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Capt Kaiser Tufail and Wg Cdr Jamshed Khan were detailed to test fly the F-7MG
(later known as F-7PG in PAF service) in July 1997. A total of 12 sorties were planned in which
the complete flight regime was to be explored, with particular focus on the
improvements in performance of the already in-service F-7P (Chinese variant of
the early model MiG-21F-13). A similar
number of sorties were to be flown after a few months, when the GEC-Marconi
Super Skyranger radar was expected to be ready. PAF’s evaluation was the first
by a prospective foreign customer, although the aircraft had accumulated almost
10,000 hours in the PLAAF since its induction in late 1995.
Soon
after arrival in Chengdu,
the wet tropical weather of Southern China
made it quite obvious that the flight trials would take longer than
expected. Two other unusual challenges
were the language barrier and the issue of finding enough airspace over
non-populated areas. Chengdu is one of China’s largest metropolitan
centres and is located in Sichuan
Province, which happens
to have the country’s densest population per acre. In such environments, supersonic flights as
well as low-level max-speed trials left no room for area violation in the
narrow sliver that had been allocated for flight trials. The pilots had also
been told that their departure back home could be delayed in case the adjacent
Chengdu International Airport was to lodge an air violation complaint. The hint
was well registered!
Major Improvements
The
F-7MG airframe has essentially the same F-7P fuselage, tailplane, fin, and inner
wing sweepback of 57°. The outer wing section incorporates the major
change, with a reduced 42°
sweepback and automatic leading edge manoeuvring flaps. The F-7MG is powered
with an improved and more powerful WP-13 engine, which is also operational on
the F-7III (Chinese variant of the MiG-21MF).
Additionally, cockpit layout, avionics and several ancillary systems
have been changed, in line with modern trends.
The important systems that remain unchanged (compared to the F-7P) are
the fuel system, weapons payload capacity and internal guns.
Double Delta Wings
Like
the old Soviet Su-15, the Swedish J-35 Draken, as well as the more modern Rockwell/Deutsche
Aerospace X-31 post-stall manoeuvring demonstrator, the F-7MG has a
double-delta wing planform, which offers an excellent solution to a slender
delta’s inherent low aspect ratio problem.
The aspect ratio of conventional deltas is, at best, of the order of
about 2.4, with the low end notched up, surprisingly, by India’s Tejas. At 1.75
it stands behind the bat-winged double-delta Saab Draken, whose very low aspect
ratio of 1.8 was considered to be a convenient remedy to the transonic Centre
of Pressure (CP) shift, albeit at the expense of overall aerodynamic
efficiency. In contrast, the F-7MG has
an aspect ratio of 2.8.
Wing tip
stalling has never been an issue on the F-7P, but the double delta wing brings
with it an added bonus in this respect.
The strong vortex of the inner wing re-energises the boundary layer of
the outer wing, preventing span-wise flow towards the tips. This allows even more carefree manoeuvring at
ultra-low speeds.
Testing
the Wings
On the
first take-off, it was evident that the aircraft was impatient to get off the
ground and had to be held down to prevent it from getting airborne
prematurely. Compared to the F-7P’s
take-off speed of 310 kph, the MG lifted off at 280 kph with ease and the
advertised 35% improvement in take-off distance was on the mark. The sight of the auto-manoeuvring flaps at
work reminded the pilots of the F-16’s computer-controlled leading edge
devices. Packaging the servo motors and actuators within the thin leading edge
without the tell-tale bulges has certainly been a marvel of engineering at
Chengdu Aircraft Corporation (CAC).
The feel
of the aircraft was smooth in all domains, none more so than in transonic
flight. As expected, CP shift was
minimal and both the test pilots were unanimous about the decrease in stick
forces. Transonic being an important
combat flight regime, this is a welcome improvement.
A good
measure of a wing’s lifting efficiency is at high angles to the incident
airflow, as in tight turns or slow speeds.
What better than to pace the MG through a slow speed loop? Normally, a safe entry speed for a loop on
the F-7P would be between 800-900 kph (at 15,000 AMSL). In the absence of any guidelines on a slower
version of the manoeuvre, it was decided to try 700 kph at first. The MG went through smoothly without any hint
of judder or slip at the top. With full
faith in the leading edge flaps, the next loop was performed at 600 kph. Again, the same results were achieved and the
aircraft went through a perfect loop without any jitter or judder. At lower altitudes it might have done even
better, but airspace limitations at Chengdu did not permit low level
aerobatics.
Several
flights followed the first check of the aircraft’s aerodynamic efficiency. It was a most pleasant surprise to note that
the turn rates were nearer to the F-16 at medium to high altitudes, and were
exactly as advertised. A 33% improvement
over the F-7P at 5,000’ AMSL, 50% at 10,000’ and 66% at 20,000’ would certainly
call for an end to the “supersonic sports plane” sobriquet that dogged the
F-7MG’s forerunners.
The
results of the flight trials were so encouraging that the test pilots were
tempted to simulate a flamed-out engine landing pattern, a not very done thing
on delta-winged fighters. While the
Chinese flight manuals suggested a rectangular pattern that can put one’s judgement
and nerves to test, the standard overhead spiral pattern was tried out
initially from a high-key height of 15,000’ AGL. With engine idling and speed brakes out to
simulate a dead engine, the aircraft glided much like the F-16, so after a few
approaches, the high-key height was lowered to 12,000’ AGL. The sink rate was well under control, and in
fact was so well manageable that all later sorties were terminated through
practice dead-engine approaches. At
1:8.5, the glide ratio compares favourably with some of the modern Western
fighters.
Landings
on the F-7MG could be made at 270 kph, compared to about 290 kph on the
F-7P. Both the test pilots felt that the
speed could be lowered further, were it not for the length of the gear struts,
which are not long enough to allow a higher nose attitude, and could result in a tail scrape. Hydraulic brakes, though
still hand-held (like those on a bicycle handle-bar), were very effective, and
the unlimited braking facility was a welcome improvement over the somewhat
restricted pneumatics of the F-7P.
New Engine
The
WP-13F engine of the F-7MG produces 1,200 lbs more thrust than the F-7P’s WP-7, giving it a thrust-to-weight ratio of
about .9 compared to .8 of the latter in clean take-off configuration. A 50% improvement in spool-up time was found
to be a welcome feature, particularly on final approach and landing phases where
a rapid go-around may be necessitated for any reason. With a faster engine spool-up, a bad landing
is actually a thing of the past on the MG. Use of titanium alloys in compressor
blades and an increased Time Between Overhauls (TBO) are indicators of
improvements in Chinese jet-engine technology.
The
thrust increase was evidenced by a 25% improvement in acceleration time from
500 kph to 1100 kph and an equally impressive time-to-climb to 36,000’ AMSL.
All improvements were verified and were found to be as advertised or even
better. Even more remarkable was the
fact that these trials took place in hot and humid weather, well outside the
15°C, 1013 hP environments in which the specifications are usually ‘engineered’
by the manufacturer.
Miscellaneous
Systems
Used to
advanced cockpits of the F-16, both test pilots were quick to point out several
ergonomic improvements, and had detailed discussions with CAC design
bureau. Switchology changes and
relocation of several instruments led to a much improved cockpit. These changes
include a Stores Management System, which is a useful cockpit-pilot interface
to help establish the status of stores including configuration, fusing and
weapon codes, etc. A voice warning system, colour video recorder, elaborate
cockpit lighting and a more precise and jitter–free Angle Of Attack (AOA) probe
are nice-to-have improvements. The
coloured Electronic Flight Instrument System (EFIS) includes two displays, one
for the attitude indicator and the other for the heading and navigation
sub-systems like ADF, VOR, TACAN, ILS etc.
Radar
The
F-7MG was originally designed to have the GEC-Marconi Super Skyranger. At the time of initial PAF trials the radar
was not ready. Trials were held again
several months later after the prototype radar was installed. In the event the radar did not come up to PAF
specifications and GEC-Marconi was not able to surmount the problems associated
with the small nose cone, including antenna size and equipment air-conditioning
which was insufficient.
The PAF
eventually retrofitted their F-7Ps and F-7PGs with the FIAR Grifo-7. To say that miniaturisation technology is at
its best in this marvellous Italian radar would be an understatement. With an excellent pulse Doppler radar having
respectable ranges and a medium order azimuth and elevation scan, the system is
married to the all-aspect AIM-9L Sidewinder missiles, making it a lethal
combination.
FT-7PG
CAC did
not design a dual-seat version of the F-7MG.
On PAF’s request, the existing FT-7P cockpit was redesigned on lines of
the PG to ensure standardisation and the resultant dual seater was
re-designated FT-7PG.
Post-script
The
F-7MG has considerably improved subsonic and transonic flight performance. Coupled with excellent turning capability and
acceleration, the combat potential is enhanced tremendously. The Grifo-7/AIM-9L combination on board PAF’s
F-7PG brings the aircraft closer to the F-16 in close combat capabilities, and
the PAF must be credited with extracting the maximum from an innovatively
redesigned low-cost fighter.
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ASPECT RATIO AND AERODYNAMIC EFFICIENCY
Creating lift in an aircraft incurs an unavoidable penalty in the form of induced drag. Aerodynamic efficiency is achieved by designing a wing that produces maximum lift for the least drag. This is done by having a high ‘aspect ratio,’ which is the ratio of the square of the wingspan to the wing area. Since induced drag is inversely proportional to the aspect ratio, greater the wingspan, lower the induced drag. A high aspect ratio is thus an important factor in combat, as it helps in sustaining turn rates. High aspect ratio also improves endurance and ceiling and, shortens take-off/landing distances.
As fighters become faster, their aspect ratios have to be reduced to minimise profile drag for supersonic flight. This is done by presenting a smaller frontal area to the supersonic airflow with the help of a smaller wingspan, besides other profile streamlining techniques. It can thus be seen that the conflicting requirements of high-speed flight and subsonic manoeuvring flight have a bearing on the aspect ratio and, compromises invariably result.
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Exchanging notes with a Chinese test pilot.
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| Taxying out for the first sortie. |
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Lift-off.
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Chute good!
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| Debriefing a CAC engineer. |
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| PAF team with CAC engineers. |
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