Nanchang CJ-6 M14P engine upgrade ? making a trainer into a ‘fighter’ | Words: Bob Davy – Photos: Keith Wilson

Many of us who own and operate military trainers such as T-6s, Chippies and Tigers quietly hanker for a WWII fighter such as a Hurricane or Spitfire ? but we can’t afford it.

What to do? Make what you’ve got go better maybe? Lighter weight? Bigger engine?

For me the idea of putting a Russian radial in a Nanchang CJ-6 was compelling. The very first time I climbed into one in 1996 I had just flown a punchy Yak-52 with a similar all up weight but with the much more powerful 360hp Vedeneyev M14P engine.

On the first takeoff in the ’Chang I aborted because I thought there was a partial power failure! I tried again and finally got it airborne on an uphill grass strip, underwhelmed by the long takeoff roll, poor acceleration and climb rate.

But then we levelled off, the ASI crept past the 240km/h point where the Yak-52 lived… and kept on going, and going, all the way up to 300. That’s 162 knots ? 186mph! I could not believe the speed of the thing, nipping at the heels of those early WWII fighters. (At sea level a MkI Hurricane would top out at 250mph?Ed.)

After I landed I started looking around the ’Chang airframe a bit more closely: fully retractable gear, flush rivets everywhere, and a clever wing like a Jodel’s or a Robin’s, with the outer panels at a lower incidence to reduce lift-induced drag in the cruise.

Even then, more than twenty years ago I was thinking to myself what if it had the Russian engine ? after all both units are almost exactly the same weight and size (both come from the same grandparent, the 260hp Russian Ivchenko AI-14R.) I knew they had been doing the conversion in the USA for years but out there you can bolt an outboard motor to a flying carpet providing you spray ‘EXPERIMENTAL’ on it. It just didn’t seem feasible here in the UK.

I forgot about the idea, sold my Yak-52 share, got bored for a couple of years, bought a ’Chang and started a group, started a two-ship aerobatic team and bought a share in another one. And three years ago its undercarriage collapsed (no, it wasn’t badly damaged, and no, it didn’t land gear-up.)

Ironically we had just had the automotive spark plug conversion done, which added a few much-needed horsepower to what is otherwise such a fantastic aircraft.

Then I remembered that the consultant engineer Dave Starkey had said at that time: “what you could really do with is an M14P!” And now we had the need to replace the engine and prop, and an accomplice who spent his life doing major modifications to aircraft and components with CAA approval. Hmm…

I worked out that an extra thirty per cent would give a ’Chang the same power to weight ratio as an early WWII fighter. It would have very similar takeoff and climb performance to a Yak-52: they use the same airspeeds – and I found an equation in an aerodynamics book that told me the new maximum speed would be old speed times the cube root of new hp divided by old hp.

Zero-timed M14Ps coming out of Hungary these days are ‘blueprinted’ (built to closer production tolerances with polished inlet/exhaust ports and balanced components) to give at least 370hp ? plus you get a bit more if you use automotive spark plugs, and as much as 385hp, running them on western fuel.

This would mean a hike of ten per cent in cruise speed and top speed. This doesn’t sound like much until you look at the comparison below:

Power: Old Chang: 285hp | New Chang: 385hp

Takeoff distance: OC: 300m | NC: 180m

Rate of climb: OC:1,200fpm | NC: 2,000fpm

Max speed (s/l): OC:162kt | NC: 178kt

This really gets us into late 1930s/early WWII fighter territory! After finding that resident Yak/Nanchang/Sukhoi engineering guru Gena Perevedentsev was happy to take it on, I was hooked on the idea. I put in an order for a zero-timed M14P and prop via Richard Goode and Aerometal in Hungary, rented a corner of a hangar at White Waltham and we started work.

I envisaged we would take about six months and spend £10,000 on top of the price of the replacement engine/prop. The modification process eventually took more than three years, and I stopped counting when we hit £20,000. At its height there were nine people working on it simultaneously not including me, and it took 400 man hours!

Getting started

Dave Starkey and his numbers man John Tempest got busy proving that the mod was viable by doing a stress analysis from first principles and a myriad of other tasks, including an aerodynamic assessment to prove that the extra thrust wouldn’t damage the aircraft skin and structures, hardness assessment and testing of structural systems such as the forward fuselage, engine mounts and bearers, and recalculation of the weight and balance.

I had a copy of a letter from the head of design at NAMC?the Chinese company which designed the aircraft?stating that the airframe was designed to cope with 400hp engines from the outset, and this formed the beginning of our mod application to the CAA. Dave and John’s tests eventually confirmed that the existing aircraft was strong enough to take the bigger engine.

In the hangar, the first thing to do was to take off the outer wing panels to improve working space in our little corner (a ’Chang can sit on its own main undercarriage legs, located in the stub wings) then remove the old engine and prop and take stock of the aircraft condition.

Damage due to the gear collapse was light but there was a lot to do to get this ’Chang in A1 condition. Although it had been resprayed five years before, it had never been properly sorted and I could see plenty of areas where a few knots could be eked out if only things were improved.

While Gena started work on the engineering bits I began renovating anything I was allowed to touch by him. For example the characteristic engine cooling gills, or shutters at the front of the aircraft needed to be modified to take the larger diameter prop shaft of the new engine, and I used this as an excuse to strip down and repaint the old set.

Taking the fifty-year-old gills apart revealed wear and corrosion. Renovatiing and respraying this unit took two months of my days off but the alternative would have been £2,200 for a replacement!

Another job was removing old and redundant radios and antennae as well as the enormous footstep on the trailing edge of the wing, designed for short pilots rather than the taller people who fly the ’Chang today.

One very simple but effective aerodynamic fix was to renew literally hundreds of little aerodynamic, but delicate screws and fasteners which had been replaced in the past with inferior slab-sided items after being damaged by a succession of mechanics. They were all replaced with beautiful stainless fasteners which now stand out like jewels against the camouflage.

Yet another job was removal and repair of the battered wingtip fairings. Why all this work? The ‘Grey Chang’ as we used to call it (it wears correct and officially approved Sri Lankan Air Force markings) had always been ten knots slower than the slippery green one we got first, and I was on a mission to find every one of the causes.

Just chucking horsepower at the problem isn’t the answer. Take 400hp Stearman vs 220hp Stearman as an example: the extra 180hp increases the cruise speed by less than 20mph. The starting block always has to be enhancing the aerodynamics.

Ups and downs

As with any restoration and modification exercise, be it car or aircraft, there will always be swings between highs and lows. Ironically one of the most pronounced was shortly after the new engine arrived. Gena had it installed in double quick time, including the original exhaust with modified lugs, a re-plumbed oil system to allow for different locations of the inlet and outlet pipes, and a change to the carburettor air intake.

We had a big high when Gena found a new accessory drive point to power the tacho generator (for the engineers: it takes its drive from the air-start distributor) as we already knew the unit fouled the ’Chang’s engine bearers.

And then to our surprise we found the generator would still run the original rpm gauges in the cockpits?all I had to do was paint the new redline on the bezel against 2,950rpm instead of the old upper limit of 2,350. The manifold gauges both still worked too (just don’t ask me what the units are?that Russian and Chinese aircraft are metric is all I can tell you).

Now came a low. The aircraft was reassembled for weighing and balancing. Because of the new installation the ’Chang had grown in length by 40mm: and because the datum is the nose of the aircraft, the C of G envelope as stated in the sheet had now shunted forwards, and the aircraft apparently could not be loaded with full fuel and two adults without going past the aft C of G limit.

It didn’t make sense but no amount of re-jigging with the data could make the figures work, and after a delay of eight weeks I employed a new company to come and do the whole thing from first principles. This time it all came good and the numbers proved what we already knew?both the old green ’Chang and the ‘Mk2’ had almost exactly the same C of G position and limits.

One of the things you will often see is pilots hanging from a Nanchang’s tail to straighten the castoring nosewheel, and for either aircraft it takes all of my 95kg to just about raise it off the floor. And here was a pleasant surprise: despite the new engine and prop, our ‘Mk 2’ was 23kg lighter than the Mk1, with an empty weight of 1,122 v 1,145 and an improved useful load of 278.

One extra feature in the Mk2 is the addition of an oil chip-detector light. Literally translated from Russian as a ‘chips in oil’ detector, this sensor consists of two magnets mounted slightly apart in the oil sump. An excess of metal chips will make the circuit which illuminates a light in the cockpit.

The system comes as standard with an M14 and so it made sense to use it. Gena installed a beautiful little eastern-bloc adjustable lamp which matched the low volts indicator light next to it.

I spent a lot of time in the cockpit cleaning old instruments and repainting new limitations on the gauges, retouching/respraying paintwork, and polishing the old Perspex canopies. One of my last little jobs was to gap seal the undersides of the sliding canopies with foam?they had always been drafty and that equals drag.

By the end of all the work it would take an expert to spot that the aircraft had been re-engined ? only the bigger prop, the dial parameters and the chip detector give the game away. And now more than ever from the front it looks like a Yak ? when people at the airfield say that to us, we say no: it’s a Wok.

Ground testing

With all the main work complete it was finally time to ground test the engine. It was a zero-timed unit on its second cycle (one overhaul) and had already been bench-run for three hours. This would mean running on straight oil for the first fifty hours to bed in the cylinders and bearings.

With Gena pulling the prop through and me sitting on cushions in the bucket seat (no parachute) it was finally time to fire her up. The engine coughed and died on the first attempt, then started second time and settled into a fantastically throaty rumble, although much less vibration than I was used to with the old engine, no doubt due to dynamic balancing at the factory.

With the engine warmed up I released the brakes and taxied out to the middle of the field then opened up. The noise and power felt immense but my initial elation was quickly replaced by the shock of seeing a wave of oil coursing over the windscreen. I quickly pulled the throttle back and had to slide the canopy back to see outside, taxying back to the hangar, giving myself a liberal coating of oil in the process.

The original engine has a crankcase breather to stop combustion gases building up and pressurising it – and this breather is just a vertical stand-pipe sticking up out of the case. But it was apparent that M14s produce much more ‘blow by’ than the Chinese engine and it meant we would have to route a pipe from the breather back into the oil tank.

What followed was a final push involving some fabulous engineering that became my favourite part of the whole build. We sent the tank away to Air Weld to be fitted with a receiving boss for the pipe ? how they managed to weld a big chunky piece of aluminum to an oil tank wall with the thickness of a cigarette packet completely amazed me. It’s a beautiful looking weld too (my dad was a welder so I know a good weld when I see one).

Technically, after the refit of the oil tank we could have flown the aircraft ? but it was to be another year before the paperwork caught up with the spannering. You hear all sorts of negative stuff about people trying to build or modify aircraft in the UK but considering how radical this aircraft would become ? going from a 200hp per ton trainer to a 300hp per ton ‘fighter’ ? the journey wasn’t particularly fraught, just long.

My worst fear was that we would end up with a limited Permit to Fly, with the dreaded words ‘non-aerobatic only’ stamped somewhere indelibly. But Dave and John had done their job and just under three years after we started, we were given a permit to test fly.

Flight testing

It’s difficult to describe the elation I felt when I opened that throttle for the first time with the intention of going and not stopping. The acceleration was immense?very much like you get in a P-51, and also a Pitts Special.

Pulling the stick back brought the nose up immediately?no need to wait for 100km/h like in the Mk1, the M14 clawing us off the runway after less than 200 metres (before the intersection of R21 and R25, if you know White Waltham.)

As the wheels left the ground I dabbed the brakes then raised the undercarriage lever, and as the wheels went up in the wells I watched the VSI peg at 2,000fpm before I pulled back to max continuous power.

For Yak and Sukhoi drivers that would be setting 82% on the prop and eight bananas of manifold (they don’t know what the units are either). For the Mk2 it’s 2,400rpm and ‘eight manifold’ on those pleasingly original and authentic gauges. I didn’t fully appreciate how much better this aircraft was until I levelled off at 2,000 feet, just over a minute later.

Carefully trimming forward as we accelerated (just like all ’Chang drivers do) I watched the ground speed peg 300km/h and then seemingly hover for a while as if to taunt me before creeping up to 320. Reversing the turn to favour the torque, it made 325km/h. I had found those lost ten knots and another fifteen.

On later flights, as the engine loosened up we went even better ? and thirty flight hours down the line she can sustain 180 knots at full throttle if you help matters by starting from a dive and then levelling off, again just like a normal Nanchang. Mind you, it’s warbird performance on warbird fuel consumption: 128 lit/30 USgal per hour at full throttle!

After the five hours shakedown flying I handed the Mk 2 over to proper test pilot Chris Taylor for spinning tests. Everything OK there (i.e. no difference) and we also picked up an additional benefit of the mod that no one predicted: the landing roll has reduced from 350m to 270m because of the drag from that paddle prop.

Was it worth it? Most definitely yes! They say that mothers forget the pain of childbirth as soon as they are handed the baby. After my delivery I think I might have been suffering from a mild form of PTSD.

The aircraft value has now been assessed as equal to its original value plus the mod costs (but not including the man hours I put in!) I can’t afford to pay for more than a handful of P-51 hours a year to maintain currency so it’s nice to be able to fly an aircraft with warbird performance on a regular basis.

The Mk2 couldn’t catch a Spitfire, even at low level – but it would nip at the heels of a Hurricane. Look out for it on the display circuit and you will see what I mean.


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Image(s) provided by:

Keith Wilson