by Peter Montgomery
•
22 February 2022
Disclaimer - this blog is NOT instruction. It is intended as a guide to help you understand the subject of 50/50 bridles. You should only ever attempt to set up and use such equipment under the guidance of experienced and qualified instructors/coaches. Why a 50/50 bridle? Hang gliders have huge pitch stability built in. The faster you fly, the more this positive pitch stability will be trying to pitch the nose up to return the glider to trim. When you aerotow you are constantly flying fast. So on a slower, more stable glider, this means you will be fighting quite heavy bar pressures. This is exhausting and has the added negative of hugely increasing your chances of getting into a pitch/roll PIO (pilot induced oscillation). The reason for this is that as the glider yaws, one side of the control bar will also move forward relative to the other. As you are forcing the bar back you end up applying more aft pressure on the side of the control bar that has moved forward. Of course this applies an opposite roll/yaw input which sets the cycle off the other way reversing the side you are then applying greater pressure to and the cycle reverses again the other way and so on with increasing amplitude. In free flight, the solution here is to just relax up on the bar and this interrupts this inadvertent roll/yaw input. It is often understood that it is actually the bar pitch pressures that are causing this and not the pilot deliberately applying roll inputs. But you cannot just relax the bar pressures on tow as you will pop up dangerously higher than the tug and in the best case scenario, you will be given a present of ALL of the tow rope by an angry tug pilot! So the solution is to move the point on the glider where the total tow force reacts. The easiest way to do this is to split the tow force and have part of it pulling on your body and part of it on the keel of the glider. This is what we call a 50/50 tow bridle. In actuality, more tow force is still applied to your body than the glider so it is probably more like a 60/40 to. But that doesn't sound as catchy! With the 50/50 tow attached, the resultant CofG acting on the glider (a factor of both your weight hanging under it and the forces it feels from the tow rope) moves forward. The more you move it forward the lighter the bar pressures will be. But here comes the caveat - if we move it too far forward the pitch pressures will be dangerously light or even worse, the glider could become divergent. Divergent basically means the more the glider pitches nose down, the more it will want to pitch nose down! This is the reason why when using a 50/50 bridle you must only release from the top release and not from the backup harness release (except for in an emergency). So by splitting the tow forces between your harness and the keel we have split the forces nicely and will have reduced the bar pressures under tow (with the added bonus of reducing the tendency for yaw). We can fine tune how much bar pressure we get by moving the upper anchor point forward or aft on the keel. The further aft we move it, the further back that resultant force will act on the wing so the more nose up pitch force. The further forward we go, the less bar pressure we will get up until it actually reverses and the glider become divergent. As a rule of thumb, the lower aspect ratio a glider the further forward the upper anchor will need to be. Again as a rough guide, never go further forward than the crosstube junction as this will typically be too much. And only go as far forward as the crosstube on single surface gliders. Lower performing double surface gliders want the anchor at around 20-30cm from the hang strap. This is a generalization and not a rule! A high medium high performance kingposted glider will probably only want the upper anchor at the harness hang point. Higher performance gliders do not want to be towed by 50/50 as their bar pressures are already light and so there is no need for splitting the tow forces. Some manufacturers publish recommended distances along the keel for the upper anchor. Some have no idea so you will have to figure this out on your own and with some trial and error. Always use someone experienced to test fly your glider after having set the upper anchor point before you jump on and fly the glider with a new 50/50 setup! You should not be the one carrying out the experimentation until you are a competent enough tow pilot to not need a 50/50 at all (even single surface gliders will tow with chest only bridles with a really experienced pilot). As stated above, you need to have your primary release at the upper anchor when using a 50/50 bridle. These come in various types from spinnaker type releases to custom made releases. They almost all release by means of a cable that runs down the upright. Some have bike brake type levers that you pull to release. Others have a cord that is always in your hand so that you do not have to let go of the bar and reach for the release. The upper release is secured to the keel by means of a loop on the keel that is also anchored by another cord so it cannot pull forward further than the desired distance. A bungee is also then often used from the release to the nose of the glider to hold the assembly up and stop it from flopping around in the airflow when you have released at the top of the tow. The upper bridle is made long enough to go forward from the release (at the keel anchor point), through the ring on the end of the tow rope and back on itself. It then attaches to the lower bridle that is a shorter cord with each end attached to one side of the chest/shoulder of the harness. This lower cord passes through a loop in the end of the upper bridle so that by releasing one side of the lower bridle (emergency release only) if will slip out of the upper bridle allowing the upper bridle to slip out of the tow rope ring and hopefully release without snagging up or leaving you connected just to the upper anchor (which would be bad for the reasons stated above). The bridles and anchor cords must all be made from a non-stretch material. These days Dyneema is the go-to material for this. The end loops must be braided loops and not knotted or stitched. This is to ensure that they will freely slide out without snagging on things (the loop in the other bridle or tow ring for e.g.). When towing, you should always use a weak-link. This is to act as a kind of mechanical fuse to protect the glider from an overload from the tow rope and also to ‘blow’ in the event you get dragged on the ground or etc. Due to the forces in a lockout being pretty much the same as tow forces, a weak link is not there to protect you from a lockout and you should NEVER rely on a weak-link to break the tow in a lockout situation. ALWAYS release! Due to not wanting the tow to release from the harness end you should never put a weak link on the lower bride. As the pitch of the glider changes in flight (while you adjust to stay at the correct level relative to the tug), the % force felt at the upper and lower attachments will change. For this reason it is also not a good idea to put a weak link at the upper bridle. The only place to put a weak link when towing 50/50 is in the end of the tow line, attaching the tow line to the metal ring that your 50/50 bridle will go through. This is the only place that always feels the total tow force applied. Different countries have different rules on strengths of weak links so consult your country’s technical/rules manuals for this data. Finally, the emergency lower bridle release - this will typically be a barrel type release connect to one shoulder of the harness (on your dominant side). A lot of folks who only chest tow (so no upper bridle and release) will fit a barrel release to each side of the harness so they have a backup. When using 50/50 this adds to the complication and risk of an inadvertent release (and would be a 5th level release device after the upper release, lower release. weak link, tow-plane release) so is really overkill. So one side of the lower bridle should be directly connected to the harness and the other to the release pin of the barrel release.