The rough rule of thumb for dynamic hydroplaning (i.e. aquaplaning) is 9 x SQRT (Tyre pressure). If tyre pressure is in PSI, this gives the minimum hydroplaning speed in knots. If the car tyre pressure is 36 psi, hydroplaning should happen above 54 nm/hr or 100 km/hr which is roughly consistent with the figure that Ann quoted. That's why Harvards can fly in formation skimming along the water and super-cubs with 18" balloon tyres can land on the water in Alaska and roll up to the river bank (See video in http://www.alexisparkinn.com/aviation_videos.htm)
The formula is only an approximation and the tread width and tread length also has a bearing on the speed. The other types of hydroplaning such as viscous and reverted rubber (as in wombat's worst-case scenario) happen at lower speeds. There are many internet references on the subject if anybody wishes to find more information. Happy landings, Patrick Barfield > >A car will normally aquaplane when it is travelling at approximately >112 kph without cruise control on. There is nothing you can do >about it once it starts. > >It happens often on the motorways in the UK. > >Ann There is an old formula relating aquaplaning speed in MPH to some factor times Square root of (tyre pressure in PSI) - but I can't remember the factor. I thought there was a 27 there somewhere - perhaps SQRT of 27 x TP which roughly fits Ann's numbers. Water depth must also exceed ability of tread to clear the water, so worn tyres increase the risk. Anyone else remember the formula? Worst-case scenario is aquaplaning on melted rubber from locked wheels, which rapidly wears flats on the tyres. However if you lock wheels the water also accumulates more than if the wheels are turning, and the aquaplaning speed decreases dramatically - this is the origin of the Dunlop "Maxaret" anti-skid device which is the precursor of ABS brakes. Both work by releasing brake pressure if one wheel stops rotating and the others keep on turning. Wombat _______________________________________________ Aus-soaring mailing list Aus-soaring@lists.internode.on.net To check or change subscription details, visit: http://lists.internode.on.net/mailman/listinfo/aus-soaring
