Building with Craft Foamboard

[Paul TParticipant @pault84577]

Hello Ashley

The exit hole is 150mm (same diameter as the rotors) I take your point about creating a tube and I will give it a go tomorrow.

How is the cold getting on?

Paul

[ashley needhamParticipant @ashleyneedham69188]

Paul. Dragging on.. I am a bit "chesty" now, and see that a reoccurrence of the last coff i had looming,, took weeks to go.

I predict a noticeable increase in lift with a tube.. Having the tips enclosed I am sure will increase the effective pressure in the P chamber. You could also, if using the vented skirt method, try adding additional underfloor vents to release some extra air, in case there is too much of a pressure build up affecting the performance of the lift prop (for experimentation).

Ashley

[Paul TParticipant @pault84577]

The thrust tube for the lift fan made no difference so I am back to the original thought that to achieve the greater volume of air I need a multi bladed fan.

All stopped now until the rotors arrive from China.

Paul

[ashley needhamParticipant @ashleyneedham69188]

Paul. Did you try venting the chamber in case too much back pressure is your problem?? or is the bottom open still..

Ashley

(you can get pills for that,you know)

[Paul TParticipant @pault84577]

Ashley, I do have pills for that and I can confirm that my bottom is open.

[ashley needhamParticipant @ashleyneedham69188]

Went to the science mus on WEDS. Noticed hovercraft prototype model,, had a fan driven by model glow engine, but with of stack of three two blade props to make a fan.. ends cut down to fit `t hole.

Ashley

[Amy jane SeptemberParticipant @amyjaneseptember49770]

Hi there

Here is the Popular Mechcanics hover craft.

Popular Mechanics Do it your self Encyclopedia, volume 1 1968

regards

Amy jane

[ashley needhamParticipant @ashleyneedham69188]

Wow !! Like it. Decent size.

Wonder how many people made one?? and did they work as per the article!!

Ashley

[Paul TParticipant @pault84577]

Rotor arrived from China and has solved the problem

[ashley needhamParticipant @ashleyneedham69188]

Nice. Red….from china !! handy 12v or whatever motor (dc) as well??

Ashley

[TelstarParticipant @telstar]

Hi Paul, How's the progress with the new fan, has it improved the hover, or is the model now in L.E.O. I have been playing with a 65mm ducted fan fitted with a "400" size motor on 12V.

On a flat punt tethered to a spring balance the developed 700g pull.

Just finished tests with the same unit fitted with an unknown brushless motor on 12V and managed a pull of 900g It sounded like a demented vacuum.

Hope to hear from you

Cheers Tom

[Paul TParticipant @pault84577]

Hi Tom

The new fan is super and performs very well, I had toyed with using a ducted fan but I wanted to keep the construction simple.

I bet you had fun testing your unit and I know what you mean about sounding like a demented vacuum my new fan sounds like a cat in a tumble drier. (not that I have ever tried that) I will have to devise a method to test the thrust.

All the best

Paul

[ashley needhamParticipant @ashleyneedham69188]

Testing lift thrust may be tricky. In free air, no doubt normal propellor would generate the most "bollard" pull (per amp)…EDF apparently are best at high speeds and dont give out their best thrust stationary.

BUt, we are trying to generate essentially compressed air in a hovercraft and I thought that a box with a hole in it letting in thrust air and having a water tube manometer would do..BUT air is vented from under a hovercraft.. and how much to let out in a test rig?

Cockerils original model had a fan made up of a stack of (I think three) air props on a glow engine, cut down to the tunnel size.

Turbulence in the plenum chamger will also have perhaps a big bearing on the efficiency of the lift generated.

Ashley essentially suggesting a suck-it-and-see approach…

[Paul TParticipant @pault84577]

Ashley

Started the suck and see approach (or should it be blow it and see) to measure the lift fan performance but without much success.

Did take some photos of the inflated skirt.

A nice close up of Janes stiching and you can see the door seal bump strip that runs around the entire edge of the hull.

Paul

Edited By Paul T on 07/06/2013 15:33:06

[ashley needhamParticipant @ashleyneedham69188]

A picture of it hovering over water may be instructive, to see if the air is coming out more in one part than another?

I had bought, ages ago, some prop adaptors for speed 400 motors, and a few JP 5×3 three bladed props, Now, these stack very nicely on the adaptors, there being a lot of thread on them and of the correct size, and two look very mean..giving a six bladed fan. This is the look I wanted on the A90 Ecranoplan..the A90 was fitted with contra rotating props. Unfortunately the shaft on the Park 370 motor is not 2.3mm so I cannae use it.

IT APPEARS, on a hand held test, to produce MORE frust using a speed 480 motor than the 6×4.5 on a common 540. As I have said before, you need a lower pitch for better frust at low forward speeds.

I may, just out of interest, make a round platform, with a circular tunnel to house either setup..make a rudimentry foam tube skirt, and see whats what (powering the motors with a simply flying lead). Might be instructive to see the difference. I believe I have a sheet of foamboard lurking in the loft !!

Ashley

[TelstarParticipant @telstar]

Hi Ashley Your quote "BUt, we are trying to generate essentially compressed air in a hovercraft and I thought that a box with a hole in it letting in thrust air and having a water tube manometer would do..BUT air is vented from under a hovercraft.. and how much to let out in a test rig?"

These are some of my thoughts on your remark.

Firstly if we take the weight of the craft (in grams),

And the area of the base of the craft (in square centimetres), this area would be the “footprint” inside the skirts contact with the ground.

Divide the weight by the area to give pressure (grams per square centimetre).

Now Ashley’s water manometer reads mm of water and 1mmH2O = 0.1g/sq cm approx.

This should be the minimum pressure required to lift the craft.

So if we fit the lift fan to a closed box, with a manometer fitted this could give the first check to see if the fan is powerful enough.

Secondly if we measure the perimeter of the footprint (centimetres) this is the distance round the craft where the skirt touches the ground (when inflated) and multiply this by the required ride height (say 0.5cm) this would give an approximation of the size of the “losses” aperture in sq. cm.

Now if we cut a hole in the box the same area as the loss aperture and run the rig again, this would show if the fan can provide enough volume of air to maintain the minimum lift pressure once the craft has lifted off the ground.

This is purely speculative. perhaps Paul can shed some light .

cheers Tom need to lie down

[ashley needhamParticipant @ashleyneedham69188]

Tom. WOW !!! need to print that so I can digest such a thesis!!

Good work, I knew there was someone out there with the knowhow to apply my vague theories!!

I can not fault that as a course of action. Or at least, as a starting point.

There is a saying in military circles.." No plan survives contact with the enemy"; I just have that feeling…

Ashley

[Paul TParticipant @pault84577]

Tom

Your speculative theory seems to have hit the nail on the head and I will attempt a practical application later on today.

Thank you for your help.

All the best

Paul

[Amy jane SeptemberParticipant @amyjaneseptember49770]

Progres?

[Paul TParticipant @pault84577]

Hi Amy

Sorry for not closing the story.

After a little tweaking the new 150mm computer cooling fan is working very well and producing more than enough lift for the model.

My grandson has 'tested' the model and I am happy to report that the foamboard chassis and computer fan both survived.

Paul

[Amy jane SeptemberParticipant @amyjaneseptember49770]

Hi Paul

Thanks for the update, I was curious as to how you were getting on. Sounds that all is good!

[Brian WiseParticipant @brianwise57018]

Hi Everyone,

I read that you are experimenting with lift fans for hovercraft and would like to suggest this link which may help!

**LINK**

Regards,

Brian Wise.

[Author]

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