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Elevator balance spring
I installed the horizontal stabilizer, saddle piece, and connected the elevator control cables on my 601 XL B. I immediately noticed a couple things. The effort to raise the elevator to full up position was excessive, to me. It took about 4.6 lbs pressure on the "Y" control stick. Also, the elevator cables tightened considerably as the stick was moved back. The cable tightening happened mostly because the nylon fairlead holes aren't aligned with the cables very well at up elevator position. I enlarged the holes and had to move one of the nylon pieces to get it so the cable didn't rub whether the elevator was down or up. That took care of the change in cable tension. Next, I figured out a spring arrangement to balance out the weight of the elevator when it is in neutral. That reduced the up elevator stick pressure to about 2.5 lbs and it took 0.5 lbs to move from neutral to down position. It was simpler to do than I thought it would be. I used a piece of 3/8" OD aluminum rod 0.437" long and drilled a 3/16" hole through it. I turned a groove 3/16" in from on end to locate the loop on one end of the spring. I drilled a 3/16" hole through the control stick about an inch below the upper cable attach point. I fastened the spring attach "spool" to the stick with a AN3-7A bolt, washer and stop nut. The other end of the spring is secured to piece 6-B-16-2 below the hole for the elevator cable far enough so the spring doesn't interfere with the cable. I used a piece of 1/8" x 3/4" angle that was left over from the center spar upgrade. I figure this should help make the up elevator trim more effective since the trim tab won't have to lift the weight of the elevator which weighs 3 lbs at the trailing edge. Less trim tab for neutral in cruise will mean less drag = more speed. :-)
Here is a couple pictures of the balance spring. The spring is 8.5" overall length unextended, OD 0.625" and wire diameter 0.063". I bought it at a 2nd hand store, but the spring was new. Unknown lineage.
Hmmm. I can't figure out how to attach the pictures.
Comment
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Comment by Louis W. Ott on January 11, 2011 at 12:39am
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Hi Bob,
Yes, the spring adds some set value of nose up trim. This airplane is a tail dragger and I like to taxi with quite a bit of up elevator to keep the tailwheel planted. The stick force for this was quite high I thought, and this balance spring was my cure for that. Full up stick force went from 4.6 lbs to 2.5lbs. I've read that with full flaps the 601 XL tends to run out of nose up trim and I had hoped this would help that out too. The airflow centers the elevator because the trim tab positions it in center, or wherever you want. The trailing edge of the elevator weighs 3 lbs when pivoted on the hinge and the trim tab has to exert 3lbs of upward force to get it centered when flying. I think you may find that many airplanes have nearly balanced elevators by use of counterweights similar to the aileron balance weights. True, the elevators will droop down, but only because they are not 100% balanced. Also, some airplanes have a spring on the elevator as the only trim system. One end of the spring is adjustable. If this proves to cause some undesirable effect, it's easy to remove.
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Comment by Bob Pustell on January 10, 2011 at 9:53pm
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What you appear to have built is permanent nose up elevator trim. Hopefully, that is what you wanted. Pretty much all airplanes have the elevator held fairly firmly in the down position (by gravity) when the airplane is stationary. Once there is airspeed, the airflow centers the elevator in the nuetral position. Elevator trim is used to vary the location of that nuetral position. I am not completly sure what you were trying to fix but your work looks of good quality. Let us know how it works out.
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Comment by Phill Barnes on January 10, 2011 at 9:24pm
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