Message-ID: <40B1220E.6010507@nc.rr.com>
Date: Sun, 23 May 2004 18:13:34 -0400
From: Ernest Christley <echristley@nc.rr.com>
User-Agent: Mozilla/5.0 (X11; U; Linux i686; en-US; rv:1.6) Gecko/20040113
MIME-Version: 1.0
To: Rotary motors in aircraft <flyrotary@lancaironline.net>
Subject: Re: [FlyRotary] Re: Trim Adjustment was Re: [FlyRotary] Re: engine
 mount geometry
References: <list-85943@logan.com>
In-Reply-To: <list-85943@logan.com>
Content-Type: text/plain; charset=us-ascii; format=flowed
Content-Transfer-Encoding: 7bit

Finn Lassen wrote:
> Ed Anderson wrote:
> 
>>     I think this gets to be a bit of a complex aerodynamic Question. 
>>     Rudder or any trim offers some drag, however if it aligns the body
>>     of the aircraft more with the airstream, there could be an overall
>>     reduction in drag.  I would hesitate to offset the vertical stab. 
>>     Here's my reason.
>>          1. Any fixed trim is generally only good for one airspeed and
>>     power setting, so unless you never intend or think your gear box,
>>     prop or engine is going to change I wouldn't try to compensate
>>     with vertical stab.  Once it set, it a lot of work to change.
>>          2.  Cockpit   trim is of course the ideal, but then you may have
>>     to rig up something to drive the trim tab, worry about what that
>>     might do to flutter points, etc. this one takes time, money and
>>     effort to get it right.
>>          3.  An adjustable trim (such as a rudder trim tab) can be 
>> adjusted
>>     to take care of such changes as mentioned in 1 above
>>          All in all, its hard to beat a trim tab for adjustability, 
>> weight,
>>     cost, and ease of application and since a vertical stab
>>     orientation or a trim tab are both only idea for one airspeed and
>>     the tab is much easier to adjust - I would vote for it.
>>     
> 
> Years ago someone told me he was going to design his vert stab like an 
> airfoil. The idea being that as the airspeed varied the "lift" of the 
> vert stab would change.
> 
> Finn
> 

The vertical stab should be airfoil shaped, because it is in fact a 
wing.  If it is designed properly, you'll have to much in high speed 
flight.  You need more in slow flight, because it has less authority.

The fuselage will NEVER fly straight through the props slipstream, 
except with engine out.  The reason being that the act of the prop 
accellerating the air adds a rotational velocity, but the ANGLE of 
rotational velocity will be different for every speed an power setting.

In the best of worlds, the vertical stab will be turned to point 
directly into the slipstream and offer the least resistance at all 
times.  If it doesn't point directly into the slipstream, it adds a roll 
component to the flight which has to be counteracted with other forces, 
  and the airplane spends a lot of energy fighting itself.  But since 
this is the real world, and the slipstream is always somewhere other 
than where we have the stab set, designers pick an offset that will line 
up with the slipstream at one particular time.  This may be at climbout 
speeds and power, to counteract 'precession' (a term that is usually 
mis-applied), or it may be at cruise, to make the plane most efficient, 
or it may be a compromise somewhere in between.

Having left offset when you need right (or vice-versa), means that 
you'll have to use twice as much rudder as would be needed otherwise. 
This used to be called rigging (and still may), and is as much an art as 
a science.

(Everything I know about this subject I learned from "Stick and Rudder")


-- 
http://www.ernest.isa-geek.org/
"Ignorance is mankinds normal state,
   alleviated by information and experience."
                                   Veeduber