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Old Jan 02, 2014, 09:44 AM
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New RC Hovercraft build - Now complete

Hi guys,

I am building my first ever RC hovercraft from scratch for a school project.

I have decided on the size (70x40cm), but have a few questions I need answered urgently.

1. Air gap: What size should I try and make this if I want the hovercraft to run over flat surfaces and water?

2. With an air gap of 5mm (no idea if that is correct for my stated size), my calculations are:

Cushion Pressure: 105N/m^2
Escape Air Flow (air lost under bottom of skirt): 0.14m^3/s

Using a standard off the shelf propeller, could this craft actually work?! Is it possible to check whether a certain propeller will meet these conditions given only the diameter, pitch and number of blades of a propeller?

If you have any advice please could you let me know as soon as possible as I have a deadline to meet in the not too distant future looming over me!

Thanks a lot,
Charlie.
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Last edited by Charlie.; Mar 22, 2014 at 04:55 AM.
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Old Jan 03, 2014, 05:55 AM
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Hi there,
the size you have chosen (70 x 40 cm) means that you should build your hovercraft lightweight, try keeping it under 2 kg if you want to run it at high speed (over the hump) on water. On smooth and even ground, i.e. tarmac or flooring, it doesn't really matter, then much higher weights are possible. Then it only depends on the necessary pressure build-up by the lift motor and the fan or propeller.
Model hovercraft don't have a substantial pressure build up underneath the skirt, so the area covered by your propeller is the same area you will have as a gap (circumference multiplied by the gap height) - easy to work out.
If you want to go faster, or go up ramps, then you need to have sufficient thrust power. Use double the power on thrust as on lift.
Remember that we are talking about static thrust, as the hovercraft is hardly moving through the air at speed. Most standard propellers have too much pitch, therefore use propellers which are used for quadcopters, and the three-bladed ones are ideal for hovercraft. You can use these propellers for lift as well.
You can find the model SR.N6 I built here: http://www.hoverpower.de , unfortunately only in German.

Good luck with building, and keep your model LIGHT!
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Last edited by mluthard; Jan 03, 2014 at 06:28 AM.
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Old Jan 03, 2014, 08:05 AM
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Thanks for your reply and very nice build you have there by the way! Used google translate on your site and it translated almost perfectly!

So essentially what you are saying is that as long as I keep it lightweight enough, pretty much any lift propellor/fan would work? It surprised me that the lift fan you used in your SR.N6 is pretty small in relation to the rest of the craft.

Also what do you think about using a couple of EDF units say 50mm size for the thrust? They provide more than 500g thrust and weight under 100g. Could also just use one larger one which may provide a better thrust:weight ratio.

Thanks again,
Charlie
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Old Jan 04, 2014, 07:23 AM
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Also for this model size what do you think would be a suitable propeller diameter? Does it matter where on the hull of the hovercraft the lift propeller is located (ie should it be near the front, middle or rear of the hull or does it not matter).

I am designing the body now so need a size to work off.

Any help would be much appreciated!
Thanks,
Charlie.
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Last edited by Charlie.; Jan 04, 2014 at 07:32 AM.
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Old Jan 06, 2014, 05:42 AM
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Hi Charlie,

The lift fan I used is only 10% larger than to scale, because I did not want to spoil the looks. As for using EDFs in hovercraft, they are built for producing a big amount of airflow when your model plane is moving fast through the air, hence the high pitch of the impeller blades.
If you check the impeller I used for lift, you will notice that the blades of this special fan are adjustable, and I set them at a very flat angle, so the pitch is only 1/3 of the impeller diameter.
For the size of craft you are building I would recommend a three-bladed propeller with 10-15 cm diameter and low pitch for lift.

You may use EDFs for thrust, but take care of the overall weight of your craft.

Cheers, Martin
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Old Jan 06, 2014, 06:22 AM
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Thanks Martin!

I have designed what I want the body to look like, not entirely sure if it is achievable but I'll give it a shot!

The duct for the lift fan is 18cm at the moment as I wasn't sure how big it needed to be and I thought it would be better to make it bigger initially as I can always just make it smaller. The construction will be mainly foam with marine plywood where necessary.

Would you say that the bigger the lift fan I can fit in the model is better, or that the range you listed 10-15cm is more than enough?

I have attached some pictures of my proposed design. Haven't added everything to it such as the receiver, ESCs etc and I still need to work out how to attach the skirt! Please tell me what you think and if there are any glaring problems please let me know.

Thanks again,
Charlie.
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Old Jan 08, 2014, 09:18 AM
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Hi Charlie,

the design looks interesting! Reminds me of th eDe Lorean from "Back to the future" As for the lift propeller: If it is only a two-bladed one at such a big diameter then you might not be able to build up a good pressure reserve, three-bladed, low pitch, running fast ... that is what you need. It will also lower the Amps needed and give you more running time on the batteries.

As for the design, try building it completely out of foam, and just cover it with any self-adhesive model airplane iron-on foil.If you use the iron's heat carefully on yellow, green or blue foam (see building stores for that), it will not melt underneath and give you a super finish. Remember you are building a craft that is going to fly. The skirt will be able to act as your bumper!

Oh yes, and make the impeller inlet areas larger, so that only the impeller itself has the narrowest diameter. Otherwise you will loose thrust power in the narrow intake tunnels.

Cheers, Martin
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Old Jan 10, 2014, 05:54 AM
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Charlie, just noticed that you didn't show any detail on how you will get the air into your skirt (bag type presumably). Will you have a hollow body and lead part of the air through to the outer rim?
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Old Mar 22, 2014, 04:52 AM
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Finished Build

Thought I'd update this thread as I have now finished the build of this hovercraft.

Here is a video of the hovercraft in action (I have since added lights to it!)

https://www.youtube.com/watch?v=0Cvg...eature=mh_lolz
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