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        Question ESC/Motor/Prop Combo for craft plus 5lb payload?

#1 wideawake Feb 17, 2013 08:45 PM

ESC/Motor/Prop Combo for craft plus 5lb payload?
 
I have to complete a hovercraft project that is required to carry a 5lb payload plus it's own weight for under $150. I was wondering if anyone could suggest a esc/motor/prop combo for the lift fan(s) (not sure if one can do it?) that could produce enough lift? Thanks for any help in advanced!
-Mike

#2 jpcwebb Feb 18, 2013 04:15 AM

The first thing that you need to do is design the platform - the weight is only pertinent to the air pressure which is also a function of the cushion area. You can calculate the required power and flow rate from the circumference of the cushion and the desired lift height, assuming a simple bag skirt design. A quick Google search on hovercraft lift calculations will get what you need, though ignore any associated fan calculations as they will be geared up full size craft and not appropriate for model props. On the basis of a standard 1:10 Griffon 2000 type size model, a six inch prop is usually sufficient to lift around 8 pounds without a problem.

#3 wideawake Feb 18, 2013 01:29 PM

@jpcwebb thanks so much for the reply! Well the project size can't be larger than 3x3 ft and must navigate a 5ft wide course. So I was leaning towards a craft size of about 3x2ft (l x w). So slightly smaller than a 1:10 Giffon 2000 style. It would be a simple skirt design, I was thinking ductless but from my research it may need to be ducted or at least have a few holes in the skirt to let air escape. I'm also unsure of the best size for the air gap to be honest. Also the base I'm leaning towards foam instead of wood for keeping the weight down. Similar to the hovercraft seen here. This guys uses a 1400kv motor, 30amp ESC, and a 8x4 prop. Do you think that would work for my setup also? I just don't want to order a bunch of stuff that doesn't work due to my minimal budget available. Thanks again for your reply and help!
-Mike

#4 jpcwebb Feb 19, 2013 08:03 AM

If the craft is only being designed for a single purpose, i.e. completing a defined course, then your air gap is dependent on the surface in question, which if smooth only needs to be a couple of mm. In terms of the duct, I assume you are talking about for the lift fan. Experience is that with props, if you make a tight fitting circular duct, it helps greatly with keeping the plenum pressure high. An 8 inch prop seems plenty - if anything slightly oversized for the craft size. A four bladed 6 inch prop inside a duct works well for the Griffon so I don't think you'd need anything bigger for what you are describing. Motor wise, you don't need too much power - there is no point in overrating it as it will lead to wasted power and reduced run times. My first Griffon used a Speed 400 Motor! I later upgraded to a 540 size because the 400 would get hot, but found that my run times reduced for no real gain in air flow or lift. An equivalent to a 480 would probably be ideal with a reasonably high kV but low pitch prop.

#5 windnsea Feb 19, 2013 06:33 PM

if the course is over a smooth perfectly flat level surface you could probably get good results using a semi rigid foam edge instead of a skirt - the type of foam sold in hardware stores for expansion joints in concrete. just make sure you keep it perfectly straight and level
a small ducted fan unit is also a good option for lift, just locate it close to the centre with another mounted at the rear for thrust
64 mm gws brand fans are quite cheap and more than powerful enough, they come with brushed motors so you dont need esc's which will save money

I have built a number of successful models this way but they are limited to operation over smooth surfaces -

#6 wideawake Mar 01, 2013 12:27 PM

Ok guys so I ended up purchasing somethings, I have to get a prototype built and tested. Hopefully what I brought will be suffice, what do you guys think?

The Parts:
2x APC 8x4 Props
2x Hobbyking Blue Series 30AMP ESC
2x Turnigy D2830 1000kv Brushless Motors

Still looking into battery options, I'm attempting to use a Nikko 9.6v battery pack from an old R/C car I had laying around. The surface will be flat but may be commercial grade carpet or tile flooring. I want to make it with a skirt, so I bought a soft tarp to use as the skirt material. Still looking into body solutions as I didn't know foam insulation was so expensive.
-Mike

#7 wideawake Mar 08, 2013 04:11 PM

Looks like it wont provide enough lift to support the 5lb payload with the duct I built using foam. I feel like its loosing to much air. Any ideas?
-Mike

#8 windnsea Mar 09, 2013 04:42 AM

if the prop is a good fit in the duct ie only about 2 mm gap
you should have plenty of pressure to lift the 5 lb payload so maybe the skirt is the problem
- are you feeding all the air into the skirt which then exits into the cusion via a series of small holes, or is the skirt sealed ie no flow with the cusion being fed directly by the prop?
also did you build it the size you mentioned above 3ft x2ft

#9 jpcwebb Mar 09, 2013 04:44 AM

It really depends on the design - for instance is the air feed to the skirt taken off from the side of the duct or through a bottom plate, and is the air to the plenum via the skirt or straight through? A photo would help. For tight fitting ducts, the best materials are things like 1/64th birch ply (which I know is pricey) or thin styrene/ HIPS sheet which is cheap as these can be bent into near perfect circular tubes.

#10 wideawake Mar 09, 2013 12:15 PM

Quote:

Originally Posted by windnsea (Post 24368854)
if the prop is a good fit in the duct ie only about 2 mm gap
you should have plenty of pressure to lift the 5 lb payload so maybe the skirt is the problem
- are you feeding all the air into the skirt which then exits into the cusion via a series of small holes, or is the skirt sealed ie no flow with the cusion being fed directly by the prop?
also did you build it the size you mentioned above 3ft x2ft

The gap is larger unfortunately, the first attempt the duct was to large so we shrink it and just started throwing stuff together to get an idea of what our issue was and if it would work. The duct is made of foam so we made a hexagon around the prop at first, then altered that design. This is a very crude prototype atm.

The skirt is sealed with just the one hole where the prop is, which is were the air is forced in. It will support its own weight but as soon as payload is added, you can feel the air being forced back out the "duct". (Pic Below)

Size is 32 in by 20 in. Thank again!

Quote:

Originally Posted by jpcwebb (Post 24368860)
It really depends on the design - for instance is the air feed to the skirt taken off from the side of the duct or through a bottom plate, and is the air to the plenum via the skirt or straight through? A photo would help. For tight fitting ducts, the best materials are things like 1/64th birch ply (which I know is pricey) or thin styrene/ HIPS sheet which is cheap as these can be bent into near perfect circular tubes.

http://img208.imageshack.us/img208/769/img00159c.jpg

Here is a photo which may show you more about what I'm talking about. Again this is a VERY crude prototype we are just trying to get it working before we hone in on making it "look good". Thanks for any help in advanced!
-Mike

#11 wideawake Mar 09, 2013 12:16 PM

Very much similar to this one, just no hole in the skirt, which is later videos was an issue for him.
RC Hovercraft Part 1 (Building an RC Hovercraft) (10 min 46 sec)

-Mike

#12 jpcwebb Mar 09, 2013 12:42 PM

yes, I'd say you need a much better duct to ensure the air isn't forced back through. Bear in mind that props are not really designed for this purpose in that they don't really work very well in static mode and particularly as the pitch increases. Therefore you have to use the close fitting duct to compensate for the lack of pressure performance of the prop. A lower pitch helps to reduce the load on the motor as the back flow tries to counteract the rotation.

#13 wideawake Mar 09, 2013 12:54 PM

Yeah I was thinking the same thing tbh. Any suggestions for how to build a cheap tighter fitting duct? I'm not really sure what to do. I'm debating just using a closed skirt design like it is with like a leaf blower for lift. I know its not a great idea but I have to get a working prototype soon! I appreciate all the feedback.
-Mike

#14 windnsea Mar 09, 2013 03:36 PM

the easiest solution to the duct problem is a trip to the hardware store to find a plastic planter tub the size you need then just cut off the lower section. if you can't find one the exact size get one slightly smaller then carefully trim the prop to fit.there are some really good ones with a nice curved inlet if you look around. the other thing is the airflow to the cusion, with a sealed skirt system you only need to feed a small amount of air into the skirt eg 10%. the rest should feed directly to the cusion
Also the the model is slightly smaller than 3x2ft the cusion pressure will be higher which means higher pressure at the fan - if you increase the size you'll find it can lift the same weigh with much less effort. I hope this helps, once you get the lift system sorted out control is your next big challenge ie to push around a 5 lb payload with an 8 inch prop could be difficult - for what you are doing a larger prop will be much more effective

#15 wideawake Mar 09, 2013 04:39 PM

Hmm thats a pretty good idea, I may swing by the hardware store and give that a try. Thanks for the suggestion. So how would you suggest to setup that? Right now I just have the skirt attached with the one hole that feeds the skirt. So there needs to be air outlets to create a cushion? Sorry I'm such a n00b at this lol.

It is slightly smaller. Teacher said it cant be more than 3x3ft with the skirt so I shrunk it a little to make sure we werent over with the skirt attached. What would size prop do you think one would need to push it? I would have thought the 8x4 would take care of it. Hell it was fast enough to almost cut my finger off lol. You think changing the prop size and use the same 1000kv motor would work?
Thanks again,
Mike


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