The EggMonster Story (Flight Of Fantasy?)

Hello Folks,

Well I'd thought that I would make a brief return to the realms of aviation and give the bushcrafty stuff a rest for the moment.

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Going back in time to early 2010 and running parallel with all the gx200 Eggmotor development, I decided to try out a more powerful version of the concept. This time I would be using a gx390 engine (well, a 420cc variant but almost identical) which has twice the power of the gx200 (around 12hp [net] @ 3600rpm factory spec). Only down side is that it is nearly twice the weight too.

I started out with a few designs first and even make a paper-wire model in order to make a light weight yet strong chassis to hold the heavier engine.

I set about the construction using 20mm x 20mm x 2mm wall square steel section as I had plenty to hand. If more funds had been available at the time then I would have preferred 1.6mm wall thickness instead. I tested fitted the engine to the chassis while cutting and welding to ensure everything remained 'true'.

I rolled (using my tube roller) a long length of 16mm diameter round tubing for the circles and the cage arms. All these curved sections were based on a 48 inch diameter circle.

Apart from the paint job, the chassis was almost complete so let's divert our attention to the engine. As stated earlier it is a 420cc clone version of the Honda gx390 industrial engine that you tend to find in generators and small agricultural equipment. In it's standard form there wasn't sufficient power for the thrust and propeller size that I had in mind. To overcome this I removed the governor, milled the cylinder head, fitted a lighter billet alloy flywheel, re-jetted the carburettor, advanced the timing by 8 degrees, fitted a free-flow air filter and exhaust header.

I estimated the power now to have gone from 12hp at a governed 3600rpm to approximately 18hp @ 5000rpm. As you will see, my prop size & pitch option would (by aerodynamic drag) limit peak rpm to under 4000rpm so my usable power was more like 15-16hp.

Below you can see the almost completed machine set up with a HasconWing 3 blade fan assembly. It's the sort of thing used for industrial air movement but also widely used in the hovercraft industry too. The diameter of this prop (fan) is 108cm and is a composite material blade fitted into an alloy hub. These fans aren't as efficient as a true propeller but after trial and error altering the pitch angle and re-profiling the blade shape we eventually managed decent thrust.

The figures achieved with the fan blade assembly were (according to our ad-hoc bathroom scales test) 47kg (103.5lb) thrust @ a surprisingly low yet torquey 2700rpm.

As the fan assembly was quite a heavy unit when compared to a prop, plus the fact it's not as efficient. I purchased a conventional wooden propeller that was specified for a Konig SC430 engine as it seemed the best match for my engine output. This propeller is also an attractive 'scimitar' shape too and virtually the same diameter as the fan at 108cm.

[the eagle-eyed among you might have spotted the off-set harness hang points in this picture below. This was to compensate for the weight of the engine cylinder 'over-hang'.]

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Here you can watch Sam and I giving this new beast a decent thrust testing on our nearby sports field.
As you can see in the footage, we achieved 47kg (103.5lb) thrust @ 3580rpm. After this test I checked the spark plug and it was very sooty and so I fitted a slightly smaller main carb jet. Sam and I conducted a second test (not filmed unfortunately) and the bathroom scales nudged 49kg (108lb) @ 3660rpm. So improving the air/fuel ratio gained us a further 80rpm and 2kg of thrust.

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So all was looking promising then I hear you ask?

Well unfortunately, I haven't really addressed the 'Elephant In The Room' yet.

You see the gx200 Eggmotor wasn't exactly a lightweight at around 28kg including harness and fuel and so this larger gx420 version was expected to be a bit heavier. The only problem was, how much heavier?

Including only a couple of litres of fuel (the tank could hold 8 litres) and the harness as well, this EggMonster was...... a Monster!!! :( If my memory serves me well, this beast topped out at 38kg in weight. Although I could get into the harness and just about stand up unaided, I couldn't conceive attempting to run and launch with the beast especially as my lower back issues were beginning to make themselves present at this time too.

It had been an amazing and sometimes frustrating project but I had to face reality.....

With a heavy heart I disassembled the Monster and sold the various parts off to the far flung corners of England.

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You could check out the pages in the old blog:
https://eggmonster.wordpress.com/

Or view the YouTube playlist:
https://www.youtube.com/playlist?list=PL0C5571142AF3F06A

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I would greatly appreciate your support by upvoting and resteeming this post if you found it of some interest.

Many thanks,

Rob.