Testing Stick Ribs

Rib Testing

The rib is supported upside down by a jig simulating the wing spars.
The load, consisting of a single mass suitably divided by balance
beams, is applied to the upper edge of the rib (as positioned in the
jig), distributed by pallets of wood as needed to prevent stress
concentration.

The test load is divided according to the manner in which lift is
distributed across the chord of the wing. For the purpose of testing
airfoils having a thickness ratio of 18% or less intended for
airspeeds of 150mph or less, the following load distribution has been
standardized.

As measured from the nose of the airfoil:

Zone 1 = 0 to 19.1% of the chord
Zone 2 = 19.1% to 46.2% of the chord
Zone 3 = 46.2% to 90% of the chord

Note that no load is placed on the extreme trailing edge of the rib.

The test weight is distributed according to the following schedule:

Zone 1 = Half the weight
Zone 2 = One-quarter of the weight
Zone 3 = One-quarter of the weight

Testing Procedure

The mass representing the test weight is to be positioned on a tray or
pallet below the test jig, suspended from the mid-point of a balance
bar, one end of which applies its load to Zone 1, the other end being
divided by a second balance bar so as to apply its load equally to
Zones 2 & 3.

When the testing jig has been assembled and balanced, weights are
applied to the tray or pallet according to a schedule provided by the
designer. The objective is to increase the weight in a graduated
manner beginning with large amounts then tapering off with small
amounts until the rib fails, at which time the last amount added to
the pallet is subtracted from the total.

Practical Aspects.

Ensure you have enough weights on hand. A properly designed rib
weighing only a few ounces is usually capable of supporting several
hundred pounds.

Lead in the form of pigs, bars or bags of shot has proven to be the
most practical form of weights.

Each weight must be individually weighed and marked. The weight of
the balance bars, pallet and the stays connecting them must be
included in the total weight.

Proof of Concept & Quality Control.

For a new design at least ten samples should be tested in order to
define the minimum acceptable standard for strength.

For a proven design, three ribs, randomly selected from each
production batch should be tested. Should any of the samples fail to
meet the minimum acceptable strength, the entire batch must be
condemned.

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The information above has been extracted from static and dynamic
testing procedures found in the Civil Air Regulations, Part 04 (circa
1936) and the structural test section of the ‘Handbook of Instruction
for Airplane Designers,' Air Corps, U.S.Army (circa 1937)

-R.S.Hoover