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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
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