A ping to StreakEagle re F-4B

[+nele 0]

I know you are designing new one, and I tried the current realistic version. I hope you don't mind that I made two small changes (for testing purposes only). Those are:

 

- adding following quickly-modded AI, similar to that one in "my" Floggers (keeps speed)

 

[AIData]

AileronDeltaRoll=1.26

AileronRollRate=-0.9

ElevatorDeltaPitch=1.0

ElevatorPitchRate=-0.09

ThrottleDeltaSpeed=0.045

ThrottleVelocity=-0.008

DeltaSpeedForAfterburner=1.00

DeltaSpeedForAirbrakes=-95

MaxPitchForAltitude=8.0

MinPitchForAltitude=-5.0

PitchForThrottle=0.005

PitchForAltitude=0.001

PitchForVerticalVelocity=-0.01

RollForHeading=10.0

RollForHeadingRate=-0.5

MinRollHeading=5.0

MaxRollForHeading=35.0

PitchForRoll=0.15

FormationSpeedForPosition=0.8

FormationSpeedForRate=0.7

 

-Changing WetMachTable data of engines (roughly, by simply entering my Mach corrections from "Flogger", for values above M1.2) That would make it now:

 

WetMachTableData=1.000,0.991,1.295,1.683,2.473,2.975,3.627

 

As a result, your F-4b (in WOE) does not crash, keeps speed up much better and altitude behavior in AB is much better (I've set cruising altitudes in WOE over 30,000ft at home). Only one thing left-there is too much pitch authority at high speeds (which in reality is limited by AFSC on any jet airplane). For "Flogger" tweaking, I tried to transfer pitch rate and speed rate as close to the stick charts (as I have the -aerodynamic- manual for the bird).

 

Please consider this as the "experience-transfer", since I tried your current "realistic" F-4b and I like it very much, and would like that you make even better one!

 

Nele

[+kreelin 2]

Nele,

 

By changing the wetmachtable to match the floggers one, you suppose both engines have exactly the same performances at supersonic speed ... Streak used TK's wetmachtable in his FM. Of course we don't know how and where TK got his data so we cannot say if they are "realistic" or not but I think the best way to "correct" the engine tables( if needed ) is to use Streak's latest works on the F-4 HighDef aero coeff and then compare the plane overall performance with real chart in F-4 pilot's manual... The other solution is to get the real performance data of J79

Edited April 25, 2007 by kreelin

[Fubar512 1,350]

Nele,

 

I have an app that simulates "generic" turbojet and turbofan engines, provides altitude tables, fuel consumption tables, etc. If I manage to find it, I'll send it to you...but keep in mind that its calculations are generic.

 

Strangly enough, though, they pretty much agree with TK's figures....

 

I'm sure Kreelin would be interested in it, as well...

[+kreelin 2]

Strangly enough, though, they pretty much agree with TK's figures....

 

I remember a guys on checksix-fr who compared TK's F-4 and the plane basic performances. Th result was TK's figures was quite right (IRC less than 5%)

 

 

I'm sure Kreelin would be interested in it, as well...

 

He he... You know me too much Ed

Edited April 25, 2007 by kreelin

[Fubar512 1,350]

He he... You know me too much Ed

 

Check your email

[+streakeagle 867]

Anything I have released is open to anything anyone wants to do... as long as no one is selling the results for profit.

 

As for engine performance data: I have a new trick.

The NASA data I have acquired shows the drag at 1-g for the entire Mach range.

The pilot manual has the level acceleration tables.

The thrust has to be equal to the sum of the drag plus the acceleration :)

Edited April 25, 2007 by streakeagle

[+nele 0]

Hm, this is "WetMachTableData readout" from MiG-23ML aerodynamic manual:

 

M0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2.0 2.2 2.4

T 1 1 0.96 1.105 1.25 1.375 1.5 1.415 1.33 1.245 1.16 1.1215 1.083

 

It is -irrespective- to altitude, and only depends of Mach number. "T" is the multiplier to static thrust.

 

Likewise, AltitudeTableNumdata multiplier from MiG-23ML manual (1000m step, starting zero m):

1.000,0.917,0.838,0.759,0.678,0.600,0.575,0.525,0.500,0.400,0.392,0.383,0.362,0.318,0.297,0.270,0.244,0.218,0.192,0.186,0.171,0.163,0.151,0.146,0.000

... is -irrespective- of Mach number.

 

To get effective thrust of R-35 at, say, M0.8 and 4.000m AGL, the formula is simple:

EffectiveThrust=StaticThrust*AltitudeTabledata*WetMachTableData and that is 12749X0.768*1.25=12239.

 

The FM follows physics to about 6,000m. Then, -if- you use original table, engine gets short of breath and ceiling is about 11,000m (ML). And true Flogger can go dynamically over 22,000m with engine still running.

 

If you use "generic" Wetmach, situation is better, but -notice- that at M2+ Mach-dependent thrust is tripled!!! No engine does that, not even SR-71 has such engine dynamics, so it is generic correction of some sort.

 

If you try to go supersonic at higher altitude (and there is a breakpoint in FM "atmosphere" at around 12,000m), your FM will be either too slow to accelerate then original, so this "generic" correction has to be adjusted. I found that for ML I need at least 3.3 TIMES more thrust at M 2.4 than true engine generates (1.083vs.3.6).

 

Also, note that originals, for some reason, have altitude table "short" of multiplier. I got adequate performance with Flogger-and I needed it, since in WOE I have cruise altitude raised to 11,000m. The Mach/thrust behavior is probably similar to all turbojets, but is hardly the same!

 

I stand behind opinion that hi-performance airplane, that have much of ram-effect (like SR-71 or Foxbat) cannot be accurately simulated without adjusting WetMach. For all other Mach2 "peek" aircraft that rarely go even over M1.5 (virtually all modern fighter aircraft except F-22/23) behaviour can be acceptable.

 

Nele

[+streakeagle 867]

Both wet and dry thrust as a function of mach are highy sensitive to the installation.

Different aircraft with the same engine type need to have different thrust versus Mach tables.

However, I have rarely encountered tables for any aircraft showing net thrust as a function Mach number.

So, except in cases where the data is available, FMs have to use generalized tables for thrust versus Mach.

 

Likewise, thrust versus altitude does not follow the theoretical values based on air density versus altitude.

Once again, detailed data for various aircraft/engine combinations is generally not available.

So, most FMs will have to rely on generic thrust versus altitude data.

 

Typically, specific excess power is available via derivation from level acceleration charts and/or time-to-climb charts.

So I define either thrust or drag as accurately as I can and use the computed specific excess power to get the other value.

In the new F-4B fm I have been working on for quite some time, I had been using TK's thrust as the reference point, but now I have very accurate drag as a function of Mach.

So, whenever I get done working on the XB-70A flight model, I will be able to make the F-4B FM even better.

I wish I had the time to get it done now so I could enjoy the accurate takeoff, climb, acceleration, and deceleration performance.

Typically, TK's stock FMs have too little drag at 1g, so you can glide at a very shallow angle for a very long time in aircraft that were notoriously bricks.

With realistic drag values, airspeed bleeds a little faster when you idle and/or use speed brakes and it also makes for longer, slower takeoff rolls.

[+nele 0]

I agree that charts are rare, as they represent dynamic engine behavior. Generic "altitude" table(s) is "short" of thrust, although there is an other default-model issue, too-too low drag due AoA at altitude. So when you make an F-4 with drag close to realistic (both static&dynamic), there is no thrust to back it up...

 

I am currently checking the issue with current "Foxbat", that cannot hold the altitude and is truly a handful. One issue is that there is no trim, second that thrust creates too much downforce. I have a loopy idea how to fix both the trim&downforce (which is necessary at 20,000m+) but I have to check is that thought practical. It would be practical (and usable!) only for high-prf, hi-alt flying aircraft (basically those are only three or four-MiG-25/31, SR-71/YF-12, XB-70A) that I lack much...