View Full Version : Mirage build - redone thrust frame

profqwerty - n/a
29-Sep-07, 07:01 PM
Hi. I thought I'd conglomerate my various threads into just the one.


I am going to mount the lift engine like this to see what happens. It seems to fit nicely - I will put additionaly heat shielding between the cylinder head and the polystyrene. if anything the extra metal work might strengthen the front planing surface a bit.

I have a set of 3z blades for the (now) wrong direction - how well do they work in reverse? can reverse direction ones be bought?


447 on its frame - i will bolt the rubber mounts to the fibreglass with penny washers on each bolt, then put some nuts +washers on to lift them up.


the flexy joint.

I took the engine to Bill Sherlock this morning to remove the old gear. I randomly came across his website - I had no idea how he was connected!http://hovercraft.org.uk/images/icons/smiley_icons/icon_smile.gif

Now selling poly belt reduction drive (check for sale forums!)

More tomorrow *hopefully*


tonybroad - n/a
29-Sep-07, 10:37 PM
you'll need the correct direction blades, they don't work in reverse, the only issue with mounting the lift engine below the fan is access for servicing and what happens to fuel lines and electrics when the blades shatter - just a thought

the rest is looking good


Keith Oakley - n/a
30-Sep-07, 09:13 AM
Plus is the lift engine underwater when afloat?

profqwerty - n/a
30-Sep-07, 09:31 AM
Blades shattering shoudln't be an issue? There would only be two wires and the fuel line, so these could be put through a metal tube if necessary.

Water was the main issue. I am going to put essentially a tank around the engine. Not sure how though. The hot parts are high up, so any plough-ins shouldn't get it too wet! Thin ally sheeting riveted then resinned should do the trick i hope.

Exhaust is the other issue - silencer would have to be in the plenum, but how would it be affected if i put an extension onto the silencer output? I won't be just letting the exhaust gas go into the plenum though.

bryan - n/a
30-Sep-07, 01:03 PM
I noticed you are using a centerflex coupling on your 503.Thats fine but make sure you belt is alot tighter than normal. On starting your engine it may not pull through because of osolation between the engine and fan.I have had this problem in the past! I use my mirage for racing and cruising and as a F1x yours would be great. However putting that lift engine near salt water may need some more thought. Great hull looking good.

Keith Oakley - n/a
30-Sep-07, 04:47 PM
Don't forget that serious amounts of water as spray can come in through the fan. Your tank will need to cope with that.

jon_curtis - n/a
1-Oct-07, 04:58 PM
for all the hassle its worth, and the potential problems you will see with the engine mounted in the plenum. why don't you just get a set of blades the proper rotation?

hovercad - n/a
1-Oct-07, 05:54 PM

Im no expert but always look on the downside.

How close would the engine be to the hull.

In the event of hitting something at the front planing surface would it make contact with the engine.

A causing damage to the engine resulting in no lift

B causing the engine to move damageing the fan resulting in no lift

C causing the hull to split rather than flex.

Just a thought


profqwerty - n/a
1-Oct-07, 07:39 PM
Yeah these are all very serious issues that do need addressing.

The bottom/front of the engine will be pretty close to the FPS, so hitting anything solid head on would probably knock the engine (ofcourse wrecking the hull too) making the fan hit the duct.

if crashing it would either be a slightly damaged hull or a total wrecked front.

of course with careful driving (yeah right) there would be some decent advantages - more stability, and much neater.

One possibity i looked at was hanging the engine frame from the existing mount points - essentially doing it normally, then turning it over.

I had the day off of work today, so did some more - thrust engine mounted. Unfortunately my pc broke - a fan blade somehow snapped off so now it won't run (hopefully not a foretelling of things to come!). But therefore no pics.


jon_curtis - n/a
2-Oct-07, 07:14 PM
i can't see how there is any advantage mounting the engine like that! not sure how it will effect the stability, and as for neater, you won't be saying that when the engine gets wet, and you have to take the fan to bits to get to the engine. its all together a more complicated solution and far more clunky.

As for hitting something, don't think you realise how much that front planing surface could deflect, even from a plough in. how do you fancy a heavy landing pushing the engine up, this would probably pop the fan!

all for the cost of a set of fan blades! if not cut down, you might even swap them?

Don83000 - n/a
3-Oct-07, 01:03 AM
Quote :- Unfortunately my pc broke - a fan blade somehow snapped off so now it won't run.

http://hovercraft.org.uk/images/icons/smiley_icons/icon_lol.gif http://hovercraft.org.uk/images/icons/smiley_icons/icon_rolleyes.gif You should have mounted it above the CPU not under it.

profqwerty - n/a
3-Oct-07, 11:54 AM
I have changed the design slightly - instead of being sat on the bottom, it will have a frame, and will hang from the 4 mounting points on the hull.

It would be as if it were on normally, then flipped over. I'll put some seat belt over the vibro-mounts incase one pulls apart.

Without it's mounting plate, there would be about 3" of space (at least) between engine and hull. I'm making the fan boss, and it will have to be in 2 pieces as I don't have a large enough bit of metal - These would be tapped + bolted together so fan removal isn't tooo difficult!

the fan blades - I'm not bothered about buying a new set. But if anyone does have a reverse set they're willing to part with do say. I've got a 22" duct (my blades are still uncut though).

jon_curtis - n/a
3-Oct-07, 06:26 PM
you may very well have to wire lock the fixings on your two bits of boss, as you would not have a visual locking device.

Hornet - n/a
7-Oct-07, 09:20 AM
Craft looks good so far, but just a question - how are you going to access the pull start or start the engine?

Mounting the engine in the plenum could be good for looks and for maybe a lower centre of gravity.

But its going to be a real pain if you need to access the spark plug or carby and if you get water coming in through your air feed holes or over the deck its going to drown the engine.

profqwerty - n/a
7-Oct-07, 06:40 PM
These are all issues that would need addressing!

I am happy to fiddle with it to the end of time, so I think i will TRY it, and put the craft through it's paces.

I can fiddle with it to try and iron out the problems, but if it is really impossible I can turn the engine the normal way up!

The frame I've almost finished is reversible (H shape with legs), so would support the engine inverted or normally using the same fixing points.

As for starting it - feed the pull start through some metal pipe from the engine to the cockpit.

profqwerty - n/a
10-Oct-07, 07:10 PM
OK, insane update here! the new bit for my computer got lost in the post, so i gave up and snapped the fan blade opposite the already broken one off. seems to work:p

Firstly the thrust engine mountings. my M10 bolts wouldn't go through the hull with enough spare for a nut, so i produced these:


a bolt welded on:


then these sit on the hull floor, with just enough poking through for a penny washer + lock-nut!


two clean strips of metal that can simply be welded onto.


found this which is peeeerrrrrfect for clearing drilled bits of fibreglass. It doesn't have enough power to suck up bolts/nuts/washers, so is ideal really. unfortunately i have no other bags, so i have to reuse the existing one by emptying it and re-stapling it again!


PS sorry for the c**ppy pictures - a lot of the work is done after work and the darkness mucks up the camera http://hovercraft.org.uk/images/icons/smiley_icons/icon_rolleyes.gif

profqwerty - n/a
10-Oct-07, 07:43 PM
These flat plates will be welded onto the strips:


they have studs welded into them (max power top make sure the studs stick! each one is welded, then ground down, then put under high tension, welded + ground again.


an engine mount on the plate:

locknuts onto those studs do nicely.


mounts+plates welded in place. bolted down tight. can turn over the engine and the flexi coupling looks to be flat!


a random one to put here, but this is the back of the duct. I was planning to cut off the back of the cone, leave the hole, and add a rudder mount in there.

>>Is this the right way???


profqwerty - n/a
10-Oct-07, 08:28 PM
making the fan hub

luckily we found a 4" dia SOLID mild steel rod. it was about 4' long and weighed waaaaaay too much.

it took about 10 minutes to cut through it on a cold cut saw

made one HELL of a racket.


one of the technicians where i teach used to be a workshop lathe operator - so he was awesome at making these kind of things. did it for free aswell!

It took ages to remove all the unwanted metal, but a very nice finish.


Took a while and many trail +errors, but he managed to create a damned near perfect taper in it!


The other side slots into the ring (6mm flat plate - cut on a 150mm hole saw!). the idea being the main boss bit is permanently attached to the engine, and the ring is attached to the fan hub with all those botls. then there are 4 bolts that hold the ring onto the boss (because we didn't have any sufficiently large dia rod to start with).



ring in the hub:


and assembled. there are no holes in it yet; drilled it later.


profqwerty - n/a
10-Oct-07, 08:52 PM
Drilling the ring. clamped it on the drill press and lined up +drilled each hole individually.


MEGA e-kudos points / e-pints for the first person to name this tool clamped to the drill press:


the final fan hub in place!


profqwerty - n/a
10-Oct-07, 09:06 PM
right. i DID try the engine inside the hull. however, you were right it does not fit well. it was oh so close! but i decided eventually that it just wasn't as good as having it hte right way.

This is the engine frame I made. the legs are the rubber mounts extended. however with it the correct way up the legs are gotten rid of and it looks quite neat (no pics yet)


they may not be needed, but i spent ages making the damn things so here's a close up:


mounting points for the engine on the frame:


side shot - yummy welds!


how it looked - it looks quite good i thought, just the mechanics were stupidly awkward.


got the day off tomorrow - so should get some more done!


hoverchaps - n/a
10-Oct-07, 09:26 PM
How you going to start it ???????

profqwerty - n/a
10-Oct-07, 09:28 PM
Just to clarify:

I have done a major u-turn, and the lift engine will be mounted NORMALLY. with engine on TOP of fan.

so the pull start will be easily accessable. I just haven't got any pics yet of this.

also i have recently painted the frame red.


PS - i have ditched those legs, so the frame is directly attached to those cotton reel mounts, which are in the recesses on the lift duct. so wobbliness there, although i know what you mean - there actually wasn't any as they were M10 studs through 15mm of metal at the top and done up TIGHT, so no movement there.

I couldn't think of much else to do tbh. There are 6 bolts altogether thru the hull. I was planning to put some seat belt over the frame joints (and the lift flexy mounts).

Mild steel bolts - plated with shiny stuff. normal ones really. I'm gonna paint it red hammerite (along with everything else - it's the only colour i have and there's loads of it).

the boss uses they're M8 bolts. it would take qutie a bit to break those!

jon_curtis - n/a
10-Oct-07, 09:35 PM
dont want to put you down, your obviously having fun with the engineering http://hovercraft.org.uk/images/icons/smiley_icons/icon_razz.gif but not sure relying on bolts welding to the plates through the hull is sufficient. also you cant see the weld if a fracture occurs, all you will know is when the engine departs the floor! are they stainless bolts?

its funny how different people look at solving engineering problems in different ways, but i would have joined the hub with 6 bolts not 4, also you could have got away with the main body of the hub in 6082 ally, would save weight.

not sure the lift frame wont be a bit wobbly in that sorta shape! the legs are certainly looking a bit iffy! adding to the possible wobblyness!

Don83000 - n/a
10-Oct-07, 10:45 PM
http://hovercraft.org.uk/images/icons/smiley_icons/icon_eek.gif Sure looks like weight saving is not on the build menu http://hovercraft.org.uk/images/icons/smiley_icons/icon_lol.gif

Nick Long - n/a
10-Oct-07, 11:11 PM
Dividing Head


profqwerty - n/a
11-Oct-07, 08:02 AM
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Dividing Head

is the correct answer!!

how'd you know that?

It's not THAT heavy...

hovmart - n/a
11-Oct-07, 08:25 AM
dividing head, all your parts look a bit on the heavy side, will work but weight is inportant

profqwerty - n/a
11-Oct-07, 08:28 AM
Ok, For the weight, what would people recommend changing?

Within reason I can chop and change bits.

However I do not have much ally at all, but have loooads of steel kicking about:p

Nick Long - n/a
11-Oct-07, 09:25 AM
<table border="0" align="center" width="90%" cellpadding="3" cellspacing="1"><tr><td class="SmallText">Quote&#58;</td></tr><tr><td class="quote">
Dividing Head

is the correct answer!!

how'd you know that?

It's not THAT heavy...

You have to know these things when you're a hovercraft driver.

I got myself a lathe and milling machine a few years ago and have been looking for one. I've concluded though that I'd be better off with a rotary table as I'm limited on clearance.


jon_curtis - n/a
11-Oct-07, 05:13 PM
well the fan boss body, could have been ally.

those strips with the bolts welded on, could have been individual for each mount, with bolts going right thru floor either side then the studs welded in for the isolation mounts, shim the mounts up with ally if the height is not there, this would save couple of feet of steel?? doing away with the strips all together.

why didnt you use the studding thru the floor? i would have welded some nuts on the end of some studding to make long bolts. (not ideal but better than the bolts welded on) the frame structure from the engine to the mounts could be lighter, but your trying to get the mounts further apart than if just sat on angle from the engine mounting points so perhaps its worth it, the engine will be more stable on the mounts.

your lift fan frame, is not only the wrong shape but its too much metal. its likely to have side to side vibration issues due to its shape, it needs to angle out towards the mounting points.

like a four legged star fish, with the engine in the middle. this is easier with the engine outside the plenum, as you can cross under the engine with tube. i would have fabricated a sub frame from steel tube, bit of pipe bending etc to get a supportive structure. i will sketch something if i have time this evening and post to see what you think.

bit jealous of the workshop you have access to, lathes etc. is that your work?

profqwerty - n/a
11-Oct-07, 10:15 PM
I can't really change the fan boss now, and i have no ally!

I didn't have any studding at the time (and still don't!), so it seemed like a good idea. (those studs are bolts cut up...)

The issue was not having a bolt long enough, although i was thinking maybe weld 2 bolts head to head, but again, the bolts I have loads of, none seem to be of the type needed to fit well (if you see what i mean?!!)

I guess I will have to go shopping for some threaded rod.

I do quite like having the mounts further apart than on the engine studs to increase the stability.

An engine upgrade is quite likely at some point, so might it be better to just leave the strips, as i can easily grind off those mounts when the time comes; a bigger engine = moer weight, so as a % weight they wouldn't matter too much?

I (used to) teach there. however I finished on wednesday to prepare for a newer, better job. bit of a bummer as they have tonnes of bits of threaded rod lying around in stores. humph.

Surely as long as the frame is rigid, it's the rubber mounts that do the flexing? The angle is 3/4"x3/4"x3mm, so is not exactly heavy. however it's quite strong!

The welds (e.g. on the bolts) are definitely strong enough (touch wood)!

profqwerty - n/a
24-Oct-07, 09:46 PM
a bit more progress. concentrated on the trailer a bit as it needs moving off the drive before the rain comes, and it's using looooads of space which the parents aren't too happy with.


I don't know what it was before hand, but i bought it as a car transporter off ebay. You could put your foot through the ply in some places, and taking the ply off, half the welds were unstuck. it looked like somone had done it without gas on the newer ribs.

I re-welded the c**ppy ones, and you can see the wood difference. The middle stripe is the top of the old deck. The lower light bit is one of the panels turned over. The top bit is the deck I am adding. I have put wood preserver - chestnut coloured - all over the new bits, and the old deck will stay to sort of protect this deck from underneath. I removed the existing mounting bolts, and will put longer coach bolts through both bits of wood (the deck is 36mm thick!).

The damper in the hitch is also dead, so that's another story.


I also now have loads of this stuff - i can never remember what it's called though.


well. this is the existing lift fan mount. I haven't lightened it cos at least it's something that sort of works - at the moment!

This is the only way the exhaust can go without cutting it. Is it acceptale like this sticking up so much? kind of like a battering ram, althought not as bad as it seems. the ally thing is there to deflect the exhaust whilst testing. unfortunately the output points directly at the hull so I will put something onthe end to deflect it (like a drinks can).


rudder, 6mm birch ply. I was asking how to taper the trailing edge at work and the technicians didn't seem to think it was necessary with the airflow I would have...?

anyway, 3 hinges for strength. It will have the steering fitting inside the hull.


a bit overkill maybe, but the rudder is removable which should be useful.


the cuprinolling station. that stuff gets EVERYWHERE when it's sprayed (i loaded it into one of those weed sprayer things).

the engine hoist came free with the trailer. needless to say it's broken too - the top half is bent.

profqwerty - n/a
26-Oct-07, 04:59 PM
ok i've finally done something about the thrust frame being obese etc:


The larger holes are from the previous frame. I will seal something over them. The bottom part of the hull doesn't have those holes, just M10 sized ones!

They are M8 threaded rods going through out the other side.

jon_curtis - n/a
28-Oct-07, 07:32 AM
your not going for a balanced rudder then?


your frame dont look to bad in, but i would have made it more this shape!

little bit less metal and more stable sideways.

regards your engine mounts, 3 or 4mm thick ally spreaders under the mounts and on the outside of the hull would be a good idea.