How does your glider actually fly faster than trim speed?

Copyright fotoglider/Paraglider Magazine 2004

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Angle of incidence ; wings forward ; multiple pulleys ; weight shift

Use of speed systems in paragliders.

              (Or weight shift is not just for turns!) 

 While this is primarily written from a PG perspective it also applies to PPG when engine off or at low power settings. Bi-wingal pilots will find much of this strangely familiar to what happens with their HG’s.  (By the way paragliders are designated as ‘Class 3 Hang Gliders’!) 

 For simplicity the additional alteration in length to any other risers (B’s & if four risers the C’s) is largely left unmentioned in this article.

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 One of the first problems when discussing flight in general and paragliding in particular is that the English language builds in many assumptions automatically without regard to what may actually be happening to an aircraft.

 What is meant by this?  

 All paraglider pilots ‘know’ that the speed system “pulls down the A risers”, so first we need to check and see if this, as the expression implies, is true. 

 Start by thinking about the Action from Paramania - it is apparently possible to place all your weight on just the A risers without the wing ‘coming down’ and going ‘frontal’. By  looking at this example we see that as it is the wing moving forward through the air that provides the lift, what actually happens on use of the speed bar is that the A riser ‘shortens’.

 Why did I say the A’s ‘shorten’ rather than saying that if we aren’t ‘pulling down’ we must be ‘raising’ the pilot, as surely that is the alternative? Well neither lifting nor lowering the ‘load’ (pilot & harness) will do what a speed system actual manages; just ask a HG pilot if the length of his hang strap affects speed (not it’s control range) as apposed to its fore/aft position which alters trim speed. Increasing your airspeed in a glider requires one of two things to happen; either you must reduce the drag on the wing or increase the ‘thrust’. Reducing any brake applied will reduce the drag but to increase the thrust will require more of the gliders lift to be given over to provide additional thrust, once this is done, the wing will accelerate until the increased thrust is balanced by the new drag produced at that higher airspeed.

Fig.1a, 1b, 1c Showing weight shift controlling airspeed

To utilize the image above we need to quickly check the terms used. Angle of Incidence (AoI) is part of the design of the glider where the manufacturer has chosen the relative lengths of the risers including the range available for accelerated flight, unlike the fixed angle of the wing to the fuselage in a normal aircraft PG’s can adjust AoI in flight.

 With use of the brakes (flaps/ailerons) we are only altering the ‘control surfaces’ part of the wing, while when the pilot uses any trim/speed options it is a bit more like an ‘all flying’ tail plane. Pitch is controlled by the pilot i.e. do a loop and pitch runs right around 360* while the AoI would remain unaltered. For the rest of us flying more normally, gravity keeps us below the glider and ‘sets’ the pitch. Pitch governs airspeed as we all should know, while powered pilots understand power governs rate of climb/decent. Pilots appreciate that in anything but smooth air, the air its self will cause short term changes to pitch/roll with gravity acting to return us afterwards.

  Having hopefully set the basics on the design side, we now need to move on to a better look at how the speed system actually alters our airspeed. 

 By the pilot shortening the ‘A’ side of the triangle from that in Fig.1a to that in 1b we see that the centre of mass is no longer under the centre of pressure. This is an unstable situation therefore the alteration of forces on the glider results in it effectively ‘tipping’ as the pilot’s mass under gravity (or under ‘G’ force in a turn) is now in front of the centre of pressure, making the wing move forward relative to the pilot to regain balance. Due to the new angle, more lift is used as thrust and the wing accelerates its mass (and that of the air ‘trapped’ inside it, see note #1) until all forces are again back in balance i.e. new thrust = new drag.

 Why is the wing forward of the pilot in PG flight?

 Well, the pilot has no thrust, only drag, so they actually remain a little bit ‘behind’ as the wing pulls them through the air. Conversely, pre-takeoff the wing is often behind the pilot when doing a ‘committed’ launch or when a PPG is under power.

  Unlike the majority of aircraft, in PG and PPG we have a large distance between the centre of mass (at the pilot’s level) and the wing itself therefore any excessively quick change in thrust whether from a PPG engine or sudden use of some form of trim/speed/brake can result in substantial pendulum effects, especially if in rough air.

Looking at Fig 1c we see that once again ‘M’ is nearly below the centre of pressure and the wing will be stabilize at its new speed, releasing the speed system, results in the reverse of what happens when ‘pushing the bar’. 

Taking a quick look at an actual speed system as fitted on my favourite Nova Vertex (I’ve got 7 of them!) we start with the basic components, of which there are two main parts.

The Nova Vertex riser has two pulleys on each A, with webbing linking across to the B riser; the B’s are shortened by only half the amount of the A’s. The C riser length remains unchanged and due to the location of C attachments at the wing at about ¼ cord, the trailing edge moves further from the pilot on use of the bar.

 It is important that in ‘normal trim’ the brake lines have sufficient slack to meet the requirement not to cause automatic braking at full bar.

 Fig.2

 (The second half of any speed system consists of the hook, cord and speed bar as well as all the pulleys which are fitted to the paragliding harness.)

With the A riser pulleys configured as in Fig. 2 the RISER part has about 30cm of cord available, which when pulled, brings the pulleys together by about 15cm. The riser side is simple, adding the harness ‘bits’ only adds some minor complications.

  Adding the harness mounted part of the system to complete the picture, we get a mechanical advantage (MA) of 3 when pushing the bar away from the bottom harness pulley. As the speed system only moves the pilots mass forward (or back when released) not up,  to find the MA all we need do is measure the 45cm of cord passing the bottom pulley when we shortening the A riser by 15cm as we swing forward and weight-shift speed control the glider. See note #2

So how far forward do I move on the Vertex? Well ‘W’ and ‘B’ lengths are fixed, the A risers at trim is about an amazing 30cm shorter than ‘B’ while at full speed this increases to a total of 45cm shorter! With full bar my mass travels forwards by somewhere around one third of a meter compared with its initial position!

We need to ask “Why do we need multiple pulleys on the A’s?”

 The obvious answer is because we need the ‘leverage’ alone. Mmmm, by now most will have worked out that I am wary of anything ‘obvious’ in aviation. If our legs can support us and our equipment climbing the hills then they are clearly strong enough (remember when practicing ‘frontals’ your hand is strong enough!) however walking is not quite the same as holding your legs at an angle for a length of time, hence full extension is fairly easy but not holding part bar. Installing a ‘Stepped ladder’ type speed bar will help for those who do long glides on part bar, permitting retaining a comfortable position for your legs. So it comes down in part to ergonomics, in that while we only shortening the A’s by up to 15cm, it is easier (and smoother) to move the legs by about 45cm forward and a bit downwards, pushing us up and back against the harness, or if using the ‘half step’ we only shorten the A’s by say 7cm. The big plus to the MA is it also reduces the strain on the legs on a long fast glide as we have a ‘lever’ against the pressure.

What gives rise to the pressure the speedbar acts against?

 Once again we need to be aware of the risks with going for a quick answer. The pilot is producing no ‘lift induced drag’; those who are familiar with polar curves will see that by flying faster he/she only suffers from the ‘bad’ single curve up of parasitic drag, which greatly increases with airspeed. Clear benefits can be gained from ‘slippery’ streamlined harnesses of the latest type used in comps and when flying on full bar. So rather than lifting the pilot or pulling down the wing, bar pressure once speeded up acts against the forces trying to move the pilot back on his/her ‘swing’

In summery PG speed systems permit weight shift airspeed control!

Notes;

NB. In aviation things are very rarely 100% ‘cut and dried’ so please bare this in mind regarding ‘pulling down’!

 #1    Air Mass and Weight inside the inflated wing

 Taking the ‘skinny’ wing on my Vertex 24 this has a flat area of about 27 m2 while the ‘fatter’ DHV 1 wings (in the same wt. range) have a flat area around 30 m2.

 This gives a ‘guestimate’ of about say 8 m3 volume, with a sea level ISA (International Standard Atmosphere) the density of air is 1.225 kg m3, the air inside the wing has a mass of around 10kg but ‘weighs’ nothing as it is the same density as the air we are flying in.

 Climb to 7,000ft and again the contained air has no weight but now has a mass of approximately 8kg. i.e. at that altitude and a chilly -6, air has a density of about 1kg m3 if around ISA conditions.

 #2   Complex pulley systems

Because we don’t pull down the wing or lift the pilots mass unlike the math’s that would be required in working out ‘complex pulley systems’ with multiple pulleys and cords where masses are lifted/pulled in an industrial work environment, we only swing forward against air drag so can ignore everything except basic changes in riser and cord lengths when looking at mechanical advantage. J

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