In our previous videos, we have looked at several experimental US aircraft pushing aeronautical limits, but never saw service. The F-15 STOL MTD, the F-16XL, and the F-18 HARV to name a few. But there is one experimental aircraft that stands out above all the others for its cutting-edge design.

The F-22 Raptor is often credited as the most capable fighter aircraft today. Although it remains largely classified in its abilities, the air force considers it their top-of-the-line fighter for air superiority. However, as many are aware, the F-22 was not the only fighter to compete for the position it now holds. In 1990, another fifth-gen fighter – the Northrop YF-23 – was also proposed. Today, many consider it an equal – and in some cases a superior design, to the F-22.

The history of the Northrop YF-23 dates back to 1981. The air force had released a Request for Information (or RFI) to some of the top aircraft companies. Seven responded, and over the following months many proposals – 19 in all – were presented. These varied greatly in design and predicted capability. The Air Force were after a design that was stealthy, highly agile, with superior radar, avionics, and the ability to super-cruise (that is, to sustain supersonic flight without using afterburner).

In 1986, the two winners of the initial program were announced; Lockheed, with its YF-22 design, and Northrop, with its YF-23 design. Rather than sending the other five companies home, the prime contractors were given the opportunity to partner with one of them; Lockheed chose Boeing, while Northrop chose McDonnell Douglas.

Over the next four years, both Lockheed and Northrop would have the opportunity to develop their concepts into working designs. This would be part of the Advanced Tactical Fighter competition. Whatever they developed would be judged against the competing demonstrator, and the winner would have their aircraft become the air force’s preeminent air superiority fighter.

Unlike the Lockheed YF-22 design – which put much emphasis on thrust vectoring – the Northrop YF-23 would use no such technology. Northrop engineers believed that thrust vectoring benefits could be achieved through other means, and that thrust vectoring nozzles would add too much weight to the airframe. Rather, the aircraft would use Single Expansion Ramp Nozzles, (or SERNs). Thrust from these engines flow through troughs lined with transpiration cooled tiles. This method – similar to that used on the B-2 bomber – dissipates heat, thus reducing infrared image and lowering that chance of an IR missile lock. The unique shape of the SERN nozzle also hinders ground launched IR missiles from achieving lock.

The overall airframe shape itself was also unique. Diamond shaped wings and a V-tail provided better stealth capabilities without negating aerodynamic performance. It also made it lighter and faster than the F-22, although the latter point remains a matter of debate.

Two test aircraft were produced: nicknamed Grey Ghost (or PAV 1) and Spider (or PAV 2). Two different engines would be used; one would use the Pratt & Whitney YF119, and the other would use the General Electric YF120.

Both the YF-22 and the YF-23 prototypes were almost equally matched. Both had similar engine performance of 35000 pounds, top speeds of Mach 2.2, and similar combat operating ranges. However, there were some differences. For instance, the YF-23 was a lighter aircraft and its maximum range could extend out to 4800 kilometres, whilst the production F-22 had a range of 3200 kilometres.

CAPABILITIES

With its unique airframe, the YF-23 is said to have had a near-invisible radar cross section, matching the stealth abilities of the B-2 bomber. I could be argued that it had superior stealth capabilities to the F-22, offering unique possibilities for a stealthy aircraft.

In terms of speed, the airframe demonstrated excellent performance when flying transonic. In modern beyond-visual-range engagements (or BVR), such capability is important, particularly at higher altitudes. The characteristics of the airframe allowed for super-cruise up to Mach 1.6 (that is, without the need for afterburner). This capability would have allowed for better BVR performance for longer periods of time, without burning through massive amounts of fuel, while remaining stealthy. With afterburners engaged, however, the aircraft was said to achieve a top speed of Mach 2.6 – (slightly faster than the F-22s Mach 2.2), although there is no publicly available information to verify the top speeds of either aircraft.

In terms of agility, the F-22 ultimately won the competition. Northrop’s decision to remove thrust vectoring came at a cost, and this was reduced performance at lower speeds. Although the YF-23 was lighter, at near-stalling speeds, thrust vectoring could do what the airframe of the YF-23 could not; and that was to generate the necessary movements for manoeuvring.

PAUL METZ

A common source of data about the YF-23’s capability, comes from test pilot Paul Metz. A Vietnam veteran, Metz would be selected to fly the YF-23 during the Advanced Tactical Fighter program. Here he got to see just how capable the aircraft was, albeit as a demonstrator. Later, he would fly a preproduction variant of the F-22, which by then had been commissioned following its victory in the Advanced Tactical Fighter competition. Having flown both aircraft, Metz offers important insight into just how good the YF-23 was.

In terms of performance, he remembers the YF-23 as being particularly capable in high AOA situations, operating at trimmed angles of attack up to 60 degrees. The F-22 could also achieve this, albeit with the aid of thrust vectoring.

He concluded that the YF-23 not only matched, but potentially outperformed the F-22. This judgement came after Metz began flying preproduction models of the F-22, which would have been more advanced than the demonstrator used in the competition. The exact reasons for his judgment remain unclear, and we can only assume classified data about both aircraft played a part in his final verdict.

According to Metz, the Northrop engineers were perhaps the best in the business, but they were engineers, and presented everything in technical terms, as you’d expect engineers to do. In contrast, Lockheed knew that marketing and lasting impressions were important – potential purchasers may not be technically astute –  thus a focus was placed on showmanship directed toward acquisition decision-makers.

COMPETITION BEGINS

The competition would technically begin with the rollout of the two competing airframes. Northrop would present the YF-23 for the first time at a ceremony at Edwards Air Force Base, on June 22nd 1990. This first YF-23 (Grey Ghost or PAV 1) would undergo a series of ground tests over the following weeks, before taking to the sky for the first time on August 27th.  Paul Metz recalls that the flight was good; the aircraft was solid, and the F-16 escort plane had to use afterburner to keep up with his Pratt & Whitney powered aircraft. The day after this successful test, Lockheed rolled out their YF-22 prototype.

Speeding up production, Northrop then approved a mid-air refuelling test on the YF-23s fourth flight. Trailing a KC-135, the YF-23 would spend three hours connecting and disconnecting at various speeds, proving it was capable. Flight number 5, saw the first supersonic test. This too proved successful, and by flight number 11, the final pilot checks had been completed.

The second aircraft (Spider, or PAV 2) would emerge on October 26th 1990. Using the GE engines, test pilot Jim Sandberg would take it for its first flight. However, it was at this point that the Northrop team began experiencing difficulties. On October 30, 1990, test pilot Bill Lowe witnessed the outer layer of his windscreen shatter on Grey Ghost, while flying at Mach 1.5. Fortunately, the polycarbonate layer remained in place, and the aircraft returned safely. Not long after, the same issue occurred on the Spider. This was a major problem, not only was it a potentially fatal flaw, but any integrity breach to the specially designed windscreens would increase radar cross section, decreasing its stealth characteristics.

PAV 2’s second flight would also prove troublesome. After taking off, one of the GE engines experienced problems and would not accelerate, and a single engine landing had to be made. There was also an almost fatal incident during flight 3, when a plugged line resulted in the fuel tanks over pressurising. Luckily this was picked up before the aircraft climbed too high, and a safe landing was achieved.

Both PAV 1 and PAV 2 would undergo maintenance to prevent further issues, and for the most part this appears to have succeeded. Both Grey Ghost and Spider would fly together for the first and only time on November 29th 1990, piloted by Metz and Sandberg over the Mojave Desert. The next day, PAV 1 would be taken for its last flight, and then retired. With funding running out as the program neared its end, focus was placed on PAV 2 for the following month. New super-cruise capabilities were supposedly tested, with the speed results remaining unreleased to the public.

Things would soon wrap up for PAV 2 as well. On December 18th, PAV 2 would be sent up with Lockheeds YF22 prototype for a 15 minute photoshoot. This would be the only time the two aircraft would fly together. Later that same day, test pilot Ron ‘Taco’ Johnson would take the aircraft on its final flight; a two hour test mission.

For the months following this, both YF23s would remain on the ground. Beyond a number of taxi runs to keep the aircraft in flyable condition, they would never be used again.

END OF PROGRAM

The end of the YF-23 program would come on April 23, 1991, with the YF-22 becoming the aircraft of choice. The Air Force contracted Lockheed to produce the F-22 Raptor, establishing the aircraft as the premiere American fighter.

The F-22 was using proven design principles and technologies which had been integrated into other designs. Very different to the more radical YF-23, that may have been perceived as too much of a risk. After all, this was to be the top-line fighter, and risk mitigation would have been a high priority. The YF-23 also lacked a demonstration-ready weapons bay (not being a requirement for the competition). This omission may have raised concerns about the concepts reliability and stealth capability.

Thus, both YF-23s were put into storage, and later sent to museums. Other offshoots from the program – including schematics for the production F-23 variant, and the proposed Navy variant – the NATF-23 – were shelved.

Today the YF-23 remains a legendary aircraft among enthusiasts. Its unique design, cutting-edge technology, and overall ability to hold its own against the F-22 (at least on paper) has only added to its reputation. No doubt, hypothetical comparisons between the F-22 and the YF-23 will continue for years to come, or at least until all available data is released.

The YF-23 may not have been as polished as the F-22, but it remains – according to the only pilot to have tested both aircraft – a formidable dogfighter with great potential.