RADIO CONTROL SCALE AEROBATICS
Eric Hawkinson
319 Yellowstone Ave., Billings, MT 59101
Since my last column I've received three phone calls from pilots who were frustrated with their airplanes; two were Giant Scale Aerobatics models, and the other was a Pattern aircraft.
All of the pilots were convinced that their airplanes were either designed poorly or set up incorrectly because of problems they were experiencing with one maneuver: the Stall Turn — specifically, the "pivot" at the top of the Stall Turn. (The Stall Turn and the Hammerhead are the same maneuver.)
The Stall Turn can really make a pilot question his or her abilities or aircraft. Through the years, the most common complaint I hear from aspiring Aerobatics pilots involves airplanes that "won't" Stall Turn. In most cases the pilot is local, and we meet at the field so he or she can show me the offending aircraft, complete with attempts at the maneuver that either flop or fly around a big arc in the sky.
The usual outcome of these flight demonstrations is that the pilot hands me the transmitter in disgust. I do a few Stall Turns in both directions, then land. Sometimes the pilot is relieved to see the airplane behaving well; other times he or she is visibly annoyed with me for being able to fly the maneuver with the model. I guess it's more desirable to hear someone agree that the equipment is the problem, rather than demonstrating that perhaps it's lack of technique.
It's exceedingly rare to find a four-channel airplane that cannot do a decent Stall Turn. I don't recall finding a sport or Aerobatics model that couldn't do the maneuver quite nicely.
Not many setup and design issues affect the Stall Turn. Airplanes with very small rudder area will be more difficult, and may require a larger rudder. Airplanes with very low amounts of rudder deflection will have problems, but that's simple to rectify with servo travel, output arm, or control horn changes.
It's also possible for the center of gravity (CG) to affect the Stall Turn. My experience has been that an airplane that flies well in other maneuvers has a CG location suitable for performing that maneuver; therefore CG is usually not a big factor.
Assuming that the CG is reasonably located and that the rudder is of adequate size and throws 30° to 35° at full stick, what else causes problems with the pivot portion of the Stall Turn? Timing and airflow.
Timing and airflow
By definition, the pivot at the top of the Stall Turn is performed as the airplane has almost halted its forward speed on the vertical line. One critical factor is the timing of the rudder input; if you put the rudder in too early, the airplane will fly an arc at the top rather than pivot. This is subject to various degrees of downgrade: a complete arc will earn a zero (and looks bad), while a semi-pivot more than one wingspan wide earns a smaller downgrade.
Ideally, the rudder will be input just before the model reaches zero airspeed. Wait too long—especially on the larger models—and you are likely to enter a short tail slide that will develop into a flop. That results in another "zero" score.
"The Stall Turn can really make a pilot question his/her abilities or aircraft."
Plus your pals may snicker. (If you don't care about scores, just think of your efforts as learning to do a Stall Turn the "correct" way!)
Now it's time to look at airflow: the more important factor in the pivot, and the most commonly overlooked. Without adequate air moving over the rudder, it cannot produce enough yawing power to make the airplane pivot.
The answer to the airflow problem is obvious: you need to ensure that the propeller is moving enough air over the rudder to make it effective. Since the forward speed is almost nil, there is not enough airflow for the rudder to be effective without the air from the propeller.
On some airplanes it will be easy because very large and powerful rudders coupled with large propellers produce a fair amount of thrust even at idle settings. In the case of these airplanes, proper timing alone can result in a picture-perfect pivot.
On most airplanes, the only way to reliably achieve the perfect pivot is to have some power applied as you "kick" the rudder. This will allow good airflow over the rudder and let it do its job of swinging the tail around.
Power-setting methods
There are a few ways to ensure that you have adequate power settings for the pivot.
- Use an idle-up function: It can increase the idle speed just for Stall Turns or decrease the low-stick idle speed for maneuvers that require a minimum tickover (spins and landing). Either option works as long as you always know what idle-up functions you have active. A disadvantage is that you must flip a switch and keep track of which "idle" setting you have active.
- Use the throttle stick as primary control (my method): This doesn't require any programming or switch-flipping and is infinitely variable. There are two basic options when using the throttle stick; which you use depends mostly on aircraft performance and personal preference:
- For an airplane with unlimited vertical performance: Reduce the throttle smoothly to idle as the airplane approaches the height at which you want to perform the pivot. As the airplane stops, smoothly add a small amount of throttle and "kick" in full rudder. As the nose passes 90° of rotation, you can reduce the throttle back to idle, since most airplanes need very little power to fly straight down. This approach allows the most precise control over positioning and performing the pivot. The drawback is that the throttle and rudder must be input correctly, so it's slightly more difficult to master.
- For heavier, draggier, or less-overpowered models: Leave the throttle at full and perform the pivot at the top as the airplane slows to a stop on its own. The biggest drawback is that if you wait a split-second too long to apply rudder, the model might start to tail-slide, and there's nothing you can do about it since you have already applied full throttle. The advantage is that there is only one real variable: the timing of the rudder input. This can be simpler if you're just learning.
If you are having problems with the pivot, try adding about 25% throttle at the same time you hit the rudder, as the airplane slows almost to a stop. If that causes the airplane to climb while it pivots, then you have added slightly too much power. When the airplane is vertical, you will be able to find a combination of forward speed, throttle setting, and timing of rudder input that will work.
Then the only problem you might have is a tendency for the airplane to roll when it pivots. This is usually only a problem with sport-trainer-type models and ones with a lot of dihedral. Either mix this roll coupling out, or learn to input aileron as needed to keep the wings level.
More notes on the Stall Turn
All Stall Turns start out the same, with a quarter-loop from horizontal to vertical flight. The vertical line is established, and it may or may not contain a roll or fraction of a roll. At the top of the maneuver is the "pivot," then a vertical down line (which again may include some portion of a roll), and recovery to level flight.
Three problem areas are inherent to the quarter-loop transition from horizontal to vertical flight in the basic (non-rolling) Stall Turn:
- The wings need to be level when the quarter-loop is initiated; otherwise the airplane will not transition to a true vertical line.
- The vertical line may need to be corrected for wind conditions. (See my former article or AMA Competition Regulations for IMAC [International Miniature Aerobatic Club] rules regarding wind correcting and vertical lines.)
- The performance of the quarter-loop needs to be constant radius and sized to fit the maneuver and the performance of the aircraft. The novice tendency is to pull too hard, making the quarter-loop smaller than optimum. A smooth pull will cause fewer problems and give more time for subtle corrections.
While the quarter-loop provides numerous challenges to a pilot who wants to perfect the Stall Turn, most pilots can work on any problems in this portion of the maneuver and eventually get good results. I don't get many calls on the quarter-loop, because it's somewhat easier to see where the problems are and what solutions are needed.
I'm out of room. Let me know if you like this sort of column. Until next time, have fun and fly safely!
Transcribed from original scans by AI. Minor OCR errors may remain.
