Flexhook user notes

This is a collection of notes about using the Flexhook. It assumes that you are using an electronic timer that matches the functional requirements list.


gust filter, gust timer
These are interchangeable terms. They refer to a short period timer. This starts whenever the line tension exceeds a threshold that is similar to the unlatch tension of a conventional hook. If the line tension drops below the threshold before the gust timer expires, the model has been towed through turbulence or a gust. No action is needed apart from leaving the hook latched and resetting the gust timer. If the tension is still high when the gust timer expires, the electronic timer logic starts to execute the pre-launch sequence, typically by opening the towhook latch.
bailout timer
This is analogous to the traditional bunt bailout timer. It is started when the line tension threshold is exceeded or when the latch opens (this should be configurable). If the hook moves back before it expires the model has been intentionally launched and the electronic timer should execute the post-launch sequence for a bunt or even a zoom launch. If the hook is still forward when it expires the launch has been abandoned and the electronic timer should take the configured bailout action: typically re-latching the hook, dethermalising immediately or skipping to glide trim.

Hook logic

An idea from Chris Edge:

The basic hook logic is that when the load on the hook exceeds unlatch tension, the gust timer starts:

  1. If the hook tension threshold is exceeded until the gust timer expires, the hook unlatches and the bailout timer is started. Then:
    • if the hook moves back before the bailout timer expires, the launch sequence starts
    • if hook stays forward after the bailout timer has expired then the hook relatches
  2. If the hook tension drops below the threshold before the gust timer expires, the gust timer is reset.

Response by Allard van Wallene:

The bump or gust filter and the relatch features should be seen as two independent entities. Item (1) is correct. This is how a gust filter should operate. Mind you, the OLA steering functions MUST be used while the gust filter timer is running, otherwise the steering kicks in after the bump filter timer expires and a consistent OLA trajectory is not possible. The only item the bump filter controls is the latch of the hook.

Item (2) is also correct. If the tension drops below the threshold, the bump filter timer should be reset. Mind you, some OLA's last as little as 1 second or maybe even less. So make sure you don't set it too long. About 0.5 second is quite sufficient (but see below).

Arno Hacken has been playing a lot with his Flexhook set up and has tried several time settings for both the bump filter and the bailout timer. We are planning to define a safe window for both time settings. Wrong timing will give you a false sense of security. The bailout timer should start when the bump filter has expired AND the latch is open AND subsequently the tension has eased off below the line tension trigger threshold.

The more common approach is to start the bailout function if the line tension threshold is exceeded. This will result in long bailout times because it must be set to OLA time with ease off time added.

Using the Bauer approach, where the bailout timer starts after the line tension threshold has been exceeded AND the tension drops below trigger level, can conflict with the time the OLA sequence takes to complete. This can vary depending on weather. The time between line tension drop and hook swing backward during a regular launch is indeed very short: the bailout time can be set at less than 0.5 sec, making a far more safe setup. Make sure the on board beeper sounds whenever the latch is open. This lets you know when to initiate a circle without crashing the model by bunting at your feet.

Setting the hook re-latch delay

From Allard van Wallene:

Under some circumstances the line tension will drop during the last part of the OLA. In particular in windstill conditions, the highest line tension will be directly after starting the OLA and will gradually drop. Of course, under such conditions, a bail out time of a few tenths of seconds might not be enough as the hook will relatch before releasing the line. That is why the bail out time should be around 1 second. It won't be much of a problem to hold on to the line for 1 second to trigger a re-latch.