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Author Topic: Engine crutch  (Read 501 times)

Offline Peter in Fairfax, VA

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Engine crutch
« on: January 10, 2022, 12:41:17 PM »
So, I am scratch building four profile trainers.  Has building technology introduced a "crutch" into profiles where the grain between the motor bearers runs vertical, similar to some combat designs, like the Nemesis II?  If so, how would this be integrated?  In a related note, cutting insets for bearers is so tedious, I am tempted to glue them to the side of the fuse for future trainers.

thanks,

Peter

Offline Dave Hull

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Re: Engine crutch
« Reply #1 on: January 10, 2022, 02:33:10 PM »
Peter,

Not sure I understand your comment on "insets."  Can you explain?

I have used vertically oriented grain on some racing planes with good results. It is assembled with 30 minute epoxy, and the plywood doublers are used to unify the structure. I have also used a maple block with the grain parallel to better ensure an optimum glue joint. Let me see if I can find a picture or two.

The advantage of orienting the balsa one way versus another is due to material anisotropy--the strength of balsa is very different between along grain and across grain. Orienting it between bearers with the grain at 90 degrees it is intended to increase the crushing strength between the two bearers. Say, if you buried the nose of the plane in the ground coming in at a 30 degree angle. The bottom motor mount would like to displace upwards, which would require the front of the cross piece to crush; or, the bond joint at the back to separate. Note that with a properly glued and unified front end, the ply doublers would have to shear and/or buckle for the mount to move. That's what you want--all the pieces contributing to taking up loads.

Remember that combining all these wood types and in different orientations is the very definition of composite construction. When you consider a certain design load--like a nose-in crash--you are imagining where the load gets applied, how much it will be, and so on. Note that the stiffest piece in the composite arrangement will take up load first and may fail despite all the "heavy reinforcement" buried somewhere else. You need to think of "maximum fiber" which is the engineering concept for the outermost material surface from the neutral bending axis. So in this case, the hardwoods and the plywoods are much stiffer than the balsa, whether the balsa is measured with- or cross-grain. With plywood doublers on the outside, then what the balsa core is mostly doing is preventing buckling of the plywood and carrying internal shear from bending loads.

On another plane, I used another piece of hardwood where this crossgrain balsa separator might go. also worked, and the weight up front didn't hurt anything. And, it provided a place to install bolt-thru landing gear. More functionality from the same increment of weight.

The problem usually with "overbuilding" is that we stiffen one area which then focuses loads/bending in another area and the end result is less durable than it was. Classic trial by error "whack-a-mole design...."

Dave

Offline Peter in Fairfax, VA

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Re: Engine crutch
« Reply #2 on: January 10, 2022, 03:37:33 PM »
By "inset," I mean cutting slots in the fuse plank for the bearers.  For example, the attached Ringmaster plan has long bearers.  A design influence in what I am building is the Streak Trainer, which has even longer bearers.

Offline Dave Hull

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Re: Engine crutch
« Reply #3 on: January 10, 2022, 04:02:05 PM »
Got it. Good description.

How are you cutting your insets?

For a balsa fuse, I'd clamp the plank to the edge of a solid table, overhanging. Then take a coping saw with a new, unmangled blade and saw to the line. Pretty easy to cut if you keep your eye on being vertical. If you have a scrollsaw, even easier. Bandsaw works too, but sometimes the blade in the saw is a bit too aggressive (but I'm too lazy to change it....)

I shoot for a slide-in fit and leave any fuzzies from the sawing as epoxy-holding filler.

Things get a bit trickier for odd shapes like a tapered stub spar in a solid wing as on a trainer or racing plane. Here's a before and after pic.

Offline Dave Hull

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Re: Engine crutch
« Reply #4 on: January 10, 2022, 04:06:40 PM »
Here's the cutout. This was done by "tracing" the stub spar perimeter onto the wing blank using a no. 11 blade. In essence, I used the spar as the ruler to make the cut. Once marked out, keep cutting.....

Offline Dave Hull

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Re: Engine crutch
« Reply #5 on: January 10, 2022, 04:15:28 PM »

Offline Peter in Fairfax, VA

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Re: Engine crutch
« Reply #6 on: January 10, 2022, 09:23:03 PM »
Dave,

Several thoughts.

First, after reading your response and giving some thought, I want to retain fore and aft grain direction, as typical.  Profiles break just forward of the nose, and fore/aft grain resists that.

Second, your sub-spar is interesting.  I had not seen that.  I was planning a 1" wide band of plywood, similar to the Aldrich Streak Trainer.  But your inset looks superior.

Third, I did use a scroll saw for one side of the bearer cutout, then did precisely what you did for your sub-spar for the second side.  Used the bearer as a ruler, traced with a #11 blade.  Made four fuselages; all the bearers are glued in with Titebond.  Poplar, btw, not maple.

thanks for the consultation,

Peter

Offline Dave Hull

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Re: Engine crutch
« Reply #7 on: January 10, 2022, 11:10:22 PM »
Sounds like you have a plan, Peter.

Just for fun, I made a table up of woods that we commonly use on models and included the most important properties. For our purposes, birch is virtually as good as eastern maple for mounts. What you get with these is superior crush strength when you bolt thru them.

If you use a thru-fuselage spacer or a threaded spacer, you are no longer trying to crush the wood cross-grain, and poplar turns out to work pretty well. Sure has on the planes I have used it on. Here's a picture of one example.

Most people that hear this, immediately think of "poplar lite-ply."  Not even remotely the same stuff.

Using a stub spar of maple or my preference lately--poplar--gives a good bellcrank mount, takes up the bending loads on the wing in a crash, and isn't that much weight. Just decide your tradeoff on spar length and weight and chop to fit. I like the tapered diamond since it is better structural transition than a block-ended piece of motor mount stock. One tip, though. Don't glue it in until all the balsa shaping is done. The easiest way I have found is to shape the wing section completely. The cut the stub-spar to planform, leaving it thicker than the wing. Trace the spar onto the wing blank and complete the hole in the wing. Then press in the spar and scribe the sanding line on the side of the protruding spar. Push it back out and cut and sand down to the line. Keep test fitting until you have everything blended. Do all of the sanding before gluing. Then you can finish the wing. Here is a picture of glassing the wing at the same time as bonding in the spar.

Dave

Offline Dave Hull

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Re: Engine crutch
« Reply #8 on: January 10, 2022, 11:31:13 PM »
Ok, the second picture was too big. Let's try this....

Offline BillP

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Re: Engine crutch
« Reply #9 on: January 11, 2022, 09:28:45 AM »
My thoughts: Those plans are far from the original RM but it appears the mods have taken care of the problem already by extending the beams 2x longer than the original RM...so I'm not sure what the gain is by doing more. Use a band saw or basic wide blade hobby saw instead of the scroll saw and cutting long slots will be way less tedious. 
Bill P.

Online Tim Wescott

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Re: Engine crutch
« Reply #10 on: January 11, 2022, 10:59:38 AM »
My thoughts: Those plans are far from the original RM but it appears the mods have taken care of the problem already by extending the beams 2x longer than the original RM...so I'm not sure what the gain is by doing more. Use a band saw or basic wide blade hobby saw instead of the scroll saw and cutting long slots will be way less tedious. 

For cutting slots for engine bearers I drill 3/8" holes at the back of the slot, then use a straight edge and a hobby knife.  If you're cutting along the grain, and you take several cuts, you can slice a 1/2" thick balsa sheet, and get a nice straight line to boot.

I wouldn't expect the balsa between the bearers to contribute much strength to resist breaking, so making up a crutch ahead of time and inserting it into a rectangular cutout in the fuselage would probably work, and may make for an easier hole to cut in the fuselage.
AMA 64232

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