Stabs

Wed Oct 19 20:28:27 AKDT 2005

I built two EU-1A's in the spring of 1984 with full-flying stabs.

My goals in trying this were 1) to not have to worry about getting 
the stab incidence "exactly" right (this was before you could buy an 
adjustable stab mechanism), and 2) have removable stabs for ease of 
transport to the Nats when I needed to fly there, which was most of 
the time (and still is).

I got ~25 flights on the first one before the stab control horn 
failed at the bottom of the non-rolling M in the old Advanced 
pattern.  The plane deep-stalled to the ground in a stable inverted 
attitude with the stab intact but oriented +45 degrees to the wing. 
Of course this occurred the Thursday before the Reno Nats.  :-(   I 
finished the second EU-1A with a plug-in non-adjustable stab but I 
neglected to replace the original stab tube and it failed at the 
bottom of the third of three inside loops on the 28th flight.  The 
stab tube had an unused hole in the center where the original 
flying-stab control horn mounted to it.  This is of course, very bad 
from a stress concentration/flight loads point of view.  Dumb, dumb, 
dumb.  :-[

In spite of these problems I did have enough time on the two planes 
to draw some conclusions about the pros/cons of a full-flying stab vs 
conventional elevators with a plug-in stab.

1)  There was no significant performance advantage to having a 
full-flying stab.  At high speed I couldn't tell any difference in 
handling qualities.  It flew like my other EU-1A's (which is to say 
great as long as you were doing rolls).  At low speed it was OK once 
you realized you needed more throw for the spins and to land.

2)  It took more control deflection than you'd think (IIRC about +/-6 
degrees vs +/-10 degrees for conventional elevators) to yield similar 
pitch response/control harmony, and greater control deflection for 
spins and landings (+/-10 degrees or 40% more for the flying stab vs 
+/-12 degrees or 20% more for conventional elevators.  This is the 
low speed issue Earl mentioned below.

3)  From a control system/linkage perspective it was difficult to get 
the slop in the stab down to an acceptable level due to the 
combination of small surface deflections used and a need to keep the 
stab pivot point as free as possible.  There was always more slop 
than I wanted to see but there wasn't much I could do about it.  BTW, 
the servos I was using were very tight - JR NES4001 coreless servos, 
about as tight as todays nylon gear train digitals but with less 
torque and holding power around neutral (the servos weren't the 
problem).  Since I was driving the stab tube directly in torque there 
was only so much I could do to eliminate slop and hysteresis.  It was 
a flawed design.

If I were to do it again (and to be honest I wouldn't... but if I 
did), I'd have the stab halves pivot with the stab tube about 
(inside) a center sleeve mounted to the fuselage, and drive the stab 
halves in pitch with separate servos mounted just below the stabs 
with short ball linkages direct connected vertically to each stab 
half at, or near, the stab leading edge root rib (the linkage would 
be oriented perpendicular to the stab chord line and the stab root 
rib decomes the control horn).  This would let you use the full servo 
output while giving maximum control leverage with minimal achievable 
slop.  There's no doubt in my mind that it'd work.  I just don't see 
any real advantage to it.

Anyway, there's my $0.02.

Jerry

>In the '70's George Albright designed the Utopia (probably the first 
>complete ready to paint pattern offering) which used a flying stab. 
>I flew them for a couple of seasons - the servos / linkages really 
>weren't good enough back then and stab flutter was a concern, even 
>with proper stab pivot placement and stab balance. As obvious from 
>the number of jets flying successfully with flying stabs, that 
>problem is solvable. (Although I've seen several flying stab flutter 
>failures on fun-fli airplanes.)
>
>However, while the neutral feel was fine, low speed effectiveness 
>was poor - requiring large angles of deflection for take-off, 
>landing, and spin entry. (I know - this is counter intuitive.) Of 
>course, this exasperated the linkage / servo strength issues. I 
>retrofitted one of these airplanes with a conventional stab / 
>elevator and it flew the same - except for much better low speed 
>elevator response. Bottom line - the flying stab provided fewer 
>advantages than disadvantages for pattern in the days of light speed 
>60-size pattern. With today's equipment and slower speeds - maybe?
>
>Earl
>
>
>----- Original Message -----
>
>From: <mailto:bob at toprudder.com>Bob Richards
>To: <mailto:discussion at nsrca.org>discussion
>Sent: Tuesday, October 18, 2005 10:45 PM
>Subject: Re: Stabs
>
>It has been tried before, way back in the 70s as I recall. I think 
>Bob Violet might have sold some hardware to do it. I know he 
>designed a pattern plane that was featured in one of the magazines 
>back then that had a flying stab. I know the glider guys use full 
>flying stabs a lot, but for drag reduction mainly. Not sure why it 
>would not work, but I suspect you would not get the same "feel" as 
>you would with a conventional stab/elevator.
>
>With a conventional setup, you are able to play with the camber of 
>the horizontal surfaces, which can be used to trim the downlines for 
>hands off.
>
>Someone correct me if I am wrong, but when wing tubes became 
>prevalent, I believe someone (I think Chip Hyde) experiemented with 
>wings that rotated on the tubes instead of using ailerons.  That did 
>not work well, from what I understand.
>
>Bob R.
>
>
>
>jeffghughes at comcast.net wrote:
>
>Now that a lot of 2M planes are going to removeable stabs, it seems 
>a short step to full flying stabs. Is there any advantage to this 
>type of stab? Seems like it almost would be easier set up, you 
>wouldn't need to worry about wing to stab incidence.

-- 
___________
Jerry Budd
Budd Engineering
(661) 722-5669 Voice/Fax
(661) 435-0358 Cell Phone
mailto:jerry at buddengineering.com
http://www.buddengineering.com
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