Comment by paulmd | Hacker News Reader

Comment by paulmd | Hacker News Reader

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> You can also stiffen the wing by using a carbon fiber spar

this isn't really optional imo, this is a "should" and it should be at least one continuous spar going across the majority of the wing (don't put two ending in the middle, that's missing the point here). If you want, you can even do two - a shorter one in front and a longer one in back, or do a triangle-shape to strengthen the wingtips.

This is because EPP/EPS doesn't have enough structural strength on its own and with brushless motors you can definitely go fast enough to cause the wing to "flap", and that will result in the elevons flexing in the opposite direction (as the servo pushrod is attached to the wing) causing a "vertical fishtailing" effect in midair.

https://www.rcgroups.com/forums/showthread.php?1500999-The-d...

https://www.youtube.com/watch?v=yYfT0tQm668

(you can see this design is problematic because of the high sweep of the wings... there is frequently a little bit of wingtip that you can't reinforce, but, most of that wing is just hanging in midair, and those types are VERY prone to flapping.)

You can dremel out a spar channel and put the spar in that. The filler material of choice back in the day was Elmers' ProBond Polyurethene glue - mix it with some water and it'll foam up like crazy and you just pour it into the channel and wipe off any excess as it dries. Not to be confused with Elmers' ProBond wood glue though - that won't foam like that.

https://www.homedepot.com/p/Elmer-s-ProBond-4-oz-Advanced-Mu...

Also don't use EPS. EPP is far far more durable. And DO use a shrink-down covering like monokote or ultracote - the shrink-down covering will provide additional rigidity in addition to the spar. I tried using packing tape at one point instead, and it was definitely noticeably less rigid than ultracote and I ending up removing and replacing it.

https://www.horizonhobby.com/airplanes/supplies-and-bench-ac...

As a practical matter, I highly recommend contrasting color schemes - like red on bottom, blue with a wide reflective stripe on top, or something like that. Because from the ground it's super easy to lose perspective if you can't distinguish which side is up.

I have used carbon fiber tape in place of rods to good effect. I agree that a proper spar is critical for handling the loads. To be more precise, you need to deal with shear and torsional loads. These super-light cheap planes will easily turn into a taco in flight without care given to forces that will crush the foam. Even a layer of packing tape on top and bottom surfaces is better than just foamboard. I've had a couple of these planes over the years. I don't really enjoy them. I'm into high performance gliders (e-powered), ducted fan jets or helicopters.

> I'm into high performance gliders (e-powered)

that's neat, what is the sailplane community up to these days?

It really depends on where you look. Tastes in the US are different from what's popular in Europe. It also depends on the audience. As an engineer I tend to levitate (fly?) towards the more technically sophisticated and challenging aircraft.

For example, I have been building and flying what used to be known as "F5B" aircraft. Back when we were using NiCd batteries I had planes that used 27 cells to power very compact multi-horsepower motors in these gliders that often are not any larger than about 2.4 meters in span. Back then (I am talking '80's '90's) these planes could climb out of sight in 5 seconds. They are, aerodynamically speaking, highly efficient. After the climb the competition consisted in completing as many laps of a circuit as possible without power, gliding at high speed. Once done, you get to use the motor again, climb to altitude and thermal (in some cases they don't to this). Finally, there's a requirement for precision landing --the closest to land to the center of a target the more points you earn.

Aside from that, I am very much into what some call "full house gliders", this to mean having a full set of control surfaces: two ailerons, two flaps, sometimes two spoilers, rudder and elevator. These are not typically high speed monsters. These days you use electric motors for the climb. I still have a winch (which was the traditional climb-to-altitude method) but have not used it in decades. With a full house glider the objective is to find and exploit thermals and, if in competition, usually have a precision landing just as explained above. I have had the experience of successfully thermalling for over two hours laying on my back listening to music, landing only due to just being tired more so than anything else.

There are a lot of great options these days to get into the hobby. You don't have to have a full-house glider to experience thermalling like a bird. One can get started with modest kits that are pretty much ready to fly. The more technically minded will eventually want to learn how to build from scratch or seriously modify aircraft (or both). While I haven't done it in a while, I used to regularly make custom planes using a range of materials, from balsa wood for normal performance planes to designing molds to vacuum-bag fiberglass, kevlar or carbon fiber designs.

Here are a few examples of the above:

F5B (I have several planes that look and perform just like these). The motor is only used to climb.

https://www.youtube.com/watch?v=T0z9iy2SJA0

https://www.youtube.com/watch?v=59V9ICM75co

https://www.youtube.com/watch?v=d4J8VQ4iNwQ

https://www.youtube.com/watch?v=MNydX3aOTr8

This type of glider/event seems to be far more popular in the US:

https://www.youtube.com/watch?v=uoZ78c-tIDY

Adding to that, here's slope soaring, which is really cool. I haven't really done a lot of this due to having to travel far to have access to this type of geography. Beautiful silent flight, no motors required. They do add motors to some of the planes just so you have backup in case you lose lift. Here's an excellent intro to the sport:

https://www.youtube.com/watch?v=Ev5fSVi1PrA

And then there's the absolutely crazy/demented --in a good way-- dynamic slope soaring. I have yet to experience this. This is the domain of sophisticated carbon fiber and top-quality components. You can find videos of planes exploding in flight due to loads.

This is a long video with a detailed technical explanation of dynamic soaring:

https://www.youtube.com/watch?v=nv7-YM4wno8

548 mph (882 kph) run:

https://www.youtube.com/watch?v=0nyYaL0dGAA

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