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Aircraft Geometry

H

High Iron

Messages
6
Country
ca-alberta
Hi folks, I'm new here and look forward to learning from the community.

I was a commercial pilot and ex-chief flight instructor before my company folded and now have taken up a challenge in the mean time of building an aircraft native to Prepar3D. I have a good understanding of aerodynamic theory and have been having much enjoyment being able to use it outside of the aviation industry.

I have several aircraft .air file editors. I use Aircraft Airfile Manager primarily but also have Airwrench to get a different display of the .air file info. One of the items confounding me has been information in Airwrench regarding the aircraft length. In the "Dimensions" and "Balance" section pictorial the aircraft length shows as 79 feet when in fact the fuselage is over 100 feet in length. I check other .air files for other aircraft and they display the correct lengths. I can find no data entry in either the config file or the .air file that allows entry for this value. Where is Airwrench getting this information from, or is it just calculating it based on values of other components and "guessing" what the length is based on those values? Currently a lot of the values are incorrect, and just being used to get it flying within Prepar3D. Any info would be greatly appreciated.
 
Hi,

I moved your thread to the aircraft design forum. I know you ask for Prepar3D specific, but making an aircraft for FSX is almost exactly the same. Hopefully in this forum some more people will see the question.
 
Aircraft length is used by AirWrench to calculate MOI. The initial value is probably an estimate, but in Edit mode you can enter data on the Dimensions page.

Airwrench is really designed for building an aircraft from scratch. Depending on what Aircraft Type you enter in the Tuning page, you will get initial estimates for things like static margin and values for Cl vs AOA.

I find AirWrench very useful for a sanity check of the big picture, but it will alter the air file if you save after edit and you may not like the changes it makes. So I do not use it other than to do a first cut at the aerodynamics because I usually want a specific airfoil wing, sweep combination.

If you are into aerodynamics, the Mudpond Workbook is far better, though more complicated at first. For instance, if you enter wing sweep and choose the best match for your airfoil then the derived coefficients will give you correct interpretation for induced drag, high AOA characteristics.

The FS flight model works on a simple rectangular wing planform, but the various coefficients then make that wing fly like it should. You can not change the way it flies by adjusting all but a few of the entries under {airplane geometry] in the config file. For instance, you can put elevator_area to zero and it has no effect because the relevent coefficient is in the air file. You will see an effect if you increase the elevator_up_limit because it is a scalar on the coefficient.

The Workbook will produce accurate aerodynamic coeffficients based on what you enter in the Data page. Taking the elevator as an example, the Workbook calculates the relevent coefficients depending on the area and location of the elevator that you enter. If you enter data that is correct for a specific aircraft you will find it flies like that aircraft.

If you want engine performance like your aircraft you will have to go further, not because oof a problem with the workbook, but because FS calculates thrust versus altitude according the pressure ratio while it actually varies with density ratio. This toic has been extensively covered in previous posts in this forum.

To sum up, Workbook sets up correct coefficients according to the data you entry. AirWrench does it based on the type of generic aircraft you choose. The various air file editors let you adjust the coefficients, but you are basically fiddling with them unless you know exactly what you are doing.

Hope this helps
Roy
 
Hi Roy, thank you for your informative reply.

Roy Holmes said:
Aircraft length is used by AirWrench to calculate MOI.
Click to expand...

Interesting note. I have used real world calculated MOI's in the .cfg file. Is AirWrench reading these? If so, I have very bad data or formulas, or AirWrench can't cope properly with RW data.

I find AirWrench very useful for a sanity check of the big picture, but it will alter the air file if you save after edit and you may not like the changes it makes.
Click to expand...

That's sort of what I was using AirWrench for as well. Of interest to me, was the "Balance" section that gave a visual of the aircraft. I do not have a registered version of AirWrench so I can not edit files of the aircraft. I've been using AAM for the .air file entries.

For instance, you can put elevator_area to zero and it has no effect because the relevent coefficient is in the air file. You will see an effect if you increase the elevator_up_limit because it is a scalar on the coefficient.
Click to expand...

Another interesting point. I was under the impression the .cfg files overruled any values set in the .air file. At the same time, I was wondering how Prepar3D/FSX determined the evectiveness of the elevator in relationship to surface area. Strange that the elevator_area has no effect on aircraft handling.

If you want engine performance like your aircraft you will have to go further, not because oof a problem with the workbook, but because FS calculates thrust versus altitude according the pressure ratio while it actually varies with density ratio. This toic has been extensively covered in previous posts in this forum.
Click to expand...

This I seem to be concluding myself with each test flight. It's rather strange they would use pressure ratio rather than density ratio as that would allow temperature to factor in on engine performance, and aerodynamics for that matter as well. To further compound the problem the engines are turbo prop and this area seems to have multiple limitations on engine/propeller performance. Seeing the performance of other add-on's leaves me to believe there are work around's for these limitations, but that will involve a programmer I'm sure.

Thanks again for your info, it will be helpful for sure. It confirms my conclusion that I will need to spend more time now in the .air file to fix some of the handling anomalies I am encountering, particularly in the "pitch" department. The length error displayed in AirWrench led me to conclude this is related to my pitch problem. It's also interesting to note that AAM mentions no such information in regards to "interpreted" length. If not for the info in AirWrench, I would never had any concern over it.
 
Hi Roy,

I did recall looking at this thread in the recent past and also found it informative. Thank you again for the link to re-read.

My concern is also as n4gix pointed out. I must be careful as this aircraft is a native Prepar3D aircraft. My worry is the L.M. dev team may have again altered what is relevant and what is not in the .cfg file. It is forcing me to be cautious of using FSX info when developing for Prepar3D. Perhaps I could also conduct the same testing as you did to confirm the similarities or differences. Prepar3D also has a Mooney Bravo ready for torture.
 
LM hasnt done a thing to the basic ESP SDK beyond giving it a pretty face. As long as your dealing with only the basics ( airframe and powerplant ) you'll have no issues.. I can fly FSX aircraft in prepar3d and vice versa.. :)
enjoy.
Pam
 
It may be true that up to now LM hasn't made any changes, but surely that's likely to change and there will be future differences between P3D and FSX. The practical consequences are unknown - they may be trivial or they may be show-stoppers.
 
mgh said:
It may be true that up to now LM hasn't made any changes, but surely that's likely to change and there will be future differences between P3D and FSX. The practical consequences are unknown - they may be trivial or they may be show-stoppers.
Click to expand...

wnt deny anytin you say there. Your certainly no fool, but right now FSX and P3D are identical under the skin. I just now had P3D complain that i had another lockheed martin program running and it refused to start. The other program was FSX.. Curious no??

undoubtedly there will be changes, just like flight sim has always changed. only now instead of microsoft who wants to rule the world via your desktop, we have lockheed martin who makes thermo nuclear missiles for a living..

We wont know what those changes will be, and we dont know when they will be. All we can do, is enjoy what we have at the present.. And what we have, is a stay of execution for FSX.. :)..
 
I doubt that LM would do much to aircraft flight dynamics. They have far better simulations for aircraft.

My impression is that it is the scenery simulation they are most interested in since the ability to construct models that can be used for military pre-mission simulation would be right up their street.

Roy
 
Indeed. You take your threat database with accurate radar search and weapon volumes and compute the terrain masking from the mesh and use that when doing your mission rehearsal. That's what we had for Tomahawk (that was done by Boeing /McDonnell Douglas Air/ rather than L-M though). Lockheed (Lockheed Missile and Space at the time) did over-water planning, but you don't need a scenery engine for that :)

scott s.
.
 
Warchild said:
wnt deny anytin you say there. Your certainly no fool, but right now FSX and P3D are identical under the skin. I just now had P3D complain that i had another lockheed martin program running and it refused to start. The other program was FSX.. Curious no??
Click to expand...

Actually, there has been numerous statements from the program engineers at L.M. that a lot has been changed under the skin of Prepar3D. It just appears there is not because it is still so compatible with FSX material, but very much is different under the hood. In the past two years, L.M. has spent very much time reworking the old ESP code that it has little resemblance to the original.

I believe that the problem with FSX and Prepar3D running simultaneously is due to the the SimConnect module that both platforms utilize. You can not have two instances of the module running at the same time. Prepar3D code is much more strict than that of FSX and does not allow for sloppiness. Prepar3D can not see that FSX is running, but it does see that SimConnect is and will not launch. It think's because it see's SimConnect active that you have Prepar3D already running. Try launching FSX twice, you will not be able to do it.
 
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