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Feb 08, 2021, 08:09 AM
Boaters are nice people.

Single motor skirtless effort.

Thank you for the elaborate answer!

I'm aiming for the simple two flaps on the rear to start with, though I see the benefits of having flaps on all four sides and have control over it like on a quadcopter.
But, I'm already making my life 'complicated' by trying to fly Tricopters, using KK-boards as a flightcontroller.

When ram air speed makes the front wanting to lift, I understand the slanted down front on Ralph's construction.

I added some pictures of my bottom plate and rounded ducts, I milled out of foam a number of years ago and dusted off last weekend.
From what I've read over the years after cutting it, the inside edge should be rounded off too, as 'air doesn't like sharp corners'.

Yesterday I did a hairraising test with a 3536-910Kv outrunner turning an 11 x 4,7 prop on 3S.
Clamped firmly in the vice it created a minor dust storm in my workshed that wanted to relocate anything that would move in the turbulence...

I'm planning a duct, at least as high as twice the foam thickness to increase efficiency and reduce bleed off, as it's just a two bladed prop. Should be a nice job for my router, as soon as I can work outside again without getting covered in snow.

Your thoughts please.

Regards, Jan.
Last edited by pompebled; Feb 08, 2021 at 09:41 AM.
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Feb 08, 2021, 11:32 AM
Boaters are nice people.

Just checking.

Before I start cutting, sanding and glueing, I want to check a few things:

- When rounding the sharp 315 angle on the top of the bottom plate, this more or less solves my issue of having a wideing exit, as taking off material will widen the top of the plenum.

I have a couple of different router bits and would like to use one of those to make a uniform rounded edge.

- The air intake with the 11" prop will be 615 cm. If memory serves me well, 1/3 of that should be the plenum surface, and 2/3 used for propulsion.

With a total plenum length of 228 cm, that would mean a slot at the bottom of 9 mm wide.
Does that sound about right?

Regards, Jan.
Feb 09, 2021, 06:52 PM
Registered User
I experimented a bit with variable width peripheral jets on a servo and found the width doesn't actually matter very much. I'd avoid very narrow (less than 5mm) or very wide (20mm+) but in the range of 5-15mm there was no difference in hover height or stability. I've been using 10mm for all my builds, nice neat number.

I also found no difference in having a rounded 135* transition compared to the sharp 45* bevelled edge. We know from aircraft that air at this scale behaves differently to full size aircraft, I suspect a similar effect is at play here for the way air doesn't seem to care about rounded vs sharp transitions. The airflow inside these craft is all sorts of turbulent anyway so I suspect the 'extra' drag round sharp corners means nothing in practice. I tried a build with rounded transtions, lots of streamlining inside etc and it behaved exactly the same as an indentical model built with sharp corners and blunt surfaces.

What I will say is you don't want anything outside of the outside of the peripheral jet slot (if that makes sense). I'll point to this sketch I made in the other thread about slot designs I tried. So you don't want that flat bottom surface outside the slot as you have on your photo.

I've used a 3536 1000kv and 10x4.5" on 3S successfuly in a past model. If anything it was a bit under-propped. You're better off with a lower diameter 3 blade prop than a larger diameter 2 blade. Instead of 11x4.7 2 blade look for a 10x5" 3 blade, Master Airscrew makes one. 2 blade does work but 3+ blades is better for holding the pressure back.
Feb 11, 2021, 04:59 AM
Boaters are nice people.
Thanks FiftySlicks,

So I've made a lot of dust for nothing... Guess I'll use the parts anyway, as they were made.
The flat surface outside the peripheral jet slot wasn't intentional, but remained after the curve was machined out, I'll slim it down according to your sketches.

I'll get a three blade 10x5, I have the 11 x 4,7 at hand, hence my testing with that one, smaller would be better, as the 11"needs a huge 28 cm hole in the top.

While I'm at it; would a ducted fan like this work?
I know ducted fans are designed for airplanes and need a high airflow for optimal efficiency, but the 14 blades should hold the pressure better than my current two blade.

I've seen something similar on a large Mark Porter design on a German forum and it had no problems inflating the skirt, with 1/3 throttle on an outrunner motor.
Or wouldn't this work due to the fact a skirtless design needs less airpressure but more volume to stay afloat?

Regards, Jan.
Feb 11, 2021, 09:22 AM
Registered User
I've never tried an EDF for lift. Just looking at it that's a 70mm diameter fan disc on a plenum with dozens of times the area. Compared to a propeller that is maybe only 1/2 the area. EDFs are also extremely power demanding. I don't see it working but I remain to be proven wrong.

There's a compromise going on between swept blade area, number of blades, and blade speed. Two blade props are completely viable, my most successful skirtless craft have used them. 3 blade gives better solidity. EDF takes that to the extreme and that's not a good thing.

Low blade pitch is vital. I've tested this out, I suspect the propeller is always a bit stalled in operation, but too much pitch draws lots of power with little or negative effects. I compared a 10x3.8" prop against a 10x6" on the same craft/motor/battery. The 10x6" increased power consumption by 40% or so and actually reduced hover height and thrust. That extra power was uselessly churning up air and turbulence in the plenum. EDFs are even worse, you have very small blades with comparitavely high pitch, makes sense once the fan unloads and unstalls at high forward airspeed, but seems the worst choice for skirtless hovercraft lift. I reckon you can get away with it on skirted craft since the volume/power demands are far lower.

I have a skirted craft that can easily handle 5kg+ of weight on a 6" prop drawing 50W of power. So the requirement can't really be compared to a skirtless craft.

What I'd like is a 10x4" 3 or 4 blade prop for testing. Or 9x3.8" 3/4 blade. They don't exist. All the multiblade props at that diameter have too much pitch. Makes sense as a 10x4" 3 blade wouldn't suit many aircraft well.

I've been toying with printing some centrifugal fans for testing. There's no way I'd put 300W+ through one, would suit<100W power draw quite well I think. Dual counter-rotating centrif fans on a smaller craft may be interesting. Centrif fans are in theory better for hovercraft lift than props, but again we get into full size fan performance not matching up to what we're doing.
Feb 11, 2021, 10:40 AM
Boaters are nice people.
The 70 mm is just an example of what's available, the larger fans tend to get pricy though and when the pitch doesn't comply with the needs of a skirtless craft, it seems like a waste of money.

What If I were to make a rig to hold the hub and use heat to deform the root of the blade to lower the pitch?
The rig makes sure each blade gets the same lower pitch and the prop stil runs true.

Or have two three blade 6" or 8" props on the same shaft, forming a 6 blade prop?
Balancing somthing like that may be an issue though...

I do have a spare fan from my car that works like a charm, but seeing the 140 x 70 sized fan on my bottom board makes me think I need a bigger craft, on 12V it moves a lot of air and I have a large outrunner sitting idle...
The weight of the brushed motor will be an issue regarding the weight high up, with a smaller outrunner this should be less.

Regards, Jan.
Last edited by pompebled; Feb 11, 2021 at 11:12 AM.
Feb 11, 2021, 11:28 AM
Registered User
I wouldn't get obsessed over blade counts. 2 or 3 blades at low pitch on the right motor works fine. Better to focus on construction and control of the rest of the craft. You can always adapt another motor or prop later on.

Edit: I would shift the motor/prop nearer to the middle of the base board. Having it too close to the back creates a lot of issues as I found out on one of my earlier attempts.
Feb 12, 2021, 06:08 PM
Boaters are nice people.
That circle on the bottom section came from balancing the 3635-910Kv motor and 4S Lipo, so it would stay level when supported in the middle.

Looking at your designs, I had my questions on having the prop so far back, with that confirmed, I'll move it more towards the center and worry about balance later.

Did some test glassing on the output ducts to see if that will reinforce the curve enough to be able to remove the flat section that would mess with the plenum.

Regards, Jan.
Feb 13, 2021, 05:54 PM
Boaters are nice people.
After the epoxy has set, it looks like one coat of epoxy with the light 24g/cmglasscloth does a nice job in smoothening the surface and reinforce the foam once the excess material is cut away on the outside of the slot.

I think I'm going to cut the slot pieces to fit and pre-glass them before fitting them on the bottom plate.
The outside temperature needs to get a bit higher, as it slows the hardening process substantually...

Regards, Jan.
Last edited by pompebled; Feb 18, 2021 at 05:21 AM. Reason: typo
Feb 28, 2021, 05:17 PM
Registered User
Twin motor skirtless hovercraft - now with more flaps! (5 min 56 sec)

Octagonal hovercraft - third test (2 min 42 sec)

Two recent builds.

Twin motor one is the best hover height I've ever got. It could go a bit further with alternate props but I'm on the margin of stability. Control is entirely through the vents along the sides and rear. Very good agility though forwards thrust is a bit lacking for reasons I think I can sort out.

Octagonal craft is closer to the ideal circular planform in terms of efficiency (least perimeter for cushion area). Stability is again a bit of an issue. It originally had only the side wall vents for control. They're great for turning and not so good for forward speed. As I said in a previous post you need some way of holding the nose down as speed increases. I added the ramped flap at the back and it works quite well. I'm going to build another bigger one with the same power and battery.
May 20, 2021, 07:03 PM
Registered User
Bit quiet in here eh

Still building and testing, though time for this hobby is slipping away. I won't be stopping but may have to deprioritise a few things.

Twin motor skirtless hovercraft - water test (11 min 10 sec)

Octagonal Hovercraft - Water test (5 min 25 sec)

3D Printed skirtless hovercraft test (2 min 24 sec)

The files for the 3D printed craft are ready, now the task of publishing them. May create a thingiverse account and probably host them on my Dropbox too.
Aug 04, 2021, 12:57 AM
Scratchbuild Aviation
speedactyl's Avatar

Skirtless Twin motor

interesting to see a resurgence of Skirtless Hovercraft especially those using multirotor derive technologies im exploring into constructing one too . with a flight controller augmenting roll and pitch functions . I think the reason for the skirts are the lift fans and powerplant of the 70s and 80s Era and manualy piloting these things are a hand full .

with a flight controller I can mitigate the stability and drift issue associated with skirtless designs. given with new BL motors and fans increasing hover heights.

current hover height is around 20mm I can push it to 30mm briefly on full power but on normal flat surfaces i keep it at 5mm-10mm
Aug 05, 2021, 05:31 PM
Registered User
Looks interesting - would like to see a video when possible. Are you trying out ground effect as well with the outboard wings and motors?

Indeed we can cheat our way around the inefficiency of skirtless with brushless motors and lipos (and a whole lot of noise depending on the setup...). I don't consider my designs to be better than conventional skirts overall, it's just a different set of tradeoffs. The big air gap and frictionless levitation over thick grass counts for a lot though!

That said I wouldn't go chasing hover height above all else.

I had a few requests through YT for some design and build information. I suppose I'm guilty of thinking the design is 'obvious' from pictures and video, though I can't blame anyone for not wanting to trawl through this thread and the others for all the details. There are a few particular details that took a bit of experimenting to nail down so it's good to spare others the frustration of trial and error. As I say in the video I wouldn't claim this as the definitive design, it's what works for me. That's another way of saying keep experimenting and improving this venture.

Looking back over the many iterations and failed designs, I've had builds with fantastic hover height but terrible thrust, then one with excellent thrust and agility but middling hover height, another that was stable enough in the garden but useless on any real rough surface....after all this I'm honing in on the best balance of hover, thrust and stability. Then you add wings for ground effect and even OGE adventures and the fun starts all over again.

Skirtless Hovercraft - Design & Build information (11 min 28 sec)
Sep 13, 2021, 01:43 PM
Registered User
Street racer's Avatar
Thread OP
Hey, everyone! So much good stuff since last time I visited!

@ Speedactyl I really like that craft! love the design.

@ pompebled I like that you're using a centrifugal fan, looking forward to see what you come up with

I had some crazy ideas again last month, so I made a sketchup model again.

I have actually looked at some real centrifugal fans (approx. 0.7-0.9 m(2.2-2.9 feet) diameter) but I'm not sure what kind of power level would be needed to power them. There are some injection molded plastic centrifugal fans and they're stronger than metal fans of the same size according to the manufacturer, but coulnt find them in a larger diameter, the larger fans are only avalible in aluminum apperently.

this is the fan type I modeled in my sketchup model,

Would probably be better to have the thrust engines integrated in the hull, then they could be used to drive the lift fans too, like in the F-35.

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Last edited by Street racer; Sep 13, 2021 at 02:27 PM.
Sep 17, 2021, 08:51 AM
Registered User
Triple motor skirtless hovercraft - quick test (4 min 7 sec)

Skirtless Hovercraft - ground effect trials (2 min 38 sec)

Twin motor skirtless hovercraft - forest tests (7 min 22 sec)

Yet more videos

Working on full elevon wings for the flying one, need to get it out of ground effect.

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