A RC hovercraft saga - the Qoum 8
The "Qoum 8.X" are a series of hovercraft models made with Graupner aeromodelling turbines, from 2003 to 2014.
This text was translated from French to English, please forgive the imperfect translation. You can enlarge the pictures by right-clicking on them and select "open in a new tab" or "display image".
The final result of this saga is the Qoum 8.4 that you can see on the video. The skirt was made with paraglider canvas and the hull was made with bathroom insulation foam.
The main improvement of the Qoum 8.4, compared to the previous Qoum, is the fully rounded skirt at the front and back, which required more calculations. The lift motor (a hairdryer turbine) is located at the front. All the electronics are placed in a waterproof compartment at the center of the hull.
The result is a very reactive machine : little friction, quick acceleration and an impressive top speed. This model still uses a heavy NIMH battery which could be replaced with a lighter LIPO battery to improve the performances.
Firsts steps : Qoum 8 (2003)
THE IDEA - I created Qoum 8 because I came out of the manufacturing of a very complex model the 1 : 48 Zubr class (superb Russian machine which was not finished yet) and I wanted something simple and quick to build, still with a few challenges to solve.
The specifications included: a compact 500 mm electric machine, with 2-way radio control and reverses, much more fun to design and build, with 15 minutes autonomy at least. With the Zubr I used Graupner turbines and Speed 400 motors with good results. The advantage of turbines: compactness for good thrust. Given the motor power, I could hope to lift between 2 and 3 kg. When making a first weight estimation, I was under 2 kg.
FIRST STEPS - I wanted to invest in three Graupner 1379 turbines equipped with Speed 400 7.2V. I will keep their voltage nominal so they don't heat up while operating for a long time. Problem, the merchant in Paris had only two of them, but he sold me a 1380 cambered turbine, more expensive but immediatly available. I took it all with a Hitec 2-way radio, two Speed Plus 400 7.2V and one Speed 400 7.2V. Thanks to the Navirc forum, I knew that we could draw plans with RcCaD V2. I pulled out the plans from the base of the craft. The dimensions follow some simple laws which link the length, width and height of the skirt. I'll give you the calculations. We arrive at a flat hull 500 mm long, 300 mm wide, with a thickness of 25 mm.
THE HULL - I made a wooden form from the RcCad V2 plans. To transform it into a mold I use a special technique. Of course the resin purists will be shocked but it allows me to go fast. When I want a beautiful piece, I do like everyone else, but for a hovercraft shell that nobody sees, it allows you to have a shape to mold in 10 minutes. I take brown packaging tape, I cover my board, with a blow of electric heat gun I stretch it, I hardly wax it and I make the resin. A single layer of 300 g glass mat. We let it dry well and it comes off almost by itself.
Shell and mold
A layer of glass mat placed on waxed packing tape surrounding the wooden form.
For the moment I am not talking about the superstructures, it is normal, at that time I had not yet decided what exact shape I wanted to give to my machine. The advantage of a shell like that is that you can do whatever you want on it. I chose turbines to integrate but we can also make a central fuselage and put two faired turbines outside with a front / rear control on the engines. To reverse with streamlined turbines by guiding the air seemed too unattractive and more fragile. I decided to put the vertical lift turbine (horizontal it is possible, but that complicates the air channel a little) and integrate the turbines into the body by opening them on the side for my reverses.
THE SKIRT - Under a shell you need a skirt, there we have two solutions: we make a simple shape and we make it, or we calculate. I calculated and made paper patterns. As I had a stock of gray Kway fabric, in one evening with my paper forms, I traced, glued to the superglue and sewed the pieces. Why glued and sewn? Because I learned to be careful, a blown skirt like this does not resist the release of a collage and the machine becomes almost unmanageable.
REFLECTION - So let's summarize: on one side a raw mold shell, on the other a ready-made skirt and in the middle the pile of engines, turbines and servos. We have to organize. I said a vertical turbine for lift, two horizontal on either side for propulsion.
Good but what about the general shape? On paper, I did a lot of sketches until I found something I liked that could contain the elements. As you can see, all the active weight is behind, fortunately with the electric, there are the batteries which are there to make the adjustment. So before these are the batteries, the exact place will be determined during the tests.
The 6 skirt pieces before sewing, cut from a Kway-style fabric.
Full glued-sewn skirt
THE LIFT - For mounting, I use polypropylene ("plastic cardboard"). I found it at a supplier of material for traders, he uses it to make signs like building permits for example. This is sold in sheets of 1m 10 by 80cm. I work with a cutter, an electric file, a soldering gun, hot glue. For Qoum 8, I cut the first two partitions and I glued them hot on the hull.
I then prepared the lifting. It looks simple, but there are a few rules to follow. You calculate the outlet area S of your turbine, you divide it into S1 = 2 / 3xS and S2 = 1 / 3xS. You draw S1 on the center line 80 mm from the rear edge of the shell, you draw S2 20 mm from the rear edge. Next picture, at the center, shows this well (a little fuzzy, but we understand anyway).
Then you pierce. Above is the air distributor. See the diagram below for help. The non-return valves are not compulsory as there is only one turbine. The air distributor has two functions: guide the air in the skirt and in the cushion (this is the central part under the machine), and support the turbine. I took a plastic box that I drilled to the diameter of the turbine, and which covered the two air outlets well. I cut the rear part of the turbine flush with the engine and kept the top of the fairing. There you have to adjust all the elements a little so that they get along, without forgetting the deflector under the turbine to supply air to the opening of the skirt back. It is a bulkhead inclined at 45 ° which sends a third of the air from the lift turbine to the rear. For more security, I stuck a reinforcement leg on both sides of the turbine.
Diagram of a lift well
The turbine engine assembly is not drawn.
Open air distributor: we see the rear hole for the skirt and the front hole 2 times larger for the cushion
Turbine in place, closed distributor, skirt briefly fixed. the weight is made by batteries for the tests.
Once everything is adjusted and glued, the skirt must be put on for testing. I take my packaging tape (I like it a lot), I do a temporary assembly and I add some batteries to make weight.
First tears of joy, he raises as expected his 2.5 kg. We can move on. Gluing the skirt requires precision (a little) and adjustment (a lot). We start by drawing under the hull a line 7.5 cm from the rear edge and the two side edges. For the front it is 10.5 cm. The bonding begins with the upper part of the skirt on the upper part of the shell. I mean on top of the hull. This allows it to be better stretched. Then when it’s dry, we go back and glue the back on the line, then the front. It is important because the air which arrives in the skirt must return in the cushion by passages which I leave on the front at the level of the pieces front right and left and of the beginning of the sides. You make a mark on the side lines 10.5 cm from the front and 7.5 cm from the edge. You glue the edges. If the skirt hits too much during the tests (but we will talk about it again), additional holes must be made near the interior hooking. Do not forget one or two holes at the point of contact of the rear with the ground to allow water that could enter to come out, without forgetting to stick a small flap to prevent scooping when moving forward. After several unsuccessful experiences with glues I add a few screws to hold the lower fixing.
THE PROPULSION - You now have a lifted platform which will move on its own depending on the wind and the slope. You have to be able to govern it. There are two solutions: the simplest comprises two turbines with front-rear control on the engines. You go up and it's ready. The most complicated is to reverse in guiding the air. We need “reverses”.
I had no preconceived ideas on the matter. So I developed as and when editing. The problem is, with a servo and only one, being able to control the right / left direction and reverse by acting only on the air flow. After much thought and sketches I came to a 180 ° order. The normal travel: 45 ° to the right, 45 ° to the left is used for the normal maneuverability of the machine. Then at the end of the race on one side as on the other, by closing the nozzle, we get reverse gear. The solution is to put shutters completely closing the turbines and make lateral exits so that the air escapes. Good: there is nothing more to do.
We cut the turbines flush with the engines, keeping the rear. Be very careful when cutting them, they are very fragile, the slightest crack requiring only to enlarge. I cut in my partitions the space necessary for the introduction of the turbines, reinforcing the whole while doubling the front partition. I had transparent polypropylene: it’s good. If I had to redo them, I would make a mold and I would make the resin reverses, because I had a lot of trouble sticking the polypropylene on the resin of the turbines. I solved the problem with white "Rubson" for the bathroom. With a transparent sheet and "plastic cardboard" for the fixed shutters, I made the central part which allows the return of air. On the photo, we can see that the rear engine cone has disappeared, that the rear part of the turbine is indented on the outside, that the holes for the shutters are made. The lower fixing is made with a partition of "plastic cardboard", which is also found under the fixed flaps of the reverse.
Then come the second tests. I put the batteries and the receiver and contact in bulk. Good takeoff of the machine, test of the propulsion engines: good thrust since the machine goes up a slope of 10 °. On the axes of the shutters I stuck packing tape to close the turbines: good push back, Qoum 8 backs up without problem. Second tears of joy.
CONTROL SURFACES - Now, how to order the shutter. First problem the travel of the servos is only 90 °, I need 180 °, second problem, the servos sold with the radio control are too big for the space I have.
I sketched it out and tried to solve it all. Some solutions were ineffective, others too complicated for my technical means, others too fragile.
Then came the show. At the World Cup I met the editor-in-chief of Fly International which I had the pleasure of boarding on my personal hovercraft for a baptism. He advised me very well on servos. At Graupner a Micro-power C261 servo had the dimensions and the power compatible with my needs. I bought them right away.
There remained the delicate problem of travel. I thought of putting gears but that implied to stick the servos under the turbines. Once Qoum 8 had a body, we could no longer reach them. While looking for what existed as compatible sprockets to mount between my axes, I discovered a transmission kit for cars with toothed pulleys and toothed belt. I tilted. On vertical axes crossing the turbines, I glued a toothed pulley. Then I stuck a round seal capable of closing the turbine outlet on the axes. I calculated the size of the lever needed for the servo and fixed everything. The belt helps absorb vibrations. A precaution to take is to put a flange around the belt on the pulley to prevent it from jumping during an impact. I did it in transparent polypropylene (like the turbine walls) by pinching lightly on the side of the servo to serve as a tensioner.
The propulsion turbine
Side view of the left turbine. We can clearly see the reverse system on the transparent part.
Glued to the distributor, the servo plate (they are coupled). Transmission to the control surfaces is made by toothed belt.
The propulsion turbine
Top view of the left turbine.
TESTS - New tests: it works ... well if you like, because the slightest stress on the control surfaces causes the prototype to reverse. Mandatory stop. How to extend the control surfaces while keeping the possibility of closing the turbines? By sticking a piece of plastic bag on the lid. New tests: too short, it doesn't work, too long, doesn't work... And finally it works! Third tears of joy.
A fully charged battery, the terrace, and presto! ... more than a quarter of an hour of operation without big problem, just a small adjustment of positioning of the battery. It seems the number of holes under the skirt is good, the air circulates well. A small defect anyway: without propulsion, the machine backs up slowly.
I lend the handle to my son. As he does not know the small faults of the controls, he plays like crazy and causes a shift of the control surfaces. We stop everything, we re-adjust and we resume. Same default !! In fact, the lever arms on the servos are too generous. I solve the problem by limiting the stroke of the handle on the radio control. With experience, I realize that the flap should only rotate 160 ° / 170 ° and does not need a 180 °. Keep an eye on the following models, but the prototype works well.
DRESSING - With another white polypropylene plate, we dress Qoum 8. The sides are in raw "plastic cardboard", as well as the inlets of the propulsion turbines. The edges are not horizontal but slightly inclined to improve the aesthetics. The reverses are dressed first then the top, making the junction at the level of the lift turbine. We hide the connection lines with the decoration strips. The decorations are simple self-adhesive blue nets.
WATER TESTS - Then comes the big day. The weather is nice, a friend is there with his digital camera to capture the scene, the pond is without waves.
Radio contact, lift contact, launch.
First meters, first problem: poor front-rear balance. Back on bank, moving the battery and starting again, twice, but it works. We push the joysticks to the bottom: good directionality, a bit of speed, but disappointment: it does not take the water out. Back to the bank, the skirt is full of water. I forgot anti-scooping flaps (a small piece of skirt that closes the drain holes made on the rear segment of the skirt). I tinker a bit and put it back in the water. Same defect but we continue anyway as long as there is battery. Suddenly, Qoum8 takes a leaning look. Back to the bank, look under the skirt... the glue did not hold. I should have put the screws before testing!
Back at the workbench, I finish the skirt: peeling off and re-gluing the skirt with neoprene glue, and blocking with screws through small sticks of plastic cardboard.
Retry a few days later with a stronger battery: 8.2 v 3300 mA. Launched on the water, Qoum8 picks up speed, begins to climb on its bow wave, but refuses to overtake it. On land 5-7 km / h, on water half : disappointment ! Back to the test bench: we take a small scale, we pinch it in front of the machine and put it all the way down. Front thrust: 125 g for 1,450 kg, rear thrust: 100 g. It's not enough thrust for too much weight. If I compare with other machines, we must either double the thrust, or divide the weight by two !
Well ! I declare it suitable for indoors, when crossing puddles once launched, but no possible to take the water out of the skirt.
CONCLUSION - I have to rethink the prototype, make it simpler, without the reverses; lighter too, so change the lift. But this is for the next time.
Rear view - we can see the pieces of plastic bag stuck on the lids.
Front view - the superstructures are a white polypropylene casing glued with a glue gun. The decoration is done with self-adhesive strips.
Big improvements : Qoum 8.1 (2003)
INTRODUCTION - Qoum 8 works well except on water, where its speed of 3-4 km/h makes it uninteresting. On land, its speed is good and its motors allowing reverse are excellent for piloting. I was thinking of making a versatile machine, but the Qoum 8 is not conclusive. A logical evolution would be a machine that is as comfortable on the water without changing the specifications too much.
I will keep the hull 500 mm long and three turbines, but simplifying the machine to go below 1000 g - while with its batteries, Qoum 8 weighs 1400 g.
Incompressible weights: three motors (80 g × 3), the three turbines (50g × 3), the receiver and the variators 50 g, the battery 340 g: a total of 780g. This gives us a skirt, a shell and a body for 220 g.
A hovercraft is a lot of air with partitions around it. The simplification will cover everything that is not essential.
Rigidity on water it is not as necessary as on land. As the machine will be used mostly on water, I can remove the sides of the shell to keep only what holds the skirt. For buoyancy, I found large air bubbles to wedge the packages. The skirt must also undergo a slimming cure. The fabric is a bit heavy, I have to choose another material.
The skirt calculations are done, I already have the patterns. It is necessary to resume a little the calculations of center of gravity to center the masses in order to decrease the electrical wiring. It will make less weight. The air distribution of the lift will have to be simplified. The turbine will therefore no longer be vertical but at 45 °.
One last thing: make a model that any beginner can do without breaking his head. It means thinking to keep it simple. I decided to make two parts: a "two in one" skirt and shell, and an upper part, the motor support. An advantage: in case of problems I could act on one of the parts without going over the other.
These are the main lines of the project, the realization will settle the details.
The original Qoum 8
Finished Qoum 8.1 - much better performances
THE SKIRT - The KWay fabric being too heavy, I turned to plastic bags. I did different tests. The most successful was with a 100 liter trash bag.
The assembly is simple: I have the patterns, I cut each piece leaving a centimeter at each end to connect it to the neighbor. The assembly is done by welding the pieces together. Using a soldering iron equipped with a plastic cutting tip, I put one of the cardboard joining shapes on the two prepositioned pieces and I cut the excess, which automatically welds the two pieces together.
Cutting and welding took an hour and a half. On the picture : (Patterns for cutting, cardboard shapes for welding and trash bag for the skirt.)
SHELL - The bottom is not used, neither are the edges. I deleted them. From a polypropylene plate, I cut the upper shape, essential to fix the skirt (500 mm long, 300 mm wide). The lower fixing is made from a cut in the upper plate. 80mm from the back and sides, I drew a 320mm long rectangle. Then I drew (25 mm for each) and hot-bended angles on which the sides of the bottom of the skirt will come. For the lower front and rear fixing, I welded reinforcements in the form of small plates. I cut out the space needed for the air intake. The cut was also folded hot to break the speed of the air leaving the turbine and to support the weight of the lift motor just above. I added two small tabs on the front of the air intake to further support the engine weight and three horizontal angles to carry the battery while keeping the gap. A last angle is added under the nose of the hull to stiffen it.
MOUNTING - This is where the sticky packaging tape comes in. First I stuck the two air bubbles. Their shape is perfect and their 30 mm diameter will allow me to use them as load-bearing and stiffening elements. 3 pieces of tape per bubble and we go to the next step.
The bonding of the skirt begins from the top, always with packing tape (I did some tests, all this is water resistant). The original roll is too wide (50 mm), I cut it in half to have narrower strips, easier to use. In order, we fix the nose then the front sides, then the sides to finish at the back. The band is glued to the shell and the skirt adjusts to the brim.
The bottom of the skirt is glued, starting from the back. A little tip: as the air comes into force on this collage, the two sides (green and black) are entitled to a collage. Then the sides at the front. The lower parts of the front sides are not glued.
The verdict of the scales: 125 g. I'm in the quote. I'm going to be able to tackle the realization of the other part: the engine support.
Almost nothing here but air - the important thing is in the cuts and the reinforcements
Shell mounting - skirt seen from above
The skirt is green and black, the sticky band is brown, all that is transparent is the shell.
The underside of the skirt
You can see the large opening that is used to feed the skirt.
MOTORS - Second part of the machine : all that is mechanical and controls. Double-sided tape on the shell-skirt assembly will allow me to stick a white polypropylene board. On this board I will install the three turbines and the controls.
In front of me, three 1380 graupner faired turbines, two speed 400 6v motors, a Speed 400 7.2v motor, a two-way receiver, two Forward/Reverse speed controllors and a switch.
LIFTING - The first turbine will be cut just behind the engine for the inside and 2 cm from the front for the outside. After welding the wires, the Speed 400 7.2 v motor is set in place with its propeller. Then in the hole provided for this purpose and in the symmetrical hole that I drilled, I will stick small threaded rods for the front support.
On the plate, a kind of pyramid will receive the back of the turbine which will be glued with hot glue. The front of the turbine will be supported by two "plastic cardboard" legs. The wires will exit through holes drilled for this purpose, the black wire towards the back for the switch and the red wire towards the front and the battery.
PROPULSION - I set the two speed 400 6 v in place, I take out the wires which will be soldered in place of the thimbles of the variators, in the fixing holes of the turbines, I stick a piece of threaded rod. It’s ready to go. I cut 4 triangles from "plastic cardboard". They will serve as a lateral engine support. I cut three stiffening tabs and two pieces of carbon tube (used for kites) as well as two small tabs to put under the turbines (they will be glued on the tray but not on the turbines).
The three turbines: rear view
The lift and lift switch will be behind the center turbine.
The three turbines seen from the front
The rigidity of the engine supports is reinforced by two carbon tubes. There is a small support under each powertrain. We see the ESC in red and the lift switch on the back.
Everything will be sticked with hot glue. Once the two lateral supports and the lower support have been glued, the powertrain is put in place and glued to the side. Then I glue the front reinforcement tube, it comes to thread on the front threaded rods. I glue the central tab (which will be cut flush with the tube after). I adjust the side tabs so that the threaded rods of the lift and the rear rod of the propulsion can be glued into the compartments of the tongue. Finally, I glue the second carbon tube back on the side tabs and on the lift turbine.
Then I solder together all the red wires, and all the black wires with a battery plug. The receiver, wrapped in packaging bubbles, will be glued with double-sided tape on the back of the lift pyramid. Right above I put the lift motor switch so I can reach it from the back.
I do some static tests, everything seems to work. The next step is to bring the two parts together properly, to test on dry ground and then on water before finishing the bodywork.
The body has been sacrificed for weight, keeping the front lip for fixing.
The carrier plate is not complete and the hole in the hull is blocked with packing tape, but it works!
Motor support detail
We see the dimmer under the central leg.
THE FINISHES OF THE SUPERSTRUCTURES - On my right, a skirt-shell, on my left, a set of motor-units. The mission: put them together and make them work.
The first step was to make a provisional assembly and some tests. If the flight satisfies me, there are still some problems. The reinforcement bars are too tight and are not where they are most needed. Disassembly and correction: the new angles are 20 × 15. After checking, the white plate no longer bends under the weight of the battery whatever its position. I'm already thinking about the flight adjustment. It is the position of the battery that will balance the machine. A little further forward, left or right depending on the trim.
When everything seems good to me I go on to the final fixing. I chose double-sided tape. I will do it twice : the first one follows as closely as possible the cut-outs of the skirt-shell to maintain and seal. The second follows the edges of the motor platform to secure it firmly with the skirt-shell. Last detail: I add a handle on the front grab bar, a simple piece of piano wire, but essential to carry it.
You all know the thrill of putting something on the water for the first tests. Not far from my home, on a small discreet body of water, I put Qoum 8.1. Contact: left motor OK, right motor OK, start of lift. Howling turbines and off you go!
Nothing to do with Qoum 8. It works well, very well even for a first try. The rear is a little too much in the water but it does not matter : it does 10-12 km/h anyway. Brief return to the edge and off we went for 15 minutes of superb skiing.
It reacts well to commands, the right motor gives a little more than the right. It is therefore by modulating its speed that we adjust the straight line fully. It works so well that I came home to get another battery and invited my friend Alain. Without him you would not have had the pictures !
Almost half an hour later, check the elements: no peeling, no water in the skirt, I can come back satisfied.
At the workshop, I take my white polypropylene roller to make a minimum body above the battery. Two rails and a cover will suffice.
The hull, all made with polypropylene sheets. The front part can slide to set or remove the battery.
Final assembly. We can see the position of the battery which will be refined during the tests.
On the water : side view
On the water : front view
Good speed, a little too much weight on the right. The battery needs to be moved.
Now is the time for review and comparison. What technical differences with Qoum 8 ? Qoum 8.1 weighs 950g against 1400g for Qoum 8. Its forward thrust is 180g against 125g. The reverse thrust of 10g for Qoum 8.1 against 100g for Qoum 8, is enough to reverse on water but not on land. The turbines could therefore be improved next time.
Qoum 8.2 (2007) and Qoum 8.3 (2008)
No further explanations at this time - the pictures are posted here to give you other techniques and more ideas.
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