Maintenance and mods
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Found old trailer top deck for scout lting out back so i decided to add some wheels and give the Scout it's very own trailer!
Re-checked thrust - it was only reading about 60lbs at 3500rpm. The belt was slipping badly - it looked pretty shiny so so I bought a new belt. the new belt was a different make at was about 1" shorter than the old one - It had to be very tight to stop the lower prop tips from striking the engine support frame. I then got a slightly longer belt and re-made the frame uprights to increase the prop centre height by around 16mm (I also made up a belt tensioner that lifts the prop hub up rather than having to mess around with the idler pulley positions). I swapped prop pitch blocks from 13 to 18 degrees and re-tested. the engine was reaching 2950rpm and the thrust was 88lbs. Changed the pitch to 15 degrees which increased the engine to 3190rpm and the thrust to 90lbs. I've left it at this as the engine is now running below it's peak rpm as is the lift fan (a major noise source)
Yearly service - changed air and oil filters. I also took off the cover and checked the ignition coils. They showed slight signs of corrosion so i took them off and treated them with conformal coating (a lacquer) after sealing the boots to the HT lead. I noticed that there is a potential problem are on these particular coils with the stop terminal - basically it is very close to an earth tab welded to the core
I could quite easily visualise a drop of salt water bridging these bare terminals and causing the ignition system to fail! I have insulated the earth terminal with polymer sealant.
In addition, the wiring connector that plugs onto the stop terminal is completely uninsulated - once again, some sealant makes the joint waterproof.
While doing some thrust testing for Ian Brooks the end cap of the silencer box came apart like a bean tin! I paid a visit to my local friendly lawn mower repair shop (the owner has had a ride on one of my craft!) and raked around their parts bins. I came up with a slightly used silencer box from a 3 cylinder diesel Kubota tractor - well made with thick (i.e. salt water resistant!) metal. It had an exit pointing straight up - ideal for the Scout as it would be near the hub rather than the prop blades. I fitted it and tested it. I had a bit of a shock when first starting the engine as clouds of black smoke came out of the exhaust!!! After smelling it I realised that it was just diesel soot that had been dislodged by the welding, etc I had done on the box. Overall, the box is significantly quieter - the typical lawn mower putt-putt noise has almost disappeared - you can hear the mechanical crankcase noise above the exhaust at idle. A short video clip is HERE
Discovered that the lower front plough plane had delaminated from the foam on the inside over about a 6" diameter area. As you will see above, I had to replace the floor panel a couple of years ago for the same reason. I cut out the internal skin and scraped out the foam. I used a new type foam as a replacement - Airex C52.60 (3.7lb/ft3 density modified PU skinned foam. I punched small holes in the foam to allow air to escape when compressing the foam onto the under hull skin.
The replacement foam is 10mm rather than 12mm thick - I've skinned the inside with CSM & woven mat to add some strength (although the new type foam is stronger than the standard PU)
Changed skirt attachment onto splitter bar - it is now attached to the underside of the bar rather than to the front face. the Scout has been prone to water coming up through the fan duct when stopping. It seems to get scooped up by the rear skirt when you start to drop off hover - in theory, attaching the skirt to the underside should deflect at least some of this water. The real solution is to use the skirt vent to lower then nose when stopping - this keeps the hull level rather than the heavier back end dropping first.
After trailering the Scout 220 miles to Ardfern on holiday I discovered the fuel solenoid valve had failed!! This valve is fitted to the bottom of the carb fuel bowl and a spring loaded plastic tipped plunger blocks the main jet pickup when power is removed. I think it is intended to stop unburnt fuel being sucked through the engine when it is stopped (no spark and the governor fully opens the throttle as the engine spins down).
After thinking about this I decided to try to just block to hole in the bottom of the carb bowl where the solenoid was fitted. The local marina didn't have the correct bolt thread (some weird AF thread) so I took apart the solenoid valve and blocked the internal hole using a self tap screw, some Loctite and superglue! I also fitted a length of string directly to the throttle butterfly that I could pull to hold the throttle closed while turning the engine off. It worked!! Not a very professional looking repair but it lasted all week and is still fitted.
I forgot to hold the throttle "string" a few times during switch off but it doesn't seem to make any difference. The engine doesn't backfire or run on and is just as easy to re-start. I am tempted to make this repair a permanent feature (with a proper bolt, of course) as it's one less thing that can go wrong in future
After buying some spare prop blades (and a 4 blade prop for the Prospector) together with a few sets of pitch blocks I decided to do some more experimenting with the Scout pitch setup.
Firstly I re-measured the thrust - it was 92lbs. That's with a 1.7:1 ratio at 3400rpm on the engine (and a 48" 14 degree Ultraprop). I then changed the prop pitch to 15 degrees and got 88lbs thrust at 3400rpm - weird! I'm not convinced the measurements are that repeatable - in particular it's very difficult to get an accurate thrust measurement. I tested the craft on my hilly field - uphill against a very strong headwind it would only manage about 3-5mph (only just making it to the top)
One odd thing I found on the Ultraprop charts (maybe not that odd!) is that the peak thrust for each particular HP rating seems to be at the same RPM on both 52" and 59" prop sizes. I extrapolated down from the 52" to a 48" prop and reckoned I should be able to get around 115lbs thrust in theory (the 52" shows about 123lbs at 15hp). The thrust curve peaks at around 2250rpm for 15hp. I changed prop pulley from 224mm to 200 to give a ratio of 1.5:1 and reduced the prop pitch to 13 degrees - all as per the charts. On a static thrust test the engine now gets to 3260rpm and the thrust is 104lbs. That tells me that the engine is fully loaded (lower rpm) and that, when moving, the RPM should rise to nearer the 3400 rpm I was aiming at. It also tells me that either the Ultraprop charts are optimistic OR the engine manufacturer HP is optimistic (probably both!). In any case, I'm quite happy with this setup. I won't be fiddling around under the engine for a while as I managed to drop a cast pulley on my face (no space to get out of the way)!
The uphill test was much better with the new setup - about 10-16mph all the way up to the top.I replaced the SPA section belt with a higher power QXPA version cogged raw edge type. This belt was slightly shorter than the 2360 length I was using so I had to adjust both the idlers and the top bearing mount. After I had done this the belt got thrown off when the engine was stopping - it did this several times. Eventually I realised that the idler alignment to the smaller top pulley was way out (the pulley had been reduced from 224mm to 200 diameter. I fixed this and that stopped the belt being thrown.
At the Ardgartan Hoverin, Keith Oakley, with his 3D sound equipment, identified a noise hot-spot from the prop where it passed close by to one of the belt anti-flap bars. I decided to move the prop about 25mm further back to reduce this effect. I had to get a longer shaft and couldn't get the bearings off the original shaft so new bearings were ordered. I replaced the shaft with a stainless one. Surprisingly the noise dropped noticeably (a distinctive chop had gone) - thanks Keith!22/2/07
Fitted skirt and trimmed on flat garage floor. Ready for testing over water! I also serviced the engine (full service, valve clearances, new plugs, etc). The old plugs were pretty sooty and black - this seems to be 'normal' for these engines if they are worked hard.
I also carried out a thrust measurement while the craft was outside - it now measures just over 45Kg (about 100lbs). This is higher than the last time I checked it (it was about 34Kg - 75lbs). Since then I've adjusted the governor to allow the engine to reach higher revs (about 3200rpm now) maybe that has made the big difference?
Completed skirt ready for fitting - I've documented the skirt making in Making a Sev Skirt
Flipped craft hull over to carry out a skirt inspection - first time for a long while! The skirt had several cuts and holes in it and also wear marks where the side meets the front quarter. I decided to re-make the skirt rather than messing around putting patches on - a lot of the patches would be on the ground contact point which always results in more wear. The rear corners are also pretty messy due to the various mods I made while trying to get the craft to work properly.
I removed the skirt to discover damage to the rear panel - the exposed section between the two flotation compartments. This is the second time this panel has been damaged - I suspect it's caused by the lower edge catching on the ground surface. I replaced the foam core with PVC foam and re-glassed the entire panel. I also added a vertical stiffener halfway along the panel.
I also found minor de-lamination of the glass skin from the foam where the craft was "bent" when it destroyed the UH18 duct last year. The de-lamination was on the underside of the side plough planes. I repaired it using epoxy/filler and clamps.
I drew up the skirt layout to work out the correct dimension for the rear panel (the plans are incorrect).
The lower tab (that connects to the skirt splitter bar) should be 26.88" wide rather than the 30.5 specified on the plans. The rear quarter skirts now join to the rear panel properly.
After testing again at Berwick I've reduced the prop pitch to 13 degrees (was 14). With the 14 degree blocks the engine was hitting around 3050rpm static - the lower blocks should allow it to rev to around 3400 and will also give more lift and less thrust at lower rpm - useful for maneuvering. I also looked at the back end skirt again as the craft is still dragging the corners on water and is scooping up into the lift duct when stopping on water. I've moved the inner skirt attach point backwards a couple of inches and shortened the flap up to the the splitter bar. I'm not convinced the back end is right - next water test will tell hopefully!
The craft is producing 34Kg (75lbs) of static thrust with the current setup - it is much easier to drive now.
No hovering for the last couple of weeks - other stuff has been getting in the way! I have spent a few odd hours installing the B&S engine into the Scout. I thought I would have to re-build the thrust stand as you are meant to fit the twin engines heads forwards. I took the stand apart and then discovered that the engine I had bought had the two closest holes under the heads rather than along one side (the standard position)! This may be the reason why the engine was cheap!! It was probably a special made for a particular lawn tractor or other machine - who knows? At the end of the day I've just mounted the engine heads sideways - I could just have swapped out the Tecumseh without touching anything else!
While I had everything apart I re-weighed the components:
Hull (including 1/2 full tank and battery = 54.2Kg (119lbs)
Air drive assembly complete with rudders = 64.2Kg (141lbs)
all up weight = 118.4Kg
The 21hp engine only weighed 36.2Kg dry - this surprised me as I was expecting it to be 40-44Kg. Fitted to the craft the air drive weight rose by the expected10.6Kg (23lbs) - a pretty small increase for a large improvement in power!
I have changed the prop pulley size (reduced from 224mm to 200mm - ratio changes from 1.66:1 to 1.5:1) - max prop rpm changes from 2168 to 2400rpm which should give a bit more thrust. I also left off the three anti-belt slap mechanisms as they shouldn't be needed with the smoother twin engine. The engine is much smoother than the single cylinder when running.
I tested the craft (without front mesh guard) in my sloping cut grass field today. The dust was terrible! The craft now powers up the hill from a standing start without any real trouble - I can't wait to try it on water.
After a half hour running it the dust bowl!
After a good wash down
Engine installation with cylinder heads sideways
I got hold of an old Sachs air-cooled rotary engine a local guy had lying in the garage. After looking at it I thought it would make an ideal engine for the Scout - mounted on the thrust stand uprights - lightweight and can be easily upgraded to 25+hp
It is a single rotor Wankel engine with 18hp available and weighs only 21Kg (46lbs). It can be mounted in any orientation as it has a diaphragm carburetor. This engine had no starter so I bought one from an friend of a friend who uses this engine to drive his lift system. I serviced the engine and fitted the starter.
After pouring some fuel in, I clamped it to a bench vice and gave it a couple of pulls - it started on the third pull! It vibrated a lot and filled the workshop with smoke (it uses a two stroke oil mix) before running out of fuel and stopping.
I built a simple thrust stand and clamped it to a pallet. I then ran the engine a few more times but the vibration was terrible - it shook the exhaust loose eventually and walked the pallet across the floor. I discovered that an external flywheel was missing - hence the vibration! Parts for this engine are very hard to get as it was last made over 30 years ago. After a bit more thinking, I decided that is wasn't ideal for the Scout after all. It would be very difficult to re-start the engine on water a the pull start comes out the side - and I couldn't fit an electric starter (about £220!!) as the engine had no generator. I eventually sold the engine to Gary - the friend of a friend who supplied the starter. I have ordered a B&S Intek 21hp off eBay at a good price (for the UK that is!) and will fit that instead
After several emails and pics, Barry agreed that the plans had an error. The depth dimension of the rear skirt panel is incorrect - making it impossible to join to the rear corner panels without deforming the skirt profile. The proper solution is to increase the 10.1" dimension to 15.2" and the overall panel depth from 50.7 to 55.8".
Removed the lift duct because the glass layer had de-laminated at a couple of points internally and the fan was rubbing. The de-lamination was caused by damage when removing and replacing the engine module from the craft for trailering. The duct was in worse condition than it looked so I decide that it was easier to make a new one rather than try to fix it! I made the new duct using PVC foam instead of PU. I've discovered a major problem with PVC foam - it's very hard to sand!
Re-trimmed rear inner skirt corners by adding triangular shaped fill panels. Took Scout to Paxton on river. Water very smooth. One or other rear corner is definitely dragging - you can't steer at times unless you shift weight to opposite side - craft then pops up at rear and accelerates. Made a paper model of the skirt from the plans and discovered that the rear panel to corner panel joint doesn't line up as implied by the drawings. Emailed Barry to ask for clarification!
Just got a Supertrapp exhaust (3S with resonator and 4 discs). It's supposed to be much quieter than the standard exhaust. After fitting it I was very disappointed - it has a distinctive loud bark when compared to the standard silencer - it measures exactly the same sound level. Another failed experiment! I had an old twin cylinder Briggs exhaust (12" x 4" box) so I sealed one cylinder entry and used it instead - it's much quieter than either the original Tecumseh or the Supertrapp systems - although it's a bit heavier. I also replaced the ignition toggle switch with a key switch and replaced the trailer electrical plug and socket (used to disconnect the engine/fan assembly) with a smaller IP65 sealed plug/socket.
Floor repairs I started by cutting out the inside floor glass skin and scraping out the foam core (leaving the outside skin intact). The foam core had sheared near the skids as suspected - it had also compressed in these areas probably prior to shearing.
I then cut out the replacement floor panel from Airex C70.55 PVC foam (even at 3.7lbs/ft2 it's twice as strong as 5lb Polyurethane foam).
I perforated the sheet every 4" or so before laminating it to the existing floor skin (the perforations allows any trapped air or excess resin the escape).
I used weights to press the panel onto the floor skin. The PVC foam absorbs a lot more resin than the PU foam -it has a much larger open cell structure. It doesn't powder like the PU foam though! The major downside is the cost - it's almost twice the price of PU foam! A better plan would be to use PVC foam in high load areas like the floor and front plow planes and use PU foam for the remainder of the craft.
I then removed the resin that had been ejected from the perforations and forced filler into any small gaps between the side and floor panels and re-glassed to top side using the specified CSM/woven mix and reinforced the side seams as per plan.
I probably used around 2Kg of resin repairing the floor - this adds extra weight but does result in a much stronger floor.
You can also see at the top of this pic my brake control - it's a bike thumb gear lever mounted on the steering wheel. Under the dash is a simple single arm lever that increase the travel of the gear lever to fully open the front skirt vent. You can now drive with one hand on the steering and one on the throttle.
Discovered that the floor foam has started to compress and crumble inside the glass skins. It's happened next to where the skids are attached - I assume this is the point of worst shear load when standing on the floor. The glass skins are undamaged at present - the floor just flexes more than it should. I will need to cut out the top floor skin, extract the broken foam, and replace it with stronger stuff over the winter. I can't blame the plans as I deliberately used lower density foam! I've carefully examined all of the other foam panels and there is no damage whatsoever (the floor is probably more prone to damage - mainly from standing on it!).
Replaced rudder control wires and engine throttle rod with push/pull cable - much smoother to operate. Moved brake control to steering wheel - used a pushbike gear leaver with a short travel compensator under bow to get skirt flap to open fully. Also made up nylon bushes for the rudder pivots to prevent the steel bolts wearing through the aluminium support frame.