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Modifying aircraft drag

E

eric_marciano

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france
If some of you have flew the Wilco Airbus Volume 1 (A318 to A321), you may have noticed that at high altitude, descending on DES or OP DES mode makes the aircraft dive like crazy with V/S up to 5000 ft/min and higher.
These modes just put the engines on idle and adjust the pitch to make the aircraft descend using its weight only. When starting the descent, the aircraft is at cruise altitude (ofthen more than 30,000 feet) and cruise speed is around 300 knots or Mach 0.78. In such conditions, the aircraft pitches down to 5000 ft/min (or more) to maintain the desired speed.

I know these aircrafts were released a long time ago, but I would like to try to fix this. I know almost nothing about flight dynamics but I'm sure it can be fixed because the Volume 2 aircrafts (A330/A340) work much better regarding that aspect.
Now how can this be fixed? I was thinking about reducing the parasite drag, do you think it is a good idea? I know it will have an impact on the rest of the performance, so I may have to reduce the engine power. Should I use the thrust_scalar for this?

Any input is greatly welcome. I must say I do this for my personal usage only. Even though I am the developer of these series, there will be no other upgrade from Wilco, this is for my personal use only.

Thanks for any help,
Eric
 
A quick back-of-the-envelope calculations suggests that order of descent rate would be expected on reducing the thrust to idle.
 
Best solves the problem with AAM (Aircraft Airfile Manager) in the AIR file, table 1506th.

1280x1024f.jpg


Scale the thrust of the IDLE speed, so that the descent works.
 
Agree with this solution that has been adopted by quite a lot of designers..considering there are not so many if you don't want to touch the parasitic drag (CDi is usually low on descent profile). However you will have to carefully check the idle CN1 range to adjust on table 1506 and test it so as not to interfere with cruise power settings. Also expect some changes in idle power roll behavior but it could be an advantage (at least for users that do not change friction parameters via SIM1.dll or FSUIPC Lua scripts)
 
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Here's the problem.

We don't know the idle thrust of the real aircraft and so we guess (or the designers guessed)... Then everything else is screwed.

All you need is the static thrust for the engine in question. You can probably find one online. If not I will look into it if you ask.

Once you have that, the descent rates are published in airbus AOMs for comparison. You could use the Oswald e method posted in the other thread to check CDi and adjust CDp to make up the difference.

You would want to do this for descents at all speed ranges and all altitudes.

Then and only then will it be accurate. Otherwise, people can only randomly suggest things that are a 50/50 toss-up chance of fixing the problem.


Step one, set correct idle thrust at all speeds.
Step two, set Oswald e correctly
Step three, double check wing geometry
Step four, compare descent rates at all speeds/weights
Step five, adjust CDp as needed***


***I strongly suggest setting CD0 to a value of one, then use the Mach drag table to adjust CD0 for each speed range. Once completed with the process, you can subtract the lowest Mach drag value to zero, then add that amount back to CD0 if you like....or leave it. Either way it's correct.
 
Thank you all for your help.
It appears that it is more complex that I was expecting and flight dynamics is a science... I was afraid that changing a value somewhere in the model has an impact on others, thus modifying the aircraft behavior. Your explanations confirm this fear, so I will carefully try to change the model but I'm not sure I will be able to reach the expected result.

Thanks,
Eric
 
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