Jeff's Rocketry Postal Contest Entry

December 2010 -- NAR A Helicopter Duration

13mm A-powered Rotaroc in recovery mode.

December 2010 -- NAR A Helicopter Duration

This month's event was A Helicopter Duration -- a model powered by an A motor that must use helicopter rotors as a recovery device. I happened to have a decent model, so I thought I would participate, if the weather cooperated.

This model has an interesting history. Early in my contest rocketry career, in the early 1980s, I flew with the PULSAR section in downstate NY (Rockland County.) The section was run by Art Rose, who is perhaps best remembered for his Rose-roc helicopter design. PULSAR flew six open meets per year in a relatively small park. We flew about six duration events per meet, most flown with mini engines to make recovery feasible on this field. So we flew a lot of mini helicopter events, mostly 1/4A, with some 1/2A mixed in. Art's 13mm Rose-roc was perfect for this, being optimized for low mass and low drag and high performance from small motors. The funny thing is, everyone (Randy Ringner, Charlie Sykos, Geoff Landis, myself, and others) beat Art once in these events -- but only once! He won about 75% of the two dozen times we flew these events! I tried to beat him for a few years with a model that was similar to the Rose-roc but using torque rods instead of rubber bands to deploy the rotors. After a while, I saw the futility of trying to beat Art, so I built a good 13mm Rotaroc that would get a decent time (particularly when piston-launched) and usually place second. I think Art was disappointed that I gave up on this event, but I felt that my efforts where better spent on the other 4-5 events each meet where I had a better chance of success. (Of course, that trauma has persisted and since then the helicopter events have almost always been flown by my teammate Chuck Weiss. :)

Back to today, that same helicopter from 25+ years ago is the one that I flew for this event. One advantage of a mediocre contest model is that it almost always comes back to you. :) It's been patched up so many times its hard to tell what is original and what is replacement parts, but it still keeps flying (although I worry that someday I might get a separation DQ for some stray bit shedding off it. :)

I built my model based on plans in the March 1980 issue of the Model Rocketeer (now out of print.) My model used Estes BT-5 tubing (13mm) and 1/16" balsa for its fins and 1" x 12" rotors. I used Estes A3-4T motors for my contest flights, but the model can use any mini motor with a short delay. George Gassaway has made instructions and plans available for building the 13mm Rotaroc:

We had a brief interval of nice weather on Saturday, December 11 (the day before we had a trace of snow, the day after we had 3/4" of rain.) I thought the model and my equipment was pretty much ready, so I didn't do much prep until the morning I decided to fly. Terrill talks about having the your equipment ready and adjusting to conditions, and this would play a role here.

Having just the one model to fly, my plan was to prep it, take it out into the field and fly it, bring it back to the car, and repeat. So I headed out into the field with just my model, piston launcher, launch controller, and a few accessories. My piston has a 1/8" wooden dowel that I stab into the ground to secure it. I go to plant it and find that since my launch last month, the ground has frozen (doh!) and I can't get it to penetrate the ground (I eventually broke it off trying.) And I don't have my range box and any tools or anything else to put into the ground to attach the piston to (unless I hike back to the car.) So I looked around and finally found a small patch of snow that I packed down and managed to mount the piston shaft in.

The model took off quickly, but its trajectory was about 15 degrees off vertical towards the northeast. That meant the flight was more of a high-speed parabolic arc than the traditional up-stop-down path. Due to the higher speed at ejection, the model did not "flip" as usual and begin a tail-down descent, but instead came down nose-first. (It could have been worse, many helicopters that eject at high speed don't even open their rotors.) So, in spite of the higher (or at least farther) boost from the piston launcher, the upside-down descent was less efficient and resulted in a sub-par 32.53 second flight. Back at the launch site, the piston rod was tilted at a similar angle, making it look like the poor anchoring of the piston was the cause for the wayward boost (not some random piston tip-off, which sometimes occurs.)

For the second flight, I decided to play it safe and fly off a rod. The boost wasn't as energetic, but perfectly vertical, the model deployed near apogee, and made a textbook tail-down recovery. The flight time was 50.29 seconds. (Which makes me want to kick myself, since I'm sure it would have gotten over a minute with a proper piston boost. Oh well, another lesson learned... :)

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Photo Gallery

(click on an image to view a larger version)
Overall view of the slide-pod B RC R/G

Jeff Vincent posing with his ancient 13mm Rotaroc (we even match! :).

Back again at the local high school. The weather was mostly sunny, with light winds, and temperature around 40 degrees F.

Flight one ready for launch from the piston.

The piston ended up to be a poor choice and resulted in a boost about 15 degrees off vertical and the model descended upside down. Duration was 33 seconds.

The recovery of flight one.

No damage to the model, but descending inverted cut efficiency and descent time.

The model posing between flights in its deployed configuration.

At this point in its life, the primary construction material is either cyano-acrylate or masking tape. :) It just keeps going, though.

Flight 2 ready to launch from a 1/8" rod.

The flight was nominal and resulted in a 50 second duration. Even though it didn't boost as far (and probably even not as high) as with the piston, the descent was more efficient and slower.

Flight 2 during recovery, showing proper Rotaroc form.


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