48
FLUIDES AVEC FROTTEMENTS
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VISCOUS FLUIDS
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ТЕЧНОСТИ С ВИСКОЗИТЕТ
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Chiche…
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Here goes -
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Ето така...
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Hop !
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- Ooops!
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Хоп!
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Comme ces assiettes, les couches superposées de gaz ne glissent les unes par rapport aux autres qu’avec un certain frottement.
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Like these plates, the superimposed layers of gas do not slide over each other without friction.
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Както и тези чинии, газовите слоеве, наложени един върху друг, се плъзгат един в друг с известно триене.
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Figurons un objet immobile sur lequel arrivent des molécules que nous allons représenter comme situées dans des boites cubiques.
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En l’absence de tout frottement, après avoir contourné l’objet, les molécules se retrouvent empilées les unes sur les autres, comme en amont.
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En revanche le frottement va ralentir les molécules situées près de l’objet. En aval, les « boites » seront décalées. L’objet freine le gaz, réciproquement le gaz exerce une force F sur l’objet : LA TRAINEE DE FROTTEMENT.
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Imagine a fixed object in a stream of gas molecules, which we can represent by little cubical boxes.
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In the absence of any friction, after circumnavigating the object, the molecules end up piled on top of each other like they were at the start.
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In contrast, friction slows down molecules that pass close to the object downstream, the boxes are shifted, the object slows the gas down, and conversely the gas exerts a force F on the object: FRICTIONAL DRAG.
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Да си представим един неподвижен обект, който се намира в течение от газови молекули, които ще изобразим като намиращи се в малки кубически кутии.
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При неналичие на всякакво триене, след като обградят обекта, молекулите с наслагва една върху друга, както в началното си положение.
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В обратния случай триенето ще забави молекулите, преминаващи близо до обекта. „Кутиите” ще бъдат разместени. Обектът ще забави газа, съответно газът ще упражни сила F върху обекта – СЪПРОТИВЛЕНИЕ НА ТРИЕНЕ.
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49
Ouais, tout cela est bien compliqué. Je vais aller me détendre un peu en jouant au tennis. Ça, au moins, c’est de la mécanique toute bête, de la balistique. On tape sur une balle, boum. Et si on calcule bien, elle tombe dans le court.
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It’s all too complicated for me. I think I’ll relax a bit and play some tennis. The mechanics of that, at least, is easy. Ballistics. You hit the ball – Boom! And if you’ve calculated it correctly, it lands in the court.
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Да, доста е сложно. Малко ще си отдъхна, ще поиграя тенис. Това поне е лесно – балистична механика. Удряш топчето и – бум. И ако си се прицелил добре, пада в игрището.
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LA BALLE LIFTEE
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SERVICE GAME…
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СЕРВИС С ТОП-СПИН (*)
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Je vais m’inscrire. Voyons, voilà une place libre. Bjorn Borg…connais pas.
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I’ll put my name down for a game…good, here’s a spare place. Bjorn Borg…Don’t know him.
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Ще се запиша. Ето тук има свободно място. Бьорн Борг... Не го познавам.
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Ready ?
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Ready?
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Готови!
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CHPOK
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ТУП
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* топ-спин (англ.) - вид удар, при който топката се удря отгоре, придавайки силно въртене по посоката на удара.
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50
Bon sang, je n’en touche pas une. Ce type a une façon bizarre de relever sa quand il frappe. Cela devrait pourtant faire monter les balles.
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Good grief – I haven’t even touched one yet! This guy has a way of lifting the racquet when he serves that ought to make the ball lift too…
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По дяволите, не съм докоснал нито една топка. Този тип има странен маниер на замахване с ракетата. Сигурно това е причината топките да излитат.
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En fait ça les fait descendre !
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It doesn’t. it drops.
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И ето тук да падат.
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Comment faites-vous cela ?
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How do you do that?
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Как го правите?
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C’est simple : je fais tourner la balle dans ce sens.
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Easy. I make the ball spin like this.
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Много просто – подавам топката ето така.
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Elle tend à descendre. Cela me permet de taper plus fort, en la mettant dans le court.
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It tends to drop. That lets me hit it harder without going off the court.
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Тя се спуска. Това ми позволява да ударя по-силно, за да я приземя на корта.
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Oui…bien sûr
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Yes, sure.
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Да, разбира се.
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PFFF…
6-0, 6-0 enfin…
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6-0, 6-0 down…
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УФ!
6-0, 6-0, най-после…
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Clair comme du jus de pipe
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Clear as mud…
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Ясно като бял ден.
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51
Voyons, Borg envoie la balle de gauche à droite sur la figure de la page précédente. Je vais faire arriver l’air sur la balle de droite à gauche, ce qui revient au même.
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Let’s see. Borg sent the ball from left to right in the picture on the last page. I’ll make the air hit the ball from right to left – that ought to come to the same thing.
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Да видим. Борг хвърля топката отляво надясно, както е изобразено на предходната страница. Ще задам движение на топката отдясно наляво, но пак ще се получи същото.
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Anselme fabrique une soufflerie.
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Archie builds a wind – tunnel.
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Анселм си прави аеродинамична тръба.
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Tu vois, Sophie, la fumée des pipes matérialisera les filets d’air.
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You see, Sophie, the smoke from the pipes makes the airstream visible.
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Виждаш ли, Софи, димът от лулите ще направи видимо въздушното течение.
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Il reste à assurer la rotation de la balle. Ceci devrait convenir
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Now all I need is to make sure the ball will rotate. This ought to do it…
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Трябва само да измисля и как топката да се върне обратно. Ето с това трябва да стане.
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Voilà, ça marche très bien !
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Yes – that whisks it around beautifully!
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Да, става!
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52
Tiens, la rotation de la balle projette la fumée vers le haut et, en même temps je sens une force qui tire la balle vers le bas.
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Gosh. The rotation of the ball makes the smoke rise, and at the same time I can feel a force pushing the ball downwards.
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Я виж ти, завъртането на топката издига дима нагоре, а в същото време усещам сила, която бута топката надолу.
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L’explication : grâce au frottement, la rotation de la balle entraîne l’air. Ceci crée une SURVITESSE en A et une SOUS VITESSE en B.
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Let me explain. Because of friction, the rotation of the ball pulls the air with it. This gives an EXCESS SPEED at A and REDUCED SPEED at B.
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Обяснението: заради триенето, въртящата се топка повлича въздух. Това предизвиква СУПЕР ВИСОКА СКОРОСТ в точка А и СУПЕР НИСКА СКОРОСТ в точка Б.
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Balle en rotation
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Rotating ball
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Въртяща се топка.
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Il ne reste plus qu’à appliquer la loi de Bernoulli.
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Now all you have to do is use Bernoulli’s law…
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Сега не ни остава нищо друго, освен да приложим Закона на Бернули.
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53
SOUS VITESSE – SURPRESSION
FORCE Vitesse de l’air
SURVITESSE - DEPRESSION
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REDUCED SPEED – HIGH PRESSURE
FORCE Air speed
EXCESS SPEED – LOW PRESSURE
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ПОНИЖЕНА СКОРОСТ – ПОВИШЕНО НАЛЯГАНЕ
СИЛА Скорост на въздуха
ПОВИШЕНА СКОРОСТ – ПОНИЖЕНО НАЛЯГАНЕ
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Pression et vitesse varient inversement. Donc, sur le dessous = DEPRESSION.
Sur le dessus = SURPRESSION, d’où le sen de la force aérodynamique.
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Pressure and speed vary inversely. So underneath the pressure is LOW, on top it is HIGH, from this it’s clear in which direction the aerodynamic force acts!
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Налягане и скорост се променят обратно пропорционално едно на друго. Следователно отдолу имаме ПОНИЖЕНО НАЛЯГАНЕ, а отгоре – ПОВИШЕНО НАЛЯГАНЕ, откъдето и действието на аеродинамичните сили.
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Tout ceci n’est possible qu’à cause du frottement de l’air sur al balle. Dans une atmosphère SUPERFLUIDE, exempte de frottement, vous ne pourriez plus faire vos balles liftées.
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Now that only happens because of friction with the ball. In a SUPERFLUID atmosphere, without friction, you wouldn’t be able to make a tennis-ball swerve.
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Всичко това е възможно само при наличието на триене на въздуха в топката. В среда на СУПЕРФЛУИД, лишена от триене, не можете да използвате топ-спин при сервис.
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Tiens, en inversant le sens de rotation, la fumée est soufflée vers le bas et la force s’inverse. Cela me donne une PORTANCE.
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Hey! If I reverse the direction of rotation, the smoke is bent downwards, and the force is reversed that gives me a LIFT.
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Виж ти, като променя посоката на въртене, пушекът се насочва надолу, а силата мени посоката си. Това е ПОДЕМНА СИЛА.
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Ce qui marche avec une sphère marcherait peut-être avec un cylindre en rotation ?
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What works with a sphere also works with a rotating cylinder?
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Това, което се случва с една сфера, може да се случи и с цилиндър при въртене?
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Chiche !
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Uh – huh!
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Ами!
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54
LE ROTOR DE FLETTNER
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THE FLETTNER ROTOR
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РОТОРЪТ НА ФЛЕТНЕР
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Formidable !
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Amazing!
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Страхотно!
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Effectivement
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Revolutionary!
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Да, наистина!
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Chers collègues et amis, examinons ensemble ce qui se passe dans le SILLAGE.
La rotation u cylindre produit des vitesses différentes entre l’écoulement supérieur et l’écoulement inférieur.
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Ladies, gentlemen, and others – let us look more closely in the wake of this discovery.
The rotation of the cylinder produces different speeds in the flows over the top and underneath.
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Скъпи колеги и приятели, да наблюдаваме заедно това, което се случва в СТРУЯТА. Завъртането на цилиндъра предизвиква различни скорости между горното и долното течение.
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En aval de ce cylindre, lorsque les deux couches d’air se rejoignent, elles frottent l’une contre l’autre.
Ceci a pour effet :
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de créer des microtourbillons.
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de supprimer progressivement la différence entre les vitesses.
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Downstream from the cylinder, although the two layers of air join up again, they rub against each other.
As a result:
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tiny eddies form.
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The difference in speed is progressively diminished.
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Под цилиндъра, когато се доближат два въздушни слоя, те се трият един в друг. Резултатът е следният:
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създават се микровихри;
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разликата между скоростите постепенно намалява.
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Il existe une différence de pression entre la partie supérieure de la nappe et sa partie inférieure, liée à l’écart entre les vitesses (Bernoulli). C’est ce qui explique la courbure des filets d’air en aval.
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There is a pressure difference between the top of the layer and the bottom, related to the difference in the speeds (Bernoulli). This is why the airstream is curved, downstream of the rotor.
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Има разлика в налягането в горната и долната част, което е свързано с разликата между скоростите (според Закона на Бернули). Това обяснява изкривяването на въздушните струи вляво от цилиндъра.
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Haut
Bas
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Top
Bottom
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Горе
Долу
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55
En déplaçant dans l’air un cylindre en rotation, j’obtiens une PORTANCE. Ceci me donne une idée : je devrais pouvoir fabriquer une machine volante.
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By rotating a cylinder in the air, I’ve managed to get some lift. That gives me an idea! I ought to be able to make a fly machine.
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Премествайки във въздуха въртящ се цилиндър, получавам ПОДЕМНА СИЛА. Това ме навежда на мисълта, че трябва да мога да направя летяща машина.
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Qu’est-ce qu’il fabrique ?
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Wot’s ‘e muckin’ abart wiv now?
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Какво прави?
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KLONK
KLONK
WSIIIIII
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ЧУК
ЧУК
ФШТ
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Je vais adapter une propulsion par réaction
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I’m trying to adapt my reaction motor.
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Ще направя реактивен двигател.
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Ça a l’air compliqué!
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It looks fiendishly complicated.
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Много сложно ми се струва!
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56
Voudrais-tu être assez aimable pour allumer la fusée, à l’arrière ?
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Would you be so kind as to light the blue touch-paper at the rear, my dear friend?
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Ще бъдеш ли така любезен да запалиш задната ракета?
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En combinant les deux mouvements et en soufflant l’air vers le bas, je crée une portance.
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By combining the two movements and blowing air downwards I can get some lift.
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Когато цилиндърът се върти при движението на моята ракета, според Закона на Барнули създавам подемна сила.
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Ça marche ! Je vole ! (*)
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It works! I’m flying! (*)
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Работи! Аз летя! (*)
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JE VOLE !
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I’M FLYING!
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АЗ ЛЕТЯ!
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(*) En mettant la puissance ad hoc, ça pourrait très bien marcher !
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(*) If the power is high enough, it actually works very well!
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(*) При подходяща мощност може наистина да работи много добре.
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