Min Sink

Copyright Murray ‘fotoglider’ Hay/Paraglider Magazine 2003-5 (flying hours etc. in this article have been updated to the summer 2008 totals)

Santa, the Tooth Fairy . . . and Minimum Sink

By Murray *“Fotoglider”  Note: If Sammel Weis wishes to get in touch I will explain his error to him. NB Pressure NOT ‘pull’ on risers!

Ref PG Forum: With luck someone will explain all this to incompetent pilots like Mario who don’t understand the term ‘Slight Application’ (of brakes)... clearly a slight application of brakes will never slow a paraglider down to 1/3rd of its trim airspeed... 7kts being well below the speed required for flight! Clearly some pilots need to gain understanding of the difference between Vertical speed (i.e. lift/sink) and Airspeed AND Ground speed... and why changing your Airspeed when in remaining in ridge lift requires an alteration of heading (so altering the time spent at any one section of a ridge) to maintain your course over ground.

NB. It is important when writing a paragliding technical article, in this case on ridge lift, that the reader has a reasonable level of basic understanding of the aspect of PG flight! under review, it is obviously also important they read what is actually written!

The quick version: As my students know & fully understand, there is a net gain in altitude attainable (climb) when flying with a glider configured for slightly less lift efficiently (higher sink rate) provided that the gain due to a lower ground speed results in the aircraft remaining in lift for a proportionally greater TOTAL (lift rate x time). The’trick’ to this rests in the ‘heading v course’ variation when flying in ridge lift i.e. slow down slightly in Airspeed and you need to use more of your airspeed to counter drift to avoid your course taking you into the ridge Etc. etc! This is of course slightly different from the situation flying in ‘open air’ while well away from any ridge lift. 

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 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!

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