Model Engine Maker
Engines => From Plans => Topic started by: moerman on September 12, 2014, 10:19:27 AM
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I have decided which compressed air engine I will build. It is a design which I've looked at for many years and I think it is a good first engine to build. It is the Rotary Valve Compressed Air Engine by David Kerzel. this engine is based on a 3 cylinder engine which was designed by Joseph Ott in 1933 for model flying purposes: http://www.floridaame.org/GalleryPages/g1Non0173.htm (http://www.floridaame.org/GalleryPages/g1Non0173.htm)
Here is a picture from that website:
(http://www.floridaame.org/GalleryPages/IMG2004/DFK/Kerzel%20Tube%20CO2%20%2003147.JPG)
I am collecting materials, I already have brass tubing for the piston, cylinder and cranckshaft case. I also have aluminium for the crankrod. In my box with bits and pieces there is sufficient material for the other bits and pieces, so I am ready to go.
Using tubing for the cylinder and piston feels a bit like cheating, after all I do own a lathe since three weeks. But brass tubing for the cylinder is lighter than anything I can turn on the lathe and light weight is a prime design consideration. But the thought of tubing for the piston is open for debate. The tubing I have fits loosely in the cylinder tubing, so some air will be lost. This fit will give very little friction and I don't need compression, so this might not be bad. but I also can turn a piston from aluminium with a closer fit. Guys, what do you advise?
For the crankshaft I have the choice of using brass or steel. What will be best? I will use a M3 bolt as extension to mount a propeller.
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That is an interesting little engine 8)
Crank: you want dissimilar metals for the crank/bearing. As the bearing looks to be brass that would mean the crank should be steel. But the original design got away with using a brass screw
Jo
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Here are some pictures of an original engine by Joseph Ott, found on http://www.worthpoint.com/worthopedia/compressed-air-motor-joe-ott-21173144 (http://www.worthpoint.com/worthopedia/compressed-air-motor-joe-ott-21173144).
The accompanying text:
This is an Extremely Rare 1928 Joe Ott Featherweight, Compressed Air Motor (CoA). It is a 6-cyl, .73 displacement air motor. Bert Pond (longtime CoA guru) told me that Joe hand built 10 of these, long stroke, lightweight motors, and only sold them to the top competitors, and to his friends. It is 6" diameter, and weighs only 3 ounces!! . In photo #2, look at the craftsmanship, and detail, of the connecting rods! This motor is 84 years old , and I was told that it was never flown. This motor is shown in the America Model Engine Encyclopedia, on page 201. These CoA motor/tanks, are inflated by a normal automotive tire pump, either a Hand Pump, or 12V Electric Automatic Pump. Also included in this auction is a pre-carved balsa wood prop, 2 formed, thin brass end caps, air needle valve, and air filler valve, and a coil of .005" x 60" brass shim stock to build a 30" long brass air tank. Also included are easy to follow instructions from Bert Pond's fine CoA book, on how to build a CoA air tank. It is pretty easy to form the brass around a mailing tibe, and solder the seam, which builds a tank capable of holding 150 psi. The tank can be fabricated in 2-3 hours, by any modeler.
I will not use the brass tank but use an aluminium beer can instead, as in http://www.flysteam.co.uk/compair.htm (http://www.flysteam.co.uk/compair.htm)
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Intriguing. It would seem that there is very little stress anywhere so why not aluminum for the crank to save weight? 7075 should be more than strong enough. 6061 would probably also work.
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Let's go out of the box for a moment. If we put historical accuracy aside and look at more contemporary materials, just how light could we build this engine?
Compressed air. No heat issues.
Delrin or Teflon bearing? Tiny ball bearings?
Graphite piston?
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Jo, thanks for your input. I will use steel for the crank.
sshire,
I have been thinking along the same lines. But that's for the MK2 version. I'm thinking about using carbon fibre, aluminium and 1/64" aircraft grade birch plywood. No soldering but using epoxy to merge it all together. But first I will build the 'original' version, which is already a modern adaptation of the Ott design.
BTW, I've bought some bronze rod (and other goodies). I can make the piston top out of bronze and make it a good fit with the brass tube cylinder. Another decision made!
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Stan, Wout,
If memory serves correct sometime in the late nineties there was a commercially available compressed air motor made mostly of plastics. I can't remember quite how the commercial tank was formed but again if memory serves - Doug Mc'Hard, a very prominent free flight scale model maker designed a scale flying model using a P.E.T. plastic 'lemonade' type bottle as the tank which was housed inside the boxy fuselage. It was featured in 'Aeromodeller' magazine but which year and issue is, I'm afraid, beyond recollection.
Perhaps that might be worth pursuing Wout :)
Ramon
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This has stirred my brain cell
Did Cox not make a small engine for free flight that was powered by a Sparklet type co2 tank the type that is used in a soda syphon
But again I may be wrong on all counts
Stuart
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There were several small CO2 motors for peanut or walnut-sized models, mostly made on a cottage-industry basis in places like Czechoslovakia (Gasparin?) and Poland although there was the Bill Brown one (american?) and a mass-produced one in the UK called the "Telco". The Telco was unusual in that it was made mostly of moulded nylon, only using metal for the crankshaft, excentric main bearing, rod and head IIRC. I vaguely remember the Telco was also available as a twin, and there was also (I think) a 5-cylinder "radial" *actually 5 motors geared to a central shaft).
AS
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AirHogs may lean slightly more toward the 'toy' category but this video shows they certainly were (are?) flyers:
https://www.youtube.com/watch?v=FDcfDTsvAzs
--Tim
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I have built a couple of these engines ... the single and the twin.
They were built using K & S brass and copper tubing and flat brass from a hobby store. .... if the right sizes are selected ...it is possible to get a satisfactory sliding fit for use in the piston and barrel construction.
Recently it was suggested that I should try a 'hiker' or 'power-walker' cylindrical, thick walled, aluminium water bottle as a power source .. with a pushbike tyre valve and stem attached so the tank can be charged up with a bicycle air pump.
The engines seem to always be a talking point and when polished to an inch of their lives, are visually attractive as a shelf engine.
Cheers........
Collbee
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Thanks for that Air Hogs video! That is indeed what I'm after, although I prefer a more gentle and less erretic kind of flight. There indeed used to be a variety of CO2 engines in the past, and father and son Gasparin still make a serie of high end engines for CO2. I met them a few month ago here in Nijmegen, very kind people and impressive workmanship. But compressed air engines are a completely different animal!
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Hi Wout - Further to my previous post I have just found this
Part 2 moves on to modern times, beginning with air engines made from plastics. Chris Stoddart explores the patents and "toy" engines from the 80's that can be adapted to more aerodynamic airframes. Articles reprinted from Aeromodeller by the late Doug McHard show what amazing things can be accomplished with the Italian Z model engine, including how to make a twin cylinder engine which, using a 2 litre drink bottle, wrapped with glass fibre packing tape, was pumped up to 9 bar pressure (132 psi) to give a 1:30 min engine run.
This comes from a page on Ron Chernich's MEN site here http://modelenginenews.org/ed.2012.08.html (http://modelenginenews.org/ed.2012.08.html) It's under the heading New Books and Magazines and refers to the engine and article I mentioned
If I remember correctly, should it be desired, the PET plastic bottles can be reduced in size uniformly by heating when the 'memory' in the plastic will allow it to shrink. This does not affect the neck (and thread) apparently so this can still utilised with a modified cap. The pressure that these bottles can withstand is considerable - possibly more than a drinks can if a little heavier - well worth investigating.
Hope that's of interest
Ramon
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Remember these guys? Had a lot of fun with these as a kid...its amazing what a little air pressure can do.
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Bill, I use to have many fun hours playing with mine. I also had the air hog plane. Great memories flooded back With these videos
Dave
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Dave,
When I was in high school, a small group of us really got into .049 control line airplanes too. Back then we would get out of school and head to the playground of the local elementary school. I can only imagine that today they would arrest us for doing that...kind of sad in a way. Any progress on your half scale radial?
Bill
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.049. I had forgotten about mine. Perpetually bruised finger from prop hits
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I've done some measurements. The cylinder I have is 7.23 mm internal diameter, the piston is 7.15 external diameter. So that's is a 0.08 mm clearance, or 0.003". Not to bad for of the shelf tubing, but a bit much for a good functioning engine of this size, I think. I'll try to turn a piston cap with a better fit so the top of the piston is slightly wider than the rest of the piston.
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Think of the fits as a percentage of size
Eg
25 mm dia with 0.05 mm slop would not be to bad
But a 5 mm dia 0.05 would be massive
For a air engine of that size you will need a lapped fit of bore to piston but with dissimilar metals
Eg brass with bronze or drawn bronze and cast bronze would work as as would a CI bore with a Ali piston
Hope that helps it's not a exact rule but the percentage idead will help
Stuart
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I've done it! I machined a bronze top for the piston which fits snugly in the cylinder. and has a step to fit in the piston. No measurements done as yet, I did the last part of the turning until I had a good fit in the cylinder. It took me about a cm of brass to make this 3 mm thick part. First tries were undersized.
The piston gets an aluminium center which takes the conrod and pin. I already machined a rod of alu in the right diameter. As the aluminium will be glued in the piston I think I will glue the bronze piston top as well. I am thinking about using cyano-glue for this.
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I just received a 2nd hand Moore&Wright 1965M micrometer, which makes measuring a lot easier. I see that the brass piston tube is 7.16 mm in diameter, while the bronze top is 7.22 mm diameter. I started worrying if the top was sufficiently larger in diameter because I noticed some grooves in the surface which will be polished away. But now I'm confident that also after polishing the diameter is larger than the brass tube piston.
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Yesterday I finished the parts for the piston and a connecting rod. The piston is build up from brass tube, a bronze top and an aluminium core. The core is slotted and drilled to take the conrod and pin. I decided to drill through the bronze top to get some weight reduction. The aluminium core is stepped to fit the hole in the bronze. It all fits beautifully! I'm really learning a lot by making these parts on my new lathe.
I made the conrod out of 1 mm dural instead of 2 mm hobby shop aluminium. I think it will be strong enough.
Pictures will follow later.
Wout
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Here is a picture of the progress I've made. Not much .... But I learned a lot!
The piston and connecting rod are now together and the piston fits the cylinder nicely. The black line is oil which turned black during lapping. That is, if you can call moving the piston through the cylinder up and down by hand lapping. The brass bearing is still twice the length it should be. I have drilled it to 4 mm ID now and it seems the original ID was not constant. The drill went in easily but found some resistance at some parts of the length. Making a fitting shaft is a lot easier now. The shaft is still a bit undersized at 3.94 mm after polishing.
It rocks slightly in the bearing. I hope the next one will be perfect!
Wout
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YES, the next one was perfect. WAS .... The fit is very good but when I wanted to part the extra length of the axle it went wrong. It bent and broke just outside the lathe jaws. I have parted the useable remaining part of the axle, but it is a few mm short. Now I have two options: make a new axle or use the shorter axle. I first will try the second option. On the plus side: it saves weight! I can make the engine just a few mm shorter.
Next question is: how shall I connect the iron cranck to the iron axle? Is silver soldering required? Haven't done that before, but that has never stopped me. I heard there is only one rule: cleanliness, is that right? But another method of joining is welcome, too. Any suggestions?
Wout
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Another rule of silver soldering, get the joint hot enough to melt the solder, don't melt the solder with the torch. Another way is to arrange the crankshaft so you can assemble it with Loctite.
Ian S C
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for your light engine I would go with loctite to
you can undo it with a small amount of heat so so easy with SS
I use 638 or 603 your choice don't buy a big bottle its to expensive , it does not go off but cost is cost
Stuart
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I have not done any work on the CA engine because I first had to tackle some issues with my lathe. But I'm back on the job now. First I will make another cranckshaft and front bearing. For the bearing I have bought a set of 4 mm reamers and a 3.9 mm drill, so the diameter of the shaft will be 4.00 mm. I also bought a length of 2.5 mm brass tubing so now I have everything at hand to build the engine. now just hoping I will find the time this weekend....
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I love this simple Compressed Air Engine! I am going to add it to my "to do list"! I make simple engines as well as some a bit more complex such as Elmer's Geared Steam Engine #5, which is now in my lathe! I cheated a bit on this one though; I spent the "big buck" and bought my gears! Regards, CM