FSXA. Afterburners.

[TooLowGear]

Dealing with an exotic aircraft requires a bit of a firm hand on occasion. With the advent of Acceleration a few new variables were introduced to the cfg file.

[TurbineEngineData]
static_thrust= 34000.000 
[COLOR="Red"]afterburner_available= 0-??(number of AB stages)
afterburner_throttle_threshold = 0.50 (min throttle percent /100 where ab 
                                                             range begins)[/COLOR]
ThrustSpecificFuelConsumption = 0.9 (valid at all power settings)
AfterBurnThrustSpecificFuelConsumption = 1.9 (valid only with AB engaged)

Here is the engine data I am working with

Specification of J58-P4
Thrust (dry): 25,000 lb
Thrust (wet): 34,000 lb
SFC (wet): 1.9 lb/(lbf-h)
SFC (dry): 0.9 lb/(lbf-h)
Core airflow: 450 lb/s

I am assuming dry means without AB and wet is with AB

I noticed an interesting occurrence in the aircraft. As the aircraft throttles up, the FF expectedly goes up until 50% throttle where the AB kicks in and the FF drops to some absurdly low level from about 30,000lbs/hr/engine to 250lbs/hr/engine. as the throttle is increased the FF advances all the way up to 63,000lbs/hr/engine. I have no idea why it does this. The F-18 does it as well but to a somewhat lesser degree. The SR-71 at 80,000ft would need to operate in a close to max ab condition but so far the aircraft does it without the use of AB.

Also, does Static thrust refer to the max thrust at Sea level with or without afterburner?

Thanks in advance for any help!

 

[TooLowGear]

Using the black box tool. I have deduced the following thank you.

The F-18 shows about a bit more 1000lb thrust increase with each of its six afterburner stages topping out at a static ~19,600lbs at 79.9999% throttle right before the AB ignites, the plane showed a 12,373lbs of thrust. The static thrust from the cfg is shown as 11,000.

The SR-71 I am working with is showing at 49.01% throttle 33,940lbs of thrust at 100% throttle it shows the same. What did I miss?

F18

[TurbineEngineData]
fuel_flow_gain          = 0.006                 //Gain on fuel flow
inlet_area              = 4.2895                //Square Feet, engine nacelle inlet area
rated_N2_rpm            = 16728                 //RPM, second stage compressor rated value
static_thrust           = 11000                 //Lbs, max rated static thrust at Sea Level
afterburner_available   = 6                     //Afterburner stages available?
afterburner_throttle_threshold = 0.80           //Percent of throttle range where a/b begins
reverser_available      = 0                     //Thrust reverser available?
ThrustSpecificFuelConsumption = 0.81             //Thrust specific fuel consumption (Jets)
AfterBurnThrustSpecificFuelConsumption = 1.74    //TSFC with afterburn/reheat engaged

SR-71

[TurbineEngineData]
fuel_flow_gain= 0.004 
inlet_area= 12.560 
rated_N2_rpm= 7350.000 
static_thrust= 34000.000 
afterburner_available= 50
afterburner_throttle_threshold = 0.50
reverser_available= 0.000
ThrustSpecificFuelConsumption = 0.9
AfterBurnThrustSpecificFuelConsumption = 1.9

I used afterburner = 50 because the AB range is so large. the xml i made shows the ab stages are working but there is no thrust change.

 

[sergio]

Hope this may help:

I think, the first step is to set the thrust properly:
The static thrust dry is the thrust at speed 0 and 100 % rpm or N2.
The statc thrust wet is the thrust at the same condition with full AB.

In your case the static_thrust is not 34000 but:

static_thrust= 25000.000

Usually the effective AB thrust varies with speed. This behavior is coded in the AIR file. For setting the static thrust wet you have to edit table 1524.

To get the required static thrust wet (34.000) you have to move the first point of the table to 1.36, because 25000 x 1.36 = 34000.

Ciao, Sergio

 

[TooLowGear]

Thank you very much Sergio.

Is there a way to determine what the AB thrust vs Mach would be? Static would be 1.36 but at higher speeds what would that value equate to on a standard day? Do the values reflect the multiplier effect of sea level?

Thanks again

 

[sergio]

I'm not sure but if I remember correctly the flight handbook of the real SR-71 has some data about this. There was a little graph or table about this matter. Sadly at the moment I'm not able to check it. I have trouble to reactivate my e-book copy.

Ciao, Sergio

 

[TooLowGear]

The graph you are referring to is the KEAS climb profile. With the modifications I have made to the airfile and to the engines the plane faithfully can fly the profile as well as FS will allow. http://www.sr-71.org/blackbird/manual/2/2-40.php I never understood if you had to be at max ab just for the climb-descend maneuver or if anytime for the climb above FL300.

Initially from what I have read in books the plane's initial climb rate after fueling was only about 3000fpm and as low as 1200 as it approached FL800