Radio Control Aerobatics - 2009/05
By Eric Henderson <[email protected]>
The meaning and effect of precision
Primary intent of "precision aerobatics"
The primary intent of calling an event “precision aerobatics” is to illustrate that there is a requirement for accuracy in the maneuvers that are performed. You can find this detail in the AMA Competition Regulations, which is available to AMA members upon request.
A secondary meaning of the word “precision” overflowed into the design and selection of materials used in RC Aerobatics (Pattern) model construction.
Precision in building and human behavior
This transference from the goal of precise flying to precise building is a fairly normal piece of human behavior. I spent four years of my professional career as a quality assurance manager. Did the job come home with me? It did not take long before it spilled over into my car-maintenance schedules. Soon I was upgrading the home heating system and improving the windows and roof insulation.
If you compete in RC Pattern, it probably won’t be long before natural progression takes over and precision becomes a big part of your airplane construction. I doubt if it would be hard to accept the next jump either.
If you are into precision, there is only a short hop to buying more expensive, ergo higher-quality components. You will tend to buy the “best.”
What “the best” is can be debated forever, but a Pattern pilot will tend to use the latest hardware, lightest materials, tightest connections, most precise servos, etc., wherever possible. It could be a better hinge or a connector that has less slop in the pushrod-to-horn connection. Exotic pushrod materials are common, as are the almost standard carbon-fiber wing tubes.
Endless attempts are made to add lightness, and all are aimed at keeping the model’s all-up weight below the maximum. A ball-race clevis (not to be confused with ball-link clevises) has great accuracy because it exhibits no detectable slop between the horn and the clevis.
Pattern pilots go to extraordinary lengths to set precise engine alignment and wing and stabilizer incidence. For precise flying, these settings are critical and can take many days or even weeks of test-flying to get right.
The neutral airplane
Designers are trying to create the neutral airplane. In brief, this model stays at the same angle and attitude until instructed otherwise. It will not roll level, as a trainer will, if left alone.
A neutral airplane is almost impossible to achieve with propeller-driven models (FAI Pattern does not allow jets). You have to deal with P-factor, or reaction to the propeller’s rotation against the air. Some enterprising individuals came up with counter-rotating propellers, but so far the side-thrust/down-thrust solution is the only one holding its own.
It may surprise you to know that mechanical/physical rules have little effect on the design of Pattern airplanes. Some may argue that the change to Turnaround schedules in roughly 1985 had a significant effect on the competition models that were subsequently designed. That is probably true, but the construction rules did not motivate or prevent the changes. There is still nothing in the sporting code to hold back your imagination when looking to design a Pattern aircraft.
I must accept that Pattern may give you the impression of having many rules, but most of them are related to competition issues—not aerodynamic design. It is a misconception that Pattern design constraints can stop you from making your model meet any aerodynamic shape that your heart desires.
Some influences are competitive, such as symmetrical wings. These do not create lift at a zero angle of attack.
Pattern airplanes do maneuvers that need a wing to fly at all altitudes and angles, especially inverted. To fly right-side-up, we do so with the wing at a slightly positive angle of attack. This works great until you go inverted. Then, to stay level, the angle of attack is adjusted back to positive with the elevator. The less elevator or rudder needed, the better.
When not required to provide lift, the symmetrical wing cuts through the air at a zero angle of attack. This would be the case in knife edge or going straight up or down.
The best Pattern airplanes I have flown were neutral in all aspects of flight, except for a little pull to the canopy in a long power-off downline. This was easy to fix with slight down-elevator trim mix at low throttle using a computer radio.
Rolls, losses of height, and design responses
Have you ever wondered why your model drops its nose in a roll, sometimes radically? Try a really slow roll (with a bit of altitude, please), and you will see a pronounced drop in height at a certain moment as the airplane rolls over.
What Pattern aircraft designers did first was figure out what was happening. Most pilots can perform a roll with a sport airplane, but what kind of roll? Anyone can bang the aileron stick over and hold his or her breath.
A precision Aerobatics slow roll has some requirements that the pilot must meet. It needs to last a minimum of three “long” seconds. (A second can feel like a long time when there is the potential of your pride and joy plummeting to earth because you deliberately made it go inverted.)
For grins, add the constraint that the model must roll around its central axis (often called an axial roll) in a straight line, with no weaving up or down along its track. Then add that it must be flown parallel to the runway at roughly 150 yards out, no matter what the wind is doing. The roll rate must also be constant; a quick flip out at the end would not score well.
Great pilots can get most models to roll close to the needs of the scoring rules. However, it would not be easy with an airplane such as, say, a Piper Cub. It has a quasi-flat-bottom wing section and responds with a rolling action to rudder input. That means the roll rate will accelerate and decelerate when you add the rudder correction to keep the nose up.
A superhuman pilot could anticipate all of this interaction and adjust the controls all the way through the roll. The rest of us normal humans would need some help from the airplane. That’s precisely what Pattern model designers decided to do: make aircraft that help you do better maneuvers.
Initially, the design thinking focused on the side area of the fuselage, so fuselages got taller. This was because this extra area helped in the part of the roll where the model was mainly flying on its side. The airplanes still dropped their noses when rolling onto their sides, but the corrective rudder inputs got much smaller.
There have been many designs in which extra side area was created using a central vertical fin, as shown at the Pattern Nats a couple of years ago by Richard Lewis. He could have put another fin on the bottom and created an X-plane if he could have come up with a way to land that idea! There is nothing in the rules to stop his design, other than the total span could not be more than 2 meters.
As designers got more feedback, they realized that as an airplane rolls there is a moment between the wing providing the lift and the fuselage taking over where the lift disappears. A square edge on a flat-bottomed fuselage causes a sudden change in lift. What they did was increase the girth of the fuselage and make it oval in section; they made the corners go away.
In today’s designs, just as the rolling wing stops providing the necessary lift, the fuselage smoothly takes over. The current fashion of oval, large-side-area fuselages handles the transition to the knife-edge position. All of this takes some pressure off of the pilot and helps produce more precise maneuver execution.
The new designs still need corrective input from the tail feathers, but if you do a slow roll with no other inputs, the overall height loss is small compared to that of their predecessors. These designs help the top pilots do better maneuvers and make it easier for others to advance their flying skills more quickly.
Pattern models for sport flying
In the past, I have compared precision Aerobatics to Formula 1 cars. I have had to rethink this position when it comes to ease of driving/flying. In Formula 1, the cars may go much faster, but I doubt if they have been made easier to drive than a sports car.
If you try a Pattern model for general sport-flying, you will get an airplane that is smooth and does what you tell it to do. It may be a bit hard to slow down, resulting in longer landings, but it will be precise.
Practical flying tips
Try these at your field:
- Entering a loop: Why enter a loop at full speed? All you really do is skid into the loop and quickly burn off speed. Try entering at three-quarters throttle and adding full power as your airplane goes up. You might be pleasantly surprised by how good the loop works out.
- Stall Turn: Instead of cutting the power and waiting to apply the rudder, when climbing the vertical line drop back to half throttle and then feed in the rudder just as the airplane stops. As it comes over, go to low throttle. It will rotate on a dime and look professional. You can dictate which way the stall goes and not be at the wind’s mercy.
Transcribed from original scans by AI. Minor OCR errors may remain.
