talk to me about how jet engines behave

[scary_pigeon 0]

we continue to improve cockpit, now that I become focused on the the gauges:

 

fuel flow

and engine RPM for the harrier

 

I am not quite sure what happens.

 

i presume the engine is started and it ticks over at some RPM or other.

 

Then there is fuel flow and rpm.

 

I presume throttle regulates fuel flow (fairly rapidly) and then engine RPM climbs or falls in some way. - there may also be a delay before a surge in fuel flow gets to affect the engine?

 

until now its just been, instant, throttle sets the thrust in the FM.

 

so if anybody has any thoughts on this, release them here :)

[Ivankuturkokoff 0]

Scary Jet engines are fairly straight forward. The biggest controling variable is Fuel flow. the pilots throttle can be thought of as a a Fuel flow control, a bit like a gate valve. More Fuel Flow more thrust ... but :)

 

However a number of modifying devices exist between the Throttle and the the engine. These devices control things like Engine acceleration, Fuel air ratios, initial ignition, and Jet pipe temperature limitation.

 

Given that there is a large rotational mass in the jet engine especially fan type engines like the Pegasus you dont have instant engine control. It is imperative that the demanded Fuel flow (by the pilot) is matched with engine RPM. If its not nasty things like compressor stalls and surges ocurr.

essentially because you are feeding too much fuel into the engine for not enough air flow. Since Air Flow is a function of RPM.

 

To control this is a device called an Acceleration Fuel Ratio Control unit or AFRCU exists. It takes the pilot demanded Fuel Flow and then smothly matches this to engine RPM via a predetermined schedule known as an operating line. Altitude corrections are also feed in to ensure stable Fuel Air ratios are always maintained.

 

The engine also has its own dedicated High pressure fuel pump to ensure pressure fuel is available for the other control units to deliver to the burners.

In the case of Afterburning aeroplanes (Ak Mirage/Dagger) an aditional AB or "Turbo fuel pump" is there dedicted solely to AB operation.

 

IDLE/MAX RPM

IDLE limits are set to ensure stable minimum thrust values. This Idle value will also vary with altitude to ensure enough Bleed air is available to maintain cockpit pressuristion. Max RPM is typically there as a structural limit

and to ensure reasonable engine life. Some real numbers. The Mirage ATAR 9C has an idle of 2900+-200RPM. Max RPM is 8460RPM and 8710 when Overspeed is permitted. Typical JPT limits 680deg C to 740DegC. The Mirage uses actual RPM values but most other aeroplanes (Harrier) uses %RPM. However typically max RPM is usually more than 100%.

 

Harrier limits (these are TMK4 values however... Dante has these as well) MAX RPM No water injection 103.5%, JPT 715deg C.

With water Injection 107.0% and 745deg C. These are very time limited vales ..no more than say 1.5mins. Maximum Continous 89% 540deg C.

Absolutely No Sh.. RPM limit 108% momentarily. I will chase up some idle values for you as well.

 

SPOOL UP TIMES

Because there is a large rotational mass and that Acceleration must follow a schedule you dont have instantaenous RPM control in a jet engine. A typical value from Idle to 100% will take 8 seconds or so. This rate is also not linear, it gets faster at the upper end.

 

From an effciency point of view The Jet engine likes to operate up around the 90% mark. Any faster or slower will vary thrust but effciency drops off.

 

JET PIPE TEMPERARTURE LIMITS (JPT)

JPT control is essential, Without it things like turbines can get cooked really quickly. The simplest method is by the pilot. He attempts to select say maximum thrust and observes JPT. In most cases it will be fine as careful design should prevent gross exceedences. If tHe pilot sees a limit approaching he must limit fuel flow. This will have an almost immediate response on the JPT guage (Though RPM will have a certain lag).

 

GOOD REFRENCES

If you can get hold of a second hand copy of the Rollys Royce publication

The "Jet Engine" you will have most of the theoretical info you need.

 

Hope this helps

Edited December 30, 2004 by Ivankuturkokoff

[Nutty 0]

Check out http://www.eflightmanuals.com for Harrier Flight manuals.

 

Cheers

 

James

[scary_pigeon 0]

Scary Jet engines are fairly straight forward. The biggest controling variable is Fuel flow. the pilots throttle can be thought of as a a Fuel flow control, a bit like a gate valve. More Fuel Flow more thrust ... but :)

 

However a number of modifying devices exist between the Throttle and the the engine. These devices control things like Engine acceleration, Fuel air ratios, initial ignition, and Jet pipe temperature limitation...

 

<{POST_SNAPBACK}>

 

you seem to know your stuff. I dont suppose you can program in C++ or LUA? :-)

 

I am rapidly trying to develop stuff now so will create a system that somewhat resembles how things should be with view of returning to perfect things.

[Ivankuturkokoff 0]

I wish :) I once was profecient in Reverse Polish on an HP41C but thats it .

 

Happy New Year

Edited December 31, 2004 by Ivankuturkokoff

[+Fates 63]

Scary Jet engines are fairly straight forward. The biggest controling variable is Fuel flow. the pilots throttle can be thought of as a a Fuel flow control, a bit like a gate valve. More Fuel Flow more thrust ... but :)

 

However a number of modifying devices exist between the Throttle and the the engine. These devices control things like Engine acceleration, Fuel air ratios, initial ignition, and Jet pipe temperature limitation.

 

Given that there is a large rotational mass in the jet engine especially fan type engines like the Pegasus you dont have instant engine control. It is imperative that the demanded Fuel flow (by the pilot) is matched with engine RPM. If its not nasty things like compressor stalls and surges ocurr.

essentially because you are feeding too much fuel into the engine for not enough air flow. Since Air Flow is a function of RPM.

 

To control this is a device called an Acceleration Fuel Ratio Control unit or AFRCU exists. It takes the pilot demanded Fuel Flow and then smothly matches this to engine RPM via a predetermined schedule known as an operating line. Altitude corrections are also feed in to ensure stable Fuel Air ratios are always maintained.

 

The engine also has its own dedicated High pressure fuel pump to ensure pressure fuel is available for the other control units to deliver to the burners.

In the case of Afterburning aeroplanes (Ak Mirage/Dagger) an aditional AB or "Turbo fuel pump" is there dedicted solely to AB operation.

 

Most all aircraft that I've worked on have LP (low pressure) pumps that siphon fuel from the tanks and bring it back to the engine.  The HP (High pres) pump is usually an engine driven pump.  Thus, loss of electrical power may cause the engine to starve because there is no head pressure on the HP pump.  THis can be solved by the pilot selected CROSSFEED, and allowing the working LP pump to feed both engine HP pumps.

 

IDLE/MAX RPM

IDLE limits are set to ensure stable minimum thrust values. This Idle value will also vary with altitude to ensure enough Bleed air is available to maintain cockpit pressuristion. Max RPM is typically there as a structural limit

and to ensure reasonable engine life.  Some real numbers. The Mirage ATAR 9C has an idle of 2900+-200RPM. Max RPM is 8460RPM and 8710 when Overspeed is permitted. Typical JPT limits 680deg C to 740DegC. The Mirage uses actual RPM values but most other aeroplanes (Harrier) uses %RPM. However typically max RPM is usually more than 100%. 

 

Harrier limits (these are TMK4 values however... Dante has these as well) MAX RPM No water injection 103.5%, JPT 715deg C.

With water Injection 107.0% and 745deg C. These are very time limited vales ..no more than say 1.5mins. Maximum Continous 89% 540deg C.

Absolutely No Sh.. RPM limit 108% momentarily. I will chase up some idle values for you as well.

 

SPOOL UP TIMES

Because there is a large rotational mass and that Acceleration must follow a schedule you dont have instantaenous RPM control in a jet engine. A typical value from Idle to 100% will take 8 seconds or so. This rate is also not linear, it gets faster at the upper end.

 

From an effciency point of view The Jet engine likes to operate up around the 90% mark. Any faster or slower will vary thrust but effciency drops off.

 

Correct.  The majority of the power produced by a jet engine is done between 80-100% RPM.  The power curve increase exponentially with engine RPM.  Going from Idle to Full power (as sim pilots may often due) is  something I don't think I've seen modeled well in sims.  The engines will have a tendancy to overshoot or better verbage would be OVERSPEED.  The Overspeed governor (typical), although many engines are using computers to regulate this, can not react quick enough and you may see your engine hit 107% before dropping back to the max rpm stop.  Computers can regulate this much better and actually sense the rate of acceleration and catch this much faster.  I dno't know what's in the Harrier.  THis OVerspeed is going to be associated with an overtemp as well.

 

JET PIPE TEMPERARTURE LIMITS (JPT)

JPT control is essential, Without it things like turbines can get cooked really quickly. The simplest method is by the pilot. He attempts to select say maximum thrust and observes JPT. In most cases it will be fine as careful design should prevent gross exceedences. If tHe pilot sees a limit approaching he must limit fuel flow. This will have an almost immediate response on the JPT guage (Though RPM will have a certain lag).

 

In the US I call this EGT (Exhaust Gas Temperature).  The aircraft (non-military) that I've worked on have ITT (inner-turbine temperature), but these are Bypass-Fan engines.  The pure Jet engines I've seen EGT.

 

GOOD REFRENCES

If you can get hold of a second hand copy of the Rollys Royce publication

The "Jet Engine" you will have most of the theoretical info you need.

 

Hope this helps

<{POST_SNAPBACK}>