F14 and overspeed

Flying in the dogfight server I have been having problems slowing down. After running out of ammo, instead of dumping the plane and getting a new one, I have been burning max speed back to the blue airport. Once there, I can’t get the engines to slow down. It comes out of burner but they just don’t seem to want to throttle back. I have tried to re assign throttles, sign out and back in, always the same. When I enter the server it works great, but after battle and a long burn home nada. Are they just running wild due to

Airspeed at this point or have I stumbled onto an oddity?

Thoughts?

If you're supersonic the engines won't drop below 80% to prevent compressor stalls. Go nose high or pull some G and you'll hear the engines cut back after you lose enough speed.

Thanks!!!!

It's not compressor stalls that's the main culprit. It's heat. After a long burner run the engine is hot. All jet engines need to push air through to cool themselves. So after being supersonic, the jet will need to cool itself. The pilot wants less thrust, so the nozzles will open, and less power overall is produced, but the turbines need to spin at a high rate to cool the engine. This is done through fuel. When this happens the fuel rate is high. The computer adds fuel to keep the rpm up on the engine until the engine reaches a safe temperature. For you car guys this is NOT the same as running rich. When a safe temperature is reached that's when you hear and see the engine spool down and fuel flow return to normal. This happens even in today's current fighter engines.

Thats an extremely cool addition to the flight model!

There are a lot of misconeception in this post so I will try clear things up

All jet engines need to push air through to cool themselves

Jet engines are cooled by various systems. The bearings e.g. are cooled by oil. The engine nacelle is cooled by air circulating around it. The most critical part however is the high pressure turbine, as it sits directly behind the combustion chamber. The HP turbine blades are usually cooled with compressor bleed air circulating inside small channels inside the turbine blades. Additionally, bleed air is expelled through small holes in the blades directly onto the blades' surface (so called film cooling), although I'm not sure which of these cooling techniques the F110 actaully uses.

The computer adds fuel to keep the rpm up on the engine until the engine reaches a safe temperature

Increasing fuel supply increases turbine inlet temperature (TIT) and does not help cooling the turbine blades at all. Setting throttles to idle is the only way to reduce TIT and thus turbine temperature. This is also reflected in the engine overheat emergency procedures.

but the turbines need to spin at a high rate to cool the engine

The air (or rather combustion gases, which are very hot) around the turbine blades are heating them up regardless of RPM. Again, the most important factor is TIT.

All of this however has nothing to do with the engines staying well above idle at supersonic speeds, since the turbine blade cooling system provides enough cooling at any RPM. If this was not the case, the manual would prescribe a maximum amount of time the engines can be used at MIL/MAX power, as was the case in older engines, where cooling at high power settings was insufficient and turbine overheat damage possible (see the Huey).

It's not compressor stalls that's the main culprit

It actually is. The physics behind this are quite complex and I am also not sure how exactly the F110 would become unstable in such a situation. But basically, engine compressor stability margin (i.e. how much random inlet velocity/pressure fluctuations the compressor can handle without stalling) is reduced at supersonic speeds compared to ground idle due to inlet disturbances and compressibility effects. Increasing engine RPM increases that stability margin and greatly reduces the probability of an engine stall. This feature is called "idle lockup protection" and is used in some form on every supersonic aircraft.

My apologies sLYFa, but I'm on target with this.

1. You explain a process that cannot happen efficiently without the blades spinning at a high rate. How do you think the engine can increase its RPM? Jet engines can increase RPM without increasing power. The primary catalyst for this process is fuel. More fuel is used to increase RPM without increasing power, therefore heat is not linked to engine RPM. They can be manged separately.

2. Just because you have increased fuel doesn't mean that you've increased temperature. More fuel does not equate to more combustion in a jet engine.

3. The air that has been heated through combustion has already departed the engine from the back. The components are still hot and as fresh air enters that heat will transfer to it. This transfer process happens slower at high altitude than lower altitudes. Why? Less air molecules. The higher you go the more difficult it is to dissipate heat because there is less medium to dissipate through. The is why the RPMs stay up for longer at high altitude than low altitude. It takes longer for things to cool down.

4. I agree. It's complex. The "idle reset" is a mode that the computer enters when the throttles reach the idle stop. This forces a bit check that will monitor the temperatures, alt, speed, etc. The job of this process is engine protection. Protection from heat is number one on the list. The inlet doors are the primary mechanism to protect the engine, not the RPM. There are other methods as I'm sure you are aware. We are just scratching the surface, but running burner creates a lot of heat, and that heat must be managed for consistent combustion control.

Scrape, how do you make your engine spin faster without producing more power? Burning more fuel to give higher RPMs will increase output power, where else is the energy going to go?

Sent from my G8341 using Tapatalk

nozzle and bypass (if it has those things) If you were to just suddenly reduce the RPM in a fast spinning turbine at high speed and temperatures, all of that hot gas and pressure in the combustion chamber will try to flow FORWARD (vacuum of low pressure where you have reduced the incoming air) instead of back thru the nozzle. This will result in stagnation and torching your engine, even if you cut off fuel flow completely (this is a compressor stall/stagnation). You have to keep air flow and positive pressure through the engine in order to get it to cool down and blow that temperature away.

Go hop in a n ME262 in another popular modern flight sim and after you get the engines running and are airborne try moving the throttle quickly to idle while at high speed, note exactly how quickly your engines catch fire, and then realize that is what the tomcats systems (and all modern jet engines) are preventing from happening.

I don't think heat has anything to do with it, at least not in our engines. RPM is maintained until speed drops below Mach 1.1, regardless of how long you were in burner. And TIT drops by around 100 C or so when it starts to spool down so there's definitely more heat while it's keeping RPM up.

My apologies sLYFa, but I'm on target with this.

 

1. You explain a process that cannot happen efficiently without the blades spinning at a high rate. How do you think the engine can increase its RPM? Jet engines can increase RPM without increasing power. The primary catalyst for this process is fuel. More fuel is used to increase RPM without increasing power, therefore heat is not linked to engine RPM. They can be manged separately.

RPM equals compression ratio equals power. There are ways to change power output without significantly changing rpm but for the scope of this discussion I can assure you that power, rpm and fuel flow are closely linked.

2. Just because you have increased fuel doesn't mean that you've increased temperature. More fuel does not equate to more combustion in a jet engine.

Yes it does.

3. The air that has been heated through combustion has already departed the engine from the back. The components are still hot and as fresh air enters that heat will transfer to it.

You do realize that jet engines combust continously? There is is no fresh air entering the turbine, its hot combustion gases all the time. I described how turbine cooling works but I assume you don't care to read/understand.

4. I agree. It's complex. The "idle reset" is a mode that the computer enters when the throttles reach the idle stop. This forces a bit check that will monitor the temperatures, alt, speed, etc. The job of this process is engine protection. Protection from heat is number one on the list. The inlet doors are the primary mechanism to protect the engine, not the RPM. There are other methods as I'm sure you are aware. We are just scratching the surface, but running burner creates a lot of heat, and that heat must be managed for consistent combustion contro

That statement above is just pure nonsense. There are no inlet doors, they are called inlet ramps. They have nothing to do with temperature protection but with slowing the inlet air to subsonic speeds. Burner does create a lot of heat but this is not the main problem as I explained.

My apologies sLYFa, but I'm on target with this.

No you are not and a lot of what you posted suggests that you have little understanding about how a jet engine works, which is OK, I just don't understand why you feel the need to spread those misconceptions on the forums.

What is modeled here is the idle lockup protection. Cooling etc is also modeled, but in this case not the "limiting" factor. Overheating etc in the F110s becomes more of a problem if you take direct damage to the engine.

oops

@sLYFa...are you always like this..? Just asking.

I mean you seem to have some knowledge, but like every good sea-lawyer, it is just talk, and especially here where no one can see or check your credentials. In then end you seem to mostly be an argumentative, condescending guy with maybe more knowledge than the average bear but...

I look forward to your response to Iron Mike who had the support of ACTUAL sme's while building the simulation of the Tomcat.

RPM equals compression ratio equals power. There are ways to change power output without significantly changing rpm but for the scope of this discussion I can assure you that power, rpm and fuel flow are closely linked.

 

Yes it does.

 

You do realize that jet engines combust continously? There is is no fresh air entering the turbine, its hot combustion gases all the time. I described how turbine cooling works but I assume you don't care to read/understand.

 

 

That statement above is just pure nonsense. There are no inlet doors, they are called inlet ramps. They have nothing to do with temperature protection but with slowing the inlet air to subsonic speeds. Burner does create a lot of heat but this is not the main problem as I explained.

 

 

No you are not and a lot of what you posted suggests that you have little understanding about how a jet engine works, which is OK, I just don't understand why you feel the need to spread those misconceptions on the forums.

Aren't Mike and sLYFa agreeing? They both said that heat is not the reason RPM is kept high. Also, section 2 page 13 of the F-14D NATOPS says that loss of Mach signal from the CADC results in loss of idle lockup protection and this may result in pop stalls or inlet buzz. Ergo, idle lockup protection is preventing pop stalls and inlet buzz, not overheating of the engine.

Idle Stop Reset or similar programs by another name run no matter where the aircraft is, or what it's doing. The throttle returns to the idle position is all that's required for the program to initiate regardless of where the aircraft is. Even on the ground with the parking brake set the Idle Stop Reset program will run. The first, not only, but first thing it checks for is heat. Once that is solved it moves on to other things. If the heat problem isn't solved it will address that first before moving on.

@sLYFa...are you always like this..? Just asking.

What exactly did you find so offensive in my posts? I'm sorry if calling someone out on his nonsense appears condescending, I'm not here to look down upon anyone.

it is just talk

I have to disagree on this. Besides the usual "just talk", I find a lot of intersesting subject matter knowledge from various people (pilots, maintainance guys etc.) on this forum and I am sure I'm not the only one. Now if somebody interested in this topic but with little knowledge about jet engines comes here and reads Scrape's posts, he gets a lot of misinformation. I just want to set things straight.

and especially here where no one can see or check your credentials.

Thats why I always try to provide sources or at least some reasonable line of argument, which IMO I also did here. If you think I'm talking BS, please prove me wrong, I always like to learn something new.

I look forward to your response to Iron Mike who had the support of ACTUAL sme's while building the simulation of the Tomcat.

I didn't get the impression that we disagreed.

Idle Stop Reset or similar programs...

Man I give up, this is pointless

 

 

Man I give up, this is pointless

If you don't know, you don't know.

The idle lockup is there to smoothen out the deceleration process and make a life of air intake engineer a little bit easier. What happened when the engine rpm goes idle is a sudden decrease in mass flow rate to the engine. This mean the intake has to dump unwanted unwanted air to somewhere else. For example, concorde has the spill door at the inlet floor to dump this unnecessary air if it has to perform an emergency descend at mach 2.0.

If air intake configuration left uncheck (ie, config for mill/max power) or is not properly config to cope with a sudden reduced in mass flow, the shock wave system inside the intake has to be stronger to decelerate the flow fast enough to match the low mass flow demand from the engine. The unwanted byproduct of stronger shockwave is a “dirty” or turbulence air fed toward the engine. This dirty air will reduced a compressor stall margin or even worst stall the engine. You can try stow the intake ramp in HB DCS F14 and fly supersonic to see what it is like. In the extreme case, the shockwave will become so strong that it will detached from the intake ramp and become unstart. Unstart may or may not solve itself depend on intake configuration and occasionally required an air intake controller to perform some action to return the flow to its original state.

If the idle lock up is used, the sudden decrease in mass flow issue will be eliminated entirely.

Here is a little story from Concorde side on their state of the art intake system from pprune and the purpose of idle lockup protection.

“If I may, I would now like to mention the 'some oil lamps and diesel oil' story. This is a true story told to me by Dr Ted Talbot, the father of the Concorde Intake, brilliant aerodynamicist and all round amazing gentleman. Ted had been invited in 1975 to speak to the US test pilots at Edwards Air Force Base in California, and after he landed he was invited to take a tour through one of the top secret hangars there, and in this hangar were a few glistening Mach 2.5 design B1A development aircraft. Now Ted had heard that Rockwell were having major difficulties with the engine intakes, and obviously had more than a passing interest in such things, and was allowed to take a close look. Just above and slightly forward of each intake he observed several beautiful made precision total pressure probes mounted under the wings, and although he had a good idea what they were for, said nothing at the time.

That evening, Ted gives his presentation speech to the assembled Test pilots, explaining in fair detail how the Concorde engine intake operated, and that the fact that unlike most other supersonic designs, the engine power was more or less freely variable at Mach 2 and above, even to the extent that if necessary the throttle could be closed all the way to the idle stop. There allegedly many gasps of amazement and disbelief in the room at this, and one B1A pilot was heard to ask his boss 'why the hell can't WE do that John'?. (It should be borne in mind here that the 'traditional' way of slowing down Mach 2+ aircraft is not to touch the throttles initially, and just cut the afterburners. If you don't do it this way many designs will drive into unstart and even flame-out).

After the audience had asked Ted several questions about Concorde, Ted was then invited to ask the assembled USAF and Grumman personnel about the B1A programme, which would be honestly answered within the confines of security considerations. Ted said that he only had one real point to raise; 'I see that you are having major difficulties with wing boundary level interference at the engine inlets'. There was now a gasp of horror from various members of the USAF entourage, 'That's top secret, how the hell do you know that?'. Ted chortled 'it's easy, I saw that you have a multitude of precision pressure sensors under the wing forward of the intakes, that I assume are to measure the wing boundary flows'. Ted then unhelpfully comes up with 'Oh, and you've got the design completely wrong, your intakes are mounted sideways, and that allows the intake shocks to rip into the wing boundary layer, which will completely screw up your inlets at high supersonic speeds. That in my opinion is where most of your problems lie, with wing boundary level interference, but I think that your probes for measuring boundary layer are beautiful, we never had such things'. According to Ted there was not so much uproar at the meeting as much as horror and amazement that this (even then) quite senior in years British aerodynamicist had in a few seconds observed the fundamental design flaw in an otherwise superb but top secret aircraft, and could even see what they were trying to do about it. Ted was asked, 'so you had no boundary layer issues with Concorde then?' Oh we had a few, mainly with the diverter section mounted above the intake' replies Ted, 'but we sorted out the problems relatively easily. 'You said that you did not use precision pressure probes under the wing to measure boundary layer flow fields, so what DID you use then?', asks a Rockwell designer. 'Some oil lamps and diesel oil' replies Ted. The room is now filled with laughter from all those assembled, but Ted shouts 'I am serious, it's an old wind tunnel trick. You mix up diesel oil with lamp black, which you then paint over the wing surface forward of the intakes, where it forms a really thick 'goo', which sticks like glue to the wing'. The pilots in particular seem quite fascinated now, and Ted goes on; 'You fly in as cold air that you can find (we flew out of Tangiers and Casablanca) and flew as fast as you could. As the skin temperature increases with Mach number, the diesel and lamp black 'paint goo' becomes quite fluid, and start to follow the boundary layer flow field. You then decelerated as rapidly as possible, and the flow field 'picture; is frozen into the now again solid 'goo'. After we landed we just took lots of pictures, repeated the process for several flights until we know everything that we needed to know about our difficulties. After doing some redesign work we then repeated the exercise again several times, eventually proving that we'd got things right'. The audience asked Ted if this technique might help them with the B1A, but he replied that although it might help them with accurately illustrating the problem, in his opinion it was irelevant, 'because the intakes are the wrong way round'.

The B1A intake problems were never resolved, and in 1977 the project was cancelled, due to performance and cost issues. However the project was reborn as the B1B, not entering service until 1986. Although an amazing aircraft, with astonishing low altitude performance and capability, it is a fixed intake design, limited to Mach 1.6 at altitude. Ted was right it seems.”

From : https://www.pprune.org/tech-log/print-426900-concorde-engine-intake-thrust-4.html

So, again, we've reached the conclusion that the idle lockup protection is to prevent inlet instabilities such as stalls and unstarts, yes? And it's not to prevent overheating the tailpipe or turbines, yes?

Besides the usual "just talk", I find a lot of intersesting subject matter knowledge from various people (pilots, maintainance guys etc.) on this forum and I am sure I'm not the only one. Now if somebody interested in this topic but with little knowledge about jet engines comes here and reads Scrape's posts, he gets a lot of misinformation. I just want to set things straight.

Yes, I do that a lot and hate to go through piles of guesses or upright wrong answers to finally find the right answer at the end of pages of pointless arguing. People should just not take part in discussion if they are unsure of their knowledge and should listen to the ones in the know, not argue more. I for one try not to go beyond my knowledge and always inform whenever it is only "afaik" or even educated guess. Misinformation is very bad habit and spread like disease.

although I'm not sure which of these cooling techniques the F110 actaully uses.

Yes piles of guesses and upright wrong answers indeed, do happen.

I'll take partial responsibility for apparently not choosing the right words to explain. It's clear they were mis-read from my intended message. I'm not taking them back, just saying a couple people didn't understand what I was talking about.

Simply put, sometimes you have to run an engine to cool it. If an engine (car/jet/whatever) becomes too hot, turning it off (in this case low idle speed) will result in a temperature spike. The only way to prevent this temp spike is to run the engine. At some point the engine is at a nominal temperature that the ignition can be switched off (in this case idle speed is reduced). Not all idles are at the same speed for fighter engines. Technically what's observed in the Tomcat is idle as far as the computer is concerned. If it's modeled or not on purpose I can't say, but this does happen on modern multi-stage fighter engines, so either it's modeled on purpose or happens to mimic real life by accident. Yes a high speed idle is possible, current fighters do the same.

Guys, my comment was towards what was modeled (and was sensible to model from our perspective), not meant as a reply to anyone.

I see you all like to have these disputes, just remember: it doesn't serve much, if at the end both cannot be the wiser for it. We all are learning together, the Tomcat has so much knowledge packed into one aircraft, it really is a rabbit hole. I would not expect anyone to know it all, not even SMEs. Well, maybe blacklion knows everything hahahaha.

That said, please be kind to each other. We're all fans of the same thing. :-)