Aquaplaning Formulae Why the Diff???

[160knots]

There are 2 formulae for calculating aquaplaning speeds. One is for a rotating the other for a stationary wheel as in a take-off(rotating), stationary(approach).
Can anybody explain the physics behind the difference in formulae??

[BANANASBANANAS]

I am not the best person to answer this but I understand that it is mainly because a rotating tyre acts like a pump and is able to remove water between tyre and runway to a certain depth/speed. If the tyre isn't turning it isn't "pumping" and will therefore aquaplane at a different speed.

Thats a very simplistic answer and I am sure someone with more tech knowledge than I will be able to fill in the gaps for you.
:ok:

[TheOddOne]

160kt,
I only know of one formula:
Tyre pressure (Tp) in Pounds per Square Inch (psi) speed in Miles Per Hour (mph)
for a smooth tyre, the aquaplaning speed = 9 x √Tp
Thus, for a tyre inflated to 25psi, speed = √25 x 9 = 5 x 9 = 45mph
for a tyre inflated to 100 psi, speed = √100 x 9 = 10 x 9 = 100 mph.
Hence, the higher the tyre pressure, the higher the speed before it will aquaplane.
What's the 'other' formula?
Cheers,
TheOddOne

[green granite]

160kt,

for a tyre inflated to 100 psi, speed = √100 x 9 = 10 x 9 = 100 mph.

TheOddOne
I think u need a new calculator :D

seriously could someone explain why its dependant on trye pressure :confused:

[TheOddOne]

Green Granite,

Oh Dear, yes, how embarrassing! Should have been 90mph!!! I have left the original post so that folk understand what you meant!

Now, as to WHY the aquaplaning speed is dependent on tyre pressure. I believe that a lower-pressure tyre, when confronted with a wedge of water in front of it, will deform more readily, allowing the wedge to further separate the tyre from the surface.

Once the tyre leaves the surface, it will tend to stop rotating, but I've never seen a formula to suggest any kind of modelling of what happens in this regime.

TheOddOne (mathematically challenged this morning!)

[green granite]

the odd one

yes that makes sense ta

[effortless]

The flatter the tyre the greater the surface area presented to the water.

[The Puzzler]

The 2 formulas are 9xsquare root of the tire px for take off and 7.7xsquare root of the tire px for landing. As you can see the aquaplaning speed for landing is lower than for take off. The difference in speeds is due to the wheel either spinning down (rejected take off) or spinning up (landing).

When the tire hits the fluid, momentary fluid stagnation occurs when the tire tread pattern or the runway surface profile does not allow the fluid to escape. The fluid pressure build up is directly proportional to the fluid density and the ground speed squared. Therefore this speed can be accurately predicted and occurs when the stagnation pressure equals tire pressure. It also explains why a slippery surface and poor tread increases the likelihood of aquaplaning.

Hope this helps. :)

[green granite]

The flatter the tyre the greater the surface area presented to the water.


the pressure is surley dependant upon the weight of the vehicle for a given tyre profile
I was thinking more of correctly inflated tyres, i,e the ones on a car @ 35psi
are no flatter than ones on aircraft @ 200psi ,

[TheOddOne]

GG,
Apparently, not so. Look URL deleted - commercial site here for an excellent article on the subject.
Cheers,
The Odd One

[glekichi]

I know it is THE accepted formula for aquaplaning, but I find it very hard to accept that tyre pressure is the only variable for aquaplaning at a given speed...

The writer in the above link explains that weight/mass is not a factor and then goes on to say that the best technique to avoid aquaplaning is to use full back pressure to put maximum weight on the wheels?

What speed does a non pneumatic tyre aquaplane at?
What about one with very very thick rubber walls and a very small tube?
?

Very grateful if somebody could explain it for me...

[westhawk]

What follows is a series of simplified suppositions which may explain the derivation of this formula. I look forward to reading the comments of some of the accomplished physics and engineering types on this forum!

1) The velocity at which an object will aquaplane is dependant upon the area in contact with the water surface and the downward force applied upon the object. (area x pressure = force applied.) The vertical component of the hydrodynamic force vector must exceed this value in order to lift the object to the surface or support it there.

2) Pressure generated at the point of contact is proportional to the square of the velocity the object moves through or across the water.

3) Each pneumatic tire has a design footprint area and design load carrying capability. The design inflation pressure of the tire is likely of a certain ratio (perhaps even equal?) to the pressure across the footprint area and is sufficient to support the tire structure at that load under the environmental conditions the tire is designed to operate under.

If actual tire loading determines aquaplaning speed and inflation pressure is in proportion to design tire loading, then design inflation pressure would also have a relationship to aquaplaning speed! Using the tire inflation pressure is certainly a more convenient method for pilots to calculate aproximate aquaplaning speed than the alternative.

Well, that's one theory anyway. Let's hear yours. Fire away!

Best regards,

Westhawk

[Elroy Jettson]

Not trying to hijack the thread, but if it was as simple as the formula suggests, why dont we just develop tyres that use pressures that would put the hydroplaning speed outside of the aircrafts possible range of ground operating speeds? IE above all the V speeds for take off at MTOW and above the Vref for maximum landing weight? :confused:

[Alex Whittingham]

As far as I can determine the two formulae were empirically derived by NASA in the late 1960s/early 1970s. They only tested on a flooded runway and with pneumatic tyres.