How does spar protrusion affect performance

Derrick Steed wrote:
The later PIKs suffered primarily from an improper profile due to
mold problems, not the usual shrinkage from a correct profile. It
was so severe, Dick had to increase the chord to get the profile
thickness percentage right.

It's possible they had less shrinkage once out of the mold, due to
the high temperature curing.

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Eric Greenwell Washington State USA


I recall that Dick Johnson found the profile near the leading edge to
be too blunt

It was more than just a too blunt leading edge: the airfoil was
significantly thicker at it's maximum than it was supposed be ,
requiring a larger chord to bring the thickness percentage to the
correct value.

I believe the problem was due to molds that slowly warped over time,
perhaps because of the higher than normal curing temperatures required
of the resin used on the glider.

and some friend of his (at JPL, I think) also checked
the published airfoil figures and found them to be slightly off and
corrected them, he re-profiled a 'B to this and tested it at higher
Reynolds numbers (add water, inside rather than outside the wing) -
he reported maximum 45:1 and average 43:1. The repairer who works on
my glider always cures the repairs he does at elevated temperature as
recommended by the resin manufacturer - I believe PIK did just the
same.

The PIK used a resin that cured at a substantially higher temperature
than almost all other gliders; in fact, they could be painted colors
other than white. There was an entirely yellow one in California/Nevada
many years ago that I flew with several times.
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Change "netto" to "net" to email me directly

Eric Greenwell
Washington State
USA