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skydawg
10-Sep-09, 02:50 PM
Ok so here in the states (at least in north carolina) we have to complete a senior research project to graduate high school, i am now a junior. I have chosen a racing hovercraft build.
My basic design is a 10'6"x6'6" craft although if i could get away with something a bit smaller that would be good too (by good i mean cheaper). I've calculated that for an integrated system i need 5095cfm for lift alone. If this has to be 33% and the other 66% is thrust then i need a total of 15,285cfm from my fan to power my lift and thrust. And since i plan on using a 40hp 8000rpm engine i would need at least 1700ft/s tip speed on the fan.
My dad owns a industrial supply warehouse where they sell fans. I have found a 4 blade 15,300cfm fan but it doesnt show the tip speed. What i dont really understand is that it uses a 1hp electric motor run on a pulley system that powers the fan. If a 1hp motor can provide the 15285cfm that i need than why is it everything i find says i need at least a 22hp engine? would a 1hp engine not suffice if it can move 15,300cfm? please answer this question for me.

My Build: I am using 2" rigid foam insulation for my core covered in fiberglass (6oz?) for structural integrity and a section of steel towards the back to support the engine and fan weight. I have not decided on an engine or fan yet but i have a 500cc 1972 suzuki motorcycle engine that im debating on using. Since i'm on an extreme budget i cannot afford to order rubber coated nylon for my skirt so i am using heavy duty pocho sewn together with vinyl shower curtain for my skirt (segmented). I am using a handlebar steering sytem with a twist grip throttle and tandem style seating for the driver +1 passenger.

Please give me any helpful advice as it is grately appreciated and i wont have to go back and change anything since i havent started the build yet.

By the way it would be awesome if i could use this build for the occasional race

I really like this concept
http://www.originalwonders.com/wp-content/uploads/2008/01/hovercraft-web.jpg

But i think attaching the skirt and providing enough clearance would be a true pain so i plan on sticking with the traditional hovercraft body style

All constructive critism is appreciated

Thanks! =]

curtis5420
10-Sep-09, 04:38 PM
dont use the fan from your dads DIY shop. it will breakup chucking bits of nylon all over the place. me thinks you have your sums wrong if you think a 1hp fan will do the job.

http://www.multi-wing.com/zseries/index.htm

4 z or 5 z depending on the size of the duct.

i think it would be a good idea for you to buy a copy of the hovercraft constructors guide, which will show you a simple hovercraft design, that will fly around a lake.

skydawg
10-Sep-09, 06:42 PM
How much are the z series impellers?

And by the way it is not a "DIY" shop. It is a business that import industrial steel blade fans 48" in diameter to cool machines in factories. they are electric and belt driven by a 1 hp motor and outputs 15,300cfm. an 800 lbs hover only needs 5095cfm for lift alone. this is 33% the other 66% plus that 33% for lift = 15,285cfm.
If you misread it is an industrial steel bladed fan and outputs 15,300cfm.
Meeting and surpassing the requirements. And finally this is not a project to build a "cruise around the lake craft" it is for a fully operational 40mph race craft. Hence i do not need the simple plans. I'm and engineering major and have fully calculated the mathematics and designs for the craft.
my only question was that since this fan outputs 15,300cfm on a 1hp motor, why would i need anything larger? i only need 15,285cfm the lift and thrust the craft.

firth5731
10-Sep-09, 07:08 PM
sounds like you already know everthing

skydawg
10-Sep-09, 07:13 PM
No i just dont get why a 1hp motor and this fan can provide 15,300cfm but when my hovercraft only needs 15,285cfm i need a 40hp engine. Im sure theres some logic behind it, i just want to know it

Keith Oakley
10-Sep-09, 07:15 PM
You haven't mentioned cushion pressure in your calculations so far. Taking your 800lbs weight (including payload) on a craft 10.5ft*6.5 ft = 68 sq ft (ignoring skirt toe-in) that would give a cushion pressure of 11.7lbs/sqft. According to my faded 30 year old copy of the design handbook you need a flow rate (in cu ft/min) of 60*3.5*craft length * sqrt(cushion pressure) which would be 7500 cuft/min. So you need to find a fan that delivers 7500cfm at (at least) 11.7lbs/sqft cushion pressure. - in fact at a higher pressure because of the losses you'll get in the duct between the fan and cushion.
I can't comment on your industrial fans but you do need to look at the flow rate it will deliver at a given pressure. Most craft of this size use between 5 & 25bhp for lift - racers tend towards the high end of that range. Most integrated craft allocate around 20-30% of their fan area for lift.
Personally I'd be a bit nervous about a steel fan because of the damage it might do in the event of a failure but you'd need to check with the Hoverclub of America what their rules are on that.
Good luck with your project.
Keith

skydawg
10-Sep-09, 07:34 PM
Here is the link to one of the fans my father sells

http://www.globalindustrial.com/gcs/group/4661/productsPerPicGroups.web#gridAnchor

As you can see the 48" diameter steel blade fan uses a 1hp motor and provides 15,200 cfm. as you mentioned i only need "you need to find a fan that delivers 7500cfm at (at least)" This fan provides that with only 1hp. Im just trying to figure out how 1hp provides twice of what i need and yet everything ive found say that i need a 20hp engine.

Keith Oakley
10-Sep-09, 07:52 PM
The technical specs your looking at are presumably at no back pressure. You need to know what flow rate they will deliver at (for example) 11.7lbs sqft back pressure. Thats at least part of your conundrum. I'm sure more fan techy folk will be along to explain.
Keith

skydawg
10-Sep-09, 07:59 PM
okay i kinda sorta get what you mean. so in that case i would end up needing like a 25 hp engine at least right? how do you go about calculating back pressure?
And whats the cheapest fan/prop i could get that would meet my requirements?

KipMac
10-Sep-09, 08:14 PM
Skydawg,
You will find the answer to yor questions and so much more if you have a really good look on HCGB website.
You will find so much regarding lift and thrust fan details.
As Keith says, you are only looking at one of the many aspects of lift fan design.
Pressure most certainly plays a major part in considerably increasing the HP required.
Again I would also caution the use of a steel fan.
No one as as far as I know uses such a fan due to the possible danger that could arise for a variety of reasons.
Keep reading and Good Luck.

Kip

skydawg
10-Sep-09, 09:41 PM
Ok one option I have considered is the fan off of an 18 wheelers radiator or the fan off the back of a chicken house( huge!). Opinions?

gavinparson
11-Sep-09, 05:10 AM
Ok one option I have considered is the fan off of an 18 wheelers radiator or the fan off the back of a chicken house( huge!). Opinions?
What, like KFC? :lol:
No seriously, back in the old days people would adapt fans from anything for hovercraft and this isn't junkyard wars. There are plenty of suitable fans and props available that do the job properly. As the very experienced members have mentioned below, there's more to it than just CFM.
A steel fan spinning in free air has no load on it. In an integrated hovercraft it has backpressure, spin up and spin down speed changes, cornering airflow changes and various other factors to contend with. It would probably destroy itself and you'd have chunks of steel like scythes flying around and you'll probably die!
Nylon (PAG type) bladed fans are most robust and reliable and if they break you're unlikely to injure yourself.
I would thoroughly recommend buying something that is proven to work and in a craft that size you'd need a 25hp engine (motor) minimum. Many of us in the UK are getting superb performance from the Briggs and Stratton 35hp so I'd choose that or maybe the new Kohler 40Hp.

If you plan to race then you'd have to use a fan that's compliant with the Hoverclub of America rules. Have you checked them? I doubt that a steel fan would comply. Best check out the rules and speak to the nearest club members.

Ross Floyd
11-Sep-09, 07:20 AM
Having had a shard of nylon fan sticking out of my lifejacket like a throwing knife after a fan burst ( it managed to come thro the guard) the thought of a metal fan scares me witless.

R

skydawg
11-Sep-09, 12:36 PM
Thank you guys so much for all the advice. I will definitley not skimp on my prop. Alot of people in the us use hanson fans. Opinions? It would be very helpful if you guys could give me an idea of the prices for nylonprops? 200 or less?

curtis5420
11-Sep-09, 01:45 PM
why dont you have a good read, thru the stuff on this site, (as previously mentioned) there are uk suppliers that will sell the bits you need!

I ask the question why are you not exploring the US club site, as that surely could give you ideas and suppliers in the US?

also its not a prop its a fan, a prop is designed to run in free air, and works in a different way, to a fan which is designed to run in a duct.

get a copy if the US construction regulations!

gavinparson
11-Sep-09, 03:28 PM
Thank you guys so much for all the advice. I will definitley not skimp on my prop. Alot of people in the us use hanson fans. Opinions? It would be very helpful if you guys could give me an idea of the prices for nylonprops? 200 or less?

I assume you mean Hasconwing?