Control Line Aerobatics
Ted Fancher
Welcome back, Stunt fans. In the last column prior to the Nats issue, my parting comment was a promise to discuss control handles and the ways in which they affect how your Stunter flies—both the obvious and, in some cases, perhaps, mysterious considerations. As I started to outline the data, it rapidly grew too voluminous to be covered in a partial column. It also became apparent that some illustrations would be necessary. Since I lacked the time this month to prepare the material properly, and since I have some loose ends to cover based on previous columns, I have decided to clean up the leftovers now and to devote most of the next two months to control handles.
October's column on P-factor and gyroscopic precession, coincidentally, appeared the same month as Windy Urtnowski's discussion of supposedly the same phenomenon in his Flying Models column. Unfortunately, other than the term "gyroscopic precession," there seemed to be no common ground between the two subject matters. I was discussing yaw effects, and Windy the deleterious effects on the rate of turn. "Are these two living on the same planet?" you might well ask.
In addition, I just received a very clever and well-thought-out letter from Gary Bickers (Riverside, CA) bemoaning the fact that he had just installed the bellcrank in his new Stunter with the up leadout in front, as had such luminaries as Bob Aaron, Al Rabe, Tom Dixon, and Stan Powell. These men have published very successful designs. He was shocked to read my column which suggested this was wrong. He asked, tongue in cheek, if his new ship was destined to failure, since a one-time Nats Stunt champ may have had the natural laws of aerodynamics altered to suit his specifications.
I'm almost sorry I brought this up again. Almost, but not quite. The subject, although interesting, is assuming proportions far in excess of its importance. In order to put this to rest, I'll devote just a few paragraphs to a short discussion of the basic properties of the gyroscope and an admission that, "Yes, Gary, you can trim a Stunt ship to fly with a front up line. Although it will be 'better trimmed' if built my way, with a rear up line."
Gyroscope basics
A gyroscope exhibits two basic properties:
- Gyroscopic Inertia (GI): GI is the property which makes a gyroscope appear to defy gravity. The spinning mass resists forces applied to it and tends to displace its plane of rotation away from the applied torque. The heavier the mass or the greater the rpm, the more it resists. GI is so dominant a force that it is used as the primary stabilizing device for airplanes, space vehicles, and ocean liners.
- Gyroscopic Precession (GP): GP is the phenomenon by which an applied force produces a response 90° further along in the plane of rotation. A down-to-earth example is the inertial effects of the spinning wheels of a bicycle. The faster you go, the more stable it becomes. Come to a stop, and it falls over. To eliminate the confusion between my column and Windy's, GI is the phenomenon which inhibits rate of turn, not GP. Windy's analysis is no less correct: adding mass to the prop/spinner—or increasing rpm—does retard the rate of turn. Unfortunately, he mislabelled the forces as gyroscopic precession.
To quickly touch base with GP, return briefly to your bike ride for an enlightening revelation. Remember trying to impress your pre-teen heartthrob with your prowess at riding your Schwinn with no hands? Anybody could ride straight ahead (good old GI at work), but you could make turns as well. Simply by leaning to the left or right, your two-wheeled gyro obediently and predictably followed suit. Your weight created a force which caused the wheels to tilt despite their inertial reluctance to do so. This force, applied at the top of the wheels, propagated a movement in the same direction but at a point 90° further on in the plane of rotation, thus turning the wheel to the left when you leaned left, and vice versa. Voila! GP at work!
None of the foregoing, by the way, alters my position on the relative importance of P-factor — i.e., much more significant than GP.
To satisfy Gary's inquiry, however, I must state that we are dealing in an area of subtle trim refinements, the whole subject being better grist for the après-Stunt pizza-and-beer together than for "tear your airplane apart and start over!" material. There are a great many very fine-flying Stunters around the world with front up lines! (Yes, Gary, I did know it was being done.) It's just that they would have been a wee bit better had they been built the other way.
As a teaser, I think that the upcoming discussion on control handles will show that a lot of the trim problems for which we are inveighing the evils of GP, GI, P-factor and other such esoterica will be shown to be correctable by application of a thorough understanding of the three basics of the handle: response rate, control feel, and neutrality. The handle does a lot more than just keep those skinny wires from cutting into your sweaty palms.
Now that I've accused my good friend Windy of confusing the Stunt world, it only seems fair that I own up to some unwarranted overkill on my own part. Knowing me (as you all do by now), I'm sure it'll come as no surprise to learn that I've got an explanation. (Read that last bit as "excuse".)
Prior to the Reno Nats, I belabored the problem of density altitude and the debilitating effect it would have on our conventional Stunters. Throughout the land, the cry was for "helium Stunt," and I was the head cheerleader. Believing totally in my infallibility as a wizened veteran of both the Stunt wars and "real airplanes," I invested six months of my life building an invincible, ultralight entry, designed especially for Reno's thin air. Off the board at 41 ounces, into the air at 43, off to Reno at 44 ... and into the closet the day before competition began — there to join most of the other superlights.
Windy Urtnowski, Paul Walker, and I all opted to fly our older, heavier ships rather than the ultralites we had been convinced were necessary. Needless to say, we were exasperated, and at least I was embarrassed after all the hype and cry in print. What happened? Why didn't we need the lighter ships, and why did the old planes apparently fly so well?
Upon reflection, I feel that the following analysis to be of merit:
- We did, in fact, have to make some adjustments. Almost everyone did fly faster and with either more nitro (Paul Walker and Bob Whiteley over 30%), more pitch in the prop, or both. No one competitive was flying over five seconds a lap. Jim Casale was consistently timed as fast as 4.5 seconds per lap on max-length lines — incredible reflexes! So that part of the analysis was correct. What was surprising was that pilots found themselves able to fly — and fly well — at such speeds.
- We never flew in the heat of the afternoon, and thus my worst fears were never realized. The one time I tried to practice in the 100°-plus afternoon heat, I fried my only good motor trying to pull out enough oomph to get through a pattern.
- The historical perspective. In the past 10 years, since the inception of PAMPA's influence in 1974, Nats-level Stunt has become much more competitive. Realization of the need for improved performance has made the 60-plus-ounce, .46-powered ship and the oversized and overweight .35 ships of any type into Stunt dinosaurs — an extinct species.
Because this progression was gradual, it took a kick in the head such as Reno to bring to light the overall improved wing and power knowledge now almost universal among knowledgeable precision aerobatic pilots. Today's Stunt competitor is to be admired for his ability to adapt to the competitive needs of the event. The nearly 15% reduction in average wing loadings and the improved powerplants developed in the last 10 years made additional quantum improvements in those areas unnecessary in order to compete at Reno's high altitude. The deficiencies of the ultralights, primarily lack of rigidity in flying surfaces and engine platform, overcame any possible advantage which might have been realized by the additional slight decrease in loading.
Anybody want to buy a slightly used Stunt ship? Write to:
Flexi-Flier, Aircraft Division 158 Flying Cloud Isle Foster City, CA 94404 Attn. Ted Fancher, Head Designer
Transcribed from original scans by AI. Minor OCR errors may remain.
