JFS LIMITS

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evilrider

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Unread post22 Oct 2019, 15:52

Hi guys!

Can someone comment on why the JFS is ops limited to 400KTS and 20.000 feet. Am new to the forum, may be it has already been talked about.. several times.

Thanks
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outlaw162

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Unread post22 Oct 2019, 16:15

Just a general answer. Been awhile.

Altitude restrictions on any air-start are generally related to air density required for a stable restart. Don't remember specifically what the speed limit implies, but in some aircraft it's tied in to windmilling RPM and gearing to engage the starter.

Above certain speeds, windmilling RPM itself is adequate to effect a restart without a JFS. Turbofans usually need higher speeds than turbojets.

One of the more current F-16 drivers or engineers here can correct me if required.
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Meteor

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Unread post23 Oct 2019, 17:35

My guess: The JFS intake and exhaust are on the side of the aircraft. There is no air scoop into the relative wind. The JFS was primarily intended to be used on the ground at zero airspeed, with no high speed airflow across either the intake or exhaust. Above the certified airspeed and altitude limits, the local airflow across either or both vents might create an environment that is not conducive to JFS operation. Example: A 500 KIAS airflow across the flush JFS intake might create a vacuum that the JFS cannot overcome. If the JFS attempts to start with insufficient airflow into the compressor, then adds fuel to the vacuum, there is a potential for hung start or even an inflight fire.
F-4C/D, F-16A/B/C/D, 727, DC-10, MD-80, A321
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jbgator

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Unread post23 Oct 2019, 23:36

Most pilots never started the JFS inflight If they did it meant they were in a glider. The exception was during Functional Check Flights (FCF) where the JFS was tested. I flew hundreds of FCFs so I had a lot of diverse experience with the low tech jet engine which is the JFS. As Meteor said, my understanding was that the limitation was the JFS doors which extended laterally out into the airflow and sufficient airflow for the motor to run at higher altitudes. As I recall from the test cards (been a while) you leveled from the vertical climb about 15-16K feet at about 200 KCAS. I stayed in burner to accelerate and climb to 20K, 400 KCAS where I would start the JFS to make sure it started and ran at the limit speed and altitude. I never had one fail. Then you shut it down and climbed for the 40K foot checks. Interestingly I had heard during my FCF checkout prep that many times the FCF pilot forgot to turn the JFS off and found it still running at higher altitudes. I only did this once and it was still running climbing through about 25K but I never forgot it again. But I did start purposely leaving it on occasionally to try to find out when it would stop on its own (probably not a good idea as the speed limit was possibly for the structural integrity of the doors). I found it usually stopped not too long above 20K so I am convinced the 20K 400KCAS was a conservative limit to prevent the delay resulting from premature start attempt that would bleed the bottles and cause a delay in a reattempt. It takes a long time to recharge the JFS accumulators on windmilling RPM so no reason to try too soon and shoot your bolt...so to speak. At higher altitudes the engine RPM would decay much faster than low altitude so it took higher airspeed to insure sufficient RPM for restart but also to turn the hydraulic pumps enough to recharge the accumulators. So I believe 20K 400KCAS was not a hard ceiling but a parameter to insure a high probability of light off.
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weasel1962

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Unread post24 Oct 2019, 01:46

This was 1982.

https://apps.dtic.mil/dtic/tr/fulltext/u2/a129578.pdf

Of primary concern are starts at altitudes above 20,000 ft. where present jet fuel starters (altitude sensitive) are subject to rapid reductions in output power capability.


I suspect there was a problem with the accumulators powering the JFS above 20k ft. Not sure whether its still a problem today but once these parameters get stuck in the flight manuals.....
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outlaw162

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Unread post24 Oct 2019, 03:22

I would wonder if the JFS can increase engine RPM any more than it's already windmilling at 400 KCAS. Is 400 a max or a speed beyond which the JFS offers no advantage?

Generally with an engine failure, folks don't dive to 400, they pick up L/D max where decreased engine RPM might lend itself to a starter assist, with the appropriate air density.

caveat: I've never started the JFS in flight, so I defer to experience.
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evilrider

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Unread post07 Nov 2019, 16:41

Thanks guys !

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