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Difficult landing training


draconus

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I don't have NTTR yet and somehow I couldn't create my own mission with 25T so I was just abusing the brakes in training and instant missions. I tried my best but failed to break anything on a straight line.

You can't select the T-bird in the ME to create a mission for yourself? Have you considered a repair?

 

At any rate, coming in heavy (96% of max gross weight) and standing on the brakes with the temp set to 104F (40C) causes no issues for me either. Track attached as well.

 

 

 

EDIT: Temp does make a difference as to when tires blow, though. Heavy aircraft accelerating run down the runway with temp at +40°C (104°F), all three blew within a second of each other at about 395 kph. Same runway with temp set to -12.4°C (9.7°F), I was at roughly 440 kph and increasing when I ran off the runway with intact tires.

Braking -Hot & Heavy.trk


Edited by Ironhand

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Well, I guess that settles that then. Braking, at least on the 25T must NOT cause overheat. Seems high speed does, though, unless it's the tiny movements causing it.

 

I dunno. The 25T is the oldest and least updated model, so that must not be in there. I wonder if the others do, though.

high speed run.trk


Edited by zhukov032186

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Well, I guess that settles that then. Braking, at least on the 25T must NOT cause overheat. Seems high speed does, though, unless it's the tiny movements causing it.

 

I dunno. The 25T is the oldest and least updated model, so that must not be in there. I wonder if the others do, though.

Not really seeing it in the Su-27 either. Hot day landing, heavy. About 360 kph down the chute but slowed to 310 kph at touchdown--I'm always conflicted about breaking this aircraft and couldn't talk myself into landing faster--but immediately stood on the brakes and held them there until the stop. No issues.

Su-27 Landing_+45C & Heavy-No Blown Tires.trk

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...

It is there. Ambient temperature 42' and full fuel and four drop tanks, I started down the runway, no wind, no brakes. By the time I got to the end I was doing 500kmh, lifted off, popped into external view, and all I had left were little nubs with shreds of tire. All three wheels were gone...

 

 

Guys... really? Come on. Take your car down a road a 500kph and see what happens. That has nothing to do with weight or braking or temperature, you are twice as fast as what they are designed for and literally blowing them apart. You should never be on the runway over 300kph. You shouldn't really even have your gear down bellow 350kph, and I'm pretty sure (it's been a while since I tested it) over 450kph you are going to damage the gear just from having them down in flight.

 

 

The brakes on the SU-25 are pathetic. You could almost brake faster by sticking your arm out the window. You cannot lock the wheels unless you are basically still flying and only have like 500kg on the runway. I don't know if brake temperature and failure is modeled in the game (I doubt it is), but in the last maybe 50hr or more of flying the 25 and 25T I have not blown a tire. I do not use the drag chutes unless it's a short runway, I apply brakes as soon as I lower the nose wheel, and I do not stop braking until I am at a safe taxi speed. Basically on every landing brakes come on around 230-250kph and are on down to 30-40kph.

 

 

The only way you are going to blow tires on the su-25 are from slamming it on the runway or from sliding. If you slam the rudder to one side you will slide the nose wheel (it turns instantly). If you want to make a sharp turn, you have to smoothly and slowly increase the nose wheel angle, and the same goes for stopping a turn.

 

 

The maximum landing weight is there for safety, but does not mean the plane can't be landed above that weight. When a maximum landing weight is established, it is done so for average pilot skill and allowing for difficult conditions. If you put the plane down gently, you will not damage it.

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Not really seeing it in the Su-27 either. Hot day landing, heavy. About 360 kph down the chute but slowed to 310 kph at touchdown--I'm always conflicted about breaking this aircraft and couldn't talk myself into landing faster--but immediately stood on the brakes and held them there until the stop. No issues.

 

 

SU-27 brakes are much better than the 25, but unless it's a slippery runway I don't think you'll ever lock them. I have no problem using full brakes, though I have less experience in the 27 than the 25. Another thing to consider is the 27 is almost exactly twice the weight of the 25A (max gross) and 13500kg heavier than the 25T. So even if blowing tires due to breaking were a thing on the 27, it has both higher weight and much much stronger brakes (stopping more weight much faster).

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If you guys really feel the need to test it, go find the longest runway on a hot day, load up to max gross, take-off, land at the proper speed (10degrees AoA) and apply the brakes as soon as you put the nose wheel down, then increase throttle to maintain that speed under braking for the entire length of the runway. If you go off the end of the runway before you blow a tire, you will never blow a tire due to braking.

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Guys... really? Come on. Take your car down a road a 500kph and see what happens.

 

 

No, really? I never guessed, especially since I wasn't braking. That was the whole point, was to determine if speed affected them. It does, as expected.

 

 

that has nothing to do with weight or braking or temperature
It has nothing to do with braking. It does have to do with weight and temperature, specifically the weight on and temp of the tires. It's called physics. Enjoy. We're also not talking about physically locking the brakes and skidding. We're referring to applying maximum braking and not letting off. Regardless, we've safely established brake temp is not modeled, and tires may or may not, at least on the 25T.

 

 

And lastly, Ironhand already did a full braking test under a loaded aircraft. So... thanks.


Edited by zhukov032186

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No, really? I never guessed, especially since I wasn't braking. That was the whole point, was to determine if speed affected them. It does, as expected.

 

 

It has nothing to do with braking. It does have to do with weight and temperature, specifically the weight on and temp of the tires. It's called physics. Enjoy. We're also not talking about physically locking the brakes and skidding. We're referring to applying maximum braking and not letting off. Regardless, we've safely established brake temp is not modeled, and tires may or may not, at least on the 25T.

 

 

And lastly, Ironhand already did a full braking test under a loaded aircraft. So... thanks.

 

 

 

 

My god you're hostile! But seeing the same rude attitude with your replies to draconus's other thread I'm not surprised. That or you just enjoy sarcasm a little too much. I know you don't want to hear it, but I'm actually going to type some of my thoughts and opinions into this here box.

 

 

 

I know you weren't braking in your test. But that's what 90% of the thread is about. Otherwise I wouldn't have gone on to talk about braking...

 

 

 

While you say you aren't talking about locking the brakes and skidding... that is far more likely to cause a tire failure than overheating. The whole point of the thread was what is causing tire failure on his landings (and then could braking too hard do it). If you cannot come close to locking the wheels (which in the 25 you absolutely can't), you probably cannot create enough heat to blow the tires anyway. When you see an airbus blow a tire with the brakes flaming, it's because it has (and was using at the time) an anti-skid system so it can apply maximum braking force without locking the wheels. That tremendous amount of energy is what overheats the tires, not gently braking (again, what the 25 does). If it didn't have an anti-skid, it would possibly lock one or multiple wheels, which would still cause tire failure, but those brakes would not be overheated.

 

 

So the 25 can't lock up, that's one way it's brakes can't cause tire failure. It also can't brake very hard, so it's unlikely they can overheat the tires. That's the only two ways brakes can cause a tire failure ruled out. So brakes on the 25 can't blow a tire, therefore the only other two things are hard landing or skidding.

 

 

Pulsing the brakes, brought up in this thread, is completely unnecessary.

 

 

 

And while Ironhand did test landing at max gross, he did not do a complete braking test to determine if brake temperature is modeled. All he did is verify that the brakes are capable of stopping a fully loaded aircraft without failure, not that they cannot fail. That's why I suggested what I did, as it's the maximum amount of brake energy that you could create on the 25 within the limitations of the sim without causing failure by rolling at ridiculous speeds. Again, I doubt they modeled that anwyay.

 

 

I wasn't twisting your arm to do the test, nor was I suggesting that you and you alone have the responsibility of proving it. I have no desire to do it because I never blow tires so it really doesn't matter to me. But if someone else really wants to find out if it's possible, that's how.

 

 

If you lock the gear up it will lurch to one side, which also increases weight on that side, which increases heat, which increases likelihood of blowout. It also makes it hard to control.

 

 

While my comments are rarely addressed to one sole person (I don't provide 15 quotes for each and every topic I discuss), you yourself did specifically say not to brake too hard on the 25 or you will lock the brakes. You can't lock the brakes. It does not lurch to one side, there are no differential brakes on the 25 in DCS and there is no other way for that to happen with regards to braking force in a straight line (you certainly can with steering though). You cannot overheat the tires through braking (to the extent that Ironhand tested). While these are concerns that may exist in the real world, and I don't fault you for expressing them, they don't exist here.

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Sideslip,

 

What I did do after the landing, however, was to make two max weight takeoffs in my trusty Su-27–one with brakes engaged for the duration in full AB and one without. In both cases, the tires blew at roughly the same speed and sequence. Front first @ around 375 and the rest at around 380 (at least I think those were the numbers).


Edited by Ironhand

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Sideslip,

 

What I did do after the landing, however, was to make two max weight takeoffs in my trusty Su-27–one with brakes engaged for the duration in full AB and one without. In both cases, the tires blew at roughly the same speed and sequence. Front first @ around 375 and the rest at around 380 (at least I think those were the numbers).

 

 

Yes but was that accelerating or at a constant speed? Ie, did they blow because they hit a predetermined speed of failure in the sim or because heat from the braking action caused them to fail? If you maintain a constant speed and they fail after a time, you can figure they failed from a build of up heat (or wear). If they just fail at a set speed, it might have nothing to do with the brakes being applied and could just be weight/maxgross X OAT/stdTemp X tirespeedrating = speed at which tires fail.

 

 

If you did the same but stopped accelerating at 350 and they still failed, just a little later, that would be pretty good evidence that heat caused it. Of course that's tires and not brakes. I still doubt the brakes are modeled.

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Accelerating all the way down the runway. In both cases, they failed at roughly the same speed—one with and one without brakes applied. I took it as an indication that it had nothing to do with the brakes.

 

EDIT: Given that tire failure occurs sooner on a hot runway than on a cold one, I imagine that tire failure is based on heat buildup in the tires rather than tied to a particular speed for a given aircraft weight, etc.


Edited by Ironhand

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Either heat build-up, or a simple calculation of a predetermined speed taking OAT into account. It'd be cool if it's really heat, but I'm still not sure. If you are accelerating it's hard to tell which it is.

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Either heat build-up, or a simple calculation of a predetermined speed taking OAT into account. It'd be cool if it's really heat, but I'm still not sure. If you are accelerating it's hard to tell which it is.

True. If I have time tomorrow, I’ll do a better controlled test.

 

I hadn’t really thought about it much before. Now I’m curious.

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You can't select the T-bird in the ME to create a mission for yourself? Have you considered a repair?
Naah, rookie mistake. Starting time was set to a few minutes later so I was invited to look at the map instead of fly.

 

Tl;dr is you can brake however you want but don't you dare touching rudder during touchdown and be extra gentle on the runway with it until slowed down. I did my own tests with braking 100% heavy aircraft and have yet to do "Sideslip test" because it's fun :) No braking induced failures too. It's hard also because the damn thing is never going straight and you almost run into PIO too.

 

Btw: no hard feelings for zhukov032186. I like him the way he is ;) No need to be oversensitive.

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I was wrong, it happens ) Not very often mind you ;)

Anyway, it's pretty clear brakes aren't a factor in overheating. I stand by everything else I said though.

 

Just to find out if tire failure is scripted at certain speeds. Will test later.

 

@sideslip

You threw chum in the water, what can I say ;)

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Although most simmers do not care about things like landing weight, for the real Su-25 it is specified as follows:

 

It is absolutely forbidden to land the aircraft with total weight exceeding 13 300 kg, such landing is allowable only in emergency and all weapons that are jettison-able have to be jettisoned. The manual moreover states that landings with total weight more than 12 200 kg are allowable only in rare cases and can form only 3% of all landings, if this number is exceeded the aircraft has to undergo a major technical overhaul.

 

Of course it is possible to land Su-25 even at 19500 kg, but you are putting the gear assembly tires and airframe over their projected stress levels.

 

When landing with weight below 12200 kg, you can touch the runway safely at 230 - 240 km/h without problems and land on real wheels, aerobrake and than release chute without ever touching brakes, just like RL. DCS has a very good FM for Su-25 very close to published numbers.

 

People often moan about flight models, minor cockpit details or small systems errors, but operate their aircraft in a way, which would be unimaginable in RL.

 

This. The tyres on Russian aircraft are very susceptible to over-stress and bursting under overweight landing conditions. If you're landing over the recommended max weight then I'd recommend a very gentle touchdown and aerobraking / use of the drag chute as much as possible prior to brake application.

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As far as I'm aware brake heat and its propensity to burst tyres is not modelled currently. In any case, brake heating caused by energy dissipation doesn't burst tyres immediately - it tends to happen some time after landing as the heat soaks in to the tyres from the hot brake disks. There was a thread on this in the F-15C sub-forum. IIRC SOP for the F-15C is for any aircraft that's experienced brake application above a certain rolling speed (60 or 90 Kts I think) is separated from other aircraft and is allowed to cool for some hours before anyone goes anywhere near it in case of a tyre bursting and killing members of the ground crew.

 

I would imagine that the Russian aircraft in reality also have upper limits on rolling speed above which the wheel brakes are not to be used.


Edited by DarkFire

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Either heat build-up, or a simple calculation of a predetermined speed taking OAT into account. It'd be cool if it's really heat, but I'm still not sure. If you are accelerating it's hard to tell which it is.

OK. Some very preliminary data. This is based on a single sampling at a rough speed held during the rollout. And it's for the Su-27 rather than the Su-25T just because the -27's tires will fail sooner than the Su-25T's and I have fewer control issues than I have with the Su-25T at very high speeds on the ground.

 

 

 

All tests were conducted on a +40° C day with the Su-27 at 93% gross weight. Method: Record landing track of Su-27 at 93% gross weight. Thereafter, play back track until touchdown, then take control of aircraft and set desired rollout speed. Hold that speed as closely as possible for the duration.

 

Here are the results--again, this is an extremely small sampling. The times and distances are: Time/distance since main gear contacted runway.:

 

attachment.php?attachmentid=191796&stc=1&d=1533920612

 

Using the same touchdown for the tests should control for any differences in stresses at the point of touchtown. Even though the sampling is small, there definitely seem to be differences in when/if tire failure occurs based on speed and duration. On the surface, it looks as if heat buildup in the tire is being modeled. It doesn't seem to be a matter of "a simple calculation of a predetermined speed taking OAT into account".

 

Original track attached.

 

Also interesting to note is that, taking off under the same conditions--constantly increasing speed--tire failure occurs in the range of roughly 350-390 kph.

1708596444_Su-27TireFailure-1.jpg.abdb9987255370154d9c39057954519a.jpg

Tire Test-Su-27_Heavy-Landing_+40C-02.trk


Edited by Ironhand

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You should have been a scientist if you aren't already Ironhand. That's very interesting. It definitely seems like heat must be playing a role there. Also interesting how the right wheel blew before left (by a huge margin) 2 out of 3 times, crosswind maybe?

 

 

That makes a pretty good case to make sure to let the plane slow down aerodynamically a little bit before applying brakes. I'll still hammer on them for the 25 though, I think the lower weight and braking force will make it much less of an issue.

 

 

For the higher take-off failure speed, it might be that there is still a lot of residual heat in the tires from the previous takeoff that makes the landings fail sooner (I assume getting to 390kph takes a little more than 8 seconds). Air being a poor conductor of heat, I have no idea how long they would take to cool down.

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@sideslip

You threw chum in the water, what can I say ;)

 

 

No worries. Text is just a very bad format for expressing intent or emotion. Imagine all the flame wars across the internet that started from miscommunication through text.

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You should have been a scientist if you aren't already Ironhand. That's very interesting. It definitely seems like heat must be playing a role there. Also interesting how the right wheel blew before left (by a huge margin) 2 out of 3 times, crosswind maybe?

 

 

That makes a pretty good case to make sure to let the plane slow down aerodynamically a little bit before applying brakes. I'll still hammer on them for the 25 though, I think the lower weight and braking force will make it much less of an issue.

:) In my early years my profession required me to be very aware of the validity of various studies and published results. It's something that's followed me through life.

 

No crosswinds. Weather is the ME default except for the temperature. I noticed that the right tire always seemed to blow first as well. But remember, the landing is identical in each trial. So if the landing abused the right tire more than the left (and if this stuff is actually modeled), then you could reasonably expect it to fail first in every trial. OTOH, I have 3 other landings recorded, one of which I bounced. It'll be interesting to see if the right tire continues to be the first to fail in those as well.

 

Braking isn't involved and I only use the rudder, if at all, to gently change direction after a blown tire starts to change my course in order to stay on the runway. I'm trying to minimize variables as much as possible. The only adjustments I'm consistently making are with the throttle to try and maintain a particular speed after the landing.

 

Anyway, this was just a preliminary test to see if 1) this approach and methodology was practical and 2) whether there might be something to learn using this approach. While both seem to be true, I wouldn't read too much into it just yet.

 

 

For the higher take-off failure speed, it might be that there is still a lot of residual heat in the tires from the previous takeoff that makes the landings fail sooner (I assume getting to 390kph takes a little more than 8 seconds). Air being a poor conductor of heat, I have no idea how long they would take to cool down.

Residual heat is a non-factor in the trials or should be. This mission is an air start and after each flight, I restart the flight for the next trial from the Main Menu. The entire takeoff roll configured as I am takes about 30 sec in full burner from "0" to first tire failure. It's about 13 seconds from the 200 kph mark to first tire failure as speed increases.

 

Anyway, if I have a chance today, I'll look more deeply into this.

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@Ironhand

Well, in my "long runway runs", with care I achieved both left and right tires blowing near simultaneously (less than a second apart). So there's no inherent weakness there. However, I can imagine any tiny bobbles, or anything that throws weight on one more than the other would gradually accumulate over the entire takeoff/shutdown run. Maybe you're just right hand biased :P

 

 

@Sideslip

You should see what "non english speaking" does to the mix. I had a Finnish friend we would play fight sometimes, I jokingly started teasing him on my forum once about spamming viagra ads or something and after the few back and forths he started getting violently aggressive. Had to lock the thread and try to pull him to one side, even still we ended up not speaking for like 6+ months after that, and lost a half dozen clan members because they were fed up with "the drama" :P One of the more bizarrely "blown out of proportion" encounters I've ever had...

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@Ironhand

Well, in my "long runway runs", with care I achieved both left and right tires blowing near simultaneously (less than a second apart). So there's no inherent weakness there. However, I can imagine any tiny bobbles, or anything that throws weight on one more than the other would gradually accumulate over the entire takeoff/shutdown run. Maybe you're just right hand biased...

:) Naw... Never touched the stick after touchdown for just that reason. Trying to keep variables to a minimum.

 

Spent a few minutes using the same methodology with the Su-25T at Groom Lake. It was just a quick look but I'm beginning to suspect that the Su-27's tire failure modeling is more sophisticated than that of the Su-25T. Time will tell, I guess.

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Win 10 Pro x64, ASUS Z97 Pro MoBo, Intel i7-4790K, EVGA GTX 970 4GB, HyperX Savage 32GB, Samsung 850 EVO 250 GB SSD, 2x Seagate Hybrid Drive 2TB Raid 0.

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It wouldn't surprise me. The 25T is showing it's age pretty severely these days. Look at poly count, for one :

 

 

Su-25T- 35k polys (if I remember correctly)

"average" aircraft poly count is 150k, with many of the newer ones going over 200k, and the MiG-29 well over 300k. Even some of the older AI models are higher, like the MiG-31, Tu-22M

Де вороги, знайдуться козаки їх перемогти.

5800x3d * 3090 * 64gb * Reverb G2

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Braking isn't involved and I only use the rudder, if at all, to gently change direction after a blown tire starts to change my course in order to stay on the runway. I'm trying to minimize variables as much as possible. The only adjustments I'm consistently making are with the throttle to try and maintain a particular speed after the landing.

 

 

I misunderstood. I thought you were holding speed with the brakes applied. Would be interesting to see how braking would affect the lower numbers.

System specs: i7 3820 @4.75Ghz, Asus P9X79LE, EVGA GTX1080SC @2100mhz, 16GB Gskil DDR3 @ 2000mhz, 512GB 960EVO m.2, 2 X 512GB 860EVO SATA3 in RAID0, EVGA Supernova 850W G2, Phantek Entho Luxe White. CPU and GPU custom water-cooled with 420mm rad and lots of Noctua fans.

ASUS PG348Q. VKB Gladiator Pro w/MCG, X-55 throttle and MFG Crosswind.

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