Message-ID: <39706D27.F98A5B2C@regandesigns.com>
Date: Sat, 15 Jul 2000 08:54:47 -0500
From: Brent Regan <brent@regandesigns.com>
To: Lancair List <lancair.list@olsusa.com>
Subject: Washers and other hardware items
Reply-To: lancair.list@olsusa.com
Mime-Version: 1.0

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A recent post incorrectly cited me as recommending that steel engine mount
washers be replaced with aluminum ones. While it is true that many steel
washers supplied in the kits can be replaced with aluminum AN washers , I
would not recommend that the washers used at the firewall be replaced
without careful consideration and proper design.

A washer can perform two functions, a spacer and a means of distributing
load. Spacer washers used in conjunction with aluminum or flexible steel
parts (as in the flight control linkages) can often be replaced with
aluminum ones with little fear of detrimental results. Load distributing
washers require careful consideration.

Aluminum and steel have approximately the same specific modulus, that is to
say that for a given section, pound for pound they have the same stiffness.
Aluminum has 1/3 the mass and 1/3 the modulus (stiffness) per cubic inch.
The section modulus varies as the square of the thickness so in order for an
aluminum washer to be as stiff as a steel washer it must be 1.75 times as
thick and weigh 57% as much.

If you need to distribute a load then the washer needs to be significantly
stiffer than the material the load is being distributed into. Picture a
rubber washer on a steel plate, not much good.

You can save several pounds off of your aircraft if you judiciously replace
the AN960 steel washers with AN960xxPD aluminum ones and replace all the
AN364/AN365 elastic stop nuts with MS21042s. I kept a bin of all the
hardware I replaced during construction. It weighed 7.5 lbs, about half of
that translated to weight saved off of the airframe, not a lot, but
worthwhile.

It is my opinion and practice that lock washers of any sort have no place in
an aircraft and should be relegated to agricultural implements and Cessnas
;) Proper torque is the best defense against vibration and loosening. If
adequate torque cannot be achieved then fastener locking is best achieved
with cotter pins (where required), all metal stop nuts (replaced after every
use) and Loctite (properly primed and applied). A fellow Lancair builder
once said to me that he did not trust Loctite. I replied " Friend, now is
not the time to loose faith in adhesives.".

Regards
Brent Regan

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