[jhb] Re: Which Speed?
- From: "bones" <bones@xxxxxxx>
- To: <jhb@xxxxxxxxxxxxx>
- Date: Sun, 1 Jul 2007 17:15:33 +0100
Record the IAS speed at the point the rate of descent suddenly increases.
Ideally autopilot should be off as it masks stall onset but I doubt this is
critical in FS - it's pretty useless at low speed handling and aerodynamics.
Once you have the IAS value it should be relatively similar at any altitude.
If not then the FS flight model is ropey.
bones
-----Original Message-----
From: jhb-bounce@xxxxxxxxxxxxx [mailto:jhb-bounce@xxxxxxxxxxxxx] On Behalf
Of Gerry Winskill
Sent: 01 July 2007 16:22
To: jhb@xxxxxxxxxxxxx
Subject: [jhb] Re: Which Speed?
I checked the Stall, all down, by holding 5000' with the autopilot and
keeping my eyes glued to the didgital rate of ascent / descent figures,
on the Misc page of my Checks gauge, whilst reducing speed in 2 kias
increments. The angle of attack was, indeed, high. I decided it had
stalled when an increasing rate of descent commenced. I used the Map
feature to note the speed at which the onset of stall, or mush to be
more realistic, commenced.
I agree the difference between the figure for MLW and my lowest weight
is smaller than I'd expected. By contrast, when measuring the speed at
which a positive rate of ascent is detected, in takeoff configuration,
the difference between high and low wt is 50 kts.
Gerry Winskill
bones wrote:
>Determining stall speeds for real aircraft is extremely complex and
>doing the same for FS aircraft is worse - not helped by the fact that
>stall emulation in FS is pretty awful.
>
>You can forget the aircraft.cfg value - it has no relevance to actual
>aircraft performance. The only way to measure stall is by test flights
>to record the values. The downside is that at high angles of attack you
>get pitot/static errors that make IAS highly inaccurate so the first
>task is to create a graph to show the IAS/CAS relationship. The CAS is
>then converted to TAS by calculation of Density Altitude (easier in FS
>because we can alter atmospheric values). Finally the figures are
>reduced to sea level, standard ISA conditions as this is the universal
>yardstick for aircraft performance tables.
>
>The stall range you quote from 80 to 99 kts is lower than I would have
>expected. Even a Cherokee has significant stall speed variation from
>full to empty weight - roughly 1kt per 100lb. At MAUW of 2550lb the
>stall is 55kts
>(PA28-181) and at 1765lb (pilot and minimal fuel) the stall is down to
>45.8kts.
>
>bones
>
>-----Original Message-----
>From: jhb-bounce@xxxxxxxxxxxxx [mailto:jhb-bounce@xxxxxxxxxxxxx] On
>Behalf Of Gerry Winskill
>Sent: 01 July 2007 09:32
>To: jhb@xxxxxxxxxxxxx
>Subject: [jhb] Which Speed?
>
>
>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
>
>
>
>
>
>
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