Tuesday, April 3rd, 2012 11:19 am GMT -6 Tuesday, April 3rd, 2012 11:19 am GMT -6Tuesday, April 3rd, 2012 11:19 am GMT -6
 
Pickerel logo

This small tailless is designed to go together very quickly.

 

 

 

Motivation

A couple of years ago I designed and flew a very simple unswept flying wing glider. These are commonly called planks. I was just trying to satisfy my curiosity. I went to a local park and threw it around for a couple of hours.

I thought the basic idea had a lot of promise. I always meant to follow it up with a powered version, but other projects kept me busy until now.

Pickerel model airplane design logo

About the Name

Tailless airplanes tend to have low pitch moments. That makes them pitch sensitive if you are not careful. With a low aspect ratio, I expect this design to have a fast roll rate. A pickerel is a young or small adult pike fish. Given the feisty reputation of the pike, the name seemed appropriate.

I want to try flying this on a slope as an unpowered glider. I also plan to scale it down for an indoor version. There is no reason why it could not be scaled up, either.

Kilo3D

After a few days of working on this new model airplane design, the power system for Kilo3D finally arrived. Most likely, I will be building and test flying prototypes for these two designs at the same time.

Pickerel is a smaller and simpler design than Kilo3D. If you are new to scratch-building, it is a much better choice as a first project.

Design Goals

With every new design, I like to push the boundaries of what I have done before. I also apply the lessons I have learned, which by itself usually pushes me in new directions.

With Pickerel, I am pushing the boundaries of simplicity, cost, and versatility. Yes, I know that simple foam delta-type flying wings have been done many times before. But I believe I can bring something new to the concept.

Power System

The power system for Pickerel is exactly the same as for ModiFly. A 25 gram motor, 500 mAh 2S LiPo, 10A ESC, two 5 gram servos, and a 8×3.8 propeller. This power system is inexpensive, readily available, and proven.

Foamboard

Simple and inexpensive always tend to be major design goals for me, specially on fun projects like this one. I decided to only rely on foamboard. It can almost be built from a single 20×30 inch foamboard sheet.

Long skinny strip ailerons made out of foam do not work well. A thin piece of foam is just too flexible. I decided to opt instead for squarer elevons (combination ailerons and elevators).

Pickerel diagram

Guide to Diagram

Each square on the diagram represents one square inch. The design only uses 3/16 inch (5 mm) foam from foamboard. Remove the paper surfaces from the foam. The orange area has two layers, with a second layer on top. It is not a symmetrical wing like Kilo3D. The green part is a single piece of foam almost the size of an entire sheet of foamboard. Yellow are the elevons.

For strength, it should not be hard to see that there are lots of triangles in the design. Also, all the pieces of wood are tied to each other and form a very strong skeleton.

Three types of wood sticks are used for reinforcements. Small round hardwood dowels, 1/8 inch (3 mm) in diameter, are used to reinforce the leading edges against crash damage.

The main spar is a single piece of spruce. Installed vertically, it measures 30 x 1/8 x 3/8 inch. So it is just as tall as the two layers of foam. A similar piece of spruce, also installed vertically, goes from the main spar to the nose of the airplane. I expect this piece of wood to dramatically improve on the crash resistance of the model.

The plan is to use a stick mount for the motor. The mount raises the motor slightly and clamps onto a 3/8 inch (10 mm) square dowel.

That means that I have to use a relatively large (read: heavy) square dowel for at least part of the build. For the sake of simplicity, I extend this square dowel forward all the way to the wing spar. Note that since the green area of the foam is only a single layer, half of this square dowel will be exposed.

Designing a Tailless

Because the horizontal tail is part of the wing trailing edge, tailless designs have more constraints than a conventional airplane.

A key decision to be made was how much wing sweep to use. More sweep is generally better for a flying wing. There is no dihedral in Pickerel, so I needed sweep for lateral stability. I wanted to minimize the size of the vertical stabilizers, which sweep helps with. Sweep also lowers the need for washout, which in this case is expressed by using slightly upturned elevons.

Too much sweep leads to structurally weaker designs and less efficient wings. The sweet spot is in the 20 to 25 degree range, with 15 to 30 being a good practical range. Even though the leading edge of Pickerel sweeps back at 34 degrees, the actual sweep angle for the entire wing is 26.6. The taper ratio is 0.5, which is also in the recommended range of 0.5 to 0.7.

Pickerel weight estimates

Design Challenges

Adding up the weights for the components, I came up with about eight ounces (225 g). I have to allow for the possibility of the actual flying weight being closer to nine ounces. The planned power system should be able to handle an airplane this heavy, but it is definitely at the limit of what I am comfortable with.

This model has a large wing area, about twice the size of ModiFly, but the flying weight is only slightly higher. Coming up with a lightweight but stiff design was not easy.

Instead of using winglets, I opted for vertical stabilizers at mid-span. The trailing edge is not swept, so there is no loss of directional stability by placing them closer to the wing center line. Just the opposite. Because of the elevons, winglets would have to be located closer to the leading edge.

Putting the vertical stabilizers at mid-span makes them act like wing fences. You see, when a wing starts to stall there is a lot of cross flow along its span. The fences keep a stall from spreading to the elevons, maintaining their effectiveness.

Why not a symmetrical airfoil? I wanted to be able to handle a wide range of flying speeds. A design goal was having a thin, high lift airfoil. Symmetrical airfoils are not as good at producing lift.

As shown in the diagram, the airfoil is very thin. At the tip it is just 3.5% and at the root it is a skinny 1.75%.

Work in Progress

Just like Kilo3D, this is a work in progress. If all you want is a well-tested design to build, wait until I am done. If you want to learn about airplane design, you have come to the right place.

I already have changes in mind beyond what I showed in the diagram. For example, I plan to add a third layer of foam at the nose to increase the lifting power of the wing. Shown by a dotted line, it will only add a small amount of weight.

Am I on the right track? What would you do differently?

Pickerel article index and discussion thread in forums.

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