[NSRCA-discussion] pitch

[Dean Pappas]

George,
This is only one of several varaible involved.
The side area we are most concerned with, in this discussion, is in front of the stab, though the fin/rudder counts too.
The real point is that no serious Pattern designer would start out to put the stab on the very top, or very bottom, of the fuse.
You might end up there, though!
Proof is found in the common mod to the EU1-A: dropping the stab to the very bottom of the plane fixed a vertyical canopy-pull, which doesn't make sense in the context of this discussion. Sometimes, you gotta cut and try.

Dean

Dean Pappas
Sr. Design Engineer
Kodeos Communications
111 Corporate Blvd.
South Plainfield, N.J. 07080
(908) 222-7817 phone
(908) 222-2392 fax
d.pappas at kodeos.com


-----Original Message-----
From: nsrca-discussion-bounces at lists.nsrca.org
[mailto:nsrca-discussion-bounces at lists.nsrca.org]On Behalf Of George
Kennie
Sent: Wednesday, February 01, 2006 11:46 AM
To: nsrca-discussion at lists.nsrca.org
Subject: [NSRCA-discussion] pitch


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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