More on Venturis

By Alwyn Smith, August 2005.

[Ron: This page was very kindly provided by Mr Alwyn Smith, a retired Airbus Captain, life-long modeller, and generous friend. He can be contacted at Over to Alwyn...

I was asked by Derek Pickard to talk to the members of Knox Club after their September meeting. One of the subjects I covered was Venturi Diameters and areas, and I took along the photocopies of the article by Claus Makis from the February 1990 Aeromodeller.

Adrian Neuman asked if he could have the copies to reproduce in October 1994 Victorian Control Line News. I suggested to Adrian that the readers may like a further article to cover the information discussed at the Knox meeting.

Eddie Farmer from the Doncaster Club built a Top Flite Peacemaker (with the moulded balsa fusilage) from a kit. The only engine he had available was an O.S. Max S 35, which was far too much power for the 46" model. Derek Pickard had a surplus O.S. Max S 30, and I swapped him a Taipan Black Head Ballraced 2.5 for the S 30, then gave the S 30 to Ed.

The S30 was a much more suitable engine for this model but would not run smoothly during manouvers. A little history would probably be appropriate at this stage. The O.S. Max S 35 was put on the market by O.S. in 1963. It had a relatively small crankcase volume and was supplied with a 7.5mm venturi. O.S. large spraybars are 4mm the same as Enya. The O.S. S 30 R/C and S 35 R/C were put out in 1964.

World engines in the USA were the O.S. agents and in the late 60s or early 70s an article appeared in one of the American magazines telling how World Engines had asked O.S. if they could make a batch of S 30s for sport or stunt to be used for models of about 42" to 48" wingspan, as a number of the modellers on the West Coast had purchased the S 30 R/C, were removing the R/C throttle and were fitting the S 35 Venturi and N/V assembly. O.S. were not making a 29/30 ( 5.00cc ) engine for C/L at this stage.

I read this article and obtained two of these engines. I still have one unrun in its original box. This is the blue box with the white star for the size. This box has no size marked. I swapped the other of these two engines in Perth about 20 years ago. The O.S. Max S 30 C/L engine was eventually put on the market by O.S. in 1977, thirteen years after the S 30 R/C.

When Ed Farmer was looking for a method of improving his engine run the above history was discussed and the Aeromodeller article by Claus Makis was searched out. I worked out the figures at the bottom of the article last month headed "Venturi Effective Choke Areas". You will see that Claus Makis recommended 10 to 12sq mm of venturi area for a 35 size engine. The O.S. S 35 venturi at 7.5mm diameter gives 14.18 sq mm of area which is 18.16% over the recommended 12 sq mm or 41.8% over the minimum area of 10 sq mm.

Ed then made up a number of venturis at 7.25mm, 7.0mm, 6.75mm and 6.5mm diameter. The 7.0mm venturi is a 26% decrease in area on the 7.5mm venturi. There is probably a small RPM drop with the smaller venturi but the better run vastly outweighs the slight loss in power, and anyway who operates a stunt engine at peak power or even peak RPM for the prop in use?

O.S. for many years have supplied their engines with various venturis. I recently obtained an old O.S. Max 1 29 ( 1954 ) and it still had the original O.S. needle and spray bar with a venturi insert and a second insert in the box. Enya have always supplied their 09 111 with a 6mm insert and the 4mm spray bar gives 4.27 sq mm effective choke area. Their 15 and 19 have 6.0mm and 6.5mm venturis and their 29/35s, 6.75, 7.5 and 8.3mm venturis. O.S. 15s have 6.0 and 6.5mm, their O.S.19 was 5.5mm and 6.75mm and the O.S. Max 20 5.5mm and 7.0mm. These O.S. engines all have 3.0mm spray bars. All Enya engines have 4.0mm spraybars.

Another story which may be of interest. I still have my FROG 500 which I bought in about 1950. In 1951 it was fitted into an enlarged Happy Harold from the December 1949 Aeromodeller page 773. The plan was only 1/5th full size. I multiplied the plan X 7 and with enlarged wingtips came up with about 44" span. The tank was too low in the fusilage and the engine cut out every time a loop was attempted. The tank was then moved up to the top of the fusilage but showed little improvement. I was probably also running the engine at peak RPM on the ground. I then made a venturi insert by wrapping a piece of shim brass round the stem of a drill (17/64 or 6.7mm). I used to make my tanks out of .007 (7 thou) shim brass in those days so probably used that size. I soldered the shim brass into a small tube, slipped it off the drill, drilled a hole for the spray bar and fitted the whole assembly to to the FROG. The running problem was now fixed. This insert is still in my FROG 500. If I used two layers of brass the 7.5mm venturi (Too large for a 5.0cc engine according to the Claus Makis figures on the chart) would have been decreased by about 0.75mm to 6.75 mm, just below the size recommended on the chart but improving the fuel suction.

Another Victorian modeller visited the Doncaster Club field. He had a Hearns Hobbies Demon fittrd with a FROG 500 and a Master 10" x 6" prop. We substituted a Master 9" x 6" prop, a new glow plug and 10% nitro fuel. The FROG was now running faster but was very rich on inside loops and running flat out on the outside loops. A venturi insert was made and the FROG was raised by 1/8" (3.2mm) by fitting aluminium plates under the engine mounts. The venturi was now 6.75mm and the FROG ran well throughout all aerobatic manouvers. Did you know that the engine mounts on the FROG 500 do not align with the centre of the crankshaft? Allowance must be made when setting up the FROG 500 and the fuel tank. Many of the older engines were made this way. The D.C. Merlin .75 Diesel is another. If you have an Aeromodeller "Calamity Jane" plan designed by Pete Weaver in Brisbane and published in Aeromodeller in October 1952, look at the nose shape carefully, drawn to align with th e FROG 500 shown. The nose shape must be altered to align with modern engines with the centre of the crankshaft on the bottom of the engine mounts. Another old time modeller commented how the FROG 500 seemed to run better when the original spraybar (1/8") was replaced with a Taipan unit. The Taipan spraybar is much thicker than the original FROG and had the same effect as fitting the venturi insert

Frank Goding, another of the Doncaster members built a " Calamity Jane " with an O.S. Max S 30 with the standard 7.5mm venturi. The engine was running poorly during manouvers and was rich on inside loops and lean on outside loops (Tank too high). An Ed Farmer 7.0mm venturi was fitted and the engine raised 1/8". The O.S. Max S 30 now runs well during all manouvers.

I have an Aeromodeller "Cougar" with a profile fusilage. It has been flown many times over a number of years with an Enya 29 fitted with the black plastic (Nylon?) venturi of 7.5mmdiameter.It was brought out to be flown after a long break. On take off the engine leaned out and stopped after about 1/2 a lap. Richening the mixture was no help. The glow plug was changed, and then the fuel, but with no improvement. The venturi was checked and it was found to be 7.5mm. This engine as I said earlier in this article is supplied with three venturi's. I then made a sleve 7.0mm internal diameter for the 8.3mm venturi. The running problems were now fixed.

Some of the older modellers may remember a practice in the late 50s of jamming a piece of balsa down the venturi past the spray bar to block off half the venturi. Not really recommended but it did appear to be a tempory fix. Are you aware that the current O.S. FP 35 S and FP 40 S are fitted with 7.0mm venturi's but the spray bars have been reduced in diameter to 3.5mm so the effective area of the venturis are still too large at 14.0sq mm These two engines will probably give a better run during aerobatics with a 6.75mm venturi.

I hope all this information explains the reason for the Claus Makis article being reproduced for the V.C.L.N. readers and prompts you to experiment with different venturi sizes to improve your engine runs. Robin Hiern and Ed Farmer can both make venturi's to order Since originally writing this article I made venturi's or inserts for a number of engines, to fix running problems. My friends in Narromine in NSW had a Claus Makis Commodore fitted with an Enya 35 (5224) Special (The twin BB engine) They could not get the engine to run It had been fitted with a 10mm venturi. I made a 7.0mm venturi for this engine and the engine did 11,000 RPM on a Master 10" x 6" prop, and the Commodore flew beautifully.

I have an Aeromodeller plan Slick Chick combat model fitted with an O.S. Pet Mk 111. This engine has a 5.5mm venturi and would not suck the fuel correctly. On T/O it would scream and stop on launch. I made a sleve for it 5.0mm diameter. The sleve is only 10 thou thick or adding the two sides 20 thou which is very close to .5mm. with this small change in the venturi diameter the engine suction problem was fixed. The 5.5mm venturi has an effective area of 7.25sq mm and the 5.0mm insert reduces the effective area to 4.63sq mm, which is a 36.1% decrease in area for only the .5mm decrease in venturi diameter. The engine can now be set on the N/V to just below peak RPM before launch, and the engine holds the peak setting in the air after leaning out after T/O and holds the setting right throughout the flight. A uniflow tank is used.

The plan for the "Rookie" shown in the V.C.L.N. for February 1996 suggests using an Enya 19 venturi in an O.S. Max S 35. The venturi's fitted to an Enya 19 are 6.0mm and 6.5mm.

I have fixed the running problems on a number of engines for modellers since I originally wrote this article. It does appear that the manufacturers fit the biggest venturi that they can to get the highest power as it looks good in their advertising, but often these engines can be made to run much better by fitting a slightly smaller venturi.




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