RCSE'rs,
Many thanks to the folks who replied privately to my post yesterday
(ARROWSTEEL, R Kinser, Scobie). I've learned quite a bit from your responses.
Mainly:
1) You _CAN_ have aileron differential with only 1 servo
2) I was reasoning backwards WRT my decalage angle
3) I shouldn't need to add a third servo to clean my ship up
Scobie's reply seemed to sum up others', and to add a lot of great detail.
Read his post below and learn something ... I know I did! :)
Cheers,
Brad
-=-=-=-=-=-=-=-=-= Scobie's Reply Below -=-=-=-=-=-=-=-=-=
From: "Scobie Puchtler or Sarah Felstiner" <[EMAIL PROTECTED]> | Block address
To: "Brad Willoughby" <[EMAIL PROTECTED]>
Subject: RE: [RCSE] Bullet First Flight (long)
Date: Wed, 14 Feb 2001 23:18:42 -0800
> 2) There is only one aileron servo, so I can't add
> aileron differential.
You CAN add differential with only one aileron servo by making a simple
change to the mechanical link! It won't be programmable on your radio, but
as long as you can stand that, it's a great system, and you can try
differing the setup to find whatever you consider to be your 'sweet'
differential. It's done a few different ways depending on exactly how your
one servo is set up, so I won't try to describe it in exact detail, but it
is easy.
Here's a chain of logic to the basic concept, in case you don't already know
what I'm getting at.
When you want any control surface to move equally in both directions, you
carfully make sure that the link to the servo is dead center on the wheel,
or in the case of the arm, you make sure that the arm is exactly
perpendicular to the servo at it's neutral point. IF the servo arm starts
way off to one side in the neutral position, then the surface will only move
a little bit one direction, but a LOT the other direction. Bummer, unless
that's what you actually WANT, as in the case of aileron differential!
Most single servo aileron setups are done with torque rods or pushrods from
the ailerons linked to either side of a single servo wheel. If you move both
links back or forward on the wheel so that they start out not at a
perpendicular neutral point but in an offset neutral point, you'll begin to
get differential. (depending on your setup, either back from the
perpendicular point or forward of it will be correct) Adding mechanical
differential in this way will involve changing the length of the connections
or the position of the servo so that your base trim for flight is still
correct, but really no big deal.
You may still prefer to add an extra servo, of course. Then you have all
kinds of flexibility right in your radio programming. You can try flaperon,
camber, spoileron AND differential, but I thought you might appreciate
knowing this simple mechanical option also exists and works great for lot's
of folks.
> I think it'll be
> easier to add differential since the plane has no rudder and I'd
> have to build
> one. Thoughts?
By all means try the mechanical or 'one servo' differential first. It will
do wonders for your yaw issues, keep your plane simple, etc.
Aileron/elevator should be a really great system for a sloper if the design
and diff. are right.
> Decalage (Dive Test Results):
> -----------------------------
>
> The plane pulled out the dive test right away, basically just performing a
> small scoop. This suggests that I've got a large decalage angle
> since my CG is
> at the back-end of the recommended location (6-7cm, mine's at 7).
Agreed.
> looks like I was a bit overzealous when I taped
> the top of the
> fuse to the bottom, and I really pulled the back of the stab
> down.
Now you're losing me. If you truly pulled the BACK of the stab DOWN 'too
much' then you have a SMALLER than designed decalage, and your plane should
be nosing over in the dive test, not scooping out. If you want to correct
for the scoop in the dive test, you'll need to pull the back of the stab
DOWN MORE, not the other way around.
> this created the large decalage angle and that if I loosen this
> tape, and apply
> strong tape in the front of the stab, this should lower the
> decalage.
NO. What you're describing will INCREASE the decalage. You have the right
diagnosis, but your solution is reversed, as I mention above. Think of the
stab as a 'full flying' elevator. If the plane scoops in a dive, you have
too much up elevator in your 'full flying stab', right? UP elevator in a
full flying stab means the trailing edge (back) of the elevator is UP. You
want the trailing edge of it more DOWN, so that you have a more neutral
'full flying stab' in a dive situation, and the dive will be nice and
neutral with no tendency to pull out, and no tendency to nose under.
> What's
> the easiest way to view the decalage angle?
by amplifying it. Tape a light, straight balsa stick across the top of the
stab extending forwards almost to the trailing edge of the wing. Put a mark
on the fuse where the stick is. then make a change in the stab mount angle.
(Based on your dive test, you'll want the balsa stick to move UP some) Now
make another mark and go out and do another good set of dive tests. Tweak
till perfect.
> Any thoughts on alternative
> methods for changing it?
Not really. On a big, spongy ship, retensioning the tape will probably do
the trick, (heck you can even run new (tensioned) strips of tape along the
top or bottom of the fuse to bend the whole fuse up or down (In your case,
you'll want to bend the fuse down a bit if you use this method) These
methods should work for a small tweak, unless it's really way off in which
case, you'll need to remount it, cutting a newly angled 'bed' for it to sit
in.
> Stalls:
> -------
>
> The plane seems very, very sensitive to angle of attack. It
> seems that even
> with the slightest up elevator, it would perform little tiny
> stalls, over and
> over;
This behavior description sounds right on the money, given your rear CG
(increases pitch sensitivity) and too much decalage angle(leads to
'porpoising' in flight, scoop in dive test)
My recommendation is to get your decalage closer first, then before changing
the CG, try exponential on your elevator if the plane still feels pitch
sensitive. If things are still too touchy for your taste, move the CG
forward. Ideally, any change in CG should be accompanied by another tweak to
the decalage, though most folks just make due with elevator trim change when
they move CG, in order not to be so fussy.
Hope all this helps. If you like it, you can post it to the list yourself.
Lift,
Scobie in Seattle
-=-=-=-=-=-=-=-=-= End Scobie's Reply -=-=-=-=-=-=-=-=-=
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