Which is the fastest aircraft in the world, ever made?

 The answer to this question depends heavily on what you consider qualifies as an aircraft, and that distinction changes everything. The Space Shuttle Discovery holds the record for the fastest manmade object to fly as an aircraft.

The Space Shuttle was unique in that it took off on the back of a rocket but re-entered as an aircraft. Orbital velocity is over 17,000 mph, and spacecraft such as Discovery re-enter with the majority of this velocity.

This translates to speeds of nearly Mach 25 in the atmosphere, a speed unmatched by any other aircraft. From the moment the Shuttles graze the upper edge of the atmosphere, they begin to strike an aircraft-like angle of attack to generate lift. The reason why Discovery in particular claims the record for the fastest aircraft speed is its Hubble Telescope Missions, which were the highest orbits flown by the Shuttles.

For a manned, powered aircraft flying under its own thrust, the record belongs to the North American X-15. It holds the official world record for the highest speed ever reached by a manned, powered aircraft, having flown at Mach 6.70 (4,519 mph) on the 3rd of October 1967. The X-15 set speed and altitude records in the 1960s, reaching the edge of outer space and returning with valuable data used in aircraft and spacecraft design. Pilots that flew this rocket powered beast were awarded Astronaut's Wings, and the research gained from this program helped greatly in making the Space Shuttle possible. The X-15 was launched by a B-52 and then accelerated, which leads some to argue it does not count since it did not take off on its own.

If you also consider unpiloted aircraft, the X-43 reached a speed of Mach 9.8 (12,144 km/h; 7,546 mph) using scramjet technology with rocket assist. NASA built this unmanned aircraft and the key distinction is that it was uncrewed.

For a jet-powered aircraft that could take off and land on its own and fly operational missions, the Lockheed SR-71 Blackbird is the clear answer. The SR-71 can achieve Mach 3 plus speed and still holds several official air speed records. The U.S. Air Force has never revealed the ultimate maximum speed of the SR-71, but the ease with which it re-established records when broken suggests much higher operational speeds had been achieved. None were ever shot down by enemies, which speaks to its ability to simply outrun threats during missions.

What is the highest altitude a passenger aircraft can safely fly?

 At the top of its service ceiling, the amount of air over the wings makes any turbulence a little more troublesome, either pushing the plane into its critical high-speed regime or perhaps nudging it back to stall speeds. The coffin corner is the flight region where these two speeds are very close. Fortunately, many airliners are unable to get to this altitude at normal operating weights because the engines just don't have the poop up there.

The service ceiling is defined as the maximum altitude at which the aircraft can still generate a positive rate of climb of 100 feet per minute. The absolute ceiling is the maximum height the aircraft can reach, beyond its service ceiling, where the air density is extremely low so that the climb rate of the aircraft drops to zero. Normally service ceiling of a typical passenger aircraft ranges from 41000 to 43000 feet, though it can be 51000 feet for private jets.

The heavier the plane, the faster it had to go. So, the heavier the plane, the lower its maximum altitude, because when you go higher, the minimum stall speed exceeds the maximum speed of the plane. At high altitude, you have to go faster to stay above the stall speed, and the faster you go, the closer you get to the high speed limit where Mach buffet stall occurs.

The FAA has over the years tightened up the certification process for new aircraft and about twenty years ago placed a strict requirement that subsonic commercial aircraft are not to exceed altitudes of 40,000 feet unless the structure is certified to not have any type of decompression. NOTE: FAA sets a maximum certification altitude of 51,000 feet. Each aircraft is certified to fly at a maximum altitude, which for commercial aircraft is around 40,000 feet. Some business jets are certified to around 45,000 feet but any higher requires special equipment and also increases the amount of solar radiation that a person receives.

Concorde had a maximum cruise altitude of 18,300 meters (60,039 ft) and an average cruise speed of Mach 2.02, about 1155 knots. The current airliners rarely travel above 41,000 feet.

Which gas is used to fill the wheels of an aircraft, and why?

 Nitrogen is used. It is an inert gas so high pressures and temperatures do not affect it as with air. It is also more compatible with rubber, and does not corrode like air containing oxygen and water vapor does. More importantly it will not support combustion or explosion as does air containing oxygen.

The FAA requires nitrogen in all commercial aircraft tires to eliminate the potential for water vapor from freezing at high altitudes. At altitude, air temps at the tires reach -30F. Any moisture in the tire can freeze, and if there is enough moisture in the tire, it can form ice and that can put the tire out of balance. Nitrogen doesn't form a liquid till -173C and pure nitrogen has almost no moisture.

Nitrogen does not expand or contract as much as oxygen, so the pressure inside the tyre does not alter as much between minus 50C at altitude and plus 200C or so when the tyre heats up as it hits the runway, compared to a tyre filled with air. Oxygen also reacts with rubber, and when this corrosion starts, the small particles break off and form rust and dust, causing them to leak. Nitrogen is far less reactive and it is not corrosive. Wheel surfaces stay smooth and clean, rubber remains supple and resilient.

Boeing has received reports of confirmed cases in which a wheel and tire assembly exploded when the oxygen in air-filled tires combined with volatile gases given off by a severely overheated tire. As a result, the U.S. Federal Aviation Administration issued Airworthiness Directive 87-08-09 requiring that only nitrogen be used to inflate airplane tires on braked wheels. However, tires may be topped off with air in remote locations where nitrogen may not be available if the oxygen content in the tire does not exceed 5 percent by volume.

How does an aircraft reduce its speed for landing?

 Drag is what slows an airplane in the air. One thing that surprises many people is that when you reduce engine power, you don't actually slow down. Instead, when you pull the throttles back, the plane descends, but at basically the same speed. The real way to slow an airplane down is to increase angle of attack.

Essentially, pull the nose up, and the plane slows down. In an airplane, every control input always effects the others, so you must coordinate reducing power, increasing angle of attack, and adjusting trim.

The real job of wing flaps isn't to slow down the airplane, although they do slow you down in some cases by a whole lot. Instead, it's to keep the plane both flying and controllable once you do slow down. Every airplane's wing is designed with a certain goal in mind. In the case of a typical airliner, it's to fly really fast and high, with an eye towards fuel savings. Unfortunately, fast high-flying wings are really bad at flying low and slow. Since every successful flight ends low and slow, we need the flaps to essentially change the shape of the wing to one that will allow us to approach the runway at a safe speed. As a bonus, flaps allow us to point the nose of the airplane more downwards, giving a better view of the runway on approach.

Spoilers are almost cheating. What spoilers do is to stick up into the airflow, spoiling the lift generated by the wing, essentially making it quite inefficient. The spoilers will create some drag of their own, but really, they're making the plane descend quicker. Extending landing gear also provides more drag and helps slow the aircraft.

Once on the runway, airliners primarily use wheel brakes to decelerate after landing. Wing spoilers extend on the top of the wings which add aerodynamic drag, but they primarily kill the lift on the wings which places more weight on the wheels and improves braking. Thrust reversers are deployed on touchdown to both attenuate the thrust and to direct the thrust forward for deceleration purposes. Once the airspeed decelerates to about 60 knots, the reverse thrust is reduced to idle because that's when the possibility of the reverse exhaust flow could ingest foreign objects into the inlet of the engines.

Why has it been so hard for China to build aircraft jet engines?

 Empirical evidence proved it is more difficult to design and build aircraft engines than it is to build atomic bombs or send satellites to space. The jet engine is HARD.

It took many decades for US, British and French engine makers to perfect the technologies that they use today. China couldn't just acquire the plans and start making them. Creating turbine parts that can survive extreme heat has been a major engineering challenge. Meeting it has required fundamentally rethinking the material structure of the turbine blades, making metals do things that they do not normally do in nature. Turbine blades may be operating in temperatures far exceeding their melting point, and thus must be cooled to typically 85% of melting temperature to maintain integrity. Blades must be cast with intricate internal passages and surface hole patterns needed to channel and direct cooling air within and over their exterior surfaces.

The turbine blades in a modern jet are a single metal crystal. Even if you have a jet and can take it apart that tells you nothing about how to cast the turbine blades, and you can have a turbine blade that looks fine but self destructs after 200 hours. As an example, the load on a single turbine blade that's only 3 inches long can be as much as 35 TONS due to the centrifugal force on a turbine spinning at 20,000 rpm at temperatures over 1,100F. Only single crystal nickle-steel can sustain that kind of load for 2,000 hours without fail.

One of the biggest reasons why China is so lagging behind in jet engine is the Culture Revolution, which thousands of scientist/engineers were purged, thus created a talent gap which China took a generation to pick up. By the time China got its sanity back, it's already in the 1970s. Chinese engines have to compete with mature jets already in the market place, and US and UK engines didn't. This let them make profit while they were making junky engines. China doesn't have that luxury.

China has been making jet engines for decades, starting with WP-5 in 1956, which is a licensed copy of Soviet VK-1, which itself a knock-off from Rolls Royce Nene. China and UK signed a license deal in 1974 for the Rolls Royce Spey engine, but since UK committed to transfer technology, but not manufacturing know-how, by the time China overcame all the technical difficulties associated with manufacturing the jet engine, it was already mid-2003. China probably will surpass France by end of the 2020s, as China has multiple engine developments in the pipeline.

Which is the largest aircraft in the world, and is it possible to land that in all airports?

 Not even close.

The largest aircraft in the world is the Antonov AN-225 “Mriya”.

Photo from Reddit

This behemoth has a landing distance of 2,400 meters when empty. That is around 1.5 miles, or about 2/3 the length of Central Park in New York.

Now that that’s covered, the world’s shortest commercial runway is Juancho E Yrausquin Airport, Saba coming in at a whopping 1,312 feet long, or about 400 meters.

Photo from Charismatic Planet

The 225 would touch down and promptly fall into the ocean on the other side of the runway. It would probably be safer to ditch in the ocean beside the airport.

Without using the runway above, there are many, many regional and municipal airports where the 225 simply could not land. For instance, Skyline Airport in Idaho has a runway length of only 400 feet. It is so small, I couldn’t find a picture of it. Either way, the AN225 is 275’ long, over half the length of the airport’s runway. No matter which way you cut it, the 225 isn’t going to be landing there anytime soon.