Thoughts on Aerobatic Glider Design
|“To be fully aerobatic a model aircraft should have, among other attributes, (i) three independent axes of control and (ii) the same control responses for upright and inverted flight.”
–Jef Raskin, Design Considerations for an Aerobatic Sailplane, RC Soaring Digest, Dec. 1992
Performance objectives for aerobatic gliders
We need to begin by clarifying what is meant by “aerobatic glider.” As noted in the introduction, most modern slopers are capable of some amount of aerobatic flight. However, as shown in the Aresti tutorial, there is a lot more to aerobatics than loops, rolls and stall turns. This section is devoted to gliders capable of “unlimited” aerobatics, or as the French say, Voltige Totale (literally “total aerobatics”).
“Voltige Totale” & airfoil selection
Voltige totale… This wonderful catchphrase sums up Jef Raskin’s thoughts from 1992 perfectly. A glider capable of voltige totale should be able to perform the same maneuvers whether upright or inverted – including square and octagonal loops, snaps, spins, and all the rest. It goes without saying that the glider should have a rudder in order to be able to do figures properly (especially snaps, spins and knife edge, but point rolls and rolling circles, too). The vertical stabilizer needs to be appropriately sized as well–otherwise the rudder may prove inadequate to the task.
One of the biggest advances since 1992 and one of the things that makes voltige totale very accessible today is the prevalence of inexpensive and powerful computer radios. Flaperons, and by extension snap flaps, allow the use of symmetrical airfoils without major concessions of gliding performance.
The well known MG05 airfoil by Marcel Guwang is a perfect example. By using flaperons and snap flap mixing, the symmetrical MG05′s performance envelope can be greatly extended into light lift regimes and square / angular loops that might otherwise be rather difficult for a symmetrically-airfoiled glider to perform. And of course, since the airfoil is symmetrical, the plane doesn’t know whether it’s upright or inverted… thermalling upside down really is such a treat!
This isn’t to say that semi-symmetrical airfoils are unusable for slope aerobatics, but what they gain in terms of upright performance (usually speed and greater coefficient of lift) they give up for in inverted and outside performance. One “happy medium” can be found in the use of a blend of semi-symmetrical airfoil at the root, with a symmetrical airfoil at the tip (one example: the popular SB96V / SB96VS blend found on the MiniToons, Le Fish, and other gliders). The semi-symmetrical root gives the plane a good fundamental ability to generate lift, while the symmetrical tips help inverted performance and, very importantly, ensure that snaps and spins can be entered with the same facility whether upright or inverted. (Full span semi-symmetrical foils will snap and spin more easily inverted than upright, leading to a difference in control response and a reduction in overall potential to achieve voltige totale)
“Something’s fishy”: Fuselage design
A glance at most any of the current crop of aerobatic slopers will reveal a common theme: tall, narrow fuselages with distinctively “fish” shaped side profiles. No, designers of slope aerobatics gliders are not closet fish fetishists; the large side area afforded by the shape enables the glider to fly knife edge and other side area-intensive figures like rolling circles more effectively.
When considering fuselage side area, it’s important to think of the fuselage as a flying wing, because when flying knife edge, that’s effectively what it is. The top-to-bottom dimension is the wing span, and the nose-to-tail dimension is the wing chord. Ideally, the fuselage should be designed such that the plane’s center of gravity will give the “flying wing” of the fuselage a reasonable center of gravity location… something between 25-40% back along the chord (fuselage length) seems to work reasonably well, based on the designs currently on the market today, but more research in this area is certainly warranted.
Logically following from the above is the need to give the top profile of the fuselage an airfoil shape… this will give the fuselage the ability to generate lift when flying in knife edge. Obviously a symmetrical airfoil should be chosen for this application, and beyond that, one which provides sufficient thickness in the tail area is also desirable so as to ensure the best material strength in the tailboom.
As of this writing it seems that some of the more recent designs are moving away from the fish shaped fuselage somewhat. Specifically, the Madslide and the Minitoons v2 feature much shorter nose moments and decidedly less fish-shaped fuses than the Voltij and Minitoons v1. In theory, this would allow the CG of the fuselage to potentially be in the same location as that of the wing, optimizing the fuselage’s performance as a wing (assuming suitable rudder throws & similar).
Assuming a symmetrical foil has been chosen, the glider should be balanced so that straight & level flight–whether upright or inverted–can be flown hands-off. This is essential, otherwise the pilot has to continuously fight the aircraft and cannot focus on the accurate performance of aerobatic figures.
Towards this end, it’s also important to pay attention to the lateral and vertical balance of the glider during its design and construction. If one wing is heavier than the other, the plane will constantly roll in that direction and will require trim to fly level – which will negatively impact flight performance, making figures different to perform when rolling (or snapping or spinning) to the right than left.
Likewise, the plane needs to be vertically balanced so that when it is flying knife edge it will have no (or very little) tendency to roll to the canopy or to the belly. While this tendency can be influenced by other factors, a low or high wing plane will normally exhibit greater tendency to roll in knife edge than a midwing plane. One should pay careful attention when designing and building to keep the radio gear as close to the vertical centerline of the fuse as possible, and (if space allows) even try to counterbalance the weight of the wing if the wing is slightly above or below the centerline.
Gliders can also exhibit a tendency, when in knife edge, to pitch either to belly or canopy. The effect is the same as if you had rolled to knife edge and then either pulled or pushed the elevator. This tendency can be affected by a number of factors, including center of gravity, rudder design, piloting technique, and more.