The Insane Engineering of the F-117 Nighthawk

Two F-117s are approaching their target Over Baghdad, beneath the cloud cover, radar-guided surface-to-air missiles scan the sky for signs of an imminent threat, but see little more than a small, fading flash. In the background of the noise, the cargo compartment doors open to reveal two GPS-guided missiles destined for the supposed location of then-Iraqi President Saddam Hussein. This simple act may well have revealed the radar stealth plane beneath its exterior. It was shaped by precise mathematical equations, each facet was designed to disperse and absorb the electromagnetic energy surrounding the skies around it with the missiles launched and their location potentially revealed.

The helpless planes quickly turned to escape enemy airspace. This was a highly specialized designed to carry only two missiles in its payload bay an aircraft that looked like a fighter jet, but if it complied Enemy fighter jets, subsonic aircraft armed with bombs intended for ground targets, stood little chance. He relied entirely on stealth to get behind enemy lines and attack unseen, something he did again and again during the Gulf War, attacking 1,600 high-value targets without losing a single aircraft. The design began as a simple Diamond that Lockheed Skunkworks aerodynamicists dubbed the Hopeless Diamond, a template for stealth.

A shape that famous former Skunkworks frontman Kelly Johnson doubted he would ever be able to fly. Turning this concept into a functional aircraft would require the best aerodynamicists. Propulsion Engineers Structural engineers and radar specialists that Skunk Works had to offer carefully cut out the simple but revolutionary shape to form a masterpiece. This is the

engineering

of the F117 Nighthawk. The F-17's journey near Hawk began here on Ben's desk. Rich, the newly appointed director of Skunk Works, replacing the eminent Kelly Johnson. This article, a Russian scientific article translated by the Air Force's foreign technology division, held the key to a new generation of warplanes, it is a constantly evolving game of cat and mouse and in the last decade the planes had changed its role from predator to prey the development of Soviet radar-guided missiles took the American industrial machine by surprise a sudden realization during the Yong Kapor War of 1973 Israeli pilots armed with the latest and greatest American attack aircraft lost 109 aircraft In just 18 days, these planes were mostly shot down by missiles supplied by the Soviets to the Egyptian and Syrian armies.

These crews required a fraction of the training required to train a fighter pilot and the systems themselves were much cheaper than the Russian ones. invested billions in the development of radar-guided surface missiles like the SA5. These missiles could reach 125,000 feet in altitude and could even be armed with nuclear warheads to destroy the Raiders from the sky. The extreme altitude would mean that the Russians on the ground would be protected from the heat, the shock waves and strong winds of the upper atmosphere would carry the consequences to Western Europe, the effectiveness of these missiles was not due to speculation, the balance of the field of battle had shifted towards the Soviet Air Force.

Planners extrapolated these attrition rates. to an all-out war with the Soviets in Eastern Europe and saw the writing on the wall that the Soviets would wipe out the entire US Air Force in just 17 days. Two strategies emerged to counteract this advance. The conventional wisdom of the time pointed towards the design of one aircraft specifically. Designed to hug the terrain and be able to fly long-range missions at extremely low altitudes, hiding from radar on the ground, this resulted in the B1 Lancer. Its variable sweep wing allowed it to adapt its aerodynamics for both low-speed, low-altitude and high-speed, high-altitude flights.

However, the flight system would soon be challenged thanks to a brilliant scientist, Dennis Overhauser, who on one fateful day placed this document on Ben Rich's desk. A long technical document full of complicated equations and theories, but for Dennis Skunkworks, the Dome radar expert, it had great meaning. holds the key to unlocking a new way of designing aircraft a method to calculate the defraction of electromagnetic waves stealth design was not a new concept the first flight of the sr-71 predates the first day of development of the F-117 by more than 10 years and Some of the first stealth concepts that relied heavily on the laws of physical optics, reflection and refraction were incorporated, preventing corner reflectors from being a known method of reducing the radar signature long before to be theorized about the F-117.

What this document unlocked was much more complicated than previously thought. too complicated to solve defraction imagine waves approaching a harbor Breakwater is designed to protect ships inside from the energy of the sea with only the laws of reflection ships must completely ignore the presence of waves outside these walls and, However, through defraction the waves finding their way into this is a chaotic and messy problem with too many variables to consider for a simple analysis and it only gets more complicated with electromagnetic waves, but this article, unbeknownst to them the Soviets who allowed it to be published internationally, held the key to the technology that would allow planes to once again become the top predator Ben Rich, the newly appointed skunkworks leader under pressure to take reigns from Kelly Johnson, approved the development of a computer program and in 3 months a working program was created that allowed analyzing simple shapes called echo1 allowed them to theorize about the Hopeless diamond this, of course, was not an airplane, it lacked everything that an airplane needs to fly it had no control surfaces, it had no propulsion, it had no payload, it even lacked a wing.

Dennis was not an aerodynamicist, he was the local Skunk en Raad specialist who designed the nose cones that housed his aircraft's radar. This design was a template that could be molded by the most expert aviation sculptors. The first step was to discover your mission profile. This would decide several things. Maximum payload and propulsion range, with stealth being a priority for the F-117. It could not have external hard points, this was going to limit its weapons capacity, it would have two internal weapons and the plane would be a precision bomber. Its weapon of choice was laser-guided missiles inside the nose of the plane, hiding a steerable turret containing a dual field-of-view infrared sensor, while a second infrared sensor was located to the right of the nose wheel well. and both sensors came with a laser that could be pointed at the target to guide the missile with extreme precision, the sensors would lock on and track the targets with help. from the internal navigation system, switching from the forward-facing system to the downward-facing system when passing over the target, these systems were of course highly dependent on the weather, as the aircraft could not guide the missile through the cloud cover, but GPS-guided missiles were also an option.

If they were the only weapons on board, the aircraft would be completely defenseless against aerial threats and would therefore be completely reliant on stealth and because of this, the F-117 was going to be a subsonic aircraft that would produce a sonic boom, it was a good idea. way to be detected, not by radar, but anyone in the vicinity, higher speeds also result in greater aerodynamic heating that could allow the plane to be targeted by infrared-guided systems, so the engines did not need afterburners or intakes Specialized engine blocks to slow supersonic airflow. This simplified engine. selection and the General Electric j85 was selected for the medium blue demonstrator which would later be upgraded to the General Electric f404 of the Fa18 Hornet, an affordable, reliable and easy to maintain engine along with the engine air intake that needed to take shape here things were become particularly unique to the F-117 Radar bouncing around inside an engine inlet is like a person screaming in a huge cave: the engine inlet would need to take the same facet angles as the rest of the plane, so the engine inlet engine was covered with a radar reflective grille that the grille had. spacing of only 1.5 CM this spacing was key to avoiding radar transmission.

Any wavelength longer than the spaces in the mesh would be prevented from passing through. Some wavelengths will still be able to pass through, but the inside of the inlet was also coated with radar absorbing material or Ram that covered the entire surface of the aircraft, we normally don't see covers on engine inlets for several incredibly important reasons, number one , reduces pressure recovery, pressure recovery is the ratio between the average total pressure at the outlet of the inlet and the average total. pressure in the freest air stream, the absolute maximum value is that at which no pressure is lost and, therefore, no energy is lost.

This is a way to evaluate engine performance. The inlet has a mesh that blocks a large portion of the inlet and drastically reduces pressure. recovery and reduces the thrust the engine can produce as a result, the secondary intake doors located here could be opened at lower power settings to increase airflow, but opening them during flight was not an option, they were automatically controlled and programmed to open when the plane's speed was below Mach 0.5 and the doors gradually closed as the plane accelerated. This was also how the crew accessed the engine to maintain it, as the front inlet was completely covered.

This grid created another, much bigger problem during flight. Ice can accumulate on the plane. Smaller devices like the air velocity control tubes have heating elements and water drains inside them to prevent this critical sensor from becoming blocked. To prevent this from happening to the intake grille. The plane came with an anti-icing kit on board with what was essentially a windshield wiper. A windshield wiper would emerge from the The lip below the intake and spray a glycol-based antifreeze mixture to the side of the engine had to be molded the outlet and once again the F-17 has an incredibly unique design;

The engineers wanted to minimize the heat signature of the plane as well as the radar signature. To do this they needed to cool the exhaust and distribute it over a wider area so it could mix with room temperature air faster. The exhaust outlet changed from a round duct in the turbine exhaust to a flat 17 to 1 rectangular shaped duct. The duct was also divided horizontally into smaller channels below the exhaust. The fuselage extended at a slight upward angle in what was called a platypus tail. This shape distributes the heat from the exhaust over a larger area, allowing it to mix with the ambient air and cool. down faster, the extension was also covered with heat-absorbing ceramic tiles that were continuously cooled by bypass air coming from around the engines.

However, this exhaust configuration caused some instability problems that were only discovered during the mid-blue thrust demonstration test flights of these engines. The nozzles were angled inward, toward the centerline of the aircraft; however, the angle of this confidence vector was often not symmetric. The thrust angle between each exhaust could differ by up to 8° in some flight conditions, resulting in differential thrust that could push the aircraft sideways. The angle was affected by engine power, speed of attack angle, but in particular Sid slip. Sid slip occurs when the relative wind hitting the aircraft does not match the direction of travel, which a It can often happen on takeoffs and landings with crosswinds.

This became a problem for the Medium Blue Test Flights because the flight computer was using a lateral accelerometer to measure Sid's slide, but this asymmetric thrust was pushing the aircraft sideways, causing the accelerometer to detect the side slip. The flight computer then attempted to correct four Sid glides that did not actually exist and created them. Sid's real slip which increased asymmetric thrust which increased Sid's false slip signal a positive feedback loop of instability. This problem was solved by taking actual Sid glide measurements using a beta vein beta is just the aeronautical

engineering

term for Sid glide with the selected engine and payload capacity determined the wings that needed to be sized and designed.

An aerodynamic has many thingsWhat to consider when designing a wing. The sweep angle, AO foil, dihedral angle design and wing area. Typically, these decisions are made based on the aircraft's primary responsibilities. The aircraft selects a wing that operates most efficiently at its expected cruising altitude and speed. A fighter aircraft like the F-16 will be designed to maximize maneuverability and a high angle of attack elevation; However, F-117 engineers had their hands tied with many of the options. They could usually make the sweep angle that describes the rearward tilt of the wings largely outside their control. The sweep angle is generally determined by the speed of the aircraft.

Slower civilian aircraft like Cessna do not need sweeping as they do not travel remotely close. at supersonic speeds a passenger plane The wing is tilted back to increase the critical simulated number. This is the speed at which supersonic flow begins to appear over the wings. Tilting the wings back allows planes to fly faster before this becomes a problem. The 787 has a sweep angle of 32° Supersonic aircraft, like fighter jets, also have their sweep angle determined by speed, but this time it is to prevent oblique shock waves emanating from the nose and the body of the plane intersect with its wings, which would cause a large amount of drag and loss of lift. an F-16 has a sweep angle of 40°, even the fastest aircraft like the SR71 had a sweep angle of only 53°, the medium blue demo plane had a sweep angle of 72.5 De ​​and the f17 was to be a subsonic aircraft flying at speeds similar to those of a passenger aircraft, a sweep of approximately 32° would have been sufficient.

This extreme sweep angle was being driven by its stealth requirements determined by the echo1 program and this had problematic consequences: a swept wing produced less lift than a swept wing equivalent at this time. This was further compounded by the rigid stealth design requirements on the AER foil itself. The F-117 airfoil was composed of simple, two flat faceted shapes, three on the top and two on the bottom, if we map the pressure distribution of a regular aircraft. airfoil can look like this, while the faceted shape looks like this with The peaks that occurred in the facet lines resulted in low lift and drag ratios and caused problems with aerodynamic flapping where the wing would vibrate and be damaged by lift. reduced caused by the sweep angle.

In addition to these suboptimal swept ailerons, the wings had to be quite large to produce enough lift. The designers increased the wing area with extensions. Here this increased the wing area of ​​the F-117 to 72 M squared, a huge area for an aircraft. So light the F-17 had a maximum takeoff weight of 23,800 kg compared to an F15. The F-15 was designed with low wing loading in mind, meaning a low ratio of weight to wing area and However, the F-15 had a smaller wing area and higher maximum takeoff weight than the F-117, the F-15 could simply produce more lift with less wing area.

Designing an aircraft around these flat faceted sheets was a great challenge not only in lift but also in stability. The huge 72.5 sweep on the blue demonstrator caused pitch instability if the aircraft banked at an angle of attack greater than 17°, the aircraft would not be able to descend again as the elevator also lost control. Authority as the angle of attack increased a concerning pitch instability that required the flight control computer to intervene on the pilot's behalf with specialized control. The blue surface turned the Platypus's tail extension into a flight control surface; It automatically deflected downward when a 13° angle of attack was exceeded, but this also brought about some unforeseen problems that ended Bill Parks' test flying career when the control surface automatically deployed just 1 M from the runway. and crashed the plane into the ground bending the landing gear and forcing the pilot to eject this platypus tail was eliminated for the final design of the f117 the F-117 was different from the blue in several aspects in addition to being much larger The internal CED , all of the hav blue's moving fins were changed to a tail and the fins themselves were enlarged to increase control authority.

These internal CED fins matched the 30° contours of the fuselage and served as rudder and elevator. Internal flaps like those on the SR71 and external open flaps like those on the F35 work in the same way and we can see how, by examining the resultant force generated when the control surfaces are actuated in different positions, we can actuate them in opposite directions to generate a horizontal resultant force that provides yaw control like a vertical rudder W or we can deflect them in the same direction to provide pitch control like the horizontal elevator would; however, the inner fins of the blue middle were protected from airflow through the fuselage during altitude. angle of attack The maneuvers that reduced their power to control the plane since they received less air to create lift.

The F-117's larger outer CED fins helped increase control authority. The platypus's tail was removed, but the F-17 gained a second exterior elevation. where the blue had only one elevon on each wing, these huge full span elevons were sized to provide more control in pitching, the wing sweep of 72.5' was also reduced to 67.5' while still being a swept Extremely high wing height for a subsonic aircraft, this 5 degree lowering helped increase wing lift and reduce the pitch instabilities that the Medium Blue demonstrator experienced with two large lifts. The F-117 Vil no longer had to control both pitch and yaw, it was dedicated only to yaw control by having a control surface like this.

Dual duty can cause problems if the aircraft needs to control pitch and yaw at the same time. The flight computer has to figure out how to manage competing control inputs on a single controlled surface. It is far from ideal. Also increasing redundancy in the event of a failure is a much better design than the medium blue demonstrator, which was destroyed in accidents with the F117's extremely poor lift-to-drag ratio. Its maximum takeoff weight was limited. The fuel tanks were located on top. and behind the main weapons bay with additional wing storage for a total fuel capacity of 8.2 metric tons of JP8 fuel, giving the aircraft a range of 1,720 km to reach enemy territory.

The plane needed frequent aerial refueling. The original trip from Langley Air Force Base to Saudi Arabia for Operation Desert Storm took 15 hours. and required seven refuelings along the way. Air-to-air refueling receptacles were mounted on the top of the fuselage and rotated into position. Due to the poor low-speed flight characteristics of the aircraft, there was little possibility of taking off and landing on aircraft carriers. In fact, Loy did. propose a transport version of the Nighthawk, the F-117n, which added a tailhook, lowered the wing sweep, added a full-motion horizontal stabilizer, increased power with a larger combustion engine, and increased payload capacity with more hard points.

The proposal was rejected, however, The F-117 was a moment of inspired brilliance that the United States Air Force eventually learned would have a limited useful life, they kept it secret for as long as possible to maintain their advantage, but Once their adversaries learned of their existence, they began to counter strategies. developed by taking the simple model of stealth and trying to adapt it for the next generation of multi-role fighters was never going to work the worst of both worlds, ultimately a whole new generation of aircraft was needed during Desert Storm, the Force Air Base 36 F-117s in Saudi Arabia and only needed refueling tankers as support.

These planes operated with impunity over highly contested airspace. On the first night of their deployment they attacked 26 high-value targets, many of which included the country's surface and missile defense systems. The 117 were so effective that the Iraqi air defense system was unable to stop them. What was once a Soviet trump card became useless. Not a single F-117 was lost. This infographic was produced by the Air Force after Desert Storm to illustrate the value of this. new technology estimating the cost of 20 years of acquiring and operating this Precision and Stealth Doctrine at $1.5 billion the standard package of aircraft needed before stealth was developed was much more expensive and required specialized fighter-bomber escorts to protect them electronic jamming aircraft to suppress air defenses and a much larger fleet of refueling tankers to support them all, costing $6.5 billion;

However, the Air Force was well aware that this golden period of stealth would be short-lived and knew it was only a matter of time until the enemy developed a strategy. and technologies to counter stealth A large part of the F-117 mission planning revolved around knowing the positions and types of radar systems in enemy airspace and planning a route through them that would keep the aircraft out of range the first and only f117 was shot down Serbia in 1999 here a tactic was used that involved moving radars and missile launchers regularly and only turning them on for a short period of time so that their positions could not be tracked, making it impossible plan a route around them.

This tactic allowed them to block successfully. on the F-117 and destroy it, the pilot was rescued but the military made no attempt to destroy the remains of the plane at this point, the F-22 had already made its first flight developed with the latest in stealth technology and being a true fighter plane. Capable of defending itself and incorporating even more advanced technology that the public is probably not yet aware of today, the F-35 employs an entirely new doctrine of interconnected battlefield information, a hive mind of stealth aircraft exchanging information with each other, the F-117 was the first truly stealthy aircraft, but as stealth technology has advanced, stealth is not something that is expected as a specialization, but rather is expected to be a standard with modern technology.

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