Building with Craft Foamboard

[Amy jane SeptemberParticipant @amyjaneseptember49770]

Hello again

Hubby (Sam) has taken an interest in your hover craft, and has done a couple off rough sketches of his flymo arangement, just in case it may be of interest? He says you've got it easy, as your hover craft isn't expected to cut long,wet grass as well! (cheeky blighter)

The fan is made from sheet metal, and he says the solid disk bottom helps to deflect the air flow against the outer edge, which keeps it stable. The mower will happily cope with large puddles.

Sam says he hopes he's not being too cheeky!

[Amy jane SeptemberParticipant @amyjaneseptember49770]

[Paul TParticipant @pault84577]

Not the photo that Ashley asked for (modesty forbids)

These images show how the skirt fits to the base of the hovercraft and how it is inflated via the 2 ports cut into the side walls of the lift fan chamber and the sketch indicates air flow.

Model hovercraft differ from their full size counterparts in the way the skirt is inflated and operates as the skirt is more akin to a childs rubber ring.

Amy

Thanks for the flymo sketch if the current set of multi blade lift fans fail to get this thing off the ground I will have a shot at using the centrifugal fans that you are suggesting…Thanks to Sam as well for his input and please tell him there is no such thing as being cheeky when offering good advice.

Paul

 

Edited By Paul T on 18/05/2013 09:09:14

[TelstarParticipant @telstar]

Hi Paul Your Air Flow diag. reminded me of a drawing I had seen in Wiki.

**LINK**

they show things slightly like A J.s thoughts.

Your diag looks as if your balancing the craft on a centeral jet/stream of air from the fan, (a bit like a pogo stick) directing the air round the outer edges gives a broader base.

Cheers Tom

Edited By Telstar on 18/05/2013 11:17:42

[ashley needhamParticipant @ashleyneedham69188]

Paul, in addition, you mat be losing lift thrust to turbulence if the prop in not IN the hole cutout. You may benefit from making the exit hole larger.

A small deflector may be all that is required underneath the craft as suggeted, to push some air backwards.

The centrifugal fan is supposed to be best for hovercraft, but mounting it on your motor may be tricky, as will be getting a good balance on it. A lot of hair driers have this sort of fan..? Me mums Morphy Richards has exactly what you need..perhaps e-bay could be the source of a cheap drier to be dismantled?

Ashley

[ashley needhamParticipant @ashleyneedham69188]

My interest is piqued here. I may just ressurect the SRN4 project, a picture of which is in my album (the prototype)

This used two 64mm EDF units to push air under the hull , and I tried it with NO floor (as per Pauls hovercraft) and I boxed in the chamber later on and drilled holes around the edges to let out the air. Oddly enough, at the achieved low forward speed (solid skirt, mind) the nose tended to dip, and the EDF units were at the front??

I did not notice any difference in lift when the underside was boxed in, or open.

Ashley

[Paul TParticipant @pault84577]

Tried changing the fan from horizontal to vertical with no measurable change in lift also tried different type of plenum chamber but again with no effect.

Tried reducing the skirt internal volume area by inserting polystyrene blocks but no good so I have come to the conclusion that I need a greater compressed volume of air to get this thing off the ground

I have ordered some 140dia multi blade fans from China (no need for brackets Dave) so waiting until they arrive before more testing.

Paul

[ashley needhamParticipant @ashleyneedham69188]

Paul. What sort (other than multi-blade) fan?? link please

Does the hovercraft actually hover,other than being a bit down at the rear ? or does it not get off the ground at all. My prototype SRN4 at 30 by 23 inches or thereabouts, and with a nice foam skirt, positivly leapt off the ground with two GWS 64mm edf (brushed, 300 size motors) units. AND it was made from 3mm MDF, and it supported two 4.5Ahr 6v SLA batteries..

cheers Ashley

Edited By ashley needham on 21/05/2013 13:49:18

[Paul TParticipant @pault84577]

Ashley

The multi blade fan is a computer server cooling fan. The idea is to remove the rotor from its little built in motor and fix it to a 500 size motor.

The model just manages to lift itself and hover tail down, being built from foamboard it is very light so the problem is managing the lifting airflow.

Any other ideas on improving the air flow will be gratefully received.

Paul

[ashley needhamParticipant @ashleyneedham69188]

Could try simply bumping up the voltage a bit to se if the extra power is whats needed??

is the skirt sealing all the way round evenly.. try on water… even just a shallow puddle to see whats what. No other leaks from the skirt/body seal..

tried a bit of ballast?? the odd ounce here and there.

Ashley

[Paul TParticipant @pault84577]

Ashley

I increased the voltage from 9.5 to 12 then 15 and finally a motor burning 24v. Yes the blades turned faster but the air volume didn't increase by much….so its back to the number and pitch of the blades. Telstar (Dave) messaged me with a very interesting idea which I am currently crunching, the results of which could be promising.

Paul

[ashley needhamParticipant @ashleyneedham69188]

I am sure i read somewhere that the problem with using a normal prop is that they do not compress the air enough.. the compressed air (which is what we are creating) leaks backwards through the skimpy number of blades and so there is not as much compression going on as would be indicated from the wattage (or something similar).

I suggest trying a proper EDF unit.. I think a cheap 64mm unit (brushed) as seen on the J Perkins website would probably do you easily for lift….

Ashley

[Dave MilbournParticipant @davemilbourn48782]

I wonder if too much thrust in the wrong place is the problem – bear with me. That big 'ole is forward of centre. My interpretation of a hovercraft is that the air is supposed to 'bleed' out of the fan, fill the chamber between the skirts and lift the craft uniformly on a cushion of air. The air rushing out of that single big hole may be exerting an assymetric downward thrust force which is overriding the air-cushion effect. Don't care if I'm wrong but it just doesn't look right as it is. No offence to Paul's bottom intended.
(Probably rubbish but I have no-one to bounce ideas off now that the fishpond has been filled in……..)
BTW Ashley, that little Jetex Mobo hovercraft used a 3-blade airscrew and yet had no lift problems.
DM

[ashley needhamParticipant @ashleyneedham69188]

Quite right Dave. Most hovercraft models seem to use either a normal prop, or SRN6 types sometimes use split air from just one rear prop.

THATS why i asked for a picture of Paul`s bottom.. as I had realised I had not seen pictures of the undersides of most hovercraft. I have suggested boxing in the lift air a bit…

Reminds me I have an old srn6 type plan at home from an old mag… will fish it out for perusal.

Ashley

[Amy jane SeptemberParticipant @amyjaneseptember49770]

Back in the '60s, Popular Mechanics published plans for a small hover craft. If memory serves, it was powered by a cox 049 glow engine, turning a 3 bladed sheet metal fan in a vertical duct. The air flow was directed through a narrow slot around the entire perimiter of the body, and it didn't have a skirt. Forward drive was provided by a deflector vane and a horizontal duct out the rear. I would be happy to look it up for you, a put on a couple of photos if it is of any help?

Amy jane

[ashley needhamParticipant @ashleyneedham69188]

Very interested, I mean, yes Paul will be very interested. thanks.. Ashley

[Paul TParticipant @pault84577]

Amy

Thank you for the input and your interest in my problem, the model that you are describing sounds like a version of SRN 1, unfortunately this type of requires an electric ducted fan such as those used by the model aircraft chaps and its a piece of kit that I am trying to avoid using.

To add to Ashleys feigned lack of enthusiasm he wont be interested in knowing more about the craft……

The SRN 1 was designed by Christopher Cockerell as a demonstration platform for his momentum curtain principal

The original was built in 1959 by Saunders-Roe under the auspices of the National Research Development Corporation (Hence SRN1)

Thanks again Amy and if you can think of anything else please let me know.

Paul

[ashley needhamParticipant @ashleyneedham69188]

Paul, what a rubbish picture.

i can JUST imagine a glo powered hovercraft (SRN1 style) with no radio, working.. but not having a skirt would present a serious problem for a model, especially using electric power.

To get something as large as the original hovering, especially over an undulating ocean surface was a major technical feat. There were rows of parallel ribs front-back underneath the original.. wonder what they were for? possibly just stiffening .

Ashley

[Paul TParticipant @pault84577]

Ashley

The ribs do look like stiffeners

Following the same principal as the stiffeners on turbine or centrifugal pump casings.

 

 

Edited By Paul T on 24/05/2013 19:46:20

[ashley needhamParticipant @ashleyneedham69188]

Paul. One would have thought that the ribs on the underside would interfere with the cushion of air?

Obviously not enough as it worked ok.

I have found the mag with the SRN6 article. Cant remember (short term mem loss) what mag or date/who by.. BUT.. the underside was boxed in, and the skirt was made from polyester lining material, which is very "porus", and ALL the lift air went into this tube skirt..and the underside was filled with air seeping from the skirt. The chap coated the outside of the skirt with rubber solution so that it did not lose air on the outer surface.

No pictures of it running, would not have thought that this would give sufficient air underneath the craft, but what do I know.

Ashley

[Amy jane SeptemberParticipant @amyjaneseptember49770]

Hi there

I think the book with the plans is down at the crib, but I will be there in a few days and will look it up.It wasn't a SRN 1, but the concept is very similar.

Amy jane

[TelstarParticipant @telstar]

Hi This was a modification suggestion for Paul's existing design, Current posts seem to be heading the same way. (Original drawing copyright Paul)

Cheers Tom

[ashley needhamParticipant @ashleyneedham69188]

Paul. I still think that you need your lift prop in a bit of a DUCT and not just in a hole. Not sure from your pictures if the prop is IN the hole or NEAR it. Would make some difference I think.

Ashley

[Paul TParticipant @pault84577]

Ashley

The prop is in a chamber that feeds air into the skirt and provides the main lift so at the moment it cant be fitted in a tube but I take your point.

Tom's idea has certainly given me food for thought with the air bleeding out of ports in the skirt but I am going to wait until the rotors arrive from China before I start changing the design.

Paul

[ashley needhamParticipant @ashleyneedham69188]

Paul. perhaps not a full duct but I am sure some sort of annular ring could be made to stop turbulence from the prop tips totally destroying the column of air it is creating?? even a tapered funnel arrangement… almost certainly it is worth a try…thin card and hot glue my choice..easy to make and easy to dismantle if of no use.

Is the exit hole in the base the full size of the prop?

Ashley

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