Venturi size

[Bootlegger]

   Fellow's I am looking for a venturi size that I can use in FP and Tower 40 engines that will give me reasonable power and economy.
   I am using ST nva's with T/bolt long reach idle bar plugs, and 10/22 fuel, probably an 11x5 apc prop, airplane weight lower to upper 40 oz range at sea level and 62' lines.
  I am thinking about .266 size, any suggestions will be appreciated, oh the engines are stock...
    Thanks a lot,

[Tim Wescott]

I knew I remembered seeing this:
http://stunthanger.com/smf/index.php?topic=6285.msg51626#msg51626

If you go to the Stunthanger forum main page and do a search on 'Venturi Size' you'll come up with lots of hits.  Basically, there's a rule of thumb that many people swear by.

[Bootlegger]

         Thanks Tim, got it...

[Geoff Goodworth]

Simple answer, the FP 20/25 venturi from Tower.

It is being used unmodified in LA 46s and FP 40s but it is also a good, cheap starting point for opening up just a fraction if you feel the need.

[Brett Buck]

I knew I remembered seeing this:
http://stunthanger.com/smf/index.php?topic=6285.msg51626#msg51626

If you go to the Stunthanger forum main page and do a search on 'Venturi Size' you'll come up with lots of hits.  Basically, there's a rule of thumb that many people swear by.

   But it's not right, and the underlying theory (that the venturi size is some direct function of the displacement) is certainly not right. The choke area of the 20FP venturi is substantially larger than the choke area David Fitzgerald uses on his WC-winning PA75.

     In fact, the choke area equivalent to the 20FP venturi is almost a universal number - about 0.0184 square inches. That's a .257 diameter with the small OS spraybar, and about a .270 with an ST spraybar. That's a good starting point for just about any stunt engine on an airplane in the typical range. That's right in the ballpark of what works for a 40VF and the same choke area is also very close to what I have on the RO-Jett 61 (and had on the PA).

     Brett

[Tim Wescott]

  But it's not right, and the underlying theory (that the venturi size is some direct function of the displacement) is certainly not right. The choke area of the 20FP venturi is substantially larger than the choke area David Fitzgerald uses on his WC-winning PA75.

     In fact, the choke area equivalent to the 20FP venturi is almost a universal number - about 0.0184 square inches. That's a .257 diameter with the small OS spraybar, and about a .270 with an ST spraybar. That's a good starting point for just about any stunt engine on an airplane in the typical range. That's right in the ballpark of what works for a 40VF and the same choke area is also very close to what I have on the RO-Jett 61 (and had on the PA).

I'm highly suspicious of the idea of a "one size fits all" choke size.  If that's valid at all, it's because we all fly about the same sized airplanes, about the same way, and once the engine's breathing is primarily limited by the choke size there's going to be a strong correlation between choke size and power.

Interestingly, for a 40 the "wrong number" that I get from the .045/inch * displacement is 0.018 square inches, which is pretty close to your "about right for everything" area of the FP.  Scale the airplane down to six ounces and shove an 049 on it, however, and .045/inch * displacement gets an area of .0022 square inches, which is about half of the venturi size of a Golden Bee -- but that FP venturi is about 4.5 times bigger than the venturi in a Golden Bee, and if you could get the plane to start I don't think you'd like how it ran.  While the "some constant times displacement" rule doesn't work, I think your "just use a constant area" rule works even less well.

The bottom line, of course, is that like anything else involving fluid dynamics, the answer is just more complicated than most modelers want to deal with.  I suspect that given a constant model weight the optimal venturi size is going to be fairly constant, and if it's dependent on engine displacement at all it's quite likely inversely dependent (imagine, if you will, the desirability of having a 75 in a 64oz model, pumping out 3.5 times as much power as a 20FP in a 40 ounce model -- I don't think that's what you want at all).

So  I think the reason your "FP venturi size works for everything" rule is because you've unconsciously tossed out anything from your personal set of "everything" that doesn't work with an FP venturi size.  (Relatively) little stunters fly good on screaming 20 FPs (or 25 LAs), kind of like a kid on a crotch rocket.  Big stunters mosey around the circle with a big, slow, spindly motor up front, like a tractor pulling stumps.  But you can only scale things so far before the model breaks, and you're helped along by current stunt practice which favors all the planes being roughly the same size.

[Air Ministry .]

They all say .272 with the ST spray bar . which is the stock .25 intake , in the .40 .

I supose this is for a A/C of near 50 Oz. One closer to 36 Oz. ( 1 Kilo ) wouldnt want it as big.

The Iron FP in the 50 + Oz. Yak used the 25 intake. 10 x 4 three blade Tornado .Chopped the
plane up as was to repaired, Dunno if the overspeeding was the split tank or the FP .

Running right it was superb , but steady speed. The H 40 gave more of a load responce run though.

[Brett Buck]

So  I think the reason your "FP venturi size works for everything" rule is because you've unconsciously tossed out anything from your personal set of "everything" that doesn't work with an FP venturi size.

   Yes, I limited myself to current typical stunt engines from 20 to 76 and current typical airplanes from 400 to 700 square inches, and competition setups.   That seems to cover a lot of ground to me, but I suppose I should defer to your experience.

One thing that will certainly not work well for stunt is scaling the venturi size by the displacement. That's what led various people to think a 20FP needed such a tiny venturi, which then led to people concluding it didn't have enough power, which then led to people saying that the stock 20FP engine setup that we have used on a few dozen airplanes was "unworkable" and that it needed extensive grinding to be made to work.  The stock venturi is indeed to big if you try to bog it down with a 10-6 and run it at 7500 RPM. It would also lead to such gigantic venturis on 75s that you might conclude that they were uncontrollable monsters that needed 1200 square inch airplanes and 120 foot lines.

   Brett


  

[FLOYD CARTER]

I've ruined a couple of venturis trying to open them up using the closest drill I happen to have!  Bad idea! Now I'm more careful.  The last one I modified using a wood dowel with some 400 sandpaper glued on.  run in an electric drill and remove not more than 3 or 4 thou at a time.  That's enough!  A little bit goes a long way!

Floyd

[Brett Buck]

I've ruined a couple of venturis trying to open them up using the closest drill I happen to have!  Bad idea! Now I'm more careful.  The last one I modified using a wood dowel with some 400 sandpaper glued on.  run in an electric drill and remove not more than 3 or 4 thou at a time.  That's enough!  A little bit goes a long way!

   .005 is the biggest change I will make at one time, unless it's just obviously way off.

     Brett

[Jim Thomerson]

I had very poor results with an APC 11 x 5 on my Tower 40 (which surprised me), so try some other props as well.  The venturi was made by Russ Gifford.  What worked best for me was a Supercool carbon fiber Petal Blade 11 x 5. 

[Chris Wilson]

I had very poor results with an APC 11 x 5 on my Tower 40 (which surprised me), so try some other props as well.  The venturi was made by Russ Gifford.  What worked best for me was a Supercool carbon fiber Petal Blade 11 x 5. 

Bigger blade area at that critical 70% station on the Petal Blade prop Jim?

[Andrew Tinsley]

Hello,
  Looks as though I have got things wrong! I am using the OS venturi intended for the OS 46 LA. The one supplied with the 46 LAS. To be honest, I can't see much wrong with that venturi. I run the LA 46 in a wet two and it seems to be fine. Tell me, what improvement would I get with the one OS used for the FP20? The engine is completely stock and one OS headshim.

Regards,

Andrew.

[fred krueger]

Claus Maikis has a list of recommended starting sizes for stunt venturis.  I've found this to be extremely useful.

http://www.clacro.de/

Click on "Design", then on "Venturi Size"

Fred

[Brett Buck]

Hello,
  Looks as though I have got things wrong! I am using the OS venturi intended for the OS 46 LA. The one supplied with the 46 LAS. To be honest, I can't see much wrong with that venturi. I run the LA 46 in a wet two and it seems to be fine. Tell me, what improvement would I get with the one OS used for the FP20? The engine is completely stock and one OS headshim.

  Nobody said that you should change anything. All I am saying is that the idea that the venturi size is some direct function of displacement *isn't right*. And then gave the examples that proves that it's not right.

   What I get awfully tired of is the endless "rules of thumb" that just aren't right and are demonstrably not right but nonetheless passed around as some sort of gospel. For example, it predicts that you need .009 sq in of choke area for a .20, when in fact it runs dandy with over twice that. It predicts a that a PA75 needs .0334 square inches, when the engine that won the world championship has a little less than *half* that. If you use .0334 choke area on your PA75 you are going to need drop tanks because is manages to suck up 8+ oz of fuel a flight with the about 0.0175 square inches.

    You have to choose the venturi based on the type of run you want, the type of airplane it is on, and the temperature and altitude, the fuel, and a bunch of other factors. Essentially, you need trial and error. If you need a starting point, I would suggest about .0184 square inches and adjust from there. That's not a hard-and-fast rule but it will tend to get you a lot closer than the magic formula for a huge range of engines.

  If your engine comes with a venturi, you *already have a starting point* and thus need neither a formula nor a 20FP venturi.

    Brett

[Bootlegger]

   Thanks Guy's I sure do appreciate all the info...

[Dave Adamisin]

It's not how large your vent is, it's how much the engine traps. The amount the engine traps controls the bmep. The rpm controls the power (how many times the bmep happens). Venturis cluster in a narrow range because the power requirements cluster. Has anyone quantified how much of the total load the lines (drag and weight) contribute to the equation? If the lines are a large component, the narrow power requirements would make some sense..

[Brett Buck]

It's not how large your vent is, it's how much the engine traps. The amount the engine traps controls the bmep. The rpm controls the power (how many times the bmep happens). Venturis cluster in a narrow range because the power requirements cluster. Has anyone quantified how much of the total load the lines (drag and weight) contribute to the equation? If the lines are a large component, the narrow power requirements would make some sense..

  Precisely. The HP required on average is in the range from around .25-.4 hp. The lines are, as I recall, about 15-20% of the total for stunt planes. LINEII computes that, I think.

     Brett

[Dave Adamisin]

  Precisely. The HP required on average is in the range from around .25-.4 hp. The lines are, as I recall, about 15-20% of the total for stunt planes. LINEII computes that, I think.

     Brett

Thanks Brett. Where is LINEII available? I'll check the programs link....