The stall speed increases with the square root of any load on the glider. For example, if the weight or load factor on the
glider doubles, the stall speed increases by the square root of 2 or 1.41. If a 540-pound glider has a stalling speed of 40
knots and the pilot adds 300 pounds of water ballast increasing the total weight to 840 pounds, the stall speed increases to
approximately 50 knots (40 x √(840/540)).
The speed that results in the lowest altitude loss over time, the minimum sink airspeed, also increases with weight. At any
given bank angle, a heavier glider uses higher airspeeds for efficiency, which result in larger diameter circles. The best
lift in thermals often occurs in a narrow cylinder near the core, and large diameter circles generally reduce the glider's
capability to exploit the strongest lift. [Figure 5-8]
Lightweight glider thermal circle
Heavy (ballasted) glider thermal cycle
Figure 5-8. Effect of added weight on thermaling turn radius.
Increasing the operating weight of a given glider not only increases the stall airspeed and minimum sink airspeed, it also
increases the best L/D airspeed [Figure 5-9]. Although a heavier glider sinks faster, it glides the same horizontal distance
(at a higher speed) as a lighter glider with the same glide ratio and starting altitude.
Operating Weight
800 pounds
1,200 pounds
1,600 pounds
Stall Airspeed
36 knots
44 knots
50 knots
Minimum Sink
48 knots
58 knots
68 knots
Best L/D Airspeed
60 knots
73 knots
83 knots
Figure 5-9. Effect of added weight on performance airspeeds.
Figure 5-9 above shows that increasing weight from 800 to 1,200 pounds increases the best L/D airspeed from 60 knots
to 73 knots. A glider with more weight can fly faster while maintaining the same lift-to-drag (L/D) ratio (glide ratio). The
advantage of the heavier weight becomes apparent during faster flight between thermals in sink. In strong lift the heavy
glider can climb reasonably well, and the advantage during the cruising portion of flight may outweigh the disadvantage
during climbs.
To fly faster with efficiency, some gliders have water tanks that allow the pilot to add weight as ballast. The pilot normally
jettisons the water ballast before entering the traffic pattern or earlier if conditions necessitate a higher climb rate. Reducing
the weight of the glider before landing allows the pilot to make a normal approach, normal landing, and reduces load on
the landing gear.
Rate of Climb
Rate of climb for a ground-launched glider depends on the power of the ground-launch equipment. With a powerful
winch or tow vehicle, rate of climb can exceed 2,000 feet per minute (fpm). For an aerotow, the rate of climb depends on
the power of the towplane. The towplane should have sufficient power to tow the glider safely, considering the existing
conditions, which include glider weight.
The rate of climb of self-launching gliders may vary from as low as 200 fpm to as much as 800 fpm or more. The
pilot should consult the GFM/POH to determine rate of climb under the existing conditions.
Flight Manuals & Placards
The GFM/POH provides the pilot with the necessary performance information to operate the glider safely. A GFM/POH
may include the following information:
• Description of glider primary components
• Glider assembly instructions
• Weight and balance data
• Description of glider systems
• Glider performance data
• Operating limitations
Placards
Placards attached to the glider provide the pilot with essential information for safe operation. The GFM/POH lists all the
required placards.
The amount of information that placards convey to the pilot increases as the complexity of the glider increases. High-
performance gliders may have wing flaps, retractable landing gear, a water ballast system, drogue chute for use in the
landing approach, and other features to enhance performance. [Figure 5-10]
Glider Self-Launching Glider
After inflight
engine shutdown,
feather propeller
DO NOT EXCEED
6,400
Engine rpm
OFF
EMERGENCY CANOPY
JETTISON
Minimum pilot seat weight—154 lb (70 kg)
Maximum pilot seat weight—264 lb (120 kg)
INTENTIONAL
SPINNING
PROHIBITED
Stall speed—35 knots
Maximum ground launch speed—74 knots
Maneuvering speed—86 knots
Maximum aerotow speed—94 knots
VNE—136 knots
Figure 5-10. Typical placards for gliders.
Performance Information
The GFM/POH provided by the manufacturer contains glider performance information. The GFM/POH lists specific
airspeeds such as stall speed, minimum sink airspeed, best L/D airspeed, maneuvering speed, rough airspeed, maximum
aerotow speed, maximum ground launch speed, and the never exceed speed (VNE). Some performance airspeeds apply only
to gliders with certain equipment. For instance, gliders with wing flaps have a maximum permitted flap extended airspeed
(VFE).
Manuals for self-launching gliders include performance information about powered operations. These include rate of
climb, engine and propeller limitations, fuel consumption, endurance, and cruise.
Glider Polars
The manufacturer provides information about the rate of sink in terms of airspeed summarized in a graph called a polar
curve, or simply a polar.
The vertical axis of a polar shows the sink rate (increasing sink downwards), while the horizontal axis shows airspeed in
the same units (knots). Every type of glider has a characteristic polar derived either from theoretical calculations or by
actual inflight measurement of the sink rate at different speeds. The polar of each individual glider varies (even from other
gliders of the same type) by a few percent depending on relative smoothness of the wing surface, the sealing around control
surfaces, and even the number of bugs on the wing’s leading edge. The polar forms the basis for speed to fly and strategies
discussed in Chapter 11, Cross-Country Soaring.
The peak of the blue sink rate curve determines minimum sink rate. [Figure 5-11] In this example, a minimum sink of 1.9
knots occurs at 40 knots. Note that the sink rate increases between minimum sink speed and the stall speed (the left end
point of the blue curve). A tangent from the origin to the polar indicates the best glide speed (best L/D). The best L/D speed
is 50 knots with a sink speed of 2.1 knots. The glide ratio at best L/D speed is determined by dividing the best L/D speed
by the sink rate at that speed, or 50/2.1, which is approximately 24 in this example. Thus, this glider has a best glide ratio
in calm air (no lift or sink and no headwind or tailwind) of 24:1 at 50 knots.
Sink rate (knots)
Airspeed (knots)
0 10 20 30 40 50 60 70 80 90 100 110 120 130
Sink rate (knots)
Airspeed (knots)
0 10 20 30 40 50 60 70 80 90 100 110 120 130
Sink rate (knots)
Airspeed (knots)
0 10 20 30 40 50 60 70 80 90 100 110 120 130
Dual (heavier)
Solo (lighter)
Minimum sink airspeed
40 knots
Best glide speed
Headwind change
L/D maximum
50 knots
{
Figure 5-11. Minimum sink airspeed and maximum L/D speed.
To determine the best speed to fly for distance over the ground in a headwind, the pilot can shift the origin to the right along
the horizontal axis by the speed of the headwind and draw a new tangent line to the polar. For tailwinds, the pilot shifts the
origin to the left of the zero mark on the horizontal axis.
Figure 5-12 shows an example for a 20-knot headwind. The new tangent indicates 60 knots as the best glide speed. By
repeating the procedure for different headwinds, the data show that flying faster as headwinds increase results in a greater
distance traveled over the ground. Analysis of the data from many gliders leads to the following general rule: the pilot can
add half the headwind component to the zero wind L/D to obtain maximum distance.
Sink rate (knots)
Airspeed (knots)
0 10 20 30 40 50 60 70 80 90 100 110 120 130
Sink rate (knots)
Airspeed (knots)
0 10 20 30 40 50 60 70 80 90 100 110 120 130
Sink rate (knots)
Airspeed (knots)
0 10 20 30 40 50 60 70 80 90 100 110 120 130
Dual (heavier)
Solo (lighter)
Minimum sink airspeed
40 knots
Best glide speed
Headwind change
L/D maximum
50 knots
{
Figure 5-12. Best speed to fly in a 20-knot headwind.
The speed to fly in a tailwind lies between minimum sink and best L/D, but never lower than minimum sink speed.
Sinking air often exists between thermals and flying faster than best L/D can result in less time in sinking air and an
increase in efficiency. The pilot can determine how much faster to fly using the glider polar, as illustrated in Figure 5-13 for
an airmass sinking at 3 knots between thermals. In this case, the pilot draws a tangent line to the polar that begins 3 knots
above the origin to read the best speed to fly, 60 knots in this case. Note, that in this situation, the variometer would show
a total sink of 5 knots (3 knots from sinking air and 2 knots for the constant aircraft descent) as highlighted in the figure.
Sink rate (knots) Lift rate (knots)
Airspeed (knots)
–1
–2
–3
–4
–5
–6
–7
10 20 30 40 50 60 70 80 90 100 110 120 130
Best speed to fly is 60 knots
Total sink
{
Figure 5-13. Best speed to fly in sinking air.
If the glider has water ballast, wing flaps, or wingtip extensions, the polar depicts the performance characteristics of the
glider in those configurations. [Figure 5-14, Figure 5-15, and Figure 5-16] Comparing the polar with and without ballast
shows that the minimum sink increases and occurs at a higher speed after adding weight. As a result, working weak
thermals becomes more challenging with ballast. In addition, ballast lowers the sink rate at higher speeds. [ Figure 5-14]
The best glide ratio remains the same, but it occurs at a higher speed. Note that as expected, the stall speed increases with
added ballast.
Sink rate (knots)
Airspeed (knots)
0 10 20 30 40 50 60 70 80 90 100 110 120 130
Notice higher airspeed with ballast for same sink rate
70 knots
50 knots
= performance without water ballast
= performance with 300 lb water ballast
Figure 5-14. Effect of water ballast on performance polar.
Flaps with a negative setting as opposed to a 0-degree setting during cruise also reduce the sink rate at higher speeds, as
shown in the polar. [ Figure 5-15] Therefore, when cruising at or above 70 knots, setting the flaps to -8° would provide
an advantage. The polar with flaps set at -8° does not extend to speeds lower than 70 knots since the negative flap setting
loses its advantage there.
Sink rate (knots)
Airspeed (knots)
0 10 20 30 40 50 60 70 80 90 100 110 120 130
With flaps set to –8 degrees, sink rate does not
change significantly but airspeed increases.
56 knots
85 knots
= performance, flaps set to 0°
= performance, flaps set to −8°
Figure 5-15. Performance polar with flaps at 0° and –8°.
Wingtip extensions also alter the polar data, as shown in Figure 5-16. The illustration shows that the additional 3 meters of
wingspan creates an advantage at all speeds. In some gliders, the low-speed performance improves with the tip extensions,
while high-speed performance diminishes slightly.
Sink rate (knots)
Airspeed (knots)
0 10 20 30 40 50 60 70 80 90 100 110 120 130
43 knots
40 knots
= 15-meter wingspan performance
= 18-meter wingspan performance
Figure 5-16. Performance polar with 15-meter and 18-meter wingspan configurations. (Matt please add tangent lines to intersect the
curves at the 43 and 40 knot annotated points.)
Limitations
Regardless of glider complexity, designers and manufacturers provide operating limitations to ensure the safety of flight.
The glider VG diagram provides the pilot with information on the design limitations, such as limiting airspeeds and load
factors (L.F. in Figure 5-17). Pilots should become familiar with all the operating limitations of each glider flown and
should not operate outside the limits. Figure 5-17 shows different possible limiting conditions and the basic flight envelope
for a high-performance glider.
N Limit wing load factor (“G” units)
V = Velocity (mph)
+6.0
+5.0
+4.0
+3.0
+2.0
+1.0
0.0
−1.0
−2.0
−3.0
−4.0
SGS 1-35 Basic Flight Envelope
High Performance Class
Cond 3 Pos. Gust L.F. = 3.55
4 Cond 4 Neg. Gust L.F. = 3.55
Cond 2 Neg. Max. L.F. = 2.67
Cond 1 Pos. Max. L.F. = 5.33
VG = 154 MPH
POSITIVE 24 FPS GUST FACTOR
NEGATIVE 24 FPS GUST FACTOR
0 20 40 60 80 100 120 140 160 180 200 220
L.F. – Load Factor
Neg. – Negative
Pos. – Positive
FPS – Feet per second
Figure 5-17. Sample glider flight envelope.