Okay, really long post, explaining in excruciating detail for everyone OTHER than FlyGuy8675309, who apparently is missing many things in addition to Jenny's number, how this efficiency stuff works.
First, "efficiency" is an abstract term. We must define what we mean by efficiency. Generally, when discussing transportation, the most important efficiency measure is distance traveled per unit fuel consumed. This is because we care are successfully getting places far more than we care about when we get there. The latter doesn't matter if we don't get there at all.
For a quad RC aircraft, the "fuel" is stored electrical energy measure in mAh, and peak efficiency is maximizing how far we can travel for each mAh we consume.
Efficiency is a very, very, VERY complex property of any mechanical system on the order of the kinds of things we are talking about (cars, aircraft, etc.). It's impacted by dozens of factors. I'll discuss a few to give an idea.
Friction with air flowing over a moving object presents a drag force that power from the motors must be use to counter. Drag force increases as the square of speed -- double your speed, and the drag on your aircraft will increase by a factor of 4. Obviously, then, 4 times as much power is then going into heating the air as it passes over your aircraft as was being used uselessly for that purpose going half as fast.
Power from motors are used to create force to accelerate the aircraft, as well as fight drag. Because of the nonlinear relationship between speed and drag, at some point the drag will be so great that all the power of the motor is simply opposing drag, balancing it out, so the aircraft can move at a constant (peak) speed. You can't make it go any faster, because there's no more power available -- it's all working against the drag. Without getting into all the graphs and equations, flying at this speed -- fast speed possible -- turns out to be extremely inefficient, dumping way too much battery energy into fighting drag. So, even though you are going really fast, your battery will run out even faster, and you won't make it as far as at a slower speed. Put simply, you're wasting a lot of battery energy heating up the air with friction, energy that then isn't available to move the aircraft.
By contrast, at the other end of flight "efficiency" is hovering, where your making no progress at all, but still burning fuel. Eventually, the battery will run out and the aircraft will drop to the ground, having consumed the entire "tank of gas" without having made progress anywhere. Obviously incredibly inefficient.
So the speed/efficiency graph looks like a curve, a hump, with speed on the X axis, and efficiency in ft/mAh on the Y axis, similar to this (this is not an actual performance graph for the MP, just a visualization of it's general shape and parameters:
To fully explain why the graph looks this way is far too deep for this discussion. There are so many variables that affect it, like the nonlinear behavior or drag, for instance. But you get the idea.
Back to the gobbley goop, to save my face. So, why does wind have no bearing on efficiency? I must be crazy. If you're flying back in a headwind, OF COURSE it won't travel as far, for the same amount of juice consumed, so therefore less efficient -- right?
Wrong. Here's why: The aircraft is not pushing against the ground. It's pushing against the air mass it's flying in. In terms of physics -- which is what we're talking about with efficiency -- it doesn't even know the ground is there. It's irrelevant.
Let's get back to that headwind. Say it's 10mph. And say the most efficient airspeed for the aircraft is 30mph. Let's further say that, with 1500mAh left, you can fly 8000ft in still, windless conditions, at 30mph.
Now, all conditions being the same, since this is the peak efficiency airspeed, if you were to go 20mph you'd fly longer (better duration efficiency), but find that you could only go a total distance of 7000ft before the battery ran out. At 39mph, you would cover the distance in less time, but find again that the battery ran out 1500ft short, and you only got 6500ft out of that 1500mAh.
I don't think there's any controvery over this -- that's the entire subject of this thread. So let's get back to how the whole situation changes with wind.
10mph headwind, straight on. What is the most efficient speed to fly? I.e., cover the most distance with available fuel? Still 30mph AIR speed. With a 10mph headwind, 20mph GROUND speed. This will result in the greatest distance traveled through the airmass, and over the ground. This speed is the point in the peak of the graph above.
Easiest way to see this is to understand that a constant wind speed is simply displacing your aircraft without affecting its flight behavior in any way. It is exactly the same as you flying for 20 minutes at 30 mph in still wind, land 10 miles away, and I pick it up and move it back 3.3 miles so it's only 6.7 miles away from you. Did it just fly less efficiently because some external actor, having nothing to do with the aircraft at all, its operation, motors, battery charge speed, etc. -- moved it?
Would you say it flew less efficiently, or that someone picked it up and moved it?
Well, that is LITERALLY what the wind is doing. Instead of me, a human being, moving it, a large mass of air it's flying IN moved it.
So, while wind frustrates your human goals, that has nothing to do with aerodynamic and power plant efficiency. As I said many posts ago, when picking apart the physics of this it is a simple reference frame translation, which preserves things like efficiency -- it just changes your spatial relationship to the aircraft.
One more way to try and help people understand this, because the physics of translating between references frames is not intuitive. This thought experiment kind of drives the point home.
Empty space. Utterly and completely empty. You, and a friend, are floating around in space suits near each other, holding hands, contemplating existence.
Suddenly, another friend seems to come rocketing out of nowhere to speed past you two at some incredible speed. She flies past in milliseconds and in a few seconds is gone -- so tiny you can't see her any more.
Question: Who's speeding through space at incredible speeds? Who has the kinetic (motion) energy?
You and your buddy holding hands would say the other guy. Yet, what was her experience? What did she see? Who does she think is loaded with kinetic energy?
If thinking about this face-saving gobbley goop isn't messing with your head, you aren't even beginning to understand it.
Now, just because there's a planet below, air all around, sunlight beating down, it doesn't change any of the fundamental physics of the Gedanken we just pondered, nor the physics of the Mavic Pro when it's flying either.
If there is still any confusion, ask questions. I will answer any that are sincere and civil. I have a face to save, and probably haven't blathered out enough gobbley goop yet to have full salvation.
