New Hovercraft for `97 - Modified for `98
During the 1996 season our craft J11 (registration 695) was powered by a Yamaha RD250LC which was reliable but compared to the quick craft of Kristie, Stephen and Daniel it did not have enough power/weight ratio.
To try and keep up we reduced the height of the splitter plate to give maximum thrust whilst still leaving just sufficient to provide lift. This was fine at full throttle - the problems started when using half power when slowing for any reason. Being an integrated craft the moment there wasn't maximum power the lift went.
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The picture left shows the craft virtually grounded when easing off for Kristie to take the inside line. On water this situation usually meant a plough in, hence some (not all! - Dad) of the problems at Stanford Hall exit from the water straight. |
| The new craft (J7) is going to be powered by a Yamaha TZR 250 power valve so we can afford have a larger lift box to give eight inches of lift at full power and some reserve lift for slower corners. The larger duct also requires a larger opening for lift to keep the ratios the same. |
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J7 will be a mix of designs. The hull is basically Eagle F3
design, the same as last year, but much lighter. The rear end of the craft consisting of
the lift box and duct are from an Eagle F25. The seat and steering console is from an
Eagle F1!! I know of two other sister craft but neither are in FJ - still, the season is still three months away. |
| If you have wondered what was inside the skirt - now you
know!! Each segmented skirt is spread and bolted to the top either side of the air hole and tied to the wire loops at the bottom. Very boring fitting 54 of them the first time. The clever bit of all this is the skirt bag. Two designs are used on the craft, the ones at the rear are nicknamed the chip bags. They have a flap sewn inside to prevent water being scooped up inside. An alternative is a drag flap which is a carpet type flap attached to the middle/rear of the craft on which the normal standard bags ride. |
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The motor bike engine is converted from a parallel to an in
line twin by discarding the chain drive and fitting a timing belt drive direct to the fan
(right side of engine). The belt drive pulley is a one off design (as far as we know) and
enables us to run the engine at up to 11000 rpm with a 900mm duct.The very low gearing
still keeps the fan blade tip speed inside the competition/safety regulations. The gearbox as well as coolant, oil and fuel pumps are cut off and chucked away, saving loads of weight. The coolant pump is replaced with an electric one. If I ever forget to switch it on before starting the engine - Oh boy!! The original generator and magneto were also discarded when the competition crank was fitted complete with light weight magneto - no generator!!! We will have to take several batteries to a race meeting.. |
| The engine is carried on a frame which also has the plummer
block and axle for the fan, the fan assembly, radiator and coolant pump as well as the air
filter, so it is self contained. Detaching the exhaust one fuel line 3 quick fit
electrical connectors and 6 bolts and the whole engine and fan assembly can be removed
from the craft. Under 10 minutes in race conditions. Quick removal of the engine is important so access to the fans for fine tuning/blade replacement can take place between races which sometimes can be very tight on time. |
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The arrangement of the drive belt is visible here with a blade of the fan in front. The exhaust stacks were fabricated from original motor bike parts but the rest of the system was specially made to fit the hovercraft but still retain the expansion properties of the original. The weight of the system compared to the original bike muffler is reduced by nearly half as single skin light gauge metal is used. The muffler relies on baffles instead of packing to reduce noise. |
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So far testing has been restricted to running in the engine
and limiting the engine revs to 7000 rpm. Before the engine was fully bedded in the
ignition system failed !! so our testing/preparation programme is a bit behind as at
31/3/97 The craft is airborne and hovering comfortably at 3500/4000 rpm and the power valve kicks in at 6000 plus and makes a big difference. The exhaust has a real ringing sound to it when the power valve is active but overall the craft is significantly quieter than old 695 |
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Design errors so far are two fold - one the master
electrical switch was sited on the instrument panel so that when leaning over the
handlebars on a practise start my body turned the switch off!!! The switch is now situated
on the back of the seat base along side and protected by the power valve servo motor.
The other item is the run of the throttle cable - we are working on this but getting the ignition sorted and some more hours on the engine is taking priority. |
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It is great to be able to do effortless multiple 360's -
you can just drift down the test field spinning round and round all the way 05/04/97 Update Ignition failure due to the coil wiring breaking apart in many places - eventually the coil connections internally failed. The reason was that the coil was attached to part of the engine frame which at certain engine speeds vibrates visibly and literally shakes apart anything attached to it. We have re-sited the new coil in line with the crankshaft on a softer mount - fingers crossed. We will also look at the vibration problem of the engine frame as a separate issue.
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The accelerator cable problem is also licked - new plastic
inserts fitted to the carburetor top guide tubes. I have a new design throttle lever which
has a greater travel and more leverage, so full throttle without cramped and hurting
fingers at the end of a race will be great. Outstanding teething problems as at 05/04/97 is a malfunctioning coolant temp gauge - a faulty earth is thought to be the problem. |
 Early testing days at Stubton Airstrip
Barry Staples at Rother Valley II 2002 - the last outing
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Update 06/04/97 We were allowed to use a private airstrip for high speed testing - flat out down runway 10 !!! Yipeee I had nearly three hours with a full airstrip to play with - Yeah! Lift performance even with a passenger, is great. Final tuning of the blades is now required (struggling to get 9000 rpm against the target of 10000 rpm) as well as fitting race numbers etc. a final technical problem concerns the new coil mount. Final 19/04/97 Final setting of the blades has taken the revs upto 10,500 rpm approx actually 9000 into a stiffish breeze and 10800 downwind, we are in two minds whether to increase the blade angle a bit to ease the revs back to the target value or increase the target value to a level used by other competitors in formula 3. There is some evidence of fuel starvation on prolonged full throttle bursts (revs slide by 400 rpm), but we are going to leave this for further investigation at more high speed testing. For now the spark plugs seem exactly the right colour (we increased the main jet size as part of the hoverisation of the engine). Final testing at Stubton Park airfield 20/04/97 - Racing now 2 weeks away. Final note 20/04/97 Testing went well - revs set at 10500. I ran over a hare on one flat out run - it came out from under the craft ran round in a few circles and then legged it back the way it came! One advantage of a hovercraft is not chewing up the grass or flattening the wild life!! 20/20 hindsight 23/01/98 The revs sliding 400rpm off peak (see above) wasn't fuel starvation but the effects of an exhaust gas standing wave at the exhaust port on the front cylinder. It took us several burnt pistons and engine rebuilds and a lot of frustration to work out that we needed to increase the specially made exhaust stinger pipe diameter and a small increase in the length of exhaust system. Fortuneately the mods worked first time in Lucon!! During the season we modified the lift system by fitting a variable splitter plate. The massive amount of lift we started the season with could now be converted to thrust at will and gave me a much more competitive craft. 20/08/2002 Five hard seasons later now in the hands of Barry and Toni Staples 1127 drove its last race at the Rother Valley II event in 2002. Even in its old age given the right conditions it could still produce a podium..... R.I.P.
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