The Und Nu plan published in the last issue was a great success. Most people are building them without rudders which makes for some interesting ground handling. That's not to say that a rudder solves everything, though. Our Web Wisdom this time is from Don Stackhouse, posted to the EFLT list in response to a query about a ground looping Cub:
Regarding this thread on ground looping, there are several things to consider. One of the biggest factors is the fore-and-aft position of the main wheels relative to the C/G. The main wheels of a tailwheel aircraft are ahead of the C/G by some amount, and their steering force during a yaw tends to make the yaw worse. Also, the force of the ground against them tends to push the nose back up at the moment of touchdown on landing, which can cause bouncing. If you move the C/G forward, or the main wheels back, you can reduce both of these tendencies.
However, if you move the wheels back too far, the airplane will be prone to nosing over. The generally accepted compromise is to find the vertical and horizontal location of the C/G, draw a line downward and angled forward by between 10 and 15 degrees, and put the main wheels somewhere along that line (usually determined by the prop's ground clearance requirements).
Dynamic stability of the aircraft is also a big factor. An airplane with a high wing loading and lots of mass in the extremities will be very difficult to stop once a serious yaw gets started. The biggest positive factor in improving dynamic stability is a very long tail moment arm. Lots of tail area also helps, but not nearly as much as lots of moment arm.
The damping effects from a generously long tail can nip a bounce or a groundloop in the bud. The Spitfire is considered by experts such as the late Jeff Ethell as probably the easiest of the WW II tailwheel fighters to land. It has the wheels fairly well ahead of the C/G (good noseover resistance), but is has a very long tail moment arm. We see the same effects in our Roadkill Series indoor/backyard Spitfire.
One of the easiest tailwheel R/C models I've flown is the Peck Polymers "Prairie Bird". Its mains are WAY ahead of the C/G, giving it outstanding noseover resistance. We would normally expect that to make it a bouncing and groundlooping monster. However, it is blessed with the humongous tail moment and enormous tail surfaces it inherited from its free flight ancestors. As a result, bounces and ground loops never have a chance to get started. It's better behaved on the ground than many of the tricycle-geared airplanes I've flown.
The full-scale J-3 Cub has the mains well forward of the C/G, but relatively low mass in the extremities and a moderately long tail moment. It does like to bounce, although ground loops usually aren't a serious problem unless there's a lot of slop in the tailwheel steering linkage (one more item to check on models as well). Generally takeoffs aren't a big problem, it's the landings. You do have to pay attention, and it does not like pilots with "lazy feet". A full-scale Cub is an excellent trainer, both for flying and on the ground, because although it will rarely let you get into enough trouble to break something, it also will not cover up your mistakes. It won't let you hurt yourself, but it will embarrass the daylights out of you for anything less than perfect piloting technique. It's a gentle but firm teacher, with high expectations for each of its students.
Toe-in can also tame a recalcitrant ground-looper. Canting the tops of the wheel outward can help a little as well. About 2 degrees of toe-in and outward cant on each wheel seems to be about right in my experience. When the airplane starts to yaw, the wheel on the inside of the yaw starts to roll more easily, while the one on the outside of the yaw starts to slide. The resulting braking effect on the outside wheel tries to correct the yaw. It's helpful for stopping small yaws early in the process, but once a serious ground loop gets going, toe-in is of little further help. Prevention is the best medicine in this case. Just don't let things get that far out of control to begin with.
Gyros
There are two kinds of gyros. One is called an "attitude
gyro" the type used for artificial horizons in full-scale
aircraft. This type is sensitive to position. The gyro holds
a particular attitude relative to the universe, and if the airplane
changes bank angle or pitch attitude, the gyro doesn't, and the
resulting difference between the gyro's attitude and the airplane's
attitude shows up on the gyro's display.Since the human body
can't tell the difference between centrifugal force and gravity,
you need an instrument like an attitude gyro to tell which way
is "up" when flying inside of clouds.
The gyros we use in models for improving stability are the other type, what's called a "rate gyro". These are gimballed a little differently, with a spring that tries to hold the gyro in a fixed position relative to the airplane. The turn indicator in a full scale aircraft is also a rate gyro. Instead of measuring the aircraft's attitude, these gyros measure how fast (i.e. the "rate") the aircraft's attitude is changing. In the case of a rudder gyro, when the aircraft starts yawing at some rate, the precession forces in the gyro's rotor cause it to fight back against the centering spring. The spring gets deflected by this precession force.
The faster the airplane is yawing, the greater the precession force and therefore the greater the deflection in the spring. This is then measured, and a command is sent to the rudder servo to try to stop the yawing motion. The amount of rudder correction depends on how rapidly the airplane is yawing, and on the "gain" setting (i.e. sensitivity setting) of the gyro.
Piezo gyros use a tiny vibrating crystal pendulum to accomplish the same thing with a lot less moving parts. They can be either rate or attitude gyros depending on their design.
Houseflies have a similar gyro system to help them maintain their sense of orientation. The aft pair of wings has evolved into a pair of tiny rods with knobs on their ends. You can see them just behind and below the wing roots if you look very closely. These rods vibrate in flight, and the plane of that vibration tends to stay constant relative to the earth. If the fly's attitude changes, it can sense this through the change in the relative orientation between it and the plane of vibration of the rods.
Thanks to Don Stackhouse for permission to print this article. To read more of this sort of useful stuff, go to Don's web page and browse the "Ask Joe and Don" section.