[Léon] ::
Mozes li nam reci nesto vise o tome sto te slike predstavljaju odnosno o pozitivnim stranam bas takvog "vorteksa"?
Regarding instantaneous turn rate.
TVC can actually help in this regard because there is turn rate enhancement at speeds below corner velocity. Above the corner velocity the aircraft will be load factor limited, and no advantage from TVC will be gained.
But another advantage of TVC is added stability/controllability above standard FBW limits. For example, Su-27 is limited to about 24° AoA but its wings produce Clmax at 28° AoA. With stability provided through thrust vectoring, FBW restriction is eliminated maximizing inherent aircraft aerodynamics. Because of that the plane can produce more lift at given speed which will in turn increase max instantaneous turn rate and reduce turn radius.
Now, instantaneous turn performance is a function of the lift capability and the structural strength of the airframe and demands high CLmax and low wing loading for attaining high load factors. Neglecting thrust effects, the load factor is a function of lift coefficient, dynamic pressure, and wing loading. The maximum lift coefficient is limited by aerodynamic stall, or any of a number of adverse flying qualities.
In other words high aerodynamic efficiency and low wing loading are of essence for high instantaneous turn rates.
Base Su-27 is very good example of great aerodynamic efficiency. Compared to F-15C (PW220 engined) it has higher wing loading and lower T/W ratio, but at the sea level (clean and with 50% of fuel) they can both pull 21°/s at about 575 km/h (flight manual data).
Only difference is that Su-27, while carrying almost 1700kg of fuel more than F-15C, can sustain that turn rate and F-15 is having negative SEP (he is losing speed during the turn). For the same speed F-15 can pull only 18°/s in order to have sustained turn rate. If they were to have the same range for more objective comparison, the difference would be even greater in Su-27`s favor.
We can see that even with somewhat worse wing loading and T/W ratio Su-27 is superior to F-15C because it has much better L/D ratio (subsonic conditions) which in turn will offer better SEP (note, Su-27 has enormous 9°/s ITR advatage compared to F-15C at the sea level when both are going 575 km/h).
We can also take example of F-16 when talking about wing loading and its effect it has on instantaneous turn rate.
F-16 block30 vs F-16 block50 sea level, both clean with 50% of fuel (F-16 block50 has greater empty weight = higher wing loading)
At Mach 0,4 F-16 block50 can pull close to 4,9G and about 20,5°/s.
At Mach 0,4 F-16 block30 can pull close to 5,5G and about 23,4°/s.
Both will have about the same negative SEP at that speed but F-16 block30 can also pull about 21,6°/s (at the same speed) and he can sustain that turn rate, or he can pull the same 20,5°/s as block50 but it will have positive SEP (he has power reserve to accelerate in a turn).
We can see that aerodynamically almost identicall planes have very different performance because of different wing loading. (note, Su-27 has about 7,6°/s max. ITR advantage compared to block50 F-16, which is huge!)
Now, according to Sukhoi chief test pilot PAK FA has much better turn performance, acceleration, climb rate etc. than clean Su-27 and Su-35S. Sukhoi engineers worked very hard on PAK FA to achieve better aerodynamic efficiency, lower wing loading and higher T/W ratio.
The thing that stands out beside TVC are LEVCON`s.
LEVCON`s are automatically trimmed to give optimal lift-to-drag (L/D) ratios for all Cg positions, Mach number and AOA. They can also increase AoA (generating more lift in front of Cg) during take off and can be used asymmetrically at extremely high AoA`s for directional control. In the case of TVC malfunction they can be used for pitch/yaw control at extremely high AoA.
PAK FA has highly relaxed longitudinal stability which is good for better trans-sonic lift to drag characteristics (L/D), supersonic L/D capabilities, low-speed lift characteristics and take-off and landing characteristics.
But highly relaxed longitudinal stability demands high pitch down control capabilities (specially at high AOA) and because positive load pitch down control surfaces (such as vertical stabilizers that are placed behind CG) tend to saturate under high AOA conditions, using negative load pitch down control surfaces (such as LEVCON that is placed in front of CG) is more effective.
The other thing is directional stability and controllability.
PAK FA has very small vertical stabilizers (decreased RCS, drag and weight), because relaxing static yaw stability and applying all moving rudders in combination with LEVCON and TVC can give very good results.
At high AOA vertical tails and rudders are placed in the low energy wake of the wing and fuselage. At certain AOA deflecting LEVCON`s can postpone creation of fuselage wake so the rudders can maintain effective. At some point (when further increasing AOA) low energy wake will form so the rudders become ineffective. Using asymmetrical LEVCON deflection can produce rolling or yawing moments at high AOA.
At high AOA that would cause controlled wind-axis roll. One more thing is LERX vortex. Asymmetric LEVCON deflection will create differential/asymmetric vortex pressure distribution which in turn will cause additional yawing moments. In other words we will have controlled differential vortex pressure points. At high AOA`s asymmetric LEVCON deflection in combination with other control devices such as TVC can increase stability/controllability and yawing moment/rate and can generate direct side force (extreme maneuverability). And in case of TVC malfunction LEVCON represents an alternate form of pitch/yaw control that can provide redundancy.
LEVCON`s can also increase instantaneous turn rate not just by providing optimal lift-to-drag conditions but also by delaying wing stall.
LEVCON can also be observed as moving part in front of the LERX. LERX generates a vortex that attaches to the top of the wing. The vortex action maintains a smooth airflow over the wing surface well past the normal stall point at which the airflow would otherwise break up. At some critical AoA point vortex eventually bursts and stall occurs.
Deflecting LEVCON`s are "feeding" LERX with a smooth airflow well beyond vortex burst point. That way the plane can increase AoA even more without stall and can generate more lift.
That can be observed nicely in this video at 8:48 sec. where PAK FA is doing high AoA loop. We can see extremely powerful vortex formation at very high angle of attack. In similar conditions Su-27 would stall its wings because of vortex burst.
So, on one side we have PAK FA (that is superior to both Su-35S and Su-27) with very efficient aerodynamics, TVC, LEVCON, low wing loading, high T/R ratio and on the other side we have small but heavy F-35 with very high wing loading, lower T/W ratio that is struggling to come close to clean F-16 block50 turn performance.
Do you think that they can have even remotely close instantaneous turn rates?
I don`t think so!
Nadam se da razumes engleski.
Ovo je jedan deo teksta sa "F-16.net" foruma gde sam pisao na ovu temu.
wow
nisam dosad video ovo..verovatno mi je promaklo..
pogledajte na 0:57 kako na vrhu manevra zvono dodaje gas i ostaje u toj visini..
ili mi se tako cini
