zondag 7 december 2008

POF week 11

Principles Of Flight

Deze week alleen Principles of flight lessen, oftewel aerodynamica.

Maandag hebben we het vooral gehad over de rotorbladen, wat dan misschien ook wel 1 van de belangrijkste onderdelen is natuurlijk.

De liftkracht die de heli in de lucht houdt is afhankelijk van de grootte en de hoek van de 'relatieve' wind op het rotorblad.
In formule:
Ftotaal = 1/2 rV2 S Cr

r = soortelijk gewicht van de lucht
S = de oppervlakte
Cr = wrijvingscoefficient van het oppervlak

Ftotaal kan dan weer ontbonden worden in de liftkracht, en een drag. De liftkracht is natuurlijk belangrijk en deze is rechtevenredig met de de snelheid V in het kwadraat, het soortelijk gewicht, oppervlak en wrijvingscoefficient zijn immers constant.

Dus de liftkracht van een rotoblad is een functie van de luchtsnelheid die er op inslaat.

Dit is een belangrijke conclusie die ervoor zorgde dat de eerste helicopters omkiepten zodra ze voorwaartse snelheid kregen. Waarom?

Even uitgaande van een R22 met 2 rotorbladen voor het gemak, 1 rotorblad komt naar voren (voorwaarts blad), terwijl het andere tegelijkertijd naar achteren beweegt (achterwaarts blad). Logisch.

1. De rotors draaien rond, de heli hangt stil. De snelheid van de lucht over de rotors is aan beide kanten gelijk. Niets aan de hand.

2. De heli begint voorwaarts te bewegen. De snelheid van de luchtstroming bij het voorwaartse blad wordt hoger, op het achterwaartse blad kleiner! Het verschil is gelijk aan de snelheid van de heli, of het zou ook een kopse wind kunnen zijn. Dit maakt voor de bladen geen verschil.

Aangezien de lift evenredig is met de snelheid in het kwadraat, is het dus vanzelfsprekend dat de liftkracht op het voorwaartse blad stijgt en bij het achterwaartse blad daalt --> de heli rolt gewoon om en komt op een zij de liggen.
Dat willen we natuurlijk niet.
Oplossing is het klopscharnier. Hierdoor kunnen de bladen naar boven en naar beneden bewegen, dit noemt men; flapping. Als de liftkracht stijg, bij het voorwaartse blad, dan gaat dit blad omhoog. Tijdens het omhoog gaan veranderd de angle of attack tov de luchtstroom waardoor de liftkracht uiteindelijk daalt. Een rotorblad gaat dus omhoog, omlaag, omhoog -> flapping.
Donderdag hebben we het nog gehad over de praktische werking van de het rotorsysteem, met de cyclic en collective.
Hier staan de rotoronderdelen nog eens met uitleg erbij.
In feite is het allemaal erg theoretisch maar wel handig om te weten hoe het in elkaar zit.
Deze week geen les gehad omdat er een technisch mankement was aan de heli die natuurlijk voor mij gereserveerd was. Bert belde zaterdag rond 9.45u op met het slechte nieuws...

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