paner

Hi, I'm building a hovercraft which is 6ft 10in x 4ft peice of 1/4in plywood. Its powered by a 25cc 2/4 hp weed whacker engine which the prop will be directly mounted to. Im lookin for a 24 x 10, 12, or 14 inch blade but I don't know alot about pitch types for my application. I know smaller pitch = less torque but more airspeed and large pitch = more torque but less airspeed. In theory the craft will be frisctionless but I'll probablly have a smalle amount of it with my hackjob. Therefore would more torque give me more volume of air to push giving me more rough terrain abilities? On a flat surface would I be able to move faster with a lower pitch with higher RPM?

Thanks in Advance.

greyhound man

My Feedback: (5)

I believe most 25 cc engines like that can only swing an 18 or maybe a 20 inch prop on a good day. Most 50 cc engines only swing 22-23 inch props and with my Evolution 58 i'm swinging a 24-10 at 6900 rpm's. Also I would assume that you will need a reverse pitch prop to be used as a pusher prop, not impossible to find...but not common. The higher the pitch the faster the speed as long as the engine can swing it with out bogging down

ICE_MAN

What he said ^^

Whats the expected weight? A 6ft Piece of ply is not a light load for a 25cc wead eater. Is it RC? Or do you plan on riding it? If you plan on riding it I would expect much of anything out of a 25cc motor.

3W 80

paner

Hmm, I'v been looking at plans and it calls for a 24 inch blade but it doesn't specify pitch. It uses a weed wacker engine in the diagram which could range from 25-30cc I think. This guy claims he can get up to 25 mph riding on it since its near frictionless and the 30cc 420 cfm leafblower engine i have mounted should be more than enough to lift me(170lbs) the board(10-15) about 20 lbs of engines and maybe another 12 lbs of various hardware.

[email protected]

Does anyone have a short video of a model hovercraft. Needed urgently for a school class lesson


Colin Stdney australia

ICE_MAN

OK Paner,

I just re read your post.. And I think you may have ptches confused. A Higher pitch (12, 14 etc.) will yield a higher airspeed, and less torque. It usually won't push as much air static and will require a shoter blade that a low pitch to aquire comparable RPM's. A Low pitch prop (8,10) will flow more air and produce more torque at slow speeds. It will also use a longer blade.

So If you have seperate engines, one for lift, and another for foward thrust. You definately want the torque of a lower pitch prop on the lifting engine. Also the foward thrust engine might do better with a low pitched prop as it will accelerate faster. I'd be affraid that a high pitched prop wouldnt have enough torque to get you up to speed fast enough.

Cheers!

DKjens

My Feedback: (50)

Why are you even using the terminology torque? Torque is force x arm, and has nothing to do with propellers. A large diameter, small pitch prop develops more thrust at a lower speed. A small diameter prop, large pitch prop develops less thrust, but a higher speed.

What purpose will the engine serve? Will it lift the vehicle of the ground, or will it propel the vehicle forward, or will it do both. For lifting the vehicle of the ground I have no idea what prop would be best. I would think it would be all about moving as much air as possible and not so much about the speed of the air you move. For propelling the vehicle forward, since you will get nowhere near the top speed of a 10" pitch prop turning 6,000 rpm (approximately 100 mph), again I would think you need as small pitch as possible.

I seriously doubt a 25-30cc engine will be able to lift you very far of the ground, so I would guess this vehicle will only work on a very smooth surface, such as on roads.

DKjens

pizza

My Feedback: (24)

in my experience with hovering, I choose a prop with a flat pitch that will allow the engine to reach the manufacturers max rpm quickly. now in order to attain this in most cases on 2 stroke you need a tunned pipe to add power. for example my os46ax reaches its max horsepower at 15,000 rpm. 12x4 I get 14,500 on a tunned pipe which is about as close as I can get. this give me a nice wide power band, explosive pull out when needed for quick adjustments and this large diameter flat prop spinning at a high rpm acts like a giro to lock in my plane in verticle axes = all this makes it much easier to hover and torque roll. I see many pilots try to hover with an on/off throttle switch rather than a smooth linear throttle stick.

the roto 25 gas engine has a manufactures suggested max hp rpm of 8,000, I would put on a tunned pipe and try some different props with a 5 or 6 pitch untill you hit 8,000.

you may loose a little verticle performance and speed with this setup but it will be the best for hovering all day, good luck