Hot Lemon (oz15497)

About this Plan

Hot Lemon. Radio control sport model, for electric power with 280 motor. Wingspan 32 in.

Quote: "Speed 280. That's a misnomer if ever there was one. Let's face it, most 280 powered models waft around halls, or venture out to the local park only on the calmest of evenings. The specifications of this little plane may look 'park-flyer' but the performance is 'big-field'. Low drag is the reason.

In the last few years, motors, batteries - and our ability to match them to each other, have improved greatly. Just about anything can now be made to fly electric. But commercial designs, despite Star Wars looks, often have WW1 drag. Foam is floppy stuff that must be used in thick, blunt chunks. Drag increases quickly with speed. Even with high power, a draggy design will never go very fast.

A clean design flies differently to a high powered, draggy one. Not that Hot Lemon is under-powered. It jumps off the ground with great eagerness and has a good rate of climb. It will do large, round loops from level flight. But it really shows its character when you lower the nose. The mdtor unloads and winds up to a turbine-like whine. It does a fast pass worth the name (aren't you sick of 'Was that a fast or slow pass?') - and you can do a fine victory roll and climb-out on the momentum.

Low drag increases duration too. Hot Lemon needs only a whiff of power to stay aloft. At high speed the current draw is also lower, because the unloaded motor revs higher. This in turn reduces the heat generated by the motor, ESC, and battery. Less cooling is needed, which reduces drag even more. A virtuous circle.

It's rather nice to know that you are achieving this performance with the cheapest and simplest power system available. Sort of following the footsteps of Rutan and MacCready. It's also satisfying that using your building skills you can fly faster, higher, and longer than mass produced products.

Missing Ailerons? There is a misconception that ailerons are essential for precise control and aerobatics. I think this is because people associate R/C with the 'trainers' that they learned to fly on. These often have insufficient dihedral, a rudder in the most disturbed portion of the slipstream (which is really disturbed because the fuselage has similar aerodynamics to the box it came in), a rudder that often has insufficient area and moment arm, and is placed so as to oppose the roll it is supposed to induce. I may have more to say on "trainers" one day if allowed. We wouldn't teach people to drive in Standard Vanguards.

Full-size aircraft need ailerons to control cross-wind takeoffs and landings. Scale models usually need ailerons if they are to retain scale dihedral. Chip, Christophe, and Quique need ailerons because they must be able to do every possible manoeuvre - but Hot Lemon is better without them.

Hot Lemon does excellent rolls. The impression that a roll is axial has more to do with speed, CG, and elevator control than the means used to induce it. Many small aileron craft that look as if they should, can't! Granted, without ailerons, extended inverted flight becomes dicey and rolling circles and knife edge are out, but Hot Lemon still has an extensive repertoire of manoeuvres, many of which its speed enables it to do better than lesser aileron craft. Landing and takeoff control is better too. (How many RC pilots can do full-size type cross-wind corrections?) Spectators will have no idea that you don't have ailerons.

When we forgo ailerons, many advantages follow. We save the weight of at least one servo and the weight of the ailerons, supporting structure, hinges, and linkage. We save the drag of a very long hinge line. The wing does not need to be so stiff - more weight saving. Aileron wings must resist the ailerons twisting the wing the 'wrong' way at high speed. If the wing isn't stiff enough, this causes non-linear control response if you are lucky, and control reversal if you're not. The structure of Hot Lemon's wings would need substantial revision to carry ailerons.

Aileron aircraft often need exaggerated washout to avoid adverse yaw near the stall. This increases drag at high speed, because the outer wings are then lifting downwards. The inner wings have to lift harder to compensate, creating even more drag.

Preliminaries: Before you get out the building board, make three copies of each of the drawings. Two are to cut up and stick together so that you have the wing and fuselage in their entirety. Tape them together over a window with the light behind. The third copy is handy for patterns.

Wood sections are quite small, so medium balsa is OK for everything except the wing main-spar, which should be hard, and the tail surfaces which should be light.

Make a foam pattern for the upper ribs, or better still, cut the pattern from hard timber with a band-saw. Laminated ribs are much stronger than the usual sliced ones. Bind four sheets of wet 0.8mm balsa to the form with cotton tape. Leave for a couple of days to dry. Remove and coat very thinly with epoxy glue, spreading the glue with an old credit card or similar. You would think that PVA or aliphatic would be OK, but in practice, you will find that the finished lamination can warp badly due, I think, to varying rates of drying of the inner and outer layers.

Bind to the form again and leave for the glue to harden. Clamp a fence onto your Dremel saw and in about thirty seconds you will have enough 1.5mm ribs plus a few spares..."

Hot Lemon from R/C Model Flyer, May 2003.

Direct submission to Outerzone.

Supplementary file notes

Article pages, thanks to RFJ.

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