Stabs

[Ed Alt]

One thing to consider, re the throw needed to get the job done - consider 
many of the commonplace 3D thingys available today. Oftentimes, the stab is 
little more than a convenient appendage to hang the elevator from, i.e., 
it's a popsicle stick compared to the elevator area.  Nevertherless, you 
still need an ordinary amount of elevator throw to do ordinary flying and 
still  need 40 degrees or more of throw to do the 3D wildthing.

Ed

----- Original Message ----- 
From: "Jerry Budd" <jerry at buddengineering.com>
To: <discussion at nsrca.org>
Sent: Thursday, October 20, 2005 12:28 AM
Subject: Re: Stabs


>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.
>
>
> -- 
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> Jerry Budd
> Budd Engineering
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