near disaster towing today.

[kjkavaney]

After 16 inches of snow last weekend...we saw a window to tow today and get things going... I brought my trusty SIg 1/3rd scale Space Walker with 78 cc brisson twin for towing the crowd.
After starting the engine...it seemed to shake more at Idle than I ever remembered, but with a few clicks up on the throttle all was smooth...Just to be sure, I checked the engine mounting bolts which are safety wired...all the wiring was intact so of course the engine mounting bolts can't possibility be loose !!! RIGHT ???

after a few take offs and landings and a few mock aero tows...I landed and was ready for the first glider when I just knew something was wrong...I killed the engine...bad news.

after a lengthy examination, two out of the four engine mounting bolts were sheared right INSIDE the T nut....wondering if maybe better to let engine mounting bolts
free to turn or ??? confused why this happened.

These 10-32 allen bolts were Dubro bought from hobby shop...probably good enough for most everything but a buzzing 2 cylinder engine ... dang...
who knew ???
kevin

[Mosquito]

My neighbor girls came over to watch on their trusty steed Dakota. A horse that likes to watch aerotowing is a great horse!

This was a classic first time out event. We all had issues from forgotten wing rods, to gliders with batteries removed to engines coming lose. I even had an issue with a tow plane where the aileron hinges were worn out of the wing! Oh well. Better to work out these bugs now than at that "Big show" that you drove 9 hours to get to. We in the north were just happy to be 90 percent snow free and to see the sun again. Oh, those two gliders you see never did fly.......

[lenb]

Kevin,

For what its worth, I'll always use 1/4-20 bolts for any motor 50cc and up. even if it means drilling out the motor mounting lugs, But thats just me. I'm certain some engineards will say its not needed ...blah blah blah, but then again I've never had a bolt break.
Good job shutting down and checking it out. Its a great reminder for everyone to do some preflights before leaving for an event and being disappointed.

[JimD]

1/4-20 bolts

[lenb]

thank you Jim, correction made.

[kjkavaney]

I had thought about drilling out to 1/4-20 when I first put this bird together, but there really is not very much meat left on that mounting plate....an engineer friend suggested I check and see if I have some slight misalignment between these holes and the T nuts since the bolts ALL fractured right in their thread right in the middle of the T nut...there could be excessive side loads...that makes sense...will have to carbide drill and try the extractor bit and see if I can get the spuds out...they are really hard to see beyond that 1/2" of plywood fire wall...
Each day is another lesson learned...so thankful it did not let go in the air...might have taken out a glider behind it as well...
it's all good...thanks.

[ARUP]

Drill it! Plenty of material. As Len said... good call shutting it down and inspecting. Put hex nuts on those bolts, too! Get bolts that have a shank that is a bit wider than the metal plate through which they pass.

[gunny11]

If there broken in the T nuts, pop the T nuts out and replace them with 1/4-20. And if the T nuts are epoxy glued into place, then heat up music wire and use the wire to melt the glue away.

[kjkavaney]

Here is part of the problem...you are looking at the fuselage upside down as the Space Walker is a low wing airplane and the access to the fuselage is through it's bottom. Hence you work on things upside down... YOu can see the two T nuts in this picture are actually for the two lower bolts for the motor...the two upper T nuts are below that wooden ledge in the picture and you can not even see them with your eyes, you can barely even feel them with your fingers....hence making working on them quite difficult..
I will probably try to drill out the T nuts and heat may allow the epoxy to let go enough to get them out. I just don't want to pound them out and break the firewall or loosen it...
The Space Walker was designed for like a 35 cc so to go to more than double that, things had to be reinforced a lot...
Thanks for your input and suggestions guys...
kevin

[ARUP]

Put bolts into the T-nuts then put a torch to the bolt heads. Leave the bolts proud of the fire wall by an inch or so. The bolt will heat then transfer heat to T-nut which will then soften the (looks like epoxy) glue. Hit the bolt with a hammer to push the T-nut out.

[Steve P]

Potential solution. Bolt the motor back in with fresh, quality bolt stock. Clean up the old bird so it's lookin' fine. SELL IT. And, buy a new Bidule 111 and hang a DA100 or 120 on the nose. Problem solved. Small, powerful tug that's easy to transport for casual tow sessions. No need to thank me. That smile on your face is thanks enough. ;-)

[Mosquito]

I'm with Steve on this one! Great idea. Kevin, get on it!

[kjkavaney]

Perfect...I will try to sell this one to the wife after buying the Czmelak, swallow, 1-26, another DA-150 and all parts to finish the Czmelak and pay for that month in Africa....hmmm...math is not working.

[kjkavaney]

thanks Mike...probably exactly what I will do...torch ready !!!!

[lenb]

You're going to torch it?? Wow, thats over the top.. Try to sell it first.

[Al_M]

Judging by the length of the bolt from the break to the underside of the washer, it appears the bolt(s) failed from bending fatigue. The failure mode is similar to bending a paper clip to failure, only the amplitude (deflection) is much lower, on the order of a few ten-thousandths of an inch, but the number of bending cycles has surpassed the fatigue limit. Further compounding the problem is the thread itself, which creats a notch, which creates a stress concentation area, which locally increases the stress and further reduces the fatigue limit.

For those of you who enjoy math, the link below connects to an engineering paper on fatigue, S-N curves, endurance limit, etc. There is nothing more complicated than high school algebra in the paper, so if you like greek symbols and graphs, go for it.

BTW, Jim and Len are both right. Increasing the bolt diameter from 10-24 to 1/4-20 increases the diameter, hence the cross-sectional area of the bolt, thus reducing the stress (load per unit area) and increasing the fatigue limit (number of stress reversal cycles to failure). It’s all in the paper.

The first page in the paper shows a curved line, decresing from left to right and technically going out to the right without ever touching the horizontal line labeled the endurance limit. The curve is known as an S-N curve, and is a plot of stress, S, and the number of stress reversals to failure, N. The higher the stress the lower the number of cycles to failure.

The S-N curve is asymtotic, that is, it goes off to the right to infinity, never actually touching the horizontal line, or endurance limit. Stress levels below this line can undergo an infinite number of stress reversals without failure. There are two ways to accomplish improved fatigue life: reduce the stress by either increasing the size of the bolt or reducing the load. In this case, the load is constant and cannot be reduced, so the solution is increasing the diameter of the bolt. Or, one could retain the 10-24 bolts, replacing them periodically, say after each season, but I would not recommend this approach, because one can not know how many stress reversal cycles, which are cumulative, have occurred during the course of a season.

[Mosquito]

I had just typed the same response almost verbatim and you beat me to it! Thanks Al for the great information. Love it!

[kjkavaney]

I was more than convinced to do the 1/4-20 conversion...I used the Torch like Mike Kelly suggested....NO Len...I did NOT burn the plane up after all...
the T nuts came out easily after heating up the bolt to cherry red... and now I have step drilled out the motor mounts to 1/4" so all is well...engine is installed and ready to fly again..

Thanks for everyone's input and support...Larry Sorenson, a mechanical engineer, also felt the likely culprit was side loading due to slight misalignment but also since the fire wall on a SIG Space
Walker is NOT angled...having to use the washer gig at the firewall to get the right angle and some down also clearly would put even more stressors on those 10-32 bolts...

Plus my less than perfect flying skills probably was the last insult to the plane...

a third scale SIG space Walker with 80 cc is really a pretty nice small tow plane...very stable and forgiving...glad she is still in my stable...
kevin

[lenb]

Excellent Kevin !
Return to service....

Thanks for documenting this little incident. These are the kind of things folks have happen but don't talk about, however there's always something, some little tidbit of information that someone may learn from reading along.

[Mosquito]

We are going to put her back in action on Saturday and she how she pulls! Aerotow practice session take 2!

[kjkavaney]

Session number two is history...The space walker performed flawlessly today, it was I who was a bit flawed. The guys said they had the best thermal flying they have ever had...thermals that took them to the moon endlessly...Tim Mattsson had well over a hour flight ...the other pilots did equally well....BUT...for the tow
pilot it meant getting the heck kicked out of the tow plane every time......the thermals were so violent it was amazing...they would flip me 90 degrees in a heart beat....or push me vertical into a stall quickly...multiple times the sailplanes would bail on me when I would get flipped around...Flying a bigger heavier tow plane like my 108% Pegasus is so much easier than a smaller scale ship with less than half the weight and 30% of the power...
I found myself really struggling at times today much to the sailplane pilots dislike... all good refresher for my first rodeo of the season...

Tim's private airstrip is a jewel and it was so generous of him to have us up there again...it is a wild life preserve, nearly 400 acres mostly wooded and animals love it there...

[ARUP]

Of course you animals loved it there but how does the wildlife cope with the noise? lol

[lenb]

A Bucking Bronco kind a day.