So, you want to build a trike?
At Aerial Pursuits we used to distribute plans for a single seat trike, and often get queries from people who want to use these or other plans as the basis for designing their own trike and using an existing hang glider wing. Sometimes we get queries from people wanting to even build their own wing from scratch, thinking that this will be cheap and easy. (It's not.)
As a result, I thought it might be of interest publishing some of the guidelines and considerations we homebuilders used back in the early 1980's when trikes were relatively new.
PLEASE NOTE: Most of us in the early 1980's who converted hang gliders intended to fly them in benign conditions, not in turbulence or thermals. There is a good reason for this. Most of the modifications were never formally tested to make sure that they were actually stronger, and whether they were compromising the stability of the wing. This still holds true today. I DO NOT RECOMMEND THE USE OF A MODIFIED HANG GLIDER FOR USE IN THERMAL OR TURBULENT CONDITIONS. If you want to do this, choose a hang glider already designed for the required load.
I HIGHLY RECOMMEND THAT UNLESS YOU HAVE SOME YEARS OF EXPERIENCE AND A GOOD GROUNDING IN THE ENGINEERING AND AERODYMAMIC CHARACTERISTICS OF FLEX WINGS THAT YOU DO NOT UNDERTAKE FLEX WING MODIFICATION. Caveat Emptor!
If you're interested in other, safer options, you might like to take a look at the Nanolight section on these pages, which details a plethora of ways to power a hang glider.
Two Places?
Well, in Australia, you can't build a two place trike out of a hang glider yourself and fly it unless it's a certified kitset, or you're willing to get the thing certified to an appropriate CAO airworthiness requirement. And believe the folks at Airborne, you don't want to go through that unless you're a damn sight richer than usual with lots of time to spare and have a few loose nuts up top.
However, there are a couple of alternatives. One is to import a Section-S certified kit, like the Mainair Mercury, and the other is to build from approved plans. For example, the Huntwing is an example of a 2-seat trike that can be built from plans (except for the sail, which is available commercially).
However, unlike the UK, Australia has no Inspector network to assure that a machine built from plans or a kit actually conforms to the required standard. At the time of writing, no-one has tried to build a Huntwing here and fly it with 2 seats.
If you're reading this in the US, your regs (when we last looked) don't have a lot of scope for two-seaters anyhow unless you're training people (in which case you're probably going to use a decent certified/commercially built aircraft if only to avoid being sued for negligence in the event of a mishap).
Some countries have more options; notably the eastern European states.
The Wing
Of course, the wing is the heart of the trike. Your average hang glider wing has been designed to fly a 80-85kg or so payload at a best glide in the region of 30-35 mph, with a maximum payload in the region of 120kg to stay within certification limits (some go as high as 135kg). If you add a basic trike without doing anything else, it's like going tandem in a wing not designed for it. The additional load will cause the leading edges to flex upwards and backwards at the tip, changing handling and speed range, and you run the risk of placing the aircraft outside its placarded "g" limits in steep turns or turbulence. There's also the concern of localised stress at hang points, etc.
So, realistically, you need to strengthen the wing frame, usually by sleeving the tubes and doubling up on sidewires, at least. This should not be done lightly, as, if sleeves are placed incorrectly, you can actually weaken the frame by introducing localised stress points rather than strengthening it! In addition, you should be aware that you may be modifying the wings pitch stability outside certification standards.
Some wings may not even be appropriate due to the leading edge construction. For example, they may have an outboard section made of fibreglass, which will have been engineered to flex just right for the normal range of pilot load.
Many wings today are designed with the pilot hanging from a hang strap suspended from a few inches up the kingpost, and the keel is not designed to take any sort of direct suspension.
On top of this, you may need to adjust the control bar rake angle and move the rear wire attachment point forward to allow more prop clearance.
Scary? Can be! So we suggest your first port of call should be the wing manufacturer. After they have got through telling you that they don't condone the practice of adding a trike to the wing and have suggested you buy a commercial unit, you may be able to elicit some "suggestions" as to the best way to beef up your own particular wing. They may also be willing to do the mods for you - as long as you take responsibility for the result.
Also, don't forget the sail. There's going to be more tension, so assure that all stitching is in good condition and that the sail is not faded due to UV exposure. The manufacurer may also recommend some re-inforcement, particularly around the root chord at the end of the keel.
The Trike
If you've managed to solve the wing problem, the trike part is relatively easy, but full of fiddly bits. The wing is critical, but has a low parts count. The trike is easier to engineer, but has lots of parts. Here are some things you will need to consider:
The angle of the dangle
When you're flying along, you want the wing control bar to be in a position that allows adequate pull-in or push-out. You also want the nosewheel to be the last thing to touch the ground when you're landing without power.
The latter requirement means that if you hang the trike assembly from the roof of your garage, and sit in it with no fuel on board, the nosewheel should be at least a couple of inches higher than the mains. In general, if you're landing with any power and have any fuel on board, the nosewheel will be higher than this, but you have to plan for the worst case.
Most hang gliders are set up for flying prone. If you want to fly them seated, you will find that the control bar is usually too far forward for comfort. The reason for this is that in prone, the body's centre of gravity is behind the shoulders (between hips and belly button) and the bar position is usually set so that the bar is about level with the chin at trim. This makes it possible to pull in or push out the bar almost an entire arms length in either direction.
When you sit in a trike, assuming that you're sitting fairly upright, depending on the weight of the engine, the trike centre of gravity is usually somewhere aft of your bum. The heavier the engine, the futher aft it will be, but for a light engine, say, a Solo, the position will seldom be aft of your shoulders. This will not give you much ability to push the bar out for takeoff or while thermalling. And it's even worse if you prefer a more laid-back position.
With some faster wings, heavier engines and a bolt-upright position, it's often enough to use a speed bar with the belly reversed, but on floaters with light engines and a laid back position, it can be a real problem.
The position can be approached in a couple of ways. You can either just hope that it will all work out, and have an "interesting" first flight if you get it wrong, or do a bit of homework first.
One approach is to fly the wing as a hang glider (before beefing it up), and measure, using a builders dial guage, the angle the control bar makes to the vertical in a glide. From this, you can calculate the angle the keel is at to the horizontal.
Now, put the trike nosewheel on a block so that it is sitting at the same angle it was when hanging from the ceiling, then mount the wing to the top of the trike at the same angle you measured when flying as a hang glider. Sit in the trike and see if the control bar appears to be in a good spot, with room to both pull in and push out. If not, you'll need to make new front and rear cables for the wing to adjust the rake of the control bar.
This is not done lightly, as you may also need to make new side wires, because unless the top of the control bar is directly in line with the crossbar cable attachment points, you will increase or decrease the anhedral in the frame by moving the control bar basetube position. Have this work done by a professional cable crimper.
In our experience, the new hang point will be forward of the normal connection point for flying as a hang glider. We've seen them as far as 4-6" forward of the original point, usually because the pilot wants more speed out of the wing than it would normally trim at. It's generally best to err on the side of speed, as you do not want to be stalling all over the sky on the test flight! (we speak from almost-bitter experience here!) Most commercial fast trikes are trimmed more than 15mph above their stall speed. However, if you're building a soaring trike, you will want to trim somewhere between best sink and best glide.
Needless to say, the keel should have been well re-inforced to take the extra load. We recommend sleeving the full keel length, with triple-sleeve around the load area!
Thrust line
Try to get the engine and thrust line so that the thrust line is relatively high, as best you can, remembering clearance for keel and rear wires. This has a dual benefit. Firstly, you'll have less stones through the prop, and secondly, there will not be a massive swing-through of the trike base on takeoff.
In addition, the thrust line should be angled so that in normal flight with normal thrust, the thrust line will be essentially horizontal. That is, on the ground, the prop will be pointing slightly upward. Now, in flight there are tow forces opposing the thrust of the engine on the base. these are
- Gravity, because you are trying to rotate the trike higher; and
- The drag of the trike & pilot.
You can fairly easily determine (from similar installations) the amount of thrust likely to be available, and you can also determine the amount necessary for straight & level flight. (Based on estimated cruising speed and estimated weights and glide ratio)
The hardest thing to determine will be the drag on the trike/pilot, and you may have to do some empirical tests to determine it.
This in hand, you can calculate the resulting force (thrust-drag), and use a spring balance and a strong friend, with the base, you and engine hanging from the roof of the garage to determine the angle the base will swing through under power, hence the engine thrust line angle...
Wheels, wheels, wheels!
We used to use a lot of BMX wheels and fibreglass rod suspensions. Unfortunately, these wheels don't take side loads real well, and I've seen those fibreglass rods shimmy something frightful. I'd suggest looking at some wheels from one of the local ultralight or powered 'chute manufacturers, or if you want some rough field capability, Falshaw make some nice 16 inch balloon wheels. Also, a good nosewheel brake comes in useful - give Bill Sullivan at Holbrook a call! Another good investment is a dashpot damper for the nosewheel steering, just in case it's a wee bit more sensitive than you planned!
Engines and props
Early trikes were plagued by bad engine mountings. There are enough good examples now on commercial trikes available to copy that you should not have to re-invent one. The other great innovation is the availability of the IvoProp, with fully adjustable pitch. It used to be that once you'd settled on a wooden prop, you were stuck with it and the performance it gave. The Ivo puts an end to that, and as an added advantage, you can be sure of a nice, well-balanced fan that won't shake your trike apart, especially if you opt for a 3-blader.
In Australia, unless you can foot-launch and land your trike, you will have to register it under the "designer/builder" category, CAO 95.10. You will need to supply some basic information and drawings of the structure, but these are not too difficult. There is a new "experimental" aircraft category in the works which may also apply.