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 post25 Jan 2023, 03:08

That 100% depends on what the Saudis wanted/ordered/paid for. It's possible physically, CG range is 23-27% notionally IIRC on the Eagle and if that is WITH ballast in the nose on the A and the EX has 1,000lb more weight in the engines it could be shifted to maybe 30%. But again, did the Saudis pay for that when they got the FBW? We don't know.
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Unread post25 Jan 2023, 11:17

henshao wrote:
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


On the F-15ACTIVE where the two canards were applied, the center of the lifting force moved forward, even if the weight is also had to move a little bit forward as well. Because of this additional aerodynamic surface, the stability went to slightly negative, so all the flaps and airelons were acting as a single large flaps, like on the Hornet:

http://www.airvectors.net/avf15_1_10.jpg

On the Hornet, the large, full wingspan flaps incorporate with the really long LERX, to balancing with the forward moved lifting force by their own "nose-heavy" torque effect.
If the EX would have a lighter nose section, while the engines are still heavier than the P&W engined airframes, the strutcure is going to be rear-heavy, so the stability is negative. If so, both of the flaps and airelons are needed to generate a larger downward torque, where the benefit is a larger lifting force in the meantime. So it could be a win-win situation, even if we do not have any additional canards at the front section.
I think, this could be compensate the lack of the leading edge flaps a little bit, so add some AoA room for the aircraft.

To keep the current wing, avoinding the extra weight and complexity of the leading edge flaps, maybe a sharper LERX also would be helpful. But in this case the sharper surface will be more sensitive to the asymmiterical design, which cased be the right side mounted cannon slot.
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