PDA

View Full Version : Will this pitch be enough?



Flaring Afro
11-Jun-09, 03:36 AM
Hi. Im currently planning my first hovercraft. In a quick summary, it will be shaped with foam and then fiberglassed and will have 2 16" ducts with an elevon system. Havent bought engines yet but Im fairly sure thrust will be about 15-20hp. Hopefully it'll look similar to this but for 3-4 people.
http://4wings.com.phtemp.com/crafts/image/hovercrafta2_6.jpg


I was planning on maybe getting these propellers for the thrust on amazon.
http://www.amazon.com/Electric-Propeller-16-x-12E/dp/B000BOCBGQ/ref=sr_1_114?ie=UTF8&s=toys-and-games&qid=1244686076&sr=1-114

And something like this for the lift.
http://www.amazon.com/4-Blade-Propeller-11-9/dp/B0006O35L6/ref=sr_1_6?ie=UTF8&s=toys-and-games&qid=1244686253&sr=1-6

(forum rules arent too clear on links, so mods edit them out if they arent)

Anyways, the thrust ones are 2 blade, 16" and have a 12" pitch. The lift is 4 blade, 11" with 9" pitch.

I know higher pitch = less acceleration and higher top speed but I cant find anything online on what pitch is generally used, at least not for 2 small fans. I have found 30+ for large ones but I assume 2 smaller will be just as good? Or are these propellers designed for model planes not powerful enough? Thanks.

atters
11-Jun-09, 06:11 AM
A bit Square for my liking, and doesn't look very practical.

For a first time project I would go for something already available and that is going to work.

Get plans and stick to them. !!!!!!!!!!!!!!!!!!

You have a great idea here but for a first time I would go another route. OK maybe I am not the person for this as my first attempt sank..

BUT I did learn that the advice given on this forum is worth its worth in GOLD.

And one of the things mentioned to all mewbies is, STICK TO THE PLAN.

I am now on my second build after 5 years of happy hovering with an odd problem here and there just by following the advice given here.

Jon Pert
11-Jun-09, 07:58 AM
Welcome http://www.jonrb.com/emoticons/wavey.gif

I have to say I don't think 15-20hp will be enough to be honest!

How much will the craft weigh (approx)?

ETA: Cracking username by the way. http://www.jonrb.com/emoticons/hehe.gif

kevthehover
11-Jun-09, 08:40 AM
"You have a great idea here but for a first time I would go another route. OK maybe I am not the person for this as my first attempt sank.."

First Hoversub by Atters ..lol :Dhttp://www.hovercraft.org.uk/images/icons/icon10.gifhttp://www.hovercraft.org.uk/images/icons/icon10.gif

Flaring Afro
11-Jun-09, 02:18 PM
I have to say I don't think 15-20hp will be enough to be honest!

How much will the craft weigh (approx)?

ETA: Cracking username by the way. http://www.jonrb.com/emoticons/hehe.gif

Im not sure on the weight, I guess the 3 passengers would be 150 pounds each plus the 2 motors and clear platics vinyl for windows and fiberglass (epoxy) body... Those propellers are fiberglass too so it should be pretty light? It will also mostly be used on pretty calm water.

Next week i'm going to a local guys place that has a lawn mower and lawn tractor junkyard. I was going to see if they had any with horizontal shafts (which did exist from what i've read) but thats why I guessed it would have 15-20 at thrust.

Also that image is just a model. It wont look exactly like it, simplified a bit in structure, but will lift up on car hatch shocks (like an X-Wing i guess). My friend and I want to build our own since thats part of the fun for us, as a hobby. Oh, and yeah I have a red afro hence the name lol.

KipMac
11-Jun-09, 02:25 PM
The only pitch you want is enough to pitch it in the trash bin.
Yet another daft design that does not have a cat in hells chance of working.
Over the years we have seen many such totally impractical fantasies where appearance is all and reality is left way behind.

I strongly suggest you spend sometime looking at the wealth of information available on this forum .
There is an accumalation of countless years of experience here.
I also suggest you do a lot more research on the basic physics of the hovercraft principle.

We never used 16" ducts 50 years ago let alone today, and 20 hp for a 3-4 seater!!!!!

IF ONLY.

Regards,
Kip

Jon Pert
11-Jun-09, 02:41 PM
The only pitch you want is enough to pitch it in the trash bin.
Yet another daft design that does not have a cat in hells chance of working.
Over the years we have seen many such totally impractical fantasies where appearance is all and reality is left way behind.

I strongly suggest you spend sometime looking at the wealth of information available on this forum .
There is an accumalation of countless years of experience here.
I also suggest you do a lot more research on the basic physics of the hovercraft principle.

We never used 16" ducts 50 years ago let alone today, and 20 hp for a 3-4 seater!!!!!

IF ONLY.

Regards,
Kip

The man speaks sense (bit harsh but fair http://www.jonrb.com/emoticons/wink.gif)!

If you wish to build then I would seriously recommend buying a set of plans. Build exactly to the plans and you will get a hover that actually works and that you will have instant gratification from.

Trying to build a craft from scratch without much knowledge will only end up in an unfinished project neglected in the corner of your yard.

Please don't take these as negative comments, only realistic ones. We have many members who have built from plans (Sevtec, ASV etc) and have fantastically capable craft and as Kip says this site has a wealth of information available both in the articles published and with the membership. There is an archive of our magazine going back some 35 years and more forums available to club members with information and tips.

If you have anymore questions then please ask away!

Flaring Afro
11-Jun-09, 02:53 PM
Na I dont take offense easily, and I'd rather someone be harsh than me build something that could never work. I'll look into plans more closely to see if any site has something very appealing.

I was wondering though, why are most hovercrafts 1 thrust propeller (or fan) instead of two in an elevan system? The elevan seems just as easy or easier and it has reverse and sharper turning, right? Also, if I did 2 duct propellers, what would be about the minimal diameter then because its really hard to find any designs to model that have 2 smaller instead of 1 big.

curtis5420
11-Jun-09, 04:09 PM
are you making a toy, for scale model people? or do you really want to fly this thing?

the only way those model aeroplane props will push you along, if is you change your name to luke and use the power of the force to aid in propulsion!

3-4 people, assuming not made of plastic in a craft made of an upturned foam meat tray, look more at craft with eccess of 50hp for thrust and 15hp pluss for lift. the design of the craft and its size will dictate the engines you require.

John Robertson
11-Jun-09, 04:10 PM
Na I dont take offense easily, and I'd rather someone be harsh than me build something that could never work.

In that case I'll have a go as well :D

I would suggest that you buy plans and use them to construct a platform for your creation. You shouldn't make any attempt to re-design the lower hull, skirt system or the thrust and lift systems - build them EXACTLY to plan and they will work as specified. You can do whatever you like with the top of the hull provided you keep within the overall weight limits for the design.


I was wondering though, why are most hovercrafts 1 thrust propeller (or fan) instead of two in an elevan system? .... Also, if I did 2 duct propellers, what would be about the minimal diameter then because its really hard to find any designs to model that have 2 smaller instead of 1 big.

Some basic thrust theory:

Thrust is air volume x velocity

Increasing the amount of air OR the speed at which it is "thrown" backwards will result in more thrust - pretty obvious really :)

The amount of power needed to do the job:

Power = 0.5 x air volume x velocity^2

From the above is should be obvious that increasing the air velocity causes a large increase in power to develop the same thrust when compared with increasing the volume of air.

So....
Larger (moves more air volume) or slower (accelerates the air less) fans/props are significantly more efficient than smaller. high velocity fans/props. The performance hit of using a small fan/prop is huge which is why you don't see twin fan/prop craft.

It is possible to make a practical 3 seat craft with only 20HP but it needs to be very carefully designed and built and you would need extensive hover-engineering knowledge and experience to have any hope of it working (and, I doubt that many on this forum would even attempt it!).

This is an example of such a craft that you can even buy plans for:
3888