[NSRCA-discussion] pitch

[George Kennie]

Like I said, I'm confused.  I only become dangerous when I don't
think I'm confused.

Bob Richards wrote:

> George, It would be real interesting to stick (no pun intended) a
> "stick" in a wind tunnel and see how the flow actually changes
> with yaw.  Same with a Cap21. I think the effect of the vertical
> fin as you described is reduced by the fact that the rudder is
> deflected. In other word, the deflected rudder creates camber in
> the overall vertical surface, and also causes high pressure on the
> side of the fin where the rudder is deflected. If the fuselage
> could be yawed without deflecting the rudder, then it might act as
> you describe. Bob R.
>
> George Kennie <geobet at gis.net> wrote:
>
>      I'm having some problems with this one. Nothing serious,
>      mind you,
>      but just a little confusion.
>      If we take this stab/fuse joint pressure build up to be
>      causative,
>      then it should logically follow that in order to achieve
>
>      equilibrium, the rudder area above and below the stab
>      should be
>      equal.
>      Then if we take the Stick, everything (area) is above
>      the stab,
>      which lends credence to the hypothesis, but if we go
>      back to the
>      Cap, the area is now closer to equal, but probably
>      weighted slightly
>      in one direction or the other, but closer to the
>      equality that we
>      are seeking, and yet the reaction is just as violent
>      except in the
>      opposing direction.
>      Therefore, we must assume that the point of equilibrium
>      is at some
>      point between the two locations.
>      With our thoeretical airplane with it's adjustable stab,
>      we end up
>      determining that indeed the point of equilibrium appears
>      to be at a
>      much lower point (relative to the rudder area) than we
>      would have
>      originally anticipated. So we, at this point find
>      ourselves doing
>      some serious head scratchin'.
>      On the other hand, if we take the two airframes together
>      and analize
>      the force arrangements we find that they are basically
>      inverted
>      mirror images of one another,i.e., Stick, ........wing
>      on top, stab
>      on bottom. Cap, wing on bottom, stab on top. And yet the
>      rudder area
>      intersect points are definitely not mirror images.For
>      that to be the
>      case, the Cap would have to be a T-Tail. Something
>      doesn't jibe!
>      Here we have the Cap with close to a balanced area
>      scenario and yet
>      we have the dreaded pitch to the belly. If we now turn
>      the Cap
>      upside down and cut off the canopy and glue it to the
>      belly
>      pretending that the belly is now the top and fly the
>      airplane it now
>      pitches to the canopy( new top, but still really to the
>      belly). The
>      problem with this scenario is that, in this inverted
>      position the
>      Cap's fin and rudder become equivalent to the biggest
>      sub-fin,
>      ventral, strake, whatever you want to call it and yet it
>      doesn't
>      correct the pitching problem.
>      I have strong feelings that the dynamics are located in
>      a different
>      area and would contend that a poorly designed force
>      arrangement
>      cannot be corrected with a band-aid approach.
>      This is not intended to raise anybody's hackles, just my
>      two cents.
>      G.
>
>
>
>
>
>      Since were still guessing at cause of pull to top in
>      knife edge,
>      Here is my Suspect -
>      Stab is on bottom of fuse- true with this design?
>      When rudder is applied, air pressure builds at
>      intersection of fuse
>      & Fin,
>      with the top of the stab. Pressure on top of stab
>      creates a nose up
>      condition. There is no equivalent pressure on bottom,
>      cause there
>      is little or no fuse and fin.
>
>      If that is the cause, adding a strake to bottom might
>      improve it.
>
>      Later, Ron Lockhart
>
>
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>
>


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