would Boxing in the spars add more strength? at the cost of?


What about just boxed in certain areas such as root and strut attach? Would that then induce less flexibility and a greater chance of fracture?

If you are a qualified aircraft designer/engineer then go ahead and make changes that you see fit.  If not, maybe you shouldn't fix what isn't broken.

I agree with Radfordc, this design has stood the test of time. There things about it that are designed for safety and you don’t want to mess with those. Beefing up one area just moves the stress to another area.
I had an engine seizure with my first Minimax and landed in a plowed corn field. I was so thankful that the landing gear was designed to fail in a hard landing. There was no damage to the fuselage at all, just the easily built landing gear. They broke just like they were supposed to leaving the cockpit area where I was sitting, perfectly intact. Something to be thankful for.
As builders it’s in our nature to modify and improve, that’s what makes us builders. I fight the urge to modify anything on a proven design.
It’s a good sign you think like that, means you have some passion for it. What I try and do is ask myself, What is this piece doing, what is it supporting, where does the load go, and so on. Where you end up is a brilliantly designed little airplane.

Boxing in the spars adds a few pounds and nearly doubles the shear strength. It doesn't do much for bending or torsion.  There's very little shear at the root (or the tip), it's highest at the strut.  The curved plywood forming the 'D' box does provide additional shear resistance while making other contributions so adding more shear web to the spar is pure redundancy.

Quoted from radfordc

If you are a qualified aircraft designer/engineer then go ahead and make changes that you see fit.  If not, maybe you shouldn't fix what isn't broken.

Well said. Couldn't have said it better myself.

If I wanted to strengthen the design, the wings are an obvious place to start.  Any additional weight in the wings is largely offset by inertial relief. Of course any change in gross weight means a change in power required for the same performance.  Boxing in the spars increases shear resistance and shear rises from the ends of the wing to a maximum near the strut attachment.  So I would use 1/16" ply around the strut and thinner 1/32" ply out to the ends being careful not to reduce the bearing strength at the root attachment fittings.  Reducing the ply thickness of both webs in the tip and root sections while increasing the thickness near the strut could make the spar stronger where it needs it with little weight gain. However, a box spar with two shear webs doesn't increase the strength as much as doubling the 'C' spar web thickness.  For bending the situation is much the same.  Increasing the size of the spar caps makes the wing stronger, again with the highest bending stress at the strut attachment.  With low bending at the tip and root, moving material from those areas to the area around the strut attachment makes the design stronger without adding weight.  In both cases gradual transition from thin to thicker is the best practice. The shear and bending moment beam diagrams would be a guide.  This isn't rocket science.  The design problems are statics problems. There's beam calculators on the web to play with.  Read ANC-18, especially  chapter three, a few times.  It's never been easier to get the necessary knowledge.  You don't need an engineering degree to know how a beam works.

I thought they did away with box spars years ago because the interior is not inspect-able, but at my age I might have made that up. Please do correct me if I am wrong.

It may be that a significant point needs more emphasis here.  I think the idea was to increase the strength of the spar.  However most of what most people think of as strength is in simply carrying the load imposed by the aircraft on the spar and in resisting bending.  That is mostly a function of the spar caps.  It really doesn't take that much to resist the sheer loads.  Adding another layer of plywood web really doesn't gain you much in any sense other than adding weight.

I work with these types of calculations all the time and it is very easy to make an "obvious" improvement which can be a really bad idea.  Unless you are prepared to read several books on spar design, and learn about testing properly with scale models and full sized prototypes, I would suggest not changing anything.  I've got a lot of books on this type of work, have done it for years, and I still just bought another book, the new "Composite Design Manual" by Jim Marske of Marske Aircraft to gain knowledge from his expertise.  This may not be rocket science literally but it certainly requires all the care and knowledge you can muster on materials science and structural analysis.  If that is fun for you and you will happily spend plenty of time studying and calculating, that's great.  Otherwise I'd just build to the plans and play it safe.

Tom

While I concur with the general sentiments of careful, dedicated study in Tom's post, the title of ANC-18 is literally "Design of Wood Aircraft Structures".  It goes into plenty of detail on various spar designs and includes a chapter on stress analysis with an example of a two-spar, strut-braced, fabric-covered monoplane. It was a reference for the design of the Minimax.  When TEAM chose to increase the performance of later models it is obvious they built on the work already done.  But if one is unwilling or unable to read ANC-18 and understand its content, then they should resist the temptation to make changes to the Minimax.

Wing strength depends on the type of flying being done.I presume most Minimax ultralights are flown sedately like transport aircraft with bank angles of upto 30 degrees only.
It would certainly be more fun to fly the aircraft in a more sprightly manner which is not possible.When we are flying model planes we throw them around easily with a few loops and a couple of rolls etc even with the trainer type aircraft.
But you cant do the same with a minimax.
I think its about time for a minimax which can be flown more like a sport plane rather than a transport aircraft.

Isn't the max rated to something like over 4g's?  Seems like with careful maneuvering you could pull some interesting moves with that much strength!  Don't ask me to try though..  

When I was building my Airbike I spoke with one of the guys at the TEAM factory.  He said that before the Airbike was allowed to fly in Germany that the wing had to be tested to destruction.  They built a strong fixture to hold the wing and started adding weight.  He said at 7.5 G the fixture broke....the wing was still undamaged.

Quoted from mullacharjak

It would certainly be more fun to fly the aircraft in a more sprightly manner which is not possible.When we are flying model planes we throw them around easily with a few loops and a couple of rolls etc even with the trainer type aircraft.
But you cant do the same with a minimax.

At one time there was a guy who did an aerobatic routine with a Minimax.  He did all the loops, rolls, spins you would expect.  As the story goes he wanted the plane to spin easier and added weight to the tail to move the CG back.  He entered a spin and couldn't recover....the plane spun flat into the ground.  The guy was sitting in a pile of sticks and pieces and just stood up uninjured and climbed out of the wreckage.  It's a good story anyway.

I hope my first crash goes that well!

My hope is you never crash....
Closest I ever came was a partial ground loop on landing when the tail spring broke.
Came within a foot of the right side fence. only control I had was the brakes.
I only had time and sense enough to grab the brake handle, squeeze it hard and with the other hand i slapped the off switch.
about 2 seconds.

I have differential brakes so I hope I can build my instincts up to be able to correct for that kind of problem..

How hard is it to nosedive a max while on the ground? I've only been handling mine with no tail so it's extremely easy to tip forwards.

By the way, take a look at this video below:
https://youtu.be/eyMZgLtibYs

been a while since I posed that question. Gotta say that was a fun read! one of the best that i have had on here. breezed through ANC-18 and will be going forward with my modifications. adding 1.2lb with minimal increase in sheer at the strut and wing attach points. But It still makes me feel good and thats all that matters. ill let you decide which parts of that statement are true. thanks for all the feedback!

Talking of spars I was wondering whether a ladder style ultralight wing with tube spars would work on the minimax.Just like the Rans S9.

Most of these ultralights dont have D cells and are good upto 100 mph. A major advantage of these wings is they are very quick and easy to

build.

Many years ago the guy who welded Airbike fuselages for TEAM (Jim Doyle) put an aluminum spar wing on an Airbike and it worked very well.

Quoted from radfordc

Many years ago the guy who welded Airbike fuselages for TEAM (Jim Doyle) put an aluminum spar wing on an Airbike and it worked very well.

Thats great news.There was a parasol conversion by Bob (Bob Daly) dont know if he flew it .That combination with the tube wings would make the construction very easy and quick.