Author Topic: Lightweight compressed air engine  (Read 10408 times)

Offline moerman

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Lightweight compressed air engine
« on: September 12, 2014, 10:19:27 AM »
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

Here is a picture from that website:


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.
« Last Edit: September 12, 2014, 10:34:51 AM by moerman »

Offline Jo

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Re: Lightweight compressed air engine
« Reply #1 on: September 12, 2014, 10:29:42 AM »
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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Offline moerman

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Re: Lightweight compressed air engine
« Reply #2 on: September 12, 2014, 10:38:18 AM »
Here are some pictures of an original engine by Joseph Ott, found on http://www.worthpoint.com/worthopedia/compressed-air-motor-joe-ott-21173144.



The accompanying text:
Quote
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
« Last Edit: September 13, 2014, 08:19:19 AM by moerman »

Offline sshire

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Re: Lightweight compressed air engine
« Reply #3 on: September 12, 2014, 01:45:58 PM »
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.
Best,
Stan

Offline sshire

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Re: Lightweight compressed air engine
« Reply #4 on: September 12, 2014, 01:54:36 PM »
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?
Best,
Stan

Offline moerman

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Re: Lightweight compressed air engine
« Reply #5 on: September 12, 2014, 02:59:34 PM »
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!
« Last Edit: September 13, 2014, 08:15:00 AM by moerman »

Offline Ramon

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Re: Lightweight compressed air engine
« Reply #6 on: September 12, 2014, 04:50:35 PM »
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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Offline Stuart

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Re: Lightweight compressed air engine
« Reply #7 on: September 12, 2014, 06:13:10 PM »
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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Offline Allen Smithee

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Re: Lightweight compressed air engine
« Reply #8 on: September 12, 2014, 06:34:31 PM »
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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Offline tvoght

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Re: Lightweight compressed air engine
« Reply #9 on: September 12, 2014, 07:03:42 PM »
AirHogs may lean slightly more toward the 'toy' category but this video shows they certainly were (are?) flyers:

<a href="http://www.youtube.com/watch?v=FDcfDTsvAzs" target="_blank">http://www.youtube.com/watch?v=FDcfDTsvAzs</a>

--Tim

Offline collbee

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Re: Lightweight compressed air engine
« Reply #10 on: September 12, 2014, 10:05:22 PM »
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

Offline moerman

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Re: Lightweight compressed air engine
« Reply #11 on: September 12, 2014, 10:15:26 PM »
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!

Offline Ramon

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Re: Lightweight compressed air engine
« Reply #12 on: September 12, 2014, 10:26:36 PM »
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 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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Online b.lindsey

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Re: Lightweight compressed air engine
« Reply #13 on: September 13, 2014, 12:29:31 AM »
Remember these guys? Had a lot of fun with these as a kid...its amazing what a little air pressure can do.


Offline Dreeves

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Re: Lightweight compressed air engine
« Reply #14 on: September 13, 2014, 01:46:23 AM »
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