The canard. Why such a configuration? Credit : Jean-François Edange

Transcription

1 The canard Why such a configuration? Credit : Jean-François Edange

2 N obody doubtless knows that a great majority of light or heavy planes share a common design. Schematically, we find a fuselage, wings to provide lift, and empennage at the rear. Mobil elements on the wings provide roll control, and other on empennage (elevator and fin) pitch and yaw. More heavy planes can have engines on the wings rather than in the fuselage axis, tractive or propellant, however aerodynamic formula is the same. Besides control elements, wings and stabilizer had a less visible but essential function : natural stability on the longitudinal axis. On flying wings (planes without fuselage) stability is acquired either with a very special profile, but incompatible with high performances, or thanks to a very complex computer, like the impressive US bomber B-2 (artificial stability). Indeed, except for this kind of plane, wings only are not sufficient for longitudinal stability. The lift is produced by difference between intrados and extrados airflow, and pitch axis roll around the gravity center, pitching the nose down. That is why aft stabilizer act like a small wing, but in an inverse law. It produces an inverse lift, a kind of aerodynamic force toward down and adjusted with pitch by the pilot. This essential action is for counterbalancing diving tendencies. Although this formula is widely used nowadays, there is a major drawback. Indeed, in this case, the wing had to produce more lift. This one must be equal to plane weight plus the tail lift (inverse lift produce by the stabilizer).

3 S ome planes use of a different formula said «Canard» (it s a French word for «Duck»). This one is uncommon nowadays, but we can see it on the first plane in the world : the famous Flyer of the Wright brothers in Nowadays, some military planes (Dassault Rafale ) or civil business planes (Beech StarShip, Piaggio Avanti ) try to take advantage of the formula. The goal is to minus this drawback. To achieve this, the stabilizer is put in front of the wing instead of behind. Like this, the necessary lift to balance the diving tendency of the main wing, will be in the same way. Consequently, at a same weight, a Canard need a smaller area for the wing because it needs minus total lift. Front stabilizer adds to wing instead of subtracting. Smaller wing means less weight, less drag it looks like a virtuous circle.

4 A century after airplane invention, why Canard planes are so few? Obviously, because aerodynamic law is not as simple as that. And secondary effects can spoil this perfect plan. The stall conditions change the problem data. In fact, with a classic plane, angle of attack of the main wing is such as it stall first, before the stabilizer. In any case, even if the stabilizer stall, the plane pitch down, and the pilot can control the plane because the airspeed increase immediately. Of course, for canard planes, effects are exactly inverted, and stall occur with a pitch up movement. In spite of the fact that stabilizer stay usable by the pilot, the situation is very dangerous and can lead to a quickly lost of control. To balance this important inconvenient, we need the wing never stall before the stabilizer anyway. To do this, two possibilities : - Play with the area wing versus stabilizer ratio. Increasing wing area will make the lift more important and allow stabilizer stall first. This way, like the conventional airplane, a down pitch movement is activated, increasing speed and aborting stall. But the dark side of this method is a bigger wing, with more weight and more drag. And this wipe out near all the formula advantages. - Play with relative angle of attack (AOA) between wing and stabilizer. This one is adjusted with an AOA more important without increasing area neither wing nor stabilizer. This way, stall of the stabilizer occurs first, increasing airspeed during pitch down movement. Advantages of the formula are preserved, there is no extra weight or extra drag. Note that a canard plane doesn t really stall. In fact, when a lack of lift occurs first on the stabilizer area, the plane pitch down and quickly recover alone a correct speed, even with full aft stick position. So it descends slowly like a leaf in a permanent oscillation. Other important characteristic : spin is practically impossible, cause of full authority in the transversal axis during the stall. Wing ailerons are efficient anyway to roll the plane, and maintain lateral stability with no tendency to engage a turn in any side. Meanwhile there is another bad characteristic about canard formula : a greater sensibility in gusts and turbulence than classic planes. Indeed, remember that a rear stabilizer induces a force in the opposite direction than the wing. So the up movement of the wing is just about balanced by the down movement of the stabilizer. The front one of the canard plane, one more time, act in the same way, at the opposite direction versus classical formula. This accentuates turbulence effect and make the plane more unstable and more difficult to pilot.

5 Sure, perfect plane doesn t exist. Just like in every field, all is a matter of compromise. To summarize : Pros canard : - aerodynamic more efficient - less weight and drag - more cruise speed for the same power - less fuel consumption for the same speed - stall characteristics very safe (particularly spin conditions) Cons canard : - More speed needed for takeoff and landing - Hyper sustentation (flaps) almost impossible or very complicated - Minus AOA control in landing configuration - Longer runways needed

6 In practice Especially for landing, all these differences between classical and canard plane lead us towards very different characteristics. With a classical plane, wheels touch don t really result in a stall very low, near the ground, but the flare is relatively easy and progressive by increasing slowly AOA. If correct, this action leads to a smooth touchdown on the main wheels. A canard plane doesn t allow such a precision during flare. There is minus progressiveness in the pilot control. Don t forget than the AOA stabilizer is adjusted for stall first. If the flare movement is too strong, front part of the plane will be quickly falling down and touch on the front wheel. This wrong move is seriously not advisable. In concrete terms, a canard plane needs much more speed in landing configuration. Final approach must be flatter than the standard plan, with an AOA less important, thus the roll distance will be higher and the braking action stronger. A canard plane is definitely not a STOL! and few accommodate small runways. Beyond this, use of flaps is complicated. We must have both on wing and stabilizer, a complex arrangement especially for the front area (mixed with elevator). More difficult will be the balance between aft and front. Few canard planes have flaps, and the Piaggio Avanti of XPlane is a rare exception. (Easier to make in a simulator than in the real life!). On the other hand, some of them have airbrakes, but this is not at all like flaps, and they don t produce extra lift, only extra drag. This is useful for a quick descent to preserve engine temperature, or to lower the speed and take a slope stronger without increasing velocity, but it doesn t really help in the landing process. The Velocity V-Twin of our package has no flaps and no airbrakes, just like the real plane. This is why it s important to understand all these special characteristics before the first landing (although you neither don t risk life and limb, nor break your new toy, of course). Equally, please, respect rigorously recommended speed (see the POH). The flight model of this plane is well reproduced as indicated in the manual, thus this is an interesting challenge to come to grips with this exotic and unusual formula.