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[congo]

Most tailwheel aircraft depending upon prop rotation tend to turn easier in one direction than the the other because of P-factor and asymetrical thrust. For american built aircraft the turn to the left is the easiest.

[Herra Tohtori]

Ok, this is a huge physics dump but I hope you can follow and understand it, as I also think it's quite interesting.




Tires are usually attached to the rims securely enough that they can handle quite a bit of sideways force without pulling off from the rims.


To explain what is going on, I need to delve a little bit into simple mechanics, but bear with me.

It's a complicated question HOW much you could turn the wheel and still keep the wheel undamaged. The traction forces are caused by static friction that breaks and re-forms very fast as the tire has very small slip angle. As you might know, static friction coefficient is usually slightly higher than kinetic friction coefficient - you can demonstrate this with any object on a table.

The mouse is a good example. As it rests on your desk, no sideways forces are applied to it. If you start lightly pushing the mouse with your fingertip you can feel the pressure your finger is exerting on the mouse. Your finger's point that touches the mouse is the contact patch, and the force to mouse is delivered with pressure on that contact patch, so you can directly feel the amount of force applied with your finger, making it a very good physics demonstration since anyone can do it with almost no equipment.

As you slowly increase the pressure, you'll notice the mouse does not immediately move. You have to increase the pressure against a certain point, and THEN the mouse starts to move and you'll probably notice that the pressure you feel with your finger reduces at this point.

This is because you have crossed over the static friction barrier, and the friction is now defined by kinetic friction coefficient which produces less resistance to movement than static friction coefficient. You'll notice a "notch", so to speak, a sharp reduction in the pressure between finger and mouse as the mouse "jumps" forward.


This is, incidentally, the reason why "drifting" in turns is always an inefficient way of doing things, because a slipping tire always produces less friction forces (acceleration, braking, or steering) than a wheel that is rolling on the surface with a static friction contact patch. In other words if you understeer or oversteer in a curve, you will not be able to go as fast through the bend as you could without any significant drift. There are some exceptions in low friction environments, such as gravel or snow compared to tarmac. If you look at WRC cars, you'll notice that they hardly ever drift on high friction events, because it reduces the cornering speeds. By comparison, on loose surfaces the WRC cars are actually sliding almost all the time, and instead of steering the cars through bends with the cornering force, they turn the car towards the inside of the corner and use the car's forward acceleration to produce centripetal acceleration.

In theory, this method of cornering is not as fast as static friction cornering. However in practice it usually produces better results on loose surface, as the vehicle can achieve positions not possible for straight steering, and moreover the driver has more control since they don't need to worry about the breaking point of static friction - they are continuously operating on kinetic friction so they're not worried about the abrupt shift from static friction to kinetic friction, which is what usually causes drivers to lose control of the vehicle at high speeds.


But, to return to the original point. How much sideways slipping can a tire take before it is damaged, pulled off the rim or otherwise?

It depends on tire structure, surface friction, speed, slip angle and duration of the slip.

Usually, tires are not pulled off the rims by sideways slipping. If you understood what I wrote before, you'll notice that the friction force is always at its greatest when the static friction breaks. When the object starts to slide on the surface, the force does not increase but instead decreases.


Let's vision a tricycle gear aircraft sitting on the tarmac. The pilot, being a noob, has turned the nose wheel 90 degrees around and is enthusiastically revving the engines, which are quite powerful indeed.

As the pilot increases throttle and wonders why the plane isn't moving - he has cleared parking brakes and removed choks - he is increasing the force that is applied to the tarmac through the contact patch between front tire and the surface. But the tire doesn't immediately start to screech forward on the tarmac, because there's static friction stopping it from moving.

When you increase the force pushing the aircraft forward, the stress between the tire and road increases, until at some point the static traction point is lost and the tire starts to move sideways on the tarmac. But, at this point, the friction force between tire and tarmac is reduced, so the tire experiences less sideways stress as it starts to move.

Therefore, if it wasn't pulled off the rim before that point it won't be pulled away after it starts to move.


Now, the secondary effects of dragging the tire across the tarmac start to affect things. Namely, you're dragging a rubber hunk on tarmac with a lot of weight on top of it. This causes a lot of kinetic friction, which causes a lot of physical wear on the tire surface, and a lot of heat. In other words, the front tire will fast develop a flat patch that gets deeper and deeper, and meanwhile the tire is also heating rapidly. The heating increases the amount of wear as the rubber gets softer. As the hapless pilot continues to speed away on the surface, the damage on the tire's surface gets deeper. At some point, all the rubber on the tire's surface is worn off, leaving a patch of the tire's weaved structure to be in direct contact with the road.

At this point, the friction between the road and tire usually drops off sharply because canvas and steel are not as "grippy" as vulcanized rubber. But it doesn't stop damage from occurring - the nose gear just slides a bit easier from this point on.

After some time of sliding on the "naked" tire, the structure of the tire will fail and it will deflate. So now the aircraft starts to slip on the nose gear rim, as the torn wheel usually rips off from the rim at this point. This is metal-tarmac friction, not very significant in force but the heat will be incredible and sparks will be flying, and the pilot will finally realize something is rather wrong as the aircraft's vibrations will sharply increase (an underestimation of the century, but whatever).


Of course this is assuming that the landing gear doesn't collapse due to unintended prolonged "dragging" before this point. Usually they're designed to be pretty sturdy, but I have no idea about the specific tolerances of P-38 landing gear for example.



This example also assumes that the tire is already at 90 degrees slip angle when it starts moving. In your example, the aircraft would be rolling when the tire would be turned. In that example it's hard to know what exactly would happen. It would depend on the speed you're rolling at, and how fast you turn the nose wheel around.

What would happen in real life is that as you start to turn the nose wheel, the slip angle suddenly increases, and the nose of your plane will start to turn into that direction as the tire wants to keep rolling on the surface instead of sliding. As you increase the tire's angle, the aircraft is turning faster and faster, until at some point (and this depends on your speed) it will either break its traction again and start sliding (at which point your aircraft stops moving in an arc and starts moving more or less forward although it may still be rotating), and after this point it doesn't really matter if you turn the tire to full 90 degrees deflection or keep it at that critical angle. Your aircraft will likely also start to slow down at this point because of the nose gear friction. If you keep the tire at the critical angle, the aircraft will slow down and attain grip at some point again, starting to turn your plane nicely again. If you turn the nose gear to full deflection, then I suppose your plane would just skid to a halt.



I hope you could follow all that, and the important thing is - the sideways force of traction or friction itself is unlikely to damage the tire. Landing gear collapse due to sideways stress is more likely, but the tire itself will probably be able to take the punishment - for a while. It will fail due to wear and heat in an extended slide, but not immediately.


Here is a video of a situation where an airliner had to land with nose gear turned 90 degrees due to mechanical failure. You'll notice that the tire doesn't immediately fail, but a lot of white smoke is generated, then when the tire deflates its remains are torn off from the rim and the metal comes into contact with the runway.

Jet Blue Airliner, KLAX, 9/22/05
Jet Blue Airways Flight 292 (wikipedia article of the incident)



By the way, all aircraft have specified slip angle limits which, crucially, come into play in crosswind landings. The landing gear is designed to tolerate certain amount of sideways stress as the tires come to contact with runway at an angle. This critical angle determines whether the aircraft can safely land at any specific crosswind or not.

[congo]

Typically the crosswind component for landing an aircraft is to protect ham footed pilots who carry the slip or crab angle right to touchdown. You normally maintain the slip to keep your aircraft lined up with the runway heading but should straighten the aircraft just prior to touchdown to reduce the lateral stress on the landing gear.

[Herra Tohtori]

Turns out the Friction value in FMB is related to ZUTI modification and the only thing it does is change the "bumpiness" of the designated area, including runways, taxiways and the grass between them.

[Legoland]

lock on taxi is extremely unrealistic with everything other than the su25 that is not glued to the ground

[HundertzehnGustav]

yea, but remember that famous 747 landing... the guy had the jet in a 30 degree crosswind position, and while he turned the nose down the runway, still not lined up, the wheels touched the ground with what.. 20° offset to the main axis of movement?
you can do that with the short legs of a 747 or a hercules maybe... but do it on a P-38/39... it might be a different story.

Can the maximum crosswind values (surely found in pilot handbooks?) be of any use to determine how much side slip is tolerable on aircraft landing gears before damage can occur?

i distinctively remember the B-25 Maintenance and rigging manual indicating that the plane can not be revolved around a main gear leg (gear stays in aposition) but must include forward movement too.
(negative, it was in the pilots manual, but referred to the pivoting wheel digging itself into the ground due to soft underground)

[HundertzehnGustav]

and thats an other factor.
In the above example, we are talking about tarmac. beton, macadam... the modern stuff.
Flying a P-38/39/Mitchell in the Med, on dirt/earth, or in wet conditions, MUD... or Sand... over grass or wet grass... or even ice makes a complete analysis of how things were back then even more difficult.

how the hell are we sposed to find out and implement in the game? scheisse. too much for tonight. very interesting subject, and a good example with the mouse.

but thanks for the inspiration, i spent about half an hour dragging my nose thorough B-25 manuals. That plane abso f*ing lutely rocks.
(ugh, that sounded like i am a weirdo. oh well. maybe i am)
tomorrow, gents...

[Herra Tohtori]

Well, easiest way to actually do it, I mean for real, is to simulate the forces between individual tires and the ground. The ground would have traction values appropriate to the material, and if you wanted to be really fancy you would have tire- and surface temperature also affect the friction, as well as things such as water on the runway.

Then you could model in the physics for the langing gear suspension (oleo movement, shock absorbers and springs) as well as the joints that attach the landing gear to the aircraft itself.


I think I know the landing you refer to. It was a cargo 747 making a botched approach at Kai Tak old airport in Hong Kong, but the pilot (for whatever reasons) decided to go ahead with the landing instead of the sensible decision to go-around. Hence the abrupt corrective turn very late during the final approach, and subsequently the pilot found themselves unable to right the aircraft before touchdown. The landing gear took the stress in a stride, or so it seemed, but I suspect a thorough inspection of the landing gear had to be done and possibly some parts had to be changed for safety concerns, because the landing most likely did exceed the maximum slip angle allowed.

Remember, though, that the "stubbyness" of a 747 landing gear is deceptive. Sure, the landing gear is wide (it has many wheels) but the main vertical part connecting each gear to the fuselage is very tall due to the entire plane being freaking huge.

The sideways torque on the joint is enormous with a crosswind landing, multiplied by the mass of the aircraft itself and comparatively higher landing velocity than a P-38. I think it is easier to make something like P-38's landing gear tolerate higher slip angles on landing, than a big airliner's landing gear (if you want more detail on why, look up square-cube law and how structural integrity of different parts and materials changes when you scale them up or down).


In short: It is premature to be thinking about the failure modes you would have with more realistic taxiing. It would be good to HAVE more realistic taxiing physics in the first place, THEN you could adapt the physics routines to also include the maximum torques that the landing gear joints can take, and if they are exceeded the gear would be bent, detached, or collapsed, causing different types of damage.

But we don't have such physics modeling, so don't skin the bear before you've caught and killed it...

[Wildchild]

If i remember correctly, Big 4-engine bombers only used there outboard engines to taxi...

[The_Jester]

I have a second question if one of you doesn't mind answering. When I was first starting out learning to takeoff on IL2, I made the error of applying too much elevator too early as I thought I was moving fast enough to get up. I managed to drag my wingtip down the runway a good 100 metres before righting the aircraft, however all was not well. As a result of dragging my wingtip, the central fuel tank of my D.XXI burst into flames. More recently I had remembered the incident and attempted to recreate it in several other aircraft to see if the problem occurred in them as well. Of the few I tried, the Spit was the only one to develop a fuel tank fire. What I'd like to know is why exactly this happens in game. I know it's physically possible for this sort of thing to happen if vibrations from the wing scraping caused some fuel to leak out onto the engine or onto poorly maintained electricals.

[HundertzehnGustav]

taxiing speeds?

[Wildchild]

I have a second question if one of you doesn't mind answering. When I was first starting out learning to takeoff on IL2, I made the error of applying too much elevator too early as I thought I was moving fast enough to get up. I managed to drag my wingtip down the runway a good 100 metres before righting the aircraft, however all was not well. As a result of dragging my wingtip, the central fuel tank of my D.XXI burst into flames. More recently I had remembered the incident and attempted to recreate it in several other aircraft to see if the problem occurred in them as well. Of the few I tried, the Spit was the only one to develop a fuel tank fire. What I'd like to know is why exactly this happens in game. I know it's physically possible for this sort of thing to happen if vibrations from the wing scraping caused some fuel to leak out onto the engine or onto poorly maintained electricals.

That happens to me to, but in alot more planes. The game doesn't even realize that your on fire, till you explode