NGH GF38

The acquisition of a NGH 38cc four stroke started after a friend gifted me an old part-built Sid King Privateer, a large model low wing trainer from the 1980s designed for up to 38cc engines, and specifically the Zenoah 38cc. Having dabbled with a 32cc two stroke petrol engine a couple of years previously, I found that getting a two-stroke petrol engine to achieve our club noise regulations was not practical. So, I was on the look-out for a suitable large four stroke.

After a bit of research, and feedback from users on the Model Flying forum (thanks all), I plumped for the NGH 38cc four stroke and called Just Engines, the UK importer, with my order, which arrived a couple of days later. This review has come about as the Editor was at the flying field as I was test flying the Privateer and asked me if I could do a user review of the engine, focusing on its ease of use and practicality from a user’s perspective.

Description
The NGH 38cc four stroke is a purpose designed four stroke petrol engine that was introduced in 2012 to complement the range of two stroke 'gas' engines offered by NGH. More recently it was joined by a smaller 30cc single and a 60cc inline twin four stroke. The design is fairly conventional, using a front mounted camshaft driven by a helical gear off the crankshaft and push rod operated overhead valves. The rear mounted Walbro carb was modified for use with this engine, the internal diaphragm pump being operated by the pressure pulses in the crankcase.

feb21ngh2

NGH GF38 installed in the Privateer, showing the silicon tubing extension and needed because the exhaust gases are very hot and will burn anything they come into close contact with, in this case the undercarriage.

There are two Tygon tubes connected to the rear of the crankcase, one to operate the carburettor pump and a second to recycle crankcase oil back to the inlet manifold. A third connection, between the main bearings allows some excess oil to vent this way and is probably located to ensure a portion of the oil is circulated through to the bearings and camshaft. The recommended oil content is 35:1, just less than 3% oil, and to accommodate this low oil content the connecting rod dispenses with plain bearings and instead uses roller bearings at both ends.

The overall finish of the engine is very good with nice smooth casings and blue anodised push rod tubes, carburettor inlet trumpet and propeller driver.

feb21ngh3

The engine is supplied in sturdy box and well protected with foam.

The engine is supplied with a small piece of tubing on the propeller retaining thread. Don’t lose this as the driver is keyed onto the crankshaft with a small woodruff key and if you are doing any work on the model without the propeller, or this tube fitted, there is a chance the key will fall out and be difficult to find again. The propeller is held on with the typical dual tapered lock nut system used on many glow four stroke engines so no additional holes need to be drilled into the propeller hub.

As standard the engine is supplied with a couple of metal beam mounts, blue anodised again, and a small exhaust stub. Just Engines offer a very nicely made compact silencer and mounting adaptor as an optional extra, which I ordered to help achieve the club noise limit of 82 dBA at 7m.

feb21ngh4

Auto advanced ignition unit is manufactured by RCEXL for NGH.

Ignition is by an RCEXL manufactured NGH CDI unit, triggered by a magnetic pick up that takes its signal from a magnet embedded in the prop hub so the spark plug fires on the exhaust stroke as well as the compression stoke, just like some Honda motor cycles I had back in the 1970s. A small CM6 spark plug is supplied to light the fire.

Installation
Fitting the engine is no more difficult than fitting a typical four stroke glow engine. The only additional things to consider are positioning the ignition unit and battery, running the sensor and spark plug leads and operation of the choke.

While the throttle arm on the Walbro carb operates in line with the engine, the choke lever operates at 90 degrees to this. This is handy if you are operating the choke with a manual lever as it can be positioned to come out above or below the engine. But if you wanted to operate the choke from a servo in the fuselage you would need to transfer the action through 90 degrees.

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This engine is nicely finished, with a generous cooling fin area. Pushrod tubes and propeller driver are blue anodised for extra bling.

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Standard exhaust stack is useful for a scale model to connect up to an internal silencer.

Installation in the Privateer was quite straightforward, with the engine mounted inverted on the beam mounts and the throttle operated by a standard Futaba 3003 servo. The choke was manually operated by a lever routed through the top of the cowl. The latter worked okay but fitting the operating rod was a pain after the cowl had been fitted so I reversed it to come out of the bottom of the model where the cowl was cut away to allow the hot air out. I cut a hole in the cowl for the cylinder head with a small air inlet directly in front of the cylinder and a larger outlet on the underside of the cowl, the outlet being approximately three times larger than the inlet. A hole was also cut for the exhaust, ensuring adequate clearance all the way around.

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Carburettor is a Walbro WT962. The dual tubes to the crankcase are for carburettor pump operation and oil return from the crankcase back to the inlet manifold.

A 400cc fuel tank was fitted in the fuselage and the ignition unit was placed alongside powered by 4.8v 2000 mAh NiMh battery, as I had one to hand. A 2S LiPo could also be used if preferred. I used a mechanical ignition switch in combination with an RCEXL electronic switch to allow the ignition to be activated from my transmitter; I set up a three-position switch to act as a dual throttle cut and ignition switch as follows:

Position 1 - throttle cut and ignition off
Position 2 - throttle active and ignition off
Position 3 - throttle active and ignition on

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Robust connecting rod has roller bearings at both ends.

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Connecting rod, piston and liner. Crankshaft uses a helical gear to drive the camshaft.

Once the engine was installed, I did a quick check of the valve clearances and took the opportunity to introduce a little oil into the cylinder head and the crankcase. I fitted the recommended 18" x 8" running in propeller and made up some 25:1 fuel for the initial runs. I also fitted a telemetry temperature sensor, with one temperature probe measuring the cylinder temperature at the rear of the engine and a second probe measuring the in-cowl air temperature. I also have an rpm sensor, which takes its signal from the connection on the ignition unit, for the optional tachometer display.

Running In
NGH recommend breaking in the engine by a series of medium speed (less than 4,000 rpm) runs followed by short high-speed runs for three to five cycles before running the engine on larger propellers or at full throttle for extended periods.

After a few attempts at trying to start the engine by hand we gave up and resorted to an electric starter. With the choke on it fired pretty quickly and then stopped. Choke off and the engine restarted immediately and settled down into a steady tick over. Following the breaking in instructions the engine was run for over 40 minutes on around 2/3rds of the 400cc tank. One thing we noticed during this was how hot the exhaust gases where and as these were directed near the carbon fibre undercarriage an extension pipe was fitted to route the gases around the undercarriage leg.

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Piston fitted to the crankshaft. The liner is fitted inside the finned cylinder block. Although the piston has three ring grooves only two piston rings are fitted.

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RPM and temperature readouts on the author's Multiplex Profi transmitter.

On the factory setting the engine was running a little rich and at the end of the running in period we noted a maximum rpm of 6,800 rpm on the 18" x 8" Turnigy Aerostar propeller. Without the cowl the cylinder temperature was getting to 140+°C at short full throttle bursts. The cowl was then fitted and on the first suitable day, which was one of the hottest days of summer, with an ambient temp of 24°C as measured by the in-cowl air temperature probe, the model and engine were prepared for their maiden flight. Once fuelled up with the 25:1 mix and with the choke on the engine fired up at the first spin of the starter. Choke off and the restart was immediate. It was then allowed to warm up until the cylinder temperature was over 100°C before committing to flight.

Confession time now... The Privateer ended up being around 0.5kg above the maximum recommended weight of 8kg and you will note in the photos the large amounts of church roof fitted above the engine to achieve the correct Centre of Gravity.

After a long take off run the Privateer gingerly took to the air, but once up and at flying speed the throttle could be reduced to around 50%. At this stage the engine was hunting and hesitating a little due to being slightly rich. The temperature probes noted an in-flight cylinder maximum temperature of around 155°C and maximum in cowl air temperature of around 40°C. A 15-minute flight used less than half the 400cc tank's capacity.

Following the maiden flight we did a quick noise test and at 6,800 rpm on the 18" x 8" propeller at 7m the engine was loudest from the front, with a reading of 81 dBA, and quietest from the right-hand side of 77 dBA. All of these readings were with the cowl fitted and a 150mm silicon tube extension on the exhaust.

Optimisation
After three flights it was decided that the engine was now run in enough to reduce the oil content to the recommended 35:1 and to tune the engine for maximum performance. So, with the cowl off, to allow access the mixture adjustment screws, a selection of propellers and a baggage weighing scale fixed to the tail end to get an idea of static thrust, the following performance was noted:

Propeller Material Size Maximum rpm Approx Static Thrust
Turnigy Aerostar Wood 18 x 8 7,200 6.1
Turnigy Aerostar Wood 18 x 10 6,800 6.8
Turnigy Wood 19 x 8 6,900 7.0
Punctilio Airflow Wood 20 x 8 6,100 7.1
Master Classic Composite 20 x 8 6,100 7.2

For all the propellers the top end mixture was slightly leaned out from the factory settings. But the bottom end seemed okay, with a steady idle and pick up from just below 2000 rpm. Any slower and the NGH became a bit lumpy.

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Valve clearances were checked after a couple of hours operation and found to be okay and well lubricated. Black staining beside the exhaust adaptor is because it became slightly loose and allowed some exhaust gas leakage.

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Internal shot of the cylinder head showing the valve guides and valve seats.

The cowl was refitted and the Privateer was flown with the 19" x 8" propeller. As expected, the performance was improved and the take-off run slightly shortened. In flight telemetry showed that the engine temp was getting to around 160°C max, with an in-cowl air temperature less than 45°C. With the mixture leaned out the mid-range hesitancy had disappeared, and the engine ran cleanly at all throttle settings. One thing noted was that in the air, when flying straight and level at maximum throttle, the engine unloaded to give around 7,900 rpm. And even at half throttle the engine was still exceeding 6,000 rpm.

Summary
The NGH 38cc isn’t the most powerful of 38cc engines and if you are looking for outright performance you would probably be better looking elsewhere. But I have found it to be an easy going and surprisingly smooth engine to operate. It starts easily on the starter and is quite economical, consuming less than 200cc in a 15-minute flight. It also powers my 8.5 kg 82-inch wingspan Privateer quite adequately.

It’s early days yet but the only issue I have come across was that the retaining bolts on the exhaust adaptor came slightly loose. They can’t come out as the exhaust tube locknut keeps them in place, so I have now used some thread lock to keep them in position.

Best of all, using the Just Engines optional silencer with the correct propeller selection, meeting the recommended 82 dBA at 7m is quite achievable and, of course, it does sound so much nicer in flight than a noisy two stroke. In a largish scale model the NGH 38cc four stroke would make a good replacement for a 150 to 180 four stroke glow engine or a 26 to 30cc petrol two stroke.

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Cylinder head alongside the inlet and exhaust valves.

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Cowled in engine on the Privateer with an air inlet directly in front of the cylinder and cut outs for the cylinder head and silencer.

The only downside I have really found so far, over a glow four stroke, is the smell of petrol and the need to make sure all the fuel containers and fuel tank are well sealed, or they will cause a stink in your car. But, on the upside, there is hardly any oil that comes out of the engine and exhaust so keeping the model clean is very easy.

Finally, a word of thanks to Just Engines. As noted in the introduction the engine was a personal purchase and as the Editor asked me to do a review after I’d installed it in the model there was not the opportunity to photograph it in its factory supplied condition. A quick phone call to Just Engines and they set up a studio and took not only photos of an engine from stock but also various parts from the spares they keep in stock. It's also worth noting that all NGH engines supplied by Just Engines come with a one-year guarantee.

There’s a short video of the Privateer maiden flight fitted with the NGH 38cc at:

Datafile
Name: NGH GF38
Engine type: Overhead valve petrol four stroke
Sold by: Just Engines
www.justengines.co.uk
RRP: From £309.60
Bore & Stroke 39mm x 31.8mm
Quoted Max Output 3.58HP @ 8,300 rpm
Recommended RPM range 1,800 - 8,000 rpm
Ignition Power 4.8 to 6V
Ignition Auto advanced RCEXL DC-CDI
Spark Plug CM6
Fuel 92 Octane unleaded petrol with fully synthetic two stroke oil (Power Model 2T- Synthetic oil used)
Mixing ratio Run in 25:1, general use 35:1
Recommended Propellers 18x8/10, 19x8/10, 20x8 (two bladed)
Weight 1310g without ignition, 1580g complete

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