Following the successful low-altitude test two weeks ago, we re-assembled this past weekend for another round of testing – this time to full altitude, and integrating all steps of the operation.

Just like last time – everything worked straight out of the box. Set-up was quick (less than an hour) and we were ready for the helicopter. Doug Uttecht was flying for Northwest Helicopters again, and he seems to have been practicing this in his mind over the last two weeks, since we were able to dive right into it.

First flight was a warm-up flight, duplicating last week’s flight, just to make sure we haven’t forgotten anything since then. We additionally rehearsed radio commands so that we will later be comfortable positioning the helicopter.

We then practiced climber pick-up and lay-down, which are now a bit more complicated than they would have been with the winch-based design. This is done with two simple tools that allow us to handle the cable without really getting uncomfortably close to it.

We next performed a series of measurements in order to correlate helicopter positions and lasing angles. The trick is to have the climber within the allowed 15-degree lasing angle throughout the climb, while at the same time maintaining its separation from the helicopter. Not-too-steep, not-too-shallow, and actually, we need to drift the helicopter during the climb since there’s no single position that satisfied all conditions. Given the practice we’ve had, this was almost trivial to do, and what’s more important, since wind conditions  will likely be different during the games, we know we can adjust in real time to different cable sags.

Finally, we did an end-to-end test with battery powered climbers. Only USST and KCSP had climbers ready to go, and KCSP suffered from control related issues and did not have their van full of spare parts with them, so to Brian’s endless misery, they were out of the game. USST was the last climber standing, and on their second try, they put the pedal to the metal and completed the 1 km climb with no problems. Meanwhile, Lasermotive who were out with their beam director, confirmed that tracking was feasible within the 15-degree cone I mentioned.

Not much more to say then – the vertical raceway is now ready and waiting for the teams. More information on the upcoming schedule coming your way soon.

The results of this test flight were nominal – just what we wanted. Happiness is three taut chains!

• GPS position keeping worked flawlessly, with the pilot maintaining a horizontal envelope of <40 m irrespective of flight altitude.
• Virtual Bob, in all configurations, worked exactly as intended, keeping the altitude to within several feet, controlling cable tension, and damping the whole structure.
• Deployment off of the figure 8 was smooth.
• Workload on the pilot was reduced significantly, with heads-in operation proving completely feasible.

In all honesty, this should not have been so difficult to do, but the best laid plans, etc.

A big part of getting it right was finding the right crew:

Doug Uttecht and John Peden of Northwest Helicopters

Our next step is to follow up with a high altitude flight to validate the end-to-end procedures. We need to work on a technique for graciously retreiving the climber when folding the pyramid, and we need to decide on whether cables will be reused once laid on the ground.

If all goes well, we’ll be able to do this flight in two weeks, and then turn our attention to running the games.

Linear Bob, single-stranded

The apex with breakaway link

Single-Strand Bob, full view

Implementing Bob turned out to be very easy, an exercise in “junkyard engineering”. After looking at the weight and strength requirements, we chose to forgo the thick cable in favor of  steel chain, and place it only at the bottom end of the pyramid. We thus connected three 3/16” steel cables (same as the climb cable) directly to the breakaway link, and added 100 feet of 3/8” drag chain at the end of each. The total weight of the chain us 400 lbs.

The dimensions of the chain were chosen based on the rate of mass-accumulation we want to achieve – for example, if the chain weights 2 pound per foot, than as the helicopter rises one foot, it lifts 3 lengths of chains, 1.4 feet each, plus a bit of sag – a total of about 4.5 feet, and so is accumulating mass at a rate of 9 pounds per foot.

Seattle is a good town for finding cheap chain. A few phone calls to used marine equipment stores, and there it was – a barrel of 100 m and 400 pounds of 3/8” chain, weighing about 1.2 pounds per foot. Perfect – we can use it as is, or double it up. This chain has shorter links than a standard trade chain, which means it weighs more per foot. Maybe an old anchor chain. Perfecter.

The point masses for step Bob should weigh about 500 pounds total – 167 pounds each, and should be sturdy enough to be beaten around a little bit, and cheap. Truck tires did the trick, and Tires Inc were happy to donate a few used ones to our cause. We ended up taking only three 75 lbs tires, so were on the light side. (this will show in the video of the flight)

Since the forces at the end of the chains are now very low, we used soft line to tie the ends of the chains to our cars. We deployed it on this beautiful field not far from Northwest’s HQ, hooked up the helicopter, and in no time were ready for the first test flight.

The sequence of images clearly shows how Bob works  - The helicopter picks up the cable from a figure-8 coil we set up, and after the coil is exhausted it picks up the apex of the pyramid. The pyramid “stands up” until the chain begin to rise, at which point the rate of pickup decreases and equilibrium is reached.  The pilot doesn’t have to stop the helicopter – it does it all by itself.

Double Stranded Bob	Double Stranded, Step Bob

Step Bob and Linear Bob

When looking for a softer cable arrestor device, Dryden’s John Kelly came up with the concept of Bob.

Bob is a weight that hangs at the end of the cable, lifted by the helicopter, so that once airborne, the tension in the cable is determined by Bob, independent of the altitude of the flight, which is determined by the helicopter, and can deviate considerably, as long as Bob does not touch the ground.

In order to prevent Bob from potentially becoming, well, a wrecking-bob, we would need to attach slanted stay-lines to him, limiting his motion.  The stay lines must be close to horizontal, so that possible vertical motion of Bob will not be hindered.

Of course the helicopter will now be lifting the stay lines as well, so their weight gets added to Bob’s weight. Actually, if we make the stay lines heavy enough, we don’t really need Bob anymore – the weight will be distributed along the stay lines, and no point mass will be hanging over our heads. Thus was coined the term “Virtual Bob system” – a bobless bob!

Next, we make the heavy stay lines (three of them) slant at a full 45 degrees. Since they are heavy, they will sag, and some portion of their length will lie on the ground. If the helicopter moves upwards, the amount of airborne weight increases, thus pulling the helicopter down. If the helicopter moves downwards, the amount of airborne mass decreases, and the helicopter floats back up. Since this is a very gradual stabilizing force, we call this configuration “Linear Bob”.

If we further place point masses a certain distance up the stay cables, we will give Bob a distinct “notch” for the pilot to pull against – we call this configuration “Step bob”. Ideally the point masses add up to the extra lifting capacity of the helicopter at that altitude, so that the only way it can pull them up is to bounce against the end of travel – thus giving us a very soft, resettable force fuse that is coupled to a fixed altitude – Once the helicopter exhausts its inertia, the weights come back down to the ground, resetting the helicopter’s altitude.

This keeps us with the paradigm of flying constant tension, except the system now has a large, self-correcting sweet-spot.

Following Mike Kapitzke’s suggestion, we also moved the breakaway link to the apex of the stay-line pyramid. Now, since the Virtual Bob system fully determines the position of the apex, if the breakaway link were to pop, everything will fall within the base of the pyramid, and so anyone standing outside the edges is not in the fall zone – very convenient for us.

On the right you can see diagrams of Linear Bob and Step Bob. They work well in theory – all that remains (again) is to try them in real life.

The Dynon D10-A display screen ...

... installed in the MD-500

The world’s most boring video clip

As you recall, one of the difficulties we had to solvel when designing the vertical raceway for the games was the requirement to hover over a specific point while at high altitude. The problem is that while flying up high, the pilot cannot really judge the vehicle’s location or speed – imagine looking down from a jetliner (right about the time when you’re told to fold up your food tray and return  your seat to the right up position) and seeing how everything down below is ant-sized… If you look downwards, every slight tilt of the plane, or every motion of your head, will result in very large apparent motion.

To solve this, our aviation consultant Keith Mackey worked with instrument maker Dynon to create a GPS based hover aid, which tells the pilot where he is situated relative to the desired hover point. The helicopter can be 5000 feet above and 5 feet to the left of the hover point, and the instrument will dutifully tell the pilot to move 5 foot to the left.

Having briefed the pilot on the instrument setup on Friday, the first task was to translate theory into practice. Understanding the instrument is one thing, but learning to fly it is another – the pilot has to train himself to properly react to the information the device is giving him – match the size and timing of the control inputs he’s making so as not to lag too much, nor over-compensate.

Luckily for us, Doug Uttecht, our pilot, is experienced in precision flying while pulling power lines, where he is constantly feeding off of instrument readings, and so was a perfect candidate for this job. Keith and Doug finished installing the GPS in the helicopter on Friday and we were all ready to go.

The first thing Keith did was take Doug for a test flight – in his car!  They drove around the helipad, learning to operate the Dynon and getting a feel for the responsiveness of the GPS needle. (This was also significantly cheaper!)

They then took the helicopter on a 10’ hover, and replicated the car exercise, flying around the imaginary waypoint and watching the needle pointing at it and flipping around every time they passed over it. Then the same thing over again at 200 feet, where visual reference is still a viable way to hold position, except by then Doug was flying completely “heads in” – based solely on instruments. 2000 feet, no problem either.

Horizontal station keeping was typically within 10 m, and within 40 m on rare occasions. Vertical station keeping was similar. 20 minutes later they came back to the helipad, saying “well that was easy – what’s next?”

During the hover, Keith recorded this video of the instrument readouts. Boring indeed, since nothing is changing – as it should be.

Dynon have stepped up and are loaning us the instruments needed for the games, and we are very grateful for that – we could not have done this without them.

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.

Hi Folks – As the status message on the right indicates, we’ve had a good “return to flight” weekend, with the successful demonstration of a helicopter-borne vertical raceway. This should not have been so difficult to begin with, but sometimes execution of a plan takes an unexpected detour, as was the case here.

In the next several posts I will cover what we did over the weekend, why we did it the way we did, and how it all turned out.

Special thanks to Greg Schoenbachler of Silver Stream Organics and Cattle Company for the generous permission to use their land, and to Doug Uttecht and John Peden from Northwest Helicopters for pulling extra hard on that helicopter.

In preparing the follow-up report for the test, I also found this gem in my archives – a helicopter flight video from youTube, demonstrating the basic ability of a helicopter controlling its position while maintaining a constant pulling force on a long line – this is what convinced us (almost a year ago) that in principle this operation is feasible with a helicopter using standard practices. However, as the saying goes, the devil’s in the details, and the past year was spent mostly on ironing these out.

Stay Tuned for more information, coming your way soon.

Ben