And do it again (and do it… and do it again…)

I was going to write this post after I finished my first prototype. I had a lot of things I wanted to say about designing under constraints and rapid prototyping.

And then I spent one week completely overhauling the entire design and building it all from scratch. So, it’s been a long time coming, but here it is. The complete redesign, and build, all in 1 week.

To add to the awkwardness I got really busy and forgot to post this :/

So now reacap of 3 redesigns 2 rebuilds, and a lot of crashes!

But First, the author admits where she went wrong

To be perfectly honest, I was really happy with where the design had gone. I had a sleek fuselage, some parts had been printed, I was getting ready to sand, and then reality hit.

my beautiful, beautiful parts ignore the real bad attempt at unsupported edges

Plastic is heavy. Real heavy. Although I had thought I had done an adequate job reducing the thickness and infill, cutting down on weight, just 2 pieces of an eventual 10 weighed 180 grams. For a pretty small design, that is insane. For context, the entire fuselage in the current iteration weighed that much. This meant I had to make a huge change almost immediately. The whole fuselage needed a redesign

Design, Redesign, Redesign again

It’s always good to think about why things aren’t good. The previous iteration of the fuselage was full of empty space, had unnecessarily complicated wing mounting features, was very draggy, and was, well, way too heavy. So how do we fix it?

Aerodynamic design is where I feel most comfortable, so I started by addressing ways to cut down on drag. the front vent, despite its nice ability to keep the battery and esc cool, added tons of height to the fuselage, as well as drag. So I reduced the profile of the fuselage to the bare minimum: enough to fit components inside. the nose has a hole just big enough for the small motors that could power the aircraft, the interior large enough for the battery esc and receiver, and to cut down on drag, servos that fit within the fuselage.

The nice carbon spars running through the wings have slots setup in the fuselage for each one to fit and get epoxied into.

gotta make it as light as possible!

As you can see above the first version of the new nose relied on small pegs that would line up to the other parts (i had plans for threaded inserts). After a failed print I discovered these were too small and a bit of torque would shear them right off. To fix this i added a thick curved tab to the base of the fuselage with a hole for a threaded insert, and kept the two small pegs up top for alignment.

The wing mounts are designed to have a rough airfoil shape, while having a thick base for epoxying wings to, a slot for the carbon shim, and a built in slot that 9g servos can fit right into. Each servo slot also has built in wire routing.

In order to mount the motor, I left a nice open face, and a slot going through the nose, allowing a custom motor mount to slide in from the front, and a friction fit pin to hold it in. This was chosen due to the simplicity of assembly, light weight, and the fact that in flight it would require a substantial amount of deformation for the motor to even misalign itself.

the cone was too thin and didn’t end up printing.

For propulsion I’m running a grayson microjet v3 and a 7 inch folding prop. In theory I shouldn’t be running such a big prop… but it folds and I don’t need max throttle according to the design. Speaking of which…

In which the author gets himself on a tangent about design

So minor spoilers, the fuselage redesign coincided with a redesign of the wings. After some analysis and looking through similar airfoils I jumped for the bb3 airfoil, (which for some reason can’t be found online and I recreated and customized slightly from an image in a design report for a previous plane made by MIT DBF) (also the reason I chose the bb3 was I found it had better performance at the lower Reynolds numbers). Anyways one big change was resizing the chord of the wing, down from 5 inches to 3.5. This was done to provide enough lift to still fly at a manageable speed while massively cutting down on drag.

One of the big advantages that can be achieved with a tandem wing/box wing design is being able to use lower area or higher aspect ratio wings, (so long as the designer remembers to make incidence angle adjustments to ensure the aircraft can fly level without trying to pitch down from moment).

Unfortunately, design isn’t about focusing on each element independently (aero, prop, avionics, structure, etc.) It’s all about those overlapping characteristics. Spoilers for the section after the next, but I failed to consider how drastically the propwash would effect lift on only the front wing of the aircraft, and when the aircraft’s stability was focused around a balance between the lift on each wing, that being upset made things a bit, unstable.

Back to build! (Wing Edition)

For the construction of the wing I went for a foam core fiberglass/carbon tow skinned wing using carbon shims as mount points.

Wings with shims installed, ready for layups

All parts ready before the layup!

Wings after getting integrated into fuselage (splotches from filler used due to complications)

The carbon tow fishnet pattern ended up being a lot messier than I planned, and resulted in a pretty messy look, however the overall wing stiffness in bending and twist was phenomenal.

After some testing it became evident that the plane had some yaw stability issues, so two vertical stabilizers were added to keep the plane flying straight.

Fully integrated airframe!

First Flight test (of this one)

So foreshadowing strikes! The propwash meant that regardless of trim on the rear wing, at low speeds during takeoff the velocity over the front of the wing was so much higher that during takeoff the plane would enter a backflip.

After many attempts at using trim to keep the plane level or even trying to power through the backflip and build up enough speed to enter level flight, I realized it was time for a full redesign.

Lighter. Stronger. Slower

After lots of redesign, using thinner walled prints and a fuselage designed for fitting the battery and receiver neatly inside with access panels, keeping the removable motor system and breaking it down into 4 parts I was able to drastically reduce the weight of the fuselage.

Some new considerations were also made, including using built in channels in the fuselage for mounting the wings to make aligning wing incidence easier.

Sooooo much more aerodynamic

Up next was wing redesign. I started the aero sizing and stability design from scratch, using a slightly smaller wingspan to accomodate for the expected weight drop, Making the rear wing a NACA 0010 foil instead of a bb3, and adding a vertical stabilizer to support the rear wing.

I did some experimenting with wing manufacturing once again. To make a much cleaner surface I used waxed mylar sheets in the layup process, and tried reinforcing the rear wing using spar caps of carbon tow. The result was some of the nicest looking wings I have ever made.

Some bagging issues occurred where the lower sheet of carbon crosshatching slid as the vacuum pressurized, causing the tow to wander off of the wing

In addition to the fuselage and wing changes the propulsion system was significantly downsized to match the required thrust much closer than the motor used out of convenience before.

After some wire bending and gluing, the whole plane came together quite nicely! I also traded the 4 control surface with linkage setup for 2 elevons directly bonded to the servo horns, This was to reduce control complexity, as well as cut down on weight.

Upgrades people, Upgrades


Some final issues I encountered during the flight test was the angle of incidence on takeoff was too low, meaning the ground roll took the entirety of the ground strip I found, which meant I lost line of sight during climb, forcing me to ground the plane to avoid unintentional collisions. Unfortunately this spelt the end for the airframe.

Its hard to see, but it managed to take flight!

Whats next?

After a long break (summer internship was really busy but also very exciting!) Im starting work on a bigger (and hopefully better) plane using a hybrid of foaming pla, foam, and composites. So far, so good :).


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