Re: Aerodynamic question for you engineers

<Dan_Thomas_nospam@xxxxxxxxx> wrote in message news:532e84e3-c288-47df-b2dc-d0473cb234d0@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
On Jan 26, 5:19 am, Stealth Pilot <notranspon...@xxxxxxxxxxxxxxxxx>
wrote:
On Fri, 25 Jan 2008 17:03:04 -0800 (PST), Dan_Thomas_nos...@xxxxxxxxx
wrote:

> And if the CG is moving horizontally at 100 knots, where
>is the rotation point now? As soon as rotation starts, the aircraft
>begins to change its flight path, and any determination of rotation
>point, whether it's the CG or CP or any other point, becomes very hard
>to determine and might be irrelevant. I would prefer to think of the
>fixed end of the flight path radius (which is also changing) as the
>airplane rotates, just like one of those complicated cabinet door
>hinges that has two arms and four pivot points. Where is the rotation
>point of that door? There is no fixed point.

> Dan

clueless. you'd never have understood the propeller like the wrights
did because you cant simplify situations until the problem becomes
solveable. finding a solution to a problem is often a matter of
thinking about it with just the relevant factors at play.

the answer to your door problem lies in understanding that there are
multiple hinge points that each act in a simple manner.

the frame of reference is moving at 100 knots. the aircraft rotates
about it's centre of gravity (centre of mass).

You are assuming that the aircraft rotates about its CG as it
would in space. But we're NOT in space, and lift and drag and various
vectors all come into play here. As the aircraft rotates nose-up, CL
(coefficient of lift) increases; CP shifts forward; the relationship
between CG and CP changes because of the CP shift, and especially so
if the CG is well above or below the CP; the tail's downforce
increases, adding another vector to the whole thing. Then we have
thrust and drag trying to rotate the airplane around some other point.
We can't consider mass alone if we're trying to figure a rotational
point. If mass is all you're concerned with, then the CG is good
enough. I'd rather have more than mass acting on my airplane; it flies
better that way.
Kerschner likely has something to say on it. I'll have a look.

Dan


All those aerodynamic forces act on the CG (center of mass) to move it in a different direction, but it is still that center of mass that needs to move, so the aircraft (or any other free body for that matter) will move around that point.


.

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