Mach limits at altitude in FSX

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bullfox
Technical Sergeant
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Joined: 24 Dec 2004, 14:50

Mach limits at altitude in FSX

Post by bullfox »

I managed to get the WB-57F stable at FL 60 by playing around with its config file. However I have run into a problem going higher.

At FL 60 Mach 1 should be about 570 kts. However in FSX at FL 60 it appears that Mach 1 is only about 200 kts. The Mach limit of this plane is .8. Much higher in FSX and the plane's Mach limit is less than it's stall speed.

Is this discrepancy in the FSX value of Mach 1 at high altitude just an FSX bug or am I misunderstanding something about these numbers?
Ryzen 7 5800X3D liquid cooled, OC to 4.5 ghz, Radeon XFX 6900XT Black edition, 2 tb M2 drive, 32 gb ddr4 ram, Asus Hero Crosshair VIII mother board, and some other stuff I forget exactly what.

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AKar
A2A Master Mechanic
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Joined: 26 May 2013, 05:03

Re: Mach limits at altitude in FSX

Post by AKar »

Mach 1 at 60000 ft pressure altitude (FL600) in standard conditions yields 'bout 197 knots CAS. I think you are mixing your indicated with TAS, which is roughly 574 knots.

-Esa

bullfox
Technical Sergeant
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Joined: 24 Dec 2004, 14:50

Re: Mach limits at altitude in FSX

Post by bullfox »

Akar,
Thanks for the clarification. I had a hunch that might be the case. I got those numbers from Wikipedia,. The article did not say true or indicated airspeed.
Ryzen 7 5800X3D liquid cooled, OC to 4.5 ghz, Radeon XFX 6900XT Black edition, 2 tb M2 drive, 32 gb ddr4 ram, Asus Hero Crosshair VIII mother board, and some other stuff I forget exactly what.

bullfox
Technical Sergeant
Posts: 898
Joined: 24 Dec 2004, 14:50

Re: Mach limits at altitude in FSX

Post by bullfox »

Image

I got it up to 68.5 and paused the sim just as it started to pitch up to take this screen shot. It oscillates quite a bit in pitch at this altitude. I cant figure out how to smooth that out with the auto pilot. It seems that the indicated stall speed goes higher with increased altitude.
Ryzen 7 5800X3D liquid cooled, OC to 4.5 ghz, Radeon XFX 6900XT Black edition, 2 tb M2 drive, 32 gb ddr4 ram, Asus Hero Crosshair VIII mother board, and some other stuff I forget exactly what.

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AKar
A2A Master Mechanic
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Joined: 26 May 2013, 05:03

Re: Mach limits at altitude in FSX

Post by AKar »

bullfox wrote:Thanks for the clarification. I had a hunch that might be the case. I got those numbers from Wikipedia,. The article did not say true or indicated airspeed.
It would be true airspeed. Indicated airspeed is actually somewhat meaningless concept if strictly speaking of speeds (or velocities). It actually is a measure of pressure, not speed.
bullfox wrote:I got it up to 68.5 and paused the sim just as it started to pitch up to take this screen shot. It oscillates quite a bit in pitch at this altitude. I cant figure out how to smooth that out with the auto pilot. It seems that the indicated stall speed goes higher with increased altitude.
Correct. Indicated stall speed increases with altitude. This has to do with the higher Mach number as the most significant factor. Some performance manuals actually provide graphs for this.

-Esa

gibo
Senior Airman
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Joined: 14 May 2009, 04:23

Re: Mach limits at altitude in FSX

Post by gibo »

Here is an interesting article. An RAAF electronics engineer once explained it to me except that he called it the "death speed" http://www.boldmethod.com/learn-to-fly/ ... -mmo-meet/

bullfox
Technical Sergeant
Posts: 898
Joined: 24 Dec 2004, 14:50

Re: Mach limits at altitude in FSX

Post by bullfox »

Good article. I got the WB-57F up to FL76.4 before it stalled, pitched up and went into another inverted flat spin. It stalled at 112kts indicated and 576kts true, according to FSX. I had to go into the config file ( the only tool I have) and changed the engine thrust scalar from 1 to 1.4, cut the empty weight in half, reduced the drag scalars and doubled the pitch stability to get to that altitude. I used accufeel to set the mach limit to .9 and it was right at the limit (in the coffin corner) when it stalled. One interesting thing was when I tried to add some wing wash out by changing the wing twist from 0 to -1 it would violently pitch down in a mach tuck (I guess) when it stalled. The ceiling on the real aircraft is said to be FL82, but that is with two extra under wing jet engines. The ceiling with out the extra engines is said to be FL80.
Ryzen 7 5800X3D liquid cooled, OC to 4.5 ghz, Radeon XFX 6900XT Black edition, 2 tb M2 drive, 32 gb ddr4 ram, Asus Hero Crosshair VIII mother board, and some other stuff I forget exactly what.

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