Inconsequence

[JNASova]

Hello people.

MiG 21 BiS is a project that is very interesting to me. During instruction manual reading I was quite surprised by the degree of realism.I must admit that I was very pleased.I decided to test some things.

 

Instruction manual say next:

 

It is permissible to take off with the load variants > 10,000 kgf under no-wind conditions at or below an ambient air temperature of +15oC;

if the ambient temperature is above+15oC, takeoff is permissible only if there is head wind,every 3 m/s increment of the head wind velocity corresponding to ten degrees of temperature surplus over +15oC.

When the runway length corresponds to that of a secondclass airdrome, takeoff with the above variants of external stores shall be accomplished at second reheat power only (when full reheat power is used, the required runwaylength is 2000 m).

 

In the mision editor, I created the following scenario:

- Airport - Novorosysk. Runway lenght 1800 meters.

- Plane MiG 21 BiS. Load 2x Fab-250 M54TU, 2xBL-755, 1x800L fuel tank, ASO-2.Total weight 10 329 KG.

- Summer, + 40° C.

 

Half the length of the runway was enough to take off.Is there an explanation?

Edited January 17, 2015 by =JNA=Sova
text wrapping

[JNASova]

Sorry I forgot one thing.No wind.

I tested also with 5m/s wind in the back.I also was able to take off.

[King_Hrothgar]

I'm not really sure what you're getting at. Were you expecting it to magically blow up or something? Manuals in general describe what's safe to do and what it's designed to do but not what it can do. In regards to max takeoff weights, for some planes it comes down to what can actually be lifted off the ground with a reasonable amount of runway, with other planes (like the MiG-21) it probably has more to do with what's safe for the suspension. Given that us virtual pilots typically don't care if our virtual planes have a service life of 30 minutes or 50 years, we tend to do things we wouldn't with a real plane.

[xxJohnxx]

You also have to consider that such limits often include emergency procedures and failures.

Would you still be able to stop if the engine failed just before liftoff?

Would you be able to lift off if the afterburner got disabled just before rotation?

What about a hydraulik failure or an air pressure discharge midway?

 

These limits, normally ignored by virtual pilots, often have a big role in the real world, even if the aircraft would be able to do it if everything goes right. Remeber, there is no re-spawn.

[NoJoe]

It sounds to me like the issue Sova is bringing up is that the data suggest the required runway length for those conditions is 2000 meters, while he was able to takeoff in only about half that.

 

However, I wonder where that 2000 meters figure comes from. Is it 2000 meters of ground roll, or is it something like 2000 meters is required for safety reasons even though the actual takeoff takes less? Maybe a regulatory requirement?

 

For the real planes I fly the takeoff calculations typically deal with two different numbers: the ground roll distance (how far along the runway must the plane travel before it lifts off), and the 50-foot takeoff distance (how far along the runway it must travel to lift off and then reach 50 feet high). I wonder if the 2000 meter figure is something like the latter?

 

Or another possibility: many aircraft have the ability to calculate an accelerate-stop distance; essentially the total runway required for the plane to accelerate to liftoff speed, then brake to a stop again. I wonder if the 2000 meter distance is something like that?

[Random]

The limits are about the distance needed to STOP if need be too I suspect....

[JNASova]

In this case, the manual says the following:

"Not to exceed liftoff ground speed of 370 km/h for KT-92D LG wheels fitted with tires model 42A."

So plane must be in the air at speed 370 km/h,to avoid an explosion of tires.

Same says manual for real life pilots. At least manual used by the pilots of The Yugoslavia.

I agree with you John, I guess things you're talking about are included too. But,I want you to consider something else.

With so much weight,you need more buoyancy if you want to be in the air at 370 kh/h, if air temperature is above 15°C. That buoyancy is provided by wind. In the scenario that I created was needed frontal wind speed of 9m / s.There was no wind.In the following scenario, I even added a tail wind, 5 m / s.In both cases, I took off without a hitch.So,my question is:

* Does the manual says one thing and the game is modeled different way?

* MiG 21 Bis is able to deal with such situations without problems,but the engineers who worked mig 21 bis are afraid of being executed in case of any failure, so they deliberately tightened procedures :smartass:?

Edited January 18, 2015 by =JNA=Sova

[xxJohnxx]

* Does the manual says one thing and the game is modeled different way?

* MiG 21 Bis is able to deal with such situations without problems,but the engineers who worked mig 21 bis are afraid of being executed in case of any failure, so they deliberately tightened procedures :smartass:?

 

There is always some room for error in such limits.

Try taking off in 30°C in same conditions, would be very interesting to hear if it also works out well in such conditions.

 

Also you have to consider that the engine might not be in the same condition as we have it in the simulator. An older engine will definitely produce less power than a brand new one. Fuel quality and other factors can also can come into play here.

[JNASova]

Further discussion on this topic will have to continue with developers directly.Public discussion has not given result.

[FSKRipper]

:wassat:

 

It was an interesting disscussion so far and I enjoyed it. Are you simply not talking to us normal people any longer?

 

In this case you might consider to relay the answer of Dolphin or Cobra to us?

[xxJohnxx]

Not sure what was to be expected?

Do you think the aircraft will instantly implode when there is 2m/s less wind?

Do you think the tires explode when you go 371km/h on take-off?

Do you think everything will go wrong because a limit is exceeded?

[Addewang]

Not sure what was to be expected?

Do you think the aircraft will instantly implode when there is 2m/s less wind?

Do you think the tires explode when you go 371km/h on take-off?

Do you think everything will go wrong because a limit is exceeded?

 

If a manual says you may not do something you assume that is because it is impossible and that you would overshoot the runway etcetera.

Edited January 22, 2015 by NineLine
Removed 1.1 violation

[Derbysieger]

If a manual says there is a limit of X then you can be sure that there are safety margins.

[xxJohnxx]

If a manual says you may not do something you assume that is because it is impossible and that you would overshoot the runway etcetera.

 

Yes indeed, that is exactly what a limit is.

A maximum value after which it can get unsafe.

 

In 99% there is a very big safety margin between the limit and the point where it gets dangerous. In aviation safety margins from 25 to anywhere up to 75% or even more are not uncommon.

 

This still means (as said many times before in this thread) that if you exceed a limit by a relatively small amount, you won't actually get in danger. Not even close most of the times.

 

Also wear and tear plays a big role. You have to account for the fact that for example a tire, that when being brand new can do 500km/h without any problem, might after getting worn down will actually fall apart when doing 450km/h. With a new tire you could easily exceed the limit of 370km/h and go down the runway with 480km/h, you wouldn't even notice, however with the worn down old tire, doing 480km/h might not be so good of an idea.

 

With parts suffering wear and tear it is always a bit of guesswork too. Again with the tire, you don't really know the history of that thing. Did it hit anything or drive over something? Did it block during braking a couple of times? All stuff that engineers normally can't calculate very well. However what you can assume (and test) that if the tire is in a very bad condition it will still be able to do more than the limit of 370km/h.

 

On the other hand, limits like temperatures and wind-speeds during take-off as an example, normally don't even come into play, because often these limits are based on deviation from a normal procedure. As an example would be a engine failure during take-off roll. If the engine fails at 270km/h, just before lift-off with 1000m of runway remaining you might be able to stop, while maybe with only 800m of runway remaining (because less wind, higher temperatures) you might not be able to stop. If the engine wouldn't fail you would just simply take-off, not even noticing that you exceeded a limited that could have had consequences in described emergency.

 

I am not sure what exact results OP is expecting, but directly comparing limits with in-game performance is not going anywhere. You don't know what the guy placing that limit had in mind, as limits can have many many reasons.

[emg]

If a manual says you may not do something you assume that is because it is impossible and that you would overshoot the runway etcetera.

You should not assume that when reading real aircraft manuals.

[aaron886]

Further discussion on this topic will have to continue with developers directly.Public discussion has not given result.

 

Maybe not the result you were looking for. ;) Some good discussion, at least.

[JNASova]

FSK Ripper

I do not know just what part you enjoyed thread.The one in which I ask a question,or insofar as no one gives an answer based on facts.If you think that something like that is normal and that all should become unclear, then we have a completely different view of the situation.I turned attention to Cobra.What he will take, that's his business.

 

xxJohnxx

We all know that the tire will not explode if the instruction writes to 370 km / h, and I achieve 371.But if in certain circumstances,for example, I succed to take of after 1000 meters distance, and three MiG 21 BiS real life pilots show me diagrams according to which I should need almost 500 meters more, then this is a serious matter.At least for me.So far, no one has offered any concrete evidence, just a personal opinion.For this reason, I will not continue to participate in the discussion on this topic.

I'm not interested in opening lua scripts, cheating, statistics on the server.I am not a "gamer".I want realism.I love aviation and I love to fly.

Edited January 22, 2015 by =JNA=Sova

[JNASova]

Maybe not the result you were looking for. ;) Some good discussion, at least.

 

There is no result,thats the problem.

[Hadwell]

The problem is... when you have 20 different manuals all with different "do this, but not that" type rules in them and recollections from real pilots that differ slightly from one another... how do you know, or choose what sources are fact and what are fiction?

 

So far, the mig-21 in DCS feels pretty damn good to fly, compared to anything else capable of going up against the FC3 planes it's hands down the most realistic, that makes it sim enough for me(for now), though it still needs some fixes.

Edited January 22, 2015 by Hadwell

[Golo]

I dont know, I tried it and used aprox. 1,5km to T/O, seems reasonable to me.

 

40°C, No wind,

4 x FAB-250

800l FT

ASO-2

 

TW-10331kg

Test NR Heavy.trk

[Vitesse2l]

OP - How are you measuring takeoff distance?

 

If this is to be investigated properly we need to know loadout, fuel, weather, which airbase etc. Perhaps post the mission you are using.

 

Also your pilot friends need to provide similar info.

 

Then we might get somewhere.

[aaron886]

The fact was given to you that you are reading a recommended procedure, not an aircraft limitation.

[FSKRipper]

FSK Ripper

I do not know just what part you enjoyed thread.The one in which I ask a question,or insofar as no one gives an answer based on facts. If you think that something like that is normal and that all should become unclear, then we have a completely different view of the situation.I turned attention to Cobra.What he will take, that's his business.

.

 

I enjoyed to read some suggestions for your problem. To give you a honest answer and make tests for ourself, we need several comparing attempts. Statistics lower n=6 are no statistics. At least a track from your side would be helpful to recreate your take off behaviour. If you can provide some more information, I will take the time and test it.

It's not personal, but I won't waste my precious spare time for statements like "this is totally wrong", or "horrible FM" which are increasing the last days, especially from people comparing a PFM with the "driving on rails" behaviour of a SFM. Give us some hard evidence and we will look into it :thumbup:

 

Edit: As Vitesse2l asked: What is your method to measure TO distance?

Edited January 22, 2015 by FSKRipper

[fjacobsen]

Also the take off technique plays in.

 

At what rate do You rotate ?

How fast did You advance the throttle ?

 

And as asked by others - how was the weather - ISA conditions or high ambient temp with low barometric pressure ?

 

It´s not easy to find exact numbers and tables does also not always tell the truth.

 

In a flightsim You might also need to compromise ground performance vs flight performance.

 

FinnJ

[FSKRipper]

Since there was no reaction from you, may it be that you instructions include a specific Vref for runway requirement?

 

Vref stands for refusal speed. Refusal speed is the maximum speed during takeoff from which the air vehicle can stop within the available remaining runway length for a specified altitude, weight, and configuration. Your t/o distance is significantly shorter than the required runway lenght.

Edited January 24, 2015 by FSKRipper