Lift-to-Drag Ratio
Lift-to-drag ratio (L/D) is the ratio of lift force to total drag force at a given flight condition. It is the single most informative measure of aerodynamic efficiency: an aircraft with L/D of 20 generates 20 units of lift for every unit of drag, meaning it can glide 20 meters forward for every meter of altitude lost (hence the alias “glide ratio”).
L/D varies with airspeed and angle of attack. Maximum L/D occurs at the speed where induced drag equals parasitic drag — the minimum-drag speed. Flying faster or slower than this speed reduces L/D.
Representative maximum L/D values across the UAV spectrum:
| Platform | Max L/D | Notes |
|---|---|---|
| Indoor micro UAV | 4–6 | Low Re, high parasitic drag relative to size |
| Small quadcopter (in forward flight) | 3–5 | Rotors are inefficient in cruise |
| Small fixed-wing (2 kg, AR 6) | 8–12 | Limited by low Reynolds number |
| Tactical UAV (50 kg, AR 10) | 12–18 | Conventional planform, moderate Re |
| Shahed-class delta (AR 2) | 5–8 | Low AR drives high induced drag |
| MQ-9 Reaper (AR 17) | 18–22 | High AR, turbulent Re, clean airframe |
| RQ-4 Global Hawk (AR 25) | 28–35 | Sailplane-class efficiency at altitude |
These numbers illustrate why planform selection matters: the Global Hawk achieves 4–5× the L/D of a Shahed-class drone, translating directly to 4–5× the range for the same fuel fraction. The Shahed accepts this penalty because L/D is not its binding constraint — cost is.
For range estimation, the Breguet range equation relates L/D directly to distance: range is proportional to L/D × ln(W_initial / W_final), where the weight ratio reflects fuel consumed. Every 10% improvement in L/D yields roughly 10% more range.
Related terms
- Induced Drag — the component that dominates at low speed and low aspect ratio
- Parasitic Drag — the component that dominates at high speed
- Aspect Ratio — the geometric parameter most directly controlling maximum L/D