Load on Wheels
The load on the wheels determine the tire size. Wheel load is the aircraft weight distributed over the number of wheels. The aircraft CG position could vary depending on the extent of payload and fuel-load distribution; therefore, both the forward – most and aftmost CG positions must be considered. (Table 7.4 provides an idea of the A380 load.)
As soon as the preliminary undercarriage information is known from the methodology described in this chapter, aircraft weights and the CG can be estimated through the formal procedure described Chapter 8.
Estimating the aftmost CG with the angle в & 15 coinciding with 40% of the MAC gives a preliminary idea of the main-wheel position relative to the wing. The wing position relative to the fuselage could change when the formal weight and CG estimations are determined after the wing is sized. In that case, the wheel-load calculation must be revised. For transport aircraft design, at this stage, the forward – most CG is 20 to 25% of the MAC ahead of the aftmost CG. For the nontransport category, including combat aircraft design, at this stage the forwardmost CG is 15% of the MAC ahead of the aftmost CG. The MTOW rather than the MTOM is used in the computation because the load is a force. (A simplified approach is to divide the main – and nose-wheel loads as 90 and 10% distribution, which has a
reasonable result, but the author recommends making the formal estimation at the beginning.)
Linear distance is represented by l with associated subscripts; R represents reaction forces. For more than one wheel, the load would then be divided accordingly. The force balance gives:
MTOW = 2 x Rmain + Rnose (7.1)
To compute the maximum main-wheel load at the aftmost CG position, take the moment about the nose wheel. The moment equilibrium equation becomes:
Ibase x Rmain = Inrearcg x MTOW
or Rmain = (Inrearrg x MTOW)/Ibase (7.2)
The load per strut on the main wheel is:
LM = Rmain/number of struts (7.3)
To compute the maximum nose-wheel load at the forwardmost CG position, take the moment about the main wheel. The moment equilibrium equation becomes:
Ibase x Rnose = Imrorwardjcg x MTOW
or RNOSE = (lM-FORWARD-CG x MTOW)/lBASE (7.4)
The nose wheel typically has one strut.
Ensure that the load at the nose gear is not too high (i. e., no more than 20% of the MTOW) to avoid a high elevator load to rotate the aircraft for liftoff at takeoff. Also, it must not be too low – that is, not less than 8% of the MTOW; otherwise, there could be steering problems.
For more than one wheel per strut, the load per tire is calculated based on what each tire would produce on the same runway pavement stress at the same tire pressure as a single wheel. This is the equivalent single wheel load (ESWL) because loads are not shared equally when arranged side by side, unlike tandem arrangements. Wheel arrangements determine the ESWL as given here based on statistical means. Readers may consult the references for more details on other types of wheel arrangements.
The tandem twin wheel is:
ESWL = load per strut/2 (7.5)
The side-by-side twin wheel is:
ESWL = load per strut/(1.5 to1.33) (this book uses 1.5) (7.6)
The tandem triple wheel is:
ESWL = load per strut/3 (7.7)
The side-by-side triple wheel is:
ESWL = load per strut/(1.5 to1.33) (this book uses 1.5) (7.8)
Table 7.1. Vertical speed
VVert, = < 12 fps – FAR 23 (semi-empirical formula for exact rate, nl = 3)
VVert, = < 12 fps – FAR 25 (nl = 2)
VVert, = < 10 fps – Military transport (nl = 2)
VVert, = < 13 fps – Military trainer (nl = maximum 5)
VVert, = < 17 fps – Military land-based combat aircraft (nl = maximum 6)
VVert, = < 22 fps – Military naval (aircraft-carrier)-based combat aircraft (nl = 8)
The twin tandem is
ESWL = load per strut/(3 to 2.67) (7.9)
The main-wheel loads are calculated based on the aftmost CG position and the nose – wheel loads are based on the forwardmost CG position. The dynamic load on the wheel is 50% higher than the static load.