ESDU 90035 provides methods for estimating the drag experienced by aircraft tyres running in a layer of water or slush (with specific gravity down to 0.8) on a runway. Configurations covered are single tyre, twin-tyres (side-by- side), twin tyres in tandem, dual-tandem tyres (two pairs of twin-tyres one behind the other), a single row of multiple tyres side-by-side and two such rows one behind the other. The effects of yaw and of tyre spray alleviators are also considered. The estimated values exclude the drag of the undercarriage upper leg above the axle or undercarriage chassis and forces due to spray impingement on any other undercarriage or other parts of the airframe. The methods use graphically presented empirical factors applied to the single wheel drag. The single wheel drag is also empirically derived and presented graphically as a function of speed (as a fraction of the planing speed) and deflected tyre width at the runway and at the liquid surface (for which estimation procedures are also given). The derivation of the methods is fully discussed, together with sketches showing the comparison between estimated and test values drawn from the available literature. Worked examples illustrate the use of the methods for typical twin-tyre and dual-tandem undercarriages. Estimates are also made for the undercarriage drag of the Convair 880 in slush over a range of speeds and are compared with the widely-quoted set of full-scale test measurements.