Zenith Zodiac CH601 HDS

Graham Byass
EAA Chapter 1000 Det 10, Perth, Australia

Originally published June 1998

The name is certainly a bit of a mouthful but the end result is a very nice aeroplane. In the June 1996 Western Flyer (newsletter of the Western Australian Division of the Sport Aviation Association of Australia (SAAA)) there was an article from me where I described, in general terms, why I had decided to build the Zodiac. I still believe that I have made the right decision - unfortunately the building progress is almost negligible to this time. Maybe something like 15 weeks overseas so far this year may have something to do with it!

In this article I will comment on some of the information that I have obtained from other builders and from my trip to the USA and UK this year. There has been a lot of discussion on the Internet in the Zodiac builders forum - as can be expected a lot of this is ill-informed but most of the comments have been quite favorable.

For instance, recently there was a request from a prospective builder who asked if the Zodiac was a good aeroplane to build. The universal answer was yes, so that is a good sign.

One thing that needs to be kept in mind with this aircraft is Chris Heinz's design philosophies - keep it simple and keep it light. A further watchword of his is 'if it works then don't modify it'. These 'rules' upset some people as they are continually wondering why the design is not 'improved'.

Anyway, back to my trip. The first part of the trip was the USA and I met Fred Long at Chicago where we set out in a rented a car for the Oshkosh airshow. I won't go into details of Fred worrying about my driving - I don't think it was all that bad. I put Fred's nerves down to the fact that he was sitting in the right seat but it was missing the steering wheel. Later in the trip I gave him my new Lowrance Airmap GPS to play with and this kept him amused.

At Oshkosh this year there were two Zodiacs at the Zenair booth - one was the one featured in the August and September 1997 issues of Popular Mechanics (PM) and the other was one built in Canada by the Canadian distributors, Flypass. The PM aircraft was a tri-gear 601 HDS aircraft was built strictly according to the plans and the write-up in the PM article was very good - sufficient to say that the factory has been inundated with inquiries for information packs. The Canadian aircraft had a few minor modifications such as a revised canopy locking and slightly different wing tank arrangements but was fundamentally standard.

At the time I was seriously considering using their canopy locking system as it is a distinct improvement over the standard system which hinges to either side and has a fairly crude locking system. While at Oshkosh I saw the RANS S16 and it had a very neat forward hinged canopy and it got me thinking that it would be a better system on the Zodiac. More on this subject later.

On the Saturday evening was the annual Zenair builders dinner and, by chance, I was seated at the same table as the Zodiac designer, Chris Heinz. After the dinner I had a discussion with Chris about some of the modifications I had in mind for my Zodiac and he agreed that my ideas were quite sound. These proposed modifications were fairly minor but included a modified wing tank configuration, the 120HP Jabiru, moving the firewall forward, modified seat belt mountings and a removable instrument panel. On the subject of dual control sticks and a modified canopy hinging he could not offer any particular advice other that to reiterate his previous comments about keeping the weight down.

Zodiac 601HDS in front of the Zenith factory in Missouri.

The next stop after Oshkosh was the Zenith factory at Missouri where Fred and myself both went for test flights on the factory demonstrator 601. The weather was lousy, with a ceiling of only about 700-800 feet so that each flight was not much more than an extended circuit. Fred agreed with me that it is a pleasant aircraft to fly.

After the USA I flew over the UK and, fairly conveniently, was staying at Salisbury and I took the opportunity to visit Alan Cozen at Goodwood where he has his CH601HD hangared. This aircraft was the subject of a protracted certification exercise with the Popular Flying Association - PFA - as they claimed that it lacked longitudinal stability and insisted on a larger tailplane. In the UK the PFA is, effectively, the ruling body on homebuilt certification and they claimed that the standard HD model failed the phugoid stability test. In other words when the stick was displaced fore and aft the aircraft did not properly return to the stable flight condition. All this had been written up in the December 1996 issue of Pilot magazine and there had been a lot of discussion on the Internet before I left on my trip. At this time I did the stability calculations for the HD model and it certainly was not within the accepted stability range so I can see the reason for the PFA stance. As a matter of interest I also did the stability calculations for the HDS model (the one that I am building) and this works out fine as the wing area is smaller so the ratio of wing to tail areas is better.

Alan Cozen's Zodiac 601HD in the hangar at Goodwood (UK), with a forward-opening canopy

Anyway, this was not the main reason for my visit to see Alan. What I wanted to see was the modified control stick assembly where he now has dual sticks instead of the central stick. I was not keen on the central stick as I am right handed and this would mean that to operate the radio, GPS and similar functions would be difficult. I would either have to try to fly with my left hand on the central stick or muddle along with my left hand on the radio etc. (which presumably would also be in the centre of the aircraft). The modifications he did were quite simple and I have a set of his drawings that he submitted to the PFA for his certification so it should not be a problem here - certainly if the Experimental Category is introduced.

Alan Cozen's dual stick conversion in lieu of the standard centre stick

Another modification he did was to have a forward hinged canopy which is supported by gas struts. I like this and intend to do the same to mine. I have his drawings for this also and it is interesting to note that the weight penalty for this mod is only 2.1Kg. Alan was a very interesting person to talk with and I certainly appreciated the time he spent showing me these - and other mods he made to the aircraft.

I now have a very clear picture of what I want to do so all I have to do now is to put in the necessary hours and dollars to see my dream realised.

If anyone else has a dream of building anything else other than an RV then this design has a lot to recommend it.


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Revised -- 14 March 1999