Month: July 2016

Bright and Sunny, Roads are Closed

The roads are closed this weekend due to a cycling event, so no (fixed wing) flying this week, which is a shame because the weather looks so good.

I have spent most of the week playing with my DaVinci Aerial Screw though. I’m now on the second prototype which uses a 1/32 ply ring and three spruce spars to hold the aerial screw, which looks a lot more like the DaVinci drawing.

The aerial screw itself is still a bit of a problem. In the original drawing, it’s obviously bigger than the circular base of the pyramid frame and flatter. The smaller diameter screw affects the performance of the quadcopter rotors a lot less. I’ve been testing different sizes and shapes all week and now have a fairly good idea about the trade-off between height, diameter, screw type and spin speed. I also wanted to make the screw look more antique, which I think I’ve achieved fairly well here. This version is made from light pastel cream coloured cardboard, which is then treated to make the antique faded parchment effect. I wasn’t sure how to go about this initially. Various people had suggested coffee, which is baked in the oven to make the card take on the yellow aged effect, but at the expense of making it brittle. So, I settled myself down with a cup of tea to have a think about it and the idea suddenly hit me. I went back and got the used tea bag and dabbed it onto the cardboard aerial screw. It does make it go a bit soggy, so I had to put weights on it to press it flat while it dried out, but the effect is quite good. The black lines you can see are just marker pen.

I also had a look at creating the cords which hold the aerial screw in place and are a prominent feature of the DaVinci drawing. Unfortunately, I can’t think of any way of doing this with the current design. My original thought was to stiffen cotton thread with cyano and attach it to the screw with a dab of glue. Then the thread would run around the outside edge of the ply ring when the screw was turning. I can’t find any way of doing this without the threads getting caught and sucked into the quadcopter rotors. I’m just going to have to find a way of securing them on a rotating platform, but that’s for prototype number 3 which will replace the white 3D printed frame with something wooden.

As you can see from some of the flight tests, it’s actually quite controllable once you get the aerial screw right. And lots of fun…

This one’s interesting because the battery came loose and was dragging along the floor. From my viewpoint above the camera, I can’t see this. All I know is that suddenly it’s all over the place.

I think that’s enough playing around with quadcopters for a bit. I’ve got to go and finish off an autogyro.

Two Wings

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I was on the bike this week, so I had the “Wing Thing 2” with me. As it turned out, somebody else had a Multiplex Xeno, so we were able to compare them in the air. In places mine was considerably quicker, then something changed and the Xeno was the faster. The balance is an interesting issue with Wing Thing. The first flight was about 1cm forward of my forward C of G mark, which was perfectly flyable without any nose down tendency. The second of my two flights this morning was on the forward mark (1cm back from the previous flight), which was a bit less stable in the air, but not a huge difference in performance. It was very windy, with conditions varying between dark overcast and looking like rain, then 25 degrees, hot sunny and humid. The weather is completely crazy at the moment.

I was only brave enough for two flights, both around the 8 minute mark, but they only took about 50% out of the batteries. Landing is still a big issue as I just can’t slow this model down. It’s like trying to land the space shuttle, you’re on a steep glide slope where the speed is much too high at the point of touchdown with no ability to flare. My technique this time was to find a big clump of tall grass and crash into it. Not exactly elegant. For the record, there was nothing wrong with the motor from the plane’s last outing. I had thought it was over-revving because the motor can had moved forwards and the magnets were no longer fully in contact with the windings. Last night, when I opened it up, there was nothing wrong with it. I do need to turn on the air brake though, as the prop windmilling must be contributing to the landing problems.¬†That’s it for my flying this week, but we were also joined by a Multiplex TwinStar which had the wings fold on launch just before I arrived. There was also a HobbyZone UMX Mustang with ailerons, rudder and elevator, a UMX Pitts biplane, a foam board own design Mustang and a really excellent HobbyZone Corsair with retracts. We never got to see the retracts work though, which is a pity. There were also no drones present which has been the case for a few weeks now.

That’s all for this week, I’ve got an AutoGyro to build and a DaVinci Aerial Screw to tinker with for the rest of the day. There’s also no flying next week, so I have a fortnight of building ahead of me.

Blue Dragonflies and Helicopters

It was a quiet morning flying today as there were only three of us there. The weather was also quite pleasant as, while yesterday had been sunny and very hot, today was more overcast with a cooling breeze. It was really bumpy in the air though, as big black clouds were being blown overhead by a stiff breeze, along with patches of thermal activity.

I got to fly a helicopter, which is always a brave thing for the owner to do. It was a Micro Twister which I had flown for him before, but it seemed to have a problem with the aileron (or whatever it’s called on a helicopter). I was steering it around on the rudder, while it felt like I was having to hold in a lot of left aileron to keep it from rolling right. The ailerons didn’t seem to have any directional effect, so, luckily, I could fly it around on rudder and elevator only. I’m getting better at helicopters, but it probably was too windy for it. My landing was fairly typical and best described as more of a crash into long grass. These really lightweight models are quite hard to get down and execute what I would call a proper landing as they have no real forward momentum. You just plop them into the ground.

Anyway, the owner of the micro helicopter also had a bind and fly UMX Corsair, but unfortunately no transmitter as he had left it at home. We also had a Sniper II and a small foam Mustang which I’ve seen fly before and which goes really well. The Sniper does look very similar to my RS352, but he obviously had a lot more aileron response than I have as it looked quite lively in the air. By comparison, he has no differential on his, but the movement actually looks less than I have. It was noted that mine looks much more controllable in the air, for what is an almost identical aircraft. He was also using 1300mAh 3S packs like me, but an EMax motor compared to my Hyperion one.

I managed to get 4 flights with my RS352 this week, but I think I need to change my transmitter battery as it was showing 8.9v after the last flight. I probably shouldn’t leave it until the low voltage alarm starts going off again. The conditions were rather bumpy, but we compared how much throttle I used to cruise compared to the Sniper (about 50%), knife edge ability, loops and rolls. He hasn’t tried any 3D aerobatics yet, but I did a bit of practice on the hover (it just won’t do it in any stable fashion) and prop hanging.

By the end of the session it was starting to get very hot (33C) and sunny again, so I was quite pleased to pack up and go home. I hadn’t realised, but on the walk back the grass is quite a bit longer than it looks. It’s quite a jungle out there, which is to let the Skylarks nest in peace. I saw a few of them around as I walked down the path, along with quite a lot of little blue dragonflies.

On a final note, there were no drones again, which is two weeks running.

DaVinci Aerial Screw

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My own rough scribbling of the famous DaVinci aerial screw, circa 1493. The Wikipedia article on Leonards’s inventions has more information.

Back in December, when I got the parts for my first 250 sized quadcopter that we built for the UK Drone Show, I said that I wanted to build something unique which people would look at and go, “Wow, what is that?”. In other words, a quadcopter that doesn’t look like an all carbon killing machine straight out of a Terminator film. I had know about Leonardo DaVinci’s Aerial Screw drawing for a while and had always wanted to try it out. Now with some time on my hands, I finally managed to give it a go.

I like the smaller quacopters as they’re just right for experimenting with and I had a lot of parts for the HubSan H107 which we used for the Royal Institution “Coding for Year 9s” project in March. The main thing was a 3D printed H Frame with blade guards, which I could use for the aircraft. My own HubSan has flown so much I’ve worn it out, so the 3D printed frame, 4 new motors and new flight controller electronics were needed to get it back to flight status again. OK, I know, that’s a completely new set of kit, but to me it’s still my old version 1 HubSan.

I’ll have to upload the 3D printer CAD file later, but I had a few issues with the holes being too small for the motors, despite the measurement on the computer being 7mm exactly. I ended up having to ream and drill the holes out so that the motors were a tight enough interference fit. I’ve done this with all the 3D printed frames we made as none of the holes were the right size, so I’ve had plenty of practice.

My attempt at the Air Screw looked something like this:

What I like about this is that it’s all held together with cardboard strips, sellotape and double sided foam tape. I had originally cut myself some Depron parts to fit between the four blade guard rings and make a full circle base (you can see it on the drawing if you look carefully), but weight proved to be an issue.

All up weight for the DaVinci prototype is 42g, which is a 8g over what I would recommend for the kit being used here. As my HubSan was the original one with the 7mm motors, it’s not as powerful as the camera version with the 8.5mm motors which we used for the Royal Institution project and the 3D printed butterfly and dragonfly frames. My H Frame was the prototype designed to work with the smaller motors, so I can’t change it. It is quite a bit lighter than the later frames which we made, though, as it was built a lot thinner (2mm thick instead of 3mm).

My biggest problem was building the screw. You can see from the pictures that the vertical structure holding the screw is made from four 10mm cardboard strips which are sellotaped to the underside of the H Frame. The flight controller sits on top in the middle of the frame, held on with double sided foam tape, which also holds the battery underneath. I wouldn’t recommend this as a good solution, but, for a prototype where you can remove the battery just by unsticking it, this does make things easier. A wooden skewer (2.5mm diameter) fits through holes in the cardboard structure and the air screw fits on top. There is a piece of Depron stuck in the top of the skewer to add effect.

On the underside you can see how the sellotaped cardboard attaches and the foam sticky pad used to hold it and the motor in place.

Now, making the actual air screw itself proved to me more complicated than I thought. Originally, I assumed that it was just two 3/4 segments of a circle stuck together. I now realise how naive this was after cutting up lots of paper circles. It’s a spiral of course, so I ended up drawing a circle with 60 degree spoke lines and drawing a spiral on top which I then made into a template. The picture should make things a bit clearer.

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The template for the Air Screw. All measurements are in millimetres and all spoke angles are 60 degree segments. Note that the ‘B’ section is drawn upside down.

My first air screw was on ordinary paper, which was useful for getting the dimensions right and for enabling me to cut around it with scissors to get the spiral nice and smooth. Then I transferred the design onto the thicker grey paper which is used in the photo and videos later.

All along, the key design criteria has been that the air screw must rotate as the quadcopter flies. At this point I could claim that it was my extensive aerodynamic knowledge from designing and building radio controlled aircraft over the last 30 years. Or that I had evolved the design using complicated computational fluid dynamics software on the computer. In truth, I just figured that with all that air moving around the four rotors, anything above them which was vaguely a helical screw just had to rotate (maybe that is experience?).

The way the aerodynamics work is that the four quadcopter rotors all push air downwards to varying degrees, depending on manoeuvring thrust. This means that air is being sucked down from on top of the quadcopter. My first attempt with a flimsy paper screw didn’t work as the paper just got sucked downwards by the air. This is the first lesson learnt, the paper needs to be stiff enough to hold its shape under the pressure difference caused by the rotors sucking air underneath. My second attempt was using some thicker cardboard, which was almost successful. I ended up cutting this one down to experiment with a smaller screw diameter, plus whether slots in the screw made much of a difference, so ended up destroying this one. My third attempt is the grey one you see here. This was from the page of an old scrap book, so heavier than the paper, but not as stiff as the card. Crucially, it holds its shape under the air pressure and it spins fairly well. I have also been using a small Depron disk (13mm diameter, 1.5mm thick) which the air screw rests on. This enables it to spin freely without catching on the under side of the screw. I’ve also increased the hole in the centre of the screw to be about 1.5 times the diameter of the skewer that it spins on. Basically, you just need to ensure that it spins freely without binding and that it’s fairly well centred. Also, that it sits more or less straight and doesn’t lean to one side. One final feature I’ve added is a small tab of sellotape on the under side of the screw, sort of like a trim tab. You can just about see it in the flying shots, but I think it helps a bit in getting the screw to start spinning.

Flying

I could try and claim that this was the first flight, but I’ve been flying this all week with different air screw designs until I found one that worked. It was excellent fun, and this grey version of the air screw was working really well last night.

As you can see, it’s quite hard to get the air screw turning, but, when it does, it spins in a very scale-like fashion. At least, if there was a person standing on the platform underneath and walking around turning it with handles, then that’s about as fast as they could turn it. In order for it to fly using the air screw it needs to go a lot faster and that’s ignoring the torque reaction and control problem. Also, the area covered by the Nexus 4 camera phone which I’m using to film is only about 1 metre square. So, I’m flying an under-powered quadcopter that’s top heavy, is held together with stick tape and has a big thing on top which messes with the aerodynamics while trying to stay within a confined space. Like I said, it’s been excellent fun all week. The bit near the end where I bump the wall was where I tried putting in a small rudder input. The HubSan flight controller has always been a bit funny when you spin it around on the rudder. All the ones I’ve flown drift very badly in all directions and I can never get it to spin on the spot. The interesting thing when watching this video is the realisation that the spin is related to how the quadcopter is being flown.

For the second flight I curled up the air screw a bit more to make a better spiral and you can see that it does spin up a lot faster. The flight was cut short because I realised that the spin increases when it is drifting left, so I then tried to do a 360 degree left spin on the rudder. That’s when I bumped the wall and drifted out of shot.

By flight three, I’m getting the hang of it and understanding how to control the spin of the air screw by the control inputs I’m making. This is really starting to get fun – I’m flying the quad and the air screw on top. By this point I’m also wondering whether it makes any difference what angle the air screw is relative to the quadcopter frame. The flight ends when the sellotape finally gives up the ghost and the frame supporting the air screw collapses. This is not before I’ve noticed a pronounced lean to the right though. I’m not sure whether this makes a difference – left drift and right lean makes it spin better?

What We’ve Learnt

The air screw is simply a cardboard piece dropped onto the skewer and free to spin. It needs to be rigid enough not to deform, but, if you watch the flying shots, manoeuvring causes it to tilt, which stops the spin. If the air screw could be fixed to the skewer with a low friction bearing such that it was held rigid horizontally, then it might spin more reliably. The air screw shape also needs some more experimentation and the addition of the helical screw above where the four quadcopter rotors are drawing their air from significantly affects the performance. It’s definitely a successful first prototype though. The air screw spins and it is controllable in flight.

Homework

  • Investigate the mathematics of spirals.
  • Make a wooden version of the cardboard air screw frame.
  • Design a zero weight, frictionless, horizontally rigid bearing arrangement for the air screw.
  • Build a second prototype with the more powerful motors, which looks like it’s 500 years old.

Links

Definitely Building an Autogyro

We’ve got weird weather this week, so no flying. Bright and sunny, rain, wind, very hot, humid, then cold, heavy rain, back to Summer again. It’s completely mad. I would have got both drenched and roasted if I ventured out.

Anyway, I’ve got all my work out of the way now, so I’ve got my weekends and evenings back and I am definitely building the Autogyro. There’s just one little idea I’ve been thinking about for a while that I need to try first. I’m going to spend the rest of today looking into the feasibility, so it’s either going to work brilliantly and be the subject of the next post, or we never speak of it again. The plans are below.

IMG_20160710_173623You’ll never guess what it is. All I’m saying is that it’s got five rotors.

 

 

Perfect Weather

Sorry, no pictures this week, but the weather was absolutely perfect for once. It was warm, not too hot, a little wind and some clouds to show up the models, but no rain.

When I arrived this morning there was the UMX SpaceWalker finding a place to land in the long grass and the foam Super Cub. We were soon joined by a DJI Inspire, the clipped wing HobbyKing Easy Star clone with FPV and a home made foam board 3D aerobat. The SpaceWalker needed a little fixing after its untimely arrival in the grass. The motor shaft had been straightened a couple of weeks ago after I bent it and the pin on the end lost. The pin was replaced with some wire, but this had come loose. I never realised this before, but the whole of the top of the fuselage up to the cockpit lifts off to reveal the inner workings. Once this was fixed we sellotaped it back together and went off to fly it again. It’s actually quite hard to fly and I can see why they put the excessive dihedral on it. I found the aircraft was constantly trying to nose up, even with full down trim. I also ended up with over 50% right trim to stop it from circling. The interesting thing is that with the power off it glides beautifully. Put the power on and it’s a different prospect with the plane rocking in a quite animated fashion as it seems to fight the thrust. I just think it’s caused by vibration with a 35g model. We had actually removed the rubber spinner before the flight for that exact reason, so we might have also altered the C of G. Either way, I still had the longest flight I think I’ve ever had with a UMX plane. The conditions were such that I could just about thermal below a rain cloud, which showed the colours of the model up to perfection. When I eventually did land, I still had lots of power left.

After that I managed to get three flights with my RS352 and had a practice at hovering again. It really doesn’t want to hover and I think it’s due to the fact that you just can’t stall it. You have to judge the speed really well, otherwise too slow and it just rocks out of the nose up attitude. I could prop hang, but a 30 degree hover is still elusive. Needs more work.

Finally, I got to launch the yellow and green foam board aircraft on its maiden flight. Well, second flight actually, as the first was just a big left hand spiral into the ground. I did a fairly solid low wing launch from a point under the wing and it went off without any problem. It was a bit of a handful in the air though. The pilot decided to land after a couple of circuits and we decided that the C of G was much too far back. It’s supposed to be a 3D model, but you want it to fly first before you start moving the C of G back. Anyway, somebody lent him a heavier 1500mAh pack and we got the balance forward for another attempt. I launched the same as before and all was good as I turned round to head back to the group, but a thump from behind told me he had hit the ground. The aircraft was fine, but they think it was pilot error and he stalled it. I have no idea what happened as I was facing the wrong way. I had to leave before his next attempt, but he switched to a different motor. I’ve seen this on a lot of foam board models, but the motor is on a square tube which slides inside the fuselage. Changing the motor and electronics is simply a case of sliding it out and sliding a replacement in. It’s all rather neat.

Anyway, that’s it for this week, except to say that I hope the DJI Inspire pilot gets permission to fly around the aircraft at Farnborough.