V2: Version 2 is now available! The pinch grip has been removed and replaced with a hook to launch the glider with a rubber band. A rubber band holder/handle is also included. (Gif of V2 flying has been added at the bottom of this post!)

This glider is inspired by the stereotypical single seat fighter jet appearance and has a pinch point on its underside to allow for easier hand launching. I have future plans for a rubber band lauched version as well. The design of the glider follows several key aircraft design and 3D printing principles, more details about this are given in the Aircraft and Model Design Principles section below.

There is a bit of a knack to throwing this model, similar to throwing a dart. Once you get it, it glides quite nicely, I do however recommend that you aim for something soft or allow it to land on grass as the wings are liable to crack if you throw it into a wall!

This model consists of a main body section printed in vase mode and a nose cone printed using regular settings. The main body prints nose down with the vase mode spiral printing from cockpit to tail. The nose cone is printed separately so its weight can be adjusted using infill percentage, allowing for balancing the aircraft. I have detailed my settings and filament below, you may have to print the nose cone with different settings (print a few) and test out which works best for you depending on the density of your filament. I have also included an STL of the whole plane so you can cut it yourself if you require a different length of nose cone.

Throwing the model: It works best if you throw it quite fast, that way the aerodynamics of the model can do more. Try to angle it just slightly upwards to get the best gliding effect. 

This video shows some of my test flights. It glides very well but as it slows up it pitches back a bit. The nose is probably a tiny bit light in this video so it could maybe do with being printed at 70-80% infill instead of 60%.

Print settings

The settings used are contained in the .3mf files attached, here is a summary of the most important ones. Printed using the standard 0.4mm nozzle.

Main body:

Filament: Polyterra PLA - white, 205 degrees nozzle, 65 degrees bed

Layer height: 0.2mm

Supports: No

Vase mode: Yes, 3 solid bottom layers, accept PrusaSlicer vase mode suggestions

Nose cone:

Filament: Polyterra PLA - white, 205 degrees nozzle, 65 degrees bed

Layer height: 0.2mm

Supports: No

Vase mode: No

Infill: 60%, grid

Assembly

The nose cone needs to be glued on to the main body, I find regular super glue works well. There is a small notch on both the nose cone and main body that you need to align when glueing to get the nose cone on the right way up. Try to use a small enough amount of glue so that it doesn't squeeze over the edges as you need to align the edges by hand for a smooth nose cone to body transition.

If you are testing different nose cones, use barely enough glue to hold it on as that way you may be able to snap it off and test another one. Or use small pieces of tape to hold it on and glue the version that works best. I found that just less than 3g weight for the nose cone works well for me.

Reinforcement

The model can break when involved in a big crash as vase mode only allows for a single layer, so the main body isn't super strong. Try to land softly! However, a thin layer of glue/nail polish/tape in some key areas may help durability. These areas are on the body near the start/end of the wings/tail plane, and along the leading edge of the wings.

Aircraft and Model Design Principles

This model was designed following some general rules of aircraft design detailed below. 

Airfoil shape

The airfoil shape used was created using splines but follows the general principles of airfoil design for low speed flight. The top surface is cambered whilst the bottom surface is fairly flat. When angled into oncoming flow the bottom surface serves to “squish” the air a bit and increase pressure under the wing, whereas the top surface forces the air to bend downwards as it passes over the surface, therefore lowering the pressure above the wing. The pressure difference between the top and bottom surfaces results in lift on the wing. 

Another way to think of lift is to think of throwing an object. E.g. when you throw a bowling ball or a baseball etc. it feels heavier in your hand as you accelerate it away from you. The object is pushing back on you as you push on it. Wings do the same thing to air. They “throw” the air down which in turn “pushes” the wing upwards. The angling the wing upwards helps to generate lift as this intuitively forces air down, and at low speed flight more cambered wings are used to help curve air downwards over their upper surfaces. 

Since the model is small and flys slowly, a large chord length (leading to trailing edge distance) was used to generate enough lift for gliding. I will test even more aggressive camber and angles of attack to see if the performance can be improved.

Winglets

The curved wing tips shown in the above image are known as winglets. These help to preserve the lift generating capacity of the wing. On top of the wing there is low pressure and beneath the wing there is high pressure. As such, the air tries to roll over from the bottom of the wing to the top which can lead to loss of lift. The winglets make this harder and the wing can generate more lift for a given wing span.

Dihedral

Also shown in the above image is wing dihedral. This is where the wings are tilted towards each other slightly (each wing tip is higher than the wing root). Dihedral adds roll stability to the plane, if the plane starts to roll one way, then the wing that is getting lower “sees” the air coming in at a more aggressive angle, thus produces more lift, and this serves to correct the unwanted roll.

Wing twist

The wings start out at an inclination of 5 degrees at the root, and finishes with an inclination of 2 degrees at the tip. This design consideration is more important on large scale aircraft but it was fun to include nonetheless. Since the root of the wing is larger it produces more lift anyway, and generally has more structural integrity than wing sections near the tip. Thus, a higher angle of attack can be used here to generate more lift close to the root without wing failure. Secondly, roll control surfaces (a type of flap) are generally near the wing tip, therefore it is desirable to have the wing stall (lose grip on the air) close to the root such that the pilot can still control the plane and recover if the wings begin to stall. These are obviously not that applicable here as the model is small and has no control surfaces.

Centre of mass

Aircraft are generally more stable when the centre of mass (COM) is further forward than the centre of lift. The centre of mass and centre of lift should be fairly close together, with the centre of mass slightly further forward. Objects rotate about their COM, so with no tail plane the aircraft would tumble forward. The tail plane generates a small amount of “lift” downwards, and therefore balance the plane. Since the tail plane lift has a longer moment arm to the COM than the wing lift, only a small amount of downwards force is required to balance the plane. On large aircraft this also helps prevent the aircraft tipping backwards when its on its landing gear - on the ground no lift is produced so a far back COM would make moving aircraft around on the ground much harder.

There are many reasons to balance aircraft like this, most are only important for larger scale aircraft. Here the main reason is gliding balance is more easily achieved with the configuration shown above. If the COM was too far back the aircraft would pitch up, this is the reason for the heavy nose cone to achieve aircraft balance.

Horizontal stabiliser

As discussed above, the horizontal stabiliser serves to control the aircraft's pitch during flight.

Finger pinch

The finger pinch spot for throwing the plane is placed close to the COM to make it easier to throw.

Vertical stabiliser

The vertical stabiliser provides yaw stability. If the plane yaws the vertical stabiliser produces a counterative sideways “lift” (as it is essentially a sideways wing) that corrects the yaw and striaghtens the plane. Much like a flag in the wind, the vertical stabiliser “wants” to align with the direction of the air flow and thus provides yaw stability.

3D Modelling Considerations for Printing

The main design constraints on the model were imposed by printing it in vase mode. As such, no supports could be used, the leading edges of the wings had to be swept, and the trailing edge had to be straight. The leading edges had to be swept as a horizontal leading edge would require supports. The trailing edge had to be straight in order for vase mode to work, if the trailing edge was swept like the leading edge then vase mode could not reach the wing tips. Another solution would be to have the trailing edge swept back towards the tail, however this would make the wings very large. 

The overhang angles of the leading edges are about on the limit of the printer but with sufficient cooling and 0.2mm layer height (or less!) they print fine.

V2 flying

Bit windy but flies very well!