Meanwhile, relax with a ready to run skirtless hovercraft........

E016F (2 min 39 sec)

Skirtless Hovercraft Projects - past experiments (6 min 19 sec)

Not much activity in this place recently (well, any activity)....but I've seen my other thread steadily racking up views so it seems we have a fair number of people reading. I'm still working on a few different skirtless hovercraft, I've had a bit more time for building and some better weather for testing. I've got a lot of 'data' to report, not so much hard numbers but a bunch of observations on different designs for stability ports, flaps, rudders etc. It'll probably be a while before I can collate it all into a post, truth is my job involves sitting at a computer all day so I'm not much in the mood to spend hours writing stuff up and putting videos together in my 'free time' now

I was thinking of drawing up a proper model plan for a single motor split flap design, complete with dimensions, details of the linkages, list of suggested motors/props etc. Something to allow a reasonably working model to be built from scratch like the foamy planes are. Maybe something I can take a look at over the Christmas break.

Anyway I was going back over some old videos and realised there's some stuff I've never published. A lot of that is because it was very crude or didn't work so well, nevertheless I thought some more footage might be appreciated.

Originally Posted by FiftySlicks

https://www.youtube.com/watch?v=NcgQFk3N3kU

Not much activity in this place recently (well, any activity)..../....I was thinking of drawing up a proper model plan for a single motor split flap design, complete with dimensions, details of the linkages, list of suggested motors/props etc. Something to allow a reasonably working model to be built from scratch like the foamy planes are.

Thanks for your recent posts & info. Encouraged by that, I finally decided to try building one of these critters for my winter project. I wanted to try using $tree foam board & some of the build ideas from the airplane guys while shamelessly using the design info you & the other guys have provided.
I was able to try my craft this AM, so I figured I'd post some pics and info on the build. Hopefully, this will generate some more interest here......
The craft ended up at 38 x 66cm at the lift slot, 44 x 70cm OA (Longer than I wanted). AuWeight is 680g (24 oz) with a 2200, 3S lipo using a 2836-9, 950kv and either 9x4.5 tri blade or 9x4.5 reg.
As a proof of concept and test bed, I'm very happy with how the build came out but wouldn't recommend using the $tree board like I did as I think it's going to be fragile. The foam is too soft for this application ( I removed 70+g of paper cover where I thought I could).
IT FLIES!!......
THE GOOD: The “maiden" was just in my little Apt but I was surprised that it hovered @ less than half power & seemed stable. It will really jump up @ about 2/3.
THE BAD: My design of the port stators to counteract the prop rotation/left turn deal needs work. I can get it going straight or right but not like I want.
It's supposed to get above 40F tomorrow so I hope to get it outside......stay tuned..........

We have lost a lot of people and discussion if you look back over the older threads. Very much appreciate your interest

The hull section on the bottom left looks very good, in theory it's a much nicer turn for the airflow than just a flat plate to the bevel. Though I've found that things which are 'supposed' to be better for airflow don't seem to have much if any of an effect on these models. My older builds were full of sharp corners and obstructions to the airflow, in comparison my recent builds are much cleaner and - they work exactly the same. Haven't seen any gains in hover height from making rounded corners and streamlined sections inside the hull. Maybe the flat walls are too restrictive for any other gains to be apparent, might need a whole hull made with the rounded section to be noticeable.

The stators at the rear - I found I needed a pretty substantial curve to get the anti-torque effect. See the attached picture for what I used. That may be a little too much but the model turns pretty well against the prop. Makes me wonder what a set of servo driven vanes linked to a gyro would do.

I don't think your current flap setup will work very well, I tried the same thing previously, check the 4 minute mark on my past experiments video. If the flaps close completely to vertical when they open you'll likely have too much nose down moment and not enough forward thrust. Then again maybe it'll work and it was my setup that was wrong. My current models all use a flap angle of 60 degrees from horizontal when closed and they work well.

I started off with servos mounted on top of the hull right at the flaps and a short push linkage to close them. I tried out the internal servo with a pull rod and got it working alright but never great. It's difficult to get enough force to close the flap completely and enough travel to get them opened out. It worked acceptably on a smaller model but not on a larger one so I went back to the external servo mount. It's less neat for sure but much easier to adjust and get working.

If it's not too much hassle, try building in an adjustable front jet on a servo control. Being able to open and close it remotely is really handy for pitch trim.

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I have drawn up a build plan for a 40x60cm craft, I started going into step-by-step instructions but I'm not sure if so much detail is necessary and I don't really have the focus to write out a whole guide like that. I can look at finishing off the basic plan at least as it covers the most important design points.

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As it happens I got my larger single motor build onto a beach for the first time recently:

Single motor skirtless hovercraft - beach test (4 min 18 sec)

I've also got another triple motor build just completed and tested, hover height seems a little poor for the size of it but there are many refinements still to be made. Will get it out for a proper test once the storms clear off.

Slick; thanks for your reply. Basically, I fully agree w/ your observations and comments.
My maiden test went pretty good Sunday, it even survived it's first 2 crashes!

My replies to some of your comments:
The inside scoring required to form the rounded test hull from the foam board makes it surprisingly less sturdy than the flat sided versions so I decided to keep things simple for my 1st attempt. I want to try some wool tuft airflow studies to, hopefully, get a better idea of what goes on inside there.....

Most of my design choices are good old TLAR engineering and I got a little optimistic with the needed offset on the stators. Actually, it didn't steer too bad when I got it moving and I have room to make some easy improvements.

Trying to limit the number of sheets used kind of limited the incline on the flap setup to 80deg and I do have some down pitch problems. It was less pronounced when I opened-up the center duct for more lift but that hurts the thrust. After I try some mods to the current set-up, I may redo the rear to get near your 60deg figure. At that time I'll also take another look at mounting the servos externally.

If I haven't destroyed the craft by then, I'm thinking that a front lift flap could be controlled by a mini KK2 board running a servo like the Super Simple camera Gimble firmware on my quad. (The fewer controls my 70+ year old pilot skills(?) have to deal with, the better!)

I'm looking forward to your plan while I play with getting this one working better as the weather permits. Your larger craft in the beach test video looks like it works great....congratulations!
Ralph

First off I hope everyone is keeping safe and healthy out there. Looks like quarantine and lockdown are not far away for me.

Second of all let's revisit a little quip of mine from last year:

Originally Posted by FiftySlicks

Anyway.....this is the first time I've ran over deep water, I don't think a glorified puddle at the park quite counts. The 'beach' here isn't all that scenic either although it's a great test for this little craft - you could see a skirted craft getting hopelessly bogged down on the stones. This one occasionally bumps and scrapes but is otherwise unimpeded.

Shouldn't have been so hasty before trying a skirted craft!

Skirted and skirtless hovercraft - at the river (13 min 21 sec)

In my defense, I've seen other skirted craft struggle on this sort of thing, seems a lot are built quite heavy with larger batteries, this one is light and lean with a 3S 2200mah. Those twin motors are putting out a lot of thrust as well and there's a very generous bag volume. It carries momentum well across the 'beach' but definitely bogs down a bit while trying to get moving on the bigger stones. Still, colour me impressed with what a simple bag skirt can do.

The skirtless craft here was more intended for drifting around on hard ground but I thought I'd try it out on water. You might say 'overpowered', I say 'plenty of lift height in reserve'

I'm continuing with some other experiments:

- A large triple motor skirtless craft, the next iteration of the design from my other thread. Size is 100cm long by 80cm wide, getting around 30cm of air gap over grass with more improvements to do. Will probably end up with bixel wings at some point to get it flying.

- Another small single motor craft to try out a few ideas that have been going through my head. This one has 4 flaps at the back instead of 2 as I'm trying to improve roll stability while turning at higher speed. With 4 independent flaps you have much more options that just 'open one side, close the other'. Going through some teething problems with balancing it out as all the extra linkages and servos make it pretty back heavy.

-In the planning stages for a single motor skirtless craft with a vertically orientated prop at the front. Going to look really weird but there is a logic behind it.

-Was intending on finishing up a draft of my build plan for a single motor hovercraft, I was using Draftsight which is now no longer free. It's not expensive to buy it and much less hassle than finding another cad program, still I'll need to get on that.

Will work on putting up more photos/videos/plans here but I may not have internet for the next few days.

Just threw a short video on YouTube of a recent test session with my single motor skirt-less hover....

Skirtless hovercraft test session (0 min 54 sec)

Naturally, as I streamline and open up the thrust duct, the lift suffers. So I'm trying different configurations to find a compromise that I like. Happily, the $tree/Adams foam board construction is holding up surprisingly well.

We're going through stay-at-home orders here but there's an exception for "exercise or outdoor activities". On the nicer days I head over to the park to exercise one of my models. (I'm not within 100 yards of anyone & I only touch my stuff)
Hope everyone is getting through this OK....be well...
Ralph

how about partly covering/restricting the intake with a flap, or baffle that's slaved to the thrusting ports? that way when you open a port, the intake is opened as well, making up for the loss of lift.

Evening folks

The worst of the restrictions here are being officially lifted next week, though it's pretty evident that a lot of people have unofficially considered them eased for many weeks. I haven't been out with any models as all my usual spots are very busy, weekends don't have a meaning with schools and many workplaces shut down.

Here's some video from a frosty November last year:

Skirtless hovercraft - on frozen ground (3 min 0 sec)

That model has since been replaced with the pictured one. 4 independent flaps with a bunch of mixes set up on TX to see if I can get better turning. Based on testing in the back garden....I can't really say anything, planning on heading back to that car park soon for a more definitive test. The front jet is also adjustable by servo.

I got a bit ambitious with the size of this other one. It's got a great air gap over grass though I think it can do better with bigger props.

There's a couple of other hovercraft I'm working one, will post more as I progress and get them working properly.

Heya. I'm new here, but I actually wanted. To start an account just for this thread. I hope I can contribute some, and learn more. ^.^

Well this place has seen better days. For anyone still reading or lurking here's a recent visit to my usual water testing spot.

Skirtless Hovercraft - back to the river (10 min 50 sec)

Single motor craft is the one pictured in my last post with the 4 flaps. There was a logic behind having 2 flaps per side instead of just one. I have a long-winded explanation and a couple of diagrams I could post up explaining the advantage 4 flaps was supposed to have, my testing shows the actual advantage doesn't seem to be significant if it exists at all.

The issue is trying to turn quickly at high forward speed. The yaw inevitably turns into a roll as the outside edge of the craft lifts up from ram air pressure. Meanwhile the inside edge smacks the ground and drags. Not very elegant. This flap setup was meant to create some yaw turning torque but also a counter-roll torque to keep the craft more level in turns. As I said it doesn't seem to have made much difference in practice.

Also on test is a couple of new stabilisation port designs. My go-to has been a plain hole at 50% of prop diameter under the motor. Simple, but it creates lots of turbulence on water. I printed a cylinder at 50% prop diameter that has curved vanes inside. The vanes counteract the motor torque so it can hover on the spot without the flaps open and without rotating. This is visible at 1:50 in the video. You can make it spin fast in the same direction as the prop rotation with more curve on the vanes. You could certainly yaw the craft well with adjustable vanes. Too much complexity to deal with right now but I may try it.

The cylindrical design reduced turbulence on the water considerably. While it's still tapping off the same 50% prop diameter as the circular port the airflow seems much cleaner coming out of it. I could really see the difference when hovering stationary on the water at high power. I suspect the plain circular port creates a very turbulent stabilisation jet.

As a further attempt to reduce turbulence I printed up a cone that scales from 50% of prop diameter at the top to 75% of prop diameter at the bottom. This also has the anti-torque vanes inside it. This is fitted to the craft at 4:50. This reduces the stabilisation airflow turbulence to almost nothing as it diffuses the jet out. Photo will be coming but I need to print it again as the first version only barely held together at 0.4mm wall thickness on the whole thing.

Nothing special about the triple motor craft here. A little bigger than the previous one and again not really meant for water but it works.

More information is coming.....soon

Alright, I've had enough of playing by the rules. Watch my act of defiance!

RC Hovercraft - in the back lane (13 min 47 sec)

Yes, a hovercraft with a weird skirt thing attached and dragging everywhere

Seriously, I was impressed at how well this little one handled everything I threw at it. Especially since the skirt was very roughly cut and stuck on. There's a proper way to cut and stick the corner sections, and then there's the can't-be-bothered approach where you cut out one bit of fabric and scrunch it all up with tape at the corners. Far from neat but also far from useless.

I've seen a lot of skirted craft that seem very heavy, built with thick plywood and fixings, and sometimes lacking in lift power as well. On water they sometimes look more like boats than hovercraft, the skirt digs down into the water and they can't seem to slip sideways very well. I've kept with light foam construction and a large bag volume for a hovercraft that is sometimes too eager to lift off and bounce.

Have now constructed a larger skirted craft with properly cut and stuck corners. Indeed much neater but also fiddly and time consuming and this is for a simple 90 degree join at each corner. I've seen skirted craft with dozens of individual skirt panels to accommodate rounded corners and it seems a nightmare of tedium to construct.

Back to our usual programming in this thread:

Twin motor skirtless hovercraft with multiple control flaps - quick test (2 min 52 sec)

Latest skirtless experiment. This was never intended to leave the garden and it didn't before being scrapped. Wanted to test out the concept of twin motors with flaps at both ends. Later added the sidewall vents for banking/strafing. Forwards and backwards thrust is good but the yaw/turning seems a bit lethargic.

I'll shortly be building a new version, this time with only one flap on each end and dual flaps on each side. This will maximise turning torque.

Attached is a few pictures of my 3D printed diffuser cone used on the small skirtless craft in the past video. It diffuses the stabilisation jet out to reduce turbulence on water and also corrects for the torque of the motor using the curved vanes. It's not enough to just straighten the flow, the vanes curve out a bit more to provide counter-torque to fully nullify the lift motor torque on the whole craft. With a bit of trail and error printing the torque correction is near perfect and it scales with the motor throttle.

Hi FiftySlicks,

I've been following the skirtless developements off and on, even started making parts for a curved planum from routed foam board.
It's still on the to do pile, as my health got in the way back then...
Retired since a few months, so time to read up on the forums and try new stuff (Tricopter building and crashing/flying them, steep learning curve).

Something I wondered watching the video's about making turns; the rear flap lifts the side that is opened, causing the opposite front end to be pushed down, often touching the ground, depending on the clearance.

What if the flap(s) hinge at the bottom, or turn on a central axle, so the airflow is not being pointing down?

Would that solve the tipping in turns?

Regards, Jan.

Originally Posted by pompebled

Hi FiftySlicks,
Something I wondered watching the video's about making turns; the rear flap lifts the side that is opened, causing the opposite front end to be pushed down, often touching the ground, depending on the clearance.
...............Would that solve the tipping in turns?

Hi;
Regarding the nose down/tilting moment, in post 94 above FiftySlicks advised:
“If the flaps close completely to vertical, when they open you'll likely have too much nose down moment and not enough forward thrust. Then again maybe it'll work and it was my setup that was wrong. My current models all use a flap angle of 60 degrees from horizontal when closed and they work well.”

I have increased the forward tilt on my flaps to the recommended 60deg and have little, if any, nose down issues.
Good luck with your project...
Ralph

Originally Posted by pompebled

Hi FiftySlicks,
Something I wondered watching the video's about making turns; the rear flap lifts the side that is opened, causing the opposite front end to be pushed down, often touching the ground, depending on the clearance.

What if the flap(s) hinge at the bottom, or turn on a central axle, so the airflow is not being pointing down?

Would that solve the tipping in turns?

It's definitely an issue, the only solution I have so far is keeping turns gentle at high speed. At lower speeds you can be a bit more aggressive in turns. There's a certain envelope of speed vs turn rate that causes it to happen the most.

As for solutions - I'm not sure there is one. The downward pointing airflow from the flaps is very necessary. Just looking at forward motion, if the air exited completely horizontal you'd think that would give the best top speed, so I have tried it. What actually happens is that as speed increases ram air pressure lifts the nose up. Then the thrust vector of the air coming out the stability port and peripheral jets points a bit forward, which then counteracts the thrust from the rear flaps. The craft ends up going forwards with the nose high and the back end very close to the ground. There's no more speed to gain.

So for higher forwards speeds you need a way of keeping the nose down. You can close off the front air jet with a servo which definitely helps, but it's not a full solution. You can get clever with outboard wings or a high mounted tail and begin to make the craft into a hybrid hovercraft/ground effect vehicle. Which again does work but isn't 'pure' to being a hovercraft. So instead of having the propulsive air from the back exit horizontal, you aim that down a bit with the flaps. That allows you to split the airflow into forward propulsion and a tail-up (ie nose down) pitching moment. Typically for maximum acceleration the flaps can go to horizontal, then they have to be brought lower as forward speed increases. Since I have the flaps on the throttle stick that means maximum forward speed isn't at the full throttle position, it's usually around half throttle.

That's the cleverness of the flaps, you can split the airstream into forward thrust, nose down moment and turning torque all on 2 servos.

I've racked my brains about the way the inside corner tends to dig in. And I'm not convinced the flap angle has much to do with it. I think the effect is mostly from ram air while turning. As I said when going forwards the nose wants to lift and the tail drop from incoming air going under the leading edge. So the exact same thing applies when turning and presenting the side of the craft to the oncoming air. Outside edge goes up, inside edge goes down. Except you have no flap force to counteract it. I don't think there's any way of turning sharply at high speed that won't cause that rolling effect.

When I think about it more - alright, say the craft is going forward at moderate speed with the flaps partially open. When you turn the outside flap opens up more. Which means the air exiting that flap is going more horizontally, not less. In other words the outside flap isn't able to push the opposite corner down much at all. For a very sharp turn the outside flap can go horizontal. Which means there's next to no pitching effect from that flap, yet the inside corner still digs in as the craft rolls. In fact since the inside flap closes a bit there's a force there opposing the tendency to roll the craft, yet it still happens.

If I've not lost you - that's why I think it's ram air that rolls the craft in turns, not the airflow from the flaps. The outside flap opening and inside one closing should produce a roll torque opposite the way the inside corner rolls in, yet that still happens.

So after all this I think the rolling problem is just inherent to the split flap design. I have a bunch of ideas swirling for additional flaps to try and counteract it. Since I have little else to do at the moment I may try implementing it soon. Here's a related experiment:

Twin motor skirtless hovercraft with multiple control flaps - new version (8 min 37 sec)

Take a look at 3:15. For reference the orange fins are the back of the craft. The flaps open to move the craft to the left, and it does, but the left side is going up in the air while the right side is nearing the ground. That's why the 45* angle of the flaps is too much. 30* is about right or even vertical works quite well for the side flaps on a more recent experiment.

I have some more musings on all this that I'll write/draw up sometime. Kind of exhausted after typing all this! My experiments are still ongoing.