Got to thinking (always a dangerous undertaking) that breakaway engine bolts might save some engine damage during "lawn dart operations", and did some very quick and dirty calculations for four #6 screws made of nylon 66 (chose that material at random - unsure if nylon 66 is even used in hobby applications) Used midrange values of 6000 psi tensile yield, 8500 psi shear....

With a safety factor of two, the max loads before meeting a yield condition in the mounting screws are 127 lb in tension and 180 lb in shear... hmm... Seems that would hold the engine on the airplane under normal operations, but would allow it break free upon catastrophic impact, instead of trying to shove a 10 lb airplane through the backplate at twenty-plus g's... With a nylon throttle clevis to also break away, the engine parting company from the used-to-be airframe should keep collateral damage to a minimum...

Any materials gurus on the forum who can confirm what type of nylon is used in hobby screws? Specific mechanical properties would be even better!

Has anyone tried this?

Any concerns about fatigue or fuel exposure? Think that once-used nylon insert locknuts and steel washers would work well enough to keep the engine tight to the mount....

Thoughts/comments/snide remarks ??


Jim

Only input thet I have here is that aviation grade nylon insert lock nuts are only permitted to be used three times and then replaced. I kind of leary of any engine installation that's designed to come off on impact. Too many variables.

on small .10-.15 models i use three 4mm nylon screws to hold the aluminum backplate mount to the firewall and it works great. takes the starting loads just fine and usually breaks free in a crash without damaging the firewall.

dave

Well... we use nylon screws for holding on wings and considering I have put a couple planes through some high G's (both in air and air to ground) I would say why not, give it a try. If I were to do this though, I would replace them every so often and make sure you have everything balanced to the "T" to keep vibration down to a minimum. I would be leery to use this method on a 4 stroke though.

I would say no. Im the crash King... trust me. I've actually gone to mostly foam planes now... cause i've toasted way to many... and i'm not sure i'd want the engine breaking off with nylon bolts. First of all. That's what a glass or nylon mount does. and trust me... i've broken a few.. lol. Second... imagine even a nose over and your motor bouncing down the runway... not to mention the tangle from the throttle cable. Who knows... maybe you have an idea and dubro will sell them for ya! lol

I would say no also...

One would have to try it first of course, but of the top of my head I think nylon screws would stretch to much to securely mount an engine...

Like Zagnut said, it could work for smaller engines... but for the larger sizes... I dunno...

Also, I don't no what a #6 screw is (we define screws by the size of the screw), but I'm thinking that the holes in the engine mounting lugs are too small for a nylon screw with sufficient strength.

On some engines I have a hard time keeping the engine tight on the nylon mount with steel bolts... let alone using nylon screws as well...

But give it a try, and let us know what you come up with.

I'd think it would work. I would think if it'd hold up on an engine test stand there'd be no problems with flying it like that. My experience with hoping the fiber filled engine mounts will break first hasnt' been good. Also, I quit using two big huge 1/4" nylon bolts for wing hold downs. I normally use one #8 and have never had one break, except in a crash. Back when I used the normal two 1/4 inch bolts, everything in sight except the bolts would usually break.

Jimmbbo I think you are on the right track here and I have used M4 nylon bolts on smaller twostrokes (up to .40 size) without problems but you have to be careful and inspect them regularly. One compromise worth considering is to use brass bolts - an old idea I got from white haired CL guys flying on tarmac. That is what I use now on both 2C and 4C engines without problems at least up to .65 size.
Steel bolts is an absolute NO-NO..[:'(]... in my ships..................Cheers/Harald

Ah, yes... the Metric system - makes emphatically more sense!! If only the US would adopt it...

A #6 screw is about 3.3 mm in diameter - the numbers I crunched were for the minimum thread diameter of .116 inches (2.9mm), which would be the maximum material availabile to resist tension or shear forces...

Cheers!

Jim

Harald,

Had the same concerns about nylon's fatigue resistance and stretch - hoping some materials savvy folks can provide more relevant data

Hmm... will run some numbers on the brass screws to see what difference they make...

Cheers!

Jim

The guys flying combat with predators. Use nylon bolts to hold their motor mounts on and they save the plane major damage from impacts with the ground. Also, if you use an extra large spinner, it will save you from having to buy new engines from cracking off your carbs.

Have Fun,
Will

We have kinda sorta adopted it. Soda is now sold in liter bottles, but milk is still sold by the gallon. Cars use it for most bolts, except those removed most often, such as the wheel lugs. It is a simple system, if you forget the conversion just multiply or devide by ten till it seems right! I don't know why so many Americans are against it, after all we set up a Metric money system with dollars and cents.

I think it should work, if they don't streach. You could try two bolts of metel and two of nylon to start.

I'm still thinking about it... it is an interesting question...

The stretch in a nylon bolt could be regarded as a plus... because it will prevent nuts from coming loose... The trick will be not to over tighten them, which is not simple for someone used to steel bolts

for SSC combat, I dont use bolts. I rubber band the plywood engine mount plate to the fuse. It works fine for .15's

I think so... Still working on what the real material is so I can get a warm and fuzzy about the real strength and stretch characterisitcs...

Cheeers!

Jim

Same idea... does it save engine damage?

Cheers!

Jim

[quote]ORIGINAL: Sport_Pilot

I think it's familiarity... I can look at a bolt head on a Chevy and know it needs a 7/16 wrench, but am less familiar with the metric fasteners on my Acura, so I take three wrenches to the car...

Believe it would be a good thing to go metric, but don't think we will ever muster the political will to do it...

Cheers!

Jim

Problem is, I have to crash an airplane to test the theory

Cheers!

Jim

Cool!! Thanks, Will! [sm=thumbup.gif][sm=thumbup.gif]

Cheers!

Jim