My experience has been that most FF models (this includes R/C sailplanes)
are designed with too much decalage. I think the theory is: "better safe
than sorry" - you can survive too much - not so too little. Or, maybe: "if
a little is good, more is better." Flying stabs eliminates the problem.
Regis
-----Original Message-----
From: Scobie Puchtler or Sarah Felstiner [mailto:[EMAIL PROTECTED]]
Sent: Friday, February 16, 2001 5:52 PM
To: Dave Seay; RCSE
Subject: RE: [RCSE] porpoising solutions???
Dave writes:
> I just took a close look at my LB2 and it looks like I definitely
> have some
> 'up' where I mounted the tail.
Actually you do have to have SOME 'up', as you call it. That 'up' that you
see IS the decalage angle. And SOME decalage is a fundamental requirement of
ANY conventional glider, in fact that is the most basic purpose of having
the horizontal tailplane: to keep the wing from diving (also can be thought
of as: to keep the wing at some useful angle of attack). On all but the most
carefully designed flying wing foils, a foil (your wing) producing lift also
produces positive pitching moment, meaning the leading edge of the foil
wants to go DOWN. Release any conventional glider without its horiz. stab
attached and you will see this effect IMMEDIATELY,(don't actually try this,
just trust me). A stab mounted at a NEUTRAL angle will give some very
temporary relief to this problem, but until that stab is ACTIVELY holding
the rear of the fuselage down by having it's trailing edge higher than its
leading edge relative to the airflow, your glider is gonna dive bigtime. If
it's mounted with too MUCH angle (Stab t.e. high) then your glider is going
to constantly repeat one mild stall after another (porpoising).The trick is
to have the RIGHT amount of this angle. Too much and you porpoise, Too
little and you dive. Either problem can be crudely corrected by shoving
weight all over the place and trimming the elevator like mad, but these are
fundamentally bad solutions because they add, well, weight and drag, not to
mention their effect on maneuverability.
> How do you correct this w/o destroying the tail given the whole thing is
> balsa? I guess I could try to cut the tail off.
I am not familiar with the exact construction details of the LB2, but it
MIGHT be helpful, depending on how it's put together, to remember that you
can change the angle of the tailplane OR the angle of the wing. Is the wing
mounted in such a way as to be shim-able in it's mount? If so, shim the rear
of the wing up a bit and test (this will have the same effect as bringing
the t.e. of the stab down. Remember, it's the relative angle BETWEEN these
two surfaces that counts). Shimming the wing is actually a great way to
experiment, if it is doable on your plane, because it's much more changeable
than rebuilding your tail six times to get things just right. Good luck.
Scobie in Seattle
>
RCSE-List facilities provided by Model Airplane News. Send "subscribe" and
"unsubscribe" requests to [EMAIL PROTECTED]
RCSE-List facilities provided by Model Airplane News. Send "subscribe" and
"unsubscribe" requests to [EMAIL PROTECTED]
