Building an Integrated hovercraft, Need fan help!?
Okay so here in my section of North Carolina we have to do a senior research project in order to graduate and i have chosen building a full fledged racing hovercraft ( I aspire to being an aeronautical engineer ). My hull material is made of Rigid FoamFoam Insulation board covered in fiberglass. And since I’m on a tight budget I’m using heavy duty poncho and vinyl shower curtain for my skirt. I am using a 1972 Suzuki t-500 dual carb two stroke motorcycle engine that has 47hp @ 7,000rpm. My biggest question is on the fan. Obviously a hason wing fan would be ideal but $200 dollars is a little bit out of my price range so I’m looking for alternatives. It needs to be some sort of composite material so that if it flies apart at 7,000 rpm’s it wont slaughter me (hopefully) and it needs to be cheap and easy to find. What I’ve been considering lately is a radiator fan from a tractor trailer. It needs to be 36 inches (at least), so I’m wondering how large an 18-wheeler’s radiator fan is?
You have a good idea there, but I don’t think that is going to be big enough, nor use the available power.
A radiator fan is designed to move a large volume of air but at a slow speed and low dynamic pressure. Even the fan in a large diesel only uses about 2 horsepower from the diesel to power itself, because the fan is considered a drag on power and so only the minimal power needed is used to move air across the radiator.
If you have 47 hp you want to use a fan that can convert all that hp into moving the most air possible. A propeller from an ultralight aircraft is closer matched to your horsepower production. If you don’t have the money for a decent aircraft propeller you might want to get a template (look online for plans to make your own prop)and make one out of a knot free piece of spruce or redwood. Wood is easy to shape and balance and you can can get the right diameter and pitch to use all of that 47 hp. Plus wood really takes vibration and shock loads well and won’t fly apart at those rpm’s
I don’t know if you can find the horsepower versus torque curve for that suzuki, but it is better to run the engine at the peak torque rpm than the peak horsepower. On a two stroke it is quite a bit less usually. Possibly at 5500 rpm it might get peak torque and hp of around 35. It is safer to run at 5500 rpm of course and with the high torque you can cut a blade with a bigger chord which deflects more air downward than a narrow blade with a higher rpm. You waste a little more in drag losses, but the efficiency of moving the air goes up.
Most engines peak horsepower is kind of senseless because the slightest load on the motor pulls the rpm’s right back down and of course the horsepower drops immediately with the loss of rpm’s. It takes torque to push thru power when there is a load on the engine. The more torque available, the less the drop in rpm’s and maintainable horsepower