I'm digging into long term memory from my aero engineering days at E-RAU, so bear with me...
If you look at a plot induced drag (drag due to production of lift) and parasitic drag (pretty much all other drag, for low speed aircraft where shock and wave drag don't apply) you'll see:
1) Induced drag decreasing as Angle of Attack (AoA) decreases and airspeed increases;
2) Parasitic drag increasing as airspeed increases (and it's a square function with 2 times as much speed creating 4 times as much parasitic drag); and
3) a U shaped curve for total drag, with the bottom of the "U" where the Induced and parasitic drag curves cross, representing the airspeed where you have minimum drag.
That's the area on the curve with minimum drag. Since drag equals thrust in stable flight, it's also the airspeed where you need minimum thrust to stay airborne. In piston engined aircraft, that's also the point where you need minimum power and get minimum fuel burn (it's more complicated than that in turbine powered aircraft, where minimum fuel burn usually occurs at a higher airspeed for reasons we won't get into).
However, when the engine quits, or in a glider where there was no on board power in the first place, the "power" is produced by using gravity to convert potential energy (altitude) into kinetic energy (airspeed) to balance the drag. That means the minimum drag airspeed is also the airspeed where you need minimum loss of altitude per unit of time to maintain airspeed, and thus it's the "minimum sink" airspeed.
Unfortunately, I'm seeing this see this version of the chart in several sources while prepping for my Commercial written. This chart, and all the others like it, refer to that point where the drag curves cross as the "best glide speed" and "L/D max". That's at best over simplified.
L/D max is exactly what it says it is - the point on a plot of coefficient of lift (Cl) and coefficient of drag (Cd) versus airspeed where the ratio (the distance between the 2 curves) between Cl and Cd is at its maximum. That point on the graph is the "L/D max" or "best glide ratio speed" and the L/D max occurs at a specific AoA. The airspeed where it occurs will vary based on the weight of the aircraft. That's why you'll see differences in L/D max / best glide speed between a Schweizer 1-26A and a 1-26E. The E model is heavier, and achieves that optimum L/D max AoA at a slightly higher speed to reflect the slightly higher weight. That's also why they use ballast in higher performance gliders in strong lift conditions. Ballast increases the weight of the glider lets the glider achieve the best L/D AoA at a higher airspeed, without having to pitch down as much to achieve it. That improves penetration for the glider.
More importantly, the best L/D speed / best glide speed is almost always higher than the minimum sink speed, which occurs fairly close to the stall speed. These graphs do a better job of showing the differences between what's common for turbojet aircraft and what's common for propeller driven aircraft:
In the above graph, you'll note the sharp increases in parasitic drag (more or less a square function), versus the steep decline in induced drag in a jet, and the more gradual decline in the induced drag curve for a propeller driven aircraft. That makes the first graph shown above, and the one everyone (including the FAA) focuses on in the commercial written more emblematic of a jet aircraft, not a propeller aircraft.
In short, using the single graph posted first, and referring to the cross points for induced and parasitic drag as the "best glide speed" is flawed as:
1) it is only considering airspeed and the effect on drag production, not AoA and Cl in addition to Cd; and
2) it only approximates the situation with jet aircraft.
The second graph, does a much better job of showing differences in the induced drag curve, and the relative positions of the "minimum sink" and the "L/D max" / "best glide" speeds.
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Determining the best angle (Vx) and best rate of climb (Vy) speeds adds another wrinkle. Climb is the result of excess power, so power available (Pa) versus power required (Pr) is what's important.
Usually, Vy airspeed occurs fairly close to the "L/D Max" / "best glide speed". However it actually occurs at a point on a plot of the Pa and Pr curves versus airspeed, where the difference between power available and power required is greatest.
However, how the aircraft is propped plays a big role. If you have a climb prop where the flatter pitch allows the engine to achieve maximum horsepower at a lower speed, that moves the hump on the Pa curve back to a lower airspeed, and that then changes the airspeed where maximum difference between Pa and Pr occurs.
The maximum range speed is also normally found near the speed for "L/D max" / "best glide" where you have the most efficient AoA, but actual airspeed and range is again dependent on altitude and aircraft weight and again is dependent on how the aircraft is propped. Ideally for maximum range, the engine will be developing the required cruise power at a low rpm and comparatively high manifold pressure. If it has to turn at high rpm and low manifold pressure to attain the speed needed for that most efficient long range AoA, the increased L/D efficiency will be offset by poor fuel efficiency.
Best angle of climb is usually found close to the minimum sink speed, where the drag curves cross and the power required is at a minimum, giving a maximum angle of climb due to the slower airspeed where this occurs. If all you wanted was maximum climb, you'd prop the aircraft to achieve maximum rated power at this airspeed. It would climb like a rocket, but the engine would overspeed before you reached a normal cruise speed, so you'd have to fly slower in cruise and suffer poor fuel economy. There's no free lunch and fixed pitch props are always a compromise.
In summary then, from lowest to highest, you'll almost always find your:
- Vso
- Vs
- minimum sink and close to it Vx
- best glide and close to it Vy
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All that said, the short version is that there are a lot of variables and the best way to know your best glide speed and best rate of climb speed is to flight test in your own aircraft.
. Two shots of her panel, Im pretty proud.
