I hope that this, concerned only with staying level or going down, may be of value.
There are three basic speeds defined in level flight.
The lowest is Vmp, where the minimum power is needed to keep the aircraft flying. This is not Vs. It is a little bit above Vs, where it is difficult to fly, as you are on the wrong side of the drag curve.
The next highest is Vmd, where the drag is minimum. Remember, this is in level flight, so the thrust required to keep the aircraft flying is also minimum
The third is the speed that gives the best Speed/drag ratio. This would be 1.32 times Vmd if the theory is working right, but in real life it could be rather more or less.
If you are gliding, and there is no power or thrust delivery to worry about, then these speeds, based only on the aircraft aerodynamics, apply to pistons, jets, and gliders. To get the minimum sink rate in the glide, fly at Vmp. This will allow you to stay up longer – gliding for endurance. If you are heavy you will sink faster and vice versa. If you want to glide as far as you can then fly at Vmd. This gives you the angle of attack for best lift/drag ratio, and your best glide angle is, in fact, exactly your lift/drag ratio and as long as you fly the correct speed is not affected by aircraft weight. This is gliding for range.
These speeds are defined in level flight, and, as lift required equals weight, speeds (not best glide angle) vary as the weight of the aircraft changes. They are faster for a heavy aircraft, slower for a light aircraft. When you are gliding lift no longer equals weight. It is now the lower value, W x Cosine of the glide angle. This makes no practical difference up to glide angles of about 15deg, but in theory, you should fly in the glide at slightly lower speeds than the level flight quoted values.
Now, if you are under power, holding level flight, it makes a difference what type of power delivery you have. In piston engines, approximately, fuel flow is proportional to the Engine Horsepower (EHP) delivered. So, to use minimum fuel you would fly at the minimum power required speed, Vmp. In practice it is so difficult to do this that your Flight Manual will probably suggest a higher speed. In jet engines, fuel flow is approximately proportional to thrust delivered. In this case, to stay up longest you would fly at the minimum thrust required speed, minimum drag speed, Vmd. Again, to make things easier for you your Flight Manual may recommend a higher value
A similar difference applies if you want maximum range in level flight. Best speed/power required works out as Vmd, so Vmd is range speed for piston aircraft. Best speed/thrust required works out at 1.32Vmd and is jet range speed. For various practical reasons, including increasing engine efficiency at higher speeds, your Flight Manual may quote higher speeds, and at altitude, transonic drag problems will define range speed at a different value.
Glider heads will recognise that at the beginning of the day, with lift all over the place, they will take off with full ballast tanks. Vmd will be high, but best glide angle is unchanged, so they transit rapidly from Cu to Cu. As the lift dies, they will dump the ballast to lighten ship and fly for endurance to stay up that last minute longer.
Dick W