Wednesday, April 11th, 2012 10:13 am GMT -6 Wednesday, April 11th, 2012 10:13 am GMT -6Wednesday, April 11th, 2012 10:13 am GMT -6
 
Pickerel prototype #2 - all three wing designs

I quickly built three half-size concept models. They all show promise. Which one do you like?


 

 

 

Pickerel Prototype #1

As reported earlier, the first prototype came out very tail heavy. Even if I had mounted the motor on the nose, the center of gravity would have been too far back. Back to the drawing board!

Rethinking the Design

Why was it so tail heavy? First, flying wing (tailless) designs are often flown tail heavy out of ignorance. If you use the common rule of thumb of putting the CG at 25% of the chord, you will be flying it tail heavy. Because there is no separate tail, the equivalent position for a tailless is 15% of the chord.

The combination of a relatively high taper ratio (0.5) along with a high sweep angle was not good. This put most of the lift production in the center with little wing area in the tips to balance things out.

Flying wing design spreadsheet

Flying Wing Spreadsheet

I put together a simple spreadsheet to help me understand all the variables that control how a flying wing will fly. Supporting up to three panels per wing half led to a large number of intermediate calculations. The rows containing these calculations are hidden. Only manually edit the rows colored in blue. All the rest are computed results.

Pickerel prototype #2 - crescent wing

Crescent Wing

Looking for a simple design that did not have much wing taper, I came up with a crescent-shaped wing. It is basically just a semi-circle.

I ran into two problems right away. First, this shape is harder to analyze. It is not easy to enter its numbers into my spreadsheet. Second, despite its simplicity, it was not easy to build. I had to cut it out freehand. No, I am not very good at cutting accurate curved shapes. Most model airplane builders are not, either.

The half-scale free flight prototype flew fine. It had reasonable stability along all three axes.

Pickerel prototype #2 - joined wing with raked wing tips

Joined Wing

A design idea that has gotten a lot of attention with full-scale airplane designers during the last ten years is the joined wing. Another very hot concept is raked wing tips. I decided to combine the two into a futuristic Pickerel concept design. The front wing sweep angle is 45 degrees.

Out of the three half-scale prototypes, this is the one that proved to be directionally unstable. Whenever I launched it, it was just as likely to curve to the left as to the right.

However, after I added a vertical stabilizer it proved to be the most stable of the three. The design shows a lot of promise.

I ran into two problems with this one. First, the shape of the gap between the wings makes fitting the motor a challenge. I really want the motor to be as close to the nose as possible, and this design does not let me do that.

Second, the design is pretty wide. It is hard to build from a single sheet of foamboard.

Pickerel prototype #2 - diamond wing

Diamond Wing

Looking to fix the problems with the joined wing prototype, I decreased the amount of wing sweep and got rid of the wing extensions. That led to the diamond wing design. Here the sweep of the front wing is down to 30 degrees.

The sweep of the front wing is a little more than the wing in the rear. Surprisingly, this design proved to be directionally stable without a vertical stabilizer.

This shape is very strong and fits in well within a sheet of foamboard. It was also very easy to build.

Feedback

I wonder if the best design would be a joined wing using 30 degrees of sweep. I’ll bet it would look and fly great. I do not think it would fit into a single sheet of foamboard, though.

What do you think? Which one do you like better?

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