Thursday, May 19, 2022
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Gaui Gx4 Helicopter

This article was originally published in RC Heli Pilot December/January 2016 issue.
Words and Photos By Carey Shurley

Welcome back to another episode of GPT. In this edition I want to review the Gaui GX4 gas powered helicopter. As discussed in the past, gas helicopters are available now in many sizes; from big 900 class models down to small 400 class machines. The Gaui GX4 is a 400 class model and, as far as I know, is the smallest gas helicopter in commercial production. The funny part is, you can’t actually buy a GX4 … it’s a factory supported gas conversion!

You create a GX4 by starting with an NX4 which is the glow powered version of the model designed by none other than Bobby Watts! Then you add the Gaui T10 gas conversion set and transform it into a gas powered model. It’s a very simple transition and we’ll go over what’s involved. When you’re done, you’ve got a five-pound model that you can fly for 15-20 minutes on a tank of fuel that only costs about 50 cents. Glow fuel is about four times that cost and the corresponding glow engine will run less than 10 minutes on the same tank of fuel. Having set the stage here, let’s get on with the review.

LENGTH: 35.4 in. (900mm)
WIDTH: 6.5 in. (165mm)
HEIGHT: 12.4 in. (315mm)
WEIGHT: 2353 grams (5lb, 3oz) Without fuel
DISPLACEMENT: .56 Cubic inches
HEAD SPEED: 2200-2700 RPM

ENGINE: Gaui T10
GOVERNOR: Gaui Gasoline Governor SERVOS: Savox 1250MG/1257MG FBL: Spartan Vortex VX1N
BATTERY: 1250mAh 2S LiPo

Here’s everything used in this build.


This is my first experience with a Gaui helicopter so I didn’t really know what to expect. Knowing something about Bobby’s other designs, I had a good feeling about it though. With the exception of a couple of small items, I found the parts quality to be very good and all the parts fit together as they should. I was impressed with the amount of pre-assembly that’s been done. The entire rotor head, swashplate, main drive gear/constant drive, fuel tank, tail boom and transmission, boom supports and the canopy are all ready to install right out of the box. I checked all the parts and they were all assembled correctly with thread lock. With the exception of the cooling fan hub and engine mounts, all the parts needed for the conversion are standard NX4 helicopter kit parts.

Starting with the rotor head, it’s a flybar- less design, all aluminum and uses integrated swashplate drivers. The kit includes stiffer dampers so the head can be tuned to your flying style by choosing between damper options. The swashplate is all metal as well and turns and moves very freely with no visible play.054-2

The chassis itself is a standard two plate frame setup with two carbon fiber plates. Between the bearing blocks, spacers and connecting plates, it’s quite stiff. The bearing blocks, main drive, clutch, front T/R transmission and tail boom mounts all fit as they should and where appropriate, rotate smoothly. Things you do NOT install are the standard engine mounts, the cooling shroud, fan and fan hub and the fuel tank. Those will go in as part of the T10 conversion kit which I’ll focus on a bit later.055-4
Carbon fiber and aluminum chassis is quite stiff.
NX4 all metal flybarless head.

The landing gear are traditional in design and consist of plastic struts with aluminum skids. They assemble easily, no heat guns required to get the skids in.

The tail boom is aluminum and comes largely assembled. The rear transmission, drive gears and tube drive are all installed and operate smoothly. The boom is both clamped and pinned into the frame so unless something gets loose, it’s not going to turn or move enough to disengage the front gears. The T/R itself, which is all metal, is built-up and includes a set of plastic T/R blades. The boom supports are aluminum with plastic ends and came assembled ready to install.

Finally the canopy, which is fiberglass, comes pre-painted as you see it with all the cutouts made and grommets installed so you can just put it on the model. The only modifications needed will be for the gas conversion which will follow

As for the electronics, you need mini servos. For this build I used Savox 1250 HV and Savox 1257 servos. I’m used to building big models so I found it quite the challenge to get all the electronics to fit. It is possible to mount the FBL sensor above the tail boom mount, but most of the one-piece systems won’t fit because the frames are very narrow. I’d suggest putting it up front anyway to avoid getting coated by the exhaust, but we’ll talk more about that as well. The cyclic servo mounts are adjustable to allow for different size servos and the kit includes some very tiny spacers so you can change the position of the control balls on the servo arms as you install them. It’s really easy to get straight runs on all the control rods.

With the model largely built we can focus on the part of it that makes it a GX4 … that being the Gaui T10 engine set. This set includes the engine, a fan hub, a new set of engine mounts, a spark plug and an electronic ignition system.


Not much room to mount all the electronics.


The reason for the name T10 is that the engine is classified as a 10cc engine despite only being a .56. It’s actually made by GO engines, but if you look at the original engine, it uses a completely different cylinder head. Actually this is the second version of the engine. Originally it had a much smaller cylinder head which would fit into the cooling shroud. Unfortunately it didn’t provide enough cooling so Gaui added a much larger head which has a stepped design. It still doesn’t fit entirely in the shroud, more about that shortly.

Other than the cylinder head, the T10 is pretty traditional engine in terms of design. A little too traditional actually as most gas engines use bearings on the connecting rod which lets the engine survive with an oil mixture that is as little as 100:1. This engine uses standard bushings on the rod meaning it requires MUCH more oil. Gaui recommends a 10:1 ratio which is 13 ounces of oil for each gallon of gasoline. GO engines recommends a 6:1 ratio which is about 21 ounces of oil or about 20%. Most glow helicopter engines with bushings in the connecting rod run with 20% oil so I’ve opted to run the engine with that mix. The down side with that much oil is that it’s almost as messy as glow fuel. The fuel itself is much cheaper, though. Using this much oil you need to use a high quality lube that’s fairly thin. I’d suggest a full synthetic oil such as the Red Line 2T, Coolpower thin Synthetic or Klotz two-cycle premix and mix them thoroughly.056-2

Gaui T10 engine set includes everything to convert to gas (muffler optional).

It’s a spark ignition and to save weight, instead of using a magneto system as you might find on a Zenoah engine, the set includes an RCExL electronic ignition system. RCExL has been making these for a long time and they’ll run on voltages from 4.8 to 8.4 volts. They also require a trigger on the crankshaft so they know when to make the spark. On this engine the traditional prop washer has been made thicker so as to include a small magnet. This thicker washer means that the engine needs to sit lower in the frame. This is why the engine set includes different engine mounts and fan hub so as to account for this difference in position.

The ignition trigger includes a small sensor that mounts on the front of the crankcase and will sense the magnet in the prop hub. This has to be installed so that the spark occurs at 30 degrees of rotation before the piston reaches the top dead center of its travel. There’s a very complicated way to test this using degree wheels and testing instruments, but in fact there’s a very simple way to accomplish this as well. The sensor mounts onto the case with two bolts. As long as the sensor body is exactly centered between these two bolts, the spark timing will be correct. The fan and clutch can be installed normally. As mentioned earlier, despite the stepped cylinder head, the shroud still won’t fit over the large cooling fins, so the fan shroud needs to be shortened by 33mm. This is really the only change needed to the basic kit. Once that’s done it goes over the fan as normal.

The engine can be installed in the frame just as a glow engine would. You do have to remove the bottom plate/landing gear if they are already on the model and them reinstall when the engine is in position. Once in place the spark plug can be installed. Due to the position of the main fuel needle, the canopy will need to be modified slightly so that it doesn’t rub on it.

The EI system should be installed on the bottom of the frame and wired as shown.

This moves it as far away from the electronics as you can get, although in practice I’ve not seen any sort of interference from these with cur-rent spread spectrum radio systems. It requires power to operate and although you can install a separate battery for it, I would suggest using a power splitter and connecting it directly to the main electronics battery. I’ve used them this way for years and have yet to encounter any problem doing so. This also saves the weight penalty of a second battery.

The fuel tank is preassembled and plumbed for glow fuel with silicone tubing. Gasoline will destroy this so the entire fuel system needs to be plumbed using Tygon, neoprene or Viton tubing. I made the tubing inside the tank longer so the clunk can move around the tank easily during maneuvers. The tank is captured in the frame by two small plates with rubber isolators that fit into molded dimples in the tank. As a result it’s very simple to remove/install the tank for maintenance/inspection.

I would highly suggest using a header tank as due to the higher engine temperature from the gasoline fuel it is not tolerant of getting lean due to air bubbles in the fuel line. You shouldn’t use muffler pressure with gasoline vapors present so instead of plumbing a pressure line, you plumb a coiled loop of line as a vent. The coil prevents gasoline from running out when the model is inverted.

Finally, Gaui offers an optional gas muffler. It’s heavy duty aluminum and black for both heat dissipation and makes discoloration from oil splatter less obvious. It installs in a traditional way with the exception of the fitting for muffler pressure being blocked off. You don’t want to have a “back- fire” into the fuel tank with gasoline vapors. Make sure the muffler plug is sealed with high temperature RTV otherwise it will fall out.

That pretty much covers the conversion of the model to gasoline power.

Gaui recommends that the engine be “broken in” by letting the model sit and idle for about four tanks of fuel. With a nine- ounce tank that will take hours. I would recommend before installing the engine in the model that you put it on a test stand with a prop and run it in well. This will let you understand the needle settings and get everything seated before you ever put it in the model. It also won’t run quite as hot once it’s broken in. The factory needle settings are good enough for break-in and then you can fine tune them as it runs in. I used a 10/8 prop and ran about a gallon of fuel through it.


Gasoline engines don’t make as much power as glow engines in an equivalent displacement due to the energy differences between gasoline and methanol/nitromethane. So while the NX4 flies with a .37 sized engine, the T10 engine is a .56. The idea is to make up the power difference with displacement.

The advantage here is that using the same nine-ounce fuel tank, the GX4 will fly about 15-20 minutes on gasoline that the NX4 will fly for less than 10 minutes on. Glow fuel is quite expensive, but gasoline costs about 90% less for the same quantity so the result is to fly longer for less cost.

It’s very important to run a throttle governor on this model as given the engine displacement it’s very easy to over-speed the model. This is less of an issue for the model than it is for the engine, as with gasoline it will become quite hot. Recommended operating RPM for the model is around 2,200 RPM, however, it can be run as high as 2,700 RPM. I used the Gaui Governor for gasoline engines. It’s pretty simple to set up, but you do need a rotor head tach to set the RPM.

The model is surprisingly stable for its size and the high head speeds no doubt contribute to this. Cyclic response is also quite quick for a gasser, again, thanks to the high head speeds. There is plenty of power for whatever maneuvers you want, however, especially considering that when fully fueled, the model is a bit heavy for its physical size. This just needs to be accounted for in maneuver timing and with autorotations. Once properly tuned, the model will be very smooth without excessive vibration from the engine.

If you are used to flying 700 size or larger models, the NX4 will get small very quickly once in the air. The canopy design and colors help with this, but this is typical of any of the models that are this size.058-1

Notice the smoke from the high oil content.


Completed model – nose in.


One assembly issue I found with the kit was with the frame connecting plates. They bolt to spacers using counter-sunk bolts. Since you’re mounting radio gear or batteries on these plates the bolts should tighten down flat, but they don’t. The bolt hole bevels in the plates are cut at the wrong angle so the bolt heads protrude from the plates. This isn’t the end of the world, but it makes it harder to mount an FBL sensor, battery or receiver because these bolts heads are protruding. A simple spin with a counter-sink bit on the drill got the heads down flush to the frame in my model.

The spark plug access hole in the front frame plate needs to be a little longer to account for getting a plug wrench in to tighten the plug or to install the spark plug cap. I opened this hole up with a Dremel bit.

Because of the very high oil content required in the fuel, the exhaust will contain about as much smoke as if you were flying with glow fuel. This is different than a typical gas helicopter, but a necessary evil for using the T10 engine.


The GX4 is an economical way to get into gas powered helicopters without the expense of the typical 700 class models. The NX4 which

it’s based on is well tested and proven, having been on the market for several years. Parts are readily available and are not particularly expensive. It’s slightly heavier than the base NX4, but the advantage is its abundant power, economical operation and long flight times. If you’re looking to get into gas helicopters but don’t want to start with larger models, the GX4 might be just what you’re looking for

Until next time, fly safe.