August 1, 2019

How Many Tires Needed and Why?

When it comes to land vehicles, it is a little more understandable, but when it comes to aircraft, it becomes difficult to carry out ideas.

How many wheels do planes have and why?

There is no single answer to this question, but of course there are certain criteria. In a nutshell, we can summarize under several titles.


Material Strength

As expected, there is material strength in the first place. If the materials used on each wheel of the landing gear were not sufficient to carry the weight of the aircraft, the number of wheels could be increased. The concept of "aircraft weight" is not just the weight of the aircraft itself. When aircraft hit hard the runway during landing, wheels could be subjected to an extra pressure of up to twice the aircraft's own weight. So there has to be as needed wheels as to resist this pressure. Airbus A340 has an extra pair of wheel under the fuselage to support the weight of the airplane. 


Number of Brakes Needed

There are no brakes on the front (nose) wheels of the aircraft. This forces the brakes to be placed only on the wheels under the wings. Planes usually land at high speeds and they have to stop before the runway ends. Or they could suddenly decide to stop for an emergency while running to take off at full speed. Therefore, they need to stop at a much shorter section of the runway. For that, too, strong brakes are needed. If a certain number of brakes are not sufficient to stop the heaviest configuration of the aircraft, additional wheels may be added due to the need for additional brakes. For example, the Boeing 737 has 4 brakes, the Boeing 767 has 8 brakes while the Boeing 777 has 12 brakes. Antonov-225 cargo plane is famous with its 28 wheels as it has to stop with very heavy loads.



Runway and Taxiway Strength (or Thickness)

Yes, you didn't read it wrong, which is among the key factors. The number of wheels can be increased if an aircraft is planned to make flights to airports with relatively weak (thin) runways. Since the number of wheels is more, the pressure per wheel will decrease, so the thin runways can carry the aircraft without breaking. The Boeing 757, for example, is therefore specifically designed with 8 wheels. Tupolev 154, as well also has an extra 4 wheels (12 wheels in total) for the same reason.


These planes hit the thin runways hard or they make their way with the full load on taxiways. The pressure reduced by the extra wheels to prevents damages on the pavement.


Weight Saving

If there are no problems such as runway and/or material strength, it is aimed to have the least number of wheels. Since each wheel will cause extra weight, having as few wheels as possible means increased useful (pay) load. For this reason, instead of increasing the number of wheels or brakes on jet fighters landing on aircraft carriers, use hooks or nets so that aircraft can stop on very short runways.


Aircraft Design

Concorde aircraft had to land and take off at a high angle, so the risk of striking its tail on the runway was higher compared to other aircraft. It may not have caught your attention so far but the Concorde also had a hidden double wheel in the tail section. Of course, that was just a problem of the Concorde.

The same risk was for the wings of American B-52 and British Harrier aircraft. The risk was eliminated by putting extra wheels on the wing ends of these aircraft.