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Re: pterosaur headcrests



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.