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Some self-launch gliders are designed for extended
periods of powered cruising flight. For these selflaunch
gliders, maximum range (distance) for powered
flight and maximum duration (elapsed time aloft) for
powered flight is primarily limited by the self-launch
glider’s fuel capacity. Wind has no effect on flight duration
but does have a significant effect on range. During
powered cruising flight, a headwind reduces range, and
a tailwind increases range. The Glider Flight
Manual/Pilot’s Operating Handbook (GFM/POH) provides
recommended airspeeds and power settings to
maximize range when flying in no-win, headwind, or
tailwind conditions.
WEIGHT
In gliding, increased weight decreases takeoff and
climb performance, but increases high speed cruise
performance. During launch, a heavy glider takes
longer to accelerate to flying speed. The heavy glider
has more inertia making it more difficult to accelerate
the mass of a glider to flying speed. After takeoff, the
heavier glider takes longer to climb out because the
heavier glider has more mass to lift to altitude than
does the lighter glider (whether ground launch, aerotow
launch, or self-launch). [Figure 5-5]
The heavy glider has a higher stall speed and a higher
minimum controllable airspeed than an otherwise
identical, but lighter, glider. The stall speed of a glider
increases with the square root of the increase in
weight. If weight of the glider is doubled (multiplied
by 2.0), then the stall speed increases by more than 40
percent (1.41 is the approximate square root of 2; 1.41
times the old stall speed results in the new stall speed
at the heavier weight).
When circling in thermals to climb, the heavy glider is
at a disadvantage relative to the light glider. The
increased weight of the heavy glider means stall airspeed
and minimum sink airspeed is faster than they
would be if the glider were operating at a light weight.
At any given bank angle, the heavy glider’s faster airspeeds
mean the pilot must fly larger diameter thermaling
circles than the pilot of the light glider. Since the
best lift in thermals is often found in a narrow cylinder
near the core of the thermal, larger diameter circles
generally mean the heavy glider is unable to exploit
the strong lift of the thermal core, as well as the slower,
lightweight glider. This results in the heavy glider’s
inability to climb as fast in a thermal as the light glider.
[Figure 5-6]
The heavy glider can fly faster than the light glider
while maintaining the same glide ratio as the light
glider. The advantage of the heavier weight becomes
apparent during cruising flight. The heavy glider can
Figure 5-4. Wind effect on final approach and landing
distance.
Figure 5-5. Effect of weight on takeoff distance and
climbout rate and angle.
5-5
fly faster than the light glider and still retains the same
lift/drag (L/D) ratio.
If the operating weight of a given glider is increased,
the stall airspeed, the minimum controllable airspeed,
the minimum sink airspeed, and the best L/D airspeed
will be increased by a factor equal to the square root of
the increase in weight. [Figure 5-7]
The addition of ballast to increase weight allows the
glider to fly at faster airspeeds while maintaining its
L/D ratio. The table in Figure 5-7 shows that adding
400 pounds of water ballast increases the best L/D airspeed
from 60 knots to 73 knots. The heavy glider will
have more difficulty climbing in thermals than the light
glider, but if lift is strong enough for the heavy glider to
climb reasonably well, the heavy glider’s advantage
during the cruising portion of flight will outweigh the
heavy glider’s disadvantage during climbs.
Water is often used as ballast to increase the weight of
the glider. However, the increased weight will require
a higher airspeed during the approach and a longer
landing roll. Once the cross-country phase is completed,
the water ballast serves no further purpose. The
pilot should jettison the water ballast prior to entering
the traffic pattern. Reducing the weight of the glider
prior to landing allows the pilot to make a normal
approach and landing. The lighter landing weight also
reduces the loads that the landing gear of the glider
must support.
RATE OF CLIMB
Rate of climb for the ground-launched glider primarily
depends on the strength of the ground-launch equipment.
When ground launching, rates-of-climb generally
are quite rapid, and can exceed 2,000 feet per
minute if the winch or tow vehicle is very powerful.
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Glider Flying Handbook(46)
