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Acceleration & AoA - Some Observations.


Deano87

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...standard day so 20C at sea level...

 

 

Not trying to harass you or anything, but

 

https://en.wikipedia.org/wiki/International_Standard_Atmosphere

https://en.wikipedia.org/wiki/Standard_day

http://dictionary.dauntless-soft.com/definitions/groundschoolfaa/Standard+Day+Conditions

 

Standard Day Conditions

Conditions that have been decided upon by the ICAO for comparing all aircraft and engine performance. The most basic standard day conditions are: temperature, 15 °C or 52 °F; altitude, mean sea level; pressure, 29.92 inches of mercury.

 

source: FAA Aviation Maintenance Technician Powerplant Handbook (FAA-H-8083-32)

 

 

 

As to the use of FPM vs gun cross: if you match the FPM at the expense of an increased pitch, thus increased AOA you are not comparing similar conditions (more drag). And I suspect the higher pitch rate contributed significantly to your increased AOA.

 

Experiments like this are imperfect, best to get a significant number of them and see what you can learn. I do applaud the initiative and persistence.

 

Have fun!

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You can keep testing if that makes you happy, but this is in no way a scientific or objective comparison, starting with issues like the different Aircraft versions and too many unknown variables in regards to performance.

 

Its maybe interesting from a curiosity standpoint, but if the aim is to get ED to change the fm based on this, thats not going to work this way if you ask me.

 

 

Regards,

 

 

Snappy

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Not trying to harass you or anything, but

 

https://en.wikipedia.org/wiki/International_Standard_Atmosphere

https://en.wikipedia.org/wiki/Standard_day

http://dictionary.dauntless-soft.com/definitions/groundschoolfaa/Standard+Day+Conditions

 

Standard Day Conditions

Conditions that have been decided upon by the ICAO for comparing all aircraft and engine performance. The most basic standard day conditions are: temperature, 15 °C or 52 °F; altitude, mean sea level; pressure, 29.92 inches of mercury.

 

source: FAA Aviation Maintenance Technician Powerplant Handbook (FAA-H-8083-32)

 

 

 

As to the use of FPM vs gun cross: if you match the FPM at the expense of an increased pitch, thus increased AOA you are not comparing similar conditions (more drag). And I suspect the higher pitch rate contributed significantly to your increased AOA.

 

Experiments like this are imperfect, best to get a significant number of them and see what you can learn. I do applaud the initiative and persistence.

 

Have fun!

 

Apologies, I meant default DCS mission editor settings.

 

The DCS aircraft flies at a higher AoA to the one in the video at all speeds, If I ignore the FPM and only fly gun crosses I will not be flying the same altitude profile as shown in the video? Surely I’d end up far lower in altitude due to shallower climbs and steeper descents?

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You can keep testing if that makes you happy, but this is in no way a scientific or objective comparison, starting with issues like the different Aircraft versions and too many unknown variables in regards to performance.

 

Its maybe interesting from a curiosity standpoint, but if the aim is to get ED to change the fm based on this, thats not going to work this way if you ask me.

 

 

Regards,

 

 

Snappy

 

This thread has already discovered one major bug. So let’s not dismiss this technique too much already ;). But yes I do understand what you mean. I would never suggest that ED change FM to match anything I am doing, I’d hope they would have access to or can get some more correct numbers on their end. But you have to admit, the simulated jet should probably not take 4x longer to go from mach 1.8 to 1.9 than a real jet which had less thrust and a smaller intake. Do I exactly know how fast it should be? No, but it shouldn’t hit a brick wall above mach 1.7 like it does, imho at least.

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.. But you have to admit, the simulated jet should probably not take 4x longer to go from mach 1.8 to 1.9 than a real jet which had less thrust and a smaller intake. Do I exactly know how fast it should be? No, but it shouldn’t hit a brick wall above mach 1.7 like it does, imho at least.

Yep like there was a wall, while in the original video the mach number was constantly growing, it is as if there is too much drag or lack of thrust, perhaps the cause is the different engine.


Edited by The Falcon
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I mean if I adjust the temp so the CAS and Mach match at the same altitude as in the video then we should be good I suppose?

yes but this only if you have to increase or decrease the temperature in a realistic way, for example if to match the video you have to put a big difference of temp like 40 °c at 30k feet, then there is a problem either in DCS or in FM

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yes but this only if you have to increase or decrease the temperature in a realistic way, for example if to match the video you have to put a big difference of temp like 40 °c at 30k feet, then there is a problem either in DCS or in FM

 

The flight was flown of the cost of Southern California, I think I tested 28C at sea level and it got CAS and Mach very close at each altitude.

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The flight was flown of the cost of Southern California, I think I tested 28C at sea level and it got CAS and Mach very close at each altitude.

In fact the videos were practically the same until the F-16 of the original video passed 37000 feet where it started to accelerate much faster than before.

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Sounds good. I’ll give it a go. Clean + no pylons, Correct?

Yeah DI 0, cheat unlimited fuel to keep constant GW, throw autopilot on in altitude hold. I think I would dumb a flare every time I passed a round number on the airspeed and check the timing later on Tacview.

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I was honestly thinking of something like jet streams, but in truth I don't know enough

 

The 37000 ft Mark also coincided with the aircraft accelerating past the relatively draggy transonic zone. I'd expect to see the acceleration rate increase as transonic drag decreases.

 

Note image below not F-16 related, just for illustrative purposes.

Sonic-barriers-768x696.jpg

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Great to see you guys testing this, here's some extra info to use:

 

F-16C Blk.50 acceleration times @ SL (std. day, 15 C) @ 24,000 lbs (DI = 0) [GE engine]:

 

200 kts / M 0.30 = 0 sec (starting speed)

250 kts / M 0.38 = 2 sec

300 kts / M 0.45 = 4 sec

350 kts / M 0.53 = 6 sec

400 kts / M 0.60 = 8 sec

450 kts / M 0.68 = 9 sec

500 kts / M 0.76 = 11 sec

550 kts / M 0.83 = 13 sec

600 kts / M 0.91 = 15 sec

650 kts / M 0.98 = 17 sec

700 kts / M 1.06 = 20 sec

750 kts / M 1.13 = 25 sec

800 kts / M 1.21 = 33 sec (limit speed)

 

You can probably guess what manual the figures are from.

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Great to see you guys testing this, here's some extra info to use:

 

F-16C Blk.50 acceleration times @ SL (std. day, 15 C) @ 24,000 lbs (DI = 0) [GE engine]:

 

200 kts / M 0.30 = 0 sec (starting speed)

250 kts / M 0.38 = 2 sec

300 kts / M 0.45 = 4 sec

350 kts / M 0.53 = 6 sec

400 kts / M 0.60 = 8 sec

450 kts / M 0.68 = 9 sec

500 kts / M 0.76 = 11 sec

550 kts / M 0.83 = 13 sec

600 kts / M 0.91 = 15 sec

650 kts / M 0.98 = 17 sec

700 kts / M 1.06 = 20 sec

750 kts / M 1.13 = 25 sec

800 kts / M 1.21 = 33 sec (limit speed)

 

You can probably guess what manual the figures are from.

 

Additional info: to test acceleration you need to start acceleration below 200 kts to make sure the engine is already at full power when it reaches 200 kts.

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The pressure recovery of the inlet (P02 o P01 in the graph, i.e. inlet pressure vs outside pressure) has a major influence of the thrust above M1.4.

 

Variable-ramp-inlet-losses_zpsuishcnyt.png

 

When we come over M 1.5 for a single chock inlet like the pitot one of the F16 the pressure recovery and how the engine handles the end of compressor max pressure (if it can take it or needs to bleed off excess pressure) influences performance as much as the airframe drag does.

 

So some of the explanations for the bad performance can be small inlet plus PW engine for the F16N and big gulp and GE engine for the DCS F16.

 

When I fly the DCS F16 at 25kft or higher I'm surprised what a lame-duck it is thrust wise in military power. It could be the intake/engine model is off rather than drag being modeled off. Who knows. But it seems a bit lame.

 

But then I never flew the F16 so someone with real experience should chip in.

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Yeah DI 0, cheat unlimited fuel to keep constant GW, throw autopilot on in altitude hold. I think I would dumb a flare every time I passed a round number on the airspeed and check the timing later on Tacview.

 

You should be cautious to use unlimited fuel, it caused other PFMs to behave completely wrong.

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Great to see you guys testing this, here's some extra info to use:

 

F-16C Blk.50 acceleration times @ SL (std. day, 15 C) @ 24,000 lbs (DI = 0) [GE engine]:

 

200 kts / M 0.30 = 0 sec (starting speed)

250 kts / M 0.38 = 2 sec

300 kts / M 0.45 = 4 sec

350 kts / M 0.53 = 6 sec

400 kts / M 0.60 = 8 sec

450 kts / M 0.68 = 9 sec

500 kts / M 0.76 = 11 sec

550 kts / M 0.83 = 13 sec

600 kts / M 0.91 = 15 sec

650 kts / M 0.98 = 17 sec

700 kts / M 1.06 = 20 sec

750 kts / M 1.13 = 25 sec

800 kts / M 1.21 = 33 sec (limit speed)

 

You can probably guess what manual the figures are from.

 

I put those numbers on Excel to analyze acceleration.

There is something going on or I miscalculated. What is causing the spike?

 

Generally I had not understood how fast these jets accelerates. Over 1 g whole way to 1 M.

1389511149_F-16CBlk.50accelerationtimes.png.1e6923c98c781b4217b866e04d5614db.png

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For me there is definitely a problem. From mach 1.35 onwards it is as if DCS F-16 had this curve

Keep in mind that the drag coefficient is not the drag. The CD peaks in transonic but the drag is higher at supersonic speed than at transonic speed.

 

 

The blue line in your graph is something like what the drag actually looks like.

 

 

Anyway I've only looked into this really briefly with a clean or light AA load top speed run and I did notice the plane to be slower than I expected after Mach 1.5.

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I put those numbers on Excel to analyze acceleration.

There is something going on or I miscalculated. What is causing the spike?

 

Generally I had not understood how fast these jets accelerates. Over 1 g whole way to 1 M.

 

Spike would look smaller if it included decimals and we had the milisecs, so I think that's the main reason.

 

Also around 400 kts the engine is probably at its peak in performance.


Edited by Hummingbird
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