This invention relates to a manner of arranging grooves in the surface of a runway.

In order to prevent the onset of aqua-planing of aircraft tires when the aircraft is landing at high speed on a wet runway, the top surface of the runway may be provided with water run-off grooves.

THE PRIOR ART

Hitherto, such grooves have generally been cut by a saw. However, saw cut grooves are very costly to produce, and are only marginally effective.

Saw cut grooves can be reasonably effective at preventing aqua-planing when the groves are new, and clean. But saw cut grooves, being narrow, tend not to be self-cleaning, and have to be swept. When the runway is asphalt, the edges of a sawn groove tend to curl over in hot weather. Another disadvantage of sawn grooves is that the sawing operation, in addition to its cost, is unhealthy due to the dust created.

A tire will aqua-plane if the water cannot escape quickly enough from the contact patch. The invention is concerned

with providing as little resistance as possible to the rapid movement of large quantities of water along the grooves. Saw cut grooves, even though expensive, are only marginally effective in conducting large quantities of water away rapidly from a fast-moving tire. (To make a saw-cut groove wide enough to conduct the water away adequately and rapidly would be even more expensive.) The grooves used in the invention are aimed at removing more water, more quickly, and thus at deferring the onset of aqua-planing, but without undue expense.

In US patent no 3791699 (CLARK) 12 Feb 74 there is shown a machine for producing grooves in concrete and asphalt. The grooves as produced by the machine are also shown. The process by which material is broken and removed in this machine has come to be known as the "re lex-percussive! ¹ process. In this process, the concrete is struck with repeated blows, each blow being delivered with a heavy hammer, but the hammer is withdrawn forcibly from the surface of the concrete immediately after the blow is delivered, before the hammer can naturally bounce off the surface. The effect is that the concrete is subject to tensile forces in the area of the blow, due to the movement of the impact-induced Shockwaves within the material. The concrete is broken, not by crushing or cutting, but by tensile stresses. Concrete and asphalt of course have quite a weak resistance to tensile stresses, while being strongly resistant to co pressive and abrasive stresses.

Grooves may be cut in (soft) asphalt by the reflex-percussion process, but since the process works by the reflection of shock waves within the material, the process is better on harder materials. The improvement due to the reflex-percussion process, compared with other processes such as sawing, therefore is even .more marked when the groove is in concrete than when it is in asphalt. On the other hand, concrete (especially poorly laid concrete) sometimes can have a comparatively soft layer on its upper surface, which breaks into flakes upon impact. In this case, it would be difficult to form a clean groove in the concrete by the reflex-percussion process. The quality of concrete used for aircraft runways, however, is generally high. In asphalt, too, recent improvements in reflex-percussion techniques have made it possible to produce uniform, clean, grooves economically.

A cross-section of the grooves produced by the machine are shown in Fig 3 of "699. The as-formed groove may be described as having a skewed V-shape. The groove has two faces, a gently sloping face and a steeply sloping face. The invention makes use of the skewed V-shape grooves. The skewed V-shaped groove is open, and clear: the groove conveys water away laterally at good flow rates (so that the grooves may be widely spaced, for economy when forming the grooves); and the groove tends to be self-cleaning. The skewed V-shaped groove is not so deep as to damage the

runway structure, nor so deep as to be liable to cave in.

A discussion o£ the performance of grooves formed by reflex percussion, and having a skewed V-shape, is given in the report: Modified Reflex-Percussive Grooves for Runways, by S K Agrawal, published by the US Federal Aviation Administration, April 1984, ref DOT/FAA/PM-8 /8 and DOT/FAA/CT-8 /7.

BRIEF DESCRIPTION OF THE INVENTION

The invention is concerned with the manner of placing grooves on an aircraft runway. In the invention, the grooves lie transversely across the runway, and are asymmetrical, or skewed, when viewed in cross-section along the length of the groove.

The invention, in its broadest aspect, lies in the fact that some of the skewed grooves lie facing one end of the runway, and some lie facing the opposite end of the runway.

A runway has a Near end and a Far end, and in the invention it is preferred that the grooves that lie towards the Near end all lie facing in the same direction as each other, and the grooves that lie towards the Far end all lie facing in the opposite direction.

Preferably, in the invention, the grooves have the skewed V-shape as illustrated in US-3791699.

Preferably, in the invention, in a Near portion of the runway, being a portion which is close to the Near end of the runway, the grooves are orientated such that, of the two surfaces of the groove, the surface which lies closest to the said Near end of the runway is the steeply-sloping surface .

Preferably, in the invention, in a Far portion of the runway, being a portion which is close to the Far end of the runway, the grooves are orientated such that, of the two groove surfaces, the surface which lies closest to the said Far end is the steeply sloping surface.

Preferably, in the invention, the said portions are spaced apart on the runway, and are so arranged that the said Near portion is the portion of the runway at which an aircraft, upon landing at the Near end of the runway, touches down and decelerates to taxiing speeds. The Far portion is the portion of the runway in which an aircraft would be undergoing heavy braking, at low speeds, in an emergency such as an aborted take-off.

It is recognised in the invention that, for touchdown at high speeds, what is required of the grooves is that they convey the water away quickly, and leave large dry areas in

the contact patch, which will lead to a good resistance to aqua-planing. On the other hand, it is recognised that, for emergency braking in an aborted take off, the requirement is for the grooves to give a good tire-to-runway adhesion. It is recognised that the oppositely-orientated grooves of the invention can contribute to these requirements, in both directions of use of the runway.

Normally, the two portions will be symmetrically arranged as the two halves of the length of the runway.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENT

In order to further describe the invention, an example of an actual runway which has been provided with grooves according to the invention, will now be described, with reference to the accompanying drawings, in which:

Fig 1 shows a grooved runway, which incorporates the invention;

Fig 2 is a close-up of the surface of the runway of Fig 1, showing the disposition of the grooves;

Fig 3 is a plan view of an aircraft tire landing onto the grooves from the "right" direction;

Fig 4 is a plan view of an aircraft tire landing onto the grooves from the "wrong" direction; and

Fig 5 is a side elevation corresponding to Figs 3 and 4.

The aircraft runway 1 in Fig 1 comprises a strip of concrete 2. The runway has a Near end 3N and a Far end 3F.

Extending over the Near portion of the runway, which is the portion 4N from the end 3N to halfway along the runway, the top surface 5 of the concrete 2 is equipped with grooves 6N.

The grooves 6N run transversely across the concrete. As shown in Fig 1, the grooves extend at right angles to the line of the runway. On a runway, it would usually not be allowed for the grooves to be set at an angle to the line of the runway, because such angled grooves might cause an aircraft to veer to the side of the runway under critical- conditions.

At the Far end 3F of the runway, the top surface 5 of the concrete 2 is equipped with grooves 6F.

One of the Near end grooves 6N and one of the Far end grooves 6F are shown in Fig 2. The grooves 6 are identical/ except that, in accordance with the invention, the grooves 6F face in the opposite direction to the grooves 6N.

Each groove 6 has a steeply sloping face ^' 8, and a gently sloping face 9. The sloping faces 8,9 meet at the base 10 of the groove 6. The steeply sloping face 8 intersects with the top surface 5 at intersection 12, while the gently sloping face 9 intersects with the top surface 5 at intersection 14.

The groove 6 as shown is a skewed V-shaped groove, in cross-section. As shown, the intersection of the two faces 8,9 at the base 10 of the groove is a right angle. The steeply sloping face 8 lies at an angle of 70 degrees to the (horizontal) top surface 5, and the gently sloping surface 9 lies at an angle of 20 degrees to the top surface. As ^' shown, the external corner at the "steep" intersection 12 is quite sharp (ie the radius of the corner is small).

The arrangement of the grooves is such that, in each portion 4 of the runway, it is the steeply sloping face 8 of the groove 6 which lies nearest to the respective end 3 of the runway.

The wheel 18 of the aircraft, as it touches down, engages the top surface 5 of the runway, and then rolls over the transverse grooves.

Figs 3 and 4 show the wheel running over the same surface at the same speed, where the grooves in the surface are the same depth and width, and the same spacing. The difference

between Fig 3 and Fig 4 lies In the direction from which the wheel "attacks" the groove. Fig 3 shows an aircraft on. landing, attacking the grooves from the correct direction, in accordance with the invention. Fig 4 shows the wheel attacking the grooves from the "wrong" direction, according to the preferred feature of the invention.

The shading in Figs 3 and 4 illustrates the much improved wheel-to-ground contact area when the wheel attacks the groove in the steeply-sloping-face-first orientation. Numeral 20 indicates the contact patch or footprint of the tire on the surface. 21 is the direction of motion of the landing aircraft. 23 is the bow-wave that builds up in front of the wheel. 25 is the forwardly and sidewardly directed splash or spray from the h ^' igh speed passage of the wheel, and 27 is the after-splash of water being shed from the wheel as it breaks contact.

The dark shading 29 indicates a zone of good wheel-to-ground contact, from which water has been (virtually completely) eliminated, so that the wheel is running on dry surface in that zone. The lighter shading 30 indicates a zone in which the water has not been completely squeezed out, and marks the start of aqua-planing.

It may be seen that in the "wrong" orientation of Fig 4, traces of water tend to be led into the area 34 between successive grooves to a greater extent than in the "right"

orientation shown in Fig 3.

The difference between the Fig 3 and Fig 4 conditions is highly dependent on speed. If speeds are slow, most of the contact patch is dry, even in the Fig 4 condition. On the other hand, if speeds are high, the water can encroach into zone 34 even in the Fig 3 condition. The difference between the Fig 3 contact and the Fig 4 contact, at different speeds of the tire, therefore would not necessarily be the same as that illustrated.

The following explanations are proposed, as to why the "right" (Fig 3) orientation of the grooves should be so much better than the "wrong" (Fig 4) orientation at alleviating the onset of aqua-planing.

It is noticeable that in the Fig 3 orientation, the bow-wave 23 in front of the tire is considerably reduced in depth and extent, compared with the bow-wave in Fig 4. The smaller the bow-wave, the less bulk of water has to be cleared to create a dry contact patch. In Fig 3, the water which is projected forwards and downwards by the onrushing tire, and which strikes the ground in front of the tire, tends to strike the gently-sloping surface 9, and to bounce well forwards and clear of the tire; while by contrast, in Fig 4, water which is projected forwards and downwards tends to strike the steeply sloping face 8, and therefore tends to be reflected back into the path of the oncoming tire. This is

a possible explanation why the bow-wave in Fig 3 should be smaller than the bow wave in Fig 4. And, the larger the bow wave, the more water has be thrust aside by the tire, and therefore the lower the speed at which aqua-planing sets in.

Another possible explanation may be offerred as to why the Fig 3 orientation is more resistant to aqua-planing than the Fig 4. With the skewed grooves, the tire tends to exert a higher pressure on the runway surface in the region of the steep corner 12, than in the region of the gentle corner 14.

Therefore, it would be expected that the last place where water could intrude would be into the area immediately contiguous with the steep corner, ie the strip 36. This is borne out in Fig 4, where although water has intruded in other areas of the contact patch, that strip remains dry. In Fig 3, water is squeezed out, and the water film broken, by the increased contact pressure in the strip 36. Therefore a barrier is created, which prevents water from in front of the tire from entering the zone of contact 34. In Fig 4, on the other hand, there is no such barrier to a wedge of water driving into the contact area from in front of the tire. In Fig 4, the barrier, ie the strip 36, is ineffective, because the strip is located where water is in any event tending to leave the zone 34, whereas in Fig 3, the strip is effective, because the strip is located where water from the bow wave is seeking to enter the zone 34.

If this explanation is accurate, it is important that the

steep corner 12 be sharp. If the corner 12 starts to acquire a larger radius, or if the corner is rounded due. to wear, or if the corner is pitted by spalling, during the formation of the groove, the local increase in the contact pressure might not take place, and that may allow the water to start to enter the zone 34. For that reason, grooves in concrete are generally more efficient and longer lasting than grooves in asphalt. Concrete can retain a better sharp edge than asphalt. Concrete that has a hard matrix will provide a cleaner edge than concrete with a soft matrix: the concrete used for runways invariably has a hard matrix. On the other hand, the invention may be applied equally to asphalt runways, especially if the grooves are formed as follows.

It may be noted that with conventional processes for forming grooves, even with the conventional reflex-percussive process, it is difficult, economically, to obtain an edge that is reliably sharp. As surmised above, it is the sharpness of the edge 12 that is the important factor in creating the high contact-pressure barrier to the wedge of water. An apparatus for forming the grooves in such a way that the as-formed corner 12 is sharp, is described and claimed in our co-pending patent application, entitled: APPARATUS FOR FORMING GROOVES IN CONCRETE ETC SURFACES. It is therefore preferred, in the present invention, to form the grooves by means of the apparatus as described in the said co-pending patent application.

The apparatus described therein is aimed at producing a groove in which the steep corner 12 is free of pitting and spalling. Concrete is not generally homogeneous, but consists of hard pebbles set in a relatively soft matrix, and the apparatus is aimed at providing a clean edge especially in the case where a pebble lies partially across the line of the corner. The pebble can be cut without any tearing out of the pebble, nor of the matrix.

The angle at which the groove is skewed is important. If the steep corner 12 were to have an angle 38' of less than about 50 degrees, the concentrated tire-contact pressure in the strip 36 might be lost, and water might start to enter the zone 34. If the steep corner 12 were to have an angle 38 of more than about 70 degrees, on the other hand, the corner 12 would not be so well supported structurally, and might become vulnerable to being chipped. The angle of the gentle corner 14 is not so important, nor is it particularly important, from the point of view of the anti-aqua-planing performance of the groove, that the base of the groove be a right angle. It is important that the groove be not so deep as to weaken the concrete, nor too shallow that the water cannot be conducted away properly. A groove depth of about 6 mm has been found effective. The groove spacing is also important: when the grooves have the dimensions shown, they should preferably be set at a pitch of about 75 mm.

It is preferred that every one of the grooves be set to face in the correct direction, as set out in the invention. However, it would not matter if the occasional groove were to face the other way. It is usually specified that runway grooves must lie at right angles to the line of the runway, within quite close tolerances.

As mentioned, it is recognised for the above described reasons that an aircraft landing at high speed on a runway should attack the grooves as shown in Fig 3. On the other hand, it is recognised in the invention that the Fig 3 orientation of the. groove is not advantageous under other conditions. At low speeds, it has been found that different effects predominate, leading to different performance requirements. At -low speeds, aqua-planing is not a concern, but at low speeds the requirement for heavy braking can arise, and of course the closer the aircraft gets to the end of the runway, the more it must be ensured that: heavy braking can be applied, without the aircraft skidding or otherwise losing control. The requirement for heavy braking can occur due to an aborted take-off, or other emergency.

It has been found that, under emergency braking conditions, the highest coefficient of friction at low speeds between the tire, and the runway surface, wet or dry, arises when the tire attacks the grooves in the Fig 4 mode. This may be explained in that a tire which is undergoing heavy braking tends to dig into the steep corners 12. There is a

mechanical interaction, or cog-wheel type of engagement, of the tire with the grooves, much more in the Fig 4 mode than in the Fig 3 mode.

When a tire is rolling, but undergoing heavy braking, the observed rotational speed of the tire is considerably slower than would be expected in theory, ie from a theoretical computation using the linear speed of the aircraft and the radius of the tire. A tire under braking in fact undergoes a controlled slippage: and the more heavily braked the tire, the greater the slippage. The slippage can be stated as a percentage of the actual rotational speed compared to the computed rotational speed. It is important that the slippage be prevented from exceeding the percentage limits that the surface can support, because that would lead to skidding. By arranging that the heavily braked tire attacks the grooves in the Fig 4 manner, the tire can undergo a larger percentage slip before skidding. It should be noted that this improvement to the low-speed heavy-braking performance of the surface, caused by orientating the grooves correctly, happens whether the surface is wet or dry, although the tendency is that the "wet" performance is improved more than the "dry" performance.

To summarize, it is recognised in the invention that when an aircraft is landing at high speed, the grooves should face in one direction, to avoid aqua-planing in the wet, but that when the aircraft is travelling at low speeds under heavy

braking, the grooves should face in the other direction, to avoid premature wheel-lock.

In the invention, it is recognised that even if a runway were to be used to land and take off aircraft only ever in the one direction, it would be advantageous to provide the oppositely facing groove arrangement of the invention, the grooves at the Near end being set for maximum resistance to high-speed aquaplaning during landing, while the grooves at the Far end are set for the maximum coefficient of friction under heavy braking at low speeds.

It will be appreciated that a runway that is provided with grooves set according to the invention in fact is equally effective to protect both landings and aborted take-offs in both directions.