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 > >