P3D v4 Drag in relation to weight

#21
Ideally you need to be able simulate whether a weapon has been loaded into an internal bomb bay, or hung in the air flow, e.g. F-111.
Same weights, totally different extra drag (especially when supersonic).
yes, ideally. I'm no expert on the coding but maybe simconnect can do it by modifying the drag coefficient directly.
 
#22
In order for your sim airplane to out climb the real airplane, but maintain the same level flight speed as the real airplane (at the same thrust setting), your airplane has a mismatch in the drag balance and the thrust table has been fudged to compensate.
Indeed, there may be a problem here, because I have no official data on the induced drag of the aircraft in question.... : Very interesting remark! Thank you!
 
#23
IF the sim is adding the correct weight to the aircraft when weapons are added, and in the correct position, AND you're performance is different from the real airplane, then YOUR flight dynamics are wrong. The sim is a math engine using real world math. It only has a few specific bugs that the original ACES team got wrong. All the fundamentals are 100% correct and when the air file is correct the airplane will output identical data to the real airplane within 1 or 2%.
That's definitely not the case. It's one of the areas where FSX/P3D are way off. Just at baggage/fuel etc. at their 'correct' locations and you will notice that their effect is greatly exaggerated.
It's most likely related to the faulty FSX/P3D inertia calculation/simulation.
 
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Roy Holmes

Resource contributor
#24
There seems to be some misunderstandings here.
The original question was about how to simulate added weights.
The solution which tacpack and all the developers I know employ successfully is to use simconnect or similar to add the station weights listed in the aircraft.cfg as appropriate so that the AUW is correct, then remove them as the stores are are dropped. The performance should then be correct. As the stores are loaded and unloaded they become visible or invisible on the airplane model.

For each airplane each store has an estimated drag index. The indices are summed to give an airplane drag index. That in turn causes the spoiler to extend by an appropriate angle. Analysis and tests with some fine tuning then create a drag index which has the same effect on speed and fuel consumption as happens in the real airplane being simulated. There is a maximum amount of spoiler extension used for drag then a fixed amount added when speed brakes are extended.

So there are two things that need to be done to simulate the effects of external stores, add their weight and create the right amount of drag.
These techniques gave results within 3% across the board in the SimSkunkWorks Tornado, for which a performance manual was available. The real drag indices for each weapon was available as was the performance effects. 3% is within the range of individual airplanes performance due to engine wear and dirty surfaces.

For other airplanes like the IndiaFoxTecho Typhoon and F-35 there was no released performance data, just some vague values tossed about here and there. Prior to working on their FDE I modeled the Mirage F-1 for which data was available and I used that as a basis for the Typhoon and, recently, the F-35. And yes to answer an earlier comment, the stores in the F-35 weapon bay add weight but no drag. The doors when open add a small amount.

There is nothing magic about how to simulate external stores, it is well proven. The sim does have over-zealous reaction to asymmetrical loads especially laterally, but that can be overcome by not using the full distances when placing the weights. Normally, weapons stations are made/designed to be as close as possible fore and aft to the empty CG so CG shifts are minimised. Some airplanes like the AV-8B have asymmetrical lateral loading limits which would preclude landing but are OK between drops. My experience with the F-4 was that fuel had a larger fore-aft CG movement than weapons.
Roy
 
#25
Just at baggage/fuel etc. at their 'correct' locations and you will notice that their effect is greatly exaggerated.
I disagree with this statement. Correct weight/balance definitions with the fuel/pax/baggage defined based on actual aircraft weight/balance sheets works correctly.
 
#26
Everbody has his own opinion of course but I fully agree with Roy that both sims 'have over-zealous reaction to asymmetrical loads especially laterally'.
That's exactly my experience when comparing RW and simulated airplanes.
 
#27
That's definitely not the case. It's one of the areas where FSX/P3D are way off. Just at baggage/fuel etc. at their 'correct' locations and you will notice that their effect is greatly exaggerated.
It's most likely related to the faulty FSX/P3D inertia calculation/simulation.
no, it's garbage in garbage out. The air file MOI are wrong. FS uses the same equation that is used in real world FD.
 
#28
Hm, if FS is sooo realistic, how do you explain the awful inertia calculation/simulation?
Do have a few example FDEs you have developed with realistic MOI values?
 

Roy Holmes

Resource contributor
#29
This is what the VRS TacPack SDK says about the subject of store weights on wings
"While you technically can use actual stores weights for your token weights (e.g. ~500 lbs for a 500 lb bomb), this is most definitely not recommended. Putting real-world store weights on aircraft stations with real-world offsets (arms) would render the aircraft nearly impossible to control due to the flawed MOI modeling used by FSX FDs. " This is a bit of a exaggeration because if the stores are symmetrical there is no issue. It is only when there are more on one wing than the other that a small problem arises and that can be trimmed out.

Yves Guillaume has a lot to say about MOI in his excellent publication on MSFS aerodynamics. He concludes that the calculation is flawed. One example from the document
"Instead of summing up all station_load.n entries individually, MSFS uses the offset of the average location of all payload stations from the CG position.
This means that placing one payload of e.g. 1000 lbs 30 feet ahead of the CG and another one of 1000 lbs 30 feet behind CG results in zero MOI contribution in MSFS, because the average location is exactly at the CG. In reality both stations’ moments would sum up."

The air file MOI are wrong
There are no coefficients for MOI in the air file.
Roy
 
#30
Hello,

I'm not going to get into the MOI debate....

But I just want to confirm that, in the case that made me open this thread (Drag in relation to weight), I completely revised my model by drawing heavily on @Roy Holmes 's valuable explanations on "FS Thrust vs Altitude calculations Version 2"...
And I can confirm that, the loaded weight is indeed taken into account in FSX and P3D as @jx_ told me in a previous message: now, my plane with the weight of these bombs or the weight of its additional tanks rises much slower than in its clean version...!
Moreover, since my charges are symmetricals, the problem of the MOI does not exist...!

Thank you all for your suggestions and participation...
 
#31
There are no coefficients for MOI in the air file
Never said that...for the weight stations to give accurate MOI the body MOI needs to be set correctly. Otherwise one or the other overpowers. This will also affect performance causing questions such as "why are my control surfaces too sensitive when I use the real coefficients?"
 
#32
Hm, if FS is sooo realistic, how do you explain the awful inertia calculation/simulation?
Do have a few example FDEs you have developed with realistic MOI values?
I already explained it. Garbage in Garbage out. The sim uses the correct math for everything I have checked. ACES screwed up on inter-dependencies and static coefficients such as not adding the lift from Lift v Mach to the induced drag equation; and ground friction. The fuel Flow is calculated from Net Thrust instead of Gross Thrust. It is possible that , as Roy/Yves pointed out, MOI is calculated after each axis instead of before (haven't specifically checked that). But the calculations are correct.
 
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