What makes the BW the strongest stock reedie?

You could buy 2 used for that price on Ebay. I have several of those early 90's issue. They work well enough but, that plastic backplate leaves something to be desired. I guess if it's something you want to collect then it may be worth $70 Canadian. If you want to buy one for use then your money may be better spent elsewhere.

Yeah it probably would just stay in the box for my collection

Canso07.

Ok, kind of a shame as there is so much open space in Alberta to fly something...

Lol yes lots of room, but I also have of engines waiting for a circuit...damn snowing right now.

Canso07 wrote:Lol yes lots of room, but I also have of engines waiting for a circuit...damn snowing right now.

Just means its time to continue building.

I flew with the club in 1980, and the BW and TD were from that year or 1979.  The cylinders of both had deep dual transfer ports, but the TD had sub cylinder induction and the BW did not.
My brother, who is an expert on car carburetors, said about a reed valve back plate "I don't see a venturi."  I would have to get mine out and look at it harder to say much more, but I stick to my analysis.  I think it works mostly by viscosity, not by momentum.  That is why you can sometimes start it without priming it.  If it worked by momentum, as a venturi, there would be almost zero suction at starting speeds.

This seems like an important point to me, so here is a picture:

The fuel line fits on the cone above and a bit left.  The needle valve sticks down from above that and partly closes the jet, which is clearly visible on the top side of the carburetor chamber.  The air enters through the screen and small hole in back of that chamber.  The important point is that the jet hole is not at the narrow place, but after it where the air has already slowed down.  So Bernoulli's principle does not explain the low pressure that sucks the fuel in, as it does for a true venturi.  Rather, the viscous friction of the air coming through the small hole and screen has reduced the pressure.
This is one reason that the reedies have less power than the TD:  unlike the pressure drop in a venturi, the friction pressure drop does not revers, so there is less air taken in.  It also is a reason the reedies start easier:  the viscous resistance is about proportional to the flow, like that of the fuel coming through the jet, rather than proportional to the square of the flow like the pressure drop in a venturi.  So it works down to starting speeds.
This is an old Babe Bee tank back plate that was in my flight kit, photographed through a magnifying glass.

David in my opinion the Bernoulli law still applies. The total pressure I. e. the sum of static and dynamic pressures remains constant in an ideal stream. The total pressure outside the carb where air is stagnating is static pressure equal with the atmospheric pressure because there is no dynamic part.  Once air starts to flow inside the air channel it's total pressure splits into a static and dynamic part and the static is less  than atmospheric . Because the static pressure inside the stream is less than the atmospheric pressure inside the fuel tank, this pressure difference makes the fuel flow from the tank towards the orifice by the stem of the needle valve. (You are right in observing that the pressure drop by friction through the screen reduces the static pressure further but even without a screen the static pressure would be reduced by the formation of dynamic pressure)

Bernoulli's law applies irrespective of the shape of the air channel i.e. in  a straight channel and in a Venturi both. The only difference in a Venturi is that the air speed in the constricted throat is higher thus the static pressure is lower than in a straight channel and this provides stronger suction of fuel.

Briefly:
Bernoulli's law is still relevant, but is only a small effect. Most of the decrease in pressure due to Bernoulli's principle in the narrow opening has reversed at the jet as the opening has enlarged, but the decrease due to friction has not reversed.

David

Yes and the Borda-Carnot effect of the sudden expansion of flow downstream the entry port further contributes to static pressure drop in the stream..

David Ingham wrote:This seems like an important point to me, so here is a picture:

The fuel line fits on the cone above and a bit left.  The needle valve sticks down from above that and partly closes the jet, which is clearly visible on the top side of the carburetor chamber.  The air enters through the screen and small hole in back of that chamber.  The important point is that the jet hole is not at the narrow place, but after it where the air has already slowed down.  So Bernoulli's principle does not explain the low pressure that sucks the fuel in, as it does for a true venturi.  Rather, the viscous friction of the air coming through the small hole and screen has reduced the pressure.
This is one reason that the reedies have less power than the TD:  unlike the pressure drop in a venturi, the friction pressure drop does not revers, so there is less air taken in.  It also is a reason the reedies start easier:  the viscous resistance is about proportional to the flow, like that of the fuel coming through the jet, rather than proportional to the square of the flow like the pressure drop in a venturi.  So it works down to starting speeds.
This is an old Babe Bee tank back plate that was in my flight kit, photographed through a magnifying glass.

David.  Not wanting to sound judgmental, but how can I take seriously a man with a budgerigar sitting on his glasses?

I think the venturi effect is there. (Photos courtesy of Cox International):




Airflow is accelerated through smaller entrance opening of the back plate. A short annular chamber is created by a small gap about the width of or slightly longer than the fuel jet hole. The other side of this annular chamber is the tank air tube, an orifice first then abruptly becomes a larger diameter passage to the reed valve. There are some flow friction losses, but the reed valve engines were considered to be the lower cost sport alternative to the more expensive higher performance Medallions and Tee Dees.

Regarding performance limitations over the Tee Dee, one must also consider the reed valve. There is a point where that at higher RPMs, it starts to float, unable snap fully closed due to the elasticity and inertial effects of the material. There are also additional friction losses due to the abrupt changes in air/fuel flow when it hits the reed.

Considering the various compromises to create one of the most cost effective engines available that outsold others, the Cox reedies even outperformed older rotary valve engines of similar displacement. By the mid 1960's, Cox had dominated the market. I bought them because they were a fraction of the cost of a Tee Dee, and I wasn't into competition or cutting edge of the spectrum. The reedies were precision machined compared with the competition's engines, easy to break in and easy to adjust and use.

Heres some technical date if you haven't already read it .. http://www.mh-aerotools.de/airfoils/documents/cox_test_black_widow_aeromodeller_august_1974.pdf I think the port timing deep bypass and larger fuel / air induction is the trick.

GallopingGhostler wrote:I think the venturi effect is there. (Photos courtesy of Cox International):

I'll chime-in with something that this conversation made me think about. The Bee tanks' mix tube from rear to front is the reverse of a conventional (external) venturi-cone. This may be a dumb thought.. but we all know the effect that a megaphone produces. It is an amplifier of sound-pressure.. is it not? The air-fuel charge enters at the smaller end, immediately introducing a sound-pressure wave into the entire pressure-equation. It's likely a naturally-occurring phenomenon that may not have been an initial design-consideration.. but could lend itself to the overall efficiency of atomization through mix-expansion.. rather than air-constriction. That's just my theory.. I don't really understand the more complex scientific principals involved.. but I'm learning some new words...

"I'll have a Bernoulli on rye.. with a glass of Carnot"

Oldenginerod wrote:

David.  Not wanting to sound judgmental, but how can I take seriously a man with a budgerigar sitting on his glasses?

I didn't put him there on my finger.  He flew there.

roddie wrote:I'll chime-in with something that this conversation made me think about. The Bee tanks' mix tube from rear to front is the reverse of a conventional (external) venturi-cone. This may be a dumb thought.. but we all know the effect that a megaphone produces. It is an amplifier of sound-pressure.. is it not? The air-fuel charge enters at the smaller end, immediately introducing a sound-pressure wave into the entire pressure-equation. It's likely a naturally-occurring phenomenon that may not have been an initial design-consideration.. but could lend itself to the overall efficiency of atomization through mix-expansion.. rather than air-constriction. That's just my theory.. I don't really understand the more complex scientific principals involved.. but I'm learning some new words...

"I'll have a Bernoulli on rye.. with a glass of Carnot"  

And I'll toast to that with a glass of vintage Red Venturi and a shot of Byron's with no less than 70% alcohol content.

Don't know whether the intake system chamber lengths are tuned or not. Only difference between Babe Bee 5 cc tank and Golden Bee / Black Widow 8 cc tank is the 8 CC is longer. After the venturi, Medallions and Tee Dees have a larger chamber through crankshaft to transfer the fuel air mixture to crankcase. Larger diameter makes for reduced flow velocity and friction.

Leroy Cox had the Bee's tuned to where they produced satisfactory maximum horsepower at a particular RPM that is safe and functional. Compromises were made to ensure the engine was easier and cheaper to manufacture with ease of starting and running.

I suppose we could get into the theoretical equations with partial differential and integral calculus, but for me, it works and as to why or how, I'd rather be concerned with what airframe I put it on, props and fuel I use. No need to make life complicated. I don't need a high performance competition engine on my Q-Tee (although I'd be game with a Venom or Taipan Serpent).

GallopingGhostler wrote:I think the venturi effect is there. (Photos courtesy of Cox International):



Airflow is accelerated through smaller entrance opening of the back plate. A short annular chamber is created by a small gap about the width of or slightly longer than the fuel jet hole. The other side of this annular chamber is the tank air tube, an orifice first then abruptly becomes a larger diameter passage to the reed valve. There are some flow friction losses, but the reed valve engines were considered to be the lower cost sport alternative to the more expensive higher performance Medallions and Tee Dees.

Regarding performance limitations over the Tee Dee, one must also consider the reed valve. There is a point where that at higher RPMs, it starts to float, unable snap fully closed due to the elasticity and inertial effects of the material. There are also additional friction losses due to the abrupt changes in air/fuel flow when it hits the reed.

Considering the various compromises to create one of the most cost effective engines available that outsold others, the Cox reedies even outperformed older rotary valve engines of similar displacement. By the mid 1960's, Cox had dominated the market. I bought them because they were a fraction of the cost of a Tee Dee, and I wasn't into competition or cutting edge of the spectrum. The reedies were precision machined compared with the competition's engines, easy to break in and easy to adjust and use.

You have a point there.  Bernoulli's principle doesn't apply directly to the jet, because the air flow is slow there, but if the gap between the narrow input opening and the narrow tube shown here is small enough, then maybe it applies to that narrow gap, causing a static pressure drop at the jet.  The gap would have to be narrow enough that most of the air passes from one narrow tube to the other without slowing down and mixing with the air in the space where the jet is.

Maybe each reedie model had its own compromise between viscous pressure reduction for easy starting and wide RPM range and venturi effect for more power.  All of these dimensions then work together with sub piston induction and transfer ports to determine the properties of each type.  What bothers me about that is that the width of the gap is not easy to control accurately.  It depends on exact dimensions and gasket thicknesses.

I still will not call it a venturi, though I have always realized that there is likely some momentum dependent effect.  Its not looking like a venturi could be cost related, but it doesn't act like a venturi either.  A warm engine will start with no priming, and without opening the needle valve. A venturi does not develop suction at starting speeds.

GallopingGhostler wrote:

roddie wrote:
"I'll have a Bernoulli on rye.. with a glass of Carnot"  

And I'll toast to that with a glass of vintage Red Venturi and a shot of Byron's with no less than 70% alcohol content.

Otto the waiter will be right with you gentlemen

I think I'll buy Roddie's "megaphone" idea... The reed valve of course opens easier as the surface area of the opening is larger, aaand Bernoulli equation states that as the velocity increases, the pressure decreases and vice versa

There is also some tuning effect (or potential) present, since the flow in the venturi, or in the entire intake line, is not constant. There is a lot of pulses or pressure waves going back and forth as the reed valve opens and closes at the frequency of the rpm. That combined to the varying diameter and length in different tanks must have some effect. The only constant factor is the speed of sound at which the pressure waves travel, and even that varies depending on the average pressure in the venturi (and the pressure depends on the ambient and internal temperature, atmospheric pressure and so on). Who is the first to start experiments with a long tank, turning it gradually shorter and testing for rpms in between

All this raises (to me anyway) also the question about SPI in reedies. The benefit of course is the extra air feed into the crankcase, but is the "root cause" of this benefit the possibility to set the needle richer as the extra air necessary can be get from the SPI *OR* does the sudden burst of extra air help to "slam" the reed valve shut, preventing any of the air/fuel mixture from escaping back towards the venturi? Or both?

David Ingham wrote:

Oldenginerod wrote:

David.  Not wanting to sound judgmental, but how can I take seriously a man with a budgerigar sitting on his glasses?

I didn't put him there on my finger.  He flew there.

I think you may be related to Mssr. Henri Matisse!



Just messing with you My wife and I visited the beautiful town of Nice last weekend, and, among other places there, the Matisse museum. I got the impression that ol' Henri liked birds too.

KariFS wrote:...All this raises (to me anyway) also the question about SPI in reedies. The benefit of course is the extra air feed into the crankcase, but is the "root cause" of this benefit the possibility to set the needle richer as the extra air necessary can be get from the SPI *OR* does the sudden burst of extra air help to "slam" the reed valve shut, preventing any of the air/fuel mixture from escaping back towards the venturi? Or both?

Here's a re-print of a post I wrote on SPI:

RknRusty wrote:
How SPI(Sub Piston Induction) works

The upstroke: On the piston upstroke the crankcase pressure is relieved by the action of the piston moving up to compress and burn the previously introduced charge. In a reed valve engine, the drop in crankcase pressure releases the reed which finally opens fully drawn by the vacuum created by the displacement of the rising piston. Fuel/air is drawn into the crankcase through the venturi behind the reed. In a rotary valve engine such as a Tee Dee, the intake valve opens while the piston is rising and the vacuum it creates draws the air/fuel mix through the carburetor venturi into the crankcase.

TDC and the downstroke: As the piston goes through top dead center, the piston skirt clears the bottom of the exhaust port, opening a gap exposing the crankcase to cold air(Not recommended for engines with mufflers). The crankcase vacuum created by the rising piston causes fresh air to rush into the crankcase adding additional air as the now descending piston compresses and forces the freshly inducted air to combine with the existing fuel/air mix.** This gives the crankcase a belly full of extra pressurized f/a mix ready for the next combustion cycle. The increasing pressure of the descending piston forces the mixture to scoot up through the bypass ports until the crankcase pressure is diminished. It starts over again with the upstroke and Kaboom, it's a wild cycle. With SPI more fuel/air mix is available for combustion power. It only has its full effect at high RPM. Usually a small prop, less than 6' is needed to get to useful RPMs for SPI do have its effect.

**Obviously, the bonus air pressure added to the existing air from the venturi dilutes(leans out) the mixture, so that the needle valve must be opened more to let in extra fuel to balance the mixture. You can call it "Richer" but it's really the same optimum F/A percentage

In summary, the crankcase is charged with fuel/air mix from the venturi plus the SPI inducted air and then compressed and mixed by the descending piston. All that, plus the extra fuel from the richer needle setting to balance the F/A ratio, now you have a higher charge in the crankcase than if there had been no sub piston induction.
Rusty

At the risk of repeating myself, the effect of SPI on the mixture can also be important.

In read valve engines such as the TD that have a real venturi carburetor, the section that draws the fuel in is mostly cause by momentum effect while the friction in the fuel line is mostly viscous, so the suction increases faster than the resistance and the mixture coming out of the carburetor gets richer with engine speed.  The TD is apparently designed with the right amount of SPI to balance that so that it puts out lots of power over a wide range of speeds.  That is ideal for racing, but for stunt one needs to get rid of the SPI to keep the engine speed from increasing much when flying down.

The readies have a different type of carburetor, that most people, including Cox writing, call a venturi, but that depends much more on viscous drag to get the suction to draw in the fuel than the TD does.  That is what makes it possible to start a warm reedie without priming it:  the pressure drop caused by the viscous drag of air entering the carburetor through the small hole and screen is present even at low RPM.  It is also one of the reasons that the reedies have less power:  that pressure drop is not reversed where the opening gets bigger and reduces the flow speed.  So the increased air intake of SPI is needed more by the reedies, but the leaning out as SPI increases with engine speed is needed less.  Putting a TD cylinder on a reedie may cause the mixture to lean out too much at high RPM.

Flipping the prop on an NOS OK Cub .059 (sometimes called .06) reed valve engine, I just noticed that even the OK has SPI. SPI shows its advantages by providing a puff of cold air into the crankcase that lowers the temperature of the air fuel mixture causing a denser charge, which results in greater power.

David Ingham wrote:In read valve engines such as the TD that have a real venturi carburetor, the section that draws the fuel in is mostly cause by momentum effect while the friction in the fuel line is mostly viscous, so the suction increases faster than the resistance and the mixture coming out of the carburetor gets richer with engine speed. The TD is apparently designed with the right amount of SPI to balance that so that it puts out lots of power over a wide range of speeds.  That is ideal for racing, but for stunt one needs to get rid of the SPI to keep the engine speed from increasing much when flying down.

There is a misnomer. The classical definition of venturi for example is given in:

Merriam-Webster: Venturi

Merriam-Webster wrote:a short tube with a tapering constriction in the middle that causes an increase in the velocity of flow of a fluid and a corresponding decrease in fluid pressure and that is used especially in measuring fluid flow or for creating a suction (as for driving aircraft instruments or drawing fuel into the flow stream of a carburetor)

Venturi has been loosely defined in the model engine world as to mean location where fuel enters the air stream to mix with air. Although it may not necessarily fit the description of a "coke bottle" shaped restriction that is more efficient to reduced friction / turbulent flow, it still follows the rules of the Bernoulli Equation regarding fluid flow. There is conservation of energy.

princeton.edu - Bernoulli's Equation

Actually what is happening is a change in the mean effective area where air is entering the reduced diameter "venturi" opening prior to the fuel exit hole. The narrower opening cause an increase in air velocity with resulting air pressure drop. Fuel is being pushed into the air stream due to this lowering of pressure. The fuel on the other side inside the tank is more or less at higher atmospheric (more if pressure is used, such as muffler or crankcase pressure). Differences in fuel viscosity are compensated by throttling the fuel stream through the needle valve setting.

There are losses in momentum of air flow, which restricts the quantity of air being available for combustion in reed valve engines versus the more efficiently designed Tee Dee.

With SPI, we adjust so fuel air mixture is initially denser, with SPI adding to correct with additional amount to restore mixture to Stoichiometric.

That said, the Tee Dee is more efficient because its fuel metering system has less impediments to air intake flow to permit a greater quantity of air-fuel to enter the combustion chamber. The trumpet shaped intake reduces drag by causing a smoother transition of air flow. It also lacks the reed valve, which is another impediment to air-fuel flow.

Regarding SPI not desirable in stunt, I watched Ron Cribb's Baby Clown with Medallion .049 flown on I think 42' steel lines. It did not fly the typical 4 cycle to to 2 cycle like with the .35 engines, but still was very impressively stunt-able.

Also FYI:

Per brighthub.com Stoichiometric Ratios

Methanol has a Stoichiometric Air / Fuel Ratio of 6.4 : 1

If fuel has say 20% oil, then we need 6.4 x 0.8 = 5.1, because oil is inert and more or less doesn't combust (unless running lean and hot). This reduction in combustible fuel in the mix will correspondingly decrease the amount of air required. It also helps to illustrate why with gasoline at 14.7, diesel at similar require smaller fuel tanks than glow, even when we take into account the "inert non-combustible oil" in the mix.