Copyright fotoglider/Paraglider Magazine 2003 (flying hours etc. in this article have been updated to the spring 2004 totals)
By Murray *“Fotoglider”
Sorry to bring you the bad news, but Santa was really your mom and dad, the tooth fairy is M.I.A. and minimum sink is sunk! We don't have the details on the first two, but here goes on the whole minimum sink thing. Conventional wisdom says that if you fly with some brake on, you can stay up longer in light conditions. Well, it can be true that your electronics say sink rate is less, but is this missing the point? I think so.
If it were simply true that a slight application of the brakes 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, instead I see paragliders flying past (and down!) with obviously 'draggy' trailing edges.... true some brake leads to more lift but the key word is 'some'. It is not true that adding brake gives more lift 'full stop', the way 'min sink' is normally taught, as is implicit in the usual training methods if so that it would have to be true all the time, right?
That is any time in light lift fly with your brakes on... 'end of story'.
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
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 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!
So pulling some brake on both sides causes two sets of induced adverse yaw, along with their associated drag. 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)
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 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!
Remember slowing a LITTLE is ok.
Example based on Nova X-act rated DHV 1/2 is quoted as having a trim speed (hands off/no speed bar or brakes in use) of 20kts and a Best Glide speed of 19kts.... To fly TRULY 'MIN SINK' the pilot needs to slow down by a few knots below trim to fly at 'Min Sink'!
Apart from being a Nova fan this wing was chosen at random as a mid DHV wing to use for an example.
Source for polar information http://parapente.para2000.free.fr/wings/index.html