Liquid Nitrogen Wing Pins

Ginger Brown

unread,

Dec 16, 2022, 2:12:26 PM12/16/22

to

Has anyone used liquid nitrogen to freeze down the wing pins for easy install?

I got the idea at skin doctor who shot some liquid nitrogen onto some moles, and it burned them off. Why not 'freeze down' the wing pins to the smallest digestion possible, so they will slide right into the wing bushings? It would save hitting a hammer onto the wing pins? Any thoughts?

This may be easier than overnight freezing, then transport in a ice chest floating in ice?

ANY IDEAS?
Thanks!!

Hank Nixon

unread,

Dec 16, 2022, 2:31:39 PM12/16/22

to

Absolutely no reason to do this if everything else is correct.
Most of the time the pins don't go in due to alignment issues.
Rarely the fit is tight on a new glider. Then a little buffing usually solves the problem.
Save the dry ice for the doctor.
UH

John Godfrey

unread,

Dec 16, 2022, 2:35:54 PM12/16/22

to

On Friday, December 16, 2022 at 2:12:26 PM UTC-5, Ginger Brown wrote:

Hmmm. I recall two demonstrations. The first in high school where a hot dog was dipped in liquid nitrogen and then dropped a few inches - shatterd like grandma's good crystal. The second more recent was Richard Feynman demonstrating the fragility of O rings. So I would be very wary of any supercooling of (expensive) wing pins.

With my current glider I did an experiment, using my super whoopee doopie micrometer to measure pin diameter at 90F and 0F - no measureable difference at .001 in

You don't mention the glider involved, but FWIW, very careful alignment and good amounts of PTFE grease are a IMO a better approach.

Charlie M. (UH & 002 owner/pilot)

unread,

Dec 16, 2022, 3:09:57 PM12/16/22

to

On Friday, December 16, 2022 at 2:12:26 PM UTC-5, Ginger Brown wrote:

If this is the assembly pin/pins vs. the fuselage lifting pins, how would you get it apart after assembly and everything normalized temperature wise?
As mentioned, no rough spots, cleaned, good lube is normally enough with correct alignment.
Some new parts or gliders are rather tight at first, JS gliders come to mind.

Eric Bick (DY)

unread,

Dec 16, 2022, 5:57:23 PM12/16/22

to

Interesting metallurgical question. This is a technique used in industry to fit parts together - cryogenic shrinking of the insert rather than heating the part the insert goes into to expand it. References I found say that steel (unspecified alloy) shrinks about 0.2% at cryogenic (liquid nitrogen) temps. Doing some simple math, a 1.5" diameter pin would shrink about 0.003" (3 mils, ~0.08 mm) in diameter. However, you have to know the alloy of the rod. There are cryogenic steels (used in Arctic climes) that do not become brittle. Many steel alloys do become brittle at liquid N temps (recover to normal when warmed), so care must be used not to impact them or they will shatter. We could calculate the time it takes for the rod to warm to 0 C, which would tell you how fast you'd have to be, but I'll leave that exercise to the reader.

Of course, to do this, you'd want the pin holes in the spars to be exactly lined up (OK, you could be off by about 1.5 mil (0.04 mm), but then you'd be able to put the pins in without cryogenically shrinking them - and for a lot less hassle and cost. Of course, who knows whether I did the math right? Fun question.

AS

unread,

Dec 16, 2022, 9:44:44 PM12/16/22

to

On Friday, December 16, 2022 at 5:57:23 PM UTC-5, Eric Bick (DY) wrote:
> Interesting metallurgical question. This is a technique used in industry to fit parts together - cryogenic shrinking of the insert rather than heating the part the insert goes into to expand it. References I found say that steel (unspecified alloy) shrinks about 0.2% at cryogenic (liquid nitrogen) temps. Doing some simple math, a 1.5" diameter pin would shrink about 0.003" (3 mils, ~0.08 mm) in diameter. However, you have to know the alloy of the rod. There are cryogenic steels (used in Arctic climes) that do not become brittle. Many steel alloys do become brittle at liquid N temps (recover to normal when warmed), so care must be used not to impact them or they will shatter. We could calculate the time it takes for the rod to warm to 0 C, which would tell you how fast you'd have to be, but I'll leave that exercise to the reader.
>
> Of course, to do this, you'd want the pin holes in the spars to be exactly lined up (OK, you could be off by about 1.5 mil (0.04 mm), but then you'd be able to put the pins in without cryogenically shrinking them - and for a lot less hassle and cost. Of course, who knows whether I did the math right? Fun question.

In most modern gliders, the pin(s) get inserted into brass or bronze bushings, which are somehow connected to the spar stubs. The heat transfer into the cryogenically cooled pins will cause these bushings to shrink, too. By how much would depend on their geometry but shrink they will. I am not a fiberglass/Carbon expert but thermally stressing that joint repeatedly can't be good. Also, unless you do that cryogenic exercise in a desert environment with single digit humidity levels, you will have a nice coating of water ice on the pin's surface very quickly.
Nice thought, though ....

Uli
'AS'

Ginger Brown

unread,

Dec 16, 2022, 9:48:19 PM12/16/22

to

Thank you all for your input!

For now, I will not use this method, but continue with the normal wing pin insert procedures.....thanks!!!!

George Haeh

unread,

Dec 16, 2022, 10:33:33 PM12/16/22

to

Many days my glider snaps together. Every now and again it laughs at me.

ALIGNMENT RULES

1. Fore and aft

2. All the way in on both sides for gliders with two wing pins. I often use wide masking tape to keep the first wing in from being pushed out by the other wing.

Best not to buy gliders with pins on the spar stubs going into spherical bearings in which case you need athletic friends with strong backs. We rig our G-103 maybe every couple years. Hint: One spar stub pin is longer; it goes in first.

3. Finally there's vertical alignment. That takes fiddling with height of rigger, cradle (much easier with hydraulic pedal) and in extremis wingstand. When you do get it right, keep the wingstand, one man rigger and cradle at the same height for the next time. Ground with a different slope will throw it off.

There is a gap between the bushings in the spar stubs which makes it difficult to feel whether the bushing behind is high or low (or a bit off horizontally). A straightedge can help.

The pin goes in when the alignment is right. DO NOT WHACK IT.

John DeRosa OHM Ω http://aviation.derosaweb.net

unread,

Dec 17, 2022, 12:24:56 AM12/17/22

to

Step 4 - Place the stick, airbrake and flap handles in the correct positions.
A misplaced airbrake handle on my '27 gives me all sorts of rigging pain.

Hank Nixon

unread,

Dec 17, 2022, 8:36:51 AM12/17/22

to

If we knew what type of glider likely some useful hints would be forthcoming.
UH

BobWa43

unread,

Dec 17, 2022, 9:39:35 AM12/17/22

to

A hammer? Hopefully you are not serious.

George Haeh

unread,

Dec 17, 2022, 1:03:13 PM12/17/22

to

Step 0 - Place the stick, airbrake and flap handles in the correct positions.

A misplaced airbrake handle on my '27 gives me all sorts of rigging pain.

And yes, unlock the spoilers.

Moshe Braner

unread,

Dec 17, 2022, 2:29:09 PM12/17/22

to

Once when I was putting together the HP14 I had then, a bystander,
seeing us struggle to get the pins in, "helpfully" brought out a hammer,
and I had to stop him...

2G

unread,

Dec 18, 2022, 1:22:45 AM12/18/22

to

I recommend assembling gliders in the morning (if possible) when the temperatures are closer to what the manufacturer used to set the bushings in the spars. Hotter temperatures cause everything to expand, moving the distances between the bushings in the spars, no matter how slight. Chilling the pins just shrinks the diameter permitting them to go into bushings that are now misaligned (regardless of how much you try to align them).

Tom