DuoDiscus Wing

[Duane Eisenbeiss]

Enough time has past such that several DuoDiscus wings must have been
inspected. However, there have been no reports as to what was found during
the inspection or what, if anything had to be done. All owner should
communicate with each other so that we all can have a better handle on what
is happening to the fleet. I would like to request that all owners provide
the following information when available. It would be appropriate to obtain
data for all ships; those that passed and those that failed.
1) Serial Number
2) Any problems doing the inspection
3) Results of the inspection
4) What repairs, if any, were required
5) Cost or reimbursement from Schempp-Hirth
6) Any other info that you would care to share.

Some voids were found in the spar of the ship that I am involved with (S/N
217). In one spot a wire could be inserted for what appeared to be the full
width of the spar cap. The ship was transported to M & H for a "second
opinion" and any repairs as required. I will provide additional info when
the ship is returned home.

Duane

[Eric Greenwell]

In article <3fRab.527488$uu5.88216@sccrnsc04>,
says...
> Enough time has past such that several DuoDiscus wings must have been
> inspected. However, there have been no reports as to what was found during
> the inspection or what, if anything had to be done. All owner should
> communicate with each other so that we all can have a better handle on what
> is happening to the fleet. I would like to request that all owners provide
> the following information when available. It would be appropriate to obtain
> data for all ships; those that passed and those that failed.

There is a thread on www.gliderforum.com under the Duo Discus section
about this that might contain some of what you are after.
--
!Replace DECIMAL.POINT in my e-mail address with just a . to reply
directly

Eric Greenwell
Richland, WA (USA)

[goneill]

I am not involved with a duo but of the three inspected so far by our local
repair shop all required repairs, a 4th has still to be inspected.
gary
"Duane Eisenbeiss" > wrote in message
news:3fRab.527488$uu5.88216@sccrnsc04...
> Enough time has past such that several DuoDiscus wings must have been
> inspected. However, there have been no reports as to what was found
during
> the inspection or what, if anything had to be done. All owner should
> communicate with each other so that we all can have a better handle on
what
> is happening to the fleet. I would like to request that all owners
provide
> the following information when available. It would be appropriate to
obtain
> data for all ships; those that passed and those that failed.
> 1) Serial Number
> 2) Any problems doing the inspection
> 3) Results of the inspection
> 4) What repairs, if any, were required
> 5) Cost or reimbursement from Schempp-Hirth
> 6) Any other info that you would care to share.
>
> Some voids were found in the spar of the ship that I am involved with (S/N
> 217). In one spot a wire could be inserted for what appeared to be the
full
> width of the spar cap. The ship was transported to M & H for a "second
> opinion" and any repairs as required. I will provide additional info when
> the ship is returned home.
>
> Duane
>
>
>

[Duane Eisenbeiss]

> I would like to request that all owners provide
> the following information when available. It would be appropriate to
obtain
> data for all ships; those that passed and those that failed.
> 1) Serial Number
> 2) Any problems doing the inspection
> 3) Results of the inspection
> 4) What repairs, if any, were required
> 5) Cost or reimbursement from Schempp-Hirth
> 6) Any other info that you would care to share.
>
> Duane
>
>
Some have responded directly to me. The info is appreciated, However ....
Actually I was not trying to collect info just for me. I was hoping that
the info would be posted to the Group so that owners around the world would
also know what is being found and be better able to evaluate their Duo.

Duane

[Duane Eisenbeiss]

"Duane Eisenbeiss" > wrote in message
news:3fRab.527488$uu5.88216@sccrnsc04...
> Enough time has past such that several DuoDiscus wings must have been
> inspected. However, there have been no reports as to what was found
during
> the inspection or what, if anything had to be done. All owner should
> communicate with each other so that we all can have a better handle on
what
> is happening to the fleet.
> Duane
>>
The report for the inspection of ship S/N 217 has been received. The left
wing had 1 small void. The right wing had 4 small voids.

Has anyone had an inspection that revealed large voids? say larger than 12
inches or 30 cm.

Duane

[goneill]

One here in NZ had a void of 14-15 inchs long,the engineers here think
the problem has been from making the glue mixture to thin and it has just
flowed out of the rear side as there is a ridged edge on the front side to
make all the mix go towards the rear.
One comment made was they thought that the factory had attempted to
push some mix into that void by hand by reaching through the access point
but had only been partially successful
gary
"Duane Eisenbeiss" > wrote in message
news:rQDhb.533114$Oz4.430152@rwcrnsc54...
>
> "Duane Eisenbeiss" > wrote in message
> news:3fRab.527488$uu5.88216@sccrnsc04...
> > Enough time has past such that several DuoDiscus wings must have been
> > inspected. However, there have been no reports as to what was found
> during
> > the inspection or what, if anything had to be done. All owner should
> > communicate with each other so that we all can have a better handle on
> what
> > is happening to the fleet.
> > Duane
> >>
> The report for the inspection of ship S/N 217 has been received. The left
> wing had 1 small void. The right wing had 4 small voids.
>
> Has anyone had an inspection that revealed large voids? say larger than
12
> inches or 30 cm.
>
> Duane
>
>

[Slingsby]

"goneill" > wrote in message >...
> One here in NZ had a void of 14-15 inchs long,the engineers here think
> the problem has been from making the glue mixture to thin and it has just
> flowed out of the rear side as there is a ridged edge on the front side to
> make all the mix go towards the rear.
> One comment made was they thought that the factory had attempted to
> push some mix into that void by hand by reaching through the access point
> but had only been partially successful
> gary
************************************************** ********************************

It would be interesting to know if there is a commonality between all
of these discrepancies. Was the bonding paste always too thin or did
they mix it according to the clearances of each wing? If the mixture
really was too thin causing a void of 14-15 inches long then there are
probably many other voids which can't be seen on video. There needs
to be an ultrasonic inspection procedure developed which can map the
bonding interface.

[John Morgan]

"Slingsby" > wrote in message
>
> It would be interesting to know if there is a commonality between all
> of these discrepancies. Was the bonding paste always too thin or did
> they mix it according to the clearances of each wing? If the mixture
> really was too thin causing a void of 14-15 inches long then there are
> probably many other voids which can't be seen on video. There needs
> to be an ultrasonic inspection procedure developed which can map the
> bonding interface.


I have no first hand knowledge. Talked to the owner of a well known,
respected composite repair shop who said that cotton threads are added to
the epoxy to thicken it and keep it from running. And that apparently they
had failed to add enough cotton fiber and this resulted in epoxy running
out, creating the voids.

--
bumper - ZZ >
"Dare to be different . . . circle in sink."
to reply, the last half is right to left


---
Outgoing mail is certified Virus Free.
Checked by AVG anti-virus system (http://www.grisoft.com).
Version: 6.0.521 / Virus Database: 319 - Release Date: 9/23/2003

[Slingsby]

"John Morgan" > wrote in message >...
> "Slingsby" > wrote in message
> >
> > It would be interesting to know if there is a commonality between all
> > of these discrepancies. Was the bonding paste always too thin or did
> > they mix it according to the clearances of each wing? If the mixture
> > really was too thin causing a void of 14-15 inches long then there are
> > probably many other voids which can't be seen on video. There needs
> > to be an ultrasonic inspection procedure developed which can map the
> > bonding interface.
>
>
> I have no first hand knowledge. Talked to the owner of a well known,
> respected composite repair shop who said that cotton threads are added to
> the epoxy to thicken it and keep it from running. And that apparently they
> had failed to add enough cotton fiber and this resulted in epoxy running
> out, creating the voids.

According to the DG website the bonding paste should be about as thick
as cake icing. I suppose too much cotton would also weaken the bond
as cotton fibres aren't as strong as epoxy. I still believe that if
there are voids which can be seen and can have wires poked into them
then there must also be voids which can't be seen and are far to thin
to allow wire into them. A .0005 to .008mm thick gap between the spar
cap and shear web could not be seen on a video but it would still be
an area where there is no bond. Ultrasound would still reflect off of
the interface and show a gap. The ultrasonic technique would need to
be proven and calibrated on actual wings where voids have been found.
Filling the voids immediately makes the chances of developing another
inspection method unlikely.

[Martin Gregorie]

On 14 Oct 2003 22:56:24 -0700, (Slingsby)
wrote:

>"John Morgan" > wrote in message >...
>> "Slingsby" > wrote in message
>> >
>> > It would be interesting to know if there is a commonality between all
>> > of these discrepancies. Was the bonding paste always too thin or did
>> > they mix it according to the clearances of each wing? If the mixture
>> > really was too thin causing a void of 14-15 inches long then there are
>> > probably many other voids which can't be seen on video. There needs
>> > to be an ultrasonic inspection procedure developed which can map the
>> > bonding interface.
>>
>>
>> I have no first hand knowledge. Talked to the owner of a well known,
>> respected composite repair shop who said that cotton threads are added to
>> the epoxy to thicken it and keep it from running. And that apparently they
>> had failed to add enough cotton fiber and this resulted in epoxy running
>> out, creating the voids.
>
>According to the DG website the bonding paste should be about as thick
>as cake icing. I suppose too much cotton would also weaken the bond
>as cotton fibres aren't as strong as epoxy. I still believe that if
>there are voids which can be seen and can have wires poked into them
>then there must also be voids which can't be seen and are far to thin
>to allow wire into them. A .0005 to .008mm thick gap between the spar
>cap and shear web could not be seen on a video but it would still be
>an area where there is no bond. Ultrasound would still reflect off of
>the interface and show a gap. The ultrasonic technique would need to
>be proven and calibrated on actual wings where voids have been found.
>Filling the voids immediately makes the chances of developing another
>inspection method unlikely.

Two comments:

- viscous, sticky stuff like epoxy will tend to leave a larger void or
none at all. I think a series of deeper bubbles would be far more
likely than a single sub 0.1mm area.

- it would be extremely difficult to get epoxy into a very narrow void
unless it was rather thin

Does anybody on here know what advantage cotton threads would have
over, say, microballoons or separated glass rovings?
--
martin@ : Martin Gregorie
gregorie : Harlow, UK
demon :
co : Zappa fan & glider pilot
uk :

[Bert Willing]

First of all, it is not the epoxy which bears the load but the cotton
threads. Epoxy is just the matrix, and the rated load of cotton/epoxy is
around 7N/mm.

Microballons are much heavier and the composite formed of balloons/epoxy
doesn't hold the same load as cotton threads/balloons (balloons are spheres
so in this case it's the epoxy which finally bears the load). Microballoons
are rather used for surface cosmetics.

Seperated glass rovings would be ideal - but the beasts don't bend easily
into small radii if threaded and a paste made up of them would leave many
mm-sized voids. so it just doesn't work.

I think that people proposing utrasonic quality checks don't have an exact
idea how a wing is constructed. Utrasonic QC basically detects interfaces,
and a composite glider wing is made up from interfaces all over the place.
I'd say that the error rate in an utrasonic QC would be completely through
the roof.

I think it would be more easy (and straightforward) to do just a structural
load test up to 1.5 times max rated load :-)

--
Bert Willing

ASW20 "TW"


"Martin Gregorie" > a écrit dans le message de
...
>
> Does anybody on here know what advantage cotton threads would have
> over, say, microballoons or separated glass rovings?
> --
> martin@ : Martin Gregorie
> gregorie : Harlow, UK
> demon :
> co : Zappa fan & glider pilot
> uk :
>

[goneill]

One simple fix would be to have the entire assembly on a rotating jig and
have the leading edge
45degrees nose down while curing
"John Galloway" > wrote in
message ...
> My understanding is that any part of the rear flange
> that is not actually bonded will show as a 'white'
> area even if there is no visible or probe-able gap.
> The thin flange is being bonded to a black spar cap
> and resin bonded to both surfaces gives the see through
> effect.
>
> As the resin gets squeezed out from the front to the
> back apparently if no voids reach near the shear web
> there aren't going to be any farther forward.
>
> The factory have looked at ultrasonic inspection and
> have been that it won't work for the spar problem because
> of all the diferent layers it would have to look through
> - apparently.
>
> If there are continuous stalactites or curtains of
> resin hanging from the rear spar flange and no visible
> voids or white areas the spar is safe.
>
> All this is stuff from various reliable sources at
> various times but there is a need for a full statement
> IMHO - I hope it will happen once all the inspection
> data have been analysed
>
> John Galloway
>
>
> At 06:00 15 October 2003, Slingsby wrote:
> >'John Morgan' wrote in message news:...
> >> 'Slingsby' wrote in message
> >> >
> >> > It would be interesting to know if there is a commonality
> >>>between all
> >> > of these discrepancies. Was the bonding paste always
> >>>too thin or did
> >> > they mix it according to the clearances of each wing?
> >>> If the mixture
> >> > really was too thin causing a void of 14-15 inches
> >>>long then there are
> >> > probably many other voids which can't be seen on
> >>>video. There needs
> >> > to be an ultrasonic inspection procedure developed
> >>>which can map the
> >> > bonding interface.
> >>
> >>
> >> I have no first hand knowledge. Talked to the owner
> >>of a well known,
> >> respected composite repair shop who said that cotton
> >>threads are added to
> >> the epoxy to thicken it and keep it from running.
> >>And that apparently they
> >> had failed to add enough cotton fiber and this resulted
> >>in epoxy running
> >> out, creating the voids.
> >
> >According to the DG website the bonding paste should
> >be about as thick
> >as cake icing. I suppose too much cotton would also
> >weaken the bond
> >as cotton fibres aren't as strong as epoxy. I still
> >believe that if
> >there are voids which can be seen and can have wires
> >poked into them
> >then there must also be voids which can't be seen and
> >are far to thin
> >to allow wire into them. A .0005 to .008mm thick gap
> >between the spar
> >cap and shear web could not be seen on a video but
> >it would still be
> >an area where there is no bond. Ultrasound would still
> >reflect off of
> >the interface and show a gap. The ultrasonic technique
> >would need to
> >be proven and calibrated on actual wings where voids
> >have been found.
> >Filling the voids immediately makes the chances of
> >developing another
> >inspection method unlikely.
> >
>
>

[Paul]

Simple fix was to make sure the mix of chopped cotton and resin was thick
enough!
Maybe the problem is when they mixed the resin and cotton together, the
temperature of the resin was relativity low which made it more viscous and
while the whole thing was heat cured, the viscosity of the resin decreased
and ran before gelling. The rotating jig is a good idea but the size of the
reinforcing to keep the wing true would be problem. ( and expensive :-)
Paul
"goneill" > wrote in message
...
> One simple fix would be to have the entire assembly on a rotating jig and
> have the leading edge
> 45degrees nose down while curing
> "John Galloway" > wrote in
> message ...
> > My understanding is that any part of the rear flange
> > that is not actually bonded will show as a 'white'
> > area even if there is no visible or probe-able gap.
> > The thin flange is being bonded to a black spar cap
> > and resin bonded to both surfaces gives the see through
> > effect.
> >
> > As the resin gets squeezed out from the front to the
> > back apparently if no voids reach near the shear web
> > there aren't going to be any farther forward.
> >
> > The factory have looked at ultrasonic inspection and
> > have been that it won't work for the spar problem because
> > of all the diferent layers it would have to look through
> > - apparently.
> >
> > If there are continuous stalactites or curtains of
> > resin hanging from the rear spar flange and no visible
> > voids or white areas the spar is safe.
> >
> > All this is stuff from various reliable sources at
> > various times but there is a need for a full statement
> > IMHO - I hope it will happen once all the inspection
> > data have been analysed
> >
> > John Galloway
> >
> >
> > At 06:00 15 October 2003, Slingsby wrote:
> > >'John Morgan' wrote in message news:...
> > >> 'Slingsby' wrote in message
> > >> >
> > >> > It would be interesting to know if there is a commonality
> > >>>between all
> > >> > of these discrepancies. Was the bonding paste always
> > >>>too thin or did
> > >> > they mix it according to the clearances of each wing?
> > >>> If the mixture
> > >> > really was too thin causing a void of 14-15 inches
> > >>>long then there are
> > >> > probably many other voids which can't be seen on
> > >>>video. There needs
> > >> > to be an ultrasonic inspection procedure developed
> > >>>which can map the
> > >> > bonding interface.
> > >>
> > >>
> > >> I have no first hand knowledge. Talked to the owner
> > >>of a well known,
> > >> respected composite repair shop who said that cotton
> > >>threads are added to
> > >> the epoxy to thicken it and keep it from running.
> > >>And that apparently they
> > >> had failed to add enough cotton fiber and this resulted
> > >>in epoxy running
> > >> out, creating the voids.
> > >
> > >According to the DG website the bonding paste should
> > >be about as thick
> > >as cake icing. I suppose too much cotton would also
> > >weaken the bond
> > >as cotton fibres aren't as strong as epoxy. I still
> > >believe that if
> > >there are voids which can be seen and can have wires
> > >poked into them
> > >then there must also be voids which can't be seen and
> > >are far to thin
> > >to allow wire into them. A .0005 to .008mm thick gap
> > >between the spar
> > >cap and shear web could not be seen on a video but
> > >it would still be
> > >an area where there is no bond. Ultrasound would still
> > >reflect off of
> > >the interface and show a gap. The ultrasonic technique
> > >would need to
> > >be proven and calibrated on actual wings where voids
> > >have been found.
> > >Filling the voids immediately makes the chances of
> > >developing another
> > >inspection method unlikely.
> > >
> >
> >
>
>

[Slingsby]

"
> I think that people proposing utrasonic quality checks don't have an exact
> idea how a wing is constructed. Utrasonic QC basically detects interfaces,
> and a composite glider wing is made up from interfaces all over the place.
> I'd say that the error rate in an utrasonic QC would be completely through
> the roof.
>
> I think it would be more easy (and straightforward) to do just a structural
> load test up to 1.5 times max rated load :-)
> Bert Willing
************************************************** ********************************
I understand your point about ultrasonic testing. The sound wave
would have to go through the outer skin, foam, inner skin, carbon
fibre rovings spar cap, bonding paste, shear web of differing
compositions. I guess there could be too many interfaces although I
would think that one could find an ultrasonic frequency which didn't
reflect off of the interfaces but did reflect off of voids. Or
reflects different frequencies and giving a picture.
Any idea how composite structures are inspected on large airplanes? I
assume x-ray wouldn't be very sensitive on nonmetallic structures.
How about neutron radiation? Acoustic emission transducers? Ballistic
Recovery Chutes?

Also, if the cotton fibres carry the load, is the repair done with an
epoxy/cotton mixture? How do they get it into a blind air bubble?

[Slingsby]

John Galloway > wrote in message >...
> My understanding is that any part of the rear flange
> that is not actually bonded will show as a 'white'
> area even if there is no visible or probe-able gap.
> The thin flange is being bonded to a black spar cap
> and resin bonded to both surfaces gives the see through
> effect.
I wondered, as the fibreglass on my glider is a translucent green but
I am not sure I am looking at bonding paste which has cotton fibres in
it.

> As the resin gets squeezed out from the front to the
> back apparently if no voids reach near the shear web
> there aren't going to be any farther forward.

I would guess that if the spar caps didn't get bonded correctly on
several wings, then the ribs in the D-tube, the tops of other ribs,
the air brake box and other areas of the wing didn't get bonded
correctly on some wings also. Shemp-Hirth should be looking into this
and possibly cutting some wings open. Especially wings with known
voids in them.

> The factory have looked at ultrasonic inspection and
> have been that it won't work for the spar problem because
> of all the diferent layers it would have to look through
> - apparently.

It could also be that the only entities with ultrasonic equipment
capable of doing the inspections are major airlines and the military.

> If there are continuous stalactites or curtains of
> resin hanging from the rear spar flange and no visible
> voids or white areas the spar is safe.

Its at least as safe as the G103 fuselage which is now speed and acro
limited because the "margin of safety" is not as great as they shought
it was. Until a couple of weeks ago I would have said that the G103
was one of the strongest, most durable, over engineered, tolerant of
abuse soaring trucks ever produced.

> All this is stuff from various reliable sources at
> various times but there is a need for a full statement
> IMHO - I hope it will happen once all the inspection
> data have been analysed

> John Galloway
>
>
> At 06:00 15 October 2003, Slingsby wrote:
> >'John Morgan' wrote in message news:...
> >> 'Slingsby' wrote in message
> >> >
> >> > It would be interesting to know if there is a commonality
> >>>between all
> >> > of these discrepancies. Was the bonding paste always
> >>>too thin or did
> >> > they mix it according to the clearances of each wing?
> >>> If the mixture
> >> > really was too thin causing a void of 14-15 inches
> >>>long then there are
> >> > probably many other voids which can't be seen on
> >>>video. There needs
> >> > to be an ultrasonic inspection procedure developed
> >>>which can map the
> >> > bonding interface.
> >>
> >>
> >> I have no first hand knowledge. Talked to the owner
> >>of a well known,
> >> respected composite repair shop who said that cotton
> >>threads are added to
> >> the epoxy to thicken it and keep it from running.
> >>And that apparently they
> >> had failed to add enough cotton fiber and this resulted
> >>in epoxy running
> >> out, creating the voids.
> >
> >According to the DG website the bonding paste should
> >be about as thick
> >as cake icing. I suppose too much cotton would also
> >weaken the bond
> >as cotton fibres aren't as strong as epoxy. I still
> >believe that if
> >there are voids which can be seen and can have wires
> >poked into them
> >then there must also be voids which can't be seen and
> >are far to thin
> >to allow wire into them. A .0005 to .008mm thick gap
> >between the spar
> >cap and shear web could not be seen on a video but
> >it would still be
> >an area where there is no bond. Ultrasound would still
> >reflect off of
> >the interface and show a gap. The ultrasonic technique
> >would need to
> >be proven and calibrated on actual wings where voids
> >have been found.
> >Filling the voids immediately makes the chances of
> >developing another
> >inspection method unlikely.
> >

[Eric Greenwell]

In article >,
says...
> I understand your point about ultrasonic testing. The sound wave
> would have to go through the outer skin, foam, inner skin, carbon
> fibre rovings spar cap, bonding paste, shear web of differing
> compositions.

Is the spar cap on the Duo Discus bonded to the inner skin or the
outer skin? If it is bonded to the outer skin, it would not have the
foam to go through, which should ease the problem considerably.
--
!Replace DECIMAL.POINT in my e-mail address with just a . to reply
directly

Eric Greenwell
Richland, WA (USA)

[Martin Gregorie]

On Thu, 16 Oct 2003 08:00:49 -0700, Eric Greenwell
> wrote:

>In article >,
says...
>> I understand your point about ultrasonic testing. The sound wave
>> would have to go through the outer skin, foam, inner skin, carbon
>> fibre rovings spar cap, bonding paste, shear web of differing
>> compositions.
>
>Is the spar cap on the Duo Discus bonded to the inner skin or the
>outer skin? If it is bonded to the outer skin, it would not have the
>foam to go through, which should ease the problem considerably.

The problem is that the bond in question attaches the top spar to the
web. Any problems with this bond will drastically affect the spar
strength by allowing the top spar to peel off the web. In the DG-style
of assembly with carbon cloth between the spar and the web the bond
between the spar and the cloth is equally as critical as that between
the web and the cloth.

In the model world we prevent spar peel failures by assembling both
spars to the web and then wrapping to lot with Kevlar thread or an
epozy-wetted woven carbon tube before building the wing round the
complete spar. This gives a 30G wing. I accept that this is overkill
for full size, but my teeth still itch a bit at the lack of any
binding round both the spars except on the protruding stubs at the
root.

I flat out would not fly a glider with foam between the web and the
top spar and would hope that such a structure has never been made or
flown.

--
martin@ : Martin Gregorie
gregorie : Harlow, UK
demon :
co : Zappa fan & glider pilot
uk :

[Eric Greenwell]

I wasn't clear enough. The problem I thought would be eased was the
ultrasonic inspection problem. So, is the spar cap attached to the
outer wing skin, or is there actually foam between the outer skin and
the spar cap?

In article >,
says...
> On Thu, 16 Oct 2003 08:00:49 -0700, Eric Greenwell
> > wrote:
>
> >In article >,
> says...
> >> I understand your point about ultrasonic testing. The sound wave
> >> would have to go through the outer skin, foam, inner skin, carbon
> >> fibre rovings spar cap, bonding paste, shear web of differing
> >> compositions.
> >
> >Is the spar cap on the Duo Discus bonded to the inner skin or the
> >outer skin? If it is bonded to the outer skin, it would not have the
> >foam to go through, which should ease the problem considerably.
>
> The problem is that the bond in question attaches the top spar to the
> web. Any problems with this bond will drastically affect the spar
> strength by allowing the top spar to peel off the web. In the DG-style
> of assembly with carbon cloth between the spar and the web the bond
> between the spar and the cloth is equally as critical as that between
> the web and the cloth.
>
> In the model world we prevent spar peel failures by assembling both
> spars to the web and then wrapping to lot with Kevlar thread or an
> epozy-wetted woven carbon tube before building the wing round the
> complete spar. This gives a 30G wing. I accept that this is overkill
> for full size, but my teeth still itch a bit at the lack of any
> binding round both the spars except on the protruding stubs at the
> root.
>
> I flat out would not fly a glider with foam between the web and the
> top spar and would hope that such a structure has never been made or
> flown.
>
> --
> martin@ : Martin Gregorie
> gregorie : Harlow, UK
> demon :
> co : Zappa fan & glider pilot
> uk :
>
>

--
!Replace DECIMAL.POINT in my e-mail address with just a . to reply
directly

Eric Greenwell
Richland, WA (USA)

[Tom Seim]

> Is the spar cap on the Duo Discus bonded to the inner skin or the
> outer skin? If it is bonded to the outer skin, it would not have the
> foam to go through, which should ease the problem considerably.

Modern ultrasonic non-destructive evaluation (NDE) should have no
problem inspecting the joints in question. Rolls-Royce has developed
standoff NDE equipment that must contend with a 20 mm air gap. They
got excellent results detecting defects in carbon fiber samples:

Advances in air coupled NDE for rapid scanning applications
Farlow, R.; Kelly, S.P.; Hayward, G.;
Ultrasonics Symposium, 1994. Proceedings., 1994 IEEE , Volume: 2 , 1-4
Nov. 1994
Page(s): 1099 -1102 vol.2

Applications of through-air ultrasound for rapid NDE scanning in the
aerospace industry
Kelly, S.P.; Farlow, R.; Hayward, G.;
Ultrasonics, Ferroelectrics and Frequency Control, IEEE Transactions
on , Volume: 43 Issue: 4 , July 1996

S-H is just talking to the wrong people.

Tom

[Slingsby]

Eric Greenwell > wrote in message >...
> I wasn't clear enough. The problem I thought would be eased was the
> ultrasonic inspection problem. So, is the spar cap attached to the
> outer wing skin, or is there actually foam between the outer skin and
> the spar cap?
>

> > >
> > >Is the spar cap on the Duo Discus bonded to the inner skin or the
> > >outer skin? If it is bonded to the outer skin, it would not have the
> > >foam to go through, which should ease the problem considerably.
> >
> > The problem is that the bond in question attaches the top spar to the
> > web. Any problems with this bond will drastically affect the spar
> > strength by allowing the top spar to peel off the web. In the DG-style
> > of assembly with carbon cloth between the spar and the web the bond
> > between the spar and the cloth is equally as critical as that between
> > the web and the cloth.
> >
> > In the model world we prevent spar peel failures by assembling both
> > spars to the web and then wrapping to lot with Kevlar thread or an
> > epozy-wetted woven carbon tube before building the wing round the
> > complete spar. This gives a 30G wing. I accept that this is overkill
> > for full size, but my teeth still itch a bit at the lack of any
> > binding round both the spars except on the protruding stubs at the
> > root.
> >
> > I flat out would not fly a glider with foam between the web and the
> > top spar and would hope that such a structure has never been made or
> > flown.

************************************************** ****************************
I don't think anyone said that there is foam between the spar cap and
shear web. Foam within the shear web adds a great deal of stiffness,
however.

The spar cap of German designs is built into one wing skin while the
spar web
and opposite spar cap is bonded into the other skin. When the two wing
halves are mated the one spar cap is glued to the spar web and cap of
the
other wing half. It's pretty much a blind operation and everything had
better well match or there will be a poor or no bond.

The Genesis 2 and the LAK-17a both have the spar completely built
outside the
wing. The spar is totally wrapped in glass fiber insuring it's
integrity.
It will not debond. Following is a site which has pictures of a
Genesis spar.

www.aviation-salvage.com/aircraft/Genesis%202.htm

On the Genesis wing it appears that the spar is bonded to an inner
skin which is a foam sandwich with an outer skin. The Genesis spar in
the picture had a metal fence post cut through it and it looks like
Graphlite rods have been bashed into more of a round shaped bundle. I
believe they are spread out into more of an I-shape than the picture
would suggest. At least with this design, if the spar is not bonded
to the skin there is still a very strong spar.

[Duane Eisenbeiss]

"Eric Greenwell" > wrote in message
.. .
> In article >,
> says...
>
> Is the spar cap on the Duo Discus bonded to the inner skin or the
> outer skin? If it is bonded to the outer skin, it would not have the
> foam to go through, which should ease the problem considerably.
> --
The spar caps are bonded to the inter skin of both the upper and lower
skins.
A drawing of this is given in the appendix to the Schempp-Hirth Tech Note
for the inspection of the Duo wing. Of course the Tech Note may be viewed
on the Schempp-Hirth web site: www.schempp-hirth.com

Duane

[Bob Kuykendall]

Earlier, Eric Greenwell > wrote:

> ...So, is the spar cap attached to the
> outer wing skin, or is there actually
> foam between the outer skin and
> the spar cap?

The spar cap is glued to the inner skin, and there is foam between the
spar cap and the outer skin. The diagrams in this tech note pretty
much show it:

http://www.schempp-hirth.com/tmdocs/396-8-489.pdf

(Somehow, when I look at those photos, I smell epoxy resin.)

The big advantage of that method is that the sandwich skins are more
stable, and less prone to mirroring the spar after aging. One of the
disadvantages is that the spar is less stiff than it would be if it
went from outer skin to outer skin, and didn't have its depth reduced
by the sandwich foam.

Bob K.

[Eric Greenwell]

In article >,
says...
> The spar cap is glued to the inner skin, and there is foam between the
> spar cap and the outer skin. The diagrams in this tech note pretty
> much show it:
>
> http://www.schempp-hirth.com/tmdocs/396-8-489.pdf
>
> (Somehow, when I look at those photos, I smell epoxy resin.)
>
> The big advantage of that method is that the sandwich skins are more
> stable, and less prone to mirroring the spar after aging. One of the
> disadvantages is that the spar is less stiff than it would be if it
> went from outer skin to outer skin, and didn't have its depth reduced
> by the sandwich foam.

I don't think this is a disadvantage, but is instead an advantage,
because the wing isn't so stiff, and the pilot enjoys a smoother ride
in turbulence. The disadvantage is it likely takes more material, so
the wing is a bit heavier and more expensive.
--
!Replace DECIMAL.POINT in my e-mail address with just a . to reply
directly

Eric Greenwell
Richland, WA (USA)

[Slingsby]

> > The big advantage of that method is that the sandwich skins are more
> > stable, and less prone to mirroring the spar after aging. One of the
> > disadvantages is that the spar is less stiff than it would be if it
> > went from outer skin to outer skin, and didn't have its depth reduced
> > by the sandwich foam.
>
> I don't think this is a disadvantage, but is instead an advantage,
> because the wing isn't so stiff, and the pilot enjoys a smoother ride
> in turbulence. The disadvantage is it likely takes more material, so
> the wing is a bit heavier and more expensive.
************************************************** ********************************
I don't know Eric, that flexibility and smoother ride might be your
upper spar cap lifting off of the shear web. There is no law which
says the wing has to actually snap. It could be more benign; one wing
a little more flexible than the other or air brakes popping open
during high speed runs in turbulence. Each 4g pop might add another
..010" to the delamination between those two air bubbles in the bonding
paste.

The stiffness, or lack thereof, can still be designed into the wing by
varying the amount of carbon fibre rovings in the spar cap, or
Graphlite rods in the case of the Genesis 2 and LAK 17. I believe the
LAK 17 wing has the externally made spar bonded to the outer skin of
the wing because the designers wanted an extremely thin profile. The
spar is only 3 1/2 inches deep at the wing root of the LAK 17; bonding
it to the outer skin of the wing allows the wing to be extremely thin.
Unfortunately, some of the earlier 17's had that mirroring and the
factory paid to have the wings re-profiled. I believe that happened
to some of the earlier ASW 28 gliders, too. Still, the use of
Graphlite rods instead of carbon fibre rovings, and externally built
spars which are then wrapped in fibreglass cloth and vacuum-bagged is
a method which produces stronger, more durable and longer lasting
wings. As the current fleet of German gliders gets older I'll wager
there will be a lot of speed limiting directives because the "margin
of error" isn't what they thought it would be.
What kind of spar construction methods do the SparrowHawk, Apis and
Russia gliders use?

[Bert Willing]

In any case you will have a blind bonding applied to one side, be is spar
web to spar cap, or spar cap to wing skin - that's the backdraw of using
negative moulds.

Now if you produce voids in one case, you might as well produce voids in
the other case, no? And, by the way, how much would you allow the price of a
glider to increase in case the the manufacturer goes for x-ray or ultrasonic
QC?

--
Bert Willing

ASW20 "TW"


"Slingsby" > a écrit dans le message de
om...
Still, the use of
> Graphlite rods instead of carbon fibre rovings, and externally built
> spars which are then wrapped in fibreglass cloth and vacuum-bagged is
> a method which produces stronger, more durable and longer lasting
> wings. As the current fleet of German gliders gets older I'll wager
> there will be a lot of speed limiting directives because the "margin
> of error" isn't what they thought it would be.
> What kind of spar construction methods do the SparrowHawk, Apis and
> Russia gliders use?

[Bob Kuykendall]

Earlier, "Bert Willing"
> wrote:

> In any case you will have a blind
> bonding applied to one side, be is spar
> web to spar cap, or spar cap to wing
> skin - that's the backdraw of using
> negative moulds.
>
> Now if you produce voids in one case,
> you might as well produce voids in
> the other case, no?...

Well, no, I don't see it that way.

The loads between the skin and the spar are far lower than the loads
between the shear webs and the spar caps. Also, the stiffening of the
spar caps against buckling is absolutely critical to the itegrity of
the spar. Voids between the shear web and the spar cap leave the cap
much more vulnerable to buckling.

Bob K.





And, by the way,
> how much would you allow the price of a
> glider to increase in case the the
> manufacturer goes for x-ray or ultrasonic
> QC?
>
> --
> Bert Willing
>
> ASW20 "TW"
>
>
> "Slingsby" > a écrit dans le message de
> om...
> Still, the use of
> > Graphlite rods instead of carbon fibre rovings, and externally built
> > spars which are then wrapped in fibreglass cloth and vacuum-bagged is
> > a method which produces stronger, more durable and longer lasting
> > wings. As the current fleet of German gliders gets older I'll wager
> > there will be a lot of speed limiting directives because the "margin
> > of error" isn't what they thought it would be.
> > What kind of spar construction methods do the SparrowHawk, Apis and
> > Russia gliders use?

[Slingsby]

"Bert Willing" > wrote in message >...
> In any case you will have a blind bonding applied to one side, be is spar
> web to spar cap, or spar cap to wing skin - that's the backdraw of using
> negative moulds.
>
> Now if you produce voids in one case, you might as well produce voids in
> the other case, no? And, by the way, how much would you allow the price of a
> glider to increase in case the the manufacturer goes for x-ray or ultrasonic
> QC?
>
> --
> Bert Willing
************************************************** *******************************
"During operation the bonding areas carry high loads, but after
closing
the wing they become inaccessible forever. Therefore these "blind
bonds", as they are called, must be carefully prepared and executed
with
great precision. To achieve this, we developed a simple but very
reliable procedure:

Small strips of play dough ("Plastilin") are applied to all areas
where the upper and lower wing halves will get in contact. These are
special areas on the leading and trailing edge, the spars and ribs.
Adhesive tape protects the bonding surfaces from grease contamination
by the play dough. When the the upper and lower molds are put
together, the play dough is compressed and reflects a very accurate
image of the bonding gap. A similar technique is used for the spars: A
series of cloth pins are inserted into the foam of the shear web.
These pins are then pushed in when the molds are closed, providing an
accurate meassure of the bonding gap.

After re-seperating the molds, our quality controller inspects the
visualized thicknes of the bonding gap, which has to stay within very
tight tollerances to guarantee a long-term reliable bonding.

Before permanently closing the wing, the quality controller checks all
elements which will become inaccessible. Each individual nut of the
control linkage is verified, secured, marked and signed off on a
detailed checklist.

"Mumpe" call our workers the glue to close the wing. It is a mixture
of epoxy resin and cotton flakes, which gives it a consistency like
cookie dough. After all bonding areas have been thoroughly cleaned and
roughed, the mumpe is applied with a squeeze bag, similar as for icing
cake. The thickness of the mumpe layer reflects exactly the gap height
as previously meassured with the play dough or cloth pins, plus
additional two millimeters. The long experience of our workers shows
in the "artwork" of optimally trapezoid-shaped mumpe layers they
create with squeeze bag and wooden spates.

The right area, exact thickness and correct processing of the bonding
layer is crucial for long-term reliability. Any attempt to save weight
or cost here would compromize relaibility and safety.

The lower wing half is put onto the upper half, adjusted exactly to
the guide pins, and then compressed with many vise clamps. Both halves
must fit exactly in all places, or the profile will not be correct.

The force of the vise clamps squeezes the excess epoxy mixture out
between the wing halves. The glued wing is tempered overnight at 35
degrees Celsius. The next morning the wing is taken out of the mould,
using the crane and usually with a loud crack. In the sanding room the
excess epoxy is removed. After the moulds have been cleaned and waxed,
the whole process starts over again."

The above is copied from the DG website. The process requires a great
deal of skill, experience, attention to detail and craftsmanship.

It's not a matter of how much I will allow the price to increase. The
more important consideration for the manufacturer is whether I will
ever consider pruchasing or flying in a German designed and built
glider. I realize that Shemp Hirth is having the problem and the
broken wings are limited to Czech. manufactured SH gliders.
Nevertheless the process of blindly assembling the spar while also
blindly assembling the wing is a process which requires a great deal
of faith that it is done correctly.

[Eric Greenwell]

In article >,
says...

> Still, the use of
> Graphlite rods instead of carbon fibre rovings, and externally built
> spars which are then wrapped in fibreglass cloth and vacuum-bagged is
> a method which produces stronger, more durable and longer lasting
> wings. As the current fleet of German gliders gets older I'll wager
> there will be a lot of speed limiting directives because the "margin
> of error" isn't what they thought it would be.
> What kind of spar construction methods do the SparrowHawk, Apis and
> Russia gliders use?

I just finished speaking to Greg Cole, the designer of the
SparrowHawk, and he told me it's spar is completely laid up in a mold,
then oven cured (like the rest of the glider). An advantage of this
method, he said, it that even if there are spar to wing skin voids in
the final wing assembly, the strength is very little affected, as the
spar has all of it's strength.

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Eric Greenwell
Richland, WA (USA)