SUPER BALSA BANDIT WING SPARS
by Bob Violett

There has been one wing spar failure on a Balsa Super Bandit. That is one out of several dozens of kits and thousands of flights during the past 8 years.

NOTE: The instant that one spar fails, it is likely that the other wing spar will also fail because of the extreme loads now placed on it.

The model in question had 11 successful flights. During the 12th flight, it was subjected to lower speed and G's than during the previous flights because of a low (400 ft) ceiling, but the wings departed from the fuse.
Had the spar material been defective from the factory, most experienced jet modelers (and engineers) would agree that it would have failed in the early flights, especially since it was powered with a 27lb thrust turbine. A Bandit (of any vintage) can reach speeds in excess of 250mph with this much thrust.
The model in question did experience a gear-up landing in the grass at Florida Jets '05. Such landings can sometimes cause nothing more than grass stains on the leading edges if all goes well, or, depending on the terrain and speed upon impact, could deliver severe aft bending loads to one or both wings and stabs and their respective spar systems.
Further, the bottom of the fuse, in the area between the speed brake and the hatch had been filled with body putty and the concrete scored surface had been repaired indicating another gear up incident of some kind.

The Material Test

A .600" tall x 2.5" long section (A) was machine cut from the remnants of the broken spar, a perfectly sized duplicate (spar "B") was machine cut from off-the-shelf 1/8" thick carbon fiber stock (the same as used in spar production).

Each sample was similarly clamped in a vise and a 2 ft long steel tube fitted to the ends of the spars. The force it took to break the sample section was measured with a very accurate scale (a $450.00 instrument).

The rate of force application was not precisely controlled - done by my hands.
The result: Spar "A" failed within 5% of the force applied to spar "B". The failure occurred at 39.5 ft. lbs. of force on spar "A" and 41.0 ft. lbs. on spar "B". This is very close with the slight difference likely due to the non precise rate of force application.

Conclusion

It is my opinion that the spar(s) in question suffered some degree of delamination during the gear up landings. Any delamination in the cross hatched area would subject the spar to failure under normal loads. Low wing airplanes are more sensitive to this possibility.
A close and thorough inspection of the spars (wing removed) following the gear up impacts most likely would have revealed the problem and called for an adequate repair before flight.
The spars in question also had the (3) mounting holes enlarged and elongated by as much as .025".
While this is not desired, it probably had minimum effect on the spar's strength.
The sketch below offers some more do's and don'ts for carbon fiber spar structures.

Our factory demo Balsa Super Bandit (with P-70 power - 17lbs of thrust) has been radar clocked multiple times in the 190-205mph range and endured hundreds of passes at that speed with violent high G maneuvers as part of our normal demonstration. The prototype model was powered with a P-120 (27lbs of thrust) and flown at speeds above 220mph for testing purposes.
Many of you have seen these demos at jet fly-ins for the past several years. If you have ever witnessed Jason Somes exercise our factory model, you would have no doubts about the airworthiness of the design.
When something like this (structural failure) happens, it is inevitable that some people will speculate beyond their knowledge and experience and cause quite a stir of misinformation, especially on the internet chat rooms/forums.
Operating high performance turbine powered models requires above average skills in building, flying and maintaining these machines.
As the most safety conscious manufacturer in this business (see bvmjets.com - Safety Issues page), BVM tests our products to reasonable limits before we issue them. However, it is impossible to envision every possible variable that can be introduced by the user. That is why there are necessary disclaimers in the kit manuals. Again, flying fast, requires experience and constant vigilance in the area of maintenance.

Damaged Spar Repair
(available July 12, 2005)

BVM can supply Balsa Bandit customers with reinforcements for the forward spars, along with instructions on how to install them. These reinforcements can be installed on a finished/flying model should the need arise.

The Composite Super Bandit has a different spar design and is not of concern.
Recently, in a Latin American country, a composite Super Bandit, powered by a Titan, was flown by an internationally famous pilot against a radar gun.
The radar unit's upper limit was 500 kilometers per hour, or 310mph. On every high speed pass, the model pegged the radar unit, and, it landed undamaged.
While BVM establishes the max speed limit for each kit on the conservative side, we pass this on to you to illustrate the integrity of the design and materials.

Internet Chatter

To compare our sport and related incidents to that of full scale aviation certainly must be viewed within the limits of reality. If some chat forum active persons want FAA certified materials, design and testing, airworthiness certification, and then subsequent NTSB type accident investigations, then be prepared for prices that would deny 95% of hobbyists access to jet models.
Let us pay attention to the AMA rules, the manufacturers red line speeds (Vne), and our own limitations and enjoy our sport of building and flying model jets. In many instances, the chat room BS and speculation is for entertainment purposes only.