AirWrench: Problem after adjusting thrust/drag curves

[kack911]

Hi,

I'm using AirWrech v1.01.77 to edit the thrust/drag and afterburner thrust curves for an F-16 addon. I used a post here at Sim-Outhouse as a tutorial, but something has gone awry.

I entered the Vmax in the Specs tab, then adjusted my Vmax Altitude, Static Thrust, N2 RPM, and TSFC. Then I clicked "Repair Jet Engine" and observed changes in the Thrust Required and Thrust Available curves.

I then edited the AB Stages, AB TSFC, and the AB table. All of my curves match the desired performance, and the file saves without any errors.

However, when I go into FSX:A, the airplane will not rotate until about 320kts, and when it does, the pitch is uncontrollable and within about 3 seconds, the airplane starts flipping end over end.

I've tried not clicking on the "Repair Jet Engine" button, manually adjusting the Max Mach Drag, even resetting the Balance, but nothing fixes the issue.

For what its worth, the thrust has indeed been affected by the changes, so at least that works.

Any ideas whats going on here?

Thanks,

Nick

[sparks]

This sounds like a weight distribution/contact point location problem. What does the balance tab look like?

[kack911]

Hi Sparks,

I just did a test, and it appears that without making any manual changes within airwrench, simply clicking the Edit Mode "Enabled" radio button does something to the .air file to make it unflyable.

If I restore the original .air file, load it in AirWrench, click the "Enabled" button, then immediately click "Save Flight Dynamics", the airplane exhibits the behavior in my first post. I didn't make a single change to any fields.

Nonetheless, here's a shot of the Balance tab before clicking "Enabled", and after:

http://i292.photobucket.com/albums/mm28/kack911/F-16BalanceTabpreediting.jpg

http://i292.photobucket.com/albums/mm28/kack911/F-16BalanceTabpostediting.jpg

Thanks!

[sparks]

AirWrench was designed on the premise that the aircraft.cfg files (from FS2002 forward) contain most of the parameters necessary to create a satisfactory air file. When Edit mode is enabled, AirWrench generates a completely new air file (in memory) from the physical data and performance characteristics from its input tabs. The initial source of this data is primarily the aircraft.cfg file and not the air file. AirWrench never reuses any lift, drag, stability, engine or prop coefficients from the air file. These are always recalculated from basic principles.

Problems inevitably arise when aircraft.cfg data is wrong. This happens frequently because the sim doesn't use all of the parameters in the aircraft.cfg file. The sim does not use any of the tail surface areas or positional data - but AirWrench uses them to calculate some very critical air file coefficients. So please check your data as it appears in AirWrench for accuracy, because if all AirWrench has to go on is garbage, the air file it generates is going to be garbage.

The aircraft length should be close to 50 ft. MS did not define this dimension in the aircraft.cfg file, so it's up to you to verify and correct this dimension if necessary. You'll also have to estimate how long the nose is (basically the distance from the CoG to the nose)

As a first cut at fixing the balance problems, I'll quote from the AirWrench Users Guide:

"AirWrench will compile the flight model without changing the balance of the original flight dynamics, and will quite happily compile some very unflyable aircraft configurations. Please see the Repair Aerodynamic Balance section to learn how AirWrench can automatically balance a flight model."

The F-16 is going to be a challenging aircraft to model. The real aircraft is inherently unstable and depends on it's computer controlled fly-by-wire control system for stability. AirWrench calculates stability coefficients based on aerodynamics alone so you may have to cheat on the physical dimensions or push the tuning sliders to get a stable flight model.

[kack911]

Hey Sparks,

Thanks. Unfortunately I'm a 3D modeler, not an FDE guy.

What I wanted to do was limit the Aerosoft F-16 to about 1.2 to 1.3 Mach. I read that it's not possible to get an FSX airplane to fly at mach 2.05 at 35,000ft, but only 1.2 at sea level.

If thats truly the case, then I'd rather have an airplane that only goes 1.2 at altititude instead of one that does 2+ at sea level and has so much excess thrust that you accelerate even in a 9G turn at corner velocity.

I figured I'd just be able to "fiddle" with the thrust and leave all other parameters unchanged. If this can't be done with AirWrench, is there another program that will let me do it?

Preferably one as easy to use as AirWrench?

Thanks.

Nick

[sparks]

As I posted in another thread, FSX SP2 and FSX Acceleration are different and afterburner performance can be vastly different depending the specifics of each air file.

In general though, the maximum attainable mach number for jet flight models is usually roughly the same at high altitude as it is at sea-level in FS. Real world jets often have lower 'operational' limits at low altitude than they do at high altitude. Jets are often capable of much higher speeds at sea-level than they're allowed to fly. You may see 'max speed is 1.2 mach at sea-level and mach 2.4 at 35,000 ft', but the reality is that jet's engines are probably capable of pushing it to mach 2.4 at sea-level, but in the meantime the wings have started to melt. FS isn't going to simulate that.

If you're willing to send me what you're working on, I'll see what I can do to get it flying to specs - at least to the first order. I don't need completed visuals, and if contact points aren't critical, all I need is the aircraft.cfg file to get you started.

[kack911]

Hi Sparks,

You're right. The F-16 has an 800kt airspeed restriction that is meant to limit excessive dynamic loads on the canopy. That said, even in MAX AB an F-16 aerodynamically cannot attain mach 2+ at sea level.

What's nice about the Dash-1 performance charts and materials for the F-16, is that it explicitly defines maximum obtainable airspeed (IAS), based upon MIL or MAX thrust at various altitudes.

I'm not sure why FS models things the way it does. Is it trying to define all aircraft performance based on TAS alone or something? Why else would max attainable mach be the same at sea level as it is at 35,000ft?

It would be nice to be able to limit the airspeed to about 800KIAS throughout the entire operational envelope. That would mean max attainable mach number would be 1.209 at sea level on a "standard" day (800KIAS = ~800KTAS), but would be 2.049 at 35,000ft (800KIAS = ~1181KTAS).

Anyway, I'm clearly not well versed in Microsoft's interpretation of aerodynamic theory, so if you really don't mind, I'll take you up on your offer to check out the airfile.

Just be aware that my intent is to modify an existing airfile included in a payware addon (the Aerosoft F-16). If you're still ok with that, then launch me a PM where I can send the files.

Thanks,

Nick

[sparks]

I'm ok with helping you develop a new set of flight dynamics for an F-16 with AirWrench. How you use them on your personal computer is up to you. You've piqued my interest in this model, so I just purchased my own copy. It's downloading now...

[kack911]

Hi Sparks, (is it Jerry?)

Thanks for the offer.

I assume you've had time to check out the FDE... Is there any assistance I can render?

Regards,

Nick

[sparks]

I've had a chance to play around with things a bit, but not too seriously.

I'm not a big fan of models that come with 20+ variations - too many folders cluttering up my FSX installation. Since I do a lot of development work, the added clutter makes it harder to find the other aircraft I'm working on. I've deleted all but the clean F-16C variation. Since they all use the same air file, it's no big deal to me.

The view is great from the VC, but I need my test gauges, so my 2D panel has replaced the VC.

I really shouldn't comment on the FDE.

[sparks]

On second thought, maybe I will comment on the FDE.

The engine in this FDE does not operate normally. Engine rpm (CN1/CN2) does not change above mach 0.5, and that's not normal - not in the real world and not in the sim.

Tables 1502/1503/1504 usually determine engine rpm given mach and throttle inputs, and they look ok; however, according to the ESP SDK, table 1506 has too many columns (18 vs max of 11). It appears that FSX is tolerating the error, but it's not running normally. I'm surprised it doesn't CTD.