Back in 1981, The Experimental Aircraft Association of Canada raised concerns about the Fly Baby's internal wing bracing. Attached is a copy of the report, followed by some of my thoughts on the subject.


The Fly Baby's Builders' Manual calls up the use of 1/8" diameter 1x19 stainless steel stranded wire and 1600 lb. strength turnbuckles in the wing structure. The wire is used externally, as flying and landing wires supporting the wing. as well as internally in the drag and anti-drag wire bracing.
The Fly Baby was designed at a time when the design criteria specified 30 fps vertical gusts to be considered in designing light aircraft. Later this requirement was changed to call up 50 fps gusts as a result of experience gathered over the years. This change took place after the Fly Baby design was completed.
Analysis of the wing design by the DOT Aviation Safety Engineering Laboratory staff and the EAAC Tech Committee structural specialist shows that the anti-drag wires in the first two bays outboard of the wing root are critical. Even with the old gust criteria of 30 fps. when flying at a gross weight of 925# at a speed of 95mph, hitting a 30 fps upward gust (2.62 G) would result in negative margins of safety being obtained for the anti-drag wires in these two bays. In addition the specified 1600 lb turnbuckles would not be strong enough to cope with these loads on the wires. These findings are based on calculated results.
EAAC Safety Bulletin #SB 81-2 recommended a VNE of 95 mph pending further investigation. At a gross weight of 925#, hitting a 50 fps upward gust at this speed (3.78 G) would result in loads as shown below:
T = tension C = compression
This diagram shows only the anti-drag wires in the three bays. Using the specified 1x19 wires (2100 lb strength) would result in the following negative margins of safety:
first bay 2100/(1719 x 1.5 x 1.15) -1 = -0.29
second bay 2100/(2032 x 1.5 x 1.15)-l = -0.40
In these calculations the 1.5 figure represents the Factor of Safety. The 1.15 figure is the Fitting Factor.
Replacement of these wires is strongly recommended. This would also require examination of the associated compression tubes and the tube end fittings to which the wires are attached plus examination of the bearing of the fitting bolts against the wooden spars.
It is recommended that Fly Baby owners contact the designer regarding possible changes to the wing structure to meet current design criteria.

Peace of mind is important when flying a homebuilt airplane. Upgrading the internal bracing wire and turnbuckles on a Fly Baby has trivial impact on cost and weight. If I were building a Fly Baby, I'd do the upgrades the Canadian EAA recommends.

As I bought my Fly Baby instead of building it myself, I'm not sure whether it has these modifications. However, I'm not too concerned about it for a number of reasons:

  1. The original Fly Baby was built with 3/32" internal bracing wires... smaller than the plans call for. It has had no problems (though the bracing might have been upgraded to 1/8" during the 1982 rebuild).
  2. A number of people have flown mild aerobatics with their Fly Babies with no problems.
  3. The only wing failures I'm aware of were traced to substandard materials, improper maintenance,  and/or poor workmanship (making Nicopress compressions with a pliers instead of the approved tool).
  4. Page 8-33 of the Fly Baby plans shows a picture of a Finnish Fly Baby undergoing a government-required static load test to 5.71 Gs with no damage or permanent distortion.
  5. One Fly Baby builder (a mechanical engineer) wrote to John Ten Cate after looking at the report and said, among other things, "Seems to me that someone is using a vertical load which should be predominantly carried by the flying or landing wires and applying this load as a drag force."
Anyway, whatever the actual load factor, this doesn't seem to be causing any problems in the field. Still, if you're still building, or have to open up the wings for any reason, might as well beef up the wires and turnbuckles.

Ron Wanttaja