F-15EX

Military aircraft - Post cold war aircraft, including for example B-2, Gripen, F-18E/F Super Hornet, Rafale, and Typhoon.
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sprstdlyscottsmn

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Unread post24 Jan 2023, 00:48

And to use the slick CFT of the F-15C would require a new flight test program. Flying without CFTs is part of the F-15E/SA/QA/EX/etc as that is a configuration found in the manual, and as we saw at LN can be done with munitions. The use of the slick CFT is not part of the configuration. I never said the EX will always have the draggy CFT, I said the draggy CFT is the only one it will have due to how it was procured. A CFT-less F-15EX being operated as an F-15C sounds like exactly what the ANG wants and will be a monster in that regime.
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Unread post24 Jan 2023, 04:41

basher54321 wrote:
allesmorobranna wrote:Back to the CFT question. If the EX test flights of that precious Fly by Wire was so strict, it just closed out any deviation from the current layout, it is not allows to apply a different engine, or the lack of CFTs, if the whole test envelope was only managed with the CFTs, why the ACC wants to get them without it?





The FBW and GE-129 engines were incorporated, certified, tested and paid for by Saudi Arabia as part of the F-15SA (first flight 2013).

The F-15QA was the next version of the SA.
The EX is basically the QA with a few mods.

This had to be done on the relative cheap with a fixed budget.

If for example another customer wants to pay for PW-229 engines in it then that could happen, or Boeing put up the money themselves. Fat chance it coming from the USAF budget unless things drastically change.

Regarding the DOTE report - it states that initial EX testing was done with CFTs. The reason the first batch wont have CFTs is as it says because enough CFTs have not been procured and provisioned yet. So it might get CFTs further down the line.


The first couple of F-15EXs are actually F-15QAs with a couple minor changes for the USAF.
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allesmorobranna

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Unread post24 Jan 2023, 10:14

sprstdlyscottsmn wrote:And to use the slick CFT of the F-15C would require a new flight test program. Flying without CFTs is part of the F-15E/SA/QA/EX/etc as that is a configuration found in the manual, and as we saw at LN can be done with munitions. The use of the slick CFT is not part of the configuration. I never said the EX will always have the draggy CFT, I said the draggy CFT is the only one it will have due to how it was procured. A CFT-less F-15EX being operated as an F-15C sounds like exactly what the ANG wants and will be a monster in that regime.


All right, I understand.
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zero-one

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Unread post24 Jan 2023, 13:57

sprstdlyscottsmn wrote:A CFT-less F-15EX being operated as an F-15C sounds like exactly what the ANG wants and will be a monster in that regime.


What kind of advantages will the 129 bring to the F-15 airframe? will it bring the F-15EX back to the F-15A levels of performance? I remember you saying that the F-15A was the absolute best in terms of turn rates or basically all dogfighting relevant performance. Or will the naked EX surpass the A models?

Anyway, the only advantage of the CFT less Eagles is performance, and the F-22 already gives the USAF all the performance it wants. So I don't see the USAF wanting it, they'll find a long range, heavy weapons truck more useful. But I can see why the ANG could probably want it.
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Unread post24 Jan 2023, 15:17

zero-one wrote:What kind of advantages will the 129 bring to the F-15 airframe? will it bring the F-15EX back to the F-15A levels of performance? I remember you saying that the F-15A was the absolute best in terms of turn rates or basically all dogfighting relevant performance. Or will the naked EX surpass the A models?

Anyway, the only advantage of the CFT less Eagles is performance, and the F-22 already gives the USAF all the performance it wants. So I don't see the USAF wanting it, they'll find a long range, heavy weapons truck more useful. But I can see why the ANG could probably want it.


The F-15EX will smoke the A in acceleration and climb, but it is still heavier so it will not do as well in ITR turns. STR turns is harder to say because the 129 will bring a lot of extra power over the F100-PW-100 of the A, but the added weight is squared in terms of induced drag added. For example, an F-15A with 8AAMs and 69% fuel (to get a total fuel fraction of 20%) weighs ~41,200lb. The empty weight of the F-15EX without CFT is 6,000lbs higher (34,000 vs 28,000) for a 14.6% increase with the same payload. So for a given q (dynamic pressure) and n (G-load) the EX has 31.2% extra induced drag. It is likely higher as the way the same n is reached at the same q for the higher weight is more Cl from more AoA which lowers the e (Oswald's efficiency factor) value. Form drag will be largely the same but for these STR conditions it will account for about half the drag at peak STR (AoA for L/D max gives max lift for a given value of thrust/drag) and less than half at lower speeds. So the GE-129 needs to produce on the order of 16-25% more dynamic thrust than the PW-100 to match the F-15A STR depending on speed range.

EDIT* Forgot to answer part two. You are bang on there because once it was shoved into the budget the USAF wanted to two seater as a new Strike Eagle and the ANG wanted the single seater for air-defense
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Unread post24 Jan 2023, 15:22

Perhaps due to a lack of funds, the ANG could very well wind up with a CFT-less F-15EX. De-facto, it would almost certainly be an absolute monster once the tanks get punched. Remember, F-15EX test pilots at Northern Edge (presumably flying with CFT's.. never seen a pic without them) got "higher and faster" than F-15C's (who almost never carry them). That says a lot IMO.

We'll probably never know the true configuration of either the EX or the C's in that exercise, but it's a reasonable assumption. I know if I was sent overseas I'd want the extra oomph. Stateside, I'll take the CFT's all day long. More time on station, no worries about RCS increasing and still can get to where I need to fast (Mach 1.4, with weapons).

But an F-35 would still be better. We need to buy them in droves IMO, at least until NGAD is on the ramp.
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Unread post24 Jan 2023, 16:55

A lot of missiles on board is wonderful. But another problem arises. As it is known from the German F-35 tranche, it is planned to buy for one aircraft: 3 AIM-120C-8 AMRAAM and 2.14 AIM-9X Block II+
Why does a fighter need 12 suspension points if it has only five missiles?
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Unread post24 Jan 2023, 19:22

sprstdlyscottsmn wrote:
zero-one wrote:What kind of advantages will the 129 bring to the F-15 airframe? will it bring the F-15EX back to the F-15A levels of performance? I remember you saying that the F-15A was the absolute best in terms of turn rates or basically all dogfighting relevant performance. Or will the naked EX surpass the A models?

Anyway, the only advantage of the CFT less Eagles is performance, and the F-22 already gives the USAF all the performance it wants. So I don't see the USAF wanting it, they'll find a long range, heavy weapons truck more useful. But I can see why the ANG could probably want it.


The F-15EX will smoke the A in acceleration and climb, but it is still heavier so it will not do as well in ITR turns. STR turns is harder to say because the 129 will bring a lot of extra power over the F100-PW-100 of the A, but the added weight is squared in terms of induced drag added. For example, an F-15A with 8AAMs and 69% fuel (to get a total fuel fraction of 20%) weighs ~41,200lb. The empty weight of the F-15EX without CFT is 6,000lbs higher (34,000 vs 28,000) for a 14.6% increase with the same payload. So for a given q (dynamic pressure) and n (G-load) the EX has 31.2% extra induced drag. It is likely higher as the way the same n is reached at the same q for the higher weight is more Cl from more AoA which lowers the e (Oswald's efficiency factor) value. Form drag will be largely the same but for these STR conditions it will account for about half the drag at peak STR (AoA for L/D max gives max lift for a given value of thrust/drag) and less than half at lower speeds. So the GE-129 needs to produce on the order of 16-25% more dynamic thrust than the PW-100 to match the F-15A STR depending on speed range.

EDIT* Forgot to answer part two. You are bang on there because once it was shoved into the budget the USAF wanted to two seater as a new Strike Eagle and the ANG wanted the single seater for air-defense


From low level maneuvering standpoint the PW229 equipped Strike Eagle is also very agile at low level, airshow environment, if the CFTs are off. Th PW229 has a same static thrust preformance, while the dynamic is a different animal, even if Pratt&Whitney charts shows the same. Based on what the EX pilots said. Maybe I just missed the point and the significant difference is only comes with the speed and altitude.
But since the EX is even more heavier than the E model, the STR or especially the low speed, the high AoA maneuvering characteristic could be not so good, since the low speed thrust differnece between the PW229 vs GE129 is not so high.
The only thing what would be at the EX side is the FbW, which is able to let some room at the high AoA limits for the pilots.
When the first F-15EX 001 did the first flight, the Boeing test pilots managed some zero speed post stall maneuvering, when the fuel level let it them.
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Unread post24 Jan 2023, 20:25

I agree, but the question was would it match the light weight A model in turning, which as I showcased is impossible for ITR and difficult in STR. The Eagle was always capable at low speed and High AoA but it took skill to wring it out, the FBW will make that easy like a Hornet.
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Unread post24 Jan 2023, 20:49

the real advantage of the EX will be maneuvering in the high altitude/supersonic regime because, as mentioned "look how they massacred my boy" a 35000lb empty weight jet won't hang with a 27000lb empty weight jet at the ragged edge of maneuverability. however when you compare for instance the ability to sustain 5g at mach 1.6 between an F-15C and a -229 F-15E without the bomb racks, and then imagine how much better the -129 Eagle will be, it's kind of eye watering
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Unread post24 Jan 2023, 21:06

When compared to level flight the "excess thrust" available can be thought of as turning drag available. There are certainly regimes where a 20% increase in net thrust can be a 100% increase in excess thrust from level flight. Taken with my previous statement about needing 16-25% more thrust becomes less relevant at the upper and right edges of the envelope where a PW-100 has almost no excess thrust, where induced drag is still secondary to form drag (AoA lower than L/D max, or in regions where wave drag gets factored in as well))
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Unread post24 Jan 2023, 21:24

Also the Full fly by wire of the EX may finally allow the jet to be weight and balanced tail heavy/negative stability, with according advantages in turn performance as those big stabilators start adding net lift in cooperation with the nodding intake quasi-canards
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Unread post24 Jan 2023, 23:04

paralay wrote:A lot of missiles on board is wonderful. But another problem arises. As it is known from the German F-35 tranche, it is planned to buy for one aircraft: 3 AIM-120C-8 AMRAAM and 2.14 AIM-9X Block II+
Why does a fighter need 12 suspension points if it has only five missiles?

The Lot 4 F-35s will carry 6 AIM-120 internally, and 2 AIM-9X externally (situation dependent.)
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Unread post25 Jan 2023, 00:15

henshao wrote:Also the Full fly by wire of the EX may finally allow the jet to be weight and balanced tail heavy/negative stability, with according advantages in turn performance as those big stabilators start adding net lift in cooperation with the nodding intake quasi-canards

Do we know if the the EX (or earlier variants with digital flight controls) has or is allowed to have a more aft CG? I know there there is several hundred lbs of ballast on the front bulkhead that can be adjusted to get individual F-15C/D aircraft within the Weight & Balance limits. And the F110-129 engines are heavier than either the -220 or -229 engines. Are these weights sufficient to make the EX a relaxed static stability aircraft?
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Unread post25 Jan 2023, 01:50

f119doctor wrote:
henshao wrote:Also the Full fly by wire of the EX may finally allow the jet to be weight and balanced tail heavy/negative stability, with according advantages in turn performance as those big stabilators start adding net lift in cooperation with the nodding intake quasi-canards

Do we know if the the EX (or earlier variants with digital flight controls) has or is allowed to have a more aft CG? I know there there is several hundred lbs of ballast on the front bulkhead that can be adjusted to get individual F-15C/D aircraft within the Weight & Balance limits. And the F110-129 engines are heavier than either the -220 or -229 engines. Are these weights sufficient to make the EX a relaxed static stability aircraft?


Technically, the F-15 is already "relaxed" stability, but as far as an outright aft-cg allowed I do not know, only hypothesize. As I mentioned the F-15 was partial fly by wire from the very first F-15A, only now is it full FBW (during testing one Eagle made it home with several mechanical control lines severed; the pilot never even noticed) It would be cool if this allowed negative stability but there may be other limiting factors

There is also the matter of the tremendous AESA weight in the nose
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