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Have glow engines improved over the last 50 years?

Posted: 18 Jul 2020, 12:30
by Mike_K
I've just acquired a used Graupner Kwik Fly MK3 and was trying to decide what engine to use. It came down to either an OS 46AX (older rear carb type) or an OS MAX-H 60F GP - the gold-headed type, as I have one of each sitting on the shelf. Out of interest, I put both types on a test stand using the same Bekra 5% synthetic oil fuel, the same APC 11x7 prop I had to hand, the same optical tacho and the same scales (it only resolve to nearest 5g). The test results were interesting:

OS 46AX 2010 vintage: 11,800prm, tick-over 2,300rpm, weight 485g (17.2oz)
OS MAX-H 60F GP 1972 vintage: 11,700rpm, tick-over 2,400 rpm 490g (17.3oz)

Starting both were very easy, a couple of flicks and they were away and the transition was similar. Noise is difficult to measure when using a test stand, but at the same distance both read 86dB.

I found reviews of both engines on the internet and the OS 46AX is a few hundred rpm down, the OS MAX-H 60F is a few hundred up. The increase in rpm can be accounted for by the Bekra fuel, I found my OS MAX-H 40 had a few hundred rpm more on Bekra straight fuel than using 5% castor fuel. And before anybody worries about an old engine running on Bekra synthetic fuel, I've used it for a year now on the OS MAX-H 40 without any problems.

So using a prop size that is recommended for both engines, OS gained 100rpm at the top end, improved tick-over by 100rpm and saved 5g in weight and didn't make it any quieter over a 38 year period. And as the OS 46AX and 46AX II have about the same specification as each other, OS hasn't really improved its glow engines in 50 years! Or am I overlooking something?

So I'm going to fit the OS MAX-H 60F GP in the Kwik Fly.

Cheers

Mike

Re: Have glow engines improved over the last 50 years?

Posted: 18 Jul 2020, 13:09
by Tobe
A modern .46 is more than enough, more reliable and runs quiter than an old original Enya .60
I flew for years a F3A veteran tour when living in the US and lack of power were never an issue.
One point is that thise models today with modern equipment of that size are up to 300-400 gr lighter.
The original Phil Kraft Kwick Fly had the engine installed with a backplate and not a regular engine mount making the engine compartment extremely small with no space to spare

Re: Have glow engines improved over the last 50 years?

Posted: 18 Jul 2020, 19:22
by jackdaw
Mike,
All depends on what you mean by 'improved'. My personal take on your question, other opinions are available :D .
RPM on useable prop sizes not so much IMO unless you are buying a special motor. But, QC and consistency with modern CNC machining, Yes! Smaller engines have definitely improved (when they were still being made). With the bigger ball raced ringed engines, well all engines to be fair, there was a pursuit for ever higher BHP figures driven by advertising sales. This resulted in the power curve reaching max at ever higher revs which were mostly impracticable for normally used sizes of props. The change from 'loop scavenged' to 'shnurle' did improve fuel economy.
For reference here is a test of a 1973 Merco 61 MkIV. With silencer its only a few hundred rpm down on your engines but it weighs only 260gm with silencer. The main Sceptreflight site(glow and diesel) are very informative. http://sceptreflight.com/Model%20Engine ... %20IV.html

Re: Have glow engines improved over the last 50 years?

Posted: 19 Jul 2020, 02:17
by bluejets
Use an OS 46 in a trainer and it forever requires starting with a quick "backwards" flip on the prop.

Also in the list of favourites is an old loop scavange OS40 ring motor which starts exactly the same, idles lovely on straight 4:1 methanol/castor, has never quit on transition and is quiet into the bargain.

There may be some power differences but in my experience, most rc aircraft have about twice the amount of required power to fly quite well anyhow.

Love the sound and performance of pylon screamers but that's another story.

Re: Have glow engines improved over the last 50 years?

Posted: 19 Jul 2020, 09:22
by Pchristy
There was a major leap in engine design in the early 70's, when Schnuerle porting was introduced. Prior to that, engines were usually of a cross-flow design, with a deflector on top of the piston (mostly larger engines) or radially ported (mostly smaller engines, predominantly diesels). Schnuerle porting did away with the deflector on the piston crown, and produced a dramatic improvement in power output. However, you never get something for nothing, and the porting arrangement tended to need a bigger (more massive) crankcase to accommodate it.

I've always reckoned that an "Old School" 61, such as a Webra, Veco, Merco, etc, was probably the equivalent of a Schnuerle ported 45-50 - something your findings seem to confirm!

The first Schnuerle ported engine I can recall was the HP .61. I remember a fellow club member getting one back around 1970, and we were all staggered by its prodigious output! Indeed, we rapidly discovered that most of the popular propellers available at the time were incapable of handling the power! We had barely warned Jack not to get in line with the prop whilst adjusting the needle when it shed a blade! He had only just moved out of the way when it happened!

Manufacturers of cross-flow engines, who had invested quite a lot of money in crankcase castings, tried to play catch up with "Perry Directional Porting" - an attempt to replicate the advantages of Schnuerle porting without requiring new castings. HB were the main adopters of this, but Super-Tigre also produced some PDP engines, IIRC.

Nowadays, Schnuerle porting is pretty much standard. PAW still produce radially ported diesels, but aside from small Cox engines (also radially ported), all glow engines have been pretty much exclusively Schnuerle ported for decades.

--
Pete

Re: Have glow engines improved over the last 50 years?

Posted: 04 Aug 2020, 17:21
by stuart mackay
Mike_K wrote:
18 Jul 2020, 12:30
I've just acquired a used Graupner Kwik Fly MK3 and was trying to decide what engine to use. It came down to either an OS 46AX (older rear carb type) or an OS MAX-H 60F GP - the gold-headed type, as I have one of each sitting on the shelf. Out of interest, I put both types on a test stand using the same Bekra 5% synthetic oil fuel, the same APC 11x7 prop I had to hand, the same optical tacho and the same scales (it only resolve to nearest 5g). The test results were interesting:

OS 46AX 2010 vintage: 11,800prm, tick-over 2,300rpm, weight 485g (17.2oz)
OS MAX-H 60F GP 1972 vintage: 11,700rpm, tick-over 2,400 rpm 490g (17.3oz)

Starting both were very easy, a couple of flicks and they were away and the transition was similar. Noise is difficult to measure when using a test stand, but at the same distance both read 86dB.

I found reviews of both engines on the internet and the OS 46AX is a few hundred rpm down, the OS MAX-H 60F is a few hundred up. The increase in rpm can be accounted for by the Bekra fuel, I found my OS MAX-H 40 had a few hundred rpm more on Bekra straight fuel than using 5% castor fuel. And before anybody worries about an old engine running on Bekra synthetic fuel, I've used it for a year now on the OS MAX-H 40 without any problems.

So using a prop size that is recommended for both engines, OS gained 100rpm at the top end, improved tick-over by 100rpm and saved 5g in weight and didn't make it any quieter over a 38 year period. And as the OS 46AX and 46AX II have about the same specification as each other, OS hasn't really improved its glow engines in 50 years! Or am I overlooking something?

So I'm going to fit the OS MAX-H 60F GP in the Kwik Fly.

Cheers

Mike
Bring it to Binham, Mike and you can compare the same models, mine is electric vs the glow??