Sunday, May 15, 2016

Lavi - An Engineer's Perspective

The Lavi fighter program was the largest weapons development effort ever undertaken by the State of Israel - embodying a unique, Israeli perspective as to what elements were most essential in the design of a modern warplane.

The following summary provides a brief cross section of this development effort, with a focus on the technological, engineering side of the program - setting aside for the moment the historical chronology, as well as the political debates that also surrounded this aircraft. In developing this summary, I have drawn on my years of experience as an aerospace engineer, leveraging in particular the skills that I first learned as a teaching assistant for Aircraft Design more than two decades ago. It has been my experience that it is when you have to teach a subject to someone else, that you first truly master it, and it is on that experience that I have drawn heavily on here. Again, I point out that the opinions expressed here are my own, and do not necessarily represent those of my employers, either past or present.

As an aerospace engineer trained in the discipline of design, you come to see aircraft somewhat differently from the casual observer. The features that go into a design become recognizable as the products of conscious design trades, rather than the result of arbitrary selections or preferential aesthetics. In this manner, the Lavi bears a unique, and distinctively Israeli emphasis in its design and construction.

In order to appreciate many of the trades that went into the Lavi, however, it often helps to have a point of reference. To this end, there just so happens to be one particular aircraft that stands out as the natural point of comparison: the General Dynamics (later Lockheed Martin) F-16. The F-16, to which the Lavi was so often compared, provides a contemporary reference point that was, among other things, the most widely produced lightweight fighter of its day. It was also, however, designed with a slightly different focus to its development.

The F-16 was first developed during the 1970s as a lightweight air-to-air "day fighter", with a secondary air-to-ground capability. In this regard, the F-16 was following in the footsteps of such aircraft as the F-5E Tiger II, which set the standard during the Vietnam War era for what could be accomplished within a lightweight fighter configuration. Like the F-5E, the developers of the F-16A selected a thin, trapezoidal wing. While this fit in with the lightweight design concept behind the F-16, it also led to a wing configuration with limited volume available for fuel stowage - requiring the F-16 to carry the majority of the fuel that it needed to complete its mission inside its fuselage.

The Lavi, in contrast, had a very different emphasis. The Lavi was designed as an air-to-ground fighter-bomber first, with a secondary air-to-air mission. The antecedents for the Lavi design included such strike aircraft as the Kfir and the A-4 Skyhawk. Much like the A-4 Skyhawk before it, the Lavi selected a delta-wing configuration, with a thick wing root section. This configuration would prove crucial toward providing the fuel capacity necessary to complete the airplane's mission. Whereas the F-16 was able to reserve volume in its wing structure for only 19-percent of its total required fuel capacity, the Lavi was able to reserve space for roughly 54-percent of its internal fuel capacity within its wings - allowing crucial volume in its fuselage to be reserved for other needs.

The selection of a delta wing configuration for the Lavi had a variety of implications - above and beyond the available fuel volume. The spars that carry the span-wise load through the wing of an airplane are essentially tapered I-beams in their cross-section. As any freshman engineering student should know, the modulus of an I-beam will increase as the height of the I-beam cubed, and the bending stress of an I-beam will decrease as the height of the I-beam squared. The result was that due to its thicker wing root, the Lavi wing structure was inherently stronger, and could carry more weight than its counterparts.

It was this kind of structural trade that allowed the Lavi to achieve a maximum take-off weight that was 13-percent greater than a Block 30 F-16C, with an empty weight that was 10-percent less.  Or measured another way, which allowed the Lavi to achieve a hi-lo-hi combat radius that was 50-percent greater than a Block 40 F-16C, with an empty weight that was 20-percent less.

Added to the inherent structural advantage of the Lavi's thicker wing root section, was the incorporation of composite materials. The Lavi airframe was intended to be 22-percent composite materials by weight - compared to the roughly 2-percent of the airframe weight found on the F-16A. The incorporation of composites allowed such structures as the Lavi wing, vertical tail, canard, air brakes, and ventral strakes to be lighter. They also, however, reduced drag, and allowed for the incorporation of aeroelastic tailoring to further improve on performance.

The Lavi was intended, for example, to carry many of its weapons loads semi-conformally on under-fuselage hard points. When combined with aeroelastic tailoring to reduce stores-induced wing flutter, it was expected that aerodynamic drag could be reduced on the Lavi by up to 50-percent. This degree of refinement had simply not been available at the time that the F-16 was developed a decade before.

The selection of the Lavi wing arrangement, however, was only one of many design trades that went into the development of the airplane. Another example was provided by the Lavi's inlet arrangement. The developers of the Lavi evaluated multiple candidate inlet configurations, from the axisymmetric arrangement seen on the Kfir, to side-mounted inlets, to the ventral inlet that was eventually selected. Evaluated on the basis of inlet distortion at high angles of attack, both the side-mounted, "shielded pitot" style inlet, and the ventral inlet offered superior performance with lower inlet distortion. Eventually, however, the ventral inlet was down-selected for the Lavi by virtue of its lower structural weight.

Similarly, multiple configurations for the Lavi vertical tail were initially assessed. Among the more unusual was the tail-boom configuration, which offered superior directional stability at very high angles of attack. Once again, however, the Lavi developers down-selected to a more conventional, single vertical tail, due to its lighter weight.

Among the most central design choices made by the Lavi developers, was the selection of its canard-delta configuration. Canard fighter designs fall into two general categories: close-coupled, and long-coupled. Close-coupled canard designs place the canard in close proximity to, and slightly above the wing, to maximize the canard-wing interaction and increase the airplane's lift-to-drag ratio. This design approach includes such aircraft as the Kfir, Gripen, and Rafale - in addition to the Lavi. Long-coupled fighter designs, in contrast, attempt to leverage the high angle-of-attack control authority that the canard offers, while reducing the size and wetted area of the canard to its bare minimum. These designs, as represented by the Rockwell-MBB X-31A, or by the Eurofighter Typhoon, will tend to maximize the distance between the canard and the wing, and will position the canard at the same elevation, or perhaps slightly lower, than the wing.

Relative to its wing size, the Lavi had the largest canard of any fighter or fighter prototype yet developed. It was also positioned closer to its wing than most of its counterparts, resulting in a greater improvement in aerodynamic efficiency. This, in turn, further enhanced the Lavi's combat radius.

The other leading design decision that had to be made early in its development, was the airplane's engine selection - which was integral with the overall airplane sizing process. The Lavi fighter was originally sized around the F404 engine, at the time of the program launch in February 1980. As the requirements of the program expanded, however, it soon outgrew its original engine selection, and the design was relaunched in May of 1981 with the larger PW1120 engine. The PW1120 afforded a 28-percent increase in available engine thrust, which translated into a larger airframe, with up to a 19-percent increase in combat radius.

Despite its focus on the air-to-ground role, however, the Lavi still needed to be able to perform a secondary air-to-air function. A first order assessment for how the Lavi would have performed in this role can be obtained by comparing the thrust-to-weight ratio, and wing loading of the Lavi to similar values for its counterparts. In general, aircraft with higher thrust-to-weigh ratios will tend to offer superior acceleration, and aircraft with lower wing loadings will tend to offer superior, instantaneous turn rates. This comparison is valid for trends only, however, since it lacks any correction for differences in the aerodynamic performance of each design.

Compared in this fashion, it can be seen that the Lavi and the F-16A were developed with very different strategies in the air-to-air arena. Building on historical experience with such lightweight fighter designs as the F-5E, the F-16A sought to maximize its advantage in thrust-to-weight ratio over its peers of the day.  The Lavi, in contrast, still had to perform its primary mission as an air-to-ground platform. It could not therefore expect to meet or exceed the thrust-to-weight capability of aircraft such as the F-16, seeking instead to leverage its advantage in wing loading to attain a higher turn rate.

It can also be seen from this comparison that, as additional strike roles were added to the F-16, weight was also added in order to increase payload capability. Advances in engine technology allowed the F-16 to maintain its high thrust-to-weight ratio with each successive Block release, but its wing loading continued to climb - impacting turning performance.

A more complete story of relative aircraft performance can be achieved using an energy-maneuverability, or "E-M" diagram. The E-M diagram will portray isocontours of specific excess power, plotted against speed or Mach number on the horizontal axis, and either altitude or turn rate on the vertical axis. Regions where the specific excess power is positive, will denote zones where the airplane is able to accelerate or gain altitude. Regions where the specific excess power is negative, will denote zones where the airplane will lose either speed or altitude. The isocontour where specific excess power is zero, will identify the maximum sustained turn rate for the airplane at that particular speed, altitude, and weight combination. The E-M diagram also allows us to compare competing fighter designs, to evaluate regions of the flight envelope where each is at a relative advantage.

In the instance of both the Lavi and the F-16A, there is published data available from the open literature allowing us to develop an unofficial, approximate E-M diagram from which to make just such a comparison. As anticipated, the Lavi would be expected to achieve an advantage at elevated turn rates. The F-16A, in contrast, would be expected to achieve an advantage at lower turn rates - where it could out-accelerate the Lavi.

In summation therefore, it should be apparent that the design of the Lavi was biased from its inception towards the air-to-ground role - a factor that drove its design towards lighter-weight options that maximized the airplane's range and structural efficiency. This emphasis was very different from its contemporaries, most of which were biased towards short-range, air-to-air objectives. The influence of this emphasis can be clearly seen in the design of the Lavi, where it is stamped large onto its design configuration

Again, this has been but a brief overview of the technical side of the story surrounding the Lavi.  Additional information about this program and its history can be found in the recently published book, Lavi: The United States, Israel, and a Controversial Fighter Jet.


Golan, John. Lavi: The United States, Israel, and a Controversial Fighter Jet (Sterling, VA: Potomac, 2016).
Shmul, Menachem, Eli Erenthal, and Moshe Attar. “Lavi Flight Control System.” International Journal of Control. No. 1, 1994: 159-182.
Tsach, S., and A. Peled. “Evolution of the Lavi Fighter Aircraft,” in Proceedings of the 16th International Council of the Aeronautical Sciences (ICAS) (Jerusalem: Aug. 28 - Sept. 2, 1988): 827-841.

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