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OS Max III .29 steel-fin  Empty OS Max III .29 steel-fin

Post  roddie Tue Feb 10, 2015 7:32 pm

My uncle gave me this engine some 20 years ago. It was first introduced in 1959. This one has a "62-D" stamped under the R/H mounting lug though. There has been some speak of venturi size/shape in some engine threads lately.. and this engine has "by far" the largest venturi of any that I own.. (I don't have many engines...  Laughing ) The compression is pretty good.. whereas I can barely turn it through; by just rotating the drive-plate.

 
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Post  Ken Cook Tue Feb 10, 2015 8:45 pm

Roddie, the Max III's came with a aluminum restrictor within the venturi area. It was installed and the spraybar assembly went through it and retained it.
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Post  Surfer_kris Wed Feb 11, 2015 5:17 am

I don't have this engine model myself, but here is a picture from the web that shows the restriction;

OS Max III .29 steel-fin  U002f_10
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Post  Oldenginerod Wed Feb 11, 2015 5:27 am

The early Enya .09s use a very similar style restrictor, although obviously smaller. The restrictor is possibly for using the engine for stunt where consistent fuel draw is required, rather than speed which may suit a larger venturi. Of course, without the restrictor it may require pressure.
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Post  GWILLIEFOX Wed Feb 11, 2015 7:19 am

Beautiful engine Roddie!  Here is an RC version of your engine's father, a 1957 OS Max 29 II.  This had one of their first throttles, a sheet steel choke, no fuel metering, and a rotating barrel exhaust valve.

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Post  roddie Wed Feb 11, 2015 8:59 am

That's a nice engine (GWILLIEFOX)!! I haven't seen a throttle set-up like that before. Is it yours by chance?


@Oldenginerod wrote:The early Enya .09s use a very similar style restrictor, although obviously smaller.  The restrictor is possibly for using the engine for stunt where consistent fuel draw is required, rather than speed which may suit a larger venturi.  Of course, without the restrictor it may require pressure.

Ken, Kris and Rod, I'll have to ask my Uncle where he originally got the engine and if he remembers if it had a restrictor. I do notice now; what appears to be two grooves (machined/worn from vibration?) on the spraybar, just inside the wall of the venturi's throat. Maybe these are indicators that a restrictor was there?

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My uncle flew some stunt back in the 60's.. but especially "combat/stunt". There's a possibility that the Max III engine was on this airplane at one time.. a Veco Renegade that he gave me just recently.

OS Max III .29 steel-fin  Veco_r11

The last engine that was on this model was his Super Tiger .35 running crankcase pressure (my uncle wouldn't part with that.. Sad )
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Post  GallopingGhostler Wed Feb 11, 2015 9:07 am

@GWILLIEFOX wrote:Beautiful engine Roddie!  Here is an RC version of your engine's father, a 1957 OS Max 29 II.  This had one of their first throttles, a sheet steel choke, no fuel metering, and a rotating barrel exhaust valve.
OS Max III .29 steel-fin  Os_max10
Interesting back then that in order to obtain a reliable idle would resort to a movable exhaust restrictor baffle, back in the days that such unmuffled noise was considered as healthy noise. This is not much different than our modern Cox .049 engines using an exhaust ring restrictor for idle. The choke throttle is not much different than Bernie's Sure Start choke tube throttle.
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Post  Surfer_kris Wed Feb 11, 2015 9:25 am

@GallopingGhostler wrote:
Interesting back then that in order to obtain a reliable idle would resort to a movable exhaust restrictor baffle, back in the days that such unmuffled noise was considered as healthy noise. This is not much different than our modern Cox .049 engines using an exhaust ring restrictor for idle. The choke throttle is not much different than Bernie's Sure Start choke tube throttle.

I think the exhaust restriction was needed to maintain the heat on the plug when it was choked and running very rich. Or alternatively viewed, the choke is needed to maintain enough fuel draw at the reduced rpm from the exhaust restriction. The throttled Cox medallion engines used the same idea (see below).

And yes, more than 50 years later the sure start engines can now be choked too...

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Post  Mike1484 Thu Feb 12, 2015 12:26 pm

Restricting the exhaust is a good way to throttle a motor . So is bleeding off crank case pressure . Both ways where used in the Navy carrier events in the 50's and 60's . I used the Max.35 and Super Tigres and McCoy .60 in Navy carrier When I first started to fly in the 60's . The .29 and .35 max are still used for certain free flight events . Strong running motors they are .

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Post  chip58123 Mon Sep 21, 2015 8:58 am

I have the same OS Max III 29 but with out the muffler.  Do I need to locate one for it to run reliably or is it designed to to run with out.  I have never run it nor do I even remember where I found it.  Someone put a Perry Carb on it for RC, and I will be using it on an RC plane possibly an LT 25 Kadt.
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Post  getback Mon Sep 21, 2015 12:11 pm

chip58123 I would think it will run just fine without the muffler but .. I always run muffler w/pressure and to deflect that excess nitro/castor exhaust ! some also use an extension (hose) to point exhaust to the ground .. Welcome to the Forum! . Do a intro so we can all welcome you . Eric RC Plane
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Post  gcb Mon Sep 21, 2015 7:06 pm

A few thoughts:

Without the restrictor, you will probably need to run pressure. This can be bladder pressure like used in Combat or crankcase pressure. The place in the backplate with the number 1 above it is the recommended place for tapping for crankcase pressure. I would recommend bladder pressure.

In the fifties, engine manufacturers as well as home-made versions by modelers tried various combinations to control speed. Cox used the two needle system on the Thermal Hopper. Fox and (I think) K&B used the two needle setup also.

Some tried a flapper valve, some tried an exhaust restrictor...both worked to some extent. The version above on the Max was also used on my MAX-II .15. Results were so-so, but it DID throttle. The MAX-III .15 had a real adjustable throttle and a variable exhaust restrictor. I believe the larger versions did also.

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