Roll Lag and Turn Lag

[wonders9]

 

Roll lag and turn lag are two concepts that seem to have been overlooked in FM design and the discussion thereof in flight sims. What do they mean here?

 

Take roll lag for example. Supose an aircraft is flying straight horrizontally. As soon as the stick is suddenly (ideally instantaneously) deflected from the neutral positon to the farthest left-side or right-side positon, the aircraft starts to roll, but it will take a length of time (the delay) for the aircraft to reach its maximum roll rate. Let us call this delay its roll lag (which, like roll rate, will be different at different airspeeds.)

 

A graph is attached below to illustrate the idea more vividly. The horrizontal axis is time and the vertical axis roll rate. Here we see two curves representing roll behaviours of two aircraft with the same roll rate but different roll lags. The blue curve shows that it takes approximately 0.5 second for the "blue aircraft" to reach its max roll rate; the brown curve shows that it takes about 0.9 second for the "brown aircraft" to reach the same max roll rate. 

 

(As we can see, the roll acceleration between the start of the roll and the point in time when max roll rate is reached, is not constant; it is always changing. So I have avoided the term "roll acceleration".)

 

Despite the identity in maximum roll rate, the "blue aircraft" and the "brown aircraft" are hugely different in agility in air combat, right because they have very different roll lags. For example, image a fighter with twice the roll rate of the FW 109, but with a roll lag of 10 seconds, and with all other flight characteristics being identical to the 190, then which aircraft is more agile in combat? Of course it is the 190.

 

The same analysis goes for turn lag. In the graph below, changing the word "roll" of the vertcial axis label to "turn" will give us a similar illustration for turn lag.

 

 

Although they are certainly import concepts, I suppose nobody has ever heard of roll lag or turn lag testing for WWII fighters. 

But why were roll lag and turn lag not an issue for WWII fighters? The reason, I guess, is that they were all short enough for the fighters to present no noticeable difference.

 

But when the roll lag and the turn lag are noticeably different for different fighters, as is quite possible in computer simulation of fight, it should get the developers attention how close they are to reality.

 

 

  

   

 

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Edited December 30, 2017 by wonders9

[ACG_Smokejumper]

Neat~o.

 

What is it you are saying in layman's terms......

[JtD]

 

 

I suppose nobody has ever heard of roll lag or turn lag testing for WWII fighters.

 

Well, I have, and so has everyone who's been reading the FM section of this forum from the beginning. You may want to just browse a bit more, as some interesting things can be found.

 

Tested roll lag figures are a byproduct of the NACA handling tests, which were carried out for several aircraft, as wwell as the subject of a few smaller, dedicated studies. Turn lag figures can also be found in the NACA handling tests, even though I find the term odd. NACA, more reasonably, covers this in their stability tests when looking at elevator responsivness.

[unreasonable]

Roll lag and turn lag are two concepts that seem to have been overlooked in FM design and the discussion thereof in flight sims. What do they mean here?

 

<snip>

 

   

 

OK fine - but why do you have to say- "Although they are certainly import [sic] concepts, I suppose nobody has ever heard of roll lag or turn lag testing for WWII fighters. "

 

Do you really believe that obvious nonsense or do you just enjoy being obnoxious? Your English is good enough for this not to be a mistake. What a wonderful way to persuade people to change their minds.

 

edit - I see JtD has reappeared - beginning to worry that he had found better things to do.

 

Whether this is covered properly by BoX is another matter - nothing wrong with your conceptual analysis, but so far it is entirely fact free.

Edited December 30, 2017 by unreasonable

[wonders9]

Neat~o.

 

What is it you are saying in layman's terms......

 

I am not sure what you mean. Do you want me to explain in simpler terms?

 

If so, I can make an analogy.

 

Suppose a train can travel at a maximum speed of 100 km/h. Now the train is at rest, and you start the train, with the intention of letting it run as fast as possible. Of course the train cannot reach the speed of 100 km/h at once, but it will accelerate. If, after 50 seconds, the train is finally running at 100 km/h, its top speed, then this 50 seconds is what I call here the "lag", the length of time during which the train cannot run at full speed after you start it. 

 

An aircraft's roll speed or turn speed is just like this. When you start to roll the aircraft, it will not roll at its top speed at once, but only after a short length of time, which is the roll "lag".

Edited December 31, 2017 by wonders9

[wonders9]

Well, I have, and so has everyone who's been reading the FM section of this forum from the beginning. You may want to just browse a bit more, as some interesting things can be found.

 

Tested roll lag figures are a byproduct of the NACA handling tests, which were carried out for several aircraft, as wwell as the subject of a few smaller, dedicated studies. Turn lag figures can also be found in the NACA handling tests, even though I find the term odd. NACA, more reasonably, covers this in their stability tests when looking at elevator responsivness.

 

I am surprised that such testing did exist, but still not very much surprised, because, as I gather from your reply, it was far from common practice for the major air forces of the second world war.

 

I was not sure about that fact so I said "I suppose nobody has heard of it". Besides, I had indeed read before of a couple of forum posts that mentioned "roll acceleration", but I pointed out that I would rather avoid this term.

 

Even though the NACA tested these data for several aircraft (during WWII?), it was not a common practice for the major air forces during WWII. So I guess it is unlikely that any test data of, for example, the 109's roll lag be found. But if they do exist, that would really be great.

[wonders9]

OK fine - but why do you have to say- "Although they are certainly import [sic] concepts, I suppose nobody has ever heard of roll lag or turn lag testing for WWII fighters. "

 

Do you really believe that obvious nonsense or do you just enjoy being obnoxious? Your English is good enough for this not to be a mistake. What a wonderful way to persuade people to change their minds.

 

edit - I see JtD has reappeared - beginning to worry that he had found better things to do.

 

Whether this is covered properly by BoX is another matter - nothing wrong with your conceptual analysis, but so far it is entirely fact free.

 

I think your attitude and remarks regarding other forum members' sincere reports of possible FM mistakes were problematic. I have already pointed it out in the "Query about the new FM of bf109 " thread. By the way, there I just used your way of reasoning and some of your own words to answer your posts...

Edited December 31, 2017 by wonders9

[wonders9]

About the lack of historical test data of roll lag.

Like many other members here, I am an IL-2 1946 veteran. The 109 in IL-2 1946 has completely different roll lag from that in the current BoS. I don't think that the 109 in IL-2 1946 is totally wrong. In fact I feel the roll behaviour of the 109 in 1946 is more similar to what is seen in videos of real life 109 flights of today. Of course this is subjective, but it is my careful observation.

Thanks.

Edited December 31, 2017 by wonders9

[wonders9]

Well, I have, and so has everyone who's been reading the FM section of this forum from the beginning. You may want to just browse a bit more, as some interesting things can be found.

 

Tested roll lag figures are a byproduct of the NACA handling tests, which were carried out for several aircraft, as wwell as the subject of a few smaller, dedicated studies. Turn lag figures can also be found in the NACA handling tests, even though I find the term odd. NACA, more reasonably, covers this in their stability tests when looking at elevator responsivness.

 

In addition: If the tests were done not by the military, but only by certain research institutes, that means that the military (American, Russian, German, British, Japanese) deemed them (roll lag and turn lag) unworthy of testing (in other words, "not an issue for WWII fighters" -- quoted from post #1).