A dialog from Sven Eriksson in Sweden between Sven, Jerry and Larry Olson
Hello (Jerry)
Sorry for not answering for a while. I have been very busy with my
job at the company.Milled fiber/fiberglass joints
My advice on milled fiber/fiberglass joints being glassed over for
increased strength/leakage resistance should be on the help page.
Hardpoints Furthermore a hardpoint is very easily made by making a hole with
appropriate diameter in fiberglass/foam surface, filling with milled fiber
all the way through and then applying the needed layers of glass cloth on
the outside overlapping the fiberglass surface by one inch or more
depending on structural strength needed.
Regards Sven Eriksson
From: Sven-Åke Eriksson <se@ebox.tninet.se>
To: Jerry <gsjostrand@sierratel.com>
Cc: Copeland <bnbent@pacbell.net>; Larry <information@express-aircraft.com>
Subject: Lower fuselage setup
Date: Tuesday, September 01, 1998 1:03 PM
(to) Jerry and Larry and Bill
I have now set up my lower fuselage into its jig and leveled the
appropriate points with the same leveling instrument that I used for the
wing jig. Using this instrument being capable of a precision plus/minus
0.02 inches I can clearly see that the WL40 scribed on my fuselage has a
precision of about plus/minus 1/8 inch. I have also mounted a string above
the fuselage and used a plumb bob to determine the centerline. I can again
see that the manufacturing precision is on the order plus/minus about 1/4
inch. Do you have any idea of what kind of precision one have to expect
from the factory made parts? My wings also showed some discrepancy in
precision as I have noted before.
Reason for asking about the empenage/fuselage joining method was that I
have carefully read through that chapter in my manual and I found out that
my lower fuselage was extended to sta 244. If I am going to use the
alternate method I MUST show calculations for the aviation authorities to
proof that strength will not be impaired. Otherwise I will not be allowed
to do that. Is there any knowledge on how many layers of glass there is in
the lower aft part of the impinge and in the aft part of the fuselage?
Sven ErikssonFrom: Sven-Åke Eriksson <se@ebox.tninet.se>
To: Larry <information@express-aircraft.com>
Cc: Jerry <gsjostrand@sierratel.com>
Subject: Aircraft tolerances
Date: Wednesday, September 02, 1998 2:54 PM
Hello (Jerry and Larry) Tolerances
I aligned the fuselage saddles as per the manual instructions with a carpenters level on WL40 and across the front end of the fuselage. Afterwards I checked level on the bottom part of the aircraft and found
that leveling WL40 in a left/right way produced an offset making the bottom flat part of the fuselage being not level. I have also used a leveling instrument mounted on a tripod to determine the height of WL40 at different points along the fuselage on both sides and found that there was a slight waviness giving a maximum error about +/- 1/8"on the whole part.For the center line I put an overhead string starting at STA244 at factory scribe mark and ending at an aluminum angle situated about one inch down from most forward/upward point at EOP fwd of STA34. I used a plumb bob to align the string exactly above the factory scribe mark for centerline position at EOP/STA34. Using a plumb bob to determine centerline in different positions gave me the centerline all the way to STA244. When afterwards measuring just above the wing spar carrythrough as called out in my manual, I found an offset about 1/4" to the right also this right side offset was easy to see when making measurements from side to side at a point in the side curvature where you have a 45 degree angle to a vertical line.
I then decided to move the starting point of centerline about 3/8" to the left from factory scribe mark at EOP/STA34. Repeating centerline measurements gave much more symmetric readings for this position all the way to STA244. I found it easier and much moreaccurate to do this procedure with an overhead string.
I do not remember if I notified you about how I did set up my wing jigs. I used the tripod leveling instrument and the fact that all templates are lined up on straight lines from tip to root. I leveled rib no: 1 and 4 using carpenters level first then using tripod leveling instrument to place fore and aft points of rib 2 and 3 at a straight line with respect to corresponding points on rib 1 and 4. Afterwards weighing down the wing skins in their respective jigs gave me an accuracy over the whole wing of about plus/minus 1/16". I can also see on the finished wings that they are very straight. I also set both jigs up at the same time to be able to compare measurements between right and left wing.
About discussion on tolerances in aircraft I am well aware of that small errors are more or less negligible. I use to fly a motorized glider manufactured by Scheibe (a German company). Aircraft from Scheibe are known to be different individuals every one. By looking for example on the trailing edge on a Scheibe aircraft that are say ten to fifteen years old there usually are a waviness of at least an inch and our motorglider looks like that. Despite this it flies quite well !! Cruise is 85 knots with a 65 hp WV engine at 3.6 gph. Conclusion would be that small offsets does not matter. Reason why I asked about this is that I wanted to be sure this was not something special for my kit. On the other hand I found when building
the wings that most of the parts were a good fit and it was no problem getting the things together.
Sven Eriksson
From: LARRY OLSON <information@express-aircraft.com>
To: Sven-Åke Eriksson <se@ebox.tninet.se>
Cc: Jerry Sjostrand <gsjostrand@sierratel.com>
Subject: Manufacturing Tolerances
Date: Tuesday, September 01, 1998 8:10 PM
Sven,
Reference your question concerning the accuracy of the water line and the
fuselage center line on your aircraft. I am not cretin how you determined
your waterline was off by 1/8 inch. I recommend you use the waterline on
the right front side of the fuselage for initial alignment. Make yourself
some reference points so that you will be able to always place the fuselage
back in that position. Once you have done that you can build your airplane
to a tolerance of .001 inches if you wish. I think you will be very lucky
to find a certified aircraft that holds a tolerance of 1/8" when it comes to
wing alignment. As far as the center line of the fuselage, I am not sure
what you are measuring. If it is the foam placement, a 1/4" is ok. If it
is the side of the fuselage you should be able to correct any miss alignment
when you bond the top to the bottom. Again, 1/4" difference will have no
effect in the performance of the aircraft, so the tolerance you want to hold
is up to you. To insure proper alignment on aircraft built at the factory,
we are making a fuselage tool that the lower fuselage sets down into and
holds the sides to the proper alignment.
If you have any further questions please write.
Larry
10/03/98