Venturi.........Engineers like numbers

In the discussions on my TT 25 starting issues and venturi size, Leonard made a couple of interesting points that I hadn't thought about. So I sat down and did a few calculations and came up with what follows. Sorry this format is not all that great for writing equations, but you should be able to get the gist of it. I also know many may not be all that interested or simply could care less, but I found this quite interesting.

Area of a circle = (Pi)(D)squared/4
Where: Pi = 3.1416, D = diameter

Example: Venturi diameter = 0.173 inches,
The Area is = 3.1416 x (0.173)squared/4 = 0.0235 square inches.

The question is what diameter venturi will provide half the area?

Let 0.173 be called D-1 and the area of 0.0235 called Area-1.

Then the area equations becomes: Area-1 = (Pi)(D-1)sqaured/4 = 0.0235 square inches

Then lets call the area that is one half the size Area-2. The diameter for a venturi with half the area will be called D-2. So the equation for a venturi with half the area would be:

Area-2 = (Pi)(D-2)squared/4 which we already know is 0.0235 divided by 2 = 0.01175 inches.

Without a lot of mumbo jumbo, the diameter that will give half the area, in this case, is 0.122 inches. So, going from a 0.173" diameter venturi to a 0.122" venturi will take the area down by a factor of 2. This is because the area goes down (and up) by a factor of the diameter squared.

Reducing these equations down further you will find the general case: multiplying by a factor of the square root of 2 (which equals 1.4142) you could also find the diameter for a venturi half the size of the original.

Final example: 0.122 x 1.4142 = 0.173 inches.

Numbers never lie.

Mind you, this is only accurate for small values of 2.

>Mind you, this is only accurate for small values of 2.

That's got to be brilliantly funny, but it took me a half an hour just to figure out he was using (d/2)^2 instead of r^2. I can't exercise my brain that much these days.

Yeah,

When I was in highschool, it was Pi are square. Now, its Pi x (d/2) squared. Which is (Pi(d)squared)/4. The formatting here does not lend itself well for equations, or I just haven't figured it out yet. But Pi R still squared........not round.

By the way, the rate of change of the area with respect to the diameter is ((Pi)(d))/2. Go figure.......

John don't you have some airplanes to build? DOC

Hi Doc,

Yeah, I got airplanes to build, but the insight into this area thing seem like a good way to take a break from sanding.

>Yeah,
>
>When I was in highschool, it was Pi are square. Now, its Pi
>x (d/2) squared. Which is (Pi(d)squared)/4. The formatting
>here does not lend itself well for equations, or I just
>haven't figured it out yet. But Pi R still
>squared........not round.
>
>By the way, the rate of change of the area with respect to
>the diameter is ((Pi)(d))/2. Go figure.......
>
>Regards,

I thought PI are round.

Hey Robert,

I think Howard it right. Pie approaches round at extemely small values of 2.

I still think the point Leonard made is interesting. Although flow is also proportional to the diameter, it's much easier to see the effect, using the cross sectional area.

>Hey Robert,
>
>I think Howard it right. Pie approaches round at extemely
>small values of 2.
>
>I still think the point Leonard made is interesting.
>Although flow is also proportional to the diameter, it's
>much easier to see the effect, using the cross sectional
>area.

I was so looking for a expert BIFFLEPLATE WAFFLEPUMP opinion

There aren't too many Biffleplate experts around anymore. All the jobs were outsourced to Mexico.

>Pie approaches round at extemely small values of 2.

All of our pies have been round. If they are square or rectangular we call them a cobbler.

>Yeah,
>
>When I was in highschool, it was Pi are square. Now, its Pi
>x (d/2) squared. Which is (Pi(d)squared)/4. The formatting
>here does not lend itself well for equations, or I just
>haven't figured it out yet. But Pi R still
>squared........not round.
>
>By the way, the rate of change of the area with respect to
>the diameter is ((Pi)(d))/2. Go figure.......
>
>Regards,

Pie are round.
Cornbread are square.

Next time you take a sanding break, factor in the reduction of total cross sectional area created by a fixed diameter spraybar and watch what happens to the results.

>Next time you take a sanding break, factor in the reduction
>of total cross sectional area created by a fixed diameter
>spraybar and watch what happens to the results.

The smaller the venturi in relation to the spraybar, the more this changes. But for most venturis, the change was minimal.
I have a spreadsheet on this that was sent to me by another modeler and I will probably use it some time (when I can take a break from other activities) and update my chart.

The Thunder Tiger venturi in question does not have a spray bar going through the center, however, so will be much smaller in comparison to, say, the OS engine of the same size that does use a venturi that is choked down by the spray bar.

The key to all these calcualtions, however, is to maintain a ratio in size of the venturi (comparing actual choke area--minus spray bar if it does choke the area) to size of the engine in cubic inches. This will remain relatively constant for most of our engines and can be used to compute the size of venturi needed for that particular engine and application. The size of the venturi needed for our stunt engines under suction will vary greatly from the size needed on a similar size engine when used for speed or combat or free flight (and whether they are being run on pressure or suction).

Many of the factory supplied venturis on their "control line" engines are much larger than optimum. They do not list these as stunt engines, however, because they are more concerned with maximum rpm and horsepower ratings to outdo the figures of their competitors.

Len... What do you consider the proper ratio of net venturi area vs. displacement for engines running suction and also same question for engines running muffler pressure.

General question - Has anyone ever measured the pressure at the muffler pressure tap and if so what did you get?

I measured muffler pressure on an OS .15. I forgot what it was, but it was really low. Maybe Preston remembers how much.

>Len... What do you consider the proper ratio of net venturi
>area vs. displacement for engines running suction and also
>same question for engines running muffler pressure.
>
>General question - Has anyone ever measured the pressure at
>the muffler pressure tap and if so what did you get?

On the venturi, this will vary a bit if you are going "non-standard", but a good starting ratio is about .045 - .052 (dividing the choke area in square inches by the displacement in cubic inches). I have a whole chart that I have worked up showing where various size venturis fit in with the various engines.

Matt, for instance, was running a number 12 or number 10 venturi with his PA 61 and 13 1/2 inch 3-blade prop. Surprisingly, this is still at the bottom of the range, giving a ratio of .046 or .048. With the PA 65 he is running a 13 3/4 inch 3-blade prop with a number 10 venturi. This gives a ratio of .045 One can go bigger, but we need to find bigger props first.

I mentioned "non-standard". George Aldrich would go to extremely low compression on his engines and then open the venturi more in an attempt to restore power. This, incidentally, is the same philosophy used on the Fox 35. (You can also choke the engine down by using a highly restrictive muffler, and then resort to a larger than normal venturi to compensate. This will change the characteristic of the engine, however.)

At the other end, Paul Walker used a smaller than normal prop on his ST 60, and then used a correspondingly smaller venturi because he didn't need the power. But the above ratio will generally hold true when using a normal engine turning the size prop that it is capable of.

Brett liked a .173 venturi on his ST 46. That gives a ratio of .051.
The .265 venturi works well with a stock FP 40. That gives a ratio of .048.
The OS 35S with a stock <.155> needle works well with a .277 or .281 for a ratio of .049 and .053.
The list goes on, but it is a good rule of thumb, unless you modify or run the engine totally differently.

On the question of muffler pressure, I put a tube on a couple of different engines and stuck it in a jar of water. It would quit bubbling at somewhere around 6 inches. Since roughly 30 feet of water will be supported by our roughly 15 pounds of normal air pressure, that would make it about a quarter of a pound of additional pressure. It is significant as far as the engine setting is concerned, but doesn't greatly affect the draw of our engines, and has negligible affect on the size of the venturi as far as I am concerned.

Looked pretty good to me until you used the sine and cosine of 45 degrees

I'm an engineer and I like numbers as much as the next engineer.

But engineers must accept that there is still alot of analog stuff in the world. And there are a lot of numbers that you can't get to plug into your formuli. Examples:

Air density, temp and humidity.
Viscosity of fuel at changing temp.
Needle valve setting.

Back in the days when I wanted maximum speed in my racing and slow combat models (where suction was required), I carried several venturis and shaved spray bars.

It came down to TRIAL & ERROR on contest day. Some days the biggest hole would not suck air and some days it would. Along with aerodynamics and fluid mechanics, you still need to study zen.

>I'm an engineer and I like numbers as much as the next
>engineer.
>
>But engineers must accept that there is still alot of analog
>stuff in the world. And there are a lot of numbers that you
>can't get to plug into your formuli. Examples:
>
>Air density, temp and humidity.
>Viscosity of fuel at changing temp.
>Needle valve setting.

As an old retired engineer, I have to disagree with your supposition. With the exception of the needle valve setting (read fuel flow rate), the above variables are easily obtained with measuring instruments that cost less than $100.

>
>Back in the days when I wanted maximum speed in my racing
>and slow combat models (where suction was required), I
>carried several venturis and shaved spray bars.
>
>It came down to TRIAL & ERROR on contest day. Some days the
>biggest hole would not suck air and some days it would.
>Along with aerodynamics and fluid mechanics, you still need
>to study zen.

I agree to some extent.... BUT, given that the atmospheric conditions can be measured, it now becomes a process of arriving at optimum plug, fuel, venturi, spraybar, props, etc for a given set of conditions. That's the empirical data that is gathered by trial and error. Once "optimum" configuration is found for various conditions, on contest day it's a matter of taking a reading of the current conditions and plugging in the appropriate stuff.

Did this on our race cars for years. Once the body of data becomes large enough it becomes very predictable. Extremely important if you bracket race.

I'll take science over Zen any day

I agree as noted in my reply to "Experts" Which is pasted below.

One point of view:
This is a point well made over and over again, not just in modeling or hobby suff. It also applies to many many other areas. Theory is important, but often theory is contiually being adjusted to account for what actually happens out there in the "real" world.

I am a chemical engineer, having returned to college long after I made it out of highschool. Now have 20+ years experience. I understand and can explain a number of equations of state (PV = nRT,), thermodynamics, reaction kinetics and how to find the number of theoretical trays needed for a particular separation, but the other day when a friend at the flying field asked me about some fuel he's got that has some nitroethane in it. I didn't know. Somewhat tongue in cheek, I told him nitroethane had one more carbon atom in it than nitromethane. And, then I told him I'd have to do a little research.

Really all I could do, since I have no experience in fuel mixtures, would be to start searching the literature to see what I could find. Granted, I might know what and/or how to go abut finding the answer better than a layman, but the fact of the matter is, I am a degreed chemcial engineer with 20+ years experience and I had no idea how to answer his question.

I also recognize that one of the racers in our cub who has messed around with fuels and mixtures for years and years who has no degree of any kind could probably give him a much more practical answer than I could.

Theories are most often formed from observation. Experiments are designed with idea to prove or disprove the theroies. The data collected leads to more theroies and more experiments. The two are intimately tied together.

What I like about this forum is there are a great number of guys who've been doing this stuff for a long time and they have built up a vast amount of data and most are willing to share the knowledge they've gained. Moreover, the novice who jumps in there and participates might very well sees something new, where the "expert" is so focused on a particular detail that he misses seeing the obvious that's right in front of him. It's combined effort and it seems to work pretty well.

"Experts" don't need degrees and degrees don't make experts.

Duke Fox liked nitroethane. He said that an engine would run on almost pure nitroethane. It is enough of a lubricant that no added oil is needed.

>Duke Fox liked nitroethane. He said that an engine would
>run on almost pure nitroethane. It is enough of a lubricant
>that no added oil is needed.

The last year I flew speed in competition was 1970. To stay competitive I was using 80% nitromethane, 5% propelene oxide and the rest was UCON 625 Soy Bean Oil. Had to nickle plate the tops of the aluminum pistons to keep them from burning thru. Sometimes that didn't keep it from burning on a overlean run.

Speed Competition Flying back then was, and I'm sure still is, very expensive.