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Old 08-10-2018, 05:01 PM   #41
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Quote:
Originally Posted by Dr_Arrow View Post
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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Old 08-10-2018, 05:06 PM   #42
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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.
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Old 08-10-2018, 06:24 PM   #43
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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.:



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.
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Last edited by Ironhand; 08-10-2018 at 07:07 PM.
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Old 08-11-2018, 01:27 AM   #44
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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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Old 08-11-2018, 01:35 AM   #45
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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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Old 08-11-2018, 02:07 PM   #46
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Quote:
Originally Posted by Sideslip View Post
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.


Quote:
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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Old 08-11-2018, 03:11 PM   #47
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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


@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" One of the more bizarrely "blown out of proportion" encounters I've ever had...
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Old 08-11-2018, 05:07 PM   #48
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Quote:
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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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Old 08-11-2018, 05:14 PM   #49
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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
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Old 08-11-2018, 05:48 PM   #50
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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.
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