Ace throttle experiment for Cox Babe Bee

[roddie]

For some time now.. (decades actually..) I've been eager to try the Ace venturi-throttle designed for the Babe Bee engine while using a larger than standard-diameter propeller. I’ve experienced decent performance running 6" diameter props of varying pitches while using the Ace throttle on the Cox Babe Bee.

Years ago.. I built a self-designed 1/2A C/L biplane powered by a Babe Bee engine. One day, I decided to try bushing the hub of an 8 x 3 “Zinger” wood-prop to see if the engine would “swing-it”. It did.. albeit with a narrow margin of a needle-setting to obtain a two-stroke operation. The exhaust-note/sound was like “scale-music”.. that you’d have to hear to believe. I’d always launched my 1/2A C/L models “R.O.G.” via a pin-stooge.. and this biplane model had no problem getting airborne (off a grass field..) for “roundy-rounds” with the 8" prop.. although I never attempted any aerobatics.

This experiment is to see if and how the Babe Bee engine will respond to the Ace venturi throttle for possible-use in very lightweight 1/2A RC scale-models of the golden-age. Triplanes, Biplanes.. and early monoplane designs.. that lumbered-around the sky’s in the early 1900’s. Not the “speed-demons” of later years.

RC flight can be less demanding on a model-engine than C/L flight is. Untethered; the model is free to “float” where it wants.. unhampered.

For this reason.. I think it’s worth an experiment to see just how viable a Cox .049 Babe engine might be, for “true” 1/2A-scale applications. I already know that a healthy engine will run in a two-stroke mode with a 1”:1’ scale propeller. If it can be throttled-down and possibly “idle”… it would be the ultimate in scale-realism.

[roddie]

With some modeling-motivation watching Rusty's latest "Stunt Hangar" video-hangout show; I managed to put together a set-up for attempting to throttle a Babe Bee using the Ace/Cooney venturi-throttle.. with a larger than standard propeller.

I had three old Babe Bee's to choose from; all of which were modified for an external fuel-feed. I picked one that seemed to have better compression than the other two.. and pulled its tank to check the reed, and replace the venturi-gasket.

The reed is the copper 4-pointed star type.. which I prefer to use on the older Tanked Bee's.. so I checked its function by blowing and sucking on the front of the reed-housing with my mouth.. to find the valve to be functioning properly.    

The needle was a candidate for sealing with some fuel-tubing.. (there was notable "play" in the threads) so I did that, omitting its spring. As I recollect; running the planned 8" prop years ago, resulted in a narrow needle adjustment-window to achieve firing on every stroke.. so I figured that a reliable seal would definitely be in order for tuning within this power-band.



You'll notice that the tank-bowl has a hole drilled for the installation of a rubber-grommet. This helps to both; anchor and protect the fuel-line from pulling-off the nipple inside.. and from abrading on the tank-bowl due to vibration. A snug-fit of the line through the grommet is in order here. When reassembling the tank and back-plate; the fuel-line is "tugged-through" the rubber grommet until the slack is taken-up.. and the fuel-line takes a smooth 90 degree turn out through the side of the tank-bowl. I've had good success with this method.. and have applied it to several Babe Bee engines over the years, to increase their run-time on the bench for experiments.. just like this one.

Recapping the Ace/Cooney design; its needle/plunger (.062" music-wire)  moves in and out of the engine's air-intake by removing the small mesh-screen from the Bee engine's backplate. The Cox Bee engine is unique; in that its "fuel-jet" is located just inside the air-intake-tube, unlike a conventional "spray-bar". Because of this; the throttle's tapered-plunger actually meters both the "fuel and air" when it enters the venturi.. (albeit crudely..) rather than "choking-off only the air.

The radial engine-mount (firewall) is drilled-out for the "plunger" to pass through. This guide-hole for the plunger is exactly on-center with both; the engine's case-screws and mounting-lugs.. so it's as easy as drawing an "X" through the engine mounting-holes on the firewall, to accurately locate the guide-hole. This can then be drilled-through with a 1/16" drill-bit.

My Bee engine mounts for this throttle-design are made of PVC using a CAD program.. but that was just a matter of convenience. I made a lot of parts that way when I had access to a CNC router machine. A firewall-mount with guide-hole can easily be made with hand-tools.

Here's my set-up..



An RC servo would normally actuate the throttle.. but this experiment is to see whether the engine will swing this propeller "initially" with a wide-open throttle. If it does.. then the throttle will be closed to see if the engine can be idled-down.. and remain running.

I tied a string to the throttle-needle so I don't lose it.. in case it inadvertently falls-out of the guide during the tests.

The prop is a wooden "Zinger" 8 x 4 with bushed-hub to fit the Bee's 5-40 prop-screw. I couldn't find my 8 x 3... so the bigger pitch might be too much for the engine.

The engine cylinder is a slit-exhaust type typical of the later product-engines (dual-bypass). The glow-plug is a standard one.. and fuel will be Sig "Champion 25".

Stay tuned.........

[roddie]

Well... I have a vid for ya'll... but it's not the quality that you're used to here.. Actually; it's pretty bad.. but I did prove that the Babe Bee will run with an 8 x 4 prop.. and can be throttled-down using the Ace/Cooney style venturi-throttle.

https://www.youtube.com/watch?v=yytMAFLLxcs



Low-idle requires a glow-driver assist. I had taken some tach-readings before my wife shot the vid. The useful throttling-range was approx. between 2K-6.5K. 2K operation was fairly consistent. The lowest tach-reading was 1.6K before the engine started "loading-up".

The muffler is one of my homemade designs.. and was equipped with dual pipes drilled-out for maximum-flow. A muffler-pressure line can be seen hanging below.. but was not used.

I have a Zinger 8 x 3 woody somewhere.. but couldn't find it. I did run a Rev-Up 7 x 2.8 which developed much higher revs.. and good low-idle #'s... but still required juice to the plug to sustain.

The Ace instructions recommend using a muffler to keep the plug-lit (head hot) at low idle speeds. My muffler didn't really handle this task; having full-flow pipes installed. I hope to be able to get consistent low-idle speeds without a glow-driver assist.

I only tested the one standard Cox glow-head using one gasket.. and my fuel is pretty old.

[fit90]

Outstanding!! That is so cool to see a babe bee idling at such a low speed! A couple of things you might want to try are a merlin/insert type plug in the "hot" heat range and lots of head gaskets. I hope you are able to fly that on an R/C model some day.

[roddie]

Outstanding!! That is so cool to see a babe bee idling at such a low speed! A couple of things you might want to try are a merlin/insert type plug in the "hot" heat range and lots of head gaskets. I hope you are able to fly that on an R/C model some day.

Thanks Bob, I'll have to try that. It would be great to achieve low idle-speeds without needing the glow-driver on.

[getback]

Cool stuff !! Thats a big prop for that little bee but doesn't seem to have a problem with it for what it is , as Bob said a hott drop in may give some better results (without the glow driver ) maybe try a idle bar plug ? Keep us updated

[roddie]

Cool stuff !! Thats a big prop for that little bee but doesn't seem to have a problem with it for what it is , as Bob said a hott drop in may give some better results (without the glow driver ) maybe try a idle bar plug ? Keep us updated

Thanks Eric. It is kinda' cool messing-around with this throttle. I neglected to check this cylinder for SPI.. and it's hard to tell with the slit-exhaust port-style as opposed to the open-type. I'm falling-short on providing complete data.

I wanted to conduct some more tests today.. but the weather has turned wet and windy. One of the tests I want to conduct is running a flywheel mounted with the propeller. I made a bunch of various types of flywheels on the CNC router years ago.



The pieces were all cut with a 1/8" endmill. The hub-holes are exactly on-center. They're programmed as "drill-points".. and being 1/8" diameter; they fit the Cox's 5-40 prop-screw perfectly.. which helps with balancing. Materials are PVC, aluminum and acrylic. Some were also designed for possible marine use. Lots of different combinations of weights to try.






I may be wasting my time in the application of "flyweight" on the crankshaft to correct the low-speed loss of ignition without a glow-driver assist. I may end-up trying the running of a flywheel alone.. without the "cooling-action" of an aero-propeller. Maybe a "reduction in compression" is the key? I'd rather not retain "excess-heat" in the upper-cylinder during high-rpm operation.. if that can be avoided.

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