Technical flight question: Best Glide=Max Endurance?

Factual Questions
1

Picky technical question, so those of you who know more about the theorectical parts of flight and/or airplane design please help me out:

It is my understanding that for either best glide speed without a working engine, or the maximum endurance airspeed with engine(s) working, you want to hit the magic point where the lift-to-drag ratio is at the minimum.

So - are those two airspeeds equal? Equal all the time? Some of the time? Never? Does having a working engine (or more than one, even) affect at what speed you find the L/D minimum?

If this doesn’t make much sense let me know and I’ll try to rephrase.

Would you believe I’ve wondered about this off and on for over a year?

2

No - there are two different airspeeds here. The first is the one for minimum energy expended per unit time. This is the speed that will give maximum endurance - the longest time in the air. Another speed is that which gives maximum range. This is the speed for best lift/drag. For a fixed-wing heavier than air machine, it will always be somewhat greater than the speed for maximum endurance. If you fly this speed, you will hit the ground sooner, but at a substantially further distance through the air from your starting point.

For unpowered aircraft - gliders - these speeds go by the name of min-sink speed (the speed at which the altitude lost per minute is lowest) and best glide speed (the speed at which height is most efficiently converted to distance).

As I understand it, never. But for very low-performance aircraft, they will be quite close.

To a first approximation, no. There may be subtle effects that cause the status of the engine to have some effect - perhaps at certain speeds an inoperable engine has a big effect on the efficiency of an airfoil it its wake.

3

Ah, so that’s where my confusion came in - I doubt I’ve ever flown even a “medium” performance aircraft!

Thanks for the answer, and this post is half a >bump< in case anyone else wants to weigh in

4

Broomstick,

Xema hit the engineering pretty well. The pilots-eye view of why is this:

For unpowered best glide, you’re just looking at aerodynamic efficiency. Fly the speed that does the best job of converting gravitational potential energy (ie altitude) into lift. As you’d expect, that speed is L/D[sub]max[/sub].

Best powered endurance adds the critical issue of engine fuel effciency to the above. So you’re going to get a different answer since you have this new issue in your equations.

In particular, engines do not convert fuel into thrust/horsepower with equal efficiency at all power settings. The airplane I fly burns 400% more fuel at cruise than it does at idle, but produces 1200% more thrust.

IOW, idle is a very INefficient power setting. As a rule of thumb, straight turbines are most efficient at about 85% of max thrust, with a shallow degradation fromthere towards the high-thrust side & a steep degradation to the low-thrust side. This ROT varies a bunch depending on fan bypass ratio, but the key point is that the most efficient operating point for the engine is a fairly high power setting.

Certainly the faster you fly above L/D[sub]max[/sub], the less aerodynamically efficient you’re being. But if engine efficiency is getting better faster than aero-efficiency is getting worse, you’re still net gaining & should speed up. And so you should continue to accelerate until the marginal aero-losses out weigh the marginal fuelburn-gains.

For aircraft with steep drag curves, piston engines, & fixed-pitch props, the engine issues are minimal and best endurance is within a knot or two of L/D[sub]max[/sub], probably below the noise floor of your airspeed indicator.

But for swept-wing airplanes, particularly modern twins with BIG engines, the engine issues are huge down in the L/D[sub]max[/sub] speed regime, and best endurance can be 15-20 knots higher.

It’s still pretty slow compared to best-range speed, which is whole 'nother kettle of fish.