Re: pterosaur headcrests

[jrc <jrccea@bellsouth.net>]

Comments below.
Jim
----- Original Message ----- 
From: "don ohmes" <d_ohmes@yahoo.com>
To: <dinosaur@usc.edu>; <jrccea@bellsouth.net>
Sent: Thursday, February 02, 2006 1:22 PM
Subject: Re: pterosaur headcrests


> Er, at great risk of getting uncomfortably distant from my personal 
> lilypad--
>
> Assuming the nosewheel fairing is in a fixed position, doesn't the fact 
> that it affects the rudder (by tending to >make aircraft fly straght when 
> rudder is first applied) imply that it is functioning as a "control" 
> mechanism? Or is >the Cherokee nosewheel fairing synched w/ the rudder 
> (ie, when trailing edge of rudder moves right, leading edge >of fairing(or 
> crest) moves right)?

The Cherokee nosewheel fairing is not fixed.  It is solidly linked to the 
nosewheel fork, and nosewheel fork and steering are solidly linked to the 
rudder pedals, so that pushing the right rudder pedal turns the nosewheel 
right in flight which also deflects the nosewheel fairing right in flight. 
The problem is, that even in straight flight there are occasional, erratic 
flow reversals along the sides of the nosewheel fairing that create moments 
on the nose strut (read pterosaurs' head) and cause sporadic and erratic yaw 
darting of the entire plane that can be quite entertaining (particularly if 
you were trucking along with your feet on the floor, as no one should). 
Another consequence is that a condition can sometimes be triggered where the 
foot pressure required to deflect the rudder in flight (and therefore the 
nosewheel fairing) can sometimes diminish with increasing deflection rather 
than increase.  This is undesirable to say the least, in both airplanes and 
pterosaurs. It feels like pushing on a brake pedal that has just failed and 
gone to the floor.   If anything, it is a 'decontrol' mechanism.  It is just 
this sort of thing that led MacCready to mention to me on more than one 
occasion that pterosaur headcrests were 'like having the feathers on the 
wrong end of the arrow'.  There are only a few pterosaurian exceptions to 
this.  If  Nyctosaurus species had a membrane on that weird bony crest 
(there is no physical evidence that it did), then that headcrest would have 
been directionally stable.  The same might be true of a very few other 
species, but in general pterosaur headcrests are quite destabilizing.  For 
that reason, I would venture to speculate that no truly long-necked 
pterosaur will be found with a large headcrest unless that headcrest extends 
far enough aft that its center of pressure is aft of the center of pressure 
of the wings or at least even with it.  I tend to think that for the most 
part, headcrests were displays and the animals may have tended to sport the 
largest one that they could get away with, without losing control.

This might also be a good place to mention that rudders aren't used for 
steering planes anyway.  They are mostly used to cancel the adverse yaw 
created by the wing deflections that do start the turn.  At the risk of 
oversimplifying things, and of describing a clock to clockmakers -- as you 
initiate a right turn, you deflect the right aileron up (which reduces drag 
on the right side) and deflect the left aileron down (which increases drag 
on the left side).  This tends to yaw the plane left instead of right even 
though it is banking to the right, so you give a small amount of right 
rudder to counteract that adverse yaw.  The rudder is then quickly 
neutralized, and the ailerons are neutralized shortly thereafter, so that 
both remain undeflected throughout the turn.  The control inputs are 
reversed to stop the turn.  Animals have analogous inputs with similar 
consequences.