GT15 - Some History, Some Updates

[carey shurley]

Intro

I’ve done a couple of articles about the OS Max GT15 helicopter engine, one when it was originally introduced and then an update a couple of years later. The engine has had a problematic history and has been written off by a lot of people.

I attribute a lot of the original issues with the GT15 the result of trying to fit a square peg in a round hole. However, OS has continued to make improvements to the engine and I think it’s time to take another look at it

How did we get here?

OS is an engine manufacturer. They saw an opportunity back in 2012 to make a gasoline engine that could be fitted into 700 class helicopters. They chose to do this with a 15cc engine. Keep in mind that until we got to 30cc engines, 700 class gas helicopters typically were considered underpowered. As a comparison, here’s the original GT15 followed by a current day 30cc engine. They are a bit different looking


Original GT15 engine


TRM 300 engine

So, it started with a flawed premise which guaranteed the engine would be heavily loaded. Then OS underestimated the effects of that loading combined with the additional heat generated by using gasoline as a fuel.

On top of this, they recommended the same fuel/oil premix ratio that is typical for engines that have full ball bearings on the connecting rod even though this engine uses a bronze bushing on the piston end of the connecting rod.

What caused the problems


In the most common gas powered engines (made by CY and Zenoah), the cylinder and piston are both made of aluminum so they expand at generally the same rate and anything that moves uses a ball bearing. They are not immune to being overheated however they are more tolerant of it because of this. OS decided to use an aluminum piston but to run it inside a steel sleeve.

Obviously the piston is going to expand more than the sleeve so the piston/sleeve clearance was made much larger than would be typical. If you wonder why these engines make a lot of noise especially when they are cold, this is why.


GT15 Steel Sleeve

But the piston also had two steel parts directly associated with it. The ring locater pin (which was inserted horizontally into the ring land) and the piston pin clips are also steel circlips. In its original form, as the piston expanded significantly, it allowed the ring locator pin and or one or both pin clips to come out of the piston while the engine is running. Typically the clips or the now free to rotate ring would hook in a cylinder port and would break the piston or significantly damage it. If the piston happened to break, parts of it could fall into the crankcase, get jammed under the crankshaft weight and literally break the crankcase. At this point the engine was basically destroyed and was very expensive to repair.


GT15 Original piston

OS decided to change the ring locator pin to insert vertically into the piston so that it became very rare for it to come out however they didn’t change the pin clips. Those of us who were trying to find ways to make the engine successful discovered the existence of Smalley “spiral circlips”. These are commonly used in high performance engines and are virtually impossible to simply “fall out” in fact they are pretty difficult to get out normally. These two items pretty much resolved the main issues with the piston.


GT15 updated piston

However it still didn’t solve the overloading/heat problem due to the engine not being powerful enough for 700 class helicopters.

Changing the rules

I started testing with shorter blades and different gear ratios and found that when you got to around 600mm blades, the engine became more comfortable with the load. I created a private conversion for the Fury 55 to use the engine and found that it worked reasonably well however the engine still tended to overheat. Although this model used the Whiplash cooling system, it was still designed for a glow engine and not for a high temp gasoline engine

At this point OS decided to do two things:

1. Discontinue the original engine version and develop a “600” version of the engine which used a 50 sized crankshaft threads and a traditional glow type carburetor/pump system which made it easier to use a 600 class helicopter fan/clutch
2. create their own Trex 600 conversion which combined the Trex 600 along with the cooling system components from the Trex 700 (in fact I still have one).

GT15-600 engine


OS Trex 600 conversion kit for the GT15


OS Trex 600 converted for the GT15

Unfortunately this resulted in some new issues:

1. this still didn’t provide adequate cooling that the heat generated by gasoline as a fuel required.
2. now, the GT15-600 engine was a mismatch – it couldn’t be used in many 700 class helicopters that had threaded fans/hubs because the crankshaft threads were now the wrong size

GT15 Crankshaft differences

My Fury 55 conversion was sort of goofy because it was a combination of Fury/Whiplash parts resulted in two autorotation mechanisms. Since MA was becoming unstable around that time, I didn’t want to make parts for something that was going to disappear. By the way it looks like they are going to return the 55 to production and if they do so one way or the other we’ll have a GT15 conversion for it.

Lets go our own way

So given the potential short life of the Fury 55 as a viable production model I approached Joey Chan (JC Designs) about making a conversion for the engine to fit the Goblin 570 which was new to the market.

A main component of this was to design a cylinder head that could replace the stock head and provide much better cooling. This resulted in the JCD Warm Air Reduction (WAR) cylinder head. Although large and heavy, it immediately reduced the engine temperature by about 80 degrees F (or almost 1/3). This was a significant improvement in and of itself. We were also able to make a drive ratio which was best suited to the engine.


JCD WARHead

Then as part of the conversion, Joey used fluid dynamics modelling to come up with a cooling fan that would maximize air flow. Finally our new fan setup would work with either the GT15HZ or the GT15-600.


JCD Super efficient fan

Eventually OS found one more issue with the engine, still related to the piston expansion issue. They found that under extreme heat, the piston skirt could hit the engine backplate because the clearances there were VERY thin. So they modified the backplate assembly to allow additional clearance.


OS revised backplate

GPH in the past offered a “Sport Edition” version of the engine which included all of the improvements OS had made including the WARHead and spiral pin clips however eventually OS made some more decisions about the engine and released the GT15HZII:

• They decided to go back to a std 90 size crankshaft
• They developed an alternative to the WARHead, although its not quite as efficient (blue head)
• They dropped the glow style carburetor/pump system and returned to a Walbro carburetor
• They changed the piston design to replace the piston pin circlips with round steel wire clips

GT15 cylinder head evolution


GT15 revised pin clips

Oil is our friend

As most of you know, I’ve been working with this engine since its inception and have provided significant feedback directly to OS about it. One of the things that always bothered me was the insistence of using a 50:1 oil premix (about 3%) even though the engine was essentially a modified glow engine design. As you probably know, glow engines run between 18-22% oil in their fuel. Oil plays a role not only in lubricating the engine internals but also in carrying away heat in the form of exhaust.

Heat has always been the root problem with this engine. So I developed a running procedure that includes using oil more oil than OS has recommended. A LOT more oil. I’ve been running 10% oil now for some time (that’s 12oz per gallon of fuel). Yes there is some smoke produced but its no where near what you see from glow fuel.

Current State

OS is known to produce a number of configurations of their engines, usually each is an improvement over the last. This is the current version of the engine, the GT15HZII, I don’t know if there will be additional versions in the future.



What are we doing with it now

We still offer a “Sport Edition” of the engine which replaces the blue OS head with our more efficient WARHead


GPH Sport Edition GT15 engine

We offer two conversions that will work with any of the versions of this engine that OS has ever produced

The Goblin 570 gas conversion is still available and has been updated to accept the newer SAB plastic landing gear


JCD Goblin 570 GT15 conversion

and as I recently announced we are poised to produce a conversion for the Synergy G556.


JCD Synergy 556 GT15 conversion

The bottom line

I’ve been running two of these engines for several months now and haven’t had any issues with them. Now granted I’m not Kyle Stacy so I’m not burning the platform to the ground on every flight. Obviously its not indestructible as no engine is but with the internal changes that OS has made along with our running procedure the engine seems to be at least as durable as a glow engine. The extended flight times and reasonable power available in our 5xx platforms makes this something you should consider taking a look at again.

[HelisRock]

Awesome, thanks for the history lesson Carey!

[tonyd12]

Thanks Carey for a really great update.
Very interested especially for the fury info.

[Danker16]

i might buy into this if an oxy5 gasser conversion comes along when the oxy 5 nitro gets released!

[brw0513]

Great work as usual Carey

Are there better oils than others to use at the 10:1 ratio?

What engine speed do you target?

Can you offer high and low needle setting recommendations?

[carey shurley]

with so much oil, you want something thin.

Red Line 2T is what I use however if you can get the thin Morgan synthetic oil (the thin stuff they put in Cool Power fuel) that will work too.

wanna keep it under 14K

[kanduxa]

Originally Posted by carey shurley

Oil is our friend

As most of you know, I’ve been working with this engine since its inception and have provided significant feedback directly to OS about it.

WoW I can't believe a big company like OS/ Futaba was getting suggestions from a random hobbyist I wonder if they have factory pilots in Japan.

Originally Posted by carey shurley

One of the things that always bothered me was the insistence of using a 50:1 oil premix (about 3%) even though the engine was essentially a modified glow engine design.

this is pretty ridiculous because most airplane gas engines do run on 40:1 so I don't get the point as to why they might want to recommend 50:1 for helis
the Webra 150 did not last long in the market because of heat issues

Webra 1059Hi Speed 150 R/C Heli Ignition Engine [WEB1059Hi] - $699.95 : Hobby Club

Originally Posted by carey shurley

I’ve been running 10% oil now for some time (that’s 12oz per gallon of fuel).
Yes there is some smoke produced but its no where near what you see from glow fuel.

Yes 10% is the average oil ratio that I have seen on other little gas engines conversions that do no have con-rod bearings

Originally Posted by carey shurley

The bottom line

I’ve been running two of these engines for several months now and haven’t had any issues with them.

what about the carburetor ? do you find Walbro carb better in flight or easier to 'tweak' than the glow type carb of the GT15-600?

[carey shurley]

Actually OS was getting feedback from a number of people in different parts of the world. Testing something like an engine is best done in more than one place, with different fuels, altitudes, and skill levels

Although originally I didn't care for the OS carburetor, by the final version of it I liked it. The Walbro is a little more challenging to tune because the needles are VERY sensitive so very small changes make a diffference but it does have a built in fuel pump.

[ChuckL]

I wouldn’t exactly call Carey a random hobbyist

[brw0513]

Hi Carey.

Just wondering about a few things with this engine.

After a days flying, do you disconnect the fuel line and let the engine idle so it completely consumes all the fuel in the carby? Or some other after-run care? The 10:1 ratio is a pretty high oil content. Would the "left over" oil tend to glug up the small passages in the carby?

Tuning this engine may take some skill. Over the years, I've been curious about the Colortune "glass" spark plug:

https://www.carbtune.com/colortune.html

I've never tried it, but the idea seems to have merit. What do you think?

[carey shurley]

typically no, however I do empty the tank. Remember I'm running camper fuel

I don't see how thats of much value. You'd need to see the flame at power under load. I also don't see any evidence that its a resistor plug so even if you could see it under those conditions it'd be crashing from radio interference.

you can hear if the mixture is way off and read the plug after flights

[brw0513]

Hi Carey.

Will you be offering any tuning tips with this engine?

What do you hear if the mixture is "way off"?

[carey shurley]

I have this:

OS GT15HZII Sport Edition Operation Guidelines

like most IC engines, if its too rich it literally won't spin the head up and if its too lean it will have a cackle to the exhaust sound

[danil]

Hi everbody nad Mr Shurley.

When the Synergy conversion for the OS will be ready to buy. I am very interested with 500 size gasser heli. I just think to buy Gaui NX4 to convert GX4. But the T10 motor became obsolete. I am waiting this conversion and your built thread with great enthusiasm.

[carey shurley]

the short answer is, I don't know
the conversion has a dependency on some N7 parts. As you know Synergy parts are in short supply right now
When that gets addressed, I'll be ready to offer some

[Oggie]

Hi guys I’ve fitted the gt15 in a raptor90 3D. The 90szh I had in there was a bit juicy. I fitted it 2yrs ago but could not get it to work ok . To start with it rev’d it’s tits off and got hot (130 ‘c) . I’ve got spektrum telemetry. I played about with the throttle curves and got it hovering but maybe to low on the throttle. It was still running hot though. In the end I got frustrated with it and put the raptor in the shed and left it there . I thought it was going to be an easy swap and fly.
What sort of temp should I expect? My other glow heli’s run at 80/90’c .
After seeing these posts I was wondering if the warhead and fan would help , but don’t know if they would fit in the raptor?
Any help would be appreciated.

[Mhrfabio87]

Originally Posted by carey shurley

I have this:



OS GT15HZII Sport Edition Operation Guidelines



like most IC engines, if its too rich it literally won't spin the head up and if its too lean it will have a cackle to the exhaust sound

hello Carey, I am writing to you from Italy, I mounted a GT15HZ2 (blue head) on the Trex 700n, but I cannot understand what oil you recommend to use. I am now going with Motul 800 on road at 4%, I read that you say you make 10% mixture, isn't it too much? I also have a synthetic-castor oil wladoil kart k2, better than the motul 800?

Inviato dal mio SM-N970F utilizzando Tapatalk

[Mhrfabio87]

hello Carey, I am writing to you from Italy, I mounted a GT15HZ2 (blue head) on the Trex 700n, but I cannot understand what oil you recommend to use. I am now going with Motul 800 on road at 4%, I read that you say you make 10% mixture, isn't it too much? I also have a synthetic-castor oil wladoil kart k2, better than the motul 800?

[Ken Here]

Oggie, in this video it comes down at 150F degrees at the end of the video. Hope this helps

Fury 55 with OS Max GT15HZ (6 min 59 sec)

Mhrfabio87,
Did you find and oil that woked out for you? Thanks

[carey shurley]

hello Carey, I am writing to you from Italy, I mounted a GT15HZ2 (blue head) on the Trex 700n, but I cannot understand what oil you recommend to use. I am now going with Motul 800 on road at 4%, I read that you say you make 10% mixture, isn't it too much? I also have a synthetic-castor oil wladoil kart k2, better than the motul 800?

so I just saw this, I dunno why it wasn't highlighted as a new post

Yes I run a 10% oil mixture but its got to be a thin oil like Red Line 2T or Morgan thin oil. Motul is a thicker oil

you need a high oil concentration to address the bushing on the small end of the connecting rod. The large end has a ball bearing in it.