'Cos I'd just read an article on V numbers that said VR is typically the indicated speed, about 3 seconds before the aircraft breaks contact with the ground. I've tried a Vr derived via that route with the A350 and it produces good response.
Again, by contrast, rotating at the Vr that designers quote for their GA aircraft often sees them leave the ground about 15 to 20 knots beyond that value. This despite applying trim up.
Gerry Winskill bones wrote:
I've lost you there. If you got consistent results then why not use the speed you noted at the time the aircraft started to rotate as the VR value? bones -----Original Message----- From: jhb-bounce@xxxxxxxxxxxxx [mailto:jhb-bounce@xxxxxxxxxxxxx] On Behalf Of Gerry Winskill Sent: 01 July 2007 13:20 To: jhb@xxxxxxxxxxxxx Subject: [jhb] Re: Which Speed? Having decided not to make assumptions about Vr, I ran a series of tests, at max and minimum takeoff weights, to find the takeoff speeds at the various permissable flap settings. I ran the tests hands off, with elevators trimmed up at 60%. I've got decent rpeatabillity, so..... I know it's nowhere as simple as a linear relationship but is there a reasonable difference I can apply to the takeoff speeds, to get to Vr? V1 and V2 are not, I guess, capable of being arrived at by rule of thumb. Gerry Winskill Gerry Winskill wrote:A couple of days ago I downloaded the FSX version of the wide bodied Airbus A350. It looks good and flies well. One advantage of the Airbus familly, to users of Fsim, is that commonality of panels etc is a real aircraft feature. That left me needing to modify the Vspeed gauge, to reflect the A350's weights and V numbers. I've not managed to unearth any V number data but weights and performance are available, from the Confidential sale contract conditions that have found their way onto the Net.. For Vr I'm assuming that the numbers won't be far off those for the rest of the familly. Producing Vref data should be straightforward, since all I have to do is determine the dirty stall speed, at the same altitude and with zero wind, for a set of All Up Weights. Only it wasn't straightforward. The aircraft.cfg gives the dirty stall speed as 124 kias, without reference to any weight. In fact there seems to be no Aircraft.cfg facillity for varying stall speed with weight. The difference between the stall speeds I determined and the Aircraft.cfg figure were big, to enormous! At Max Permissable Landing Weight of 400,000 lbs it stalled at an indicated 99 kias, with the Stall Warning following a few knots below that. At the bottom end of the weights, with just the minimum permissable fuel reserves, it stalled at 80 kias. As if that isn't bad enough there was a discrepancy between the AIS / Map indicated speeds and the Ground Speed recorded in my Checks gauge. When ASI read 99 the GS was 110. With ASI at 80, GS was 88. Where does that leave me? It seems reasonable to take the actual stall speeds recorded, as the route to calculating the Vref figures for the simulated aircraft, but should I use the ASI or the higher GS figures? In passing, the figures for dirty stall speed in most of the aircraft I fly seem to be higher than the actual speed at which the stall occurs. Which explains why I can seldom hold off enough to get the Stall Warning klaxon to sound, when landing. Which makes it seem likely that the actual stall speed data is held somewhere other than the Aircraft.cfg. The fact that there is an actual variation of stall speed with weight seems to bear this out, since that ain't possible from the data held i the Aircraft.cfg. This is a serious limitation of FSX and its predecessors, since lapses of concentration allowing the speed to fall to the stall don't produce the wake up effects of a real life lapse! Gerry Winskill
