CONTROL LINE AEROBATICS
Frank McMillan, 12106 Gunter Grove, San Antonio, TX 78231
I'm going to continue the technology discussion this month. So far, I've looked at composites, structures, and power plants. The contrast between where we are now and how far we have come is astounding.
I happened to see the original drawings for Don Still's ’49 Nats-winning Stunter the other day, and I was really surprised at the contrasts in the design: huge wing, very short coupling, semi-molded fuselage construction, and it was a take-apart! But the wing construction was a true D-tube, which George Aldrich says may be the first Stunter to use that type of construction. With maneuvers under 30° and a speed well over 90 mph, George said it was the hot combination in the late forties.
After looking at these drawings and some others that George had, I've finally reached the same conclusion that others have: we should have a procedure to approve additional designs for Old-Time competition. After all, there are many worthy designs from that era that deserve to see the light of competition. With proper documentation, we can still maintain the spirit of the event.
When I looked at Don Still's design, I saw little comparison in the side view to later designs. It is tiny and had a distinctive appearance. But a look at the wing and the D-tube construction aside, the airfoil and size (696 square inches) are in the ballpark of where many of the current designs are. The fully sheeted fuselage was developed for easy construction, as George says, but it's no big stretch to compare it to much-later molded-shell efforts. The point to be derived from this bit of our history is that there is much to be learned from the past and from the pioneers of our event. An often-paraphrased quote applies here: those unaware of history are doomed to repeat it. Our next big advance may be sitting out there waiting to become a relevant piece of the puzzle.
There are also numerous examples of crossing technology from other model fields. Consider Frank Williams' unusual motor: front-exhaust, rear-drum valve O.S. .40, which is basically an RC boat engine. Tuned-exhaust systems were used in FAI Speed models, but they really came from two-stroke motorcycles. Modern aircraft technology gave us composite materials and many of the new adhesives. The quest for performance has really only begun.
For years, there have been occasional attempts to significantly improve retracting-gear performance. Harold Price made one of the earlier attempts with his Crusader design, but it enjoyed only limited success because of the mechanical design of the retraction systems.
From the late fifties through the late seventies, there were rumors that leading fliers were seriously considering a retracting-gear project. Bob Hunt was looking into it, but deferred.
Al Rabe had the Sea Hornet primarily designed for a retract system; however, he stopped competing before he completed the project. At the time, there wasn't a satisfactory retraction mechanism available off the shelf, and control of the retraction cycle had not been solved. Al was going to design and fabricate the entire system, so the Hornet sits in storage.
Recently, several competitors reopened the case. Bob Whitely has a prototype system working in a classic Rabe Mustang. He demonstrated the system at Vintage Stunt Championships VI, and it certainly looked impressive. It was impressive in flight: locked in and groovy. The retracts were commercial RC units, and the brains of the system were a Jomar Autoretract servo controller.
Bob had worked with Jomar to define the requirements. The 3/10-ounce unit controls the retract servo and the battery powers both the servo and controller. A switch activates the unit. After an adjustable delay (between five and 15 seconds) the servo retracts the gear (but only if the engine is running). The controller monitors the engine with a tiny microphone mounted on the circuit board and drops the gear when the engine stops.
Bob Hunt also has a prototype retraction system. He has been working on this completely different design for several years. Airworthiness has been proven, and all that remains is constructing a competitive airframe. A design is on paper, and some work has been done on the actual construction. As anyone who has explored retracting gear knows, the design of the installation is not trivial. Bob is using the lost-foam technique he originated to develop the rib sections after locating the retract mechanisms.
We look forward to seeing retracting-gear models in competition to evaluate the competitive edge. After all, there are positives and negatives to be evaluated. There's no question that considerable drag is eliminated by tucking in the gear. On the other hand, there is a weight penalty for the associated mechanisms and the power pack. Bob Whitely says that the weight increase is between five and seven ounces. Bob Hunt says his system should be approximately half of that.
There are important design considerations, not the least of which is the significant change in the vertical center of gravity that occurs upon retraction. Bob Hunt has made the adjustment by looking at a totally in-line configuration. This may be the ideal situation, but since there hasn't been much testing, we don't know for sure. However, competitive components now exist; perhaps more widespread use will develop retracts until—like the tuned pipe—many models use them.
In my limited experience, however, the biggest performance gains to be achieved are still in propellers. If you have never felt the "instant airplane effect," you haven't lived. I vividly remember the first time I felt it. Hi Johnson gave me a YS Q 10 x 6 to try at the '58 Nationals, and everything improved immediately: vertical line tension, corner, etc. It was just amazing! After I broke that prop, I could never get quite the same feel again.
Some years later, I had a prop for the S.T. 60 model, and although it never seemed to work in Texas, it was another story up north. I flew it in three Nationals, finally breaking it at a team trials. It was highly thinned and re-pitched. Although I have tried on numerous occasions to duplicate it—and did, to the thousandth of an inch—those efforts did not yield the same results. Perhaps the wood on that good prop flexed in a certain way.
The movement to carbon-fiber props eliminates the variations in props caused by different grains. The carbon/epoxy configuration is also significantly stiffer than other materials. Therefore, props can be thinner and more efficient. The real question is: do they work, the Bollys and Ethers? The answer is yes, but sometimes it takes work on the flier's part to tailor props to a specific model.
Now these props have been around in various forms for a number of years. The idea that they can be re-pitched using heat to soften the epoxy is significant; essentially, they can be cut and dried without being able to return to a baseline. The carbon has memory, and reapplying heat returns it to the molded pitch. Make no mistake: altering any propeller assumes risk, because the manufacturer most certainly voids any warranty.
Although it would be unrealistic to say that a 12-1/2 x 4-3/4 prop is the prop for an S.T. 60 in your model, it's probably a good candidate. I can only suggest that you work with what competitive fliers use.
There is significant performance to be gained if this is pursued, perhaps more increase than any I've discussed in this series. Refer to a discussion of the low-pitch series of props in previous columns. This is a proven approach that deserves your attention.
Workshop News
No model workshop is complete without the new airplane ceiling fans now available. Several companies are making them, and they are neat. Hunter makes a Navy Corsair nose, and another P-40 nose is carried by Sam's. Check them out!
Transcribed from original scans by AI. Minor OCR errors may remain.
