Swept-Forward Fighter (Free Plans)

Ever wondered what a plane with swept-forward wings would be like? Most of us have flown or built swept-back wings, but swinging the wing forward is far less common. I decided to design my own forward-swept plane to see if there would be any performance benefits or other interesting things.

The configuration I settled on is similar to NASA's X-29, with canards, a rear-mounted wing, and a single vertical stabilizer.

I thought this would be the most stable option because of the long moment arm between the wing and the canards. Another reason I chose this configuration is its stall characteristics. By setting the canards at 5 degrees of incidence, the canards stall at a lower overall angle of attack than the wing. This causes the nose to drop as the canards lose lift, pitching the aircraft down and recovering from the stall. The same principle was used on David Windestal's JA-37 Viggen jet.

With my design goals and configuration in mind, I sketched up some rough dimensions by hand and then refined them in Autodesk Inventor. The result was this sleek-looking aircraft:

(ok it's sideways but you get the idea)

I decided to name this aircraft the SR-3A Lionfish, after the SR-1 and -2 designs that I have already done. And "Lionfish" just sounds cool.  

Once it was modeled, I went ahead and built a chuck glider to test its stability and stall characteristics.

Surprisingly, the glider flew very well. It was very stable, just as I had predicted, and the stall characteristics I had designed it for showed through even when launched from shoulder height. The airframe is also very effiecient, with the fuselage producing some of the lift, creating an excellent glide ratio. The Lionfish turned out to be a great slow-flyer as well, maintaining a high-ratio glide with its nose way up in the air.

The few problems I noticed included a pitch-up moment even at forward CG, probably because the glider's light weight and the incidence of the canards caused an excess of lift forward of the CG. This probably won't be a problem with a large battery centered over the canards.

There also was evidence that adverse yaw could be a problem, but I later determined that this was probably because the wings and fuselage blocked airflow to the vertical stabilizer at high angles of attack.

Knowing this, I went back to the (digital) drawing board and made a few changes. I didn't want to fix what wasn't broke, so I didn't mess with many of the main design elements. The biggest thing I changed were the vertical stabilizers; the plane now has a v-tail with much larger stabs that provide both directional (yaw) and lateral (roll) stability since they are angled at 45 degrees from vertical. The stabilizers are much taller as well so that they still get plenty of airflow at high alpha.

I also added forward and aft hatches for electronics, an improved wing with a center section for extra strength, and internal braces for the canards. The nose is also removable and swappable, another page from David Windestal's book :)

The powered version of the Lionfish flew surprisingly well, just like the glider. I stuck to the plans for the most part, but the removable nose didn't work out. The plane also needed an astounding 70 grams of nose weight to get the correct CG, and I had some real concerns about climb performance because of that. Despite this, the first few flights were uneventful. The SR-3 turned out to be a fast, agile aircraft and it is a real joy to fly. In the air it reminds me of the SU-47 Berkut with its unique planform. Stalls were pretty docile because of the canards, and the plane kind of sinks down at high alpha instead of going into a full stall like most airplanes. Overall it is a very fun airplane and I definitely recommend building one.

First flights:

Free plans in PDF format are also available below, so if you want you can build your own Lionfish and start testing it! A few pointers for anyone who wants to try this:

• Keep the rates and aileron expo relatively low. It will likely have a pretty high roll rate, but it is not very sensitive around the low ranges.
• Make sure the thrust line is inline with the center of mass, or possibly a little above it. Any upward moment generated by the thrust will make the plane feel tail-heavy.
• Load it forward a little bit to counteract the canards' lift. The CG published on the plans is about 3/8" farther forward than the one that was tested, and for a first flight you may want to go farther than that. Be prepared to throw in some down trim.

Also keep in mind that the plans aren't perfect yet. The fuselage bulkheads aren't very accurate, so you'll have to measure your own. The nose is also difficult to build in the swappable form and if you build one you should just glue it onto the fuselage. If I left out an angle or something in the plans, measure it off the drawings. It will be accurate enough to use in the build.

Good luck, and stay tuned! If you have any questions about the plans or if you actually build one, let me know in the comments!

Plans page 1

Plans page 2

Plans page 3

Plans page 4

Plans page 5