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Posts Tagged ‘maneuverability’

F-35 reality check

Posted by picard578 on July 16, 2015

CLAIM: F-35 can supercruise

Incorrect. F-35 can achieve and maintain speeds just above M 1 though usage of minimum afterburner. “Supercruise” claim can be discounted by comparing the F-35A with F-16A.

F-16A has a 40* wing sweep with a laminary wing profile designed for supersonic flight. Its engine has a frontal area of 6.082 cm2 while providing 64,9 kN dry (uninstalled) thrust, giving 10,67 N/cm2. F-16A also has wing loading of 338,5 kg/m2 at combat weight, span loading of 947,2 kg/m and TWR of 0,7 at combat weight and dry thrust.

F-35A has a 33* wing sweep with a supercritical wing profile designed for transonic flight. It also has frontal area about as large as the F-18s. Its engine has a frontal area of 10.715 cm2 while providing 124,5 kN dry thrust, giving 11,62 N/cm2. F-35A has wing loading of 427,9 kg/m2 at combat weight, span loading of 1.698 kg/m and TWR of 0,7 at combat weight and dry thrust. Read the rest of this entry »

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Posted in weapons | Tagged: , , , , , , | 17 Comments »

Usefulness of thrust vectoring

Posted by picard578 on April 13, 2013

With introduction of thrust-vectoring F-22 and Su-35, many claims have appeared, such as that thrust vectoring aircraft are most maneuverable in the world and that addition of thrust vectoring alone guarantees that fighter in question will be unrivalled in maneuverability, excepting of course other thrust vectoring aircraft. These claims hold that addition of thrust vectoring by itself is enough to turn otherwise-sluggish fighter aircraft into supreme air-to-air machine. Things are more complex than that, however; effectiveness of thrust vectoring depends on aircraft’s aerodynamic configuration, speed and altitude.

To discuss thrust vectoring, we must first know how non-TVC aircraft behave. Major parameters that impact aircraft’s performance are:

  1. weight
  2. lift, which can be approximated through wing loading
  3. excess thrust, determined by thrust to weight ratio
  4. drag

One of advantages of thrust vectoring is allowing aircraft to enter and recover from a controlled flat spin, yawing aircraft without worrying about rudder, which looses effectiveness at high angles of attack. However, aircraft using close coupled canards instead of thrust vectoring have also demonstrated flat spin recovery capability, example being Saab Gripen. But while thrust vectoring reduces drag during level flight, thus increasing the range, close-coupled canards add drag and decrease lift unless aircraft is turning, thus improving the range.

But to see what impact thrust vectoring has on combat performance, we have to take a look at parameters I have defined above. Mass of aircraft determines inertia – thus, heavier the aircraft is, longer it takes to switch from one maneuver to another quickly. This results in slower transients, making it harder for pilot to get inside opponent’s OODA loop – in fact, mass is defined as a quantitative measure of an object’s resistance to acceleration (to clear common mistake in terminology, acceleration can be in any direction – in fact, what is commonly called “deceleration” is mathematically defined as “acceleration”). But to actually turn, aircraft relies on lift. Lift is what allows aircraft to remain in the air, and when turning, aircraft uses control surfaces to change direction in which lift is acting, resulting in aircraft turning around imaginary point. It can be approximated by wing loading. But turning leads to increase in angle between air flow around the aircraft and the aircraft itself (this angle is called Angle of Attack), which results in increased drag. Increasing drag means that aircraft looses energy faster, and once fighter’s level of energy decays below that of his opponent, he is fighting at disadvantage. Loss in energy can be mitigated by excess thrust, which can also be used (usually in combination with gravity, aka downwards flight) to recover lost energy. All of this leads to expression “out of ideas, energy and altitude”, which basically means “I’m in trouble and have no way out”. Nose pointing allows aircraft to gain a shot at opponent with gun, and was crucial for gaining a shot at opponent with missiles before advent of High Off Bore capability, which shifted requirements more in direction of ability to sustain maneuvers at or near corner speed (minimum speed at which aircraft can achieve maximum g loading; it is usually around M 0,6 – 0,9). It must be noted that, while lift and excess thrust of aircraft can be approximated by wing loading and thrust to weight ratio, heavier aircraft will require higher thrust to weight and lift to weight ratios to achieve same turn rates as lighter aircraft.

Thrust vectoring, as its name says, results in shifting of the thrust. Due to modern fighter aircraft’s center of gravity and center of lift never being behind its nozzles, shift in thrust results in aircraft rotating around its center of gravity, resulting in massive increase in Angle of Attack. Thus, comparision non-TVC aircraft turning and TVC-equipped aircraft turning would look like this:

TVC

This is result of forces described above acting on aircraft. In this model, assumption is that aircraft can reach angle of attack required for maximum lift both with and without thrust vectoring, which is true for all close-coupled-canard aircraft, but not necessarily for tailed and long-arm canard arrangements.

Thus, forces impacting turn ability of non-TVC and TVC aircraft would look like this:

forcesforces_resultants

It can be seen that thrust vectoring increases angle of attack, and thus drag (as entire airframe at high AoA drags far more than just control surfaces plus airframe at far lower AoA), while reducing thrust avaliable to counter the drag – and, in case of very high AoA values, lift avaliable to pull aircraft around. While TVC can improve turn rate even at combat speeds, it happens only if aircraft is unable to achieve angle of attack that is required for maximum lift, one example being F-16, which requires 32 degrees AoA for maximum lift but is restricted to 25,5 degrees by FCS due to departure concerns. Angles of attack in excess of 35 degrees are unsustainable, however, due to massive drag they cause, resulting in very large energy loss, turning fighter into a deadweight in very short order. “Benefit” of extreme AoA values is also not unique to thrust vectoring aircraft: while TVC-equipped X-31 achieved maximum controllable angles of attack of 70 degrees (compare to 60 degrees for another TVC design, F-22), whereas close-coupled-canard delta-wing Rafale and Gripen are able to achieve controllable Angles of Attack that exceed 100 degrees, with Gripen being able to sustain Angle of Attack of 70 – 80 degrees. Further, X-31 without TVC was unable to achieve more than 30 degrees of alpha, even momentarily, whereas without TVC F-22 is limited to 26 degrees, though not due to issues of lift but rather controllability. As such, TVC actually improved instanteneous (and possibly sustained) turn rates of both aircraft by allowing them to reach angle of attack required for maximum lift, which is between 30 and 40 degrees of AoA. Aircraft that use TVC during combat to achieve angles of attack beyond lifting capability of wing actually sink in the air, as opposed to turning, but if they are unable to achieve maximum lift capability without TVC, then TVC does indeed improve their turn capability. Close-coupled canard configuration, on the other hand, drags less in turning than TVC one as it achieves same lift at lower angle of attack, resulting in far lower fuel consumption. This is important as in visual-range fight, most kills have been historically made when one of aircraft fighting ran out of fuel; thus aircraft with less fuel consumption per unit of weight is (assuming similar fuel fraction) more likely to win the fight. Specifically, maximum lift for close-coupled canard is greater than that for just wing at any AoA past 10 degrees AoA; in configuration analyzed in this thesis, lift is greater than baseline value by 3,4% at 10 degrees AoA, 34% at 22 degrees AoA, 9,4% at 34 degrees AoA, 7,2% at 40 degrees AoA and 18,3% at 48 degrees AoA. Thus aircraft does not need to achieve as high AoA for same lift to weight and lift to drag values, consequently allowing pilot a choice (assuming other values are similar) wether to achieve same turn rate as opponent and outlast it due to using up fuel far slower than it is case with fuel-hungry thrust vectoring maneuvers or try to outmaneuver it with higher turn rate.

Neither is main “benefit” of thrust vectoring, post stall maneuverability, anything new. Aside from close-coupled canard designs, which have extensive post-stall maneuverability, Russian Su-27 has demonstrated stall recovery capability and post-stall maneuverability. It is also important to note that John Boyd was able to do Cobra in F-100, and other pilots did it in J-35 Draken. While TVC certainly improves post-stall capability, capability by itself is useless in multi-bogey scenario, as it bleeds energy very fast. As such, thrust vectoring is tactically useless for most fighter aircraft, especially in age of high-off bore missiles, as usage of thrust vectoring would leave then slow-moving aircraft very vulnerable. Further, Cobra – one of main “poster maneuvers” for TVC – is easy to see in advance, and if done, leaves fighter without energy and at opponent’s mercy; so while usage of TVC may surprise pilots that do not know what it allows, it is suicide agains pilots that are aware of it.

TVC does not necessarily increase security either, as resistance to departure and superstall which it provides are inherent advantages of close-coupled canard designs. However, it does allow non-close coupled canard configurations to recover from these conditions.

Using TVC for maneuvering is beneficial for tailed aircraft, however, at two regimes: at velocities well below corner speed, and during supersonic flight at high altitudes. Simple reason for that is that in these two regimes, flight surfaces are not very effective. At very low speeds (150 knots – M 0,23 – and below), large control surfaces’ deflections are required for turning due to weak air flow, thus increasing drag – and even when surfaces are fully deflected, aircraft responds comparatively slowly. This also includes takeoff and landing; as result, aircraft with thrust vectoring can take off and land at lower speeds and in shorter distance than same aircraft without thrust vectoring; this capability can be useful if parts of air strip have been bombed (though it is always smarter not to require air strip at all). During supersonic flight, tail finds itself in wake behind the wing, which reduces its effectiveness. Thus thrust vectoring can be used to compensate for this effect. Further, at high altitudes (12 000 to 15 000 meters) aerodynamic control surfaces are less effective, and there is less drag, which means that thrust vectoring provides greater benefits and less penalties. As dogfights happen at altitudes of 1 500 to 10 000 meters, and speeds that start in transonic range, thrust vectoring is obviously not effective for WVR – and, therefore, real world combat.

In level flight, thrust vectoring allows for trimming, thus increasing range due to reduced drag. 3D TVC nozzles can also reduce drag by optimising their shape. Further, thrust vectoring can add STOL capability to otherwise-CTOL aircraft, but it is always better to look at simpler, lighter and cheaper options. If aircraft lacks roll authority, TVC can be used for pitch, freeing up tail control surfaces to improve roll rate – examples of this are F-22 and Eurofighter Typhoon.

TVC (especially of 3D variety) can also provide ability to quickly point nose in a certain direction, but this is only useful in one-on-one gun-only dogfights (which do not happen in real world) as it leaves aircraft with seriously depleted energy and thus vulnerable to opponent’s wingman, and/or its target if attack was not successful. This is especially problematic in age when HOB capability is becoming increasingly common. But even in such unrealistic dogfights, TVC does not garantee victory. In upper set of images, F-22 is seen from Rafale, pulling a turn; OSF is clearly visible, showing that Rafale’s nose is pointed towards the F-22 (allegedly, Rafale won 2 out of 7 engagements; further, while IRST does have high off-bore capability, video camera is fixed):

rafale F22

F-22 does have major advantage in thrust-to-weight ratio over Rafale, however, allowing it to recover some of energy lost through TVC usage simply by flying straight and level for short time. But against aircraft with higher thrust-to-weight ratio, TVC usage will be even more problematic.

As for air shows, in this video, at 0:50, MiG-29 can be seen doing Cobra:

http://www.youtube.com/watch?v=ra3sr4HqF3E

At 1:54 and later, several S-35 Drakens can be seen doing Cobra:

http://www.youtube.com/watch?v=jqiDEcfSnXs

Reason is simple: while TVC-aircraft relies on TVC to provide both lift and forward motion, close coupled canards allow for lift production beyond 100 degrees of alpha, while forward motion is provided by inertia. Energy loss is high, but so it is with thrust vectoring, and neither version of Cobra has any real tactical application.

Edit 19. 6. 2013.

Here is link to a video recording of DACT from which upper row of screenshots in image comes (thanks to Jeneso):
http://www.youtube.com/watch?feature=player_embedded&v=oGuWadoTgkE

Another one (just recording):

www.youtube.com/watch?v=B4rNPouCggk&feature=player_embedded

Edit 22. 8. 2013.

mwuzi1

Posted in Uncategorized | Tagged: , , , , , , | 25 Comments »

Is the F-22 really superior to all other fighter aircraft

Posted by picard578 on December 1, 2012

USAF often touts F-22 as being the best fighter aircraft in the world. Is that really so? What are requirements for a good fighter aircraft?

By analyzing past wars we can see that following requirements have never changed:

  1. high agility at dogfighting speeds (currently in the medium subsonic to transsonic regime)
  2. superior situational awareness
  3. low cost
  4. high sortie rate
  5. capability to convert any split-second opportunity to the kill

High agility requires good acceleration, good turn rate, low energy loss and quick transients. Good acceleration and low energy loss require high thrust-to-weight ratio and low drag; good turn rate requires low wing loading, and quick transients require both. Energy state is important for gaining positional advantage and evading missiles.

Superior situational awareness requires not only having good situational awareness yourself, but denying it to the opponent. These requirements can only be met through use of passive sensors.

Low cost and high sortie rate are required for establishing a crucial numerical superiority over the opponent. Both are achieved by making the design as simple as possible.

Capability to convert any split-second opportunity to the kill is crucial in the dogfight, especially if multiple aircraft are involved on both sides, as it allows pilot to deny opponent the opportunity to reverse positional advantage, and allows him to kill more targets in the same timeframe.

With standard loadout of 50% fuel, 2 Sidewinder, 4 AMRAAM, F-22 has wing loading of 313,5 kg/m2 and thrust-to-weight ratio of 1,29. For comparision, with same loadout, Eurofighter Typhoon has wing loading of 284 kg/m2 and thrust-to-weight ratio of 1,28; Dassault Rafale’s values are 276 kg/m2 and 1,22. Su-27s values are 324 kg/m2 and 1,24. Thus, F-22 is inferior in wing loading to both Eurocanards, and has only slightly superior thrust-to-weight ratio compared to Typhoon. It is also only slightly superior to the Su-27 in wing loading, and somewhat more in thrust-to-weight ratio.

As such, it has slightly better turn rates than Su-27, and worse turn rates than Eurocanards. Its large weight will make it more difficult to F-22 to make transit from one turn to another, and its thrust vectoring will, if used, cause major energy losses. More about that later.

As mentioned, superior situational awareness requires not only having good situational awareness yourself, but denying it to the opponent. What this means is that aircraft must be capable of detecting and identifying the enemy completely passively. Currently, IRST and optical sensors are only types of sensors, except for Mk 1 eyeball, to posses such capability. F-22 lacks both, and as such has to either have an uplink to another platform – and such uplink can be detected and jammed – or to carry out both tasks World War II style, with pilot doing detection and identification visually. While F-22 was supposed to have FLIR, it was deleted as the cost-saving measure, and there are no plans to fit it.

Moreover, while some measures have been taken to reduce F-22s thermal signature, no major reduction was (or could have been) achieved, especially from the front. F-22 is also very large, increasing its detectability by the IRST. Thus, F-22 will be easily detected at ranges exceeding 80 kilometers by opponent using QWIP IRST.

Modern heat-seeking missiles also do not have to rely on engine exhaust for locking on the enemy aircraft, but can rather lock on to aircraft itself.

F-22 also isn’t undetectable to the modern radar, despite what some accounts say. While F-22s RCS of 0,0001 and 0,0014 m2 reduces detection range considerably, Typhoon’s radar (which has detection range of 185 km against 1m2 target) can detect it from distance of 18 to 35 kilometers. On the other hand, modern RWRs can detect LPI radars from ranges two or three times greater than such radars can detect target with RCS of 1 m2 at, thus making any use of radar an unwise course of action for F-22 (and any other fighter aircraft).

Low cost and high sortie rate are where F-22 feels least at home. Its flyaway cost is 250 million USD per unit, which is twice (205%) the flyaway cost of the most expensive non-VLO fighter aircraft – Eurofighter Typhoon – and has maintenance downtime of 45 hours per hour of flight, compared to the 8* hours for Rafale, 9* for Typhoon, 10 for Gripen and 19 for the now-ancient F-16 (* have to be confirmed). However, flyaway costs of these fighters, which are, respectively, 33%, 49%, 16% and 11-24% of F-22s, mean that it will be at 10:1 numerical disadvantage compared to Typhoon, and 26:1 disadvantage against Gripen.

F-22 is also incapable of converting split-second opportunities into kills. Reason for that is the fact that it carries all its armaments internally. It takes around half the second for gun doors to open; for missile bay doors it takes at least that much, and possibly more. Worse, Sidewinders it will be using in visual range dogfight are not simply ejected into air, but have to be lowered by mechanism; however, it is possible that such action will be performed while doors open.

Gun itself is the Gattling design. It offers maximum rate of fire of 6 600 rpm (110 rps), compared to 1 700 rpm (28 rps) for BK-27 used in Typhoon and Gripen, and 2 500 rpm (42 rps) for GIAT-30 used in Rafale. However, firing rate alone cannot be used as a measure of effectiveness.

First, Gattling gun takes some time to achieve full firing rate. While M-61A2 takes 0,25 seconds to spin up to its full firing rate, fact that F-22 has to open bay doors to fire increases that time to 0,75 seconds. For revolver cannon, time is 0,05 seconds. Thus, in first second, F-22 will have fired either 13 or 68 rounds (depending on wether gun doors were opened before or after press on trigger); Typhoon would have fired 27 rounds in the same time, and Rafale 40 rounds.

Second, aircraft now are highly resistant. Thus, per-hit damage and weight fired may be more important than number of projectiles. At projectile weight of 100 g for M-61, 260 g for Typhoon and 244 – 270 g for Rafale, F-22 fares worst in per-hit damage category. For total damage, in first second F-22 will have fired 1,3 to 6,8 kg, Typhoon 7 kg and Rafale 9,8 to 10,8 kg of ammunition.

Third, rotation of gun barrels creates vibrations, which means that Gattling design will be less accurate (more spread) than single-barreled designs, and problem will only increase as gun keeps firing.

While F-22 is supposed to kill opponent at BVR, it only carries 6 BVR missiles. With usual 0,08 Pk ratio against same-era threats, it will take two F-22s to kill a single enemy aircraft. That is made even worse by the fact that F-22 not only has to radiate in order to lock on the enemy aircraft, but has to get close enough to penetrate any jamming – distance that was regularly around 1/3 of maximum radar range; in F-22s case, it will be 50 – 80 kilometers against 1 m2 target, such as Typhoon or aircraft with comparable frontal RCS (J-10?) in air-to-air configuration.

F-22s maximum speed of Mach 1,8 – 2,25 and supercruise speed of Mach 1,5 – 1,7 are better than those of most competitors, as Eurofighter Typhoon – the second-fastest supercruiser – can achieve “only” Mach 1,3 when in combat configuration. Thus, F-22 can choose to run if it finds itself outnumbered too much, but if it does choose to attack, it will most likely be forced to engage the opponent in the visual range.

How maneuverable F-22 is

Many say that F-22 is the most maneuverable fighter aircraft by virtue of its thrust vectoring. So, I have decided to take a closer look at various claims about F-22s agility.

F-22 is the most maneuverable fighter aircraft out there

Some claim that F-22 is the most maneuverable and agile fighter aircraft out there, due to the thrust vectoring. That claim, however, is false.

To execute a turn, aircraft requires lift to pull it around the turn. Even civilian jets make sharper turns this way, by banking. Amount of lift can be roughly estimated through wing loading figures, with the caveat that LEX and close-coupled canards do provide the additional lift during high-alpha maneuvers by strengthening vortices created by the wing.

However, while F-22 does have LEX, it is not the only one. Dassault Rafale has both LEX and close-coupled canards, Saab Gripen has close-coupled canards, and Eurofighter Typhoon, while not having either, does have vortex generators at sides of the fuselage.

Thus, actual lift at high AoA could be estimated by comparing length of forward portion of the wing to the aircraft’s weight. This method is only of limited accuracy, however, it is more accurate than standard wing loading figures for high alpha maneuvers, as large portion of wing stalls in such circumstances.

F-22 has combat weight of 24 883 kg and combined wing leading edge length of cca 12,58 meters, which becomes 20,56 meters when LEX and air intake leading surface are taken into account. Thus loading value will be 1210 kg per meter. However, LEX-generated vortices will improve value.

Eurofighter Typhoon, on the other hand, has combat weight of 14 483 kg and combined wing leading edge length of ~18,3 meters along with canards. Thus its loading value will be 791 kg per meter, or slightly higher, but as with F-22, vortices will improve value – this time vortices generated by strakes at sides of Typhoon’s hull. Both Typhoon and F-22 have similar wing sweep and high-lift devices, so actual lifting area per meter will be the same, except maybe for canards.

At lower angles of attack, when entire wing area is used, F-22 will have wing loading of 319 kg/m2 in standard combat configuration, and Eurofighter Typhoon will have wing loading of 283 kg/m2. Thrust loading ratios will be 1,28 for F-22 and 1,25 for Eurofighter Typhoon.

We can thus see that, while F-22 has thrust-to-weight ratio advantage, Eurofighter Typhoon has both lower combat weight and lower wing loading at combat weight, and thus has better maneuvering performance. Dassault Rafale will have similar advantages, although its canards act more like F-22s LEX, which makes it for two aircraft that have better maneuvering performance than F-22.

F-22 is comparable to F-15C (claim made by Pierre Sprey)

Comparing it to the F-15C, we see two things: wing loading and thrust-to-weight ratio that are very similar, with F-15C having slight advantage. While F-22 is larger and heavier aircraft, it is also unstable, improving its response time and removing resustance of aircraft towards the continued turn. It also has LEX, which improves lift at high angle of attack.

While its internal missile carriage adds weight and frontal area, that is cancelled out by reduced drag due to lack of external stores.

F-22 is worse than F-16

F-22 and F-16 have two major things in common: both are relaxed-stability designs and both have LEX. As such, similar wing loading figures and thrust-to-weight ratios will result in similar maneuverability, especially since F-16 was designed to achieve optimum performance when two wingtip AAMs are present.

With 50% fuel, 2 Sidewinder and 4 AMRAAM F-16C has wing loading of 392 kg/m2, thrust-to-weight ratio of 1,186 and weights 10 936 kg. F-22 has wing loading of 313,5 kg/m2, thrust-to-weight ratio of 1,29 and weights 24 579 kg. Thus, while F-22 will suffer maneuverability penalty due to its size and weight, it is unlikely that F-16C will be able to outmaneuver it.

With F-16A it is a different story. With empty weight of 7 076 kg, it has wing loading of 349,5 kg/m2 and thrust-to-weight ratio of 1,29 (figures for 50% fuel and 2 Sidewinder). While its wing loading is higher than F-22s, F-16A is far lighter and smaller, so it is possible that it could be capable of matching the F-22.

Conclusion

To conclude, while Pierre Sprey’s notion that F-22 is no more maneuverable than F-15C is not supportable, those that insist F-22 is the most maneuverable fighter aircraft in the world are equally wrong. Indeed, new fighters such as Eurofighter Typhoon or Dassault Rafale will have better maneuvering performance with virtue of their better aerodynamics and superior attributes (wing loading, thrust-to-weight ratio, etc). F-22 also does not meet force size requirements.

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