Most people believe that water on the road surface goes through the slots in the tire tread allowing the tire to grip the road, but if you think about it that is simply not possible.

If you take a water depth of only 1/32nd of an inch, 10 inches wide and 88 feet long (the width of one of our Jag tires, and the distance covered in 1 second at 60mph) then approximately 1.5 gallons of water must be forced through the tire tread slots every second. That is physically not possible. If we go faster or the water depth increases then the amount of water that must pass through the tire treads also increases, compounding the problem. The truth is most of the water is pushed out of the way by the tire, not channeled through its tread.

A tire hydroplanes in stages. As the tire speed increases; the amount of water it must move increases, and so does the dynamic pressure it takes to move it. Dynamic pressure continues to increase with speed or water depth until it exceeds the tire pressure. At this point the tire begins to deflect, and the water begins to build a wedge between the tire and road surface. At a given speed or depth of water the pressure reaches a point where it forces the wedge completely under the tire. At this point there is no contact between the tire and the road, the tire is hydroplaning.

Given the above scenario then it would seem logical to think that increasing the tire pressure will prevent tire deflection caused by dynamic water pressure, and thereby delay the onset of hydroplaning. Interesting you should think that…

Let’s look at another vehicle that goes fast over a road like surface in the rain. Aircraft tires are, at least in tread design very simple. They only have circumferential slots cut in the casing, no clever designs to move the water, or give better grip, just slots. Aircraft calculate the speed at which a tire will hydroplane based upon tire pressure. I.E. the sq root of the tire presure in psi, times 9 = the speed in knots the tire will hydroplane. (1.15 statute miles = 1kt)

We all know commercial aircraft weigh much more than cars, but not all aircraft are that heavy and many of them still go just as fast as commercial jets on the runway. Those tires must give good grip even though the weight on them varies as the aircraft transitions from flight to ground.

It’s interesting that NASA who came up with that formulae, make no allowance for weight, or tread depth, only tire pressure to predict the speed at which the tire will hydroplane.It’s also interesting that in all the reading I’ve done on automotive hydroplaning I’ve never seen any reference to tire pressure delaying hydroplaning, the only thing ever mentioned is tread depth.
Any thoughts?
Regards,
White Bear.

 

I'm concerned about the calculation. Taking the tyre contact as roughly
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it only has to shift the water where the contacts are. Quite a bit less than the figure given.

For an aircraft (or car), tyre pressure depends on weight of vehicle, number of wheels and contact area (which reduces as you increase pressure) - at least I rather think this is likely. The weight of an aeroplane presumably reduces with lift which tends to depend on speed as I recall. Er, do aircraft/NASA help at all???

 

The formula for predicting aquaplaning gives a result for when a tyre CAN aquaplane, it does not say that it WILL aquaplane. I had always thought that the tread pattern was to discourage aquaplaning, it can't stop the physics which predicts that the tyre CAN aquaplane.
It just pays to be aware that for a tyre pressure of 35 psi say, it is possible to aquaplane at just over 60mph. Also once aquaplaning has started then it can continue below aquaplaning speeds. This is why when you land an airliner on a wet runway you do a firm landing to make sure that the wheels break through to the Tarmac in order to avoid aquaplaning. Aircraft tyre pressures are typically above 200psi.

 

In the '70s an engineer from the USAF Academy in Colorado Springs (my first time living in Denver) told me that an extensive USAF study looked into hydroplaning. Their outcome was that the minimum speed for hydroplaning was 9 times the square root of tire inflation pressure. Assuming 25 psi... square root of 25 = 5 x 9 = 45 mph minimum speed. This was almost forty years ago so please don't ask me for particulars, I'm lucky to recall this much. ;-)

 

As a retired airline pilot (big jets - no posers!), I can confirm White Bear's post. We do consider only the tire pressure. The aircraft's weight doesn't make it into the calculation. Even as an engineer who understands the rationale, I've always found it to be curious. But true.

Ciao! - Jim

 

with respect to airplanes they have several advantages over cars in this regard:

1. Airports that are made for high speed jet aircraft often have cerations cut across the runway. These are like the cuts they put on the side of the highway that cause a racket when you drift off the highway, only in the case of runways they go all the way across and their purpose are there to evacuate water not make noise to wake up a sleeping driver.

2. Runways have no turns and there is only one airplane on a runway at a time. So hydroplaning if it happens is less likely to be a concern in an airplane. If an aircraft hydroplanes on landing (and I have actually done it twice) usualy it just means you can't brake as well. The loss of lateral control is not as big a deal as in a car because at high speed your rudder is still effective at keeping the airplane pointed down the runway and there is no one else to hit. Assuming you were lined up fairly well at touch down you would have to hydroplane for a long, long time to go off the side of the runway. The bigger issue is loss of braking ability which could conceivably send you off the far end of the runway at relatively high speed.

3. Tires in jet aircrafft are inflated to hundreds of pounds of pressure. If memory serves when I was flying EA-6Bs (a long time ago) we had either 350 or 480 PSI in the main gear tires for land-based and carrier operations respectively.

Also higher pressure will not always make you hydroplane later. It reduces the contact patch and give the "wedge" you speak of a head start. Lift due to fluid pressure on the wheel will lift it off the pavement. If you look at actual hydroplanes (i.e. the boats that ride on hydrofoils) they hydroplane by design and the hydrofoil they ride on is rigid and does not deflect at all.