Best Lift-to-Drag Ratio (L/D) Speed Determination
Concept: The Best Lift-to-Drag Ratio (L/D) speed, often denoted as V(L/D)max, is the airspeed at which the aircraft achieves its maximum aerodynamic efficiency. Flying at this speed results in the minimum total drag for a given lift (equal to weight in level flight) or the shallowest glide angle when the engine is off.
Physical Origin: Total drag is the sum of parasitic drag (due to shape, skin friction, etc., increases roughly with speed squared) and induced drag (drag due to lift, decreases with speed squared). At low speeds, induced drag dominates. At high speeds, parasitic drag dominates. There is an intermediate speed where the sum of these two drag components is minimized. Since Lift equals Weight in steady level flight (or is proportional to Weight in a steady glide), minimizing total drag for a given lift is equivalent to maximizing the L/D ratio. The AoA at which L/D is maximized is related to the point where the parasitic drag equals the induced drag.
Flight Test Proposal (Glide Method):
Setup:
Select an aircraft model.
Configure mission for a high starting altitude (e.g., 2000m or higher) to allow for multiple glide segments.
Set a long data recording window (e.g., 0-300 seconds or more).
Assume constant weight for the duration of the test (or note starting weight).
Procedure (Series of Steady Glides):
Initial Trim: Trim for level flight slightly above the expected best L/D speed.
Thrust Idle/Off: Reduce thrust to idle (or zero if possible in the sim model).
Target Speed 1: Using pitch control ('A'/'Q' keys), establish a steady glide at a specific target Equivalent Airspeed (EAS). Let the altitude decrease naturally but keep the EAS constant. Start with a speed estimated to be near best L/D.
Stabilize & Record: Maintain this constant EAS glide for at least 30-60 seconds to ensure data stabilizes.
Target Speed 2: Without adding significant thrust (unless needed to reposition briefly), smoothly adjust pitch to stabilize at a different target EAS (e.g., 5 knots slower).
Stabilize & Record: Maintain this new constant EAS glide for 30-60 seconds.
Repeat: Continue this process, establishing steady glides at various EAS values, covering a range from near stall speed up to a typical cruise glide speed. Aim for 5-10 different speed points.
Recovery: After collecting data for all target speeds, recover the aircraft.
Recording: Ensure data is recorded throughout all glide segments.
Data Analysis:
Load Data: Use the visualization tool. If multiple runs were needed, load them sequentially or use the tool's comparison feature if appropriate.
Identify Steady Segments: For each target EAS glide, identify the time interval in the data where the EAS was constant and the VSI_ms (or glide angle) was also relatively constant.
Extract Average Values: For each steady segment:
Calculate the average CL_CD_ratio (this is the L/D ratio).
Note the corresponding average EAS for that segment.
Note the corresponding average alpha_DEG for that segment.
Plot L/D vs. Speed/AoA:
Create a plot of the average CL_CD_ratio (Y-axis) against the average EAS (X-axis).
(Optional) Create a plot of the average CL_CD_ratio (Y-axis) against the average alpha_DEG (X-axis).
Identify Best L/D:
Find the peak (maximum value) on the L/D vs. EAS plot. The EAS at this peak is the Best L/D Speed (V(L/D)max in EAS).
The corresponding alpha_DEG from the L/D vs. AoA plot (or looked up from the data at the peak L/D) is the angle of attack for Best L/D.
The maximum value of CL_CD_ratio itself is the aircraft's (L/D)max under these conditions.
References:
Aircraft performance sections in standard aerodynamics and flight dynamics textbooks.
Anderson, J. D. (2016). Fundamentals of Aerodynamics.
Dole, C. E., et al. (2017). Flight Theory and Aerodynamics: A Practical Guide for Operational Safety.
Pilot's Operating Handbook (POH) for real aircraft often list V(G) (Best Glide Speed, usually close to V(L/D)max) and sometimes (L/D)max itself.