DHV Safety tests of LTF A and B paragliders, Part 6

Testpilot: Harry Buntz, Simon Winkler
Text: Karl Slezak
Pictures: Harry Buntz, Simon Winkler
Data logger engineering, translation: Peter Wild

This report extends the work publicised in DHV Info 174 which can also be found here.  Details on how we have classified the gliders, the relevance of different manoeuvres to accident statistics, German airwothiness requirements (LTF certification) and other details can be found there. The DHV safety and technical department tested the following gliders in the 6th round of its ongoing safety test program:

Launch preparations
What is tested:
In particular we look at the risers and line systems, how easy they are to sort, if looping or knots in the lines are easy to see or not and the functionality and ergonomics of the risers.

Launch preparations are particulary exemplary on Skywalk's Mescal 4. The soft lines are easy to sort, the risers have good clear colour markings and the brake handles can be adjusted in size. Ozone's Buzz Z4 needs a little more care in sorting due to thinner lines, but it is positive that Ozone have not followed the trend of using unsheathed lines on this low end B glider. The lines on Niviuk's Hook 3 are also all sheathed with the exception of the top gallery, and are easy to sort. Its risers are also well arranged. The glider should be layed out in a curve to aid inflation. Gradient's Golden 4 has very soft risers which are kept simple. The plastic clips in the triangular shackles are not well dimensioned – one went missing after the first flight. Line sorting is easy, and setting up the glider is simple, although the thin lines in the top gallery require a little more care and attention when sorting. Sheathed lines are used everywhere apart from the stabiliser line on Gin Gliders' Atlas. Line sorting is easy, but the stabiliser needs a second look to check everything is ok. Its risers are kept simple without any gimmicks, a ring is used instead of a pulley for the brake lines. The risers are also colour coded for the right- and left-hand side.

Launch characteristics
What is tested:
Inflation, climb rate, requirements to stabilise and necessary brake inputs.

Skywalk's Mescal has well balanced launch characteristics. The glider climbs evenly with good internal pressure after a small impulse and needs only minimal brake input to stabilise at the zenith. Ozone's Buzz Z4 is very similar, but feedback via the risers is a little less and a little more control on the brakes is needed to stabilise at the zenith. Both gliders are easy to keep over the pilot with a little forwards movement and give plenty of time for a good control look before launching. Gradient's Golden 4 has a lightweight canopy which tends to hang a little behind on inflation and needs steady guidance. Just before the zenith it then accelerates a little and needs a jab on the brakes to stabilise it. Here, fine control is required as too much brake then results in a stall. Gin Gliders Atlas has a heavier canopy which climbs readily without any breakout tendencies, a bit of guidance makes life easier and it needs a bit of brake input to stabilise at the zenith. The canopy is easy to keep inflated at low speeds for control checks. Niviuk's Hook 3 needs to be layed out in a clean curve and inflated with the inner A line risers only, to prevent the ears overtaking the middle of the canopy on inflation. The canopy requires gentle but constant guidance and a jab on the brakes to stop acceleration just before the zenith. Keeping the canopy inflated during the control phase is not a problem, but the required ground speed is a little higher than with the other test gliders.

Test manoeuvres / recovery from instability
All test manoeuvres were filmed with onboard GoPro cameras, ground cameras and documented with the DHVs data loggers. Test manoeuvres were performed by the DHV test pilots Simon Winkler and Harry Buntz.

Info Data loggers
The pilot data logger is firmly attached to a main suspension strap on the pilots harness.
A second smaller glider logger is attached to a cell wall inside the glider using two strong magnetic plates. The best position for data collection has been determined to be where the C-gallery lines are attached to the canopy at the 70% collapse marker points. Logger data is collected continually from the beginning to the end of the test flight and the two instruments are synchronized  with each other via a low-range radio signal. Data sets are transferred from standard micro-SD memory cards to a PC after landing.
The loggers collect the following information:
 Pitch, roll and yaw angle,
 Pitch, roll and yaw acceleration,
 Vertical velocity calculated over a 0.5 second window from the barometric altitude sensor,
 Velocity: the pilot data logger contains a 5 Hz GPS, from which the velocity over ground is calculated,
 G-Force: from the accelerometers contained in the pilot data logger the G-force acting on the pilot is calculated,
 Altitude: both the barometric height (recorded at 100Hz) and the GPS height (5Hz) are recorded.
Data processing: the processing software is written to automatically recognize the beginning and end of a test manoeuver. Pilot and glider movements are processed to produce a simulated flight video from the recorded data, and this simulation is synchronized with the camera video material of the test flight. Test pilots check the synchronized results for plausibility. Data loggers are instruments to assist test pilots and provide additional objective information on parameters which are difficult to judge in the air such as roll and pitch angles, height loss,  course changes and durations.

Asymmetric Collapses
What is tested:
Asymmetric collapses are conducted at trim speed and at full speed with no pilot action. Canopies are collapsed to the top limit of the field defined in the LTF airworthiness requirements (visible from the tapes stuck to the lower sail), i.e. the maximum possible for certified gliders. During LTF testing it is also possible to certify a glider collapsed at the minimum limit of the LTF field, but this generally results in less dynamic behavior. For this reason we often see differing results here in safety testing, when compared with those of the certification tests.
Note: Paragliders in the LTF B certification class are permitted to pitch forward to 45° after collapsing. This test regulation is verified in LTF testing using only video film shot from the ground and therefore subject to large error margins, as no clear reference points can be used to calibrate the measurements in any way.  When a glider collapses assymetrically, the dive experienced is always a combination of pitching forward and rolling to one side. This again makes judging the pitch forward angle from video film very inprecise. The data loggers used by the DHV record pitching and rolling movements separately. It has been determined that only a few gliders in the LTF A and B classes actually fulfill the test requirements of diving forward to less than 45° when measured with the data loggers.

The only LTF-A glider in this test demonstrated good conformity to this class. Trimspeed collapses were accompanied with course changes of around 90° and height losses of 30 meters or less. Fully accelerated collapses at the top end of the 70% collapse field were often very similar. The largest course change was just under 180° with a height loss of less than 40 meters.
Ozone's Buzz Z4 (low-end LTF B classified) is a little more dynamic. Assymetric collapses at the maximum limit of the test field resulted in height losses of more than 40 meters.
Neither of these two gliders showed any tendencies to cravat after collapsing.
Niviuk's Hook 3 is clearly positioned as a high-end LTF B glider. Collapses at trimspeed are unproblematic and not difficult to recover from. At the top end of the speed range the glider design results in massive collapses where a large percentage of the collapsed wing stays inflated for longer and creates a large amount of resistance to the airflow. This makes it difficult to directly compare the Hook 3 with other gliders. Fully accelerated collapses were repeated several times by the test pilots to evaluate the gliders typical response characteristics.  A massive collapse presents a large resistive area and results in the canopy diving forward rapidly, and changing course rapidly. Dives and course changes are often accompanied by collapses on the opposite wingtip with further resulting course changes. In some cases the opposing collapse cravated and remained tangled in the lines requiring pilot input to recover from. It is difficult to predict if the glider will collapse as violently under real-life conditions, or if this only occurs under test conditions.
Gradient's Golden 4 has similar collapse characteristics to its big brother the Nevada. On collapsing the glider reacts slowly at first until it has turned by about 90°. After this, dynamics increase and the glider dives markedly forward. After a self-recovery the glider turns another 180°.
The Atlas fom Gin Gliders also has its own reproducable collapse behavior. Dynamics are moderate throughout the manoeuvre, but as the collapsed wing opens relatively slowly, the course change is usually between 270-360° with a higher resulting height loss. In some cases, recovery had to be aided with a little pilot input.

Front collapses
What is tested:
Front collapses are performed at trim speed and at full speed in different configurations: firstly by collapsing to a depth of only 40% of canopy (marked with tapes on the lower sail), and then by collapsing the maximum that the gliders construction presents. Maximum front collapses on full bar generally exceed the LTF certification limits. From accident analysis and diverse video footage, we know that front collapses in practice often effect 100% of the canopy. Many gliders are certified at the minimum 40% limit without looking for construction weaknesses. We often see large deviations to certification behavior when gliders are tested at the upper limit.
Pilot action to recover is only performed when the glider does not self-recover.

In past serial tests, we noted the tendency of some B-class gliders having challenging behavior to front collapses, which we can also verify in this test series.
For low end B gliders, behavior is happily much more benign, a good example for this is Ozone's Buzz Z4. After hard front collapses the glider recovers very rapidly, on occasion somewhat assymetrically. If less of the canopy is collapsed, then recovery takes a little longer and the wingtips tend to remain deflated for longer periods.
100% front collapses were not possible with the Skywalk Mescal 4, the maximum we could achieve was about 60%. Recovery is fast and the glider quickly returns to normal flight. Of the five gliders tested here, the Mescal has (as to be expected for an A class glider) the best recovery characteristics.
The remaining other three gliders, Niviuk's Hook 3, Gradient's Golden 4 and Gins' Atlas all have significantly more challenging behavior. All of these three gliders had tendencies to remain in a stable front stall after collapsing. Niviuk's Hook 3 showed this mostly after massive collapses, whereas the other two sometimes remained closed after medium sized collapses – recovery from massive collapses was usually fast and without pilot input.

Spiral dives
What is tested:
Spirals are flown such that after at least 5 seconds and before the 540° point the canopy is fully locked in a rotation. The testpilot then keeps the glider in the spiral using the brakes for a further 2 turns (720°) before releasing the inside brake to start recovery.

Spiralling is easy with Skywalk's Mescal 4. Initiating is straightforward, G-forces and sink velocities do not increase unexpectedly in the spiral phase and exiting occurs immidiately once the inner brake is released. The other four test candidates behaved as expected for their higher classifications rather more dynamically. Ozone's Buzz Z4 behaved quite moderately in the initiating phase but then accelerated rapidly in the spiral phase. To exit, the glider required almost 100 height meters, as it remained in the spiral at -20m/s for a complete turn before slowing an pitching up. Niviuk's Hook 3, Gradient's Golden 4 and Gin Gliders Atlas were all very dynamic in the initialising phase. Of all the tested LTF-B gliders, the Atlas exited and recovered fastest, while the Hook 3 had the highest G-forces.

B-Stall
What is tested:
Pitch back behavior on entry, pitch forward behavior on exiting and sink velocities.
In particular we look at the canopy stability and tendencies to deform on longer B-Stalls (>10s) and any recovery problems.

Four of the five tested gliders (Skywalk Mescal 4, Ozone Buzz Z 4, Niviuk Hook 3, Gin Gliders Atlas) had no problems with B-stalls, and this manoeuvre can be recommended for rapid descents if required. Gradient's Golden 4 deforms somewhat on initialisation and is a little unstable in the stall phase, and on exiting tends to have a delayed recovery with short deep-stall phase.

Big Ears
What is tested:
Big ears at trim speed and at full speed. Glider sink and speed is measured. Any entry difficulties or deep stall tendencies on exiting from trim speed big ears are noted.

All gliders tested here had no real issues with big ears. When accelerated, the wingtips of Gradient's Golden 4 flap constantly, and those of Gin Gliders Atlas flap occasionally. The Atlas had the highest sink rate (4.5 m/s) and needed the longest time to recover to normal flight on exiting.

Control characteristics and stall point