Glide Performance (flight-mechanics/performance/glide-performance)
Use when the task is unpowered glide analysis: glide ratio,
descent angle, sink rate, best glide speed, and time to descend
for a fixed-wing aircraft.
Domain quick reference
- Glide ratio: L/D, the lift over the drag in steady unpowered
flight, also equal to the horizontal distance covered per unit
altitude lost (E = L/D = horizontal / altitude_lost).
- Descent angle: gamma = atan(1 / (L/D)) in degrees; for small
angles gamma ~= asin(1 / (L/D)), the difference is negligible
below about 15 deg.
- Sink rate: V_sink = V * sin(gamma), with true airspeed V in m/s
and gamma in radians inside the sine.
- Time to descend: t = altitude_loss / V_sink, in seconds, for a
constant sink rate over the altitude loss in meters.
- Best glide speed: the speed that maximizes L/D (minimum sink
angle); scaled from a reference condition as
v_best = v_ref * sqrt((L/D)_max / (L/D)_ref).
- Units are SI throughout: lift L and drag D in newtons (N) (any
consistent force unit works for the ratio), speeds in m/s,
angles in degrees, altitude in meters (m), time in seconds (s).
- Glide performance sits in the FAR-25 / CS-25 context for
emergency descent and dead-stick landing considerations; the
mathematics here is standard flight mechanics.
Workflow
- Collect the lift and drag, or the horizontal distance and the
altitude lost, for the unpowered condition.
- Compute the glide ratio with glide_ratio; verify it is
positive before trusting it.
- Convert it to the descent angle with descent_angle.
- Compute the sink rate with sink_rate from the true airspeed
and the descent angle.
- Estimate the time to descend with time_to_descend from the
altitude loss and the sink rate.
- Locate the best glide speed with best_glide_speed from a
reference speed and the maximum lift to drag ratio.
- Report the glide ratio, angle, sink rate, time, and best
glide speed together for the glide assessment.
Pitfalls
- Mixing degrees and radians: descent_angle returns degrees and
sink_rate expects degrees; the sine conversion happens inside
the function, so never pre-convert.
- Feeding drag of zero: the glide ratio is undefined at D = 0;
a zero or negative drag must raise, not divide by zero.
- Using the airspeed instead of the sink rate for time to
descend: t = altitude_loss / V_sink, not altitude_loss / V.
- Forgetting the square root in best_glide_speed: the speed
scales with sqrt((L/D)_max / (L/D)_ref), not with the ratio
itself.
- Treating the descent angle as exact at steep angles: atan is
exact; the asin small-angle form is only an approximation.
- A sink rate of zero or less has no glide: the aircraft is not
descending, so time to descend is undefined.
Behavior contract (gate 3)
The glide ratio, descent angle, sink rate, best glide speed, and
time to descend logic is exercised by the gate 3 contract test:
scripts/test_glide_performance.py against
scripts/glide_performance_logic.py (stdlib unittest, offline).
Run:
python3 scripts/test_glide_performance.py
Compliance
- Standards referenced, not reproduced: FAR-25 is US government
work (public domain) and CS-25 is a free EASA download; glide
performance from lift and drag is common flight-mechanics
methodology, summary-only per standards-map.yaml.
- compliance: STANDARDS-REF, gated: false.
1---2name: glide-performance3description: Use when you must compute unpowered glide performance for a fixed-wing aircraft: derive the glide ratio from the lift and drag, convert it into the descent angle, find the sink rate from the airspeed and the descent angle, estimate the time to descend through a given altitude loss, and locate the best glide speed for the maximum lift to drag ratio. Produces the glide ratio, descent angle in degrees, sink rate in m/s, time to descend in seconds, and the best glide speed that gate the glide performance assessment. Trigger: glide ratio, sink rate, best glide speed, descent angle, time to descend, lift to drag.4license: Apache-2.05---67# Glide Performance (flight-mechanics/performance/glide-performance)89Use when the task is unpowered glide analysis: glide ratio,10descent angle, sink rate, best glide speed, and time to descend11for a fixed-wing aircraft.1213## Domain quick reference1415- Glide ratio: L/D, the lift over the drag in steady unpowered16 flight, also equal to the horizontal distance covered per unit17 altitude lost (E = L/D = horizontal / altitude_lost).18- Descent angle: gamma = atan(1 / (L/D)) in degrees; for small19 angles gamma ~= asin(1 / (L/D)), the difference is negligible20 below about 15 deg.21- Sink rate: V_sink = V * sin(gamma), with true airspeed V in m/s22 and gamma in radians inside the sine.23- Time to descend: t = altitude_loss / V_sink, in seconds, for a24 constant sink rate over the altitude loss in meters.25- Best glide speed: the speed that maximizes L/D (minimum sink26 angle); scaled from a reference condition as27 v_best = v_ref * sqrt((L/D)_max / (L/D)_ref).28- Units are SI throughout: lift L and drag D in newtons (N) (any29 consistent force unit works for the ratio), speeds in m/s,30 angles in degrees, altitude in meters (m), time in seconds (s).31- Glide performance sits in the FAR-25 / CS-25 context for32 emergency descent and dead-stick landing considerations; the33 mathematics here is standard flight mechanics.3435## Workflow36371. Collect the lift and drag, or the horizontal distance and the38 altitude lost, for the unpowered condition.392. Compute the glide ratio with glide_ratio; verify it is40 positive before trusting it.413. Convert it to the descent angle with descent_angle.424. Compute the sink rate with sink_rate from the true airspeed43 and the descent angle.445. Estimate the time to descend with time_to_descend from the45 altitude loss and the sink rate.466. Locate the best glide speed with best_glide_speed from a47 reference speed and the maximum lift to drag ratio.487. Report the glide ratio, angle, sink rate, time, and best49 glide speed together for the glide assessment.5051## Pitfalls5253- Mixing degrees and radians: descent_angle returns degrees and54 sink_rate expects degrees; the sine conversion happens inside55 the function, so never pre-convert.56- Feeding drag of zero: the glide ratio is undefined at D = 0;57 a zero or negative drag must raise, not divide by zero.58- Using the airspeed instead of the sink rate for time to59 descend: t = altitude_loss / V_sink, not altitude_loss / V.60- Forgetting the square root in best_glide_speed: the speed61 scales with sqrt((L/D)_max / (L/D)_ref), not with the ratio62 itself.63- Treating the descent angle as exact at steep angles: atan is64 exact; the asin small-angle form is only an approximation.65- A sink rate of zero or less has no glide: the aircraft is not66 descending, so time to descend is undefined.6768## Behavior contract (gate 3)6970The glide ratio, descent angle, sink rate, best glide speed, and71time to descend logic is exercised by the gate 3 contract test:72scripts/test_glide_performance.py against73scripts/glide_performance_logic.py (stdlib unittest, offline).74Run:75python3 scripts/test_glide_performance.py7677## Compliance7879- Standards referenced, not reproduced: FAR-25 is US government80 work (public domain) and CS-25 is a free EASA download; glide81 performance from lift and drag is common flight-mechanics82 methodology, summary-only per standards-map.yaml.83- compliance: STANDARDS-REF, gated: false.
ashfordeOU