We spend the last couple of weeks looking at the results of the last test flight at Dryden. First order of business was of course to sort out the sequence of events that led to the activation of the safety breakaway link.

We were able to confirm a few things pretty quickly:

• We re-tested the breakaway link and know that it pops as designed at 3000 pounds, very repeatedly and reliably. Since the helicopter was aiming to pull only 500 pounds, we are confident that the high force was a result of hitting the end of the tether at high velocity.  This is in agreement with the ground video that shows the helicopter first dropping and creating quite a bit of ground slack, and then picking it up rapidly just before the cable goes taut and disconnects.
• In the design phase, we estimated the velocity in which the helicopter has to move in order to create a snap load sufficient to pop the link when the end of the tether is reached. This was approximately 1000 feet/minute, which is in rough agreement with what see in the video.
• Based on the location of the dropped link, we know that the helicopter deviated from its prescribed hover zone. This is in agreement with the ground observations at the time of the disconnect.
• We estimate that once the helicopter deviated from its prescribed horizontal position, the pilot’s attention was diverted from keeping an eye on the tension and altitude readouts, which resulted in the behavior described above.
• The GPS hovering-aid system was not used in either of the flights, since the pilot preferred to use visual references.
• The ground winch did not pay out cable before the breakaway link separated, even though this was intended. This is a result of a combination of the bypass load being set too high (2000 pounds instead of the preferred 1000) and the inertia of the winch drum.
• We estimate that the winch can work as a slow fuse against an accidental pull by the helicopter, but is less effective against a snap load. The manufacturer now says that the setting cannot be brought down to 1000 pounds.

With these conclusions in mind, we proceeded to modify our flight setup:

• We are moving back to the original “small helicopter” model. The S-58 we used was a result of a limited choice that we had. It is heavy, and thus a) has a slower reaction time, and b) has more inertia that manifests itself as snap load once the slow reaction time causes a snap condition in the cable. We located an MD 530 (our original “weapon of choice”), and can also use s lightly heavier helicopter like a Huey H1B.
• We are working with a pilot that is experienced in doing utility power line pulling, which is similar to what we’re doing. To our endless delight, this pilot has pulled 6000’ tether spans in the construction of the recently completed Tacoma Narrows bridge project, and so is well versed in constant-tension line pulls.
• We have done away with the winch. While this is the “industry standard” way of pulling line with a helicopter, we can do better, since our cable does not have to be threaded onto power poles… more on this later, but we have created a custom “gradual arrestor” system which will mitigate snap conditions if we were to run into the same issue again.
• We are now mandating the use of the GPS hover system as the principal means for position keeping.
• At Dryden’s advice, we’ve moved the breakaway link from the top of the cable to the bottom. While still protecting the cable, the result of a breakaway now are that only a small portion of the cable drops from a low altitude, and the helicopter is left towing a long piece of weighted cable, which it can then safely deposit on the ground.
• At Dryden’s encouragement, we will perform a gradual test plan, starting out with untethered flights validating the GPS system, followed by a lower-altitude test demonstrating the system in operation to an altitude of 1000′, and then proceed to a full-height end-to-end demonstration.

The design changes were completed on the first week of the month, and the helicopter operator, Northwest Helicopters, had an opening on the weekend of 9/10. We drew out the logistics plans over labor week weekend, shipped everything out on Tuesday, and flew out on Thursday to prepare for a Saturday flight.

Dave Horn (who organized the SE conference only a few weeks ago) and Seattle-Based team LaserMotive helped with manpower at the site.

I’ll post a little bit more on each of the components list above, and then get back to the actual test flight.