Class A Aircraft - Take-off — Page 383, Lesson 476

We're now looking at the factors that affect \( V_R \) — the rotation speed. This is the speed at which you, as the pilot, begin to rotate the aeroplane to lift the nose wheel off the runway and initiate the climb. Now, I want to make this easy for you. Just like with \( V_1 \), all the factors that affect \( V_R \) can be found by examining pages 18 and 19 of section 4 in CAP 698. On those pages, look at the second table from the top. That table lists the three V speeds — including \( V_R \) — against the aeroplane mass. Let's use that table to see what mass does to \( V_R \). If the aeroplane mass is 50,000 kg, \( V_R \) would be 131 knots under density column A. But if we increase the mass to 65,000 kg, \( V_R \) increases to 155 knots. So the relationship is clear: increasing mass increases \( V_R \). That makes sense physically — a heavier aeroplane needs more speed to generate enough lift to rotate. The next factor is the aeroplane configuration — specifically, the flap setting. Page 18 lists the V speeds for 5 degrees of flap, and page 19 lists them for 15 degrees of flap. Comparing the two pages shows us the effect. On page 18, with 5 degrees of flap and a mass of 55,000 kg, \( V_R \) is 139 knots. But on page 19, with 15 degrees of flap and the same 55,000 kg mass, \( V_R \) drops to 131 knots. So increasing the flap angle decreases \( V_R \). More flap gives you more lift at a lower speed, so you can rotate earlier. The third factor is density. On page 17 of section 4 is the density graph. You may recall that band A is a high density band, whereas band F equates to lower density. We use these bands to see density's effect on \( V_R \). Returning to page 18, take a mass of 50,000 kg in band A — high density — and \( V_R \) is 131 knots. But as we move through bands B, C, D, and E, the value of \( V_R \) increases. So we can state that as density decreases, \( V_R \) increases. Lower density air is thinner, so you need more speed to produce the same lift. Now, here's the modern reality. In modern airliners, \( V_R \) is not calculated by looking at speed tables. Instead, it's computed by the aeroplane itself once the relevant data is inserted into the flight management computer, or the multipurpose computer display unit. Once you, the pilots, have calculated \( V_1 \) and \( V_R \), you enter them into the Flight Management System. From there, they're shown to you on the speed scale on the left-hand side of the Primary Flight Display — the PFD — or the Electronic Attitude Director Indicator, the EADI. So the speeds you see on that scale during the take-off roll are the ones you've entered, and the system displays them for you to fly against.