Aerobatics and Fly Babies:  An Editorial

By Ron Wanttaja

22 June 2008

The following contains myown opinions; I do not speak for the Bowers estate.  While I am an engineer, I am an electrical engineer, with no training in aeronautical engineering.   Nor, for that matter, am I any sort of expert in flying aerobatics.

When you look at the accident reports, one thing really stands out:  The Fly Baby has suffered from a fairly high rate of wing failure:  Thirteen out of about 51 accidents involved failure of the wings or bracing system.

Of that...nearly half (six) involved aircraft doing aerobatics.

Magaziine CoverPete was OK with aerobatics in the Fly Baby, performing them himself and writing about it in SPORT PILOT and other magazines (including this cover shot from SPORT PLANES).

Any plane can do aerobatics...just ask Bob Hoover!   But when you consider that over 10% of the total Fly Baby accidents involved wing failures in aerobatics, it's time to reconsider.

What sort of structural strength is required for aerobatic airplanes?  If the plane is a homebuilt, none.   There are no regulatory restrictions on flying aerobatics in a homebuilt airplane...they aren't required to meet the same requirements as production aerobatics airplanes like Decathlons.

"Yes, but...they successfully tested that Fly Baby to +6Gs/-3Gs in Finland in 1977!  Doesn't that prove the Fly Baby meets the same structural requirements as certified aerobatic airplanes?"

Well, no.  14CFR Part 23 does say that an aerobatic airplane must be capable of withstanding flight loads of +6 Gs and -3 Gs without permanent deformation, and that's what the Finnish Fly Baby was tested to.

However, Part 23 requires a 1.5x Margin of Safety.  This means that to qualify for aerobatic certification, the airplane must be shown to be able to support 9Gs for at least three seconds (and, of course 4.5Gs negative).

The only real way to prove this is to test an airframe to destruction, and the tests in Finland were to allow certification of the plane under test that the owner planned to fly.  They weren't going to keep piling on the sandbags until it broke!

For all we know, Fly Baby OX-XOS's wing would have failed at 6.1Gs.  So the only thing we can logically do is extract the margin of safety from the values it was tested to... +6Gs / 1.5 =  +4Gs,  -3Gs  / 1.5 = -2Gs.

Which is just about midway between Normal and Utility categories.

There's also another factor to consider.  The classic General Aviation aerobatics airplanes have been biplanes.  It's easy to attain significant structural strength in a wire-braced biplane, as the Landing and Flying bracing wires form an "X", and the cables can be tightened to a high degree of tension to minimize the fatigue effects on the metal fittings and the cables themselves.

However, monoplane Fly Babies can't do this.  Tightening the Landing or Flying wires draws the wing tighter to the fuselage.  If you tighten the Flying wires on the right wing, the Landing wire on top does get somewhat tighter, but there isn't the mechanical advantage the biplane has. 

headon view

BracingFly Baby pilots are accustomed to seeing the landing wires sag a bit during flight.  This is nothing to be concerned's common, and doesn't harm anything during normal flight, or even when you pull a few Gs

However, if one were pulling several positive Gs in a Fly Baby, then suddenly "pushed over" into a negative-G maneuver, the wing will jerk downward as that slack in the Landing Wires comes out.   This is certainly not ideal; the abrupt stop will cause a momentary spike in the stress in the bracing system.  Imagine you need a short piece of string but don't have a knife or scissors to cut one off from the spool.  What do you do?  You could just slowly pull on the string until it breaks, but most of us would just wrap a bit around our hands and jerk suddenly.  It's the same thing with the Fly should try to avoid sudden transitions between high positive and negative G-loading conditions.

My own Fly Baby's operating limitations prohibit aerobatics.  I've been told that due to some of Fly Baby wing failures in the '70s, the FAA had been prohibiting aerobatics for planes finished in the early '80s (Like mine was). However, I have found no evidence of this.  However, if you look though the accident reports, comments were made in at least one case that aerobatics were not approved for that particular airplane.

Allow me to emphasize my own opinion:  A Fly Baby is NOT a Pitts Special.  It's not a Great Lakes.  It's not Acroduster.  It's a fun little plane for zipping around in on nice days, but it not really meant for serious aerobatics.  If you would still like to do a few casual loops and rolls in your Fly Baby, upgrade the wing anchor bolts like Pete says, and avoid any maneuver that features a rapid transition between positive and negative Gs.

 Ron Wanttaja.

Alternate Views

I often receive email on these types of subjects.  Since I'm not an expert on structural issues, I prefer to post the messages that differ with my own conclusions....


One thought about the negative "G" theory you mentioned.  If these planes are so delicate, that going from a positive to negative G can pop the wings off, we probably shouldn't be flying them.  You would think more wings would fail during turbulence and hard landings, where the G forces change instantaneously.


I don't mean to imply that the plane can't handle a transition between one positive G and one negative G.  After all, this web page begins with a picture of N500F in inverted flight!  I think the problem might occur when a plane is pulling several positive Gs, and a sudden pushover to negative Gs is made.  Or even to several Gs in the negative direction.  I've edited the above text to make that clearer.