BLEVEN’S TECH TIPS

Tech Tips July 2004

WHEEL ALIGNMENT GEOMETRY-PART TWO

CASTER ANGLE and LEADING OR TRAILING AXLES



Most of our airplanes have three wheels for the sake of simplicity. This number is also driven partly by the physical shape of the airplane and the bulk and weight of the gear. Whether tricycle or taildragger, typically one of the wheels does the steering. The steering wheel on an airplane turns around a pivot pin called a KINGPIN. If the airplane gear uses balljoints or oleo struts, you will need to imagine this kingpin, like you had to imagine the axle in relation to the camber angle in the last tech tips article. To better understand this, it is easiest if you think of the steering fork on the typical bicycle. These forks are basically always angled forward or to the front. THIS MOVES THE STEERABLE WHEEL FORWARD OF THE PIVOT OR KINGPIN AND IS AN EXAMPLE OF POSITIVE CASTER ANGLE. This is the arrangement we almost always see on airplanes. NEGATIVE CASTER ANGLE IS ALMOST NEVER USED as it tends to erase all of the benefits of positive caster angle. Some of the benefits of positive caster angle are listed below:


  1. As the steerable wheel is turned, the wheel will camber towards the direction of the turn.

  2. This action will increase the positive angle of the camber into the direction of the turn.

  3. This effect on the camber angle will also try to turn the wheel into the turn.

  1. The positive camber angle will also cause the wheels to pull to the side of the most positive camber angle. (We can increase or decrease the amount of this pull by choosing leading or trailing axle placement. Leading axle placement will offset some of this pull and trailing edge placement will increase this pull into the turn as we lean the bike when the bike is not moving. With the bicycle moving, we lean into the direction of the turn so the comparison to our airplanes should stop here.)

The other concept we need to look at along with caster angle is that of “leading or trailing axle placement”. We now know that caster angle tells us the general location of the axle of the steerable wheel in relation to the pivot or kingpin, positive caster being ahead of the kingpin pin or pivot, negative caster being behind the kingpin or pivot. That general location is further adjusted by the choice of either leading or trailing axle placement. To understand this concept, you need to draw an imaginary line downward through the very center or core of the kingpin or pivot, a line that defines the center of rotation of the kingpin or pivot. Continue the imaginary line down to that point at which this line is abreast of the location of the axle of the steerable wheel as viewed from the side of the airplane. IF THE AXLE OF THE STEERABLE WHEEL IS AHEAD OF THIS POINT, IT IS AN EXAMPLE OF LEADING AXLE PLACEMENT. IF THE AXLE OF THE STEERABLE WHEEL IS BEHIND THIS POINT, IT IS AN EXAMPLE OF TRAILING AXLE PLACEMENT.


Our airplanes create kinetic energy when they move, and the steering components will need to absorb this energy in order to effect the turn. As the steering components absorb this energy, the effect will be that of increasing the weight or the pressure on these components and on the air pressure in the tires. (remember that one for a later part of this topic) The caster and camber angles we use are intended to balance these loads and to allow this kinetic pressure increase to force the steering wheel to track straight with a minimum of amount of pressure, so that it may be turned easily with rudder or brakes when under way. If excessive or a large amount of trailing axle is used on our airplanes (in both full swivel nose wheel and tail wheel airplanes) as the wheel is turned to one side or the other, it will twist the spring and create more negative caster angle to the kingpin as it is turned. This generally is not a good thing because it will change the angle of the kingpin in relation to the newly established turning track of the wheel. In other words, the caster will go from positive to negative as the spring twists, and will lose the likelihood of tracking straight ahead. It may even tend to turn towards the lowest position or ninety degrees from a straight forward track. For this reason, the nose wheel or the tail wheel should not be required to carry more weight than the twisting of the spring can support and still maintain positive caster when the wheel is turned ninety degrees to the side.

Next month, Toe-In or Toe-Out?

And putting all of this geometry together to optimize control.

The fourth and last article on this subject will deal with wheel shimmy and tire wear.