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OK, Im stumped on this one...

C

chevelle505

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us-florida
OK, I have a question about editing the config of an aircraft in FSX. On the IRIS Global Express, it SHOULD do .88-.89 mach with no problem, but anything over .82 i get that buffeting (nose pitching up and down about 3 degrees very fast)like its overspeeding. What do I edit to get this thing to cruise smoothly at .89 without the buffet and overspeed issues?:confused:
 
Try this

Hello

If you open the aircraft.cfg (with notepad or similar) and find the entry:

;Moments of Inertia
empty_weight_pitch_MOI = XXXXXXX

I suggest you increase this value by about 5% at a time and see what happens.
If you increase it too much the pitch response will start to feel sluggish.

Have fun


Alan
 
OK, Tired that, no joy....i ended up finally doubling the number, no change whatsoever as far as this buffet...basically it thinks it over speeding, so what do I edit so that is doesnt overspeed until mach .91?
 
Are you trying to fly a beta version of an addon by the numbers...?
 
no its actually the iris global express that was released as freeware. It comes with the default CRJ 200 Aircraft config...so i added fuel tanks, more thrust appropriate for the Global (14750 lbs) and got everything pretty close...its just when I get over .81 mach, the nose bumps up and down very fast like its over speeding or something.
 
I guess think of it this way, im trying to get a "crj 200" to cruise at .88 and have a 6200nm range. Ive got everything else figured out, but its still acting like .88 is WAY to fast for it (which it is for a crj). Sorry if I just said the same thing a different way..haha, but im just trying to figure this out, otherwise, a global express that flies with crj numbers is completely useless.

Does anyone know what exactly that buffeting is all about? I assume overspeed, but there's obviously a way to change it, just trying to figure out how.
 
The sim doesn't work that way... overspeed is NOT your problem. It is most likely a table such as 421, pitch dampening vs Mach; or 401 CL vs Mach; or a control effectiveness vs mach (such as trim or elevator moment or effectiveness).

It just depends on how the designer went about implementing the high speed buffet. The sim does not have such characteristics built in.
 
I remember that same kind of "buffeting" happening in some Project Opensky aircraft when the altitude hold autopilot was engaged. I wasn't even close to going overspeed when their 777 did this, but apparently that's not what the sim thought. :rolleyes: I've searched and haven't been able to find an answer since then.

Does this happen when you're flying it manually, without the autopilot?
 
Iris glex

I've been trying to get mine up to spec too, but having all sorts of issues. I thought I had done a good job of researching the W&B data etc but when I added fuel tanks and spread the seating out the nose buried itself in the ground. When empty and using my revised cfg, it tipped over backwards! I haven't figured out how to get the mains to touch the ground when empty. Back to the drawing board. I am mindful of advice suggesting that you really need an engineering or similar level of technical expertise to really craft the .air files and associated tables properly. Chevelle... I'm guessing that you're experiencing high speed buffet going that fast which is supposed to happen in the CRJ, and I'm wondering (new to this) about substituting an air file from an aircraft that will go at .89, or maybe getting the data from one?

This is a beautiful rendition of the GLEX and I'd love to see it flying the way the good lord intended it!
 
Hell if I know...I just added the fuel capacity to the existing tanks. Ive bumped the thrust up to damn near 16000 lbs per engine (quite a bit over the real world 14750) and still cant seem to get this thing to cruise at .88 at FL510 with full tanks. Is there a way to "reduce the drag" or something?
 
So far I have amended the aircraft.cfg with data from a variety of real-world sources. It flies, and I've solved the ground behaviour, although I'm now having to fiddle around with settings because for some interesting reason I cannot rotate until about 160 knots. I'm going to try and get some real world performance info from a buddy who flies these and then learn how to edit air files properly. Meanwhile I'm hoping that a crude blend of brute force and ingenuity will yield some results...
 
Ha! So, I managed to get a look at the performance data for the GLEX, and at 98,000 lbs in ISA, maximum cruise altitude is about FL345 for Mach 0.88, FL380 @ M0.87, FL410 @ M0.85, and at Mach 0.85, you can cruise at FL510 at a weight of 54,000 lbs, which would be a realistic weight with a few thousand pounds of gas left on board. The aircraft will however reach FL510 in ISA at a weight of about 58,500 lbs at which point the ROC has dropped to 200fpm which denotes the theoretical service ceiling at that weight. The service ceiling after a MAUW takeoff in ISA is just over FL420.

I only just got back from work, so I'm keen to see how my changes stack up. And I've got 2 air files open in AAM and AirEd at the same time, comparing the GLEX with the Eaglesoft Citation X which does 0.92. Now THAT'S a cool little machine! I'm looking for obvious discrepancies which when fixed will hopefully line the GLEX up a bit better. I can see this turning into a 5-year plan judging by the fascinating posts I have read so far..!
 
If you add more power or speed your going to wobble.
So in this case lower the drag under tuning.
For over speed you simply change the max speed under speeds section.
 
jx_ said:
The sim doesn't work that way... overspeed is NOT your problem. It is most likely a table such as 421, pitch dampening vs Mach; or 401 CL vs Mach; or a control effectiveness vs mach (such as trim or elevator moment or effectiveness).

It just depends on how the designer went about implementing the high speed buffet. The sim does not have such characteristics built in.
Click to expand...

Check Record 1101 offset (decimal) 150 for Pitch Damping.
The variable appears to be an integer32 and expects a fairly high negative value. Negative is greater damping.

On some of my CFS aircraft, I try to tune this effect to happen at a certain speed. For example, on my A6M2 Model 21 Reisen, a value of -8000 would cause an oscillation to start at 305 mph IAS. A value of -10000 would push the airspeed to 330 mph before the vibration would start.

Hope that helps.
- Ivan.
 
Ivan said:
Check Record 1101 offset (decimal) 150 for Pitch Damping.
The variable appears to be an integer32 and expects a fairly high negative value. Negative is greater damping.

On some of my CFS aircraft, I try to tune this effect to happen at a certain speed. For example, on my A6M2 Model 21 Reisen, a value of -8000 would cause an oscillation to start at 305 mph IAS. A value of -10000 would push the airspeed to 330 mph before the vibration would start.

Hope that helps.
- Ivan.
Click to expand...


Ivan, that all depends on a bunch of other variables and tables. On some models, you can use values of over -10,000,000 before the weird oscillation occurs. There are too many tables and entries involved to be able to use specific values such as -8000 without knowing the settings of the other entries.


But pitch damping is definitely the first place I would look.
 
jx_ said:
Ivan, that all depends on a bunch of other variables and tables. On some models, you can use values of over -10,000,000 before the weird oscillation occurs. There are too many tables and entries involved to be able to use specific values such as -8000 without knowing the settings of the other entries.


But pitch damping is definitely the first place I would look.
Click to expand...

Hello JX_,

I agree with you completely. I saw that you were pretty much on the right track with your post which is why I quoted it.

The CRJ we are discussing here is having weird effects near 0.9 Mach which should be pretty near 600 mph. My Zero hits structural limitations around 350 mph IAS. The maximum take-off weight of the Type Zero fighter is around 4500 pounds or about 1/10 of his jet. Those facts were not lost.

I have used this effect on a Blohm & Voss BV 141B-0 also and the numbers there were not even close to what I listed for the Zero.

I posted these number as an example of how a fairly small change can put the desired oscillation / loss of control where you want it. With the Zero, the maximum level speed at sea level is pretty close to 300 mph so I wanted the effect to appear just outside the normal operating range but still discourage exceeding the structural limits. Once the oscillations start, reducing speed will make them go away, but not until speed falls about 15-20 mph below where they started. (At least that is how it works in Combat Flight Simulator.)

Regards.
- Ivan.
 
Ahh, you are able to tweak the speed at which the oscillation occurs. I have not tested this aspect out. Do you know if an increase in the negative number raises or lowers the oscillating speed?
 
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