Venturi size and proper prop for OS III 15?

[Tim Wescott]

I've got one running with a 17/64" diameter venturi with a 1/8" needle -- that works out to an area of around 0.062 square inches if I'm getting my math right.

It seems to work OK with a Top Flight 8x4 prop; it turns about 13500 RPM when fully lean, and was doing 12000 in the video.

What's "best" for sport flying?

[Larry Renger]

Well I can’t actually make specificities recommendations but... Oh! I should just t take my own advice and shut my trap. 

Actually I did have one in college, and one later, but not a clue what prop and fuel were used, nor was I expert enough to now be the one to giving specific advice.  And don’t remember how well they performed for me.  Ummm, that was nearly 60 years ago, sorry for the memory fade.

If it was a Cox Medallion .15, the fuel would be .15 x .18 x .02 and the prop a skinney 8x4. I suspect the Max is a bit milder than that. Might be more forgiving of load than the Cox. Perhaps someone who is a current user will respond with more useful data.

It certainly will not be similar to the current run of Schnuerle ported engines.

[Dan McEntee]

    Judging from the way it sounded in the video, I would start with what you got. You have some lee way to go up in pitch and down in diameter if necessary. I would try some wood props if you have them, as those older nylon props flex and flatten out when running, but I think you are in the ball park.
   Type at you later,
  Dan McEntee

[Tim Wescott]

    Judging from the way it sounded in the video, I would start with what you got. You have some lee way to go up in pitch and down in diameter if necessary. I would try some wood props if you have them, as those older nylon props flew and flatten out when running, but I think you are in the ball park.
   Type at you later,
  Dan McEntee

Yes, it's clear that I could do worse that what I have.

The nylon prop happened to be on the motor -- I'm thinking I'll use APC props, or someone's wood props.

[Brett Buck]

Prop = 8-5 or 8-6. I don't know about the venturi.

    Brett

[GERALD WIMMER]

Hello
Spectre flight has a 1962 Model Aircraft test on the OS Max III in CL version and the standard venturi is 6.5mm with a 3mm spray bar which is right in betwwen 1/4 and 17/64th's see:
http://sceptreflight.com/
The test states that it sounds best on a 7x3 Trucut prop at 20,400rpm !
It seems an 8x4 is most practical as going any bigger with a muffler causes overheating if loaded under 12000.

Currently have 9 models (my sons mainly) with OS 15's on them mainly the drop in liner 1970's version but do run a pair of MkIII's on a Twin Showman stunter .
 8x4 work best for stunt or 7x6 (trim to 6x6) for fast models. Even used them on combat models years ago but they need pressure as the venturi size goes up.

[John Park]

The prop. I settled on as the best for an OS-Max 15-III, in a flapped stunt model (350 sq.in., 24 oz.) was a Master Airscrew 8x4.  However, that was with 5% nitro fuel.  With more nitro, I suppose it could take more pitch - but I wouldn't go over 8" diameter.

[Dave Hull]

Pizzaman,

My spreadsheet approximates your venturi area at about 0.0222 in^2.  This is about 3% larger than the factory OS .15CV-A with RC carb setup, for reference.

You should be in the ballpark.

A lot of .15s will run well on the APC 6.5"-series of props. I'd try the 5.0" pitch, myself. But if you get on the needle, be expecting it to be the fastest pizza delivery you've ever seen!

The Divot


PS--I'm having DiGiorno tonight to celebrate the completion of your design....

[Tim Wescott]

I think I'm going to make up a collection of venturis of different sizes, starting with what I have now and going up.  Reading that review, the motor pretty much came from OS ready to be set up with a pressure fuel system, and was designed to run with no venturi at all.  And as near as I can tell (not having been around then) it was a hot motor, quite unlike the OS 'S' engines that came after.

[Dan Berry]

The Max111 15 is the hot ticket for Nostalgia Gas FreeFlight. APC 7x4 is a popular prop for it. Assume they’re on pressure with a wide open venturi.  I’m not recommending this for your application. I would start with an 8x4.

[Tim Wescott]

The Max111 15 is the hot ticket for Nostalgia Gas FreeFlight. APC 7x4 is a popular prop for it. Assume they’re on pressure with a wide open venturi.  I’m not recommending this for your application. I would start with an 8x4.

Were they used in class A speed, back in the day?  Or F2A?  Had FAI settled on 2.5cc by then?

[Dan Berry]

No idea.

[Brett Buck]

Were they used in class A speed, back in the day?  Or F2A?  Had FAI settled on 2.5cc by then?

    2.5 cc is a .15. I can't imagine they were used for speed, they aren't nearly as strong as many other speed engines of the day.

     Brett

[John Park]

My OS-Max .15-III came with two venturi inseerts - the larger was 6.5mm., and the smaller, which I used for stunt, was 5.75mm.  The spraybar was the standard 3mm. OS unit.  I seem to recall that, being very light, this engine was quite popular for free flight power: I think they ran it on crankcase pressure with no venturi insert at all, screaming like crazy on something like a 7x3 prop.

[Tim Wescott]

Hey thanks John -- the size of the insert was exactly what I was looking for.

All the crankcase holes in mine are 1/8", and the one needle valve that I have which is reported to be from that engine measures out to 1/8".  The Enya 19 that I have from just a bit later in that time period also has a 1/8" spraybar.

Are you sure your venturi was 3mm, or are you just assuming?

[John Park]

Are you sure your venturi was 3mm, or are you just assuming?

We're talking about the spraybar, I assume.  The NVA of my ancient worn-out OS .15 is still in existence, so I've just measured it with my digital caliper: it comes out at 3.09mm., so I think it was supposed to be 3mm.

[Tim Wescott]

We're talking about the spraybar, I assume.  The NVA of my ancient worn-out OS .15 is still in existence, so I've just measured it with my digital caliper: it comes out at 3.09mm., so I think it was supposed to be 3mm.

Thank you!  (And yes, I meant spraybar -- it seems like all the nouns in my head are in one big bag; sometimes I reach in and grab the wrong one).

[L0U CRANE]

Tim, the .15 stunter I've mentioned in here a few times was very pleasant to fly and performed very well. Light, on 52.5' lines, 8-4 prop - stereotype specs for a .15 model - moderate to brisk wind didn't much bother it. Probably from fuselage form more than anything else...

That was in the early 1970's: adapted 370 sq in TF Combat Streak wing, full fuselage, inverted OS MAX III .15, no muffler.

Never weighed or tach'd it, but it ran steady-two with decent fuel-line-tweak higher RPM range - about like yours in the video.

Prop was most likely a wood  TF 8-4 Power Prop. APC or Master 8-4 might  rev a bit higher.

Fuel was SIG Champion 10% all castor. Plug wasn't a Fox; there were other plugs around - affordable then.

The model had a long and bizarre history. Traveled with us where Army assigned me -finally died in Germany! Its other stories include its scary takeoff, and when its lines twanged a seagull at a Lake Charles Nats...

OBTW, words re: 'spraybar' vs 'choke:' sometimes a word we want isn't there when wanted, but pops up much later. (I'm 80... this shows up more and more with years...) You know the Magic 8-Ball toy? Ask it a question: the answer floats up ...? Well, the more years we load answers into our 'Magic 8-Ball', the more crowded it gets. It takes longer for the right answer to float up... (It's about as accurate as a Ouija Board, and less work.)

[Brian Hampton]

Were they used in class A speed, back in the day?  Or F2A?  Had FAI settled on 2.5cc by then?

Short answer to this would be that when the 15 Max-1 was introduced back in 1955 it was used to win the FAI F2C (free flight power) World Championship. FAI had settled on 2.5cc 2 or 3 years previously.

[GERALD WIMMER]

Hello This post got me interested to see how the OS15MkIII compared to other hot 15's in 1962.
Looking at Spectre flights reviews the Tee Dee 15 around .47hp on 30% nitro while the K&B 15R series 61 got .48hp on 50%nitro and the Super Tigre G20/15 got .495hp on 50% but the plain bearing OSMkIII was a bit down on these at .425HP on the same 50% nitro
But the big winner in 1961 was in fact the OS15 Racing an engine based on the 15D diesel, a beefy twin ball raced engine but converted to glow that was the top engine yet tested by Model Aircraft at .515hp and this is all well before the G15's and Rossi 15's
Regards Gerald

[Brett Buck]

Hello This post got me interested to see how the OS15MkIII compared to other hot 15's in 1962.

     The Max-III 15 was the engine around which a lot of the "Junior" models was designed. The Jetco Shark 15 is typical, and Lew McFarland used a Max II or Max III in his. I recall him flying combat with somebody using Shark 15 at the Bluegrass field airshow, and the airplane hung on the wall in his shop for years.

    Brett

[Steve Helmick]

Short answer to this would be that when the 15 Max-1 was introduced back in 1955 it was used to win the FAI F2C (free flight power) World Championship. FAI had settled on 2.5cc 2 or 3 years previously.

That was F1C, actually, but we understand the confusion. There is a move afoot to have a contest event in our area (Tangent, Oregon, specifically) for "Cranfield 10" models in (I think 1959, '60 or '61), where 10 fliers were still tied after all the flyoff rounds allowed by the rulebook, and were declared "Joint Champions". Because of this, I was going to ask Tim what he intended to do with his OS Max III .15, since he has a big enough pasture to allow test glides while social distancing. 

FWIW, when I first flew my Fox .35 NW Sport Racer, I tried an OS Pet .099 spraybar/NV assembly in it. Not very successfully, I might add! That was the only part that was not required to be "stock". It never occurred to me to raise the exhaust port timing or use CF props,, like some others did. Silly me!  Steve

[Tim Wescott]

... Because of this, I was going to ask Tim what he intended to do with his OS Max III .15, since he has a big enough pasture to allow test glides while social distancing. ...

Oh, I wish I had enough time to pursue free flight.

[GregArdill]

For sport flying, stick with the venturi you have, run an APC 8X4. Fuel 10% nitro and 20 - 25% oil.

The recommendations for smaller props will have the OS singing its head off. I'll love it, but it won't be compatible with your stated desire for sport flying.

Great engine you can enjoy in many ways.