How Petter Made The Gnat Light

This and My Other Reminiscences of the Wunderkind

By Gp Capt NK Krishnan (Retd)

1. My right to claim membership of the Gnat Brotherhood could actually date back to a mid-fifties SBAC Show at Farnborough, when as a kid, I was able to snaffle an autograph of a gentleman called Petter, on a photograph of a civil registered Gnat (it could have been a Midge). But kidding aside, the Gnat was the first aircraft whose systems we learnt at the Air Force Technical College, The luck of my course was that our instructor, (then) Sqn Ldr B Mukherjee, was passionately in love with the Gnat and very experienced on the type. This was our, (certainly my), joy. The Gnat was the first jet aircraft that I had the privilege to ground run, in fact, twice in that limited availability during boot camp training. My AFTC team’s ‘Project’ work was to install and make functional the air conditioning and brake parachute systems on a Gnat shell. Some years later, my belonging to the Gnat Brotherhood was firmly established when I had the privilege of servicing an Ajeet, the HAL derivative of the Gnat, for two years (including dozens of ground runs) at an establishment where they didn’t ask you stupid questions like whether you were trained on type! Later still, my first ever job as a Flight Test Engineer was the IAF’s Preview of the Ajeet Trainer, where I worked with my first Test Pilot, (then) Flt Lt Rakesh Sharma, who had grown up on Gnats.

2. In the early fifties, Folland Aircraft of Hamble, UK, undertook a private venture to design the smallest, lightest, zippiest and, for its size, the lethalest jet fighter of its day. At that time, the UK probably led the world in aircraft design. At the head of the design team was Folland’s Managing Director, William Edward (‘Teddy’) Willoughby Petter, an eccentric genius. At that time his principal design contributions were the Westland Lysander, the English Electric Canberra (both operated by the Indian Air Force) and, later, the English Electric P1, which was developed into the Lightning–and the age of super cruise was born. (If you think that the F-22 is the world’s first super cruiser, think again. If you think that the Mirage-2000 is the IAF’s first FBW job, think again; it was the Su-7 in its AP-28 mode. The Lightning had the most revolutionary way of stuffing two re-heated Avons into its fuselage and using only one intake.) As a young designer, Petter had moved from Westland to English Electric, even though his family had founded Westland, because he preferred aeroplanes to rotorcraft.

3. At about this time, Petter became obsessed with small, light, fast, agile, accurate; he wanted a laparoscopic fighter; not a claymore. And boy, did he get it! And when I say Petter I mean his design team, of which he was the driving force.

4. Say, you want to install some system on an aircraft. Let the equipment weigh 10 kg; the figures are not real, not even realistic, but you get the drift. By the time you finish installing the system, what with its bracketry, cabling and stuff, the aircraft would get about 12 kg heavier. Petter argued conversely. Most aircraft have some dispensable junk. If you threw out about 10 kg, the aircraft could become 12 kg lighter!

5. So, what did Petter throw out? I’ll take a few examples, in no particular order.

6. It was the fashion in the UK at the time that a fighter was expected to have four ADEN cannon. The Hawker Hunter, Supermarine Swift etc. Pretty awesome firepower. Petter argued that if you had a platform that could let you draw a bead quickly and accurately enough on a target, two ADENs were more than adequate to blast anything out of the sky. We could ask the Pakistani Air Force for their views on this. He decided his fighter would have only two cannon. It was well appreciated that you could throw the empty shell cases out, but you couldn’t throw out the links. What Petter did was that, as Gun A’s ammunition tank emptied, it began to accommodate Gun B’s links and vice versa…the links and the belts being kept apart by cloth separators! No separate link collectors. Lightness.

7. The Gnat did not have a fire extinguisher. The Petter argument was that, given the Gnat’s wing profile and plan form, it wasn’t a great glider. If your single-engined jet fighter caught fire, you hit silk. The Mirage-2000 does not have a fire extinguisher. You don’t need one on a single-engined fighter. Compare this to the Su-7: you had to shut the LP cock and only then could you operate the fire extinguisher…and now you had to go for the handles! Why? Even Pavel Sukhoi wouldn’t have been able to tell you.

8. And how did you hit silk? Petter was not satisfied with what Martin-Baker had on offer. Not light enough. He designed his own Folland Light-weight Seat. A very interesting feature of this seat was that it was made safe by a hand operated bolt-like device located just below the headrest. You couldn’t inadvertently fly with the seat safe; you’d get a crick in your neck with the bolt knob poking you in the nape!

9. Can anyone clued-up on his Dicta Boelcke imagine a jet fighter without an air brake? Petter didn’t have any air brake on his Gnat. He used the undercarriage. He made the legs rearwards retracting and used the fairing doors (which haven’t been D-shaped since the Second World War) as the air brake when the legs were partially lowered to the air brake position. The undercarriage selector was a four-position sector lever on the throttle quadrant. The four positions were: up, air brake, down and emergency down. And, how did the gear come down in an emergency? Contemporary aircraft used an emergency hydraulic accumulator or an air bottle to provide emergency power. Petter thought otherwise. He argued that after a hydraulic failure, the nitrogen side of the main accumulator was just loafing. He used the main accumulator’s nitrogen base-charge pressure for emergency lowering of the undercarriage!

10. Next Petter got rid of the flaps! Well, not exactly; not entirely. He put the ailerons inboard, right next to the main undercarriage legs. This was heresy; every mutt in aviation since Alberto Santos-Dumont knows that the ailerons have to be placed as close to the wing tips as possible. (Not that Petter didn’t know; the P1 had so much wing sweep that the ailerons were the wing tips!) Petter knew that flaps were needed for take-off and landing, and also needed was the undercarriage. He put the ailerons close to the undercarriage main legs so he could link them mechanically to have the ailerons droop symmetrically when the undercarriage was lowered, to produce a ‘flaps’ effect that was good enough for take-off and landing. Perhaps the small size of the aircraft necessitated a high-wing configuration. This would add to lateral stability. The inboard ailerons would detract lateral manoeuvrability. Anyhow, with the readily visible anhedral thrown in, the Gnat was a handling delight and lateral control was never an issue.

11. And the throttle? No fancy catches or latches. Just a notch on the quadrant to select idle and prevent inadvertent shutting of the HP Cock.

12. Petter employed a turbo-starter system for engine start on the Gnat. While at English Electric, he had used a solid fuel turbo-starter for the Canberra and Hawker had gone in for a liquid fuel turbo-starter for the Hunter: and mind you, a separate liquid fuel. Electrical cranking for a quick reaction ORP type of aircraft is nonsense. A bad example is the MiG-21. I doubt if any modern fighter uses a starting system as ridiculous as electrical cranking! Certainly, the MiG-21 can look after itself, but it is generally Ground Equipment (APA) dependent. The worst example is the Gnat. It was totally dependent on an air cart. No air cart, no start, period. A high-pressure air turbo-starter, supplied with air from the ground, was the one that afforded the least dead weight on a fighter (if you exclude electrical cranking with a starter-generator; and electrical cranking is too slow), and this was chosen by Petter. Weight weighed heavily on his mind. There were no engine start automatics. With the throttle at idle, you called for air and hung onto the relight button. You were at idle inside of fifteen seconds. If you’d gotten your logistics and drills right and had adequate air carts, there was no other fighter in the world in its time, which could go from ‘Scramble’ to Angels forty quicker than the Gnat! And probably even since.

13. In a scramble situation, you ganged-in the CBs with your left elbow while getting the stupid British QRB to behave. (If only the Russian QRB could become universal!)

14. Size does matter. If you’d gotten your paint scheme and sun angle right, the Gnat was one helluva difficult proposition to spot—so vital to a dog-fighter. Those of us who have watched a Gnat on down wind must have marvelled at how fast it was. If you’ve stood on Juhu Beach and watched a Westwards taking-off 747 turning cross, you’d have sworn it was about to fall out of the sky. Both are illusions. A small aircraft just looks fast.

15. Size does matter. The Gnat was so small that the cockpit ladder came as an after thought. It was so small and light that you didn’t fool around with its steering-arm. You got one or two guys to sit on the tail plane, hooked an arm around the gun camera, lifted the nose wheels off the ground and turned the aircraft.

16. In his weight saving enthusiasm, Petter kept the brakes very simple—no anti-skid. Old-fashioned, automobile-like. Left and right foot motors (master cylinders) and left and right brake units; not connected to the hydraulic system. (Petter had used Dunlop Maxarets on the Canberra.) A light aircraft would be prone to skidding, but Petter just expected his pilots to tread lightly. Brake fade during taxying was a common complaint. But the problem was not with the brakes. Would you believe it that the culprit was the DC generator? The generator would cut in only if you had so much RPM. And, while taxying, if you kept the RPM up to keep the generator on line, you got so fast that you had to ride the brakes continually. And they faded.

17. The hydraulic pressure gauge in the cockpit was unreadable because of obstructions. Did Petter relocate it? No, he just turned the bloody thing upside down so the area of interest of the dial was visible.

18. For weight saving or not, the Gnat was probably the first British fighter to feature a Ridley tail plane. I doubt if any fighter, any where in the world, designed after the late fifties has anything other than a Ridley tail plane. (The Russians had Ridley tail planes in the early fifties.) I call it the Ridley tail plane after Jack Ridley, Charles Elwood Yeager’s FTE, who solved the problems of longitudinal control of the X-1 in transonic flight by dispensing with the elevator and making the whole goddam tail plane move. Some say he borrowed a British idea. This is commonly called a ‘flying’ or an ‘all-moving’ tail on the Eastern side of the Atlantic. While Lockheed (‘All British’) had their Servodynes (used by Petter on the Gnat’s ailerons) and Fairey had their Hydroboosters, Petter chose a hydraulic-motor operated screw jack type of powered flying control unit from Hobson for the Gnat’s tail plane. The Hobson PFCU was egregious; what with its unreliability and cracking of its end pieces, etc, it was a perfect pain. Was it foresight or was it for redundancy that Petter did not go entirely Ridley? (I haven’t the foggiest!) He kept himself an escape clause. The Gnat did have elevators! Follow me closely here: there were elevators, which were locked into the two tail planes; but these could be irreversibly (while in flight), unlocked by the pilot in what was called ‘splitting’ the tail. Once you had split the tail, you had manual control over the elevators. If there was no unserviceability of the hydraulics and the notorious Hobson Unit, you had a ‘follow-up’ tail! If you have ever been through a Gnat flip, you’d have realized that whatever the emergency you encountered in flight, about the first thing you did was split your tail! The joke was that, if a kindergarten schoolteacher was late for class, all the Gnats in the air, and for that matter–even those taxying, would have to split their tails!

19. The longitudinal trim system of the Gnat needs a little explanation. Some terms require an engineering explanation. I’ll make it as painless as possible. An aircraft is said to be in equilibrium if there are no residual forces or moments acting on it. An aircraft is said to be in trim if it is in equilibrium and the pilot’s control forces are zero. There! You didn’t have to be a TP School graduate to understand this! Any aircraft is flown by attitude. Every pilot brings his aircraft to the desired attitude and would like to maintain it so. In bringing an aircraft to a desired attitude, the pilot has to exert some control force and hold that force to maintain that attitude. Not satisfactory. So, enter the trimmer. By operating the trimmer, the pilot reduces his control force to zero without any change in attitude. To this extent, the Gnat is identical to any other aircraft. Let’s look at non-Gnats—say the MiG-21 or the Jaguar. In these aircraft, trimming is by altering the datum, the anchor point, of the artificial feel unit. This effectively re-positions the whole control run to ensure that, without moving the tail plane (to not alter attitude), the pilot’s control force is reduced to zero. The stick would remain wherever the pilot had held it before he began trimming. In the Gnat, things were a little different. The pilot still flew by attitude. He set up the desired attitude and was holding off control force. On operating the trimmer, what he was effectively doing was altering the length of the control run down-stream of the AFU, so that, without moving the tail plane (to not alter attitude) the pilot’s control force was reduced to zero, but the stick would return to its ‘neutral’ or zero artificial feel position, all the time not letting the attitude deviate. Another way of looking at this is this: if you were in a Gnat and in trim, and if you operated the trimmer, without you moving the stick, you would get a change in attitude without the stick moving. But, in the MiG-21, if you are in trim, and you operate the trimmer without you moving the stick, you will get a change in attitude with the stick moving. In the MiG-21 the stick position would correspond to the tail plane angle required for trim, but in the Gnat, while the tail plane would be at the tail plane angle required for trim, the stick would always be in the zero AFU load condition, ie at its basic position. The bottom line is, whether we’re talking Gnat or MiG-21, if you do not let your attitude slip, you will be happy. And in either situation, trim naturally and do not fight the stick.

21. Turning the Gnat into the Ajeet was, sadly, like turning the T-77 into the R-11 T-96. Fuel in the wings, more hard points, Maxarets etc which all added weight, weight, weight—quite contrary to the fundamental Petter light interceptor design concept. Petter wanted light; lightness… No gripe about the seat, though. All in all, the Gnat was the more lovable aircraft.

22. Well, so much for this stroll down memory lane for us Gnat lovers. Happy Landings!

5 thoughts on “How Petter Made The Gnat Light”

  1. A super article ! Whatever Petter did regarding the starter, it no doubt proved to be an immense advantage. The superfast startup time together with the easy access to the cockpit due to the low profile are reasons for the incredibly short scramble times the Gnat was capable of, which were consistently under two minutes, while the rest of the world prided itself on scrambling in “four and a half minutes flat”. In fact, scrambles under a minute were achieved from outside the cockpit during demos to dignitaries. Surely these are world records that will perhaps never be beaten.

  2. Super.I remember in(1982) ASTE,where Krish was the F/O oic DSS how fondly he would look at the Ajeet and take pains in keeping it flyingworthy.We would discuss the ac informally whenever we could.He wanted it to keep flying, and it did , several tp`s converted and flew it till it finally retired, like the rest.After his fte course Krish joined me and we did our (joint with HAL Ashok Yadav) dev trials cum preview on the Ajeet Trainer.-One correction machan–para 1 (last line)its F/L Ravi Kant !!

  3. A really great piece and must read article for every Gnat lover . And here I was thinking that I knew Gnat what I have learned makes me realise how little I knew . Another aspects where I think Gnat was unique was The “gun dip ” and the “Pressure Ratio Limiter ” . I do not think any other fighter in the world has such provisions to prevent “rich cut ” of the engine .
    I am in total agreement with para 21 . In making “Ajeet” they really screwed up the Gnat . So sad 🙁

    Krish sir thanks for enlightening article .

Leave a Reply

Your email address will not be published.