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Archive for the ‘technology’ Category

Why variable sweep wings or “swing wings” for fighter aircraft are not effective at air superiority

Posted by altandmain on August 19, 2017

Why not another variable sweep fighter?

There seems to be a lot of F-14 nostalgia around. While it may have had a great deal of impact on how the US Navy conducted fleet defense, we have to consider the effectiveness of the concept of variable sweep aircraft. It is human nature to always want to look up to the past.  The other reason may very well be that people find the F-14 to look visually attractive and want similar proposals.

The reason why we will not see future variable sweep fighters however is because there are very serious drawbacks compared to fixed wing aircraft.

Short Background

Variable sweep wings, known as “Swing wing” evolved as a solution for early jet engines. Experiments were being made as early as WW2 with wings that could change their sweep on the ground, such as the Messerschmitt P.1101.

Back then jet engines produced less thrust because they ran at lower inlet temperatures and were overall more primitive. Wings with a sharp sweep were desired for high top speed, but that left the aircraft vulnerable in dogfights, which as Vietnam revealed still happened, and also led to high take-off and landing speeds. High take off and landing speeds are less safe, which would result in increased number of crashes. They also led to long runways, limiting off  road mobility and making it easier to disable for enemy forces, as there would be a far larger airport to protect.

In Europe, there were two key projects, the Panavia Tornado, which entered service as a mult-role interceptor/bomber, and the Dassault Mirage G, which never entered production. The US would build the F-111, which was a very heavy variable sweep multi-role aircraft. The famous F-14 was derived from the F-111. The USSR made several variable sweep designs, most notably the  Mig-23 and the Su-24.

Bomber designs were also made by the US and USSR. The B1 Lancer from the US, along with the Tu-22 and Tu-160 from the USSR. All 3 bombers remain in service.

What do swing wing aircraft bring?

Their main advantage is that they can use that variable sweep wing to find the optimal wing swing angle (within their sweep limits) for a given airspeed.  This can allow for fuel savings on the climb and landing during a fighter sortie.

On aircraft carriers, they have the advantage of having very low sweep on take-off and very high sweep when bursting with full afterburner. Variable sweep wings can also be folded for compact storage without compromising wing’s structural integrity (as is the case with folding wings like on F-18E).

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On an aircraft carrier, deck-space is always going to be a bottleneck. While a carrier may look very large to an untrained eye, deck space is always at a very big premium.

So why not on fighter aircraft?

To achieve variable sweep aircraft, that requires a large gearbox in the fuselage of the aircraft. This gearbox adds a great deal of mass and makes the fuselage larger, causing drag. This means that fuel fraction on such aircraft is lowered a great deal.

In a dogfight, this heavy gearbox would mean that compared to a fixed wing, it would result in an unfavorable thrust to drag, even if the pilot could switch to what they felt was the optimal sweep right before combat. Switching the wing sweep during a dogfight would be risky, as it could cause a loss of energy.

This would mean:

  1. Higher wing loading due to mass of gearbox
  2. Faster fuel consumption due to gearbox
  3. Lower transient performance (very important in a dogfight)

This gearbox would also lead to lower G limits as well. On the F-14D, the symmetric limit at 50,000 lbs was 6.5G. The F-16  and F-15 were both capable of 9G. Navalized versions of the F-18 were capable of 7.5G, while certain land based variants of the F-18 could also perform 9G. For a comparison, Dassault Rafale can do 11G, with an ultimate limit of 16.5G.

The gearbox lowered the aircraft’s fuel fraction. An empty F-14D has a mass of 43,735 lb ( or about 19,838 kg) and can take on 16.200 lb of fuel. This results in a fuel fraction of 0,27, which is below 0,30 fuel fraction required for sufficient combat persistence.

Jet engines have become far more powerful than their 1960s and 1970s counterparts, allowing for much higher thrust to weight ratios. As such, they can achieve lower take distances, even more so on an aircraft carrier with a catapult. This fact somewhat negates swing-wing’s main advantage of high low-speed efficiency.

Modern computer control surfaces too have played a role in rendering variable wing sweep obsolete as they can adjust wing shape and size very rapidly, without the weight penalty.

Complexity and reliability problems

The more complex a system is, the more risk there is for failure.

When the US Navy opted to retire the F-14 in favor of the F-18, a big reason that was given was the appalling flight to maintenance ratio.

The decision to incorporate the Super Hornet and decommission the F-14 is mainly due to high amount of maintenance required to keep the Tomcats operational. On average, an F-14 requires nearly 50 maintenance hours for every flight hour, while the Super Hornet requires five to 10 maintenance hours for every flight hour.

I’ve been told that a newer F-14 would likely require 40 to 1 and on average, the F-18 requires 8 to 1, which is in line with the USN’s claims of 5-10 to 1. So in that regard, the F-18 would be able to generate much higher sortie rates. Keep in mind that the 50 to 1 is with after  the General Electric F110 engines were put on the F-14. Early F-14s suffered from an unreliable TF-30 engine that was prone to flame-outs.

Compounding the problem, the  high flight to maintenance ratios mean that there’s a good chance you will not have enough F-14s available when you need them the most (ex: if an enemy launches a surprise attack on your carrier battle group, you may need to scramble the aircraft very quickly).

There were other points of failure. Sometimes when one side of the gearbox worked properly and the other did not, it could lead to an “asymmetric wing sweep”.

f-14-asymmetricWhile the aircraft could fly in such a situation and land with some difficulty, this leaves a point of failure. This could also be a weakness in combat, as the hydraulics could be damaged.

Much like this F-14, under Australian service, the F-111 did encounter a similar incident, and the B1 did once as well. I suspect that under Warsaw Pact service, Soviet variable sweep designs may have too.

Conclusions

The cons simply outweigh the pros when it comes to variable wing sweep. There are very significant penalties in terms of mass, cost, and complexity for variable sweep wings. While they may bring some advantages in the take-off and can have the “optimal” sweep for each scenario, the drawbacks outweigh these to the point where we are not seeing variable wing sweep aircraft on modern aircraft.

They are simply a dead end as far as aircraft design goes. While they may have seemed like a good idea on paper, when implemented in combat aircraft, they carried significant drawbacks that outweighed any advantages they brought.

 

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Unconventional fast ships

Posted by Picard578 on May 16, 2017

Overview

Modern conventional fast ships rarely go over speeds of 30 knots, simply because conventional hull forms do not allow higher speeds without unacceptably powerful and complex powerplant (a la LCS), which then means less volume and displacement avaliable for other necessities. High speeds are also very rarely necessary, which means that most ships can sacrifice high speed in exchange for greater firepower, survivability, or endurance. However, combat and peacetime operations have always required ships capable of achieving high speeds, for purposes such as reconnaissance, surprise attacks and combat in general, especially in littoral waters where small hulls dominate and enemy can appear from basically anywhere. High speed is also desireable for deployment of rapid-response forces as an answer to an unexpected crisis, and as airlift is of limited capacity, high-speed sealift is desireable. This means balancing out requirements for high speed, range and high payload.

In conventional ships, developing ships with higher length-to-beam ratio allows installation of more powerful powerplant for a miniscule increase in drag, thus allowing both longer range and higher top speed. Conventional hull form also has major advantages in range, and in that they can be of basically any size. However, they cannot economically achieve very high speeds, which is why specialized – unconventional – hull forms are required for ships that have such requirement. In all these, the goal is to minimize contact with surface of the water. Hull forms can be divided into: Read the rest of this entry »

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Modern artillery munitions

Posted by Picard578 on April 16, 2017

Introduction

Munitions are used for fulfilling the primary task of artillery, which is destruction or neutralization of enemy army, as well as enabling or supporting the ground maneuver by suppressing enemy defenses. First munitions were spherical stone projectiles, launched from ballistae and catapults. Identical projectiles were also used by first gunpowder artillery. Those were typically around 8 cm in diameter. French navy used basalt, which has higher density and hardness, to achieve increased hitting power; those projectiles could penetrate ship’s wooden sides at 200 meters. Stone projectiles were also used as incidendiary projectiles by coating them with lime, followed by resin. These were superseded by lead, which was easy to shape due to low melting point. In early 13th century (cca. 1221.), Chinese were using explosive ceramic projectiles, launched from catapult or a cannon. These were filled either with gunpowder, or a combination of gunpowder and metal shrapnel. In Europe, projectiles from bronze or iron were also used. These could be homogenous, or filled with gunpowder; earliest percussion fuzes – using flint to create the spark – appeared in 1650. Another type of shot was canister shot, which was used against combat for infantry at close range, and was particularly effective against linear formations of Middle and early New Ages. But when linear formations disappeared after American Civil War, canister shot was replaced by shrapnel, which utilizes time fuze and detonates in the air. During the 19th century, two main types of fuzes were used, time delay and impact fuzes. Time fuzes were combustion types, consisting of a burning fuse train, ignited upon firing. There were various designs, but all were only accurate to approximately the nearest 1/2 or 1/4 sec at best. Read the rest of this entry »

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Air combat and stealth

Posted by Picard578 on March 11, 2017

Introduction

For a long time, visual detection was the only type of detection possible. But in World War II, two significant advances appeared: radar, as well as radar- and IR- -guided missiles. Until 1970s, these were defeated through jamming and decoys. Early German attempts at building LO aircraft – Ho-229 – never got anywhere, albeit their RAM paint utilization at snorkels and aircraft was noted. In United States, first attempt at reducing the radar signature of aircraft was on U-2, by utilizing RAM paint, but it was not very successful. First actual stealth aircraft appeared in early 1960s – SR-71 Blackbird, which utilized shaping such as canted surfaces to reduce radar signature. In 1970s a second generation of stealth aircraft appeared with B-1A, and also began a programme of development of VLO aircraft. Result of that was diamond-shaped F-117, to soon be followed by B-2. All these aircraft successfully performed against enemy air defenses, but in the case of B-1A and later aircraft, their performance against air defenses was similar or identical to performance of conventional aircraft they were deployed alongside. Fourth generation of stealth aircraft are F-22, F-35, PAK FA, J-20 and J-31. While still stealth aircraft, they sacrifice stealth characteristics for the sake of better flight characteristics, allowing them to match conventional fighters in terms of maneuverability. However, their stealth requirements make them larger and heavier than comparable conventional aircraft, thus sacrificing kinetic performance for the sake of stealth.^1 Read the rest of this entry »

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Infrared (IR) sensors in naval use

Posted by Picard578 on November 1, 2016

While they are mostly associated with fighter aircraft, IRST sensors are also used by surface forces, including navies. In naval use, IR sensors are used primarily as early warning devices, which means that shipboard IRST is integrated with ship’s self defense / point defense system. IRST detects targets and sends data to weapons systems, which then engage targets based on pre-determined priorities. In naval use at least, IRST will not replace radars because high humidity at sea level significantly reduces its range. However, it is still an important sensor, especially during the times of radar silence (e.g. threat from anti-radiation missiles). Also of importance is IRST’s immunity to jamming and passive operation, which makes them extremely important in electronically degraded environment. IRST’s shorter operational wavelength means greater resolution, and thus far greater ability to detect small targets as well as to distinguish between the low-flying targets and the background clutter. For these reasons, IRST and IR targeting systems are already in widespread use in air forces for purposes of both air-to-air and air-to-ground combat. Read the rest of this entry »

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PAK FA vs F-22

Posted by Picard578 on October 11, 2015

Introduction

This article will compare upcoming Russian PAK FA with US F-22, since both air single-purpose heavyweight air-to-air fighters. However, since PAK FA is still in a prototype stage, article will by its nature be incomplete. I should also note that while some use the term “Raptorski” for PAK FA, it is entirely inaccurate. In fact, while the F-22 clearly draws its basic design from its F-15 predecessor, utilizing some aerodynamic advances introduced by the F-16 (such as aerodynamically unstable design and LERX), PAK FA in the same measure draws its basic design from Su-27. F-22, like the F-15, has two closely set engines, air intakes on sides of the cockpit and classical wing-tail surfaces with shoulder-mounted wing. Both have standard armament of eight missiles and M61 20 mm rotary gun. Su-27 and PAK FA on the other hand both utilize large LERX, wing-body blending and spaced podded engines. They also have basic standard armament of six missiles and 30 mm revolver cannon. If comparison should be drawn, then F-22 can be described as a stealth!F-15, and PAK FA as a stealth!Su-27, as neither presents clear design departure from their predecessor that the F-16 did. They are also both hugely complex to produce due to their stealth designs, and as a result both US and Russia have decided to supplement them with large fleets of 4,0 (and, in Russia’s case, 4,5) generation fighters.

Read the rest of this entry »

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Airborne IRST properties and performance

Posted by Picard578 on June 16, 2015

Introduction

IRST is a sensory device which uses IR (infrared) radiation for detection and targeting purposes. IR radiation has wavelength of 0,75 to 1.000 microns (micrometers), longer than wavelengths of color red in the visible spectrum (visible spectrum ranges from 0,39 to 0,7 microns, with violet at 0,4 and red at 0,7 microns). It is given off by all objects above absolute zero, though objects that are below average temperature of their surroundings will absorb far more IR radiation than they will give out. Unlike FLIR which is a targeting device, IRST can be used for initial detection as well. Read the rest of this entry »

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Fighter aircraft engine comparision

Posted by Picard578 on December 6, 2014

Introduction

This article will compare several engines used in modern fighter aircraft: EJ200 (Typhoon), M88 (Rafale B/C/M), RM-12 (Gripen A/B/C/D), F-135 (F-35A/B/C), F-119 (F-22A), F404-GE-402 (F-18C/D), F-414-400 (F-18E/F, Gripen E/F), AL-31F (Su-27, Su-30, J-11). Read the rest of this entry »

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