Zodiac 601 HD - The 250 Hour Report on G-BVAC

By Alan Cozens

This report is to tell you what my aims were when choosing the tricycle HD, and how well the machine has met those aims in 250 flying hours. Also I mention one or two ideas and modifications that should be considered by builders with more than local flying ambitions. Some of the points I have mentioned in an earlier article but I make no apology for presenting one bumper package. Certainly I hope you can benefit from the experiences of an earlier builder and not fall down the same holes!

I wanted an aircraft capable of touring with two people around the UK and western Europe, and although the HD is very small my wife and I are quite light and it seemed capable of this. In fact we have already been to Belgium, Ireland, Scottish western islands, France, Switzerland, the Channel Islands, and the Scilly Islands. We have crossed the Channel 14 times, all but 2 on the long route at the western end, and the Irish sea twice. We carry both lifejackets and a liferaft! The CAA were good enough to allow me to do some of the training for my IMC rating in the machine, so I am familiar with handling it in poor visibility. Possibly the biggest challenge was flying over the Jura mountains and then up through the upper Rhone valley and Bernese Alps in Switzerland. However, all in all, it has met my original aims well, and current builders should have no concerns if they take note of what should and should not be done to meet there own aims.

The machine was built as light as possible, and every 'extra' was weighed before fitting. The optional Zenair wing locker tanks were fitted to give a very necessary 105 litres total. With the light 912 engine, vacuum pump, AI, DI, plus panel mounted GPS, radio, and transponder the empty weight was 672 lbs with empty c.g. at 11.6 inches. This provides an excellent (and just adequate for the task) payload of 528 lbs. to reach the MAUW of 1200 lbs. The propeller is the standard wooden ground adjustable 3 blade GSC. Structurally the airframe is built completely standard from the kit parts, except that I moved the firewall forward 80 mm due to the light engine and extended the tailplane span 24 inches.

The editor has given me instructions to keep this short, so I will split my key findings into a list of good points and not so good, but I must stress this is based only on touring experience.

 Good points:

• Airframe is simple, light, and structurally efficient.
• Very safe stalling characteristics and very low (40 knot) stalling speed at MAUW.
• Good rate of climb (650fpm) at MAUW with lowish prop. pitch setting (16.5 degrees).
• Excellent short field performance - very suitable for European airstrips.
• Excellent all round visibility from cockpit.
• Wide comfortable cockpit - necessary for touring.
• Very good payload when fitted with light 912 engine and built light to drawings.
• Adequately stable when carefully trimmed.
• Good range when fitted with wing tanks (350nm still air plus 45 min reserves).
• Good crosswind landing capability - once you are well experienced.
• Good landing gear dynamics - note this one is tricycle.
• Very good ground handling due to steerable nosewheel/good turn radius.
• The 912 engine has been faultless, averaging 18.5 l/hr on the long heavy hauls, 20 l/hr when transiting at FL055/065 on airways in France.

To amplify a little on the above points, if you are going to a strange foreign airfield 500 miles away, you need to be able to land safely when you get there. If the aircraft cannot cope with normal crosswinds or needs close attention on the approach, or does not have rugged wide track landing gear, you are at risk. I have made a number of landings with crosswind components of 18 to 20 knots - definitely not recommended but it is possible. Equally if you arrive at your destination and its `clagged in' you need substantial reserves of fuel to divert somewhere, perhaps returning across 70 miles of Channel. The Zodiac handles all this well.

Not so good points:

There is inevitably something of a price to pay for all those good points above, and this principally concerns the wing. Relative to the aircraft it is large and of deep section, generating massive lift and very resistant to stalling.

• Drag from the wing is substantial, hence cruising speed and fuel economy are rather low at touring weight (80 knots / 20 nautical mpg at 17.0 degrees prop. pitch)
• The deep section wing and long narrow ailerons produce heavy and not very powerful roll control. For the uninitiated this can be dangerous on a gusty approach at low level. However the rudder is powerful and judicious bootfulls co-ordinated with ailerons prove quite adequate. Lesson: you have to learn to use your feet!
• The large wing combined with a smallish short coupled tailplane results in a low tail volume coefficient - only 0.29. Mine with increased tailspan is 0.37 but ideally it should be about 0.45 Although not unstable, HD aircraft cannot meet JAR/VLA longitudinal stability specification, mainly at max. rearward c.g.
• The Zenair wing tanks are installed in the wing lockers aft of the c.g., displacing all luggage to the aft shelf. This combination moves the c.g. rearwards and reduces the longitudinal stability.
• The rudder will not self centre, due to the substantial friction of the coupled nose wheel and also perhaps not much centreing power from a rudder without a fixed fin. As with the previous 2 points, no real problem in normal flight, but avoid accidentally stumbling into IMC as the aircraft can't help you.
• Several steel working parts operate in aluminium bearings. Examples - control torque tube and nose gear leg. Normally regarded as unacceptable aviation design practice due to very rapid wear.

From the above, I can make some recommendations, but these have to be totally factored against your ambitions. For flying locally a few hours a year plus the occasional day trip with say a VW engine the design works very well - don't change anything and keep it light! For the more ambitious, the machine will perform very well as a mini tourer, but to fit essential navaids and still leave adequate payload for crew, 100+ litres of fuel, and luggage, a light engine like the 912 is required. A light engine means moving the firewall forward to give a safe c.g. position when loaded to MAUW but approved drawings are available. Apparently Chris Heintz is suggesting 2 inches for this but my experience is that 3.2 inches is only just enough but can be achieved from kit parts. There is the additional advantage that this allows the depth for a proper radio/ transponder/GPS stack in the panel. DO NOT USE THE WING LOCKER TANKS - keep the lockers free for heavier luggage as originally intended. There is adequate space for tanks in the leading edge - ahead of the c.g. Don't even consider that the 56 litres of fuel in the forward tank is sufficient for safe touring - it isn't! Regarding the low tail volume coefficient, if you are ever going to get into IMC conditions, an extended tailplane is certainly going to help and also improves handling. Regarding the scooter wheels, they are so ugly you have to cover them with fairings, but they are light and have proved (surprisingly) robust. Coming to the aluminium `bearings' they can be field repaired cheaply but not easily e.g with patch plates under the top skin of the wing for the main gear leg. Expect significant wear after 200 hours. Alternatively consider the approved nylon reinforcements which will last almost indefinitely and do not rattle. Also I found very bad wear on the aluminium main gear slides at 200 hours, caused by cross tube hammer. This is one area which is almost impossible to repair and a proper engineering solution should have been implemented during build. Ainal point is that because the wing loading is so light, tying down the aircraft for overnight stops is mandatory, and the picketing gear weighs about 4 lbs and should be stowed ahead of the c.g. I still have no solution for this one, but would have liked a locker at the firewall for the pickets plus my 3 lb flying tool kit.

To complete my HD report, I would like to mention the HDS, having flown Alec Bender's beautiful example. This provides a few lessons. It has delightfully light and responsive ailerons, flies at least 15 knots faster, and has an excellent tail volume coefficient ( 0.46 - due to larger tailplane or should I say smaller wing - get it?) Hence the aft c.g. problem is less critical. The stall speed is just within PFA upper limit of 52 knots. But would I give up that big safe HD wing - almost capable of providing a forced landing on a tennis court ? Can the HDS claw its way up and out of a cooling mountain valley to 6500 feet at MAUW, fuel almost dripping from bulging tanks? Maybe, but I'm waiting confirmation from the Swiss builders!

Finally, a plea to report performance professionally - at MAUW and at c.g. limits - not a pilot only dreamflight with a thimble full of fuel in a thermal - please!

References: Stinton: The Design of the Aeroplane p382,p391.