Sudden engine shutdown, what causes?

Hello everybody.
The man flying my helicopter this time experienced a sudden engine shutdown.
As a result of the inspection, it was confirmed that the connecting rod was damaged.
I am not sure what is first.
Confuse whether the connecting rod is broken and the engine is stopped, or if the engine is stopped and the connecting rod is broken.
What factors cause the connecting rod to break?

What was the damage? Burn up the rod big-end bearing, or the beam broke?

Typically an engine is not going toss a rod just because it quits. The rod was likely what made it quit. Big-end bearing failures are normally caused by either not warming the engine up properly before loading it, or over-revving it. Both will cause the rod big-end bearings to skid on the crankpin instead of roll.

Beam failures are usually due to either over-rev (structural stress failure) or failing to cool down at landing (thermal stress). Both of which weaken the rod and then it later fails when the engine is running.

Since the helicopter likely has an ArduPilot-powered autopilot, have you been doing auto takeoff’s and landings with it?

Hello Chris, it’s been a long time.
I think the damage I said is that the connecting rod is broken.
I’ve often seen this engine’s connecting rods break.
I haven’t looked closely yet, but I expect there are many other additional issues.

I don’t know why the connecting rod breaks frequently.
I set it according to the figures the helicopter company told me.
Correct RPM Correct engine temperature Correct engine oil
But sometimes the connecting rod is broken like this

Is it a F-50 engine in a GX9?

no, it is gaui 100cc

Not familiar with that engine. The F-50’s were famous for seizing the piston and sticking valves. As far as I know all they did with the F-100 is bolt two F-50 cylinders on to make it an opposed twin, then mount a couple muffin fans in the frame to blow air on it to hopefully keep it cool.

If it blew up might want to contact your engine manufacturer.

If it’s like the F-50 it’s got aluminum rods too, not steel. Forged steel alloy rods have a tensile strength of 200K psi. Aluminum is 95K psi. So if the engine has a problem with broken rods frequently, as you say, then somebody didn’t do their homework in designing the engine. Which they certainly didn’t do with the F-50.

BTW - I had a GX9 about 3 years ago with a F-50 in it. That engine didn’t have a TBO (Time Between Overhaul). It had a TBS (Time Between Seizures) of about 10 hours.

I finally threw that engine in the scrap heap and put a Stihl chainsaw in it. But that wasn’t ideal either because the crown gear gave out and can’t get any parts for a Gaui helicopter.

I was just about ready to give up on piston power and experimented with electrics. Electrics are toys and they didn’t work either.

Then came up with a conversion for a Zenoah engine, never looked back. Got over 500 hours on the first one those I built.

Electrics are toys and they didn’t work either.

What a statement.

You cannot call a TESLA car a toy?! What I find out through reading and comments it is only a matter of time than most Combustion engines are obsolete. The battery development is to far behind at the moment.
BTW I would trust a solid Scorpion motor more than any combustion engine.
This is my opinion.
Sure for long flying time the electric Heli or any is not ready yet. But they are not toys!!

Yes, a Tesla is certainly a novel government-subsidized toy. Let’s say I had one and I’m going to drive to Montana with it. Get to Fargo, ND and the battery is dead. Now what? Get the Honda generator out of the trunk and charge it up at the rest area on the freeway?

Drive it four years and the battery capacity is down to 70% original and it only makes it to Detroit Lakes before the battery is dead. Spend $7,000 to put a new battery in it so it can make it past the state line?

Don’t drive it in the winter when it’s 10 below zero because it won’t make it past the county line.

Over that 4 years where did the electricity come from to charge it up? Mostly coal and natural gas. What’s the energy efficiency of that process, with all the loss stages from extracting the fuel, burning it in a power plant and distributing it over a grid system that’s only ~40% efficient?

Might as well buy a piston engine car for less money (despite having no government subsidy) that gets 35 mpg. Put the fuel directly in the gas tank instead of running it thru all the loss stages in between. Spend only $3,500 for fuel over the same 4 years and 50K miles in a machine that does not lose efficiency and range as it ages.

Hang four big Scorpions on the wings of a A380 turning ducted fans, fill the cabin with batteries that are many times the fire hazard of JetA and see if you make it across the Atlantic or not with an outside air temp -40C at FL350. I would prefer to fly in the one that has Rolls Trents hanging out there.

Do you not think that practicality (or impracticality, depending on which side of the fence you’re on) doesn’t scale down to RC size? Just because your two Tattu 22A batteries for your RC helicopter only costs $940 and weighs 5.4kg to fly it for an hour? How many change/discharge cycles do you get before you have to spend another $940? If you fly 6 hours in a day, how many sets of those batteries do you have to own? And how do you charge them in the field? With a Honda generator?

That $940 buys enough gas to fly my helicopter for over 1,200 hours. Without the environmental impact (and cost) of tossing junk lithium batteries in the garbage.

All depends on the perspective as to what is a toy, and what is practical.

@FRED_GOEDDERT I originally started out with a piston nitro 700 with ArduPilot. It was the very first helicopter I ever flew with an autopilot in it. That nitro was just as much of a hobby toy as electric is. Engine would make it (best case) 50 hrs and either the main bearings were out of it, the cylinder was scored and it wouldn’t start, or the piston seized up. Another $300 for piston, sleeve and main bearings.

And the fuel cost 12x per gallon what gasoline costs, and it burns 2x more of it for the same power.

There was not much available then for gas piston power. The GX9 looked like the slick ticket to replace it. But the F-50 is a vibrator, it doesn’t last any better than the nitro and can’t get parts for a Gaui in the U.S. (at least not for the GX9).

I tried three different electric models. They were more expensive to fly than the nitro. The amount of batteries it takes to put a 700 in the air for an hour is atrocious, much less flying it 6 hours a day. I finally built one that would fly for almost an hour in a smaller 626 size. But by that time I was searching in earnest for a piston machine as I could see electric was a dead end. The maintenance on electric is way too high because of batteries. The motors themselves don’t stand up to UAV duty - if it’s not bearings going out and they start to vibrate, I busted the motor shaft off in one of them. I went thru (IIRC) 3 motors in one helicopter in one season - Scorpions. And those motors were $300 bucks a crack - more than a stock G290 engine costs brand new, and that easily lasts 500hrs if you take care of it. And then all it needs is to hone the bore and put a new ring and piston in it.

So when it comes to this:

There is no freakin way, in my experience. Maybe if you only fly them for 15-20 minute duty cycle? But when you start pushing 'em to an hour or more, 6 hours a day, the gas piston is the only thing I’ve found that handles the duty cycle. The bearings in the piston engine are constantly cooled and lubricated with oil. Those little bearings in an electric motor rely on what little lube is in them and they get HOT after an hour of running time. If you don’t catch it and the bearing starts piling up internally it will break the motor shaft.

Those motors are many times more severe duty than multicopter motors because they run at 12,000 rpm or more. They were made for a 7-10 minute full power blast in a 3D helicopter.

Which is a toy in my business.

What fuel and oil were you using?
Low octane fuel or hit octane?
Was the oil especially for air cooled engines?
What rpm were you running the engine at?
Did you run the engine in before putting it into service?

And the other questions asked, by @ChrisOlson
If you want the answers then you need to give the information.

I like internal combustion and I like electric, but I hate the batteries.
IC still has over times the energy fuel density over our best batteries.
Until then, if you are after long flight times, IC is the way to go.

As noted, by nature, Heli’s are hard on engines, running high nitro content to extract the most amount of power from the smallest power plant, sucking through a lot of fuel in the process.
But these were designed around hobby use where you couldn’t concentrate for too long or carry excessive fuel.
So in heli’s the quality of the engine is paramount and motors such

I would be looking at the new Zenoah 32 with interest.

There’s one problem with the Zenoah 320. The crankpin/rod big-end bearing. When they put the bigger cylinder on it they increased the size of the journal on the crank. Due to the increased speed (bearing rollers) they tend to skid on the crankpin and gall the journal. That engine will handle up to 10,500 rpm but no faster than that. So the gearing has to be right for it to keep the engine rpm down.

That is why I built stroker 290’s. As Zenoah increased the displacement of the original G23 they did it by progressively increasing the bore size. The increased piston weight gradually caused the vibration to get worse because they use the same crank in all the small blocks up to the G290 - 28mm stroke.

Using the 2mm stroker crank increases the weight of the crank counterweights, and with an appropriately lightened piston you can achieve the same balance as the G230. With an increase in piston displacement to 30.5cc and increase in horsepower to 4.6 (with some mild port work). Also requires decompressing it so it can burn premium pump gas. There’s three ways to decompress it - either use a 1mm shim on the cylinder base gasket, machine the combustion chamber dome, or remove some material from the piston crown (which alters the port timing).

With the 320 they stuck with the same 28mm stroke and increased the bore to 38mm, so balance is going to be a bigger issue than it is with the G290 in its stock form. That was part of the reason for going to more mass in the crank for the 320.

The stroker crank for the Zenoah small-blocks is a special order item that comes from Zenoah/Husqvarna

My answer for bigger more powerful helicopters up to 25-30kg takeoff weight and rotor diameter of 2.5m is to use two G290-based engines instead of a larger single engine. Can get the same power from two Zenoah small-blocks as a turbine at slightly more weight in engines, but radically less fuel weight than a turbine. Two small-block Zenoah’s burn ~52cc/min in a 25kg helicopter compared 190-200cc/min with a turbine.

Incidentally, RC-sized turbine engines have the same problem as electric motors - the bearings in the hot section. Those little ceramic ball bearings that cost $185 apiece only last 25 hours in a JetCat engine. All because the RC-format turbines don’t have pressure-lubricated bearing sumps like a big turbine engine has. Even with expensive ceramic balls and races, without oil to keep the bearing cool, wash out contaminants and lubricate it they have a limited life. And then the engine won’t start, or worse yet it hung-starts and you end up burning up the turbine wheel in the gas generator and you’re looking at a $1,000 engine overhaul.

Hi Chris,
do you have more information of your Zenoah conversion for GX9?

Never did a Zenoah conversion on the GX9. I put a Stihl 088 122cc chainsaw engine in the GX9, with 8.7 bhp. That engine blew the plastic gear that drives the rotor and I gave up on that helicopter because i couldn’t get a gear for it. They were initially imported to the U.S. on a special order basis for $4,400 but no parts. I don’t know the status of their distribution program now, as that was 3-4 years ago.

I went to a different model of helicopter with the Zenoah engine from a company that is based in the U.S. The drive on the Zenoah uses the standard Zenoah industrial clutch** and bell with a belt primary and geared secondary transmission, with a starter shaft that passes thru the primary shaft.

** When I say industrial clutch, it is the same clutch used on Husqvarna and Red Power outdoor power equipment. It is not the problematic lined clutch bells used on nitro engines. Zenoah was originally a Japanese company that goes back about 100 years. The engines are still built in Japan but Zenoah is owned by Husqvarna AB in Sweden.

When you buy a new engine they come either with or without the clutch (depending on how you order it), but only the clutch plate. The bell does not come with the engine. There is short and long clutch housings, and all sorts of different aftermarket clutch bells and billet clutch carriers that bolt directly to the clutch housing. The clutch housing does come with the engine, usually the short housing on a small-block Zenoah.

The advantages of the Zenoah engines:

  1. It has built-in fan cooling just like a manned helicopter piston engine
  2. It has magneto ignition just like a manned helicopter piston engine - no battery power required for it to run, no external CDI box
  3. It has high power/weight ratio compared to a four-stroke - modified engines can match a RC-format turbine for power/weight
  4. It has a 500hr TBO in UAV helicopter duty - overhaul cost is < $150
  5. It has way fewer moving parts than a four-stroke, which makes it infinitely more reliable
  6. It can tolerate ethanol blended fuel with the stainless steel fuel pump in its Walbro 990 carb
  7. With some modifications it runs as smooth as a turbine engine at rated speed


  1. They are hard to start. That engine might be little, but it is one mean SOB and will tear the skin right off your fingers and jerk the pull handle out of your hand unless you know how to start one
  2. Fuel burn is higher than a four-stroke, but usually because Zenoah-powered helicopters are grossly over-powered. A stroked G290 will easily match a so-called “10,000 watt” electric motor for power. But the Zenoah will put that power out continuous where the electric can only put it out for a brief burst until the battery voltage sags (or the ESC explodes into flames). Actual BSFC (Brake Specific Fuel Consumption) is pretty close to a four-stroke in lb/hp-hr.


thanks for your information.

@Loren_DS it is possible to do a two-stroke conversion on the GX9. But requires ability to machine some parts. The clutch and bell on a Stihl chainsaw engine fits in there, but you have to cut the drive gear off, weld a chromoly shaft to the clutch bell and fit the pinion gear on it from the original clutch. You have to machine some engine mounts for it. And come up with a way to start it because the chainsaw engine is rope-start only. So have to make a hole in the side to get to the t-handle.

When you get all done, instead of 1,160 rpm rotor speed it will now turn at 1,460. And then the crown gear won’t take it. The chainsaw engine has WAAY more power and torque than the four-stroke.

There is no easy solution for that helicopter, unfortunately. They have had continuous engine problems since they were invented.

Edit: And then there’s the afterthought thing. The particular engine I used, the Stihl 088, kids put those engines on blacktop racing go-karts and those go-karts run 100 mph and they will smolder a set of racing slicks for 200 yards. Maybe I should’ve used a littler chainsaw engine.