Hover bike - what is the minimum propeller rpm required?

[gordon13]

Hello, I read this article about someone who made a hover bike prototype from a motorbike and two propellers. Best part is, its classified as Ultralight and doesn't require a license to fly!

I can totally see myself building something like that in my back garden!

How can I calculate the minimum rpm required for two propellers (which I probably have to build myself) to produce enough thrust to lift the airframe and payload?

 

[phinds]

How can I calculate the minimum rpm required for two propellers (which I probably have to build myself) to produce enough thrust to lift the airframe and payload?

Your question is taken totally out of context and has no answer it is so vague. How heavy is the craft and payload? how big are the props? How efficient are the props? How effective is the skirt? Dude, you've hardly even STARTED to ask the questions you need to answer.

Also, is "fly" really quite the right term for a hovercraft? "Guide" seems more reasonable since they don't go very far off the water/ground. I wouldn't think you could "fly" one down the highway without the cops having serious objections.

 

[Unrest]

If you can get it classified as a bicycle it will be allowed on the road :P

Hey don't forget that 100 years ago people were making aircraft in their backyards. Using crude motors, wood and cloth. It's not really that hard if you have perseverance, foolhardiness and live on a farm.

 

[gordon13]

Ok, well obviously I have to make assumptions. I am just looking for guidance on how to go about calculating theoretical values.

for starters let's say the total mass including rider is 300kg. With propellers around 1m diameter each.
From your post I see, I need to find out more about the propeller.
So essentially I need to design my own. Theres a java applet that looks like it will be useful for that:
http://www.mh-aerotools.de/airfoils/javaprop.htm

Also its not a hovercraft, it actually flies (according to the people designing it that is..):
http://inhabitat.com/crazy-hoverbike-is-capable-of-flying-to-10000-feet/

Heres a spec sheet:
http://www.hover-bike.com/specs.html

I realize my original question was vague. I think I know what to do now though. I am going to focus on designing a propeller with the above engine specs in mind. Ill come back with more detailed questions at some point I am sure.

Unrest -- Yeah exactly! I don't think Ill be using it to go to uni/work or anything of the sort anytime soon, though. laws and regulations wouldn't allow it. but I think it could be a nice project, and a ton of fun in the country side, if it works.

 

[spanky489]

If you can get it classified as a bicycle it will be allowed on the road :P

Hey don't forget that 100 years ago people were making aircraft in their backyards. Using crude motors, wood and cloth. It's not really that hard if you have perseverance, foolhardiness and live on a farm.

lol you forgot to add that most of them died while testing them xD

 

[potatoecannon]

To answer your question,

The spec sheet you linked to lists the engine power at 7500 rpm and specifies a "custom" gear reduction of 1.5:1, so assuming the engine is operated at peak power:
---> 7500/1.5 = 5000 rpm prop speed

As far as stability goes, I'm not sure if the prop rotation will be sufficient to provide enough gyroscopic restoring force. Since they're still only performing short tethered tests, the anchor cables will be keeping it level while the engines keep the tethers tensioned.

For your weight assumption...if their design uses kevlar/carbon fiber/foam (and CAD design), and you are wanting to develop a "backyard" version, you might end up a little lower on the power/weight ratio...

edit: Keep in mind that prop design is done to keep the tip speed slightly sub-sonic to avoid shockwaves. Designing your own (without experience) can be...very dangerous. A little imbalance, or material imperfection (or design flaw), and suddenly you'll have a catastrophic failure happening about 1ft away from you.

 

[Unrest]

To answer your question,

The spec sheet you linked to lists the engine power at 7500 rpm and specifies a "custom" gear reduction of 1.5:1, so assuming the engine is operated at peak power:
---> 7500/1.5 = 5000 rpm prop speed

Here's a similar thing which seems to be at a more advanced stage of development, but also not a commercial product yet. Says 7058rpm, which I guess is the propeller speed.

http://martinjetpack.com/technical-information.aspx"

These guys apparently made a deal with an (unnamed) aircraft company last year to produce them commercially for military and search & rescue, but nothing seems to have come to fruition yet. Still, look at the impressive videos! Tho with dummy pilots :P