Control Line: Aerobatics
Ted Fancher
Introduction
Back in the air, again. We finally got the wing flying level both upright and inverted. Now let's do a few loops, both round and square, inside and outside, as we start to investigate several of our Trim Objectives more or less simultaneously: Stability, Response Rate, Tracking, and Turn Uniformity. In addition, we're going to start "thinking" about our control handle. (Editor's note: This column is the fifth of a series which started in the May 1985 issue. RMcM)
When you fly loops, does the airplane turn a consistent radius, smoothly ending each loop at the same altitude it began? Or does it jump into the maneuver and consistently finish higher? Conversely, do you have to force it out of level flight and fight to keep it out of ground at each bottom? Does it react the same inside and outside, or does it turn easily one way and hard the other? Keep these responses in mind as we wait for the gas to run out. Don't fall asleep, however, as one of the most important trim clues manifests itself when the engine quits.
Diagnosing trim from engine shutdown
Watch the reaction of the airplane closely as the engine dies. Its response, power off, is going to tell us a lot about whether the symptoms of mistrim we witnessed in flight are related to the airplane trim or if the handle may, in fact, need to be adjusted.
If your airplane took off and climbed smoothly, tracked well in level flight, responded uniformly and positively to your control inputs, chances are that when the engine quits the nose will drop just slightly, tension will remain positive (slightly reduced, of course, with power off), and you will have fairly precise control of the rate of descent and touchdown. If this is your happy state of affairs, congratulations! The hard part is almost over.
If, on the other hand, your ship leaped into the air, flew level flight everywhere except where you wanted it, maneuvered like a drunken cat chasing a mouse, etc., you have a bit more work to do. When the engine quit, did the airplane pitch nose-up? Did line tension nearly disappear? Did the landing spot just appear rather than being selected? If this is the case, you have a tail-heavy airplane. Add nose weight until it no longer behaves that way.
If the airplane behaved much like the previous example, but when the engine quit the nose did not pitch up and the line tension did not decrease while the glide control was jerky and unsmooth, your control system is most likely too sensitive. Do not add weight to the nose. Reduce the line spacing on your handle, depending on the severity of the problem. Reducing the spacing by as little as 1/8 in. is noticeable, but for this early coarse adjustment, I would go for no less than 3/8 in. at a time. Move both lines an equal amount at this point. We may yet have to bias the line spacing, but we're nowhere near ready to quit trimming the airplane yet.
If the ship had to be pulled into the air, grooved smoothly in level flight but responded slowly to control inputs and showed almost no interest in cornered maneuvers, your problem is more complex. If, upon engine flameout, the airplane noticeably drops the nose and you must react fairly quickly to pull it up for a smooth glide; if line tension remains good and if you can "whip" readily to extend the glide, the airplane is almost certainly nose-heavy. Try the following:
- Lighter muffler or spinner.
- As a last resort, add weight to the tail until the response rate is correct.
Carburetion and mechanical checks
If the preceding case fits the observed performance—except the glide is smooth and controllable with no abrupt tendency to drop the nose when the engine quits—but while flying the ship in the pattern it seems sluggish in the corners yet handles briskly on the straight-ins, the carburetion may be suspect. If the carb is set so that the engine runs too rich while the ship is in level flight, only when the throttle is opened for inside maneuvers is sufficient air drawn into the engine to reach the true mixture; as a result the engine will pull harder and the ship will suddenly spring into the inside corner. To check for this condition, lean the low-speed mixture slightly, but not enough to cause a rough idle. If the problem disappears, there is your answer.
If carburetion adjustments do not correct the sluggishness, check the control system for mechanical problems such as binding in the bellcrank area, sticky bearings or rod ends, friction in pushrods and quick-links, etc., and make sure the control throws and handle spacing are correct. If you most likely have too insensitive a control setup, increase line spacing 1/8 in. per flight until response improves.
However, if the ship took off smoothly, grooved in level flight, handled nicely after engine shutdown (or, heaven forbid, got light on the lines and pitched up), yet turns sluggishly and unpredictably, your problem is even more complicated.
Increasing response rate
For a situation like this, we must determine what is keeping the airplane from pitching at the rate we desire. We have determined (by other means) that the CG is roughly correct (smooth takeoff, good groove in level flight, proper response to engine shutdown), so simply adding tail weight isn't the best solution.
Check the table (in the May 1985 issue) for response rate; most factors for controlling pitch are built into our plane: tail volume, tail effectiveness, aspect ratio, etc. We do have a couple of aces in the hole, however.
- Reduce the ratio of Flap to Elevator travel (Trim Objective II, item 4). This accomplishes two things: it reduces the amount of flap travel (thus reducing negative pitching moment) and increases the amount of elevator deflection (Trim Objective II, item 11) for a given amount of flap travel, thereby increasing Tail Effectiveness (Trim Objective II, item 3).
- Seal the hingeline (Trim Objective II, item 7) between the stabilizer and elevator. Lifting surfaces produce lift by the pressure difference between top and bottom. If the hingeline gap is greater than about 0.1% of the tail chord, there will be significant leakage and therefore loss of lift. Sealing the gap increases lift and thus Tail Effectiveness, helping your rate of response.
- Propellers and horsepower (Trim Objective II, items 8 and 9) go hand in hand. One force that retards response rate is drag (primarily induced drag). We must have adequate thrust to overcome drag induced in sharp maneuvers. Stunt engines run well below peak-power rpm in the classic setup. You can increase airplane response rate by increasing engine rpm and thus thrust. More nitro and/or larger air intakes will increase power without necessarily increasing airspeed, but they will increase fuel consumption. Smaller-diameter props are a very useful method of increasing power and will almost always improve turn rate. Additional pitch will move more air over the tail and increase its effectiveness; however, care must be taken not to overload the engine. More pitch will also increase airspeed (if the engine can handle the load), which will also make the airplane more responsive, assuming your control system has sufficient mechanical advantage to overcome the loads on the flaps and elevators.
- As a last resort, increase elevator size or reduce flap size. Use of "kickers" is, of course, mandatory in either case. (Yucch!)
Climb/dive tendency and handle trim
Okay, we've got the thing gliding well and turning great, but we can't get it to turn and level at the same time. First, remember that a CL plane, especially one with a light wing loading, will naturally climb into the wind and dive a little downwind as the velocity of the wind adds and subtracts from airspeed and therefore lift. We can't trim that out completely.
Remember that a comparatively nose-heavy airplane must fly at a slightly higher angle of attack in order to compensate for the greater pitching moment about the CG. Therefore, the nose-heavy airplane will aggravate the climb/dive syndrome in the wind.
If we still have the problem of hunting in level flight, consider the handle neutral position (Trim Objective I, item 1). Any reasonably straight airplane should want to fly straight and level if it is in balance and stable. If it wants to do so at an elevation different from the "rule book" altitude, we have a handle-trim problem. If, in upright level flight, the airplane wants to climb and, in inverted flight, it wants to dive, you most likely have too much flap relative to the elevator. Reduce flap travel or increase elevator travel (or otherwise change the flap-to-elevator ratio) until upright and inverted level flight occur at the same handle position.
Leadout position and hingeline sealing
If the blamed thing doesn't respond to handle adjustment, we have a couple more zingers. If the leadout position (Trim Objective I, item 4) is too far aft, a vertical component of P-factor (Trim Objective II, item 10) is produced due to the airplane flying with the nose pointing outward of a line tangent to the circle. The lower half of the prop arc (upright) will produce more thrust than the top and the ship will want to climb. Control input must resist this. The reverse is true when the plane is flying inverted.
If this "leadout-too-far-aft" problem exists, it is probably manifesting itself in another form of instability. In tight corners, the airplane will jerk on the lines in a fashion similar to hinging but will not be felt by the pilot. When the leadouts are moved forward, that jerk should disappear and the hunting in level flight should decrease. If not, seal the flap hingeline (Trim Objective II, item 5).
I have personally fought sealing the flaps over recent years because I felt that, unless you are stalling your ship in corners, you have no need for additional lift (the primary byproduct of sealing any hingeline). While I still feel that, I witnessed a near miracle at Reno—one that I was able to repeat. Gary McClellan's three-year-old Stunter had steadfastly resisted all of our best efforts to eliminate an almost uncontrollable hunting problem. We had given up and he just flew it that way to our embarrassment. Yet, on arrival at Reno and ever since, that airplane flew like it was on rails in level flight. I finally asked Gary what he had done. It turns out he sealed the flap hingeline to get a little more lift for the high density-altitude. Lo and behold, it totally cured the hunting.
The last two days of that Nats, my Celebration started to hunt badly, something it had never done before. Inspection (after the Nats, of course) revealed a broken hinge which was allowing the flap to flex and open the hingeline gap. Repairing the hinge and sealing the hingeline eliminated the hunt. Take it for what it's worth.
Final thoughts on stability
If your airplane doesn't come out of corners hard and flat (that is, it turns either too much, or not enough, or it hunts) you are experiencing another form of stability problem. It is my (controversial) opinion that the conventional solution of adding nose weight is totally wrong unless you have other indications of the CG actually being too far aft. Moving the CG forward increases pitching moment and, therefore, the amount of control input required for a given corner increases.
I feel a better solution is to reduce the line spacing at the handle and to move the CG aft a little. At some point, the plane will exit the corner hard and flat in a repeatable fashion. As always, avoid extremes. Make changes a little at a time.
Ted Fancher 158 Flying Cloud Isle Foster City, CA 94404
Transcribed from original scans by AI. Minor OCR errors may remain.
