A "non-sticky" and realistic grass grip

[85th_Maverick]

Although this funny subject has been discussed before, it should be taken more seriously as time passes and it should be relatively easy to fix/model, or it will remain a great drawback and bad looking thing for a modern simulator to have the grass grip coefficient set to virtually infinite. This is mostly about turning and hard braking on grass or coming to a landing even with the tiniest sideslip relative to the ground which only makes your plane snap over in an instant. It doesn't happen like that in reality (more than obvious) unless the factors are powerful enough to make that happen such as CG height and position relative to wheelbase dimensions/position and grass grip.

 

Normally, the MIG-21 as well as the Su-25 are aircraft meant to operate on relatively low prepared grass fields. Only in DCS the grass grips you as if you're driving on rails when turning and sticks you up instantly and better than Super Glue when your ground speed reduces below some 5km/h, point from which you won't be able to start rolling again even with the F-15 in full afterburner, 2% of fuel left and clean loadout. At, least this is how badly affected the FC3 planes are. Separate fighters modules, like F-14,F-16, F-18, etc, don't remain absurdly stuck, but still turn on rails.

 

 

 

 

 

[randomTOTEN]

42 minutes ago, 85th_Maverick said:

Although this funny subject has been discussed before

And as we've said before, you can't just compare prepared grass landing surfaces to random grassy terrain. They are not the same. An unpaved runway is still a runway.

 

I think the Hornet doesn't get stuck in the grass, which is a problem which needs to be addressed if true.

Edited May 4, 2021 by randomTOTEN

[85th_Maverick]

On 5/4/2021 at 5:47 PM, randomTOTEN said:

And as we've said before, you can't just compare prepared grass landing surfaces to random grassy terrain. They are not the same. An unpaved runway is still a runway.

 

I think the Hornet doesn't get stuck in the grass, which is a problem which needs to be addressed if true.

 

LOL! So instead of making the planes be able to roll on unprepared grass terrains as well, which is absolutely normal on a dry grass land, you want to make them get stuck, thinking that that's how it is realistic...?! Why would you imagine that it should be stuck or what makes you believe that a clean loadout afterburning jetfighter, at least..., will remain stuck on dry grass terrain as long as it's wheels won't dig in more than 38% (for the F/A-8C, for example) of their radius? If you'd do a bit of trigonometry calculations you can find out the limit wheel depth (be it in sand, mud, whatever) for a given horizontal thrust where it does remain stuck. I've just calculated it right now, that for a clean F-18 weighing 24000lb (1000 above operational empty), in full AB you can start rolling if your wheels haven't dug into the terrain more than 38% of the wheel radius. I'll show you how I've calculated it if you will, so we can conclude that I'm not missing anything! There isn't much of difference between an "unprepared grass terrain" and a "prepared grass terrain" to be used as a runway, as you might believe! The "prepared" one only has the grass cut to some level and then rolled (the rolling is only slightly increasing toughness, it mostly smooths out the surface) after the terrain has been considered acceptable to be used by airplanes after checking out that there are no hidden obstacles and that it isn't too uneven/bumpy to worth the cost to level it.

 

Of course your main wheels might have a good chance to dig in with an enough radius ratio to get stuck even in full AB on a freshly plowed land, but it would be a rare occasion for that to happen in any "unprepared" grass field that has been drying in the sun for at least a week.

The down pressure needed on a wheel in order to slowly start digging in, on a relatively low toughness clean and dry soil is usually >1.5 Mpa.
https://www.sciencedirect.com/topics/agricultural-and-biological-sciences/soil-penetration-resistance

 

So now just for the sake of saying..., do you know how much down pressure is on the F-18C's main wheels (taking it that 90% (could be lower than that) of the weight would stand on the main wheels)? For a clean wings, but full internal fuel config, which is 15400kg (from wiki, with acceptable error), the down pressure is: ~1.6MPA. Thus, in this condition, for about the lowest toughness dry grassland, the F-18 would slowly start digging in, but how much...? Well, as it digs in, the contact area also increases, reducing the down pressure and stopping the sinking at some point. How quick that point occurs? Well, after a quick calculation it results that the main wheels won't continue sinking in the soil beyond 3% of the effective (effective meaning the radius of the already deformed tire under the weight of the aircraft), while this 3% sink of the effective tire radius would require a thrust increase (when compared to 0% sinking) of 11% more in order to get unstuck. Just 11% added thrust! In order to find out the 0% tire sinking (as if on tarmac) required thrust, simply multiply the weight with the friction coef (which for hard soil is about 0.15) and there goes your required thrust. Just above 11% increase over that thrust would be needed to start rolling for a 3% of effective tire radius sinking. I might have the habit of detailing too much, but how can I provide a better view of "how much" the quantities or things evolve?

Keep in mind that the roots of the grass only add mechanical strength to the earth compared to dry soil, for similar drought conditions. This is a pressure resistance bonus for grass lands alone.

 

At the moment, only for the F-14, F-16 and F-18, the thrust required to start rolling on the grass is grossly acceptable and even as it is right now it's still a bit too high to be true, but you want to make it even worse? For FC3 and all other jet aircraft modules, it's just spilled glue over the ground everywhere! The "rails" effect which flip you over with the tiniest steering input without having the tiniest lateral slipping must be addressed and do something to reduce the infinite friction coefficient to something just slightly higher than on tarmac as it would normally happen on dry grass. On wet grass and mud however, the plane should skid quite much before starting to dig in to conditions which we see now as "aircraft stuck on grass"..., only in those conditions you may have what happens right now on dry land.

Looking forward for contradictions! On the other hand..., cheers!

[Nealius]

Hmm, I might give the Su-25 a go on one of the Channel's grass strips to see if ED modeled prepared grass strips vs random grass fields. 

[Ahmed]

Oh no. This again. I just wonder how many of those defending the "airplanes can land anywhere" principle have actually landed any real plane (bonus points if bigger than a 172) in an unprepared strip.

[Rakkis]

I think theres a line between ALL AIRPLANES STICK and ALL AIRPLANES ROLL that needs to be a little greyer. There are varying degrees of "Prepared grass strips" @85th_Maverick, respectfully you're a little incorrect in your description of prepared grass. Most regularly maintained or professionally developed grass strips have either a packed clay or gravel base beneath the topsoil to "harden" the landing surface, careful consideration has also been given to drainage and a crown is usually grated into the runway for this reason. Historically Marston Matting (a perforated steel mat capable of intelinking) was used by SeeBees and the likes to allow for the rapid construction of temporary runways all the way through Vietnam with the M8A1 mat. But basically it's not just " ...cut to some level and then rolled..." All that being said a little tweak could be used, especially in the actual airport environment. It is a REAL pain when my NWS Button periodically freaks out and my Tiger rolls off the taxiway a little and its suddenly spawn a new jet time. 

[Mars Exulte]

On 5/9/2021 at 7:44 AM, Ahmed said:

Oh no. This again. I just wonder how many of those defending the "airplanes can land anywhere" principle have actually landed any real plane (bonus points if bigger than a 172) in an unprepared strip.

 

  I would say safely zero.

 

@OP

  Prepared dirt, or naturally hard (like the salt flats) is very, very different from ''grass field'' and ''running off the taxiway''. You know what's not likely to firm enough to support an aircraft? Random fields and the ditches alongside taxi/runways (which are more for drainage than anything else).

 

  DCS is *not* ''all planes stick or don't stick''. It is very simplified, but it DOES have different ground conditions.

 

You wanna know what's another major influence? Aircraft weight! If you're empty and low on gas, your odds of getting ''massively stuck'' are much lower than if you're trying to bombtruck out of a wheatfield @@

 

  Oh, and I didn't read everything, but from past experience : the ''I got stuck and firewalled the throttle and it still wouldn't move'' people are absolutely right. That's unrealistic. DCS should have the gear collapse, the plane dig into the ground and burst into flames, then it should flash ''you suck'' across the screen, automatically uninstall itself and force you to check a box saying ''I will try to suck less'' before allowing you to reinstall it.

 

  This is a flight simulator not ''Offroad Muddin' 2021'' @@

[85th_Maverick]

On 5/9/2021 at 3:37 PM, Nealius said:

Hmm, I might give the Su-25 a go on one of the Channel's grass strips to see if ED modeled prepared grass strips vs random grass fields. 

Even if you come with that provable data they'd still think they know it better that it should get rapidly stuck whatsoever!

[85th_Maverick]

On 5/14/2021 at 11:44 PM, Mars Exulte said:

...

  This is a flight simulator not ''Offroad Muddin' 2021'' @@

And it should prove so! Who talked about mud? You know there's a difference between soft mud and hard dry soil that we're actually talking about. The differences in pressure required to deform the 2 types (moisturized vs dry) by the same amount can be hundreds or thousands of times different. Unless you've read the links, the worst dry land can withstand pressures of at least 1.3MPA and can go higher than 4, while the grass itself further increases that bonus. It's not what we imagine in our minds of what happens or would happen, it's what verifiable data proves.

 

Here's an accident of a failed rejected takeoff of a Tu-22 in which you can all clearly see how the plane bumps and jumps over the totally unprepared terrain beyond the runway threshold and that nothing major happens with the landing gears until the left wing hits an obstacle:

 

Yes, unprepared grass terrains might most of the times be bumpy, but not sticky and not with 100% grip (train rails). So, until it will be able to simulate (simplified, not required to be complex) some lower friction forces between the landing gear and the grass terrains, I agree with you, it only remains a flight simulator, at least for the aircraft simulated rather correctly, cause even in the air a good part of them have more or less quickly observable flight model problems.

Edited May 24, 2021 by 85th_Maverick