If it were simply true that a slight application of the brakes always created a situation of minimum sink and that was what pilots actually did then that, then that is what I would teach, but when was the last time you saw a pilot flying 'min sink' with only a touch of brake?
I don't think I have ever really seen that very often in practice among pilots trained by the main sporting associations, instead I see lots paragliders flying past (and down!) with obviously 'draggy' trailing edges.... However it is of course true that some slight application of brake leads to less sink but the key words are 'some’, ‘slight’ & ‘sink’ (not ‘lift!’)'.
Certainly it’s not true that “adding brake gives more lift” 'full stop', the way 'min sink' is often taught, as is implicit in the usual training methods if so that it would have to be true all the time, right? Now this is not intended to imply that in training pilots are not warned that too much brake will increase the sink rate, and if held will in fact stall the glider, as of course this risk is taught world wide, rather it is the application of ‘mid-range’ brake we need to look at here for use in normal (rather than SIV/Acro) modes of flight.
So based on many year observing numerous pilots flying in light lift with clearly a significant deflection of the trailing edge the question is do instructors need to review the common practice/advice “To fly light lift, fly with your brakes on”.
Well it is true if by 'some' you mean just enough to slow the wing that small amount on the polar curve to give the lowest sink rate and not any more. Looking at polar curves on modern wings whether DHV 1 'school wings' or 'Hot' DHV 2/3's we can see that at the slow end of the graph any reduction in airspeed beyond the actual 'min sink' speed results in a rapid increase in inefficiency of the glider
NB. There are significant risks of miss-interpreting Polar Curve graphs as the vertical & horizontal scales do NOT use the same units!
Why mention efficiency?
Well compared to most aircraft with ailerons and flaps at we have a limited best glide even when flying with a aerodynamically 'clean' wing x-section. Based on the idea common in 'min sink' it would follow that some right brake only gives more lift on the right, leading to a roll to the left (this would be a primary effect). Roll will induce yaw (as a secondary effect) causing the glider to turn to the left, right? Of course, that’s wrong as we can observe paraglider don't do that!
NB. I said Yaw not 'Adverse Yaw'
So what’s up with that?
Well, anyone who stops to thinks, and applies the laws of aerodynamics to the problem, will soon realize that what’s really happening is in fact you do get some additional lift, but much more drag (parasitic and induced). The increased drag first causes adverse yaw to the right (primary effect) and then induces a roll to the right (as a secondary effect) - which is what we have all experienced! That is add right brake while in straight flight and the wing will, given time, enter a right turn.
So pulling some brake on both sides causes two sets of induced adverse yaw, along with their associated drag, with one ‘wing’ (side) trying to turn the whole glider to the right, the other to the left... as a result there is no change in heading. The result is you slowly going down and due to wind shear, descending into even less lift! Not good. In the meantime, yours truly is flying at about best glide angle (usually zero brake/trim speed on my favourite Nova) and climbing up past them on my way to logging 1,500 hours by the end of 2003! (I started paragliding about '98)
NB. The drag at the pilot & lines is only parasitic and will of course reduce with any reduction in airspeed
Now, here comes the “but” . . .
Sometimes it’s better to fly inefficiently!
Why? Well, if flying at best glide angle you pass the only area of lift at +1 over the course of a minute, and then spend another one minute in a area of -1 sink while turning, you have not gained anything. But (there it is), by slowing down and flying less efficiently (at a gain of, say only +1/2) you spend say proportionally three times as long in that lift. Your 1/2 better off! And due to windshear, as you get higher, you get more lift to play with and can fly nearer the speed you need for best glide angle.
Your only problem will be once everyone else has walked back up they will want to know where you hid the engine!