Defense Issues

Military and general security

    Advertisements
  • Follow Defense Issues on WordPress.com
  • Enter your email address to follow this blog and receive notifications of new posts by email.

    Join 253 other followers

  • October 2019
    M T W T F S S
    « Sep    
     123456
    78910111213
    14151617181920
    21222324252627
    28293031  
  • Categories

  • Advertisements

Posts Tagged ‘thrust vectoring’

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

Advertisements

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

On AviationIntel F-22 vs Typhoon article

Posted by picard578 on November 24, 2012

http://aviationintel.com/2012/07/28/in-response-to-reports-of-simulated-f-22-raptor-kills-by-german-eurofighters/

 

While author is indeed correct that training sorties do not necessarily mean that one type of aircraft is superior, multiple sorties can, when analyzed properly and assuming that setup is known, provide some information about respective fighter’s capabilities.

Huge control surfaces and thrust vectoring are useful for high-altitude and low-speed maneuvers, not in types of maneuvers required for close-in combat (transsonic low-altitude maneuvers). In fact, thrust vectoring is dangerous as it bleeds off energy, leaving fighter defenseless if it does not manage to get a kill immediately upon using it Secondly, German Typhoons in the exercise had no helmet-mounted sights, and as such had to point nose at F-22s to get a lock.

Modern radar warners, such as those carried by the Typhoons, are very capable of detecting even newest LPI radars. In any scenario where IRST-less Typhoon and F-22 went against each other with no AWACS support, both sides would be limited to visual detection.

In the end, visual-range combat is more likely than not to be decisive between fully equipped 4,5-th/5-th generation aircraft. As such, while F-22 is a capable dogfighter, it cannot be counted on to have a major impact in a war due to high cost and low sortie rate.

Posted in news | Tagged: , , , , , , , , , , , , , , , , , , , | Leave a Comment »

F-22 fact spinning on USAF website

Posted by picard578 on October 28, 2012

I was browsing http://www.af.mil, when I have found this page. While most, possibly all, of claims there have been addressed in my F-22 Analysis, I am aware that it is very long read, and as such I will examine claims here.

First claim is that “The F-22 possesses a sophisticated sensor suite allowing the pilot to track, identify, shoot and kill air-to-air threats before being detected.”. Problem with that claim is that F-22 has no sensor capable of tracking and identifying target without requiring either F-22 or enemy aircraft to actively use its radar. Thus, F-22 must either rely on (jammable) uplink from another unit or on enemies being willing to give it first strike possibility by radiating themselves. However, IRST-equipped aircraft can detect subsonic fighter aircraft from large distance, without being required to radiate themselves – Su-35 can do it from 50 kilometers head-on, and Eurofighter Typhoon from 90 kilometers, also head-on. From rear, Su-35 can detect subsonic fighters from 90 kilometers, which means that Typhoon can do the same thing from over 150 kilometers.

While F-22s radar can detect 1m2 target (which is approximately same as Typhoon’s frontal RCS when in air-to-air configuration) from 200 – 240 kilometers, jammers can reduce range required for a lock-on to be achieved to less than a third of range in non-jammed environment. That can be confirmed by recent exercises, where F-22 was unable to lock on clean-configured Typhoon from front until latter was 20 miles (32 kilometers) away; as Typhoon has frontal RCS (when clean) between 0,25 and 0,75 m2, it means that F-22’s radar range has been reduced by jammers to approximately 14,4 – 22,7 % of expected range. Thus, F-22 cannot be expected to lock on combat-configured Typhoon from range larger than 45 – 54 kilometers from front. Both ranges are well inside detection range of PIRATE IRST. With Su-35, situation is somewhat better, due to its larger RCS and lower-capability IRST; however, reduction of radar range by jammer, which means that F-22 may not be able to even launch all BVR missiles (and even if it does, 6 BVR missiles combined have Pk of 36 – 48 % against capable opponent) means that far more enemy aircraft than is assumed will be able to get to visual range with F-22.

While F-22 is a capable dogfighter for its size and weight, its low production run and high maintenance downtime mean that it will likely find itself outnumbered in any war against China – which is a primary justification for continuing production. For comparasion, while Su-35 has flyaway cost of 65 million USD at most, F-22 has flyaway cost of 250 million USD, and maintenance downtime of 45 hours per hour of flight. While I was unable to find any figures for Su-35s maintenance downtime, it most likely isn’t worse than 30 hours per hour of flight as required by USAF’s ancient F-15s. Thus, F-22 will find itself outnumbered 5:1 in best case, whereas Typhoons, with flyaway cost of 120 million USD and maintenance downtime of 10-15 hours per flight hour, might even be able to slightly outnumber Su-35s.

What is worse, Russians have air-to-air anti-radiation missile (R-27P), and are very willing to sell it over the world. As internal USAF exercises have shown during the Cold War, several aircraft equipped with anti-radiation missiles can force everyone to shut down radars. That, in turn, will force aircraft to return to visual-range dogfight, with IRST-equipped aircraft having very large advantage in situational awareness – even larger than usual.

Second claim that needs examining is the value of stealth. While I have already discussed value of stealth in air-to-air scenario, I have not addressed scenario with surface-to-air threats – mostly SAMs.

While it is true that stealth aircraft have increased survivability compared to legacy aircraft when confronted by X-band radars, it is not so with lower-frequency, long-wavelength radars. Namely, aircraft RCS depends on size and shape of aircraft, its position relative to radar waves as well as wavelength radar in question is using. Stealth aircraft are designed to scatter radar waves away from (monostatic) X-band radar, with stealth coating absorbing minor part of radar signal. However, that only works when wavelength is far shorter than dimensions of the shaping features of the aircraft. Against VHF radars, with their 1-2 meters long waves, fighter aircraft such as F-22 and F-35 will see majority of their shaping features fall into either resonance or Raleigh scattering region. In these regions, shape of feature in question becomes irrelevant, and skin becomes electrically charged by radar waves, increasing RCS even further. Against such radars, stealth aircraft are forced to use same tactics as legacy aircraft against any type of radar, making stealth irrelevant and even harmful.

Third claim is that F-22’s engines produce more thrust than any current fighter engine. While it is true, F-22 is also heaviest fighter aircraft in existence, and these powerful engines give it thrust-to-weight ratio of 1,09 at loaded weight and 1,28 with 50% fuel, 2 Sidewinders and 4 AMRAAM. Later value is same as Eurofighter Typhoon, while former is inferior to Typhoon, which has TWR of 1,14 at loaded weight. Rafale has thrust to weight ratio of 1,1 at loaded weight, and 1,23 with 2 WVR, 6 BVR missiles (all MICA) and 50% fuel.

Fourth claim is that F-22 can outmaneuver all current and projected aircraft. It cannot; thrust vectoring is only useful as help with maneuvering at speeds below 150 knots; above 150 knots aircraft ends up with drifting motion – lower aircraft has TVC, upper doesn’t – which increases drag for no decrease in turn diameter. At the onset of the turn, aircraft looses lift and sinks in mid-air, with nose rotating up. Suffice to say, both of these effects are very dangerous in visual-range dogfight, especially in era of high off-bore missiles.

Fifth claim is that “The combination of stealth, integrated avionics and supercruise drastically shrinks surface-to-air missile engagement envelopes and minimizes enemy capabilities to track and engage the F-22 .” Stealth has already been addressed  as have sensors; supercruise is of interest here. While non-afterburner supercruise is useful, as it reduces fuel expenditure and heat signature of exhaust plume, it is not a game breaker. F-22 has low fuel fraction, is heavy and with large amount of drag, limiting duration of supercruise. Moreover, aircraft supercruising at Mach 1,7 can be tracked from 10% longer range than subsonic one, which means that Su-35 will detect it from 55, and Typhoon from 100 kilometers, head on. Reduction of engagement envelope can be achieved by increasing speed, supercruise or not; however, supercruise does reduce fuel expenditure, although such reduction is not very large.

Next is the claim that F-22 will have “better reliability and maintainability than any other fighter aircraft in history”. With F-22s maintenance costs and downtime being as they are (maintenance downtime of 45 hours per hour of flight, maintenance cost of 61 000 USD per hour of flight, and availability rate of 55,5%), claim is certainly false. Indeed, while Eurofighter Typhoon is a very complex aircraft, comparing it with F-22 produces shaming numbers: maintenance downtime of 10-15 hours per hour of flight, cost of 18 000 USD per hour of flight, and availability rate from 50% for Luftwaffe to 88% for RAF during Operation Elamy, RAF participation in Libya. Dassault Rafale costs 16 500 USD per hour of flight; unfortunately, I do not have figures for either maintenance downtime or availability rates.

Last is the characteristics table. While most of it seems correct – I won’t check it now – unit price is not. When debate has been held about ending F-22 production at 187 aircraft, proposal was to buy seven more F-22s for total price of 1,75 billion USD. Since it R&D expenses have already been paid, and production line was still active, sum shows an actual F-22 flyaway cost of 250 million USD per aircraft.

Posted in Uncategorized | Tagged: , , , , , , , , , , , , | 14 Comments »

 
%d bloggers like this: