Predator (oz15990)

About this Plan

Predator. Radio control slope soarer model. Forward-swept wing layout. Wingspan 43-1/2 in.

Quote: "Hungry for a challenge? Why not try building this futuristic Forward Swept Wing canard slope soarer? Prdator, by Andy McNally.

It was whilst thumbing through one of my Experimental Aircraft books at the unearthly hour of 3am whilst waiting for my five month old son to tire himself out that I came upon (once more!) the Grumman X-29, a Forward Swept Wing (FSW) aeroplane designed purely as a research tool. The aircraft first flew in 1984. It has proven to be extremely robust, with no hint of divergence, and is exceptionally manoeuvrable, though if Grumman had asked me first I could have told them that and saved them lots of money!

The first FSW aircraft flew in 1944, and was the Junkers Ju287, a prototype
bomber. Apparently, the Germans not only discovered some of the benefits of FSW, but also the main bugbear, Aeroelastic Divergence. An FSW is prone to twisting under loads such as control surface deflections and this phenomena can be oscillatory, building up very rapidly until structural failure occurs. Incidentally, conventional sweep does not suffer in this manner as any divergence is self-damping and tends to decrease over time rather than increase.

Advantages of Forward Sweep are potentially very great. It improves low speed handling and, more importantly, greatly improves the handling at the stall. Never will you have flown such a benign aircraft at low speed. The Predator has proved to be controllable at very high angles of attack down to walking pace!

On full size aircraft Forward Sweep also delays the rise in transonic drag, though I doubt the Predator will ever be flown to these areas of the envelope, but if any rocket-heads out there feel like trying!

Anyway, after a very short nights sleep, I sat at my desk in work sketching what I thought might be a practical design for a FSW slope soarer in the impression of the X-29. After a bit of fiddling with dimensions to get a reasonable wing area, I ended up with the design you see before you. Not very much like the X-29 now, however, apart from the forward sweep and the canards but, nevertheless, an interesting shape.

When I originally started laying down balsa, I wanted to leave the option open of flying the Predator without canard foreplanes fitted and I had a set of aluminium templates handy, hence the choice of a reflexed airfoil for the main wing. I have since discovered that for pitch stability with a forward swept flying wing, using a normally cambered airfoil, it is only necessary to wash in the tips slightly to provide a forward 'pitch-up' moment. Its obvious when you think about it! Therefore, a normally cambered section could be used, which would give better inverted performance into the bargain, though the reflexed one doesn't seem to adversely affect performance.

Another decision was whether to fit 'flying' canards or tack glue them in place to ascertain the correct incidence and then fix them permanently. I chose to make them 'flying' which turned out to be unnecessary - more about that later. The rest of the model is quite conventional balsa bashing.

Building: The wings are conventional in construction. Start by pinning down the 1/2 in leading edge followed by the 1/16 leading edge sheeting, cap strips and trailing edge sheeting. Glue in place the two 1/8 x 1/4 spruce lower spars leaving a protrusion of 20mm at the root. Glue in place all ribs, setting the root rib at 2 to 3 degrees to the vertical to provide a small measure of dihedral as sweeping a wing forward reduces its effective dihedral. Glue in place the top spars. Fit the servo bearer so that it sits atop the cap strips and fit the two fillets. butting them to the ribs. Now glue in the vertical grain 1/16 balsa spar web between all ribs, followed by the leading and trailing edge 1/16 top sheeting..."

Predator from RCM&E September 1996.

Direct submission to Outerzone.

Supplementary file notes

Article pages, thanks to RFJ.

Corrections?

Did we get something wrong with these details about this plan (especially the datafile)? That happens sometimes. You can help us fix it.
Add a correction