BLEVEN’S TECH TIPS

Tech Tips June 2004

WHEEL ALIGNMENT GEOMETRY-PART ONE



Why do we need to concern ourselves with the subject of wheel alignment in our airplanes anyway? Actually, there are two very good reasons for us to have some understanding of this subject, the same two reasons we have for caring about wheel alignment in our automobiles. Firstly, wheel alignment has a direct bearing on our CONTROL of our automobiles and airplanes (while on the ground), and secondly, alignment has a lot to do with TIRE WEAR.

The geometry of wheel alignment for an airplane is dependant on a couple of factors, the type of gear we have, and the number of wheels involved, which could be one, two, three, or four. Let’s first look at how geometry varies depending on how many wheels are involved.

When we only have one wheel, as in the case of some gliders, or in a wheelbarrow, the only geometry concern we have is locating the wheel properly in relation to the center of gravity to provide the best handling control. Only if this wheel is not properly centered and/or not square does tire wear ever become a factor.

When we go from one wheel to two, and when the two wheels are connected by way of a solid axle running between them, as in an automobile or some early airplanes, we need to consider another geometric factor, camber. What is camber? Camber is the vertical angle of the wheel in relation to the horizontal line of the solid axle. Right? Imagine you are looking at your plane from in front of it, and it is in its normal upright position. The axle is approximately horizontal and the tires are approximately vertical. The angle that is formed by the axle and the tires is the camber angle. If your landing gear has no horizontal axle, you will have to imagine that it does in order to understand what is being said here.

There are two types of camber angle, positive and negative. Positive camber angle, which is the more common geometry, exists when the horizontal distance between the tires is smaller at the bottom than at the hub or the top of the tire. Only in special cases is negative camber used.


Some points to remember about positive camber:


  1. It would cause wear on the outside of the tire if it were not for some other dynamics involved

  2. It will help to hold the wheel onto the axle when under a load

  3. It will reduce the rolling and braking load upon the outer ends of the axle

  4. Due to flexing of the gear leg, camber will go from positive to negative as loads on top increase and decrease

  5. Your airplane will pull to the side with the most positive camber geometry

  6. Picturing the up and down flex, we can begin to see some of the reasoning for wanting to achieve positive camber geometry in our landing gear wheels

  7. When we are applying wheel braking, or when wheel rotation friction is increased due to soft grass or mud, the wheels are pulled backward. When this occurs, the toe out position is moved outward, and the camber angle is reduced as the wheels spread out from these loads.


These are some of the dynamics involved in setting the wheels on your airplane at certain angles. Remember, the tires do not wear until the airplane moves over the ground and various loads are imposed on the gear and tires. Not unlike most dynamics, each factor affects the others. The factors we will be considering in the world of wheel alignment geometry include: camber, caster, toe in, and the weight and angle of the gear in relation to the surface, and we will be looking at them in order to gain better control and handling of our airplanes, and to minimize tire wear.

Wheel geometry is often not considered as important as flight geometry. On the contrary, a landing accident is often more than just ego deflating.


NEXT MONTH PART TWO OF WHEEL ALIGNMENT

THREE, AND FOUR WHEELS. CASTER ANGLES.

CASTER WITH LEADING AND TRAILING AXLE PLACEMENT

(AS USED ON MOST OF OUR AIRPLANES)!!!