THE FASTEST F15, CONCLUSION

The fastest F15?

Dodging the bullet! No not the speeding F15, the enraged base commander!

In an all things considered situation, I'd rather be lucky than good any day.? That day I was just relieved.?

If you're still interested,? part two wasn't the end of the story.?

Continuing on, the jet promptly went to the tanker for a refueling then returned to base without incident.??

There were a few things I skipped over while I was telling the previous bit.? One important thing was the Flight Test Director's call to terminate the test when the jet reached 2.4 Mach. You see all pilots know every airplane they have ever flown has a sign, called a placard which? shows the never to exceed speed of any aircraft.? These placards have one purpose, saving your life. (I’ve written this before,? “There are old pilots and there are bold pilots, but there are no old bold pilots”) The "knock it off " call is the universally recognized call so cease what ever is going on.

Typically when you exceed these posted speeds, aerodynamic forces start to remove pieces you need to remain in controlled flight.??

So is the case of the 2.4 limit that was on the F15, and the F16.? It was not strictly an airframe driven requirement it was an engine issue, in fact it was the reason for these flight tests.? You see, several years earlier, an F16 had crashed when it had reached a critical speed resulting in the loss of aircraft and pilot, and all regulations are written in blood.? ?The result of the crash was an investigation into what caused the breakup of the engine.? I used to love to teach this event when the USAF crash lab at Kirtland AFB when it was more publicly accessible. Come to my crash investigation class and mention it, we can discuss the whole crash. This turned out to be a forth stage compressor blade tip stall.? The blade entered a region where the combination of it's rotational speed combined with the ram effect of the high speed caused a stall situation to develop which caused the tip region of the 4th stage blade to load and unload rapidly, this vibration drove the blades into a high cycle fatigue (HCF) condition.? (A tiny engineering lesson here..... HCF is scary stuff.? It often comes on without any warning, it isn't something you can hear or feel and worst, it goes from onset to failure in a very short time)

This day when we allowed the pilot to push the jet to Mach 2.5, our luck held as the engines were both reconfigured with the original design 4th blades, which had just been reinstalled after proving the redesign worked.? But while the aircraft withstood the extremes it was likely some of the players might not!

When the jet returned, it was not without battle scars. A couple antenna had melted off and the wingtips, being made of fiberglass were distorted and needed replacement.

The pilot was called before the base Commander and I don't know what was said to him When my turn before the commander came,? I explained that both engines had to come into the shop to have each engines complete set of new 4th blades replaced, the poo hit!? ?I explained how we got there, what occurred with regard to the engines and that Pratt&Whitney would replace the blades and pay for the maintenance.? He didn't? smile, but he didn't ban me from the base, which I was grateful for.??

When I was younger there was a news person that used to end his stories with, "Now you know the rest of the story” thanks for this appropriate ending

Mr. Paul Harvey.

For those that found this interesting enough to read it all, I left some bread crumbs along the way that might have gotten you wondering a few things.??

First, I didn't get fired, neither did our performance engineer.? ?

I did over commit Pratt&Whitney on the blade replacement though it was not only the right thing to do, it was the ONLY thing.?

To explain,? in the picture at the top,? it states,? NSMS this was a very specialized? test and I believe my team were the first to accomplish this installed in a flying aircraft.? ?What is NSMS?? It stands for Non-interference Structural Measurement System.? In a highly simplified explanation, a laser beam is focused on, in this case, the tip of a rotating compressor blade.? The laser beam records the precise time the blade arrives.? It records the time and does so for every blade in the rotor.? ?The rotor speed is also recorded to provide a reference.? Any variation between blade arrival times can be translated into vibration displacement.? (note: blade arrives early, one reference point, blade arrives late, another reference point.) This way very small displacements can be recorded.? ?This is HFC, high cycle fatigue.? ?In 1999, this type of measuring and recording was being done in a test cell with huge computing systems. Pratt&Whitney wanted to have a positive way to show that the new redesign blade eliminated HCF in flight operations.?

You see, HCF can not be detected by nondestructive inspection methods.? ?You can't tell if a part has been subjected to HCF by looking at it, furthermore even if you? could,? you couldn't tell how much of it's life had been used up.? Often, the time between the onset of HCF and component failure is measured in seconds.

Destructive testing is the only way and that's not helpful until after the failure. (another note, HCF can occur in varying degrees, and combine with other forms of stress.)

Come to my Gas Turbine Accident Investigation class at www.scsi-inc.com? to learn about this and other jet engine facts.

?So doing it in flight was the perfect way.? A lot of hard work went into this, many very intelligent people were involved.? The toughest sell on this was to the USAF customer.? You see in order to make this work we’d have to get a rather thick cable routed from inside the engine bay to a location where the laser feeds could be connected to the recorder and processor.? This was at a box, first in the wing, then to the data recorder.

The USAF was concerned because in order to access this wing box area, would necessitate putting a hole through the F15's firewall.? ?Normally a no no.? This was eventually resolved and the NSMS laser cables were routed to the interior wing box, then internally over to the data recorder where the ammo drum for the machine gun would normally be.? The gun and its ammo drum were not necessary for this test vehicle so it saved a considerable amount of the weight.?

A special thank you to the Pratt tech who endured the blazing sun of the Edward's AFB California sun mid summer.? If no one else appreciated it I did.? Pratt&Whitney people are always the best.?

So much information, but the answer to final questions if it had not been on your mind, well I would have hoped I'd done a better job, maybe writing as a new career is out, but I felt I? owed an answer to those that stayed for the credits...

THAT FINAL QUESTION:

So how did the less powerful F100-P-220 powered F15 go significantly faster than the much more powerful F100-P-229???

After all it was the exact same aircraft,? the same fuel load,? the same airfield, essentially the same weather conditions, but the -220 really shone that day.??

Modern Jet engines are marvelous inventions.? ?I spent thousands of hours testing these engines and still there are many little secrets each holds.? This little secret is called a T3 limiter.? It's not actually a secret, it's one of many ways Modern engines protect themselves.? T3 is a compressor exit temperature.? ?The electronic control uses this to regulate turbine temperature.? ?In the old days of a jet engine, thrust was a function of 3 things, Woof (Wf) PB and N2.? Wf was fuel flow, PB was burner pressure and N2 was high compressor speed.

It operated as Wf over PB versus N2.? Today's engines use other various means to regulate fuel flow.? But where a jet engine is concerned,? the easiest explanation is if you want more thrust, add more fuel.??

This is where the T3 temperature sensor on the F100-P-229 comes in.? A T3 limit is built into the control system of the -229.? This limit prevents the fuel control from adding any more fuel once this limit is reached.? No extra fuel, no extra thrust, simple right?

The -PW-220 fuel control didn't have this. (it does now)?

So as the jet accelerated,? PB went up.? As that went up the fuel control saw this and it scheduled more Wf, (fuel flow). The results were more thrust.??

There are other protections that the PW-220 had but my babies were fine tuned by the Pratt&Whitney mechanics at Edward's AFB and they were always ready to excel.? As they did, it was just that pesky potential of HCF above 2.4 Mach.? It was only a slight chance that the HCF did occur, but out of an abundance of caution Pratt&Whitney replaced both engine sets of 4th blades.

Today the engine has a new setting for Fan exit temperature,? redesigned compressor blades and won't go 2.55 Mach in case.

For those of you involved back then a special thank you and if I got some of this wrong, I’m sorry but it's been 25 years.

P.S. Maybe someday I’ll relate a short story how how I made the tether guy “blink” this is a 229 F16 story.

P.P.S In confidence the pilot on this flight told me his only concern was that the canopy was getting soft from the heat, when I asked how he knew he said he could see wavy lines in it.