Well, why are there no glider canards ("Ducks" in French) around,
in other words, why are so very few aircraft designed with
the stabilisator first, Donald asked.There are several reasons for that, some of them simple, some less so. First, the forward wing (of any plane) must always stall first. This leads to using more cambered airfoils on the front wing, or smaller chord, or higher aspect ratio, or high wingloading, and/or the use of extreme rigging angles. The Lockspeiser 3/4 scale prototype utility aircraft used three identical wing surfaces, the nose wing being set at an usual rigging angle, to lower its stalling speed, while the right and left wing panel had a pronounced dihedral, complemented with very big winglets. Its flying characteristics were described as interesting .... Not the most efficient of planes, but very cheap to manufacture, as all ribs were identical, the fuselage had constant cross section and so on. On a pure glider canard, when you are approaching stall, say in a thermal, you lose your elevator first, as it is situated on the front wing surface - which has to stall first unless you want to crash into the ground tail first - not a nice situation. So many practical canards, like the Saab Viggen just has trimming flaps on the forward wing, and have elevons/elevators on the main wing. Let's say we use a fairly high aspect ratio canard wing, of small chord, in front of the main wing. This will stall as predicted before the main wing and is possible to build without excessive costs. We now have to deal with the canard wing's tip vortices, that will hit the main wing midspan, and the difference in downwash angle for the main wing's innner sections and outer sections, a sure drag-producing way of designing an aircraft's wing arrangement. And fitting elevators to the main wing of a conventional canard means that they will be very inefficient, being so close to the CG, unless very big, so that is not a great alternative either. So let's say we have a high aspect nose wing, of greater span than the rear wing, with higher wing loading than the rear wing. Nice, good, simple. Sadly, this is just like how we design conventional, tail-equipped planes, so we already know that this works. So such a canard (following conventional design rules) probably works quite well, but we probably end up with a heavy plane (the high aspect ratio forward wing has still to be torsionally stiff, and as it is of smaller chord than the main wing it might easily weigh more than the main wing)! A bonus is that the extreme ends of the canard (beyond the span of the main wing) can be equipped with ailerons, to boost rolling power, but that is the nowm of conventional airplanes anyway, that the front wing carries the ailerons and the rear carries the elevators! Adding ailerons to the front wing of a normal canard, with a front wing of the same, or shorter span, than the main wing is a no-no, as the effects of the upwash/downwash from the moving ailerons will upset the flow over the rear wing, and might even result in control reversal! The aileron-equipped version of Lazy Bee suffers from this, to some extent, as the tail has such a big span compared to the wing, so it rolls not much better than the original version, which has no control surfaces on the wing itself. Tailerons, on the other hand, might work very well! As anyone tried it? Back to canards: So, while a canard can use flaps on both canard and main wing (the latter's doubling as elevators - flappevators?), things are much easier if we add a tail to this equation. Say we build a conventional plane with a conventional tail, but add a pair of auxilliary wing surfaces under the main wing, which we can rotate at will we are close to the ideal arrangement, as it doesn't really matter if it is the wing or theses auxilliary surfaces that stall first, as they are so very close together. This arrangement of wing surfaces is called Junkers' flaps (as used on Ju-52s and other aircraft) and is very efficient on models, too, as long as the gap is kept to a minimum. One student of Martin Hepperle did make some wind tunnel test the other year that proved that it worked well on models (have not seen the paper but had a letter from Martin about this). You can then use a main wing with almost nil camber with fullspan flaps, also of nil camber, mounted slightly below, and slightly forward, of the main wing's extreme TE. The control surfaces could as usual be subdivided into three sections, the outer acting as droopable/"crowable" ailerons, the middle as flaperons and the inner as pure flaps, or 80% flaps and 20% flaperons. Even Burt Rutan, who was initially inspired by the Saab Viggen, has stated that unless your aircraft is designed for very extreme use a conventional layout is always better. For deltas adding a delta, or swept, nose wing is sensible, as it improves the lift at high angles of attack, as it increases the wingtip vorticies. Once saw a guy with a tiny, cropped delta, say 4" in span with a small Cox up front, which had the smallest of canards I ever seen, say 3/4" long each side of the very tiny fuselage. These were fully movable, at least +-30 degrees, and slightly swept and quite simple. The elevon-equipped aircraft looped very, very tightly, but only after he added the canard, according to its designer! Now I wish you all a Happy New Year, Tord S Eriksson www.tord.nu RCSE-List facilities provided by Model Airplane News. Send "subscribe" and "unsubscribe" requests to [EMAIL PROTECTED] RCSE-List facilities provided by Model Airplane News. Send "subscribe" and "unsubscribe" requests to [EMAIL PROTECTED]
