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Hovercraft

Design and build a cheapish Hovercraft

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Martian22/03/2020 10:30:43
2485 forum posts
1163 photos

Thanks Simon , the majority of the space between the decks is open with no obstructions the short ribs are to support the top deck the aim is to maintain a constant cushion pressure I will expect to experiment maybe cutting larger holes or introducing them in the middle of the plenum . The pressure in the skirt needs to be constant whereas the cushion pressure do not need to be to high or the skirt will blow out. If the skirt pressure is to weak the craft will be unstable . The cg has to be as low down as possible also to improve stability.

Nigel R30/03/2020 11:16:31
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3750 forum posts
587 photos

Hi Martian, any update on the hovercraft?

I've been watching along yes

Martian30/03/2020 14:09:36
2485 forum posts
1163 photos

Hi Nigel a slight hiccup because of my finger but it is now functioning properly 😎 will be back on it today also am going to design a smaller single motor hovercraft about half size of this and it will be easier to build

Martian31/03/2020 21:12:48
2485 forum posts
1163 photos

Still working on closing the side decking I.ve not tried complex bending of foam board before so I still need to practice the foam board that is available in the UK is the 5mm A1 sheets that are normally available from HobbyCraft sadly closed at the moment. Any way this can be worked into bends by first removing one of the glossy sides either by peeling the top layer after damping then rubbing with your finger to remove the under layer sometimes it comes off in one go. Flite test sell a different type of foam board the has easy peelable outer layers. Then foam can then be dragged over the edge of a table whilst holding down with the other hand open palm it can be work in more than one direction essentially what is happening is the foam is being crushed where needed. Whilst I will be still working on the craft posted here I am going to make a single motor craft. The motor is mounted at the rear as a thrust motor part of the thrust generated is diverted by a splitter plate at around 20% to 25% of the thrust duct area I'm going to 3d print the duct complete with the motor mount. There are some general rules on length to width ratios which are usually width at just over 50% of the length this provides stability fore and aft. Whereas a craft can be made without a skirt it will not have much hover height and only be suitable for a smooth floor. Below are pictures taken from my light hovercraft handbook Fig a is an open-loop air is blown in through an opening in the bag skirt and allowed to bleed into the plenum chamber maintaining a positive pressure the hole size is achieved by experimentation to high a bag pressure make the bag stiff and the obstacle clearance is poor > this is the type of skirt I am using, the skirt material being polyurethane coated nylon the material used for kites.

the type b skirt is a finger skirt complex to make requiring a downward feed of air it is not very efficient. Type c is a bag and finger skirt generally regarded as the optimal but not in the way the air is fed in from the top it is really stable and effective when fed through the thrust duct via a splitter plate. very difficult to make unless you are a skilled seamstress and not really suitable for a model .

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Martian03/04/2020 19:53:18
2485 forum posts
1163 photos

Decided to hold fire on the big Hovercraft and build a half size one -- son of hover.

this one can be made from 2 sheets a1 foamboard or 1 if scaled down more. this is a single motor craft using a Turnigy 2830 1300kv, I will test various lipos. First up I cut out a piece roughly half a sheet 590x430mm and then marked out on the cutout piece the centre line then marked out 70mm from the front edge and sides and a line 40mm from the back edge. at the rear I made a cutout to accommodate the thrust duct with integral motor mount this was 3d printed at 130mm diameter .

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The thrust duct set in cutout at about 30%

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Thrust duct with motor fitted and temporary prop I will use a cut down 5 inch prop to keep the spacing tight at the side of the duct.

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40mm high duct cradle

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40mm high inner side walls which will have bottom deck glued to

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Martian03/04/2020 19:54:43
2485 forum posts
1163 photos

Duct sitting in place ready for glue

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Martian06/04/2020 20:00:15
2485 forum posts
1163 photos

Moving on or Hovering on,

Rudder parts the rudder is an STL file from a 3d ugly stick but it does a perfect job

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Duct bonded in place and splitter plate extended with a smoothed of leading-edge I can soften glue to adjust if needed. (at this point my glue gun overheated and went pop it was only cheap and glue had leaked out and created an insulating jacket around the heat tube) there is also a rudder bearing plate at the bottom

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glued and sealed underneath

the bottom deck will cover thisdscf8326.jpg

Rudder mounted using a 1.5mm piano wire through the top of duct and rudder then through bearing plate to the bottom of the duct . Using a ball joint to the top-mounted servo that will be bonded to duct

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Top view

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More soon waiting for new glue gun

Martian10/04/2020 20:11:20
2485 forum posts
1163 photos

Well, it's all but finished just the skirt to make or codge together somehow. The prop is next to useless so I am searching for a high-performance 5inch 3 or 4 blade one which might be a drone prop I don.t know anyone have any ideas it needs high thrust ?

please feel free to comment good or bad

some more piccies

Lower deck and rx esc and lipo box cover not fitted

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Servo mounted at the top of duct works very well

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Holes in the sides to feed the skirt

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Frank Skilbeck11/04/2020 08:07:03
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4684 forum posts
101 photos

About 16/17 years ago we built the Griffon Hovercraft using 2 x 600 brushed motors running off 2 x 6 cell 2,000 mah sub C Nicds. We had the lift motor on an aircraft ESC operated by the throttle stick and drive fan used a boat esc (forward and reverse) using "elevator" stick.

The skirt was made from lightweight ground sheet material, sewed together and glued on, it was fully sealed apart from a couple of ports into the lift duct to inflate it.

It actually worked well on water and if the surface was pretty smooth we could drive on and off the water.

Simon Chaddock11/04/2020 09:49:30
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5692 forum posts
3026 photos

A very neat build but I fear as a "single motor craft" there will be an issue.

A skirt needs "pressure" to inflate and resist the weight of the craft whereas thrust is obtained by airflow and mass where the actual pressure generated is tiny.

Just an observation.

Nothing wrong with experimenting. It will be interesting to see the result

Martian02/05/2020 21:06:48
2485 forum posts
1163 photos

Finished the little one and it hovers yet to give it full beans.

Simon you were right the prop would not give sufficient pressure then I remembered from my days around light hovercraft all the British ones had multi-bladed fans and the Americans used massive props a bit like their airboats so I tracked down some stl files for various fans and 3D printed some once I settled on the best I scaled it up printing it slow with maximum detail trouble is it got that much traction on the air it pulled the Collett of the motor shaft so I switched the motor round and used the bolt-on shaft. but it did work and now it will take higher rpm .The fan is 11 bladed and with the duct is somewhat like an EDF . The skirt although rudimentary work very well so hopefully in the next few days I can wind it up I see how it performs again. I should add since taking these pics I have increased the duct length to fully enclose the fan 

 

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Edited By Martian on 02/05/2020 21:08:30

Eric Robson02/05/2020 22:58:58
177 forum posts
32 photos

Hi Martian, don't know if you do this already but can you vary the deflection of the airflow so that when there is enough to lift it more can be diverted to the rear for speed?. By the way the fan looks a lot better than the two bladed prop, keep your fingers clear!

Martian03/05/2020 18:52:17
2485 forum posts
1163 photos

Thanks for commenting Eric. I could do that Eric but it does not warrant the time. There is plenty of lift now and speed is not a problem the problem is directional control in light hovercraft you lean into a turn with the rudder. but this needs three rudders I,m pretty certain the bigger hovercraft will have loads of control

Gary Manuel03/05/2020 19:06:48
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2255 forum posts
1548 photos

Martian

I know absolutely nothing about hovercraft(s?), but that won't stop me from commenting.

The rudder looks as though it will deflect the airflow from the EDF a little, but maybe not enough to push the rear end of the hovercraft around enough to steer it. Have you considered using "louvred" rudders, so that the majority of the EDF thrust is affected, rather than just a strip in the middle?

By "louvred" I mean having maybe 3 separate rudders spread across the diameter of the circular fan outlet.

Martian03/05/2020 19:53:54
2485 forum posts
1163 photos

Thanks Gary you are right as I stated in my comment above ordinarily in a light hovercraft body movement dictates the sharpness of the turn obviously that's not possible so accurate lateral balance is needed plus as you suggest at least 3 rudders of even better thrust vectoring

Christopher Morris 202/06/2020 16:10:14
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169 forum posts

Will be watching this with interest, i am just starting one myself purely as an experiment. One thing i found that was interesting. Many intakes for the lifting motor have a large flange like a doughnut with a flat bottom. This acts as a wing & when the air is sucked in over the edges, it creates a low-pressure area above & with the high-pressure underneath & creates lift. So the answer to all problems is doughnuts.. LOL but its true.

Martian02/06/2020 20:52:11
2485 forum posts
1163 photos

Hi Christopher welcome to the Hover area you can post your progress on here if you want or do your own build log and yes a donut is an answer to everything LOL I can always make modifications if needed.

I have been in the doldrums for a while ridiculous I know with all this build time I keep trying to get back in the grove well you all know how it is. Anyway, I have made progress on both Hovercraft the small one is complete and works well you can see from the pics the rudder arrangement has been changed also a new duct with a slight rearward down angle, now the kids are back at school (not mine ) that frees up a large flat tarmac area and I want to video it in action. The larger Hovercraft known as Large lol requires the skirt making and fitting I have printed a large fan blade that seriously moves some air I had fitted it using a compression fitting on the shaft at half throttle the fan lifted itself off the shaft and went spinning up in the air about a meter it was fascinating to watch it spinning in the air before it slowed down enough to lose lift. I am unable to change the shaft around to fit a screw adaptor so I am in the middle of fitting another motor. Some pics So onwards

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Martian02/06/2020 20:54:41
2485 forum posts
1163 photos

Missed one the EDF with swivel duct

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Eric Robson02/06/2020 21:27:01
177 forum posts
32 photos

Good to see you modelling again Martian,I see you still have 2012 on the camera. I hope it goes better for you this time, beware of flying fans.

Cheers Eric.

Martian02/06/2020 22:07:13
2485 forum posts
1163 photos

Thanks Eric , I can't be bothered to reset the date every time I remove the card it's just easier that way

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