4 Bolts

AggieMike88

AggieMike88

Touchdown! Greaser!
Joined
Jan 13, 2010
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Denton, TX
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The original "I don't know it all" of aviation.
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KDTO (Denton Enterprise)
With 55WB's engine dismounted and at the engine shop, the club is taking advantage of the time to do some additional maintenance items, including an inspection and painting of the engine mount frame. Today I helped our club A&P remove it.

Amazing to me that all that's keeping that 6-cyl power plant mechanically connected to the air frame is just just 4 bolts.

But like 6PC, I'm easy amazed at many things in aviation. Like seeing a Cherokee's engine on my first lesson and marvelling how that small 4cyl engine was going to take me, the instructor, and the aircraft into the air and haul us around at 110 KIAS.
 
Yeah, I just pulled and re-installed the IO540 on my RV10 (for a nose gear SB). There are 6 bolts holding the engine and nose gear mount on the rest of the airframe - 4 bolts for engine to mount.

I know steel is strong but Wow.

I find the tail surface mounting structure equally non-intuitive in terms of strength.
 
I am still very unhappy knowing that it is also only 4 bolts that hold the wing on.
There really needs to be a solid I-beam from wingtip to wingtip
 
Every ounce counts. Why add more if 4 bolts meets the strength requirements? I read somewhere on some Flybaby material that every 1lb costs you 10 fpm in the Flybaby. Not sure how true that is but I keep it in mind whenever I'm redoing something on it.
 
AggieMike88 said:
With 55WB's engine dismounted and at the engine shop, the club is taking advantage of the time to do some additional maintenance items, including an inspection and painting of the engine mount frame. Today I helped our club A&P remove it.

Amazing to me that all that's keeping that 6-cyl power plant mechanically connected to the air frame is just just 4 bolts.

But like 6PC, I'm easy amazed at many things in aviation. Like seeing a Cherokee's engine on my first lesson and marvelling how that small 4cyl engine was going to take me, the instructor, and the aircraft into the air and haul us around at 110 KIAS.
Click to expand...

Those 4 bolts are considerably more than required too. Steel is amazing.
 
SixPapaCharlie said:
Because I am not that kind of engineer so while I accept that the structure is stronger than needed, I don't have an understanding of the math behind it.
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It's a loss, and some planes use a lot more and insert spars into sockets then thru bolt there as well. Pulling the wings off an Aztec is a freaking chore.
 
SixPapaCharlie said:
Because I am not that kind of engineer so while I accept that the structure is stronger than needed, I don't have an understanding of the math behind it.
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The bolts are loaded in tension and they are probably at least 40,000 psi bolts.
 
I may have missed a thread. Why is the engine off? Time for overhaul? Prop strike?
 
Take a single 1/2" bolt. It has a 150,000 psi strength and wouldn't shear below 17K lbs. of force. So nothing on a 4,000lb. airplane is going to break four of those unless you ram it into a concrete wall at 110 knots and probably not even then.
 
On my Sky Arrow a single large wing nut holds the horizontal stabilizer on. :eek:

Well, it actually snugs it down on three tapered locating pins.

But at least it's safety-wired! ;)
 
TangoWhiskey said:
Except at 4.2 G's... ;-)
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Wouldn't a 600 lb. engine take a lot more than a 100 G's to break (4) 1/2" bolts with a combined shear of 68000 lbs? :wink2:

I worry more about the guy forgetting to tighten them and having them fall out in flight.
 
Alexb2000 said:
Wouldn't a 600 lb. engine take a lot more than a 100 G's to break (4) 1/2" bolts with a combined shear of 68000 lbs? :wink2:



I worry more about the guy forgetting to tighten them and having them fall out in flight.
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I was talking about Bryan's wing spar bolts...
 
AggieMike88 said:
With 55WB's engine dismounted and at the engine shop, the club is taking advantage of the time to do some additional maintenance items, including an inspection and painting of the engine mount frame. Today I helped our club A&P remove it.

Amazing to me that all that's keeping that 6-cyl power plant mechanically connected to the air frame is just just 4 bolts.

But like 6PC, I'm easy amazed at many things in aviation. Like seeing a Cherokee's engine on my first lesson and marvelling how that small 4cyl engine was going to take me, the instructor, and the aircraft into the air and haul us around at 110 KIAS.
Click to expand...
But but btu...they're really big bolts!

On the cherokee, the wings are bolted to the spar with 6 or 8 bolts, don't remember which.
 
Alexb2000 said:
Take a single 1/2" bolt. It has a 150,000 psi strength and wouldn't shear below 17K lbs. of force. So nothing on a 4,000lb. airplane is going to break four of those unless you ram it into a concrete wall at 110 knots and probably not even then.
Click to expand...

No way that an AN bolt is 150,000 psi...

Of the shelf, maybe you can get 100,000 psi spec material and then it needs to be de-rated for safety factor.
 
The BOLT is not the only thing holding the engine on. The bolt holds the mount together. The mount carries most of the force.
 
flyingron said:
The BOLT is not the only thing holding the engine on. The bolt holds the mount together. The mount carries most of the force.
Click to expand...

At the firewall end of most mounts I've removed the bolts are the only thing in sheer, however the geometry of the mount limits the total amount of sheer available so you will fold the firewall before you shear the bolts.
 
flyingron said:
The BOLT is not the only thing holding the engine on. The bolt holds the mount together. The mount carries most of the force.
Click to expand...

The mount is connected to the airframe at four points. Those four points carry the entire load.
 
Clark1961 said:
No way that an AN bolt is 150,000 psi...

Of the shelf, maybe you can get 100,000 psi spec material and then it needs to be de-rated for safety factor.
Click to expand...

I just used a grade 8 example from memory. I had no idea AN bolts weren't as strong. Interesting, I wonder why they don't use the stronger bolts?
 
Alexb2000 said:
I just used a grade 8 example from memory. I had no idea AN bolts weren't as strong. Interesting, I wonder why they don't use the stronger bolts?
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Because they become more brittle and less shock/vibration resistant. These bolts are stronge than the surrounding structure. The bolt need be no stronger than the firewall, it's a waste of weight, and the softer bolt matches requirements of the application environment better.
 
Henning said:
Because they become more brittle and less shock/vibration resistant. These bolts are stronge than the surrounding structure. The bolt need be no stronger than the firewall, it's a waste of weight, and the softer bolt matches requirements of the application environment better.
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That is one side of it. Another side is damage tolerance and make-up. What happens if that high-strength bolt has a stress riser? or wasn't torqued properly? The larger diameter, softer material is going to be much more tolerant.

Match the fastener to the material...and allow for errors on critical components. There's a reason ya gotta keep the old design heads on staff even though the kids are better, smarter, faster...
 
Plus the engineers try to account for manufacturing error, maintenance error during it's lifecycle, etc. There was a interesting blog post I read at one point (that perhaps Boeing made him remove, can't remember) from a young Boeing engineer stating their philosophy when they design and how much they try to account for the fact that what they state should be built isn't what is always built and leave plenty of margin for error to account for that.
 
Anyone who's concerned about the strength of aircraft should take a look at the number of structural failures that occur (it's very few) and then look into the NTSB reports and eliminate those structural failures that appeared to have been caused by some pilot error. When I did a look at the 310 as an example, there were something like 13 structural failure crashes, of which 12 were night IMC, and 1 was day IMC. IOW, you have to get disoriented and do stupid things to overstress these planes. They're overbuilt.

I went to a Twin Cessna seminar last month in which I saw a lot of heavily corroded parts that came off of airplanes, all of which had flown in. There's a lot of margin for error on airplanes.
 
Ted DuPuis said:
Anyone who's concerned about the strength of aircraft should take a look at the number of structural failures that occur (it's very few) and then look into the NTSB reports and eliminate those structural failures that appeared to have been caused by some pilot error. When I did a look at the 310 as an example, there were something like 13 structural failure crashes, of which 12 were night IMC, and 1 was day IMC. IOW, you have to get disoriented and do stupid things to overstress these planes. They're overbuilt.

I went to a Twin Cessna seminar last month in which I saw a lot of heavily corroded parts that came off of airplanes, all of which had flown in. There's a lot of margin for error on airplanes.
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When I was working at a shop in Long Beach this professor flew in in his 411 that he wanted shipped to Guam as he took some position there. When I started taking the plane apart to stick into a container :yikes: Holy crap, there was big holes corroded in crap. I asked my boss if I should continue, the plane was scrap metal. He called the owner and we ended up cutting it up instead, but it flew in from Indiana in that condition. Talk about a long time of pencil whipped annuals.:nonod:
 
TangoWhiskey said:
I may have missed a thread. Why is the engine off? Time for overhaul? Prop strike?
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Not a prop strike and Didn't post before. Overhaul being evaluated.
 
Henning said:
At the firewall end of most mounts I've removed the bolts are the only thing in sheer, however the geometry of the mount limits the total amount of sheer available so you will fold the firewall before you shear the bolts.
Click to expand...

The firewall was something else I was able to study in detail. I now have a much better understanding why a bad nose first landing in a 182 is going to result in a messed up firewall.
 
AggieMike88 said:
The firewall was something else I was able to study in detail. I now have a much better understanding why a bad nose first landing in a 182 is going to result in a messed up firewall.
Click to expand...

Not just a nose wheel landing, the speed bump at Catalina collapsed them semi regularly when people would hit it while standing on the brakes to keep from running off the end of the runway, only what they are looking at isn't the end, it's the hump/horizon in the middle.
 
I can't believe that only four 3/8" bolts hold the PA46's 350 hp engine in place. Jetprop uses the same four bolts when they install an extra 200 hp turbine engine in one. I guess they've been overbuilt.
 
kmead said:
I can't believe that only four 3/8" bolts hold the PA46's 350 hp engine in place. Jetprop uses the same four bolts when they install an extra 200 hp turbine engine in one. I guess they've been overbuilt.
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I've not seen good numbers for converting HP to thrust, and of course there are many variables. But if you figured 1:1 (which it isn't), you'd still only be looking at 310 or 510 lbs, which isn't much. Vibration and engine weight are bigger, and those also aren't much. We often forget how strong bolts really are - think about how rarely we see them broken.
 
The B-727 pod engines (#1&3) are attached using 3 bolts.

Take a look at a Boeing or Airbus with under slung engines and the engine and pylon attach points as well.
 
The thing to worry about is the 1 (one) single, only bolt on the end of the strut on many high wing airplanes. It's carrying all that load when you hit turbulence, pull, etc. And if that one bolt lets go, the whole wing folds up at the root.
 
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