VIFF-ing the Harrier

This was the start of an unforgettable friendship, with both the man and the task.

Step one was to evaluate the characteristics of the Harrier against those desired in a first-rate fighter, albeit defensive as in this case. Limiting the scenario to low to medium altitude, subsonic speeds, and dogfighting with close-in weapons seemed a reasonable approach, considering the prima(Y close air support mission of the aeroplane.

With these constraints imposed, what fell out as desirable were: (1) high acceleration capability (thrust to weight, etc); (2) good manoeuvrability or turn rate (wing loading); (3) high structural limits; (4) excellent handling characteristics; and finally (5) good pilot visibility. Who knows, with high enough values in all of these areas maybe an attack pilot could win a fight or two.

Unfortunately, the Harrier was not designed with the ACM arena in mind. It was optimised for a completely different mission. Fortunately, the characteristics demanded by the attack role closely coincided with the fighter requirements. The aeroplane possessed an extremely high thrust-to-weight ratio, even considering the fact that the engine was restricted to approximately 4/5 power once windborne. The turn radius or manoeuvrability was poor over much of the speed range due to the very high wing loading, although at higher speeds it could attain its structural ‘g’ limit, which was equal to or better than most existing fighters.

The vision was also excellent to the front but poor to the rear; good if you are winning, bad if you are losing. Finally, the handling was superb, both for the airframe and the engine. By this I mean the stall characteristics were such that pilots would not be afraid to fly the aeroplane to the edge of the published envelope. The airframe went where pointed and the engine kept running. Who could ask for more?

Overall though, the Harrier could not be considered as good fighter material. It could possibly beat any X-15s or blimps it stumbled across, but it would be in trouble with anything else.

Fortunately, the story does not stop here. The aeroplane came equipped with the ability to vector its thrust in flight (VIFF – Vector In Forward Flight, or TVC – Thrust Vector Contra The effect of this, then unknown, variable provided a new dimension in air combat.

The first look at thrust vectoring took place during the tests at NATC. Since the RAF had been using VIFF for several years and test sorties were at a premium, I decided just to look at the 500 knots, full reverse thrust point, without any intermediate checks.

Before I describe the results of this end-point test, I must explain something about my personal strap-in procedure. Due to a strong belief that it is of vital importance to be able to keep track of other aeroplanes, especially when they are behind you, I only loosely attach me to the seat, thus allowing maximum freedom of movement and visibility. The penalty, of course, is that you are free to bounce around the cockpit if the aeroplane makes any violent manoeuvres.

Now back to the 500-knot point. I tried to picture myself in a position where an imagined attacker was closing in for the kill and I had to slow down as fast as possible to cause an overshoot. To quantify the manoeuvre, I was going to time my deceleration from 500 to 300 knots.

With pencil in hand, I slammed the nozzle lever into the braking stop (reverse thrust) position. The aeroplane started decelerating at an alarming rate, the magnitude of which I could not determine because my nose was pressed up against the gunsight. A terrible way to be, even in an imagined conflict. My movements were even further constrained by the fact that the violence of the manoeuvre had dislodged me from the seat and I was now straddling the stick with my right hand extended backwards between my legs trying to hold on for dear life. Obviously positive ‘g’s’ were out of the question! As the aeroplane slowed and the forces reduced, I was able to push myself back into the seat and regain control.

Two lessons were learned. I was going to have to compromise my strap-in procedures and the potential of VIFF was awesome. If somehow the tremendous forces associated with thrust vectoring could be harnessed in a more meaningful fashion, then stand by the world, a new dogfighter was coming your way!

Due to the limited duration of the NATC trials, the next step took place at Beaufort, South Carolina, home of the Marines’ first Harrier squadron, VMA-513. By this time, we had realised that the aeroplane had some unusual characteristics when the nozzles were deflected. For one thing, the nose pitched up about 30 degrees, a fact the aerodynamicists were very unhappy with. Secondly, the reaction controls were automatically activated by the nozzles and could be used to control the aeroplane below normal stall speed. A new portion of the envelope was starting to open up – low speed.

How to develop tactics to take advantage of these characteristics now became the problem. A contractor was hired to evaluate the effects of VIFF using a computer simulation. The results indicated that the turning capability of the aeroplane could be enhanced, but the loss of energy associated with VIFF would not offset any gains. Not very encouraging.

The computer program was then inserted into the manned ACM simulator at the McDonnell Douglas (MCAIR) plant in St Louis. This simulator allowed a pilot in one cockpit to fly against another pilot in a different cockpit with the computer acting as the go-between. The results were portrayed on a wrap-around screen accompanied by sound effects so that a high degree of realism was attained.

The effort indicated that the effects of VIFF were far more impressive than those predicted by the pure computer. Several good defensive manoeuvres were identified, along with a first look at the use of full engine thrust (combat plug) in ACM.

The only anomalies encountered appeared when the nozzles were deflected at low speeds. First, the model in the computer did not turn as well as the actual aeroplane when light stick forces and small nozzle angles were used. When MCAIR changed the simulator to match the turn rates my memory recognised as being correct, I was told that it was aerodynamically impossible for the Harrier to achieve such rates.

The second and more disturbing of the anomalies occurred when violent manoeuvring was done while using thrust vectoring at low speeds. My first indication that all was not well was during a dogfight when the nose of the airplane started a slow undamped oscillation up and down when I used a large amount of VIFF and backstick. As the airspeed decayed, the amplitude of the pitch increased until, at 180 knots, the aeroplane started to tumble nose over the tail. Regardless of what I did, the simulated plane continued to flip until it hit the ground.

Although this manoeuvre completely befuddled my opponent, I did not categorise it as an enhancing characteristic of VIFF. The situation was repeated several times with the same results. I was becoming quite used to seeing alternating views of houses, sky, fields, clouds, etc.

The seriousness of this episode was that it took place in a portion of the flight envelope that had not been thoroughly explored, at least by the US, thus the simulator reaction had to be accepted as the best indication of what was to come. Undaunted, I put on my white scarf, changed my shorts and returned to Beaufort to verify the results using an actual Harrier.

Fortunately, the tumbling manoeuvre turned out to be a fault of the simulation rather than the aeroplane. However, it cost several excess sorties to determine this, since I was unwilling just to rush out and test the worst-case point, but rather chose to approach it in an incremental manner.

The turn rate discrepancy was verified by an actual flight test. Under certain circumstances, the Harrier would turn faster than theoretically possible. It wasn’t until the AV-SB wind tunnel tests were conducted that this was fully understood. It seems that when a small nozzle deflection angle is used and the aeroplane is in a buffet-free condition, a pumping action is set up that smooths the flow of air across the wing. Essentially, a form of boundary layer control (blown wing) is invoked; a positive side effect of VIFF that no one had predicted. This phenomenon is a little tricky to learn to use, but once mastered enables the pilot to perform tasks previously considered impossible.

Now the time came to tie the package together and hopefully produce something operationally useful. Preliminary results indicated that our best areas were at the top and bottom ends of the speed range. But how does one take advantage of such characteristics, especially in light of the fact that history shows the attacker would most likely be behind you before being detected?

Considering the difficulty of starting on the defensive while attempting to integrate the unusual characteristics of the Harrier, the answers came surprisingly fast. Two of the more unique manoeuvres developed to neutralise an enemy fighter who is approaching firing range are called the “defensive break” and the “flip”.

Describing the first in terms of pilot action required means: “When first detecting the enemy in your rear hemisphere, but, not in range, use the brute power of the Pegasus engine to accelerate to high speed while turning to keep enough angle off to prevent the attacker from firing. If he still closes and approaches lethal range, perform a ‘defensive break’. That is, pull to maximum turn rate and use a large amount of VIFF.”

“The Harrier will pitch into the turn by about 30°, jump vertically towards the inside of the circle and rapidly decelerate. The attacker will be forced to overshoot, and as he slides by to the outside, reverse and turn back into him. Now he is the hunted and is faced with a choice of slow fighting an airplane that can virtually stop, or trying to out-accelerate the little jet. In either case, his day is ruined. ”

The flip is “a second method that is used to dispose of an enemy who is close in the trail is to climb at a very steep angle entering the low-speed environment. Some type of spiral manoeuvre has to be employed while climbing, to avoid becoming a stationary target. As the speed further decays, pull the aeroplane through the vertical using light stick forces and small nozzle angles.

“The Harrier will ‘flip’ to a 90° nose low position. The attacker will be forced into a ballistic semi-controlled turn over the top in an attempt to match your manoeuvre. As you both start back towards earth, put in full reverse thrust. The Harrier will essentially stop relative to the attacker. With no way to prevent his acceleration, he will slide by in a rather helpless state. The rest is easy. ”

If you are fortunate enough to be the attacker, your task is simplified by VIFF. No longer do you have to worry about excessive closure rates? If the adversary detects you and performs his own defensive break, you can use thrust vectoring to control the overshoot. His only choice is to continue his break away from you or reverse and engage in a slow-speed fight. In either case, you still have the advantage.

The effects of this capability are twofold. It allows you to become a more aggressive attacker and it denies a primary defensive counter to the airplane being attacked.

With these manoeuvres in hand, we were now ready to enter the next test phase; flights against airplanes with characteristics similar to Warsaw Pact fighters. The tests were exhaustive, consisting of several hundred sorties on an instrumented range. The results enthusiastically endorsed the operational effectiveness of the Harrier and in particular, the ‘defensive break’ and the ‘flip’ over the top.

Two much-needed improvements were nevertheless identified by these tests. (1) an elimination of the restrictions_ that required the pilot to check engine parameters prior to using thrust vectoring; and (2) more thrust to allow for higher vertical manoeuvring against low wing-loaded aeroplanes.

On the positive side was the fact that the much-maligned endurance of the Harrier turned out to be a strong point. Although being able to carry less internal gas than most of the opponents, the fuel usage of the Pegasus engine was so much less than that required by the other jets to produce a comparable thrust-to-weight ratio, that they would have to terminate the fights due to low fuel states. In peacetime tests, this was not considered a big factor, but in actual combat, it could prove really embarrassing to the opposition.

Overall the Harrier was impressive. We now knew that, because of thrust vectoring, the aeroplane could survive in a hostile air environment. However, there was still that nagging feeling that we didn’t quite have our act together. There was more potential buried in the aeroplane and somehow we had to get at it.

Back to Beaufort again to work out the bugs. This time it was out of our hands though and up to the Naval Air Systems Command (NAVAIR) and the contractors to take action. We had identified what was needed, they had to provide it.

Lo and behold, will miracles never cease? Modifications arrived to beef up the nozzle system thus eliminating most of the hated restrictions. A significant improvement but not quite all that was needed.

Then it arrived! A combat plug! A screw-in turbine temperature control fuse that allowed the engine to put out significantly more thrust when winsome than we now had. To put this in perspective, the two-minute installation of the plug resulted in a higher thrust gain than the A-4 Skyhawk had seen from all its engine improvements over a 20-year life.

The plug had one restriction and one reservation associated with its use, but they did not present an operational problem. It could only be used for 2½ minutes at a time and the effect on the life of the engine was unknown.

The 2½ minutes coincided nicely with the length of an average dogfight, and the USMC tactics of short nozzle spurts during VIFF (7 sec. ) had, to date, shown no adverse effects on engine life. However, several knowledgeable people had great reservations about the latter point, not the least of which was one of the principal designers of the engine. Their thoughts had obviously influenced the deliverer of the plug since I received it wrapped in a package marked: ‘Personal, for Harry Blot only; do not reveal source; if an aeroplane crashes while the plug is installed and you get thrown clear, climb back in the fireball’. Fortunately, adventurism won out over conservatism and NAVAIR permitted the tests to continue.

It was now time to fly the modified, plug-equipped bird. It was love at first flight. The Harrier did everything that was asked of it. The airplane had crossed the line from being good to being great. This was the end of the project. All the potential was tapped. If an attacker wanted to shoot down this airplane, he was going to have to earn his hero medal.

The raw power allowed you to pitch up vertically high enough to make the pilot of a wing-loaded airplane commit himself to a turn direction, thus enabling you to flip back at him, accelerate to a gun range before he could escape and get in a good shot. No longer was it necessary to look into the cockpit to check gauges. You could concentrate on the target.

Further tests were conducted to verify the capability of the modified bird and verify they did! There was no longer any question about what it would do when all the deficiencies were corrected. The Harrier’s ACM capability was absolutely eye-watering!

Several significant results came from these trials. First, the Harrier evolved from a barely survivable attack aeroplane to an aeroplane that could not only survive but also stand an excellent chance of destroying its attacker. Secondly, thrust vectoring was proven to be a valuable, operationally significant means of enhancing the characteristics of a tactical airplane. Finally, the improvements needed to optimise the use of thrust vectoring in air combat were identified for further use.

Unfortunately, Lt Col Bud Baker died (of natural causes) before his project was complete, but those of us who are still flying are about to reap its benefits. The next version of the Harrier, the AV-8B, is a fully modified, thrust-vectoring-equipped aeroplane. With this machine in our inventory, the Free World will have a usable first-rate fighter, even if all the runways are cratered.

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Lieutenant Colonel Harry W. Blot, USMC, soon to be assigned to the Second Marine Aircraft Wing, has recently completed the Industrial College of the Armed Forces in Washington, D. C. As a pilot, he has flown more than 25 models of tactical airplanes and has seen combat in Vietnam. His technical schooling includes an engineering degree from Villanova University, a master’s degree from George Washington University and diplomas from the USAF Aerospace Research Pilots School and the Industrial College of the Armed Forces.