MPAI MiG-23 and MiG-27 Takeoff Issue

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Muckinator2005
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MPAI MiG-23 and MiG-27 Takeoff Issue

Post by Muckinator2005 »

While converting MPAI MiG-23 and MiG-27 from Mike Pearson I noticed a very strange behavior when the planes takeoff. While accelerating main gear first leaves the ground and the plane is levelling, so that the front gear still touched ground with the plane heading down the runway before getting airborne. Landing is almost a three point landing but OK.
I don't remember if the planes did the same thing in FS9 or not. There's only little clearance between the folded tail fin and ground while taxiing, so there's little room for lifting the planes nose during takeoff and this could be the reason for moving that strange.

So here's my question:
What needs to be done to make the takeoff characteristics a little more realistic or does it have to stay like that ?
Any help would be highly appreciated.

Best regards and stay healthy,

Jean Claude Drockur
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Re: MPAI MiG-23 and MiG-27 Takeoff Issue

Post by hschuit »

Sounds too familiar, this problem appears if on the ground the fuselage sits tilted with a nose up attitude but the model itself is not tilted (as can be checked in MCX). I tried several contact points setups, searched the web for solutions, no luck. My best guess is that P3D thinks these models are tail draggers and makes them lift their tail near the end of the take off roll.

When I converted the F-100 Super Sabre (which also sits nose high), I found a solution. In MCX, I rotated the converted MDL 3.0 degrees up (at keyframe 130) along the pitch X-Axis to get the wheels aligned horizontally with the surface. Then I exported the tilted MDL as a new copy, adjusted the Aircraft.cfg contact points - problem solved. Note: It is important to always keep a non-tilted, original copy of your converted MDL file, just in case you need to update/add parts later.

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Re: MPAI MiG-23 and MiG-27 Takeoff Issue

Post by Firebird »

Not having P3D I can't answer to this issue personally or even make any amendments.

What needs to be understood is that each sim stands on it's own and therefore may have certain characteristics altered from the parms that were set up to work in FS9.

An example of this is that I know some people will alter wing parms so that the aircraft park in a spot of the correct size for an aircraft, as you are aware FS9 uses the mdl size for the spot size whereas FSX/P3D use the wingspan parm.

This is not to say that you have altered anything but if you do you will have consequences. The fde is best described as a balancing act. You amend one parm for one result and you will also get other effects as most parms affect more than one area of performance.

What you have to do is make a small adjustments and see what happens throughout the entire flight envelope. You may well then need to adjust another parm, or maybe more, to counteract unwanted changes to performance. This is the reason that sometimes an fde can take a long time to develop.

Now, to your specific case. IF you have not altered anything at all from the original FS9 fde then it would seem that you have hit one of those individual sim characteristics.
From what you describe it sounds like either the wing area is too great and causing the wing to lift off early. Possibly a forward centre of lift and/or centre of gravity come into play here as well. You can tell from what I said earlier that this may well become a big job to fix quite quickly.

Now for the AoA on landing you can alter this by amending a graph in the .air file. This is not easy, should not be taken lightly and takes me a lot of trial and error to get results, so it is not something that I can say 'do this and this will work'.

IF you have altered any characteristics from the original FS9 fde then it might be easier to go back to that to improve matters and just alter your spot sizes if need be. Remember that the atc_parking_code parm is used before spot size for a parking spot match.
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Re: MPAI MiG-23 and MiG-27 Takeoff Issue

Post by gsnde »

I would like to have a confirmation if the behavior is the same in FS9. May I please ask somebody still using FS9 to check this and give feedback?

Then we can decide if and how this could be fixed.

Many thanks!


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Re: MPAI MiG-23 and MiG-27 Takeoff Issue

Post by jimrodger »

In FS9 it looks like the nosewheel sticks and the mainwheels lift levelling the airframe and then it takes off. Just like you describe after the conversion.

Video here :- https://1drv.ms/v/s!Am2Fnz2wsOPB4zvw7hF ... 4?e=RKc2el

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Re: MPAI MiG-23 and MiG-27 Takeoff Issue

Post by hschuit »

Jim, that is interesting, these FS9 packages have been around for 5 years and no one complained or figured out how to fix it... Until I see a better offer, I stick with my tilted mdl method.
BTW, every AI model I converted which has a nose gear and sits nose high on it's wheels has this problem.

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Re: MPAI MiG-23 and MiG-27 Takeoff Issue

Post by gsnde »

Your method looks perfect, Henk. Let's see if Jean Claude is willing to re-visit all these models.How cool if we had a batch conversion ability for such tasks.
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Re: MPAI MiG-23 and MiG-27 Takeoff Issue

Post by Firebird »

I can confirm what JIm sees. It is not very pronounced but you can see it on take off if you look hard enough. I randomly tested a 23 and a 27.

There is however a very good reason for this the ground clearance is very small for the folded ventral fin. If you had it any other way the chances are that people would moan that the folded fin would disappear below the surface of the runway. All in all I think it does a good job of avoiding that.

What I did notice is that in quite a few pics the fin appears to be further off the ground than with the model. I did find some pics with it that low but they appear to be static or stored examples. Which leads me to think that there is some hydraulic action that keeps a flyable one higher off the ground.
I couldn't see an obvious flaw with the complicated gear mechanism so it is probably a subtle degrees of angle thing with the main gear. In any case there is nothing we can do about that now, and to be honest if it has only just come to light is it worth the effort?

I would say about the MiGs specifically that you have probably got it about as good as you can get it without a lot of time and energy spent redefining things.

On the subject that Henk spoke about I can't be certain, because I have not done this myself, but I am pretty sure that altering the model angle will alter the pitch of the model in game and may lead to other things.
I am not sure that a blanket 'do this' is what you want to do in all cases.

I am fairly sure that all the fdes I have been involved in the models have been level on the ground so the fde has taken this into consideration.
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Re: MPAI MiG-23 and MiG-27 Takeoff Issue

Post by Muckinator2005 »

Thank for the feedback folks.

@Jim:
Yep, that's exactly what happens with the converted models. As I animated the elevators you can also see that drops the elevators until the main gear leaves ground, than levels and finally goes positive just before it gets airborne. Like Henk describes it the plane behaves like a taildragger.

@Steve:
I didn't make any changes to the fde (I wouldn't even know how). The only thing I changed is the light section, as the models have the light effects directly attached to the model and the toe_brake_scalar to reduce the abrupt braking of the airplane in P3D. Everything else I left alone.

@Henk (and Martin)
I'll give it a try. The only thing I don't quite understand is you mentioning the keyframe 130. Is this the 'Rotate' function at Transform Object we're talking about or is there another function ? I thought it would rotate the whole plane regardless of the keyframe count.
I'm just afraid that, if the plane rotates like a normal plane does the tail fin might hit ground, as there's really little space. I checked some videos and it looks like the MiG pilots take off at a very shallow angle before they're high enough to increase the climb.

Thanks for the support everybody.

Best regards and stay healthy,

Jean Claude
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Re: MPAI MiG-23 and MiG-27 Takeoff Issue

Post by Muckinator2005 »

I just missed Firebird's last post.

It's relatively simple. All the birds are converted. I'll test Henk's method on one of the planes. If it works I'll change the others as well and everybody has to wait a little longer. If it doesn't we'll use them the way they are. No waste of time here.
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Re: MPAI MiG-23 and MiG-27 Takeoff Issue

Post by hschuit »

JC, around keyframe 130, the gear animations are fully down and at a compression height which (usually) reflects how much the gear is compressed when the AI model sits on the ground in the sim. MCX has a slider on the menu bar to show the model in a desired keyframe state. This only impacts the viewport display, it does not updatre the MDL and has nothing to do with the ransform - Rotating function.

BTW: In real life the MiG23 ventral tail fin is folded sideways to the right when the gear is down.

Henk.
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Re: MPAI MiG-23 and MiG-27 Takeoff Issue

Post by John Young »

Have you checked the gear compression calculations? It looks like the nose wheel has quite a bit, but the main wheels have very little, if any. When the plane rotates the main gear oleos stay fixed and the nose wheel drops as the compression comes off.

How much extension is built into the gear legs (nose and main, in feet) and what are the values in the contact points?

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Re: MPAI MiG-23 and MiG-27 Takeoff Issue

Post by Muckinator2005 »

I checked the extension of both nose and main gear. Nose has 0.656ft (200mm), main exactly half of that (0.328ft or 100mm). Contact point are defined as this:

point.0=1, 12.1950, 0.0, -5.7484, 1181.1, 0, 0.75, 46.8, 0.7500, 2.000, 0.9100, 3.000, 3.000, 0.000, 260.000, 260.000
point.1=1, -6.8695, -4.75, -4.6216, 1574.8, 1, 1.25, 0.0, 0.7500, 2.000, 0.5456, 5.000, 5.000, 2.000, 260.000, 260.000
point.2=1, -6.8695, 4.75, -4.6216, 1574.8, 1, 1.25, 0.0, 0.7500, 2.000, 0.5456, 5.000, 5.000, 2.000, 260.000, 260.000

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Re: MPAI MiG-23 and MiG-27 Takeoff Issue

Post by gsnde »

Does this have optimization potential, John?


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Re: MPAI MiG-23 and MiG-27 Takeoff Issue

Post by John Young »

It's a bit late this evening to go through the calculations Martin, but I've taken a copy and I'll have a look in the morning.

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Re: MPAI MiG-23 and MiG-27 Takeoff Issue

Post by gsnde »

Many thanks, John


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Re: MPAI MiG-23 and MiG-27 Takeoff Issue

Post by John Young »

I spent a couple of hours on this morning.

Having had a quick look at the figures, I wanted to confirm the problem in FS9, so I downloaded Mike's package and tested the take-off. I tried this as normal speed and 1/4 speed a few times, but I couldn't see the problem described above. The take-off looked normal:

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I don't think it will make any difference to the reported problem, but for what it's worth, what I didn't see was any compression in the nose or main gear, either when braking or on lift off. There are probably 3 reasons for that:

- The physical compression in the nose leg is only 0.656 ft and 0.328 ft in the main gear. That's not really enough meat to play with. I normally give my nose wheels 1ft. The static compression is however over-stated at 0.75ft for all legs so that might compensate a bit.

- The static compression ratio for the nose wheel (static compression/maximum compression, measured as a drop from the datum/centre line to the bottom of the wheel) is 5.716ft/5.06ft = 1.023, not 2.00 as used in the Contact Points. I couldn't measure the maximum compression of the main gear from the model I imported into Gmax via MCX, because the animation is needed to do that. The value of 2.00 though also seems very high.

- The Damping Ratio of 0.91 for the nose leg is also probably too high (a value of 1.0 will kill the animation). I tried a value of 0.1 which caused a lot of oleo movement, but with the Mig bouncing all over the place. A value of 0.3 tamed it, but it then killed the oleo movement.

There is an added problem when the values are changed, even moderately - the wheel heights also change. I could reset those with new values, but at the expense of putting the tops of the main wheels through the gear doors when parked.

I could try for a few more hours to get a better result, but I can't tell if it has any influence on the reported problem, because I can't see it.

I'm wondering also, that since the model has been around for 5 years and no one has reported it, does it actually exist? Could it be that what is being seen has been induced in the conversion?

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Re: MPAI MiG-23 and MiG-27 Takeoff Issue

Post by gsnde »

Now that is weird. Jim could see the issue with the FS9 model, but John could not.

What might explain this?




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Re: MPAI MiG-23 and MiG-27 Takeoff Issue

Post by Firebird »

I saw it but sporadically and it is not very pronounced. The shadow on the ground had to be used for me to see it at all.
The landing almost has to be a three pointer due to the compactness of the main gear on the ground, as I said earlier.

It could be that the users set fps is a feature here. It sounds daft but John and I found there there was a massive effect with changes in fps settings when we were developing the Tiff. It really was unbelievable. Basically the higher the fps the better the flight characteristics.

I was not sure that John would find a simple fix for the compression. My guess is that the gear animation would probably be need to be redone and possibly even a mod of the main gear be necessary.
In essence, I think that it currently stands in a fully compressed state, i.e. what you see in static or open stored pics, in active jets it seems that the gear has more hydraulic pressure and therefore the main gear is longer on the ground.

Although I am not a modeller my theory is that the gear needs to be slightly longer and be able to be compressed, the angle of the struts may need to be altered slightly (again to slightly increase the ground clearance) and it may also need to made more complicated so that the gear will fit in the well still.

The main issue here is that actual pictorial information is required to even think about amending this and as is always the case it is never easy getting concrete evidence of Russian aircraft.

My opinion is that Mike didn't get anything wrong, he just may not have got it as accurate as it could be. This maybe due to model/information limitations.
Would I like the model(s) to be more realistic - yes. Can I live with what we have got - yes. I do try to be a realist.
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Re: MPAI MiG-23 and MiG-27 Takeoff Issue

Post by gsnde »

That would have been a nice fix.... sigh....

Now if Jean Claude is willing to try Henk‘s trick, he will tell us if the improvement is worth the extra work.


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