Control Line: Aerobatics
Ted Fancher
Response Rate and Pitching Moment
Last month we were discussing response rate and, up to the point where we left off, everything we had spoken of (namely, pitching moment) was in opposition to our desired pitch change. Let's discuss how to make it turn the right way.
The prime motivators are the elevators, to make a pun. More correctly, it is the lift produced by the tail (stabilizer and elevator) and its distance from the center of gravity (CG). The more efficient the tail at producing lift, and the further that lift is produced from the CG, the more leverage it will apply about the CG. It first overcomes the opposing pitching moment of the wing's lift and then rotates the aircraft about the CG until the pilot neutralizes the controls.
Tail Volume and Effectiveness
Once more, consider tail volume (area times average chord). The area of the stabilizer and elevator should normally be between 20% and 25% of the wing area. Both larger and smaller areas have been used, but to no discernible advantage. Area should be juggled with tail moment to arrive at the desired tail effectiveness. Longer arms are generally more effective in increasing response — but tend to make the ship tail-heavy more rapidly than using more area.
Higher aspect ratios (A.R.) of the tail surfaces will produce more lift at given angles of elevator deflection and will therefore be more responsive. Again, if the A.R. is made too high, smooth control inputs will be difficult to maintain and, for instance, round maneuvers will tend to show little flat spots as minor control corrections create comparatively major pitch changes. Notice the dichotomy between the implied preference for fairly high aspect ratios for response rate and low aspect ratios for stability. Compromise is necessary. I opt for lower aspect ratios and accept the need for more deflection and area to obtain the lift necessary for a given rate of pitch change.
Control Deflection and Flap/Elevator Ratio
It has been stated by some authorities that only 20° or 30° (or some such) degrees of control travel is necessary to perform a hard stunt corner. This statement is only true for a particular set of conditions for a given airplane, not as a general rule for all designs. The amount of control deflection for a given rate of turn is variable, based on airspeed, aspect ratios, and most especially on the CG relationship to C/L (center of lift) and the amount of pitching moment. It will take more control deflection for a given turn rate early in a flight with a full fuel tank than at the end of the flight when the CG has moved aft due to fuel having been consumed.
Because the flaps and elevator move as a unit — i.e., in a fixed relationship with each other — lift is increased by the flaps as a result of control deflection with no consideration of the amount of lift actually necessary to support the G loads of the corner. It becomes obvious that we are probably generating more lift than required at any given time. If this were not the case, we would frequently stall our stunter ships in hard corners. This additional lift increases pitching moment, which means reduced rate in pitch.
When we first began the discussion of response rate last month, I spoke of pitch rate which is predictable and controllable, in addition to being rapid. This is where it becomes important to keep the lift-required / lift-produced equation in balance. Lift in excess of that required by the G forces produced will accelerate the aircraft in the direction of change, once pitching moment is overcome.
Lift vs. Rotation — The Jumping Effect
On the surface this sounds like a good deal, but what actually occurs is that lift acts at right angles to the wing chord line, and the airplane leaps in the direction of the lift — which is not the same as rotating to a new body angle, which is what we are seeking. The result is an airplane which jumps in hard corners, making their shapes difficult to control and their rates and final position difficult to predict.
Wing Thickness
While discussing lift, let's talk about wing thickness. It has often been stated that we should use thick (18%–20% plus) wings to increase lift potential. After the foregoing, you probably realize I no longer share this opinion. While reasonable thickness is desirable (12% or more for a ballpark figure) to prevent early flow separation and stall, very thick sections are seldom needed for the sake of lift. Structural considerations are probably a more important factor. The higher the aspect ratio, the thicker the wing should be to prevent flexing. This is apt to be controversial, but since I don't feel extra thickness does any particular harm (thickness in an airfoil shape causes minimum drag increase and is much less significant to lift than is area), make the wing as thick as you like.
Aspect Ratio and Leaping Acceleration
This phenomenon also argues for moderation in our pursuit of high aspect ratios' many virtues. Because higher aspect ratios increase lift coefficient at a greater rate with angle of attack increases, it can be seen that excessively high A.R.'s will induce the same leaping-type acceleration in sharp corners. Only an educated guess, but I suspect that's part of the reason Denny Adamisin tries to fly his very-high-A.R. stunter at very slow speeds — to mitigate the very rapid buildup of lift that would result from the combination of high speed and high lift coefficient at a given angle of attack.
Design Considerations for Line Tension
Let's see what we can do in the design stages to optimize line tension. Not a whole lot, it would seem from a perusal of the Trim Table (last month's column). That's basically the truth. We can decide whether and how much of the following to use:
- tip weight
- wing asymmetry
- engine offset
- leadout position
- rudder configuration
For the most part, these items act in concert with one another more to ensure we can take advantage of the real source of line tension — the engine's horsepower — than to actually produce it themselves. It is worth stating that there is little we can do to produce line tension and a great deal we can do to lose it.
From the design standpoint, consider wing asymmetry (i.e., inboard wing larger to equalize lift, since it flies slower on the outboard). There have been national and world championship stunter ships with and without wing asymmetry, and I therefore conclude it to be not significantly important. Since equal-panel wings require more tip weight to balance lift requirements for both wings, I would opt for a certain amount of asymmetry to reduce the need for significant amounts of weight a long way from the CG.
While wing asymmetry seems a matter of taste, the outboard flap should probably always be slightly larger than the inboard. Here's why: the wing tip weight is used to balance both the lift differential of the two wings and to overcome the weight of the lines and control system, all located in — or attached to — the inboard wing. The amount of weight that is proper in level flight is increased in effectiveness by the G loads in a hard corner and will, therefore, cause the ship to roll in that direction (popularly known as hinging). By making the outboard flap a little larger, when the corner is being flown the outboard wing will have a slightly greater increase in lift, thus offsetting the increased G load on the tip weight.
Engine offset is helpful, particularly in the instance of lost line tension, as it tends to return the ship to the end of the lines where it belongs. The same can be said for a small amount of rudder offset. An adjustable rudder is probably the best alternative, although I have personally gone to a fixed surface airfoiled on the inside (per the Nobler) and have been happier for the lack of yet another adjustment to make.
The final design parameter influencing line tension is leadout location relative to the CG. Its location is the same as that established in the stability objective, roughly 3° aft of the fore-and-aft location of the CG at the fuselage centerline. An additional consideration, which is very important when building a stunter of unusual planform (such as semi-scale ships with dihedral), is to consider the leadout's exit point relative to the vertical location of the CG. They should be in line. If they aren't — say the leadouts exit above the CG — in flight the CG will align itself with the exit point, resulting in the aircraft banking toward you. The resulting bank is the single most effective way of throwing away line tension (short of running out of gas).
We'll have a lot more to say about line tension in our discussion of flying trim. For now let's look at the trim objective of speed control.
Trim Objective: Speed Control
Speed of an airplane is governed by two things: thrust and drag. Here's another irreverent statement from Fancher: reasonable amounts of form drag (from the shape of an object moving through an air mass) and parasitic drag (from skin friction, lines, wheels, etc.) is a good thing for a stunt ship. We are all flying engines which are operating significantly below their horsepower potential (i.e., at much lower rpm figures than optimum) in part to keep airspeed manageable. The sleeker we make a ship, the faster it goes for a given amount of thrust; ergo, the slower we must run the engine; ergo, the lesser amount of horsepower we're getting from the engine, and so on. By not only accepting — but even seeking — sources of drag, we develop the means whereby we can operate the engine closer to its peak horsepower without losing control of the speed. So, go ahead and put on the bombs and antennas and big wheels, etc., however, make sure you've really got the horsepower available first.
There's one other form of drag about which we can't be so cavalier, and that's induced drag (drag induced by the production of lift). Here, again, higher aspect ratios will reduce the amount of induced drag for a given amount of lift. Just be sure to consider all the caveats mentioned earlier about excessive aspect ratios.
Tracking
Tracking has several design aspects which have been mentioned previously. An explanation of tracking may be helpful. The maneuver descriptions require that consecutive maneuvers be flown concentrically — in the same spot — within two feet of the track of the first maneuver. If a stunter locks on to a rate of turn in a round maneuver and tends to follow the same radius smoothly with little control input, it is said to track well. If it requires constant control trimming to maintain the radius, and the input causes noticeable flat spots in the rounds, it is poor tracking.
Consider tail volume. If the aspect ratio of the tail is too high, angle-in lift with minor control inputs will be larger, resulting in noticeable pitch changes and flat spots.
Less simple are the effects of pitching moment (CG to C/L) and flap effects, which we will consider together.
The lift required in a loop is not constant. Going up, lift must oppose gravity, and pitching moment will be large, requiring larger control inputs to both overcome it and to establish the desired radius of turn. Over the top of the loop and coming down inverted, required lift is lessened, inasmuch as gravity and lift now work together. This reduction in lift reduces the pitching moment and requires that control be backed off to retain the radius of the loop.
As you can see, the greater the variation in pitching moment of the wing, the greater the variation in control input required from the pilot to maintain constant radius. Therefore, contrary to popular opinion, adding nose weight is not the proper solution to most tracking problems, since it will increase the arm between CG and C/L. This is another sound argument for keeping the lift produced in line with the lift required.
Still my own opinion here: the flaps have a strong effect on tracking. Particularly influential is their spanwise distribution. As flaps get shorter, tracking suffers. Flaps should be long in span and narrow in chord.
A rule of thumb: flaps should be full-span, or nearly so, and their chord should range from 15% to 20% of the wing chord, depending on aircraft weight. In addition, their hingeline should be straight. Since a stunter flies in a constant left skid, a straight hingeline assures that the airflow across the airfoil is as consistent as possible for the entire span.
Uniformity of Turn (Pitch)
The prime design factor here is decalage. Decalage is a fancy term for alignment of the wing and tail. To be blunt, there should be no decalage in a stunter. The centerlines of the wing and tail, and for that matter the thrust line, must all be parallel. Any misalignment of any of these in either design or construction will result in a plane which turns differently inside versus outside. Don't do it.
Secondarily, be aware of where you locate large draggy parts of the airplane. Uneven vertical distribution of drag relative to the CG will result in unequal turns. No rules of thumb for this one — you must eyeball the side view and make an educated guess on drag distribution. The same goes for thrust. Don't mount your engine on a pylon above the tail and expect your ship to turn well inside.
Conclusion
This about winds up the design phase. Next month we'll do a preflight and get started flying.
Ted Fancher 158 Flying Cloud Isle Foster City, CA 94404
Transcribed from original scans by AI. Minor OCR errors may remain.
