Hi all, I wanted to share the venturi I am planning to use next season on my ST51. Big advantage is the fact that the NVA will be on the opposite side of the silencer and I hope fuel draw will be improved. To be used with an Enya or OS  RC type NVA. Thanks to my club mate Jef Lemmens for making it real on his lathe..
Take care!
Paul

       Randy would suggest that what you have in your drawing isn't a "true" venturi, because it has a flat spot. Depending on other factors, that matters or doesn't, and making it smooth curve may make it "better" or "worse" depending on what parameters you think you want to maximize. I am not a big fan of the terminology -  It's not "true" or "untrue" based on what simplified drawings from elementary physics books happen to show.

   At any rate, you are most of the way to what I consider the ideal solution for stunt. Insert a spigot to inject the fuel into the middle of the stream instead of along the wall, and you have a classic Frank Williams "fuel post venturi" as described in his Stunt News article in the mid -90s'. You generally do not want to inject the fuel along the wall, that works OK most of the time, and other times, it never gets atomized and instead just dribbles down the wall. You can improve that by making the dribble hole smaller in diameter, to make sure it stays wetted across the entire diameter. The nominal hole size on the RO-Jett venturi is (as I recall) .090 or so.  But it worked much better still when I used a 1/16 OD copper tube to extend the end about .030-.040 from the wall of the cylinder to make sure it went all the way to the high-velocity part of the stream. It also probably introduces very beneficial bluff body turbulence behind it to ensure good mixing.

   The difference it makes depends on the engine. For the PA40 and 61 it was an absolute miracle as far as fixing the notorious PA characterstic of running much differently in inside and outside corners - that is, hard lean at times on outside corners. Bill Werwage seems to have discovered the value of it, David was amazed at how Billy's engine ran so much more symmetrical between insides and outsides, and, after the WC in Germany, David was looking and found it had a Frank William's fuel post venturi. He came home, made on, and it was immediately much better. I tried it on my PA40, and it was about 50% better.

  Since, that's about all we have used. I started with the original venturi on the Jett, quickly progressed through the smaller dribble hole to the spigot, and as far as inside/outside run quality, it has been completely bulletproof. David uses a fuel post venturi on the 75, it still did a few "PA" things, then he reduces the restrictions on the fuel side, and that has been pretty reliable since.

      Fuel flow restrictions and venturi optimization are the current areas where there are still experiments going on (and probably the LAST big area). and we have found great improvement in run quality that has kept us more-or-less competitive with electric for quite a while now.

    You are about two small steps from where we have ended up, I greatly encourage you to experiment, and if you carefully experiment ,you may find some big improvements.

       Brett

p.s. looking carefully at your drawing, I think all you might want to do is solder/somehow attach a piece of small tubing to the tip end of the spraybar where it screws in, maybe 1/16 OD x .050 long. You probably don't want to screw the spraybar in to the point it intrudes on the choke it's too big and you probably want sharp edges to encourage turbulence, and M4 is way too big around. This will allow you to screw it in or out to different degrees to adjust the choke area and the spigot location across the bore.

   Once you come to some solution on the venturi and spigot, I would also encourage you to experiment with diffusers on the inlet (panty hose or other filteresque materials), again, to ensure that the flow into the engine is turbulent as possible. That tends to reduce the effect of the airstream hitting the inlet at different angles and again, encourages mixing. That is contrary to maximizing the flow, or the pressure drop for a given flow rate, but those aren't important for stunt, since you have far more power than you could use anyway.

       
p.s. looking carefully at your drawing, I think all you might want to do is solder/somehow attach a piece of small tubing to the tip end of the spraybar where it screws in, maybe 1/16 OD x .050 long. You probably don't want to screw the spraybar in to the point it intrudes on the choke it's too big and you probably want sharp edges to encourage turbulence, and M4 is way too big around. This will allow you to screw it in or out to different degrees to adjust the choke area and the spigot location across the bore.

Thanks for the feedback, Brett. In attachment the correct picture of the NVA I intent to use. it will stick out 2mm in the stream. First attempt with dia 4.5 mm was a little down on power. Next attempt will be 5 mm. Static runs are promising.

Paul,

In my opinion you are blocking too much of the cross area with your fuel post. It only needs to be 0,8…1mm long.
Also, I’d recommend to make a geometry with 2 cones and a sharp intersection like in the drawing below. It will give a better stability. There is a clear difference even with my rear intake engine, so it’s not just about the variations of incoming air direction & speed.
Also, with the sharp angled type, there is a better chance to get it working well even without the fuel post. L

..of course, with very little extra work he could turn the very tip (maybe at 1/32-1/16" length) to as small diameter as possible. like hole diameter + 1mm, and then adjust the correct protrusion with the nut. L

Id think raising the bypass & boost port  would maybe be worthwhile , if theres the decent machineing available .

the thing WITH REVING IT , is that it does get all the scavenging & pulses into operation .

theres a effect like ' coming onto the cam '  , like in the old ' 2 stage cam ' days . Where its OFF ( full stop ) below a certain rpm's .

i figured Mr Garafiori might know what he was talking about . so tried a 11 x 6 FIRST . as recomended by the manufacturer .
If Bill Bell says " That runs as well as anything ive seen  " ( at the world champs ) He means it runs as well as anything he's seen .

If I recall right , it was with the .46 muffler without the tailpipe , and the 10 x 6 Grish ( Tornado . BLACK G F R N ) .

Dropping the rpm it lowers the gas speed IN THE PASSAGES ( cylinder passages ) so can nulify the scavenge effect .

If you want to turn a Lamborghini into a tractor , they made those too . But the G 51 can get a perfect switch variable on the needle
from middle of lower vertical eight loop to middle of upper vertical eight loop , consistantly . Up & down , on 1/4 needle turn .
DEAD STOCK , burning 80 cc of no nitro juice in a 70 ounce plane at 70 foot of .018 . . 5 or ten percent nitro 7 a extra gasket
has it fine till 18 kts odd turbulent air . where Id try a g 51 R. C.  and the 10 x 4 three blade . ( or maybe just add head gaskets or a harder plug ) .

The eather  11 1/4 x 5 1/2 has it mostly ' off the power . - loafing . schincking into the 2 stroke ( or lean ) just occasionally . at a 5 + sec. lap .


The G 45 ABC might be better to use as a loafer , with a bit of butchery to the ports .perhaps . As VF's have massive shaft passages ,
and the shaft passage restricts the21/46 ( and 21 / 35 - the Mk II has a 15 mm shaft ) output , sleeving it will too , most likely .

The como 51 pdp is a 15 mm shaft 1 mm overbore 21/46 more or less , but has very high sleeve timing . same wt. & fit as G 51 bar shorter nose , could be got low timed with a long skirt piston .
But like al rabe & frank mc Millan , at the end of the day , its easyer & less grief just to get something that works as you wish ' off the shelf ' straight off . theres still pdp ST 60 V to be had , unused .
Urtnowski & Modesto run 4 & 5 pitch 13 inches , to deal with flushing meadow .  so likely a saner option , if ya wanna run ' prop ' (diameter ) .

THOUGH YOU SEE " VENTURIE " is missplaced , just for now .

 

These are 1980- ish . like the last 3 window ST 60 pdp . Best thing you could do would be STROKE the 51 to 22 mm. But it wont fit . UNLESS your Frank or Al , who did it to Como 51 to 60 bored 1 mm stroked 2 mm .
So using similar ' methods ' youd get the G 51 out to 60 . If you ' do a crank ' . Thing to watch is the piston dosnt unclose the case at T D C  so skirt length must be sufficent . ( or dropping a G 51 liner ) .

G 51 & Como ) .Ive just whacked the fins down to glowplug pedastal level . A to fit a Cowl ( ive got ) B to see if it IMPROOVES the churnability like they do on the V 60 & 46 ( shave the fins to 3/32 ) .

The G 51 Ex is higher from the lug  so it wont go inwithout further surgery on the aircraft . ( After all that )  HOWEVER , THAT would indicate a .60 displacement G 51 is easier than a . 60 Como 51 . By Far .
1 mm on the crank throw & a 1 mm overbore , if you want to get your mate going . 

( Frank run the .60 Como 51 in the Martin Baker M B 5 . ) c.o. your archive research organiseation . ! 

Paul,

You are on the road to make all the mistakes that have already been made, but that’s also one way to learn. Lots of testing has already been made, to find out the best solutions. And if something is *always* better in practical tests, it does not matter if theory or your intuition says something else.
For example, as I wrote earlier, the fuel post only needs to enter 0,8…1mm to venturi bore. You just need to get it away from the unstable boundary layer. Your spraybar end is so thick and clumsy that it will change your venturi geometry far away from the theoretical ideal, that’s why I recommended to thin down to minimum the last mm or so.
And, do yourself a favour and make the double-cone true venturi instead. It is more stable.
Also, your idea of the radial turbulator wall in crankshaft passage will be counter-productive to stability. It’s ok to create some turbulence in the venturi for good atomization, but after that you’d better keep things laminar. More you disturb and slow down the flow between venturi and glow plug, the more you you can expect instability due to flow ballistics and oil accumulation. L

Thank you all for your comments. I hope  to join a contest in Italy or Switserland in 2024 to talk engines and stunt with Lauri. And you are all  invited to come to Herentals, Belgium in July were we will have a control line  Contest/Fly inn with Aerobatics, carrier, combat etc . Many circles for sport flying and practise .

I am still in doubt whether or not to participate in the World championship in the US next year. Very tempting but quite a trip...

The venturi I had made for the ST51 is derived from the adjustable Berringer venturi that Gilbert used in his Saito 51 for his Caudron or Sukhoi. Not too bad as a reference.

I am super happy with my ST51 with restrictor inlet. It pulls my ship with authority. It is super stable, but I lack braking going downhill and I experience whip-up in windy conditions. I know a pipe might help here, but that is not the way I intent to go. (nor electrics) . Therefore I want to experiment with this venturi.  Bernoulli should help me  with braking :-)  . I don't have a lathe or other machining tools, besides a drill stand. So I am limited in what I can attempt to alter to engines.
Gentlemen, I am reading your advice, absorbing it and I will extend the post with an outside  diameter of 1.5 mm sticking out 1mm max. my setup allows me to experiment with the dimensions. I will skip the turbulator in the crankshaft, but reducing the diameter is still on my list...  The fun is in experimenting and trying to understand the effects of certain changes.

Tests continue....  Take care!

further thoughts & observations cetra .

NOW the other day I saw a 60 inch or was it 58 thing waffling along nicely , a bit like a big soft woffly rather than snarling ST 60 ,
and said to the bloke " that runs well , WHATS init ." Whereapon he said a S T G 51 . So I glared at it . AND the big 12 1/2 x 5 1/2 ( or some pitch ) three blade Eather prop. Astounding !  .
He further informed us , it was ' done ' by ( i missed the name ) possably Dixon . So might be bunged bypass & Hemi Head ? ANYWAY , was a soft quite torqey reponsive run . Last thing Id expect from a G 51
in a big plane .
Being as I runnem Hard .
 Just dropped a 22 thou. old V 51 gasket in the shaved head oldGit 51 ( stripped and cleaned used ) , to give it a try , maybe on a 12 x 4 Bolly , for a soft snarl , we hope .

The 40 & 45 head match the BOLT PATTERN . Theres a ' plug to piston ' Max. according to Lew wollard ( on schneule Foxe 40  ) He carves the lower sleeve flange to drop liners . to 130 / 110 .
which is what they set the HP 40 to . Also other work good there . so the old ' Hemi , 1/16 th squish . shallow angle ( 3 or 4 deg. ) remove all sharp edges trick , if youve spare heads , could help .

Crank Timing , the REAR intake HP is 60 closeing . and works extremely well , balancing gusts & switch off down hill .

Randy Smith states the R C G 51 works well . I dont know the sleeve timing there . Bt the ducks guts is the Ro Jet 50 , on those big tapered 3 blade 12 1/2 Eathers everyone ( here ) runs . On many engines .

The Ro Jett Timing is io 40    ic 55      Ex ( 68 ) 136   trans & Boost  126   pistons  23 . 5 bore   heads 1/8 squish  4.2 deep chamber .

G 51s ive got THREE differant Crank timings . Measured .  theyre about 114n / 116 on the boost . 126 or 128 on the trans & 136 / 140 on the exhaust . Theyre were built for years . Ones a China 4 number stamp still
ITALY on case .  Just grabbed my notes . wasnt expecting the varyation . Tho ive carefully measured them a few times . A flashlight into the lower case help exactitude with the angled port upper faces .
Lew left the Fox 40 boost center intact - only liftedouter 25 or 30 % for turbulance - said to be a neccesity . on that . ( Similar port layout . so we mention it .

a China G 40 is  124 T  110 b  155 Ex .  44 - 47 crank. ( ive got 44 - 47 for a old G 51 crank. stamped 3154 ) . slotted screw driver head screws . Late Italian ones were allen head / cap screw .

NOW !  ive got written , 43 - 50   . 57 , 57 , 64 ( which is 114 & 128 ) written for G 51 from a few decades back /
ALSO 51 - 41  63 58   68 . which is 126 T 116 B & 136 Ex . So are irritaed and annoyed . as were  C L G 51 . But maybe NOT . AND ive not brung the G ( GS ? )  45 ABC figures .

Could be any explanation includeing ' test ' figures with say a 1 mm feeler guage ( or stick ) on piston to try result from dropping sleeve ? perhaps . ( Im sure all the hights in the Ex Stack match )
NOW I will have to take another look . the decompressed head with the upper fins knocked off , is ready now for test this weekend . Along with ( next ) a Rabe rudder , im hoping itll get theb 63 span 63? ounce Spitfire ' on track ' consistantly . the heads now low enough for the cylinder opening in the deeper ( of 2 ) cowl to be closed over . but'd be tight . will clean up the airfolw over it anyway .
the GARDNER G 51 was plain ringed from a old plane of his , from aniother bloke . and had had the BY PASS ( boost ) bunged . a piece of balsa , maybe tack glued - would TEST that theory . but was common.
( I debunged it in outrage    )

Id think differant squish band width Hemi'd heads would be worth a try ( alledgedly Big jim G - 51's ( or some ) were hemi'd . ive not seen ( or seen in ) one . Squish CLEARANCE . Close for effectiveness - is the trick .
at 22 thou. alledgedly ive lost the sudden B M E P jump that gives swirl & maintains ' pink free ' non Pre Ignition - runnig ( previously at high rpm on the 10 x 4 3 blade the resonance was distincnt & resonant !
cut the fins off a S T 46 muffler & the ring like blazes . THIS sounded like the sides / upper case  R E S O N A T E I N G  . Loudly . Id said that to Brian Eather on the phone . that'd be DETONATION he said .

Yes. indeed .    ahem . was the thought . detonateing the upper half into orbit it sounded like . The C L Horsepower  is rated at 11.000 ( factory Figure ) the R C at 16.000 . therefore , the C L set to run up to 15 or 1600 . the two differant G 51 LINERS . same Crankshaft  .
holding its own when it blowing viciously gusty , is in theory at least , not impossable . the H P 40 rear intake . with 130 100 timing at 40 open 60 close , on the crank ( a unrestricted passage one might say )
gets a 4 - 2 - yowl - 4 - off , run , in a wingover into a big gust up top . the old Scott motorcycle was said to Y O W L .
It coulda been called 8 - 4 - 2   run . but it was a normal 4 - 2 with another yowling faster two , under duress . And could drop straight back to braking . ' Throttle Shut ' !

Set the  carbs closed , so the idles ' on the throttle ' for max engine braking . AND YOULL KNOW WHAT i MEAN . FASTEN SEATBELTS . The old back roads were pretty rough .
After the world rally Champs stage there , the local trick was time the bounce from the drop onto the leap to the bridge right, pull off 30 mph maybe ( they went from 90 )
for Max. braking . as the hard left of it was a wall / hill cuttaway , on the right . There was no room for error .

Another trick there was to have them all stand in the long grass roadside AFTER the corner . And have someone on the gate , to wave as they went into the paddock . as they were using the spectators BEFORE the corner , for dialing in the braking points . Haa GAUGHT YA . Told it worked on most of them . Mostly just a few front runners wernt fooled . 1980s world rally champs . ford Escorts , Vauxhauls & real rear wheel drive stuff .The banks outside the bends bend or break many an axle . Then theres the pot holes . but a FORD is under control as long as your not looking at the road in front out of the rear window , as they said then .

Id think the big chamber close squish HEMI might be the trick , for Engine Breaking . the HP head matched a std. ST 46 & K&B 40 chamber . Wide Squish , deep cup - POLISHED .
NOT the as cast lower face of a G51 Head . That may improove the economy , but a CLEAN FACE likely gets a sharper more distinct shock wave as the squish closes .
The ignition ' coming off ' / Going LATE ,  maybe the thing -m  off load - for the down hill sections . ( a weber gave modulation on closing variation , for dirt rooads - UNLIKE most Carbs )
getting ' modulation ' on Engine Breaking - Off Load : might be the trick . But theres off load and right off load . As in it VARIES .
 So the decompression ( Bigger Chamber ) for the ' soft break ' with the close squish may get THAT happening, we can always hope .

SO ; Old 40 & 45 heads are suitable for butchery for tests . Also for highest standard craftsmanship . ( think of caught most misprint / miss key )

junk for experimentation in the head / COMBUSTION CHAMBER .

( Two Early ST 46 , one had a WEDGE HEAD . Just as well the other did too .  . The  WEDGE is Not uncommon . Niethers a HEMI . See TRIUMPH Twin . Riley . the Club , old Boy . )

SQUISH , wedge or Hemi . Dave Adamisins ' TEN THOU. ' recomendation .https://stunthanger.com/smf/engine-set-up-tips/os-40-fp-set-up-q's/msg570055/#msg570055

B M E P - Chamber Volume . Comp Ratio .  includeing Lew woolards " Max. Distance plug to piston ( at T D C ) or the lamp goes out . 

So , we set the SQUISH opperable . so It Does . created a shock wave . Can be supersonic to blow plugs . 

We set the Head Volume . We want so gas ( fuel Air ) in there . Lots of it ? the more the merier ? AND we DONT want it to ' hit off ' all the time . ( or it wont 4 stroke ) .
we may also want , perhaps , the Ign. ' Flash Point ' in degrees  ( we call it Timing . the MOST CRITICAL aspect in a Four Stroke - assumeing its getting sufficent fuel and has adequate heat dissapation .
We also want heat retwention . At least in the glow Plug . and we want Temperature stability . At least to a reasonable percentage . WHICH was what those Gent's were getting All worked Up About .

4 - cycle Vs 2 -  Cycle - Temperature pluctuation - revolution to revolution . ( say 10.000 Cycles per minute . ) So there Ya Go . Put that in your pipe and smoke it . i believe was the adjective .

A head or Two .



Your Big Art / adamisin . Typical WEDGE . above .




Rbt. Zambillie's ' open chamber ' ?  Hemi .


https://i.ebayimg.com/images/g/oaoAAOSwNh9j7TTF/s-l1600.jpg picture'l likely drop out . BAFFLE is irreklevat to THIS issue .
COMMON to Many . ST 40 , K&B 40 , & Irvine 40 ( Wide Squish , could all have been chamber off the same tooling . Somebody assumed somebody new someting . Or they couldnt get it better otherwise themselves .
NOTE : the Wide Squish Vs narrow - pulse magnitude at T D C increased . perhaps .

they were playing with chamber volume , hight , and Squis Widt in minuate in later racing g-15s . All Early ST ringed tended to be wedge then narrow Sq. open Hemi . then to later Wide Sq. but for R. C . 

So theres roomfor playing about . a BUTTON HEAD ( Butchered stock unit ) might speed testing . OVERCOOLING - sometimes people taped up partial intake - for CORRECT opperationg Temp .
72 Deg in a water cooled two stroke its Said . L C yamawotsits  . Which is LOTUS ( Cortina , Jensen Twin cam and a Good Few Others ) For some reason . For Max Output . Going Hard .

Like some had a will off their own with these Triumphs . The wILL being the Engines , which is better than WONT . or is it . If it wont ' it'll brake . which is what we want . we dont want it to break . any of it .

Thanks . 
( thatll keep em busy for a bit .  0 Thanks to all the F M contributors and Bob hunt for their Knowledge . And Aeromodelor . Also Freddy Dixon . and others . Also DYNAMICS of Gas Flow .