The Thistledown

While the Mosquito concept appeals to me, I get a sore neck after too long a flight in prone and I'm not comfortable with having to foot-launch and foot-land with a power pack every time. I've flown the Minimum, but found it a bit awkward. So, it made sense to revisit the trike option, particularly as I have many years of trike experience and feel fairly confident flying in a seated position.

My original design, which got all the way to the welding stage, was a space-frame (view pic) that would have had very low drag - but the half-completed project was consigned to the rafters of my carport once I realised how hard it was going to be to transport it. (Offers accepted for the remains!)

The eventual design was a very conventional trike, albeit very lightly built. I acquired a second-hand Solo 210 14hp engine for about $300, imported a light reduction drive from Adventure Paramotors, and had Mike "Heloptera" Ellinas (a local hang glider pilot) build a suitable prop. The rest of the materials were scrounged out of my aluminium graveyard, except for the wheels, which were kindly provided by Independence Technology. They're a golf-cart type wheel made of a stronger GRP plastic and weigh less than a kilogram each. The engine is mounted via 4 Lord mounts on a flat plate inserted on the pillars of the reduction drive. The normal Adventure mount has the engine floating on three rubber "sandwich" mounts, one of which is attached to the cylinder head, but I found that these start to develop cracks in the rubber fairly quickly - not very confidence-inspiring! I was also warned that similar installations had had problems with the head mount disintegrating. With the revised mount, vibration is very low as long as the prop is kept properly balanced. (I'm now using a prop from nipon-propellers which is basically the same as an Adventure F3 propeller, and a Hirth redrive. It's very smooth.)

While the trike is a conventional wire braced frame, it also folds down into a package that fits in the back of a station wagon or on the roof racks. The main mast and base tube separate into two parts, and the wheels fold along the base tube. I also deliberately built it a bit high off the ground to give better prop clearance for fields with longer grass.

I tried a normal two way joint on the Shark, and while it was fine under power, it was really inefficient in thermals. I needed the yaw capability, so I redesigned the "normal" joint to allow a limited (30 degrees either way) yaw. It now thermals very nicely. (You could not use this kind of joint on anything with more than about 25hp as your arms have to handle any torque imposed by the thrust and engine torque.)

On all the gliders, I also had to make a spare set of lower front/back cables to move the base tube back about 25cm or so - they take about 5 mins to fit if I'm swapping between hang glider and power mode. The Shark control bar base tube had to be shortened to eliminate the dihedral this put in the wing.

	Front view of the original version. Kill switch is on seat frame on RHS of trike.
	The view above is of the original, with the latest configuration with a different, more heavily cranked base tube and nosewheel system below. When this lower photo was taken, the skirt had yet to be smoothed and stiffened with foam.
	Version 2 Nosewheel assembly from two angles. Click on the pics for a higher res version. There is a steel sleeve on the aluminium tube with an angled, bushed hole. A slotted wedge at the top and nylon washers provide the joint with significant friction so that you never get nosewheel shimmy. The pic also shows the simple throttle and brake setups.
	Version 2 Hangblock assembly Click on the pic for a higher res version. The pic clearly shows that the block can be yawed relative to the mast.
	Thisledown packed ready for transport. It fits in the back of my station wagon, or on the roof rack.
	Details of the current engine mount with four Lord mounts (4 of 150P30). The mounts are captive, with snubbing washers.
	Seat Frame, cushion and main gear hasn't changed much between versions.

Of course, having read this far, you probably wonder how successful it's been.

I've had many, many thermal days in the trike. On most days I have been able to launch in the late morning, cut the engine while low, and go soaring up to cloud base or the inversion for flights of up to 5 hours duration at altitudes of up to 9000ft. I have done 50km out-and-return power-off flights on blue days. The engine-on, still-air climb rate is slow (about 250 ft/min), but unlike a tow launch, the trike allows me to fly a search pattern looking for lift almost immediately after takeoff. This doesn't have to be great lift - air that would not normally sustain an engine-off climb can be used to help gain altitude. Once at a "safe" height, if the air is suitable, just switch off and keep soaring. If the air is still too light, you can extend the pattern until you find some that isn't. Most launches require less than a litre of fuel, and I only carry a bit extra in case of an outlanding. I have had to land out a couple of times when the engine refused to restart in the air. After an uneventful landing in a field, I just restart, take off from where I've landed and hook another thermal.

Click here for a short article on flying the Thistledown at a popular hang gliding competition at Birchip.

"What about performance?" you ask. Well, with the engine off, the sink rate is a bit higher than it is for me via foot-launch, but not much. After all, the wing is designed for this weight. Takeoff takes from 5 to 40 metres depending on the wind and the ground surface. Once aloft, if I find consistent lift that yields more than 450fpm climb, engine-on, it's time to switch off. I've also flown in the same thermals as sailplanes, quite happily out climbing them to the extent that when I landed, the sailplane pilots accused me of using my engine! (which I did not!)

Another interesting observation is that workable lift is actually around quite early; for logistic reasons, when hill or tow launching, we tend to wait until things are obviously cranking before committing aviation. A nanolight allows you to explore the lift without penalty, earlier in the day - and it's amazing what you find!

Handling is excellent, with the extra weight of the trike adding control authority to get that inside wing down. You do tend to feel a bit of a pendulum effect that can make precise speed control a problem at first, and it takes a while to get used to banking the trike over as hard as you need to in tight cores, but other than that, no problems!

Visibility, of course, is great. With no pod, the view below is good, and in the seated position, it's much easier on your neck when scanning the airspace. The vario is also more visible out front on the trike strut, and I can carry maps on a knee-board, and even take notes!

Landing is a non-event. It lands very slowly, and of course, you don't have to worry about dropping the control bar! However, the machine is not designed for high winds, and is particularly vulnerable to wind on the ground, as I've (tragically) found out by having it turned over on me once by a willy-willy or gust, so this is something to be watched, particularly when parked with no-one in the seat. The 12-inch lightweight wheels I'm using ride very well over the bumps in a rough paddock, and the steerable castoring nose wheel helps a lot.

Lastly, I have to ask why this sort of machine never really got popular in the past, and what is different that makes nanolights viable now?

The only answers I can come up with are fairly personal, but they may apply to others. Firstly, we're confident with the general safety of the "trike" concept now, and like me, there are many who have plenty of experience flying the heavier trikes. (When you first get involved in power, the tendency is to take advantage of it completely - hence the number of folks buying Mosquito's etc.. and writing about their long under-power flights - now we have a critical mass of folks who've been there and done that, and are looking for a challenge again). Secondly, the paramotor movement has led to the wide availability of light power systems, reduction drives and props - even if the paramotors themselves aren't suitable for rougher air. We now also have a range of wings certified to take the extra weight without requiring modification which compromises their certification. (Unfortunately, the current crop of nanolights are still only suitable for light pilots. Sadly, if you tip the scales naked up in the 85-100kg range, it's likely to be harder to find a suitable certified wing - and the available light engines may not have adequate power - leading to the old weight/power spiral...) Mix these briskly with a little motivation, and you have a recipe for successful motor-gliding.

Are plans available? The answer is no, for now, and probably always. In particular, I am concerned about product liability issues, and I would want to be sure that not only was the unit tested to meet a recognisable standard, but that I would be protected from litigation is someone crashed one.

As a small consolation, here are some PDF files showing some of the basic assemblies on the TD. You'll need Adobe Acrobat to view them.

*What's Thistledown?
When you're thermalling here in Victoria, you often encounter wispy globes of filaments, thistledown, floating around in the thermals. These are the seeds of the thistle, cast out to ride the wind and infest yet another annoyed farmer's field. A bit like a hang glider, really... :-)