Apr 282011
 


The history of stealth aircraft design remains shrouded in secrecy.  Though selected details regarding the development of such successful stealth aircraft as the Lockheed Martin F-117, Northrop Grumman B-2 and Lockheed Martin F-22 have come to light in recent years, information on the unrealized projects and studies that preceded these aircraft has been less easy to come by, as most remain classified.

One study that has come to light is the McDonnell Douglas Model 226-458 Quiet Attack Aircraft (QAA) of 1974, a project sponsored by the Office of Naval Research that offers a snapshot of the state of stealth aircraft development in the early 1970’s.  The Model 226-458 is briefly mentioned in the book Advanced Tactical Fighter to F-22 Raptor: Origins of the 21st Century Air Dominance Fighter by David C. Aronstein et al., an excellent technical history of the F-22.  This article summarizes a declassified report covering certain aspects of the QAA study.

History

Reduction of aircraft radar cross section (RCS) became a big priority for the US military in the aftermath of the Arab-Israeli war of 1973, in which large numbers of Israeli aircraft were downed by Soviet-made surface-to-air missiles over a very short period of time.  Realizing the same fate could befall the US if it ever got into an all-out conflict with the Soviet Union, the Air Force and Navy began to invest more heavily in technologies to reduce RCS.

Initially, creating a stealthy design was a matter of trial and error.  Modeling clay was progressively applied and shaped on models, which were then sent to an RCS range for testing.  It was a slow and inexact process, but significant progress was nonetheless made.  Aircraft from this period that resulted from this approach typically featured extensive surface blending, exemplified by the portly Northrop Tacit Blue; the Model 226-458’s elegantly blended airframe suggests that it was also designed in this manner.

McDonnell Douglas was supposedly the among first US aerospace companies to arrive at a determination of what the RCS levels had to be in order for an aircraft to avoid detection while performing a useful military mission.  This was in no small part due to their ownership of the Grey Butte Microwave Measurement Facility, one of the best test ranges in existence for measuring very low RCS values.  According to Bill Sweetman, Grey Butte was originally established by McDonnell Astronautics for reentry vehicle testing. Lockheed and Northrop also conducted various tests at Grey Butte. However, unlike its rivals, McDonnell Douglas seems to have come up short in designing an aircraft that could achieve the desired RCS levels.

The QAA program was an exploratory investigation to conduct applied research on the feasibility and potential operational value of a covert, quiet, carrier-based aircraft for Navy/Marine Corps missions.  RCS reduction was achieved through the filleting of corners and blending of the wing and tail surfaces into the fuselage, employing a v-tail and providing an internal weapons bay.  At the time, previous attempts at signature reduction on military aircraft had focused on the reduction of one signature or fixes on existing aircraft, such as the application of RAM (radar absorbent material) to areas of high radar reflectivity.  The Model 226-458 was designed from the outset to reduce all signatures to a level that would permit covert operations and reduce vulnerability.

In order to develop the stealthiest aircraft possible, a baseline aircraft was required against which the impact of design changes could be compared.  McDonnell’s first study in the QAA program was the Model 226-450A, which was developed without any attempt at signature reduction, making it the smallest and lightest of the configurations studied in the program. This project was powered by one non-afterburning turbofan engine of low bypass ratio. This aircraft carried all stores externally and was in the same size and weight class of the A-4 Skyhawk.

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