<div dir="ltr"><div dir="ltr">Hi Olwen,<input name="virtru-metadata" type="hidden" value="{"email-policy":{"state":"closed","expirationUnit":"days","disableCopyPaste":false,"disablePrint":false,"disableForwarding":false,"enableNoauth":false,"persistentProtection":false,"expandedWatermarking":false,"expires":false,"isManaged":false},"attachments":{},"compose-id":"11","compose-window":{"secure":false}}"><div><br></div><div>As Peter pointed out, I don't mean "Pitch", I mean "angle of attack". Sorry, sloppy of me. </div><div>Cheers,</div><blockquote style="margin:0 0 0 40px;border:none;padding:0px"><div>Alvery</div></blockquote></div><br><div class="gmail_quote" style=""><div dir="ltr" class="gmail_attr">On Wed, 2 Dec 2020 at 13:52, Grazebrook, Alvery <<a href="mailto:alvery.grazebrook@airbus.com">alvery.grazebrook@airbus.com</a>> wrote:<br></div><blockquote class="gmail_quote" style="margin:0px 0px 0px 0.8ex;border-left:1px solid rgb(204,204,204);padding-left:1ex"><div dir="ltr"><div dir="ltr">Hi Olwen,<div><br></div><div>If I understand it correctly (and it's not our aircraft, so I'm not absolutely sure), </div><div>a) there is a rule that says the stick force must increase as pitch increases. The purpose of this is to give the pilot an intuitive feel for what he is doing to the aircraft's aerodynamics. I assume this is only measured in a steady state, because if you include rotational acceleration I can't see how any airframe would achieve this at all moments of a maneuver or in turbulence. </div><div>b) the generation of a vortex to produce lift is not necessarily sudden. It would just change the steady state position of the centre of pressure. (CoP is kind of the balancing point on the airframe in flight). I'm guessing that the engine nacelle is close to lift-neutral when straight-and-level, and provides increasing lift at increasing angle of attack. </div><div>c) the point Peter made about this feature being necessary only for certification is sort-of valid. The certification rule exists for a reason. For example, if a pilot is operating in poor visibility and with partial instrument failure, I could see the "intuitive" feel associated with a monotonically increasing stick force as a function of pitch might help reduce confusion, and therefore might make an accident less likely. It won't change whether the pilot can achieve a particular steady-state pitch if that's their intention. So you could argue that a known failure to meet this requirement in an area of the operating envelope that isn't normally used isn't truly a safety problem, just a training challenge. </div><div><br></div><div>Does this help?</div><div><br></div><div>Cheers,</div><blockquote style="margin:0px 0px 0px 40px;border:none;padding:0px"><div>Alvery</div></blockquote>** opinions expressed are my own, not necessarily those of my employer.</div><br><div class="gmail_quote"><div dir="ltr" class="gmail_attr">On Wed, 2 Dec 2020 at 12:11, Olwen Morgan <<a href="mailto:olwen@phaedsys.com" target="_blank">olwen@phaedsys.com</a>> wrote:<br></div><blockquote class="gmail_quote" style="margin:0px 0px 0px 0.8ex;border-left:1px solid rgb(204,204,204);padding-left:1ex">
<div>
<p><br>
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<p><br>
</p>
<div>On 01/12/2020 21:11, Herrell, Dave
wrote:<br>
</div>
<p><br>
</p>
<p><snip> ( emphasis in extracted text is my own - OM):<br>
</p>
<blockquote type="cite">
<pre> This new location and larger size of nacelle cause the vortex flow off the nacelle body to produce lift at high AoA. As the nacelle is ahead of the C of G, this lift <b>causes a slight pitch-up effect</b> (ie a reducing stick force) which could lead the pilot to inadvertently pull the yoke further aft than intended bringing the aircraft closer towards the stall. </pre>
</blockquote>
<p><snip></p>
<p><br>
</p>
<p>I had actually read this text on the web before Dave quoted it
here ... and yet PBL has said in a previous posting:</p>
<p><br>
</p>
<p>>>>As far as I am aware, there is no "pitch-up tendency
[which] starts to lift the aircraft's nose". <br>
</p>
<p><br>
</p>
<p>These positions appear to me to contradict each other.<br>
</p>
<p><br>
</p>
<p>Two questions:</p>
<p>1. Who is right? PBL or the Boeing people who produced the text
that Dave Herrell quoted?</p>
<p>2. If the forces on the stick change rapidly, this suggests, to
me at least, that the aerodynamic pressure on the relevant control
surfaces (thinking physics again) changes rapidly at some point,
hence the question, what causes the sudden change of pressure?
Flow separation from the HS, possibly??<br>
</p>
<p><br>
</p>
<p>And if PBL whinges again about repeating himself, I would
respectfully remind him that</p>
<p>(1) Repeating oneself and expressing oneself clearly are two
different things.</p>
<p>(2) Repeatedly asking people to say clearly what they mean is a
foul habit that I acquired by reading about the cantankerous
ancient Greek criminal, Socrates.<br>
</p>
<p><br>
</p>
<p>Still confused,</p>
<p>Olwen</p>
<p><br>
</p>
<p><br>
</p>
<p><br>
</p>
<br>
</div>
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