The CAS (Control Augmentation System on the F-15A/B/C/D) has the ability to control the aircraft using only electrical signals sensed from the stick. In this, it is the “FBW” capability of that system. Later F-15Es have digital CAS, which can already be considered a full FBW system.
It may be interesting to some that the original CAS was able to control the aircraft with more than 1% negative stability of the aircraft. Yes, the F-15 could be balanced for negative stability. To prevent this from happening, he carried weight ballast in the front.
One can even eyeball whether an aircraft is stable or not in its current state when it is performing a sustained high AOA maneuver. Ideally in a high AOA pass, to watch its elevator / stabilator deflection angle.
In a sustained high AOA flight, a statically stable aircraft would require a consistent deflection of the elevator all the way to maintain the AOA. Because of the negative Cm generated by a neutral elevator, the aircraft wants to return to low AOA upon the elevator returning to neutral.
Think of an F-15:
A statically neutral aircraft however, would only require an initial trailing-edge-up deflection to start pitching the nose, and then the elevator should be returned to a near neutral position, in order to maintain the AOA, because the Cm of a neutral elevator is also near zero at the current AOA.
Actually this is the same eyeballing method used in the previous F-16 stability report:
You can tell it’s a high alpha flight by the fully deflecting leading edge flaps / slats.
Now how about the Su-27. Can you find any video showing a deflection angle as large as the F-15, or is it similar to the F-16 in a sustained high AOA flight?
It’s very rare to see an alpha pass on an F-15, I haven’t found one yet. It also does not have a moving leading edge. But older versions are statically stable, QA/EX are different.
The F-15 images above are showing high alpha maneuvers performed by the QA at 2023 Dubai. By the looking of consistent large stabilator movements during the airshow, the aircraft seems to be statically stable.
Considering as to how the Su-27’s slats aren’t IRL on auto mode by default, I don’t think it relies on the same principle as the F-16 in sustained high AOA
The switch is defaultly guarded to ABT (Auto) position.
Управление положением носков крыла может осуществляться вручную и автоматически. При становке переключателя НОСКИ КРЫЛА в положение ВЫПУЩЕНЫ – носки в диапазоне ежимов до Vпр=860 км/ч отклоняются на угол 21°-25°, при положении УБРАНЫ – носки убираются.
В положении АВТ управление осуществляется автоматически, при этом в режиме ВЗЛЕТ-ОСАДКА носки отклонены на угол 23°, в режиме ПОЛЕТ отклонение носков производится в иапазоне углов отклонения носков (0-30°) при углах атаки 1°-15°.
A full deflection of the slats (30°) indicates an AOA of at least 15°.
i use the 27 with manual controll in sim mod and absolutly fine. damping not needed when u know how to fly whit the flankers. my problem the r73 cant hit the targets but i launch that in close range only under 800m…
It’s interesting to overlap the Cm curve with the NASA F-16 one. They form a similar shape.
The Cm curve of F-16 is at mach 0.6 with a designed center of gravity of 35% MAC. The TsAGI one has unknown conditions but I assume it’s at designed center of gravity too.
They are in the TsAGI paper, although they are not necessarily specific to the Su-27 but rather to unstable aircraft in general… presumably it is for an Su-27-esque aircraft as that was the design consensus at the time.
Look at where Cm=0 positions. It’s at around 72° AOA, which is more than the known Cm of F-16 (65°), F-18 (56°) or Jas-39 (65°). A modest Cm curve would’t allow a max attainable AOA of 100°.
I agree with MiG_23M that it is for an Su-27-esque aircraft.
Actually the static margin at low AOA, which most people like to refer to, doesn’t tell much as it is not the whole picture. But hey, you would need a wind tunnel for a whole picture. That’s why it’s fun to do speculations.
Rough translation:
Detection range for a fighter at max dry thrust with the clear sky in the background from the rear hemisphere with an aspect of 0/4 - 2/4 is about 50 km and 20-35 km with the clouds / sea / earth in the background. For a target in afterburner from the front hemisphere with aspect of 1/4 is 90 - 100 km. The locking range is about 70% of the detection range. The detection range for a full dry thrust target from the front at an angle of 5° - 15° is about 10 km. To ensure higher locking ranges from the front, increase the aspect angle above 15°.
Now, a few notes:
The fraction aspects represent the ratio of the visible length to the full length of the target, which means that the aspect angle is the inverse sine of that fraction.
Spoiler
That is, if we call \theta the aspect angle and R the aspect fraction (say, 2/4), then R = \sin (\theta) and \sin^{-1}(R) = \theta. Thus:
0/4 is 0°
1/4 is 14.48°
2/4 is 30°
3/4 is 48.59°
4/4 is 90°
For the IRST on the MiG-29, the detection ranges are used for detection and locking ranges in the game. (The manual gives a 15 km detection range for a MiG-21 flying side on), so I guess the detection ranges should be used to configure the ranges for the Su-27’s IRST.
But what is the “fighter” in the manual mentioned? I thought it could be the MiG-21 (bis?), since the MiG-29 manual uses it and the Su-27 is from a similar time frame. However, the F-15 (and sometimes the F-16) is often used for those kinds of things also (R-27T detection ranges, for example). So, what fighter is it?
There is an archived page from NIIP (the manufacturer) about the N001VEP (an upgrade of the N001 present on the Su-27S). It also has the ranges for the IRST, which is slightly upgraded from the one we have in the game. But if you look at the ranges, they match quite well. They also use quite odd aircraft for those tests: the Su-15 for rear aspect detection, and a MiG-25 flying above Mach 2.0 in afterburner for front aspect. This leads me to believe that the data is also relevant for the IRST we have in the game, as those tests seem to be using older aircraft and the ranges match the one in the Su-27SK manual.
Now, this is how the Cm curve of F-15 would look like considering it’s only able to stabilize at 35+ AOA (45+ units in cockpit). Also a downward slope means it has a positive static margin.
This the difference between a stable design and a relaxed stability design.
I’ll add a few details that may be of interest.
The Su 27 ( the original, not the new versions like the Su 35 etc) is electronically limited to 24AoA (soft limit) and above 28 AoA it is transversely undriveable. The F-16 has a hard limit of 25 or 28 AoA to avoid deep becoming region. The MiG 29 has positive stability but can cobra maneuver and the first version 9-12 had an AoA limit of 27 ( soft limit).
F-15 can’t cobra but can go above 30 AoA.
