À pleine charge, un Boeing 737 pèse environ 80 tonnes et un Boeing 747 peut peser plus de 400 tonnes au décollage. Et pourtant, ils volent !
Le secret du vol des avions réside dans la forme de leurs ailes. D’abord, elles sont bombées sur le dessus et plutôt plates en dessous. Lorsqu’une aile avec un tel profil avance, l’air passe normalement dessous, mais l’air qui passe au-dessus a une plus grande distance à parcourir, car sa trajectoire forme une légère courbe.
Mais l’air ne peut pas prendre plus de temps pour parcourir cette distance, il y aurait formation de zones » vides « , ce qui est impossible.
Alors l’air passe plus vite sur le dessus de l’aile. En 1738, un physicien suisse du nom de Daniel Bernoulli a découvert que lorsque de l’air accélère, sa pression diminue. C’est ce qu’on appelle maintenant le principe de Bernoulli.
Bref, l’air sous l’aile a une pression normale, alors que l’air sur l’aile a une pression plus basse. La pression de l’air est donc plus forte en dessous, ce qui pousse l’avion vers le haut.
Mais ce n’est pas tout. Ce phénomène à lui seul ne peut pas expliquer le vol d’un avion. L’aile de l’avion n’est pas parfaitement horizontale: elle pointe légèrement vers le haut. Lors d’une sortie en voiture, sortez votre main par une fenêtre ouverte (faites attention aux usagers de la route) et tennez-la bien à l’horizontale.
Elle reste sur place, en fendant le vent. Mais inclinez-la juste un peu vers le haut, et votre bras est tout de suite soulevé vers le haut. Votre main, comme l’aile d’un avion, pousse l’air vers le bas, et par réaction, elle se pousse elle-même vers le haut.
Ces deux effets combinés génèrent ce qu’on appelle la Portance. Mais cette portance ne peut apparaître que si l’avion avance, et plus il est lourd plus il doit avancer vite. C’est pourquoi les gros avions de ligne ont besoin de plusieurs gros moteurs très puissants… et très gourmands en carburant !
Et si nous parlons un peu d’aérodynamisme ?
L’aérodynamique étudie l’interaction entre un objet en mouvement et l’air qui l’entoure. Sans cette science, l’être humain n’aurait jamais pu voler.
Les avions ne volent pas dans le vide. On ne le voit pas, mais l’air est bien existant.
S’il est possible d’avancer dans l’air, c’est parce que les molécules de gaz sont mobiles. Dès que l’on avance d’un pas, on déplace un nombre incalculable de molécules. Quand on marche, affronter l’air n’est pas une tâche bien difficile ; mais plus la vitesse augmente, plus l’air oppose de résistance.
L’aérodynamique permet de comprendre comment cette résistance agit sur les corps en mouvement. L’air ne bouge pas n’importe comment et il faut étudier ses mouvements pour mieux s’y déplacer.
Vous vous êtes déjà demandés pourquoi les avions ont à peu près tous la même forme ou silhouette ? Ce n’est pas un hasard.
Certaines formes s’adaptent mieux à la résistance de l’air que d’autres.
En fait, plus la surface qui frappe l’air est petite, plus il est facile d’avancer.
C’est pour cette raison que les avions ont une forme allongée, plus étroite à l’avant et à l’arrière. La partie avant frappe une plus petite quantité d’air, les molécules déplacées suivent les contours de l’avion en douceur, et se replacent aisément après son passage.
Les mêmes lois s’appliquent pour les voitures, les camions, les motos, et même les vélos, mais aussi pour les corps qui se déplacent dans l’eau. On parle alors d’hydrodynamique.
Très bon vol !
PS : si vous avez des questions surtout n’hésitez pas à les écrire en commentaire. Je répondrai aussitôt que le temps me le permet.
Cdt. Habib Aboukhedoud 🇧🇯
Pilote de ligne, formateur, je crois en une aviation africaine forte par les jeunes et pour les jeunes, alors démystifions l’aviation dans nos contrées et régions en partageant et en vulgarisant l’aéronautique.
habibaboukhedoud
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