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The closest I have seen to an original source, and the numbers that seem to be quoted by most pilots:
Aquaplaning can occur when running in water and in cetain depths and densities of slush. Present indications are that simple dynamic aquaplaning is unlikely to occur in water depths much below 0.2 in. although under some conditions the minimum depth may be as low as 0.1 in. Once aquaplaning has commensed it can be sustained over areas where the water depth is less than that required to initiate aquplaning and to lower speeds that that required for initiation. A simple formula has been derived from data obtained during trials which shows the relationship between the minimum speed at which aquaplaning may commence and the aircraft's tyre pressure. This states that the minimum initiating aquaplaning speed in knots is approximately nine times the square root of the tyre pressure in pounds per square inch. Speed in this context is of course in context of true ground speed and not indicated airspeed, although it not be precisely calculated... Page 188 (Typing errors are mine) There was an old thread on this topic that had a link to a Dunlop site that was unteresting, but was lost in a server upgrade (sorry!) ------------------ Tech Log forum moderator |
Checkboard,
Fully agree with your post and J P Davies. I've spent the last couple of days since this thread re-activated trying to internalise this. For a while there, I got lost in images of a military or business jet aircraft with small high pressure tyres slicing through standing water, while the big low pressure tyres of a widebody aquaplaned on top. That didn't help me internalise it very well. Finally gave up, pulled out the books and built a spreadsheet to run some numbers. I used the L1101 and the BAC 1-11 (since I've got the data for both). Both run similar tyre pressures of 175/174 psi respectively (about 1200 kPa). At MTOW, the large main gear tyre on the L1101 has a footprint of 279 square inches, and on the BAC 1-11 the main gear tyre has a footprint of 136 square inches. Both have the calculated contact pressure of 175/174 psi (the same as the tyre pressure). Both should encounter dynamic aquaplaning at 119 knots and static aquaplaning at 102 knots. Take the load off both planes, and the weight/tyre decreases. The radius of contact (and we assume here a circular contact area aka the footprint) decreases, and the footprint of the tyre decreases. Calculate the new ground contact pressure, and it remains the same at 174/175 psi. For the BAC 1-11 at 50,000 lbs weight, the contact area is down to 68.25 square inches, and the contract pressure is 174 psi. L1011 at 200,000 lbs has a footprint of 135.71 square inches and a contact pressure of 175 psi. In all the weight changes, the aquaplaning speed doesn't change. One can intuitively visualise the onset of aquaplaning. The water is trying to push the tyre upwards off the runway, and this is being resisted by the weight on the tyre pushing down, together with the area (or footprint) of the tyre. These two are brought together as a single expression in terms of the contact pressure. The contact pressure is the same as the tyre pressure. So the aquaplaning speed is therefore represented by a function of the tyre pressure. |
Wel....I'm glad I asked.
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The discussion so far has focussed on dynmic aquaplaning...
There is, of course, viscous aquaplaning and reverted rubber aquaplaning. The former normally occurs on very smooth surfaces at low speeds. The latter can result from locked wheels superheating water and causing the tyre rubber to melt and trap steam beneath the tyre. |
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