ULBuilder, My kit is of 1997 vintage ( see image below ). Sl # is 975.
The manual clearly says to butt joint the web for both spars, but they sent out the pre-cut plywood only for the rear spar web. Probably may have been an experiment at that time to see if spar webs should be scarf jointed. Hard to know for sure after so many years.
If I were going to scarf any plywood... I would scarf the leading edge sheeting
The ply shear webs are only to keep the top and bottom spar caps from shifting sideways relative to each other. So scarfs really not adding, nor subtracting, anything.
If I were going to scarf any plywood... I would scarf the leading edge sheeting. The ply shear webs are only to keep the top and bottom spar caps from shifting sideways relative to each other. So scarfs really not adding, nor subtracting, anything.
While making the spars, I realized that the 3 mm birch plywood from my very old kit stock( RS 561 - which will be glued at the location of the spar-to-strut brackets ) was damaged due to water seepage.
While using new 3 mm birch plywood that I had purchased from Aircraft Spruce, I notice that the new ply stock has only 3 plies, where as the old ply stock from the kit had 5 plies. The new stock is aircraft grade plywood, so I am going to use it anyway, but I was wondering why one has 3 plies and the other has 5 plies , and what are the implications in strength ?
I would have to do extensive tests before I would accept thick cores with thin skins rather than all veneers being equal in thickness. Having said that, if it is the Domestic MIL-P-6070, it should meet standards. However I would be very unhappy trying to scarph it and would be inclined if scarphing it to use a very thin layer of carbon fiber and epoxy on each side of the scarph.
While using new 3 mm birch plywood that I had purchased from Aircraft Spruce, I notice that the new ply stock has only 3 plies, where as the old ply stock from the kit had 5 plies. The new stock is aircraft grade plywood, so I am going to use it anyway, but I was wondering why one has 3 plies and the other has 5 plies , and what are the implications in strength ?
Thanks for any thoughts on this.
They sell two kinds , the "Finnish" which has 5 plies and that comes in the kits, and the "domestic" which has only three. so apparently you bought the domestic one.
Yes George, I bought the domestic birch. So does that mean it may not be fully suitable for the underlay support for the spar bracket ? I have cut the pieces, but not glued them to the spar yet.
While making the spars, I realized that the 3 mm birch plywood from my very old kit stock( RS 561 - which will be glued at the location of the spar-to-strut brackets ) was damaged due to water seepage.
While using new 3 mm birch plywood that I had purchased from Aircraft Spruce, I notice that the new ply stock has only 3 plies, where as the old ply stock from the kit had 5 plies. The new stock is aircraft grade plywood, so I am going to use it anyway, but I was wondering why one has 3 plies and the other has 5 plies , and what are the implications in strength ?
Thanks for any thoughts on this.
The important strength characteristic of the ply in this case is probably the bearing strength against the bolts also known as the maximum crushing strength or ultimate compression strength. The original Minimax analysis appears to use an all-birch 3-ply for the calculation with the plies of equal thickness and a 90° orientation of the inner ply. The strength of the plywood can be determined by adding the contributions of the individual plies. So for birch we have a compression strength parallel to the grain of 8170psi and 970psi perpendicular. Then the plywood has a strength parallel to the face grain of 5770psi. Your plywood, according to Aircraft Spruce, has inner plies of either basswood or poplar and they appear twice the thickness of the outer birch plies. Poplar and basswood are about 60% and 40% respectively of the strength of birch so at best your plywood is about 75-80% the strength of the all-birch ply. Is that a problem? Well, the ply contributes only about 25% of the bearing strength here and the safety factor is 2.67 according to the TEAM analysis so using your ply would still leave you with a safety factor of 2.5. If that is no comfort it is easy to increase the strength here, just go to larger bolts or the equivalent, metal bushings.
I would have to do extensive tests before I would accept thick cores with thin skins rather than all veneers being equal in thickness. Having said that, if it is the Domestic MIL-P-6070, it should meet standards. However I would be very unhappy trying to scarph it and would be inclined if scarphing it to use a very thin layer of carbon fiber and epoxy on each side of the scarph.
Tom
Tom, Thanks for sharing your opinion. Appreciated.
The important strength characteristic of the ply in this case is probably the bearing strength against the bolts also known as the maximum crushing strength or ultimate compression strength. The original Minimax analysis appears to use an all-birch 3-ply for the calculation with the plies of equal thickness and a 90° orientation of the inner ply. The strength of the plywood can be determined by adding the contributions of the individual plies. So for birch we have a compression strength parallel to the grain of 8170psi and 970psi perpendicular. Then the plywood has a strength parallel to the face grain of 5770psi. Your plywood, according to Aircraft Spruce, has inner plies of either basswood or poplar and they appear twice the thickness of the outer birch plies. Poplar and basswood are about 60% and 40% respectively of the strength of birch so at best your plywood is about 75-80% the strength of the all-birch ply. Is that a problem? Well, the ply contributes only about 25% of the bearing strength here and the safety factor is 2.67 according to the TEAM analysis so using your ply would still leave you with a safety factor of 2.5. If that is no comfort it is easy to increase the strength here, just go to larger bolts or the equivalent, metal bushings.
Bob, very persuasive and detailed response. Much appreciated , thank you.
As it happens, out of three RS 561 strips that came with the kit, one had tell tale marks of having been exposed to water at some point, though the wood itself is not rotten. The other two seem to be intact and well preserved, and visually in very fine shape. I think I will use the well preserved strip for the wing which I am making right now.
For the second wing's spars, which I plan to make only after I am done assembling the first wing, I may just order a small 2 ft by 2 ft piece of 5 ply Finnish birch (3mm) from ACS, just to be on the safe side, although I feel the 3 ply stock which I have right now, may work just as well.
Also, looking at the plans, I wonder if these two RS561 strips on either side of the RS 11 diagonal provide some kind of a gusseting effect to make the (RS11 - RS9) joints stronger ( ? ). Could this be in addition to the role that Bob Daly just explained above ?
Bob, very persuasive and detailed response. Much appreciated , thank you.
As it happens, out of three RS 561 strips that came with the kit, one had tell tale marks of having been exposed to water at some point, though the wood itself is not rotten. The other two seem to be intact and well preserved, and visually in very fine shape. I think I will use the well preserved strip for the wing which I am making right now.
For the second wing's spars, which I plan to make only after I am done assembling the first wing, I may just order a small 2 ft by 2 ft piece of 5 ply Finnish birch (3mm) from ACS, just to be on the safe side, although I feel the 3 ply stock which I have right now, may work just as well.
Also, looking at the plans, I wonder if these two RS561 strips on either side of the RS 11 diagonal provide some kind of a gusseting effect to make the (RS11 - RS9) joints stronger ( ? ). Could this be in addition to the role that Bob Daly just explained above ?
Yes, the RS561 does help fix the RS11 piece to the spar caps but the ply composition is less critical for the job. Also, more plies doesn't necessarily mean greater strength. Bearing strength can be greater with fewer plies because of the anistropy of wood. For equal thickness plies, 3-ply beats 5-ply because 2/3 is greater than 3/5.
I have not given an update on this thread for a while. Presently I am assembling the port wing, and I figured I will give an update after the wing is assembled.
I have a question on wing assembly, on how to glue the 2 end ribs together so as to "box" the 2 ribs in. The procedure seems fairly straight forward, but while gluing the 1.5 mm inter-connecting ply to the 2 ribs, do I need to leave the marked area (between the 2 orange lines) glue-free, by inserting wax paper ?
Since the RS 17 stringers are going to come later into this space, leaving this small section un-glued may make it easier subsequently to cut the rib to make way for the RS17 stringer.
The construction manual for Max 103, shows the following sequence.
Glue ribs to spars. Glue anti-drag diagonals. Glue compression members.
Turn assembly around ( Front to back ) Glue rib ends to trailing edge RS 15. Attach square gussets. Slide in aileron spar web. Glue end ribs at this point, also glue aileron spar web to vertical RS-1 members. Glue aileron nose ribs.
Glue RS-3 stock on back-side of aileron web ( between rib stations )
Glue in aileron diagonals.
Box-in the first two and last 2 ribs. ( The above 3 steps can only be done for top of wing at this time.)
Turn the wing assy over ( i.e. Invert the wing ), and repeat the above 3 steps on the bottom of wing.
Make and glue leading edge skin to wing. After Leading edge is complete, ailerons are to be cut off. Remove aileron from wing.
RS-17 stringers are now cut and glued to trailing edge of cut-away wing. Bevel the trailing edge of RS-17 by 45 degrees.
Install / glue aileron leading edge skin.
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I asked the above question because I tried to follow the steps exactly as in manual. The possible reason why the the 2 tip ribs are boxed in as shown in manual is because the boxed in structure is more resistant to twisting and may help to keep the wing from twisting in the next operation, which is installing the leading edge skin.
Perhaps I cut my ailerons in the incorrect order from that in the manual, but it worked out fine and the plane flies great I'm told by my test pilot. (I still haven't flown it )
Just out of curiosity I re-read the Max-103 wing assembly section quickly. Step #31 lists gluing on the end-rib plywood strips (RS-534 - 2" wide strips) on the bottom of the wing, and that's after the trailing edge is installed and just before the leading edge ply in installed. I never did see instructions for just when the top strip is installed. Maybe I missed it in my haste?
In any case I've added a couple photos of cutting the ailerons loose after the top and bottom end strips were glued in. Maybe I did it incorrectly or out of order, but like I said, it worked out fine.
Happy building!
Lynn
PS...Looking at photos #2 and 3 in the posts below, I now seem to remember that when I did flip the wing over to cut the 2nd "legs" of the ailerons loose we used EXTREME caution to not stress the partially cut-loose assembly. I probably yelled at whoever was helping me flip the wing, probably my wonderful bride, and I still feel bad about probably being "testy".
Hi Lynn, Those pictures are very helpful and clarified a lot of things. I guess further to cutting away the aileron from the wing, you may have used a dremmel or other small rotary tool to cut away that 1.25 inch length of rib stock just below the 1.5 mm ply, to make way for the RS-17. By the way, your workmanship is great too. I wish I was half as good. Keith
I guess further to cutting away the aileron from the wing, you may have used a dremmel or other small rotary tool to cut away that 1.25 inch length of rib stock just below the 1.5 mm ply, to make way for the RS-17.
I can't honestly say exactly what I did for the RS-17 trailing edges of the main part of the wing. It's all covered with fabric and paint on my plane now otherwise I'd take a look and get back with you.
BTW, your kind words about the construction are appreciated. I just tried to do each step of construction as nice and accurate as I could since mistakes and messes really pile up.
Hi all, I have a question on nose ribs for the wing. Does the orientation of the plywood matter while cutting the rib ? I thought the ribs were to be cut with grain running fore and aft.
The project came to me with the nose ply ribs already made, but I found they were very pliable, may be because the ply was cut with the orientation vertical ( perpendicular to the plane of the wing ). Also the plywood looked to be much less stiff than the mahogany - it may have been poplar (?)
The wing build is progressing. Thanks for everyone's help along the way. I made the spars in May, and started assembly of left wing in June, and hope to be done with left wing by July end.
Some pictures below. In the nose rib picture, I am using the old ribs as pattern.
Thanks Lynn. Yes, the cross braces on the aileron were glued in later. Regarding ply orientation on nose rib, I posed the same question on the HBA forum and one member suggested that the grain orientation should be parallel to the chord line or within 45 degrees from the chord line. I guess I will go with that.
The answer from HBA seems reasonable so I'd think that would work. Considering (in my humble opinion) that the nose ribs biggest job is to shape the leading edge that grain direction on a +/-254# plane is not too critical.....but that is only my opinion. They are, after all, pretty much "captured" in place by the leading edge sheeting when it is glued into place and also stiffened side-to-side by the stringer at the leading edge as well.
I re-started work on the ribs in Feb this year. 22 ribs were completed last year, so I had to make 2 more ribs. After the ribs were done, it took me a month to make the aileron brackets from metal, drill holes in the 6 steel brackets, check alignment etc. Took me another month to get my work bench aligned and flat at the top. This was a frustrating exercise, as my work bench was in 2 parts and getting them to align correctly to yield a perfect flat top was hard. Moreover the floor was vinyl and not exactly flat. Any lateral movement of the bench would again send the alignment off. Finally had to fix the legs to the floor with Loctite, and then do the alignment.
Took me a month for the spar for left wing. Assembly of left wing commenced on June 01. Took 2 1/2 months to complete and varnish left wing. An injury to my back caused a month's layoff to recuperate, around Labor Day. Right wing spar commenced on Sep 20 th. Took 2 months for assembly of right wing. Right wing completed, varnished and stored away on Nov 22.
Speaking of this thread, I realized it is better to ask general questions on the build, outside of this thread because then those subjects become searchable to another builder, assuming the subject is correctly indicated. If it specific to my build, then obviously I will post it to this thread. However I will continue to post updates in this thread on progress on the build.
My workmanship is nothing to crow about, but I feel I have built the wings strong, with all glue joints properly done. Here is a link to a video on my right wing.
The wing looks great in the video, and glad you're continuing the build as time permits. My own build was interrupted with various things too, but I guess that's the way life works. Once the interruptions pass it can be a bit difficult to get back into the swing of building, but you just have to focus on the fact that each small step puts you just a bit closer to the finished MiniMAX. Seeing a completed wing you built from "sticks" is certainly an awesome feeling and sight, but just wait till you have the rush of seeing the completed plane together for the first time. I think the feeling and sense of accomplishment is beyond describing!
Lynn, Thanks for the words of encouragement. New / aspiring builders need these words to inspire them, just as much as they need information on building techniques. Dick, thanks for your comments.
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