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Definition
As an unbraked pneumatic tyre rolls along a water-covered runway, it contacts and displaces the stationary water. The resulting change in momentum of the water creates hydrodynamic pressure which reacts on the runway and tyre surfaces. This hydrodynamic pressure tends to increase as the square of the groundspeed. As the groundspeed increases, the inertia of the water tends to retard its escape from the tyre/ground contact area and a wedge of water forms which begins to lift the tyre from the ground (see Fig 1).
Fig 1 Development of Tyre Aquaplaning
Partial Aquaplaning
|
Further increase of groundspeed increases the hydrodynamic lift until the lift developed equals the weight supported by the wheel. Any further increase in groundspeed will result in the tyre being lifted off the runway surface. This is the condition of full aquaplaning. Partial aquaplaning, as its name implies, occurs when only part of the tyre is lifted from the surface as shown in Fig 2. |
Fig 2 Partial Aquaplaning |
Spin Down of Unbraked Wheel
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When aquaplaning occurs, perhaps the most unexpected feature is the condition in which free-rolling (unbraked) wheels slow down or stop completely. This wheel spin-down arises from hydrodynamic lift effects which combine to provide a total wheel spin-down moment in excess of the wheel spin-up moment caused by all tyre drag sources. In Fig 3, which illustrates the force on a tyre from a dry runway, A indicates the size of the tyre "footprint" on the runway and Dt is the drag force caused by all the tyre drag sources, which combine to form the wheel spin-up moment. Fm is the vertical load on the tyre due to aircraft mass. Vr is the wheel rotational speed. |
Fig 3 Free Rolling Wheel-Dry Runway |
Total Aquaplaning
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On a water-covered runway, as in Fig 2, a fluid wedge has started to penetrate the tyre footprint and the wheel is partially supported by the hydrodynamic force produced by this wedge. B is the tyre/water contact area, and the tyre footprint (A) is decreasing in size. Because of this, the tyre drag (Dt) is decreasing and the force Fm is moving forward of the axle line, causing a wheel spin-down moment. In Fig 4, full aquaplaning speed has been reached and the tyre has been completely lifted off the runway surface. The wheel spin-up moment (Dt) is approaching zero and the vertical force Fm has moved even further forward of the axle line. The wheel spin-down moment is thus at a maximum and the wheel's rotational speed will be decreasing fairly rapidly towards zero. |
Fig 4 Total Aquaplaning |
Factors Affecting Aquaplaning
Effect of Weight and Tyre Pressure. Changing the weight on the tyres appears to have little effect on the speed at which full aquaplaning occurs. As the weight on the tyre changes so the contact area changes and the ratio of weight to area remains constant; this is essentially due to the tyre inflation pressure. It is this pressure that the hydrodynamic lift pressure must equal over the entire contact area before total aquaplaning occurs. A simple expression based on hydrodynamic lift theory can be used to predict full aquaplaning speed; this is:
|
V |
= |
|
(lb / in2) |
|
or V |
= |
|
(bars) |
where V = full aquaplaning speed (in knots) and P = tyre pressure in units indicated. It must be remembered however, that aquaplaning is a progressive phenomenon and significant losses in braking and directional control will occur below this speed. Once aquaplaning has started, the wheels spin-down and stop fairly rapidly. However, if groundspeed is reduced the time for the wheels to spin-up again is much longer and aquaplaning will persist to a lower speed than that given by the formula; how much lower depends upon the surface texture.
Water Depth and Runway Surface. The actual depth of water required to initiate full aquaplaning depends on surface texture. Trials have shown that when puddles form to a depth of as little as 1 - 2 mm there is a potential risk of aquaplaning.
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Notes for Cranwell Aircraft Types
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AQUAPLANING SPEEDS (kts) |
|||
|
Aircraft |
Tyre |
Aquaplaning may begin above |
Aquaplaning may continue down to |
|
Dominie |
Main |
82 |
66 |
|
Nose |
75 |
61 |
|
|
Hawk |
Main |
104 |
84 |
|
Nose |
94 |
77 |
|
|
Firefly |
Main |
51 |
|
|
Nose |
63 |
||
|
Jetstream |
Main |
64 |
52 |
|
Nose |
50 |
41 |
|
|
Tutor |
Main |
50 |
|
Note: Check FRC/ODM for latest amendment state
If you have any experiences, or advise about operating a specific aircraft type we can post it on this page, and other operators within the same fleet can learn from your experiences. Send details to one of the electronic addresses below, or by mail to Executive Editor AP 3456, HQ CFS, RAF College Cranwell:
RAF Intranet: HQ CFS AP3456
Internet: editor@ap3456.connectfree.co.uk
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