONCLUSION:
Sink greatly decreases the L/D with
the greatest impact on the higher
performance ship. They all have the
glide ratio of the Space Shuttle
GLIDING PERFORMANCE WITH 10kt SINK
PLUS 40kt HEADWIND
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The best L/D of the 1-26 drops to 2.0
at 79kts
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The best L/D for the ASK 21 drops to
3.5 (3.9) at 109 (114) kts.
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CONCLUSION:
All ships soar like bricks under these
conditions.
(By comparison with maximum dive
brake plus flaps the Jantar has a
worst L/D of 14 and Pegasus a worst
value of 5 which seems very steep)
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HYPOTHETICAL RIDGE DAY WITH 40KT WIND
Drift back from lift into sink in lee of hill.
• To recover the 1-26 has to dive 60 ft to gain the best penetration
speed of 80kts in order to penetrate against the wind
• To recover the ASK21 has to dive 131ft to gain the optimum
penetration speed of 110kts
• To penetrate 500m upwind to lift requires altitude clearance above
top of hill of >420ft for the 1-26 and >200ft for the ASK21
CONCLUSION:
ASK21 has a much better chance of penetrating across the ridge to
reach the lift if there is sufficient altitude to clear the top of the hill
40KT WIND SHEAR
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For a constant low ground speed the airspeed drops with altitude
leading to stall at 100ft if the ship does not accelerate
Slowing the airspeed to try to extend the glide exacerbates the
problem
Dive 60ft will provide sufficient ground speed to maintain airspeed
throughout the wind shear
Diving to accelerate also will enable the ship to penetrate further at
lower altitudes where the headwind is lower.
DIFFERENT BEHAVIOUR AT LOW AND HIGH
WING LOADING
Low wing loading: 1-26, 1-33
Advantages: Forgiving and safe flying qualities, low airspeeds allow high climb
rates in thermals. Can land on a dime using a slip.
Disadvantages: Penetration against headwind is very poor leading to poor cross
country capabilities
High wing loading: ASK 21, Grob 102
Advantages: Excellent penetration against head winds. Cross country flight more
viable
Disadvantages: High kinetic energy requires longer landing distances
Transitions between high and low wing loading:
Very different flight characteristics requires great care when transitioning
students taught in the ASK21 to the 1-26. Invariably the new student will land
the 1-26 off field if not careful. Fortunately the 1-26 is easy to land off field.
The former teaching ship sequence [ 2-33 to 1-26 to ASK21 to Russia]
produced less surprises than the [ASK21 to 1-26 to Russia] sequence.
SUMMARY
• Wind and sink have an enormous impact on
glider performance
• Large difference in flight characteristics
between low and high wing loading ships
requires caution. They require different
flying techniques.
• Always conserve total energy; extra altitude
and airspeed are money in the bank