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1. 09:15 PM - Re: Re: Main wing bushings (Bud Yerly)
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| Subject: | Re: Main wing bushings |
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Pete,
I really enjoyed our discussions, as you and Graham have excellent
insight and the Lord knows I need a break from wiring.
Just as you and others found it interesting that the side pins on the
fuselage were designed to take all the load in lift and moments, I ran
some basic calcs one Sun n Fun and it hit me why.
If you consider the wing sockets only it is a determinant structure and
easy to calculate.
When the Europa was only 1300 lbs, like my Classic, I figured quick and
dirty the following:
The GW is 1300 less wing weight is 1200, times ultimate of 6 Gs gives
7200 lbs load and 3600 pounds load born by the pins on each side.
Figure one below shows the balance beam idea. Of course we have to add
pitching moment and the drag component doesn't go exactly through the
pins and the dimensions were rough as shown in figure two. Considering
304 stainless pins are good for about 15Ksi in shear and at 1/2 inch
diameter they can take a load of roughly 3000 lbs. The front pin holds
about 2700 pounds and the rear pin about 900. So it was a slam dunk why
it was calculated that way.
My instructors in college had a saying which was "the spar takes the
lift and the root takes the moments" and that is something we, and I am
sure the PFA, all remember. So the Europa design calculations indicate
the main spar only translates the wing bending and the wing spar pins
carry no lift load in the calculations. But who would buy an aircraft
where the wing pins are not attached to anything but the spar
themselves. After all if the wing sockets at the root carried all the
load, we would only have an inspection plate in the fuselage back and we
would rig the wings to the spar cup and side sockets only, then stick
the other wing in, pin it and go fly. What do we need all that extra
ply, aluminum and bushings for in the cockpit bulkhead? That is a lot
of beef for just the spar bending loads and lateral shifting to be
checked. That is unless it is redundant structure?
Further, if we increase the loading of the plane to 1450 lbs and then
take the gust loading into consideration as well as temperature, the
actual loading jumps up by a huge factor. We only had a reserve on that
front pin of a few hundred pounds, now that pin/socket is going to be
compromised.
If we consider the wing is supported at the main pins as a fulcrum say,
we have the sum of forces and the sum of moments but three unknowns and
we have an in-determinant structure as in figure three. If you assume
the wing pin takes all the load as a see saw, then the spar bushes only
have the moment load which is much less. Add in the point you made
about the spar bending actually unloads the load its all gravy. So it
seems without extensive strain gauge testing you can't get the design
approved on the statement "well the pin is really not overstressed". We
can't calculate the forces actually born by the pins unless we assume
(ass out of u and me) some loadings.
It is my feeling that the main wing pin does in fact take some of the
lift load, greatly relieving that front pin. Any assigned loading on
the bulkhead wing spar pins would significantly relieve the front
socket. I believe this is born out by the fact that so many aircraft
are flying well over gross weight and have flown uneventfully with no
signs of stress on the forward pin and socket. Looking at the forward
and rear socket, we also note that if a careless builder did not get the
clearances as per the manual, bending on the pin would compromise the
rib attachment as the glass could not stand the peel forces from the pin
threads and disaster would ensue. A loss of any one pin would be
catastrophic. If the front pin was removed, the rear pin may be able to
take the load, but the root rib shear web may not be quite strong enough
and the main spar cannot take any twisting so structural integrity will
be lost. So to recap, the root rib, socket attaches, and pins are all
critical.
Anyway, that is all I was commenting on. I'll leave the gross weight
increase to Dave Goddard as he is a whole lot smarter than me. Other
drawings and comments are provided below the three figures.
Regards to all,
Good night,
Bud
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----- Original Message -----
From: Pete<mailto:peterz@zutrasoft.com>
To: Bud Yerly<mailto:budyerly@msn.com>
Cc: GRAHAM SINGLETON<mailto:grahamsingleton@btinternet.com>
Sent: Saturday, January 22, 2011 7:08 PM
Subject: Re: Europa-List: Re: Main wing bushings
hi Bud,
thx very much for taking the time for your clarifications and drawings
etc. i agree with strenthening the front lift pin fwiw.
the point i was making was that although the main spar pins ideally
*should* take a considerable portion of
the g-loads, in the current implementation i believe they are not when
the wings and spars are flexing
under g-loading. for paper illustration purposes only, imagine the
main spar pins being free floating wrt
the cockpit module. also assume for illustration purposes only, that
the wing spars ar glued together,
and the shear/twisting is negligible. now draw a straight line between
the two lift pins. measure the vertical
distance between this line and the spar pins. now add g's to the
wings, and the bend thoughout the spar
increases- thus this distance measurement increases, in a downward
direction relative to the lift pins.
so the result is that when the spar pins are captured securely in the
cockpit module, they are exerting
a downward pressure on the cockpit module bushes (!).
surely not what we want from the design, but what i believe we have in
reality. so when the wings and spars
are flexing under load, all the g loading is carried by the lift
pins, and the spar pin bushings are
actually *adding* downward load to the load carrying lift pins :-(
all that said, i would never mess or compromise the design, and only
strengthen areas after very careful
anaylsis, to avoid simply moving failure points to other areas of the
structure...which why i'm happy to hear
that there may be a forward lift pin strengthening mod.
cheers& thanks again,
pete
a239
On Jan 22, 2011, at 5:44 PM, "Bud Yerly"
<budyerly@msn.com<mailto:budyerly@msn.com>> wrote:
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AH semantics, or I just don't understand the question. Fatigue is
setting in...
I failed to note your discussion was also including the root pins,
which are absolutely carrying a share of the load and cannot be
discounted.
The system needs all the pins, and it is rather scary to note that
elimination, compromise or failure of any one pin leads to catastrophic
failure of the wing...
This is why the LAA is so concerned about the glider wing and the
wing root pins. I'm afraid that the shear forces have always been of
concern on the aft pin, but in fact is also a problem on the forward pin
as well. The fuselage pins do contribute to the lift forces, but also
are the alignment devices to hold the wing in place so the root pins
stay engaged. It has to otherwise we would not have a plywood
reinforced and metal and glass interface structure holding the bulkhead
mounted bushings, we would have put an access hole in the fuselage for
the pins only and just a bolt to hold the root secure to the fuselage...
It is an integral system.
Granted, my traditional schooling always implied the spar takes the
lift and the root takes the moments and drag forces. One would only
have to consider the spars glued together and the fuselage attached to
the spar at only the two spar pins to see the point. Each spar pin
would be carrying half the fuselage weight or more accurately, the spar
pins must take the full load of the fuselage weight component with
loadings. The spar (glued together) will just be a rigid structure and
the fuselage would hang from the pins, and I say it can normally,
however, the Europa seems to count on both side pins as well to
translate the root forces and assure structural integrity.
The two side pins at the wing root do allow the root forces to lift
a component of the weight of the fuselage or as much as the root rib
will translate, it is true, but the sum of the forces and moments on the
root pins is significantly augmented by the wing fuselage pins. A
significant amount of the load, in my opinion has to be carried by the
pins in the cockpit, as you know there is considerable moments working
on the fuselage bulkhead. Of course the moments imparted by the wing
cannot be born by just this internal fuselage structure and pin
arrangement the shear web on the side of the fuselage contributes to the
strength of the aircraft. The pin sockets on the side of fuselage or
root pins take all the torque of the lift moment and the moment between
the lift vector and the main spar as well as the root rib imparted lift
and drag forces. If only the side pins were supporting the entire load,
the spar bending loads on the root rib (which unfortunately allows
flexing of the glass, as well as the attach points), would in practice
fail in bending of the socket or pin so the load cannot be 100% on the
root pins. This is quite prevalent in the motor glider. The root rib
cannot lift all of it alone nor is it strong enough if the forward pin
or socket is compromised. We are now facing the LAA wanting a much
stronger attach point on the fuselage for the forward root pins. The
sides of the fuselage are not strong enough really for my taste and I
believe they are probably correct in wanting a much larger fitting on
the forward pin and socket to take some of the load from the rear pin.
The sum of the forces then about the starboard cockpit pin indicates
huge shearing forces on the internal pins, as I alluded to, which keeps
the pins in check. (See my quick and dirty drawing.)
If the pin bush is imparting an upward force on the fuselage, in
steady flight, the downward force implies that the fuselage has a weight
(plus manuever loading) to the fuselage wing pins also. Hence my term,
hanging from the pins. Of course, the side pin forces must be included
and are considerable as the vector drawings show.
If the side attach pin points were so strong, why don't we have a
trolley for the mono with pins to lift the plane for servicing. Or, why
not build a wooden beam through the fuselage to hold the plane alone
through the pins, probably. Would it hold in either case, you bet, but
any slop and you are going to bend something badly. In reality betting
the whole design limit loading on the plane on just either is not
possible as glass and fittings bend under load. It is an integral
structure.
Anyway, that is how I understand it...
Bud<74.jpg><http://photos.msn.com/Viewing/Album.aspx?PST=8nK2AN1B!1LEiK
Sdooxx9v4CFFiF2tnPWP2mfc7Vl*sS5AKZ1XFu9bdFiq9nGbamu2smUkPeKxoqgpH5Zhql1A%
24%24>
----- Original Message -----
From: GRAHAM SINGLETON<mailto:grahamsingleton@btinternet.com>
To: Peter Zutrauen<mailto:peterz@zutrasoft.com>
Cc: Bud Yerly<mailto:budyerly@msn.com>
Sent: Friday, January 21, 2011 7:39 AM
Subject: Re: Europa-List: Re: Main wing bushings
Pete
that's how I understand it
Graham
-------------------------------------------------------------------------
-
From: Peter Zutrauen
<<mailto:peterz@zutrasoft.com>peterz@zutrasoft.com<mailto:peterz@zutrasof
t.com>>
To:
<mailto:grahamsingleton@btinternet.com>grahamsingleton@btinternet.com<mai
lto:grahamsingleton@btinternet.com>;
<mailto:budyerly@msn.com>budyerly@msn.com<mailto:budyerly@msn.com>
Sent: Friday, 21 January, 2011 11:41:12
Subject: Re: Europa-List: Re: Main wing bushings
Hi Bud,
(this email sent to just you and Graham to avoid further confusing
the issue.)
My original mental analysis was just has you have described - but
Graham's description contradicted the assumption that under positive
high-G loads, the main spar pins are indeed providing an upward force
vector to the cockpit module as originally (and intuitively) thought.
The reason being that both the forward and rear (your rear pin
under tension analysis is accurate tho) lift pins are indeed taking
*all* of the downward G-loading from the cockpit module, and the two
spar pins are actually pushing *downward* on the cockpit module
(disregarding the twisting vector on the main spar pins imparted by the
scissor action of the spars). Why? because the spars are actually
bending in curve (think a smile) just at the entire wing is, and the
root ends of the spars are actually (relative to the fuse) higher as a
result than the spar pin positions. The spar is trying to impart it's
smile shape onto the vertical cockpit module surface - which is not
going to comply.
I believe this analysis (Graham's) is correct. So the fuse is
not hanging on the spar pins during high-G loading as one would
intuitively assume.
Cheers,
Pete
On Thu, Jan 20, 2011 at 10:26 PM, Bud Yerly
<<mailto:budyerly@msn.com>budyerly@msn.com<mailto:budyerly@msn.com>>
wrote:
Sorry guys if I don't understand the actual question about the
bushes and bearing loads! I had to go back and try to picture the
question, in the mean time, Grahams analysis is spot on as usual.
To visualize the loads read on to my miscellaneous drivel.
The two spars are basically a set of scissors.. The pins lock
the spars together like a pair of scissors trying to cut a steel bar.
The pins of course must go through the fuselage and each spar to hold
the fuselage gravity forces and translate the lift forces of the wings
to the fuselage.
The bushes distribute the load and the hardened pins take the
lifting loads as shear load.
Essentially the fuselage (about 1100 lbs. loaded with down tail
forces added under G and acceleration as well as the radial Gs) hangs
from the pins.
The spars bend under the vertical load. Like a pair of
scissors, that is trying to cut through that solid steel bar stock, so
picture the force of your hands causing the thin blades to bend, as you
would expect a thin long pair of scissors to twist, bending vertical and
laterally.
Mod 52s rear socket and tube takes the forward element of the
lift vector (the lift of the wing near maximum is forward and up, really
pulling on the aft socket) and translates the load through root rib, to
the rear attach to the steel tube in fuselage which is under tension but
does not significantly contribute to the vertical lift load.
Because the aircraft is fiberglass and it does move a bit under
load, the spars twist some under load. The Mod 52s, 1/2 inch spar pip
pin, keeps the tip of the starboard spar in check and from slipping into
the control linkage under bending and twisting. The spar strap deters
twist and spar separation under load from their at rest positions. The
pins hardened strength allows it to not be sheared by the huge shearing
action of the two spars against the fuselage and each other.
So why doesn't the spar pin bend at the bush? Picture the
fuselage weight is pulling down at the left cockpit bush, the left spar
(forward) is pulling up, but the right spar (aft) is pushing down, and
the sum of the actions basically keeps the pin horizontal. All that
force though is on only one 1/2 inch in diameter pin, and that is one
heck of a lot of shearing force. Because it is over a short distance
(and our spars are tight together), we don't have to have microns of
tolerance in the pin to bushing fit.....The action of the spars against
one another brilliantly keeps the pin horizontal and not from bending
upward at the fuselage bush and slipping off the pins as one would
expect at first glance thinking of only one spar.
It is a clean neat simple design. Could it be better, you bet,
but can it be built by us simple beings easily and cost effective?
Probably not, so do I want to change it? No way, it isn't worth it.
Like you all say, it ain't broke, so don't fix it.
For you new builders, keep your bushes and spars at a reasonable
tightness and you will be fine. If you must ream the bush a bit because
the port pin is too tight, you only need to ream a couple of thousandths
normally. A heated pin through the bushes works best, as Graham said
before, to take a slightly out of align bush and right it. Don't panic
at the fuselage bush slop as the spars keep the pin horizontal. Always
use the spar strap and the pip pin adjusted as per the manual with the
washer to distribute the load on the poor spar cup. Don't substitute
bolts for the wing pins provided unless properly hardened. If you own a
classic, do a Mod 52/74 as it is great insurance.
Regards to all and I hope to see you at Sun "n Fun.
Bud Yerly
Custom Flight Creations, Inc.
<http://www.customflightcreations.com/>www.customflightcreations.com<http
://www.customflightcreations.com/>
(813) 653-4989
----- Original Message -----
From: Bob Harrison<mailto:ptag.dev@tiscali.co.uk>
To:
<mailto:europa-list@matronics.com>europa-list@matronics.com<mailto:europa
-list@matronics.com>
Sent: Thursday, January 20, 2011 6:38 AM
Subject: RE: Europa-List: Re: Main wing bushings
Hi! I understand that the late Barry Mellors did all the
fuselage calcs. IMHO he was a brilliant engineer in all things aircraft
and worked in his own self employed right doing lots of work for the
Brazillian aircraft manufacturers Combine him with Don Dykins wing
designer and you had the benefit of a wonderful combine. Unfortunately
we obviously can=92t speak to Barry anymore to discuss the items you are
interested in. However it may help you to understand that when the wing
fails it fails forward. Like has been suggested =93if it ain=92t broke
don=92t try to fix it !=94 Barry, bless him, helped immensely with the
engine mount design and PFA approval for the Jabiru 3300 engine mount
and I=92m quite sure the aircraft would never have flown without his
design skill and approach to the intransigence of the PFA engineer of
the time!
Regards
To all
Bob Harrison G-PTAG
From:
<mailto:owner-europa-list-server@matronics.com>owner-europa-list-server@m
atronics.com<mailto:owner-europa-list-server@matronics.com>
[mailto:<mailto:owner-europa-list-server@matronics.com>owner-europa-list-
server@matronics.com<mailto:owner-europa-list-server@matronics.com>] On
Behalf Of Kingsley Hurst
Sent: 20 January 2011 03:33
To:
<mailto:europa-list@matronics.com>europa-list@matronics.com<mailto:europa
-list@matronics.com>
Subject: Re: Europa-List: Re: Main wing bushings
Hello Will,
I am not suggesting there is any alternative for us as
builders. The design "as is" obviously fits the bill so the old adage,
"if it ain't broke, don't fix it" is appropriate.
From a designer's point of view however, if you inspect the
various methods employed by sailplane manufacturers, there are possibly
better ways of doing it. I have never seen a sailplane with the same
set-up as the Europa but that doesn't mean there isn't any. Ivan Shaw
did get some ideas from sailplanes so maybe he found one . . . . I
dunno!
Cheers
Kingsley
do not archive
----- Original Message -----
From: William Daniell<mailto:wdaniell@etb.net.co>
To:
<mailto:europa-list@matronics.com>europa-list@matronics.com<mailto:europa
-list@matronics.com>
Sent: Wednesday, January 19, 2011 9:57 PM
Subject: Re: Europa-List: Re: Main wing bushings
Kingsley
I don't understand due to my limited knowledge of the
engineering.
What alternative might there be?
Will
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