What are the good values in discus-launched gliders for beginners?
DJ Aerotech Chrysalis
About twenty years ago my favorite model was a DJ Aerotech Chrysalis. This was an early generation hand-launched glider. It had a peg sticking out both sides of the fuselage and you would throw it like a javelin.
This was a traditional built-up balsa design with a lightweight heat shrink covering. I paid $50 for the kit, which is still today the selling price. I remember it having a very thin wing with a lot of very light balsa ribs. On the first hard landing, all of the ribs on one wing snapped in two.
The highest launches that I could manage were about 50 feet (15 m). I did not catch too many thermals with this model, but that was not really the point. I had a great time flying it anyway.
Discuss-Launched Gliders (DLG)
Today, the rage in hand-launched gliders is discuss-launched gliders. Launch heights of 200 feet (61 m) are common. But along with increasing sophistication in designs, so have increased the prices. If you are serious about competing, be prepared to spend close to $1000 on your glider.
I do not know about you, but I do not have $1000 to spend on a model airplane. I am even less willing to spend that kind of money on a DLG. Simply put, the life of a DLG is tough. There are many launches and landings, with some landings even ending in hand catches. Each one of those is another opportunity for something to break.
The experts tell me that a $1000 DLG only makes sense have you have acquired enough skills to launch and land consistently without breaking it. Fair enough. So what then is a good beginner’s DLG?
Entry-Level DLG Market Survey
I asked some experienced friends and conducted my own research. I set an arbitrary upper price limit of $400 or so. I put the key numbers of the gliders into a spreadsheet and did some calculations.
For a couple of the gliders, a key number such as the wing area was not provided in the product description. In those cases I estimated the value based on other similar gliders. When a range was provided, such as for the flying weight, I chose the average value.
In a competition, the standard size of a DLG is 1.5 meters or 59 inches. There are several inexpensive gliders in this size. But I also noticed a large number of smaller wingspan gliders that appeared to be very affordable. Were they as good a value as they looked?
A calculated value that is often provided in product specification tables is the wing loading. This is a key performance number in a competitive DLG. A value of 9 ounces per square foot is considered excellent.
Problem is, wing loading is a two dimensional value that is being used to evaluate a three dimensional object. In other words, directly comparing the wing loadings of two gliders that are different sizes is not a fair comparison. We need a better measure.
A number I have used in the past and like very much is the scale factor. It is an indication of how an airplane will handle in the air. It does not take into account Reynolds number effects, but otherwise it works pretty well. The lower the scale factor, the slower the apparent scale flying speed of the model will be.
For a glider, a key performance metric is the smallest circle that it can fly. The smaller the circle, the better it can take advantage of thermals.
With the thin wings that model gliders have, a maximum lift coefficient of about one is a good guess. If a model has flaps, I estimate the maximum lift coefficient at 1.25. Both of these values are estimates, and if you disagree with my choices feel free to change them in the spreadsheet.
The attached spreadsheet contains the results of my calculations. The gliders are sorted by increasing turning radius. It is tempting to add a column that takes into account the glider’s cost and the turning radius or the scale factor. Feel free to do so.
Studying the table, a couple of favorites popped out right away. The first is the Elf sold by Kennedy Composites. The very low weight gives it pretty decent performance at a cost of only $199.
The other DLG that caught my eye was the TopSky 1.0C. This is an older generation glider, which sells for only $230. But it still has very respectable performance.
Of course, there are many important factors that this simple table is ignoring. For example, how much work is it to build one? How do they handle in the air? But at least, this is a good starting point for further research.