Construction

At this point we first think about what kind of plane it should be. Do we want a glider? A slope tiller or rather a high performance glider? Maybe we would prefer a powered glider with which we can go to competitions. And we build all this from wood, when everyone who knows about this subject uses GRP and carbon fiber? This somehow does not sound right.
More realistic is a PSS (Power Scale Soaring) glider made of balsa. Thermal gliders, motor planes, scale or not, all that can be built out of balsa but nothing that is "high performance" has in the name. The material is simply not good enough for that and that’s why you just don’t see wooden models on many competitions.

Research

This section is actually only important for scale models. Exemplary models should look just like the model. There are different levels, from "looks similar" to "looks like" until "the rivets are all in their proper place". Accordingly, you also need the appropriate documents. Most of the pictures are still available at Flickr. All other links are less worthwhile compared to Flickr. There is simply no "more" of pictures. Three-page views can be found with Google within minutes. However, some are inaccurate and should be compared with real photos.

If the internet research is too time-consuming (we could also use this time for building) just buy a book. Amazon and especially Moduni have good books. A cardboard kit is also suitable as a template. They are often very detailed.

Of course you can also just buy a construction plan. The publishers VTH and Neckar Verlag offer construction plans. But we wanted to construct and build an airplane by ourselves and therefore we forget that quickly again. Construction plans are for wimps :)

And last but not least: A real scale Research looks different of course. There are years of collecting books and taking pictures of the real plane. If you put in so much work, you can build in epoxy as well.

Design

Now it is about some details.

Profile

The profile should follow the concept. In particular one should consider the prospective flight speed and the weight. My great F9F-Panther PSS glider flies unfortunately only with much upwind because the profile R15 on the small stubby surfaces does not bring enough lift. Here one could have built a better flying model with a stronger lift profile.

My little HLG glider is doing quite well with the same profile and more weight though. This is simply due to a higher basic speed and more wing area.

Therefore my rule: the slower the thicker and more buoyant the airfoil. The limits are on one side with profiles with 8% thickness. This is difficult to build without massive use of composite materials. The other limit is ClarkY or MH32 with 12% thickness. If you can’t get your airplane into the air, you should build airships instead.

Stabilization

This means the tail. The tail compensates the moment of the wing. This is the force that pushes the aircraft out of normal flight attitude.

Therefore better check what kind of airfoil I have chosen. High lift airfoils have a larger (netative) momentum. S-beat profiles a positive.
Depending on this, I should choose a larger tailplane on a longer tail with more angle of attack difference. A symmetrical profile needs then rather less.

When I build a scale model, this is also the time when I modify the horizontal stabilizer. Especially with smaller models I have to increase the tail so that the model flies reasonably. A guide is 10% more area at 1:6 and 20% at 1:10. Without this measure the aircraft can not be flown.

And in exactly the same category falls the vertical stabilizer. If you are too modest here, your glider will always take the nose down in the turn. In the worst case, the airplane will then completely stall. (Rest in peace my little Spitfire) Ok, ok. A profiled rudder helps a bit but 10% more area should be planned for.

If it did happen: The glider flies with the right speed and straight, but needs constant corrections. It feels like you are balancing the plane on your fingertip. In this case land immediately and build a bigger elevator. I have lost two airplanes because I did not follow this rule.

Center of gravity

In general the center of gravity is about one third of the wing depth. You can also move it in a relatively large range. (For flying wing and duck models other rules apply). But one should be careful with swept wings. Here the center of gravity is still at 1/3, but related to the whole area. At the wing root it is then quite a bit further back. By the way, this also applies to wings that are not swept but only extend to the rear or have strongly rounded ends.

Glider

Control surfaces

I have already discussed the size of the tail, but how large must now be the rudders? With elevator I had so far few problems. A large elevator helps in extreme and slow maneuvers (i.e. landing). In the flight itself I had so far still no problems.
Rudder can also be omitted (except for large gliders or ground-launchable models).
With aileron I already had problems. For slow gliders with a large wingspan, there can be problems. I was forced to mix the flaps with the aileron on my 2.5m glider, otherwise I could not get around a turn.

Divisions

Normally we want to disassemble the airplane for transport more or less. We should now alreadyconsider which parts should be removable. At least the wing should be removable, if not also divisible. With larger models it makes sense to make the wing in three parts. This means we don’t need a plug in the heavily loaded center section and can use a single servo for the flaps. Also the flap distribution results from the division. With a high or low wing, the attachment is also mostly problem-free. Either with a plastic screw or with rubbers. The rule is: "The attachment should be broken before the wing is broken."
For purpose-built models (which sometimes have a harder landing) the wing should really be able to fly away forwards and backwards. Forward, if the plane ever hits somewhere. To the rear, in case the wing gets stuck somewhere. In both cases the surface should not be dented by the fuselage but rather by the obstacle (which hopefully is softer).
With a middle decker it becomes already more difficult. Here the wings have to be attached to the airplane somehow. And something must hold it then also still. The whole thing is much more complex to build. That’s why it’s best to avoid middle-deckers at the beginning.
A removable tailplane is also quite nice. A fixed tailplane always bugs me during transport.

Durability

A few more words about durability. Who has a "utility plane" If you want to build an airplane, it is best to leave out everything that could break off. So no antennas, especially not the small IFF antennas. Bombs and rockets are best left out as well. Planked wings are more durable than covered wings, but only if you don’t always use the lightest 1mm balsa boards you can find. Fixed tail units are also at risk. Removable just the same if you don’t take it off :)
The negative example: My SAL sailor is glued to the tail unit every few weeks. Not because it gets damaged during the spin cycle, but because I keep breaking it during transport.

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