F15C Vmax simulation

[Raider11]

I think it would be fantastic if we had the option to "click" ourselves into Vmax on the engines. Vmax (as you likely know) has been prohibited for use in standard flight in the US, but it is still available for warfighters in combat if needed.

It increases thrust by about 4% for a maximum of 6 minutes, but requires additional maintenance if used. It has a switch on the left side of the cockpit that is wired shut during normal operations. It's interesting that I have read anecdotal accounts that the Vmax top speed of an F15C could be as high as 2.8 mach. I know we have been able to get the FG model to speeds in that neighborhood under the right circumstances.

Perhaps it would be cool to have the engines fail after 6 min of Vmax flight?

[Raider11]

I touched 3.08 mach in the F15 day before yesterday in level flight at 25,000 ft. This bird can sing!

[Zaretto]

This is something I've already considered - but I couldn't really find much information about real world usage. Basically if I add the extra power it must come with the extra risk - but I'm not actually sure what that extra risk entails. From basic principles running the engine hotter and faster is more likely to cause failure - but I need to understand what the failure is, how it can been seen from the instruments before it happens so the pilot can use it without destroying the engines.

Any other data you can find would be useful

I touched 3.08 mach in the F15 day before yesterday in level flight at 25,000 ft....

Is that with the latest version ? as my first thought is that maybe the mach 2.5+ table is wrong...

[Raider11]

Yes, it was with the most recent version.

In the F15 manual, it talks about Vmax to a certain degree. I am "speculating" that the max thrust figures we have may include the Vmax thrust?

The manual says that an engine must be scoped if it runs Vmax... I think for 6 min or more.

Interestingly, the manual states the max engine rpm for military/ab thrust is 96%. So, the extra 4% must be the Vmax 4%... meaning we are already using VMax? (See tech order Figure 5-5 titled "engine limitations"). It states that the max (30 seconds) oil temp is 990 C... so I will run some tests and see where we get at both 96% and 100%... and see how that correlates.

Also, if you look at the level flight diagrams for a 36000 lb ac at 1000 ft elevation, you can see that the cut-off of 2.5 mach is not due to thrust. It shows clearly that the 2.5 number is easily acheivable at a range of altitudes and temperatures (its a cut-off line marked by "design limitations"), obviously structural and not relating to drag or thrust limits. (Level flight envelope, figure A9-1, third figure).

I have read that there are issues with the inlets that cause the AC to be limited to 2.5 machs... but that many pilots have seen over 2.8 in extreme cases (combat chase). It makes sense when you look at the BIT limits that it is designed to capture.

I think the answer is to use Oil Temperature. If a pilot is over 990 c for more than 30 seconds, damage the engines. I will test to see if realistic...

[Raider11]

Ok, I think engine heat is a good start. So, I ran it at 96% pretty much without issue on a flat course at Angels 10. Engine Oil Temp stayed below 700 even with fuel freeze running hard at 96. However, when I went to 100%, temp climbed pretty quickly into the 800s... but kinda stopped around 837. After sitting at 837 for about 30 secs, the burn-through indicators came on (on the left fire panel).

So, it appears that, even after maxing the engines at 100%, they will not elevate over 837 deg c at Angels 10. Perhaps that could be the point of failure?

[Zaretto]

The 96% vs 100% thing is more down to the engines model in JSBSim and how I've set it up.

The FTIT temp gauge measures the temperature at the fan turbine. This is the turbine at the back of the engine, also called the high pressure turbine (HPT). There is effectively no difference between FTIT and EGT, if I remember correctly P&W usually call it FTIT and GE EGT - it's down to where the sensing probes are located, but in operational terms the temperatures are going to be about the same.

The engine model we are using is a simplified PLA -> Thrust(Mn,Alt,Ab), the EGT is something I added based on very little data so it probably is wrong. I've got some work in progress to have an EPR based engine model, ideally we'd have a full gas generator model - but that requires good data which I haven't found yet.

The data presented in http://www.nasa.gov/centers/dryden/pdf/87966main_H-1200.pdf may also prove to be some use.

[Raider11]

My apologies for the misguided assumption on the source of the temperature read-out. I feel a touch silly... but I do think after looking at it again that this may be the right tree...

The FTIT sensor is what I am assuming is driving the burn-through light (engine overheat) on the fire panel... ? Assuming I am correct (which I am not claiming to be), then wouldn't the engine, during normal operation, have to be well above 837 degrees C in order to cause a burn-through or overheat condition if the max op temp for that operation is 990 deg C?

Maybe I am just dumb... feel free to tell me to shut up!

Raider1

[Zaretto]

Burn-thru is basically using FTIT with a lag filter; so if you have sufficient FTIT for long enough the lights will come on. It's not right; but at the time I had virtually no FTIT data (I still haven't got much) so the numbers are "looks about right" rather than "matches this data".

[Raider11]

So perhaps you leave the burn-through warning about where it is (seems like about 30 seconds at 830+???), but allow the heat to continue to increase (it seems to top out at 837, at least for me). Then fail the engines at 990+ IAW the manual max? Does that make sense?

[Zaretto]

It does make sense, but would need some data about FTIT temperatures at different altitudes and engine conditions; as I don't actually know what happens here.