BLEVEN’S TECH TIPS

Tech Tips August 2004

Toe-In or Toe-Out ?

Most of our airplanes only allow for a minimum of adjustment for sagging springs and old, slightly worn parts. As these planes age, it is important to have an understanding of the basic geometry of the gear in order to keep them in a safe operating condition.

Camber on the main gear should be positive and equal on both wheels. Generally, more camber angle will require more toe-in to keep the wheels from spreading apart. The tendency to cause the wheels to spread from this camber angle must be offset with a minimum amount of toe-in between these wheels. A-frame type gear with pivot bushings may have encentric type bushings that can be rotated, and bungee type springs usually will increase camber when tightened. Flat spring steel gear legs with bolt on type flat flange axles can be shimmed between the mounting bolts to change camber and/or toe-in angles. Round rod type gear will require re-bending or rotating to achieve good results. Bent, stretched, or otherwise damaged gear often is apparent to the trained or practiced eye(look around next time you are at the airport).

Equal tire pressure is critical to keep the rolling diameter of the tires equal and thus equal camber and toe-in of the main gear tire pressure of the steering gear will usually be the same as the main gear, but we will go into that next month.

The range of the angles employed in our planes is dependant upon the gear length, softness, and the load carrying requirements for your plane. Generally positive camber angle should only go to negative when over stressed, toe-in is only set enough to offset the spreading caused by the camber angle(often this angle is changed as the load of the airplane is increased or decreased and a compromise is needed). If there is too much toe-in, the tires will wear on the outside edge, too little and they wear on the inside and your plane becomes “squerrilly” or difficult to control.

It is important to remember that the main gear of your plane carries most of the weight and therefore will have most of the influence for directional control. The steerable wheel, in conjunction with the drag of an occasionally lowered aileron, is used to “aim” this load. FOR THIS REASON, you must be able to easily turn the steerable wheel, and to maintain good straight ahead tracking ability. POSITIVE CASTER ALONG WITH TRAILING AXLE will magnify the tracking effect. Too much of this will cause you to find it hard to turn. Too little of this and your plane will turn too easily and as such, it could become easy to overcontrol.

At the beginning of a turn, the kinetic energy is initially absorbed by the steering wheel(when on the ground). This energy will cause an increase in the load bearing on all of the steering components, even including an increase of air pressure in the tire if pneumatic. As the turn progresses, the outboard main gear will absorb this energy. Fortunately, with our planes, high speed turns are also supported by the flying surfaces.

With a nose wheel airplane, as the plane is rotated(for takeoff) the camber angle and the toe-in, in relation to the new angle of the gear, will change. If this change causes a toe-out condition, the airplane could become squerrilly at this new angle. Therefore it should be measured with the airplane at this angle as well as when level. With a tail wheel plane, the raising of the tail is a rototation in reverse of a nose gear plane, and the toe-in may become excessive, thereby causing a pinching or rising effect to the main gear. This is a good way to help you to bounce when doing wheel landings. On occasion, some tail wheel planes are better off with zero camber angle and even slight toe-out in the three point position in order to perform well for wheel landings.

Next month we are going to discuss how to identify the causes of tire wear and wheel shimmy.