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Why are aircraft tyres not grooved horizontally like vehicle tyres? If the tyres are grooved horizontally won't it help in achieving more friction and reduce the chances of veering off?

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    $\begingroup$ horizontal grooves in tires has nothing to do with grip directly. They help in channeling water into the vertical grooves to avoid aquaplaning. They do increase rolling resistance which is detrimental to takeoff distance. $\endgroup$ – ratchet freak Oct 26 '15 at 10:48
  • $\begingroup$ @ratchetfreak avoid aquaplanning at $V_1$ seems more important than rolling resistance affecting take off distance in the event of a rejected takeoff on contaminated runway. $\endgroup$ – Manu H Oct 26 '15 at 10:59
  • $\begingroup$ By the time you get to V1, the airfoils are going to be effective in controlling the aircraft. $\endgroup$ – Howard Miller Oct 26 '15 at 11:50
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    $\begingroup$ @HowardMiller, at controlling yes, but the point is stopping the plane and most rejected take-offs won't be able to use thrust reversal (because the engine just failed). $\endgroup$ – Jan Hudec Oct 26 '15 at 12:36
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    $\begingroup$ Keep in mind, also, that runways with standing water (ie: puddles) deeper than 3mm are considered "contaminated" and special procedures (afaik) are required for such landings, where permitted. A wet runway should generally not have standing water, so extensive siping (or crosstreading) is not a prime requirement for water removal. The bigger concern is lateral grip (aircraft can have large lateral loads due to crosswind conditions) and the more you groove a tread the more grip you sacrifice. I expect the tread design is grooved only as much as necessary to keep the contact patch area high. $\endgroup$ – J... Oct 26 '15 at 16:56
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Almost all the aircraft operating from prepared surfaces have circumferential grooves in order to remove water between the tyre and runway surface.

From Dunlop site:

Most Dunlop aircraft tyre designs feature circumferential grooves moulded into the tread to disperse water from beneath the tread in wet runway conditions.

The tread helps to reduce the risk of aqua planing and improves traction and grip between the tread and runway surface.

In 1960s, NASA carried out a number of studies to determine the effect of tread design (among other things) for operating on wet runways and settled on the circumferential groove shape.

In case of aircraft operated from unpaved surfaces, the tyres may have some type of cross-tread pattern.

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It's certainly not universally true that all aircraft have length wise grooved tyres only.
Especially aircraft designed for use from rough and unprepared strips often have heavy duty tyres to deal with such conditions.
Think some military aircraft especially. Here's an example of a Sea Fury deployed in Korea during the war there for example:
Linked from www.3rdattackgroup.com

enter image description here

Another example is the He-111:

Wheel detail of a He-111

Picture source. The link points to the full picture.

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    $\begingroup$ I can't help but notice that the Sea Fury has not just grooved tires, but what looks like two different styles of grooved tire. $\endgroup$ – egid Oct 26 '15 at 21:13
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    $\begingroup$ @egid hadn't even noticed... Guess they fit what they have in combat, and what they had came off of another aircraft $\endgroup$ – jwenting Oct 27 '15 at 4:57
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    $\begingroup$ This one may or may not have the original tires (at least they're both the same). $\endgroup$ – Mast Oct 27 '15 at 7:27
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You have two questions here, lets just consider this for GA/commercial planes that are operated mainly from paved strips. Back country flying and grass/dirt strips lend to other tire needs.

Why are aircraft tyres not grooved horizontally like vehicle tyres?

Short answer, contrary to popular belief groves actually reduce grip. Vehicle tires are also designed to fit a different use case. Summer tires (mainly for dry times) can actually be close to aircraft tires in their cut (full slicks are not legal everywhere). All seasons are made to work well in not only rain but snow and ice as well. As such the groves are cut to displace water as well as provide limited traction on packed snow.

If the tyres are grooved horizontally won't it help in achieving more friction and reduce the chances of veering off?

No, but lets dig in. There are two cases to consider here, a wet runway and a dry runway. The coefficient of friction of rubber on dry asphalt is extremely high ~=.9 but when it comes to using tires to stop something (breaking really) you want the most rubber/asphalt contact you can get (also known as the contact patch). Thus the "grippiest" tire so to speak is whats commonly known as a racing slick since there are no groves. By adding tread you actually reduce grip by reducing the contact patch when talking about a dry runway.

Since airplanes sometimes operate in the wet (let's ignore snow), we need to account for that. To make a tire work well in the wet we need to displace the water so that the rubber can make contact with the asphalt. If you get a layer of water (even a thin film) between the tire and the asphalt you have what is commonly known as hydro planing (no, a seaplane rating won't check you out for this); you can read a full study on it here. Since you only need to provide area to displace the water to the simple groves provide enough space without adversely affecting grip as a traditinal car tread would.

Another somewhat related reason they may not be grooved laterally is to prevent the tires from spinning up which could cause other issues.

Older F1 Rain tires area actually cut very similar to aircraft tires as can be seen here

enter image description here (source)

Compare to this

enter image description here (source)

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    $\begingroup$ I looked at that second image about 5 times and thought "why do grooved drag slicks even exist?" and "that's a funny looking dragster". Yeah, main landing gear. I get it now... $\endgroup$ – FreeMan Oct 26 '15 at 19:08
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    $\begingroup$ That F1 image is misleading: those tires do have cross-grooves, the exposure time is just too long to register them, see e.g. wpmedia.driving.ca/2014/03/… $\endgroup$ – Hobbes Nov 26 '17 at 15:22
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    $\begingroup$ Thanks for the laugh at "a seaplane rating wont check you out for this". $\endgroup$ – a CVn Nov 26 '17 at 17:43
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Runways have horizontal grooves on them to allow more surface area for water/slush to "hide" in. This feature helps keep the thickness of the fluid film low.

Most airplane tires avoid using horizontal geometric features—both blocks and sipes—so that the rubber on these wheels that land at around 250 km/h do not chunk out. At these high pressures the canals or ribs are enough to take care of hydroplaning. Horizontal ends further pose the disadvantage of groove wander if the pitches on the tires match that of the runway.

Moreover, airplane landing gears are designed strategically so that in the H configuration the front tires clear the track of slush for the rear pair to grip in.

Smaller aircraft and "off road" versions—typically small and light weight—have patterned tires. Some use these on the front alone while the rear tires that touch down first are simple rib tires.

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Why are aircraft tires not grooved horizontally?

Because, unlike cars where the thrust is a force that acts on the drive wheels, in the point where they touch the ground, and has a forward orientation, the plane is pulled by propellers or pushed by jet engines and the wheels provide no thrust. So, horizontal grooves will not increase the thrust in the case of an aircraft. Even for trucks, the non drive wheels tend to have mainly vertical grooves with some minor horizontal cuts (see the picture).

http://www.falkentire.com/sites/default/files/content/tires/tire-rotator-images/493x673xRI109_Eco.png.pagespeed.ic.fdQSFQwZvn.png

Regarding "reducing the chances of veering off" the best grooves are the vertical ones because they can catch soil and increase the friction in case the plane tends to veer off course while trying to land. Horizontal grooves will have exactly the worst orientation, to the left and right, acting like skates for that direction.

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    $\begingroup$ Your first point is valid, however, braking force is applied through the wheels. (Reverse thrust cannot be counted on and is not figured into braking distance calculations.) As discussed in the other answers "veering off" is usually due to a hydroplaning on a wet surface - tires for unimproved runways do have a more "road car" like tread. $\endgroup$ – FreeMan Oct 26 '15 at 20:22
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    $\begingroup$ All of this is largely fundamentally wrong. I would accept any credible evidence you have to support this, however. $\endgroup$ – J... Oct 26 '15 at 20:42
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    $\begingroup$ The purpose of grooves in tyres is to disperse water, not to catch on roughness in the ground when the tyre starts to slide. $\endgroup$ – David Richerby Oct 26 '15 at 22:55
  • $\begingroup$ Grooves have also the role to disperse water (see: en.wikipedia.org/wiki/Tread) but the question is why the wheels of a plane possess only vertical grooves when having both vertical and horizontal seems a better option. $\endgroup$ – Robert Werner Oct 26 '15 at 23:19
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Recently I came across a Wired article which mentions:

In the first moments after a plane touches down, the tires are skidding, not > rolling. The airplane essentially drags them down the runway until their rotational velocity matches the velocity of the plane. That's why they smoke upon landing, and why Michelin uses grooves instead of the block patterns seen on your car's rubber—blocks would simply break off. (Most tire wear comes from this moment of contact—where the rubber meets the runway.) The stoutest tires are rated for speeds of up to 288 mph.

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