I've discovered an interesting phenomenon regarding how ground effect is possibly modeled with the 737 NGX during the landing phase. After quite a few repeated tests, I've found the there appears to be a critical AOA and/or pitch attitude that seems to define a sort of internal flight dynamics modeling threshold. If the aircraft exceeds this threshold during the last few moments before flare, ground effect simply never seems to happen. If, however, the aircraft remains within some kind of flight modeling threshold limit (AOA or pitch attitude range?), ground effect seems to be correctly modeled and implemented.

 

For example, if I fly a perfectly stabilized 3 degree glideslope at Vref + 5, with the 1000 ft marker on the runway as my touchdown aimpoint, the 737 NGX will actually behave quite realistically in ground effect if the standard FCOM approach and flare technique is implemented. However, if an unstabilized approach is made or a last minute correction using negative pitch (to correct for too high a glidepath angle, etc), ground effect seems to simply not happen. It's as if there is a ground effect "on/off" switch at a certain AOA or pitch attitude prior to flare in some situations. I can perfectly predict whether or not ground effect is taking place about the time I cross the runway threshold though. If my approach is stable and on both glidepath and speed, I'll see my rate of descent naturally fall off (without a change in pitch or power) as the ground effect cushion zone is entered prior to touchdown (about over the runway threshold on a normal 3 degree glidepath). In a situation when I may be below a presupposed AOA or pitch angle, however, my rate of descent over the threshold will not change as I cross the threshold - ground effect simply does not seem to happen. I'm really not talking about making a last minute dive or such just before flare in this situation, but relatively minor corrections in pitch that again seem to fall below some kind of flight modeling threshold, effectively 'turning off' ground effect modeling.

 

Obviously there are several dynamic factors which require different flare technique depending on rate of descent, density altitude, aircraft weight, etc, but none of these seem to actually come into play. Ground effect as it's currently modeled seems to be entirely based upon the a certain AOA/pitch threshold being maintained as I described earlier. I know that FSX has a lot of limitations and that PMDG had to implement some kind of non-standard flight modeling trickery to get ground effect working properly with the 737 NGX sim, and I'm just wondering if I might have come across a limitation here in terms of flight dynamics. I wonder if anyone else has noticed this?

 

Signed,

 

Andrew Hansen

PMDG's silence speaks volumes?       Nevertheless, after further extensive testing, I can confirm without one iota of doubt that ground effect is intermittent and unpredictable with the 737NGX.     Nothing is tied to either pitch or AOA and there are in fact serious flaws in the way PMDG attempted to model ground effect altogether.    I was, however, able to make modifications to both the .air and .cfg files (with the help of a very experienced person well schooled in FSX wizardry) that completely eliminates the problem.    This took a LOT of testing and trial and error on our part, but the aircraft now flies realistically and consistently, and furthermore handles beautifully when the correct published FCOM technique is applied.    No more phantom ground effect ruining otherwise perfect approaches and flares.

 

I don't work for free though, unfortunately.    If PMDG would like to private message me, I will be happy to provide the correct modifications to the files mentioned above for a modest fee to cover my time and effort.     If not, I completely understand and wish them the best.   

 

I'm thrilled I was able to finally isolate and correct the problem if only for my own use.   

 

Cheers,

 

Andrew Hansen

 

 

In a situation when I may be below a presupposed AOA or pitch angle, however, my rate of descent over the threshold will not change as I cross the threshold - ground effect simply does not seem to happen.

 

You do understand that AoA has an effect on the difference of the CL between normal flight and flight in ground effect, yes?

 

I highly doubt they designed the simulation to simulate ground effect with limits well below the operating envelope of the aircraft.

Too bad a previous poster wants to be recompensed for his "so called" modifications to the NGX.  I thought this forum was initially established for mutual benefit of the sim community based on the free and willing participation of the membership as a whole.  Apparently not!

 

Jim Driskell

Andrew

You sound quite proud of your discovery and "fix", but I have to agree with the previous post. Why throw this out there rather than take it up privately with PMDG? What reaction do you expect from the community? Pitchforks and torches marching to Las Vegas?

If you're outing PMDG and then demanding compensation, I don't think you'll drum up much support.

To each is own but personally I don't think you'll get too many pats on the back with those tactics even ?if? what you found is valid.

PMDG's silence speaks volumes? Nevertheless, after further extensive testing, I can confirm without one iota of doubt that ground effect is intermittent and unpredictable with the 737NGX. Nothing is tied to either pitch or AOA and there are in fact serious flaws in the way PMDG attempted to model ground effect altogether. I was, however, able to make modifications to both the .air and .cfg files (with the help of a very experienced person well schooled in FSX wizardry) that completely eliminates the problem. This took a LOT of testing and trial and error on our part, but the aircraft now flies realistically and consistently, and furthermore handles beautifully when the correct published FCOM technique is applied. No more phantom ground effect ruining otherwise perfect approaches and flares.

 

I don't work for free though, unfortunately. If PMDG would like to private message me, I will be happy to provide the correct modifications to the files mentioned above for a modest fee to cover my time and effort. If not, I completely understand and wish them the best.

 

I'm thrilled I was able to finally isolate and correct the problem if only for my own use.

 

Cheers,

 

Andrew Hansen

I wouldn't call what you found to be a flaw. The one flaw in all of this is that ground effect was not modeled in fsx in the first place. PMDG was able to at least put some form of ground effect into their simulation. Therefore they at least partially fixed the flaw.

PMDG's silence speaks volumes?       Nevertheless, after further extensive testing, I can confirm without one iota of doubt that ground effect is intermittent and unpredictable with the 737NGX.     Nothing is tied to either pitch or AOA and there are in fact serious flaws in the way PMDG attempted to model ground effect altogether.    I was, however, able to make modifications to both the .air and .cfg files (with the help of a very experienced person well schooled in FSX wizardry) that completely eliminates the problem.    This took a LOT of testing and trial and error on our part, but the aircraft now flies realistically and consistently, and furthermore handles beautifully when the correct published FCOM technique is applied.    No more phantom ground effect ruining otherwise perfect approaches and flares.

 

I don't work for free though, unfortunately.    If PMDG would like to private message me, I will be happy to provide the correct modifications to the files mentioned above for a modest fee to cover my time and effort.     If not, I completely understand and wish them the best.   

 

I'm thrilled I was able to finally isolate and correct the problem if only for my own use.   

 

Cheers,

 

Andrew Hansen

Boy I am glad that you now can land your airplane at unusual attitudes and have accurately simulated ground effect while doing so.  I will continue to simulate operating the 737 in a safe and efficient manner and not worry about such trivial things.

 

Hitting PMDG with a 'gotcha' and then demanding money for the 'fix' on a public forum is not going to get you very far.  Quite frankly, I'm relieved that not everybody is like you.  Just imagine if all of the repainters demanded money for their hours of meticulous work.

How much would I pay for a tweak that models ground(!) effect combined with a negative(!) aircraft pitch? I mean, pitch down so near to the ground that ground effect actually applies?

 

There are people around on these AVSIM forums for example,  who supply extensive, professionally designed and recorded voice pack replacements for high-quality payware addons - and they supply them for free. They support them, fix bugs and reply to their 'customers' within hours.

 

Now what's your ground effect tweak worth, compared to the high-class "freeware" I've received from this community?

 

And why would you try to cash in on that tweak, when the tweak actually was developed "with the help of a very experienced person well schooled in FSX wizardry".

 

If a payware developer intercepts the topic of another, competing product, there will soon be some uproar on the AVSIM forums - for good reasons.

 

What's the difference if you ask for a "fee" on this payware developer's forum?

 

Move on, troll, before you get banned ...

Gents-

 

So wait...  Let me get this straight....

 

Some guy nobody has ever heard of, parades into this forum while waiving a magic wand and calling himself an expert...

 

And I'm supposed to ignore the engineering credentials of the fellow who, in addition to working for PMDG in the production of our flight models, holds engineering degrees in the field from one of the most prestigious engineering universities in the world...  and I should also ignore the fact that he does this type of work for a living outside of simming....

 

And trust some guy who approaches us and demands money because something he did with no qualifications at all reduces the realism and effectiveness of the overall product?

 

And we are the ones who are arrogant....

 

Oh... silly me...   Don't feed the trolls.