Most important characteristics of a fighter aircraft?...It depends upon ‘when’ you ask the question(Part 1 here)
Before any discussion about whether or not a fighter is ‘good enough’ to be an ‘effective’ and therefore ‘successful’ fighter design, there has to be a discussion on WHAT makes a fighter aircraft design effective and successful in the first place. While many may think they know the ‘magic mix’ that makes a fighter design a good one, the problem with the ‘many’ thinking they ‘know something’ is that the ingredients, and to be more precise, the mix of ingredients have been evolving continuously under pressure of technological and operational developments that span the entire history of fighter design. As I indicated in Part 1, for this part of the series we will use “The Characteristics of a Fighter Aircraft”, a 1977 paper by Prof. Gero Madelung to guide us through fighter development into the 1970s. At the time of his preparing this paper, Prof Madelung was the Managing Director of Panavia, the company formed expressly to develop and build the Panavia Tornado aircraft.
The First ‘Characteristic’ Emerged QuicklyProf. Madelung tells the history of fighter development in terms of the development and application of technology in response to the operational requirements over time, beginning with the first ‘challenge’ that had to be overcome (all text in [brackets] throughout this post are mine):
The initial generation of fighter aircraft (in W.W.I) had first to solve the problem of developing an effective armament, the art of maneuvering flight having been provided by the Wright Brothers only a few years earlier. The unarmed early airplanes were nevertheless providing effective reconnaissance and were as such already "fighters”. The Wright Brothers thus delivered probably also the world’s first actual fighter to the U.S. Army Signal Corps. The "armed fighter" was only a reaction to an earlier airborne threat [Zeppelins] to the land and naval forces.Prof. Madelung further observed that the thrust of fighter design for the next two decades appeared to be maximizing the fighter’s “1g SEP” (Specific Excess Power) or Rate of Climb to get higher than your opponents as a priority over other parameters, which drove increasing engine horsepower by an order of magnitude (10 times the WWI horsepower ratings) during that time while increasing weight only by a “factor of 3.5”. The size of the aircraft changed little during this period and most were sized around the pilot it would carry. Prof Madelung observed that the aerodynamics of the aircraft were held “subordinate” to structural load-carrying considerations, which meant that external bracing, and fabric covering over wood and steel tubing structure remained the norm.
The initial armament by hand-held guns was soon overtaken by the aircraft-mounted machine gun, but it was difficult for the pilot to control the airplane with one hand and to point the gun with the other, especially if a propeller in the front was in the way of the natural line of sight.
The solution of taking a weapons operator or "gunner" along was detrimental to the fighter’s rate of climb and speed. The other solution of reverting to a "pusher installation'' of the engine also resulted in a heavier airplane.
The truly ingenious solution was that of firing through the tractor-propeller with a rigidly mounted gun and to accurately point and aim by controlling the direction of flight and attitude of the aircraft. The propeller was protected first by local armour and later by a synchronizing system.
This system was, I believe, invented by R. Garros of France in 1915, met instant success and set a pattern which is still [as of 1977] valid. I am recounting this well known history because I believe that it really started the fighters as a special breed of airplanes.
To summarize this era, we find that (aside from getting as much climb performance out of the spindly early fighter designs) the single dominant characteristic of a fighter during this rather extended timeframe was ‘a machinegun aligned to the direction of flight’.
The alignment of gun and plane simplified the attacker’s problem of ‘attack geometry’ by reducing the variables involved. This simplification of the problem enabled the fighter pilots of the era to methodically plot and execute a path of attack, within some predictable level of certainty, that would at least enable him to fire his gun(s) in a direction that would place bullets on target.
Aerodynamics, Propulsion and Structural Design Matures
Prof. Madelung continues:
By the early 30's however the designers of airliners and bombers started to really apply aerodynamics including retractable landing gears and combined this with higher wing loadings and stress-skin aluminum structures. They were outspeeding the contemporary fighters which did only about 230 mhp [sic] with an engine of 600 hp. The fighter community had to react since it could not justify its existence for long by pointing out how excellent they remained in fighting their own kind. This started a revolution in fighter requirements and for the next 25 years these were reoriented towards excelling in maximum speed. It also started the introduction of mechanical complexity with all sorts of variable geometry features: retractable landing gear, hydraulic system, flaps and slats - soon used as maneuver devices, cooling flaps and variable pitch propellers.
The first fighter coming -very close to this new concept in 1939 was, I believe, the Russian Polikarpov “Rata”, which was only lacking the aluminum stress-skin and the closed canopy. In the following year appeared the ME-109 and the Curtiss P-36 followed shortly by the Spitfire… …all of which had without much increase in engine power a speed advantage of some 30% over the previous fighters. Early combat encounters proved the superiority of the new design [approach] despite its higher wing loadings.
|Complexity Drives Engineering Costs. |
If there is a recurring theme in this history, it is that requirements have, and do, drive complexity. That complexity has impacted design in different ways over time, including (in general) an increase in wing-loading as a by-product of the necessary complexity. We will see that the trend persisted to at least up until the F-15/F-16 era.
The Jet Fighter Arrives
Madelung now tells us that the constantly increasing ‘need for speed’ made the next pivotal point in fighter design recognizable beforehand:
Once the philosophy of maximizing fighter speed had been accepted, it was soon recognized that propulsion by propellers (and reciprocating engines) would be limiting this to some 450 mph. [propeller efficiency plummets as tip speed approaches the speed of sound.] Work on the first jet engines started at about the same time when the second generation fighters emerged, and took only 10 years, to the mid-40’s, to completely take over the propulsion of fighters. In this period the reciprocating engines were developed to high performance up to about 2800 hp and with turbo-supercharging for altitude performance.FADEC-equipped jet engines are simpler to operate than many light sport propeller-driven aircraft. The sudden jump in speed achievable by the jet engine performance drove fighter design right into the next technical developments that were necessary:
Yet the first operational jet aircraft in 1944, the ME-262, immediately had a speed advantage of about 100 mph with two jet engines of only 2000 lbs thrust each. Relative to the fastest bomber, the B-29, the advantage was almost 200 mph. The airframe and aerodynamics of these first jet fighters were at that not really advanced over the contemporary aircraft apart from the thinner symmetric airfoil, tapered spar caps made of steel and a nose landing gear with a breaked [sic] wheel. I was an apprentice at Messerschmitt when production of the ME-262 had started and I recall that the advent of the jet engine was welcomed as a move towards mechanically more simple fighters. The reciprocating engines with their increasing number of cylinders, already 48 cylinder engines were under discussion, their supercharging and their cooling system were getting increasingly more complicated. The jet engine had fewer moving parts and bearings, and the podded engine installation of the twin jet was mechanically very neat.
Again combat experience of the speed advantage was positive, despite another increase in wing loading. The associated disadvantage of requiring longer runways for these fighters was accepted by the Air Forces.
It was evident that further development of the jet engine would soon push the aerodynamic design concept to its mark under limit. The propulsion break-through was however followed by an aerodynamic breakthrough with the discovery of effect of wing sweep in the early 40’s. Again this technology reached the users first with fighter aircraft, that is with the F-86 and the Mig-15 in 1948. The speed was pushed right up to M 0.9, the limit of the thick swept wing, another step of about 160 mph.
Many people thought that fighter speed performance would settle for a while at that, and this may have been better in the long run…
However, the aviation world and in particular the fighter community, was in a speed craze and daring experimental airplanes in the U.S. had demonstrated by 1947 that the sonic barrier could be overcome by brute force and skillful design in terms of thrust, reduced wing thickness and powered control surfaces.
The increased thrust requirement could be met by the jet engine by reheat, which in turn required variable nozzles and resulted in additional complexity. Wing loading had to be further increased and so were the airfield requirements. Brake parachutes were required to shorten the landing run.
Prof. Madelung’s wistfulness over increasing speeds and ‘paths not taken’ is recognition that the military utility of increasing top speed past a certain point in the end provided a smaller return on time and dollars invested than it was worth, but we didn’t know it at the time:
The fighter community lost its innocence [sic] at this stage and only the major military powers, the U.S. and Russia, entered this round in the early 50’s with the introduction into service of the NA-F100 and the Mig-19, one and a half years later. The thrust of these magnificant [sic] fighters was about 3 times that of their predecessors, their speed at Mach-1 .3 about 40% higher. The single engine, single seat F-100 had about twice the take-off weight of its predecessors, at 30000 lbs equal to the twin engine medium bomber "NA-62 [North American Aviation Project Number for the B-25] Mitchell" with a crew of 4 only 12 years earlier…So Prof Madelung observed that, for the first time, the fighters’ size and weight due to the increasing complexity grew out of proportion with the speed increase. The increases in propulsion, structure, and systems complexity needed just to be able to fly at supersonic speeds drove the weight and size growth. Let us note here that as of this time in fighter development history that the ‘day-fighter’ sub-type was the norm for fighter design and that the need to make all fighters all-weather, 24-hour weapon systems was not yet the norm.
The aviation world of Britain, France and Sweden however followed suit with prototypes which were demonstrated in the mid 50’s, capable of Mach-2 and introduction into service of these fighters started 1958 to 1960. From the technology of the F-100 and the Mig-19 it was a matter of air intake development and further refinement of engine and airframe to reach the limit speed for aluminum airframes. The first fighter prototype to reach this limit was, I believe, the Lockheed F-104, with a thin unswept wing. A wing concept which was to gain prominence in future fighter designs, particularly in the U.S. "Mach-2" was to be the limit of the fighter communities’ speed craze and only special purpose aircraft, such as the Lockheed YF-12, SR-71 and the Mig-25,· were developed for yet higher speeds. The fighters which were developed in the :mid-50’s are however still now  dominating in quantity in the world’s forces, and inflation makes these complex airplanes appear inexpensive relative to anything we do in the 70’s.
It would be hard to find a better illustration that all the handwringing these days over increased costs and complexity is not a ‘new’ sport and that things will always look simpler and less expensive in retrospect than what Prof. Madelung wrote here in 1977. He then discussed the state of fighter design drivers in the 1977 milieu:
The question arises why, having reached the “ultimate performance" in terms of speed, new fighter designs were actually required. It is not surprising that the requirements picture was at first hazy for the follow-on generation, the development of which only started toward the end-60’s sand early 70"s with one notable exception [AV-8 and STOVL].Under ‘area .2’, Prof Madelung discussed the technology developments that came out of this new requirement including “fan-jet engines with greatly improved fuel consumption and the terrain following radar system”. He also made a point to emphasize the importance of, a practical scheme for the variable sweep wing” that “allowed the retention of optimum high speed/low level dash performance with a gain in cruise performance at all altitudes and greatly improved air-field performance.” Prof Madelung continued:
The following new requirement areas were however becoming apparent:
1. In the late 50’s concern was mounting relative to the vulnerability" of fighter forces relying on these long 9000 ft runways...
2. Another new requirement which became important to the fighters in their fighter-bomber role was that of low level/high speed penetration. In fact, most of the early Mach-2 fighters are usable in this role due to their high wing loading. The F-104 with appropriate navigation equipment and plenty of external fuel is  still widely in service for this task, a task which is of particular importance in Central Europe.
3. Another new requirement which emerged in the late 60"s called for a better balance of performance in air-to-air combat. The high speed capability of the Mach-2 fighters of the mid-50’s turned out to be of little practical use as there were no bomber and recce aircraft flying at such speeds (apart from special purpose aircraft which. could not be intercepted anyway by a tactical fighter), and air-to-air combat could actually be sustained only in the lower transonic regime with these airplanes. A better balance of performance could be achieved mainly by a decrease in wing loading, which would provide for higher turn rates in the speed and altitude regime of dog-fights, at the expense of increased wetted surface and of a heavier airframe, i.e. trading rate of climb and low level dash performance. It is a tribute to aviation technology that the new generation of fighters actually improve also the latter two performance regimes while making a big step forward in turn rate and as a fallout in airfield performance.
4. Finally the new fighters would require a "look-down" capability of the radars in their air-to-air role in order to be able to fight the low level intruders. …So the trend shifted for the first time in decades to not a more ‘maneuverable’ and ‘balanced’ fighter design, once the practical upper limit of aircraft speed was reached:
The four U.S. designs, the F-14, F-15, F-16 and F-18 and the Viggen have low wing loadings (50 to 70 lbs/ft2·) to optimize turn rate. The latest three U.S. designs, the F-15, F-16 and F-18 have at the same time thrust to weight ratios in excess of one, resulting in a big step forward in dog-fight capability. They employ advanced materials including composites and very advanced engines. The latter two designs are introducing a new aerodynamic feature, the "strake", to improve the lift of the thin, unswept wings at high angles of attack. In the case of the F-15 this dog-fight capability is combined with fairly long range air-to-air missile intercept capability which results in a very big fighting machine, with a wing area of 650 ft2, as big as the F-14 fleet defence fighter….…In Europe most of the forces have emphasized the requirements (1) and (2), that is low level/high speed penetration capability associated with excellent airfield performance. The defence environment of these countries requires instant and effective response, day and night and all weather in the land battle. The latest fighter engine technology with the magnificant [sic] thrust to weight of about 8.0 and variable sweep with considerable use of titanium were applied in the Tornado to improve payload-range by a factor of about two for this mission, and to cut runway requirements to 60%. This twin engine fighter with plenty of avionics, a crew of two and a wetted surface of only about 1850 ft2 is smaller than an F-4 Phantom, about in-between the big and new U.S. fighter with wetted surface of about 2800 ft2 and the small fighter of about 1400 ft2. At the same time this aircraft will provide first-class long range air-to-air capability with an air defence avionics fit and long range missiles.
What Was to Come after 1977?
Prof. Madelung then ruminated on what would be the NEXT developments in fighter aircraft design, and as it happens he was largely prescient, foreseeing most developments that have since occurred or are emergent at this time, I see only one complete ‘miss’, a tail-sitting fighter did not come about. But that may have been due to the collapse of the Warsaw Pact as much as anything else. Who knows what would have happened otherwise? But I’d say he had an amazingly complete vision of what would be the major requirements that would drive those developments.
The outlook into the more fighter-specific areas is difficult because of the attendant operational trade-offs which depend upon projected structures of threat and friendly forces. The "haze'' obscuring the real future requirements is still very thick, apart from the broad scope of ECM, the air-to-ground weapons area and the requirement to reduce unit cost.Prof Madelung predicted increased use of the post-stall flight regime:
In any case it will be increasingly necessary to “destill" [sic] the essentials for future combat effectiveness, rather than relying on the simple formulas like maximizing rate of climb or speed or turn rate. We have already seen two breaks in such simple and general formulas. The trade of quality versus quantity will remain most difficult.
In fighter aerodynamics and control we will probably open up the post-stall regime for another increase in dog-fight maneuverability….But he also saw under what conditions that post-stall maneuverability might not be so important: recognizing other development could obviate the advantage:
…As long as weapons remain installed in the classic fighter style requiring turning of the whole aircraft for pointing of the weapons, this post-stall maneuverability may also be of interest to other than dog-fight missions….He recognized the potential of thrust vectoring in exploiting the post-stall maneuvering capability:
…Post-stall maneuvering will require some form of auxiliary control such as used on VTOL aircraft or missiles, for example by thrust vectoring. It will also require an air intake and engine suitable for angles of attack of 90°. Both techniques are basically available…
|Project GunVal Concept: Cannon Turret on F-89. |
The turret rotated and the guns elevated 90 degrees. Sanity
inserted itself before the system ever flew. (Northrop Photo)
…Another next generation fighter may be (the return of) some form of pointing the weapons other than by the pointing of the entire aircraft. I hesitate to put this forward since all earlier attempts involving some form of weapons-turret and a gunner have been failures when used on fighters. The fixed guns operated by the pilot have been a tremendous success due to the light weight and low volume and due to the accuracy of firing achieved with this installation. However the rate of pointing of the fixed weapons is slow, even a fighter with a turning rate of 18°/second will take some 6 seconds for 90° change of directions. Modern weapons installations on ships and cars will do 90° in less than one second. One approach to overcome this problem is to program, with the aid of a helmet sight, the projectile or missile to turn at very high “g” after being fired from a conventional fixed launcher. It may be rewarding to find a simple way of achieving this with a gun since this form or armament is still the most economic one.Helmet Mounted Sights may not have required all that much of a leap in imagination given the then state of the art and the known initial goal of employing them on what would become the F-15, but taken in context of all his thoughts on the future it is still impressive that he thought them significant.
Stealth Was Seen as Too Hard for AircraftMadelung recognized the advantages of reduced RF and IR signatures, falling short in his vision only because he was not aware of the revolution in stealth that was underway as he spoke:
Yet another design feature may be that of reduced signature for radar and IR missiles. For a full-fledged fighter with all its other requirements these appear to me pretty difficult additional ones.However one should bear in mind the advantages of small aircraft size in this context as well as for reduced probability of visual detection and last but not least for a better chance of not being hit. The next generation of fighters should, and not only for these reasons, be of moderate size.As the control of UAVs by manned aircraft seems to be brought up more and more as ‘the future’ Prof. Madelung’s observations on the topic seem particularly ‘timely’:Finally this outlook has to cover the prospects of unmanned fighter aircraft: adding up all the interface design features which are required to allow the pilot to control an aircraft, as well as the features to provide for the appropriate environment and safety, a lot of sensors and computing capacity could be provided instead, using microprocessor technology. The ''cruise missiles" are paving the way in this direction and I expect that the fighter aircraft designers will have to take this development very serious. The manned fighter will have to concentrate on the more difficult tasks which cannot be readily programmed. One could imagine combined systems of manned fighters and unmanned aircraft like a hunting party with hounds, the latter being "programmed" to track and harass under the command of the former.Recognizing the limits to the return on investment from increasing aircraft capabilities, he foresaw a shift to more capable weapons such as AMRAAM and ASRAAM etal. And the interest in even more advanced weapons continues unabated.
The future of both the manned and unmanned fighter may however depend largely on the development of more effective weapons and methods for the air-to-ground battle in order to achieve a better balance of cost effectiveness.Prof Madelung then concluded:
Some 75 years ago the Wright Brothers had the vision, skill and persistence to develop the prototype of powered aircraft, and gave birth to a new dimension of mobility and spirit of mankind. The fighter aircraft is one of the grim but magnificant [sic] outgrowths of this new dimension and will continue to participate in a lead role of aeronautics if the "fighter community" will maintain and develop its vision, skill and persistence offering-new and cost effective qualities and performance rather than retiring to marginal improvements.
To Recap the Part 2 'SoFar'Operational requirements other than ‘maneuverability’ drove fighter design for far longer than post-stall ‘Supermaneuverability’ has been part of the definition, and ‘maneuverability’ was and still remains only one of the required hallmarks of fighter design.
Further, while ‘maneuverability’ has always been a requirement, the definition of same evolved over time. ‘Maneuverability’ only increased in importance relative to top speed and ability to climb after the option of increasing the top speed and climb rates for fighter aircraft reached their practical operational limits.
Most important to the current and near-term future of fighter design considerations are:
1. ‘Maneuverability’ as it is currently interpreted to include post-stall controllability is a relatively new construct in the history of fighter design development and even in 1977, the limitations of post-stall maneuvering, and developments that could render it less effective or even ineffective were already foreseen.
2. Low Observable aircraft were seen as unachievable by a noted aircraft designer at the same time the US was developing the first Low Observable (LO) aircraft in the form of the F-117. We do not have Prof. Madelung’s thoughts on the ramifications of this development, but he obviously grasped the significance of LO in even mentioning the possibility of LO weapons (a couple of examples of same I was supporting or flight testing by the early-mid 1980s’.)
Next UpInstead of breaking Part 2 into different sub-part posts, I’m electing to keep all of Part 2 in one post, adding material in stages. Given that ‘supermaneuverability’ appears to be the current ‘top dog’ requirement in the general public’s mind and that perception seems to be running amok in any discussion on what defines ‘maneuverability’, in the coming section we will discuss ‘supermaneuverability’ by leveraging a series of technical papers written by W.B. Herbst in the 1970s and 80s’. As Dr. Herbst coined the term ‘supermanueverability’ in the first place, his thoughts should provide a solid basis for further discussion on the benefits and limits of post-stall maneuverability and how it fits in air combat scenarios going forward.
I've been doing killer hours (60+ billable last week for example) at work and collapsing on the weekend. Outside of work and a comment elsewhere here and there all I've been doing is sleeping.
1 Sep 15: Things did not lighten up. I'm about 2/3rds the way through with the rest of Part 2. Soon....
….to be continued…