Stuka Stunt Main Forum · 30 of 30 known posts recovered
The later is sure allot easier to make but was wondering if you lost something taking the shortcut.
Thanks
Bob
Well, let's try it from this perspective: What do you see on the PA series of engines or the RO/Jett? I am a firm believer that if the "sprinkler" venturi, as it has been called, would truly prove to be true, AND if it worked better, that is what we would see on these engines. But, and quite frankly, I don't believe that the "sprinkler" really is.
What I am trying to say is the fuel will find its way into the venturi from the closest hole to the needle valve. It is not going to be spraying in from all five or six holes all around the venturi. So, you haven't gained anything here, and you have really lost. Basically, it is easier to machine a slot all the way around the venturi on a CNC type lathe than it is to make the cross slot. And, with enough holes spaced all around, the assembler of the engine just slaps it in place and moves on to the next one.
What you call a "cheater" venturi, I actually consider to be a better design in that the intake hole is much closer aligned with the hole in the needle assembly. When you make these by hand it may be easier to do it this way, but even when I have them CNC machined, that is the way I go. And I now have CNC made venturis for both the ST 46 and ST 60. As I said, just check what they are using on the PA and RO/Jett engines. They certainly are not less efficient when made that way. Oh, and the cross slot has more shoulder area for the needle to hold it more tightly in place as well. Less chance for air leakage to mess things up.
Leonard Neumann
Never seen a PA or RO Jett venturi, still 2 or 3 airplanes away from making that $tep. Maybe I should not have used "cheater" just poped in my head at the moment.
Thanks for the info
Bob
I don't know - I'm inclined to doubt it, although the size of the holes and the volume of the "manifold" would probably matter. I imagine there would be a potential for allowing fuel out some of the holes while air went in the others. I suspect this has a lot to do with the reason the TD 049 almost requires pressure - not because the venturi is too big, but because keeping positive pressure in the annular manifold makes sure it stays full of fuel. One of the old tricks was to block two of the holes and open up the third - then you could run suction. On the ST, I always ran the stock sprinker, and it was fine. But you have to be very very careful to seal everything up so that fuel doesn't go out around the bottom of the venturi around the edge below the manifold.
I do know that I have had much much better luck with a spigot in the single sprinkler hole than I do with the straight hole, even after reshaping the cross-section from where I drilled it out. Greatly reduces the inside/outside "mixture" difference on the PA, particularly the tendency to go lean on outsides.
I think the size of the hole makes some difference as well. I think it should be just barely big enough to not be the restriction in the system. That will get it to squirt out better. I got a lot better results when I bushed the big hole (~.085?) in the RO-Jett down to about .035 or so. A spigot worked even better, both on my engine, and on Bob Hunt's 40 at the team trials. A small hole encourages the fuel stream to head towards the middle of the venturi - a spigot guarantees it.
Brett
If you do not mind me asking, what was the full extent of all the modifications you made to the RO Jett? Including venturi, cylinder head, etc.
What pipe did you use?
What prop?
Fuel?
The City Smasher
No modifications aside from the venturi. This is my setup (as of Team Trials {now pulling some pond-scummy-looking airplane around for test purposes}):
Stock RO-Jett 61 BSE
Stock head button (.075 projection?) with 1 shim.
Stock venturi drilled out to #6 and with SIG eyelet spigot w/.020 projection (really complicated nested tubing driven and staked in original hole)
nylon stocking air filter/diffuser (important!)
Smith/Werewage diamondback "60" pipe (original size, not really big one since it won't fit in the airplane) with straight outlet and large DuDro stinger, 17 3/8"
Eather 12.5-3.9 flatback 3-blade (helical)
At home I run Powermaster 10% R/C fuel (just standard 18" oil kind). At the NATS and Team Trials I ran SIG 15%.
~7.0 oz of fuel in dual-taper uniflow tank (home-made) with pipe pressure into uniflow (not going to get caught out again on the wind)
I am very happy with the run consistency and mild response to adjustments. I have (thanks to RO) several head buttons that go all the way from very low compression to very high compression (like, maybe .005 deck height) and it never, ever gets wild or doesn't anything unexpectedly and doesn't significantly change with the weather. It's *always* the same in insides and outsides. When I first had it, it tended to go rich in inside corners and even just flying around, but for reasons not entirely clear but about coincident with going to the diffuser/filter made from a nylon stocking, it stopped doing that. The next thing I would try is a larger venturi
I can't get away without a comparison to the PA, despite the hazardous nature of such comments. It's much steadier in run than the PA, but does not have the same vertical performance with small props. It's much less sensitive to changes in air and setup - almost to a fault. There's no problems with the power shift from 4->2 on the RO-Jett - it's like a non-event. The PA is much more aggressive, which is why mine is set to run in a 4 all the time.
Of course, all of these characteristics are double-edged swords, depending on what you are looking for. I think between the two, the gamut of possible responses is well-covered, and let's just say that from someone who lived through the 4-2 break era and the "where do I get a stunt engine?" 80's, we are pretty well off to have multiple extraordinarily good engines that you can just go buy and stick in there.
I've flown back-to-back many times with both engines and they are two completely different animals, just nothing alike in run characterstics.
Brett
When you say stock head button, do you mean stock (like the one everyone else gets) or are you using one of the buttons provided to you special from RO. How do you know the one you are using is stock? Have you compared your engine to any others direct from Richard?
Why do you think the RO requires such extensive work in the venturi area? What were your findings that lead to changing the venturi arrangement so dramatically? What was the engine doing before you changed the venturi?
Have you ever had any problems with the thing just simply stopping like it blew out the plug?
The City Smasher
Oops! Thunderbolt 4-stroke, just like everybody else.
>When you say stock head button, do you mean stock (like the
>one everyone else gets) or are you using one of the buttons
>provided to you special from RO. How do you know the one
>you are using is stock? Have you compared your engine to
>any others direct from Richard?
Why does this sound like a cross-examination?
They looked the same to me. I can't imagine that Richard sent me anything particularly special - I just asked for whatever was going to work the best. I did later ask for more heads with higher compression, but they all looked more-or-less the same except for the position of the shoulder on the button.
>
>Why do you think the RO requires such extensive work in the
>venturi area? What were your findings that lead to changing
>the venturi arrangement so dramatically? What was the
>engine doing before you changed the venturi?
It's not that dramatically. All I did was make it close to the same as the venturi's I ran on the PA. It ran pretty good with the stock parts. The only problem I ever had was that it occasionally wanted to go rich on insides. I think that was a matter of break-in+ the diffuser (which Richard recommended and I had just not been doing because I was embarrassed to go to the drugstore and buy panty hose). By the way, this works like a charm for a filter, takes up no space at all, and also tends to intercept the cone of fuel mist that makes oil go all over the bottom of the airplane on most engines (PA, VF, and RO-Jett).
>Have you ever had any problems with the thing just simply
>stopping like it blew out the plug?
Nope. I've had that happen on other engines, but not the RO-Jett.
I've been very satisfied with my engine. I ran both the PA and the RO-JEtt pretty extensively over the last year, and I even swapped between rounds at the NATs this year and never really skipped a beat. I'm pretty picky, and it works just dandy as far as I can tell.
Brett
Are you not seeing my questions below?
Brett, Did the "color" of the panty hose make any difference? What color(s) of panty hose did you buy and was there a specific brand that you like better than others? (smiles) Being picky and all....
Regards,
John Jolley
I'm sorry Brett, I got a visual of you at the drug store counter buying panty hose and I shot Coke out of my nose is was laughing so hard.
You are Hard Core Stunt.
I went up to the counter at Longs, and the checkout guy was a maybe 22-year-old Hispanic guy. I just said " Well, I won't even try to explain this one" and that was the end of it.
By the way, it was a really cheap/not very sheer pair, and black.
BTW- Richard put me on to this, but he has someone to buy them FOR HIM!
>
>You are Hard Core Stunt.
Well, hard core something....
Brett
Thanks for taking the thread off topic...
Well, his questions were pretty juicy, I'm happy he did it.
I learned a thing or two, didn't you?
GBY!
Ion
If you put a spigot in a single sprinkler hole, how do you make sure the hole in the needle valve aligns with the spigot?
Do you angle the end of the spigot that is in the air stream or leave it straight?
How would one figure what size tubing to use for the spigot?
Thanks,
Bob
A few years ago, I asked my buddy "Team Race Tom" about spigot vents. He clued me in on using flathead rivets of an appropriate size and length. Also said to file a slight angle on the end of the rivet. It looks to me like c'sinking the single hole to an adequate depth will open up enough of a cavity that there won't be an issue with getting the hole in the spraybar lined up to the hole in the spigot...and it should not be a problem to press fit the rivet into a Delrin venturi by just drilling (center drill, then drill well undersize, then drill to final size). What I have not been able to do (at home, with limited equipment) is to put the groove in the venturi neck for the spraybar. Anybody figured that out, please 'splain. 
Steve
PS: I found some 3/32" FH rivets at work. With a .055" hole (about the same size as a spraybar), this seems about right. I'll drill them in my mini-lathe before installing in the venturi.
http://www.nwskyraiders.com/index.html
I was thinking about making a few Tigre venturies so I could have a couple different sizes, hence the original question.
Thinking It would be difficult to cut the cross slot I was either going to make the venturi out of large enough stock that I could pre-drill the cross hole and machine the venturi down to the pre-drilled hole creating a slot.
The other thought was to make a drill jig that the venturi would fit in with a guide hole to drill the cross slot. Haven't tried either way yet but leaning toward the drill jig. Once the jig is made making venturies would be easy..
Bob, I'm not ignoring you - but I can only type/draw so fast, and at a low duty cycle during work hours!
Ok, now I have something to show:
First is how I put a eyelet-based spigot in a PA venturi:
[photo not recovered: 4031aff93f3918a9.jpg]
Then, how I did it on my RO-Jett:
[photo not recovered: 4031b0223f4fdb06.jpg]
[photo not recovered: 4031b0543f88c7ad.jpg]
[photo not recovered: 4031b0703fb4b010.jpg]
In either case, I used a SIG 1/16 eyelet. As far as aligning the needle valve hole, it's actually no different, and probably easier than stock - since the clearance for the eyelet head is actually bigger than the hole. In any case, it's not very difficult.
Of course, the hole in the venturi is somewhat obstructed by the spigot, so you use bigger holes to get the same choke area - or larger, since it quite obviously sucks fuel better with the spigot than without. I think David runs a #4 AWG venturi, and I run a #5, on the PA (as opposed to maybe #9 or #10 without), and I run a #6 on a the RO-Jett and will try more the next time I get a chance. I recall Frank using a #1 or even a .250 (!) on the PA, which is one heck of a hole. I run 7.5 oz of fuel in the PA - with a 1/4" venturi, I'd have to put on drop tanks!
As for what is does, or doesn't, do, see Frank's original article. I initially looked at it, said, "hey, that Frank Williams is a pretty smart guy, but I'm running a VF and I don't need it", then promptly forgot all about it. David came back from one of the W/C and mentioned that Billy's PA never broke lean on outside turns - something we had never been able to even moderate, much less cure. After a while, David found out that Billy, ahead of the curve as usual, had tried the spigot venturi and found it evened out the inside/outside difference a lot. David and Ted tried it with excellent results. and subsequently, I used it on my PA40 with some improvement, if not total success. That was pretty much the last piece of the puzzle, and subsequently, we have run spigots exclusively.
One theory on why it worked was that we had drilled the stockers out to much that it made the "airfoil" shape less fair, with a sharp transition from the curve to the cylindrical section. Randy now makes the stockers in various sizes, but cut with the regular tool that smooths it out. David tried one and it was way better than a straight drilled-out venturi, but not quite as good as a spigot - at least in our opinion. It's pretty close, as far as I could tell.
When I first had my RO-Jett, I started with stock venturi, but it went rich occasionally on insides. The RO-Jett has a sprinker hole that is very large - RO can tell us, but it's bigger than 3/32 and less than 1/8 - probably .100?. I figured that part of the "going rich" problem was the large hole, probably a "dribbler" more than a sprinkler. So I did the above procedure to fill the hole and then put in a spigot. It helped some, as did time. The last element seems to have been the nylon stocking "filter". I don't know how much filtering it does, but it definitely seems to act as a diffuser. I think this helps in that the stock inlet is very short, and in my case, the venturi inlet is perfectly flush with the surface of the cowl. I suspect that there is a variable flow condition as the airplane turns inside and outside, and the diffuser turbulates it so it runs more consistently, as opposed being subject to laminar but sometimes separating flow around the cowl surface. Whether that's really what's happening, or not, now it runs with no tendency to go rich in either my airplane or Ted's (who is currently running it in the Pond Scum).
Everything above is just my opinion - it may be a complete load, but I am VERY happy with both the PA and the ROJett with the current setups.
Brett
I thought Brad was Mr. 4-Stroke, which makes his insistent quizzing all the more puzzling.
I'll appreciate the longer venturi - I'll pass it along to Ted, who is dilligently testing my engine. The smaller hole seems very likely to help.
BTW, Hunt seemed to get better runs with the 40 at the Team Trials with a #8 spigot. So good, I talked him out of withdrawing - and then he beats me (and you and a lot of other guys)! That's what I get for being a nice guy. 
I might also add that I am not sure exactly why it stopped going rich occasionally. I did a bunch of things, and it stopped, and runs like a metronome now. The fact that these small intake changes effect the inside/outside turn difference (like the old "5 degree taper venturi" on FSRs), makes me wonder about the Belko engine and the intake on the bottom - and buried in the fuselage. I would be interested in trying some sort of plenum or airbox with an inlet facing straight forward - like the old Fox intakes, except with a lot more volume. At least the venturi would "see" a more-or-less constant pressure from a more-or-less constant direction.
Brett
What a great post. Great pictures. I never completely understood how to do the whole spigot thing, now I understand!
As far as being only "Mr. 4 stroke". That may be true for now, but I have several friends that are very interested in the RO Jett. I too see the differences of which you spoke, betweeen it and the PA engine. Very interesting. It appears to be more docile and more "VF like" or even more "4 stroke like".
Doug and I were talking about your successes just the other night. I told him to call you. Maybe now it is not necessary.
BTW the new UHP venturis for the 4 strokes will be "post types" using RC spigot type needles.
The City Smasher
Well, it's certainly different than a PA. Better/worse is of course a matter of opinion and will vary from individual to individual. But it's really not too tough to be a stunt flier these days - PA, RO-Jett, 4-Strokes, Magnum, OS, Rustler-Merco, etc all work well and have sparkling performance. I wasn't just kidding above. For a while there in the 80's it looked like the event was going to go down the tubes just because you couldn't buy any engien that would reliably work. Now, all you have to do is plunk down some reasonable money, and not really have to think about it too much.
$350 for a PA or RO-Jett might sound like a lot to a sport flier or casual competitor. For a serious competitor, however, the availablity of such very high quality products is a Godsend, and the price is an incredible bargain given the amount of futzing around it saves. Now you can use your futzing around time to actually try things to improve it, not just get it to run at all. The "good old days" really weren't all that good.
>Doug and I were talking about your successes just the other
>night. I told him to call you. Maybe now it is not
>necessary.
I think we talked last year sometime about the same topic, but what the heck, I'll talk about most anything. I started getting very good runs with the 61 in pretty short order - and actually, it all came together about the time the bearing got chipped at the NATs. In fact, that noisy bearing flight (about the time Jerry Phelps had his problem) that everybody commented on was actually a darn near perfect run except for the noise, and I was sorely tempted to just keep running it until something let loose entirely. If I hadn't had a perfect-running PA that I could just pop in there, I probably would have. Like I say (and Ted and David can confirm), I tried the PA, and commented that "well, I guess EVERYTHING doesn't have to be difficult" or some such. First run, it fired up just like it had the year before, same launch RPM, same lap speed, same thundering 4-stroke, and I literally never skipped a beat.
>
>BTW the new UHP venturis for the 4 strokes will be "post
>types" using RC spigot type needles.
That's interesting. Sort of like the Rustler-Merco, I take it.
Brett
Only thing I can add is I'm lucky to have Brett on my side!
Goood stuff.
Ted
Reminds me of when I was making model rockets, and the local Big Lots store (Len probably has some in his area) got a huge shipment of Leggs Eggs in. The old plastic ones, which they don't use anymore. I was thinking "Giant Big Bertha nose cones", so I picked up a few (at a buck apiece, the price was right). Now, the problem was - what to do with the pantyhose INSIDE the eggs. I could've given them away to various friends of the female persuasion, except for one problem. There was a reason WHY these things ended up at Big Lots. They were all Double Queen Sized.
Diffuser? Where does the pantyhose go - over the top of the venturi? If this helps keep the excess fuel in, then I might want to use the stuff over my huge Combat venturis to increase fuel mileage...
Never did get around to building those Giant Big Berthas..
Leonard Neumann
Well, Len, the way some "modern" episcopal ministers are going I won't be surprised if they start marrying married unfaithful couples...
Ion (give me that old time religion, it's good enough for me) Muniz
Quick answer is I don't know, since I never tried it.
A more useful answer would be to use the same choke area, and make the drill hole larger enough to make up for the area taken by the spigot:
[photo not recovered: 403575c266da135c.jpg]
I ran a .173 (AWG #17) venturi in the ST46. area = pi*(.173/2)^2 = .0235 square inches. Once you put a spigot with the .020 projection, it takes up .00125 square inches. That means that the hole needs to have .0235+.00125 square inches, or .02475 square inches. A drill that's .178 (~#16 or so) will give you the same choke area. That should give similar performance, but much better fuel draw. I expect you could go a lot bigger, but I would work up to it.
If you are making your own, it would be relatively easy to make multiple venturis and fiddle with the projection of the spigot into the venturi. There's nothing magic about the .020 number, and in fact it's a crude measurement of what I am using. It could easily be .025 and I wouldn't know it.
In the drawing above, you might note that there's an extra o-ring in the bottom of the venturi. The best thing I ever did for ST46 engine runs (actually, the only modification that I ever found to be a net improvement) was to get my buddy to turn a groove for another o-ring that seals the "manifold" from the rest of the engine. This makes sure the fuel goes through the venturi instead of leaking around between the boss in the case and the bottom of the venturi.
[photo not recovered: 403575e667126cff.jpg]
With any of these systems, it's very important to seal everything up, particularly where the spraybar goes through the case. If you don't, tiny air leaks will make the engine surge or run unevenly, particularly on the ground. I can sit in the pagoda at the NATs and tell who sealed it up, and who didn't. I just clean it up and use the silicone header sealant, in TINY amounts, around the spraybar "hex" fitting and the nut and threads. Make sure and do this after you assemble it to prevent any from getting into the works, because a tiny bit if stray silicone in the spraybar hole or spigot will make it run bad and drive you crazy trying to find it. Or so I hear.
Brett