"lfk f; \"A Lt. - Airborne Systems

at
a) Canopy Tension Approach
Tsnoenr
// ..
a a
bJ Z.EqLli/ibrium Approach
Figure 7.22A Circular Approximation of Span wise Profile of the Twin Keel Parawing
Unpublished Nasa Data
Canopy Tension Approach
l/) Keel Line Loads
eiJ
Unpl.bri hed Nasa Oats
Z EqLlilbrium Approach
Figure 228 ComparisQn of Predicted end Measured Line Loads for Twin-f(eel Perawing
symmetric structL: res, despi te use of reefi n9 techniques
to reduce disymmetry dwing the ei3rly sti3ges
of the inflation process. The natL. re of the problem is
Illustrated by Kenncr 3BO In h is analysis of a twin-keel
p8r8wing that failed during the second reefed stage.
His approach was to sub-divide the surface into a
number of small triargular elements, with reinforcing
tapes defining some boundaries. The load-strain and
stiffness properties of each surface and tape element
were specified, and the state of initial stres'! in each
element was determined by an iteration process to
ensure a correct star-: on the soluton. Then the element
and system stiffness matrices corresponding to
the initial stresses and initia ' configuration were
formed, The boundary conditions were selected to
represent the flght conditions as ciosely as possible.
The critical par8wing lobe was free except for the re-
straints at the center- lobe reef point and tre suspen-
366,
Canopy
Tension
Approach
8 0.9 1.
sion line attach point to the test vehicle. The :-esult
was a 234-DOF representation of the finite element
model. Failu'e to obtain convergence of the model
on the measl.m c apolled loads by the linear incre.
mental rr,ethod made it necessary to use a piecewise
linear j:eration technique tQ obtain a solution.
Among his conclusions, Kenner stated, "altr,ough the
iterative solution cor.verged only approximately, the
results predicted stress levels sufficient to cause failure
in the region where a failure was experienced in
the drop test Lack of suspension lines at all reinforcement
tapes was indicated as the probabie cause
of fai:ure.
Another approach to the internal loads analysis of
single and twin keel parawings is illustrated in Reference
533 where Spangler and Nielsen describe a
method of predictinv the aerudynarnic perforrrance
of all flexible parawings. The spanwise profile of the