Engine mount and frame bolts orientation?

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oriol

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Hi all!

Why in some airplanes the bolts that connect the engine mount to the frame, are sometimes positioned parallel to the crankshaft, and sometimes perpendicular? Is it because sometimes what matters the most are torque loads, and sometimes it is thrust? Are not both equally important? It is rare that the position of the bolts is not always the same.

Thanks a lot for your comments,

Oriol


parallel.jpg perpendicular.jpeg
 

wsimpso1

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See above why.

In all cases, these bolted joints should be carefully analyzed for the worst case load combinations and are generally sized with generous Factor of Safety.

Loading of engine mounts includes cyclic loading and has a bunch of load sources
  • Thust/Torque/P-factor from propulsion;
  • Gyroscopics arising from rotating mass rotation rates;
  • Forces and Moments arising from Engine/prop mass and airframe acceleration;
  • Vibration of engine/ prop during start, running, shutdown;
  • Loads imposed during service work including mount/dismount work.
A thorough design process will estimate these loads, determine combined load states, and size fasteners and joined elements.

Of course, a different process may be followed, where other airplanes with similar engines/ props/ missions are reviewed and their mounts and joints copied. While this may not have the rigor of the first method, it does have the appeal of "there are lots of them flying and none are breaking". Folks using this method are copying any mistakes made as well as the things done right, and so must also rigorously copy ALL of the details, lest they accidentally build without fixes that were found needed.

BIllski
 

oriol

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Thanks guys for the replies!

Now i see. I believe to have seen an arrangement mixing bolts, perpendicular and parallel to the shaft.

One of the most intimidating things about building my own fuselage, is the connection between the engine mount and the frame. I would dare to do many things by analogy, except this and the wings.

It is always fascinating to see the multiple arrangements used to bolt the engine to the frame, particularly in ultralights.

Cheers,

Oriol
 

TFF

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Most of our stuff is pretty light and a bolt in single shear is ok. Even a 550 Continental. Get something heavy like a big radial and you have to start thinking what you can do, especially if you don’t want some super strong and expensive custom bolt. Swing away mounts are pretty cool, but kind of hesitant to swing one. Pretty tight package where 2-3” would have eliminated the complexity. And finger smashing.
 

Riggerrob

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Consider that Dynafocal mounts use mounting bolts and rubber vibration dampers all pointed at the centroid of the engine in an effort to reduce vibration.
 

Riggerrob

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See above why.

In all cases, these bolted joints should be carefully analyzed for the worst case load combinations and are generally sized with generous Factor of Safety.

Loading of engine mounts includes cyclic loading and has a bunch of load sources
  • Thust/Torque/P-factor from propulsion;
  • Gyroscopics arising from rotating mass rotation rates;
  • Forces and Moments arising from Engine/prop mass and airframe acceleration;
  • Vibration of engine/ prop during start, running, shutdown;
  • Loads imposed during service work including mount/dismount work.
A thorough design process will estimate these loads, determine combined load states, and size fasteners and joined elements.

Of course, a different process may be followed, where other airplanes with similar engines/ props/ missions are reviewed and their mounts and joints copied. While this may not have the rigor of the first method, it does have the appeal of "there are lots of them flying and none are breaking". Folks using this method are copying any mistakes made as well as the things done right, and so must also rigorously copy ALL of the details, lest they accidentally build without fixes that were found needed.

BIllski
My old boss: Sandy Reid used to say "If you don't understand something, copy it exactly." Sandy pioneered the ringed harnesses found on most modern skydiving harness/containers. Many of his plagarisers tired alternate configurations. but only the configuration on Infinity looks like an improvement on Sandy's original design for hip rings.
All the other clumsy copiers just added awkward wear points.
 

oriol

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Thanks TFF for your comment regarding shear stress on a bolt.

It is easy to picture the maximum loads on theory, but it is hard to grab stress and fatigue caused by sudden loads, like torque generated by an engine starting. Is a "swing arm mount" similar to the silent blocks used in scooters, which have the motor that too serves as a swingarm? Can you name an airplane that uses this type of arrangement?

Good point Riggerrob on the silent blocks pointing to the centroid of the engine. I did not realized that was intended to reduce vibrations!

I realize that a big radial engine, or a rolls Royce merlin belong to a different league, if compared to a two stroke. Still, it is good to know a few things about how stuff works.

Cheers,

Oriol
 

SeppoK

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The WAR FW-190 has horizontal engine mount bolts, but then you need extra screws to mount the brackets to the fire wall.
 

wsimpso1

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The second picture appears to be a swing out mount. Solidly tie down the tail and both wings, then pull the bolts from one side and swing the engine around the bolts on the other side. This does require all the connections that go to the firewall connect on the hinged side. There are engines that snuggle right up against the firewall, and there is no way to access stuff on the back without a swing-out mount. C190 and or C195 come to mind.
 

Bill-Higdon

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The second picture appears to be a swing out mount. Solidly tie down the tail and both wings, then pull the bolts from one side and swing the engine around the bolts on the other side. This does require all the connections that go to the firewall connect on the hinged side. There are engines that snuggle right up against the firewall, and there is no way to access stuff on the back without a swing-out mount. C190 and or C195 come to mind.
Correct the Cessna 190 series had a swing out engine mount, I helped a friend do a mag service on his back in the 70's. Also the BD-4 as designed had a swing out engine mount
 

SeppoK

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It seems I remembered wrong, the WAR FW-190 has vertical bolts, which now makes sense. Made this 3D model when practicing with Solidworks. However, for example the Skyranger Rotax mount uses horizontal bolts.
Capture.PNG index.jpg
 

wsimpso1

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It is easy to picture the maximum loads on theory, but it is hard to grab stress and fatigue caused by sudden loads, like torque generated by an engine starting. Is a "swing arm mount" similar to the silent blocks used in scooters, which have the motor that too serves as a swingarm? Can you name an airplane that uses this type of arrangement?
It is all amenable to analysis. You could do this in SolidWorks we used to have as an EAA member. If someone really wants to do this, we can still model the mount we have in mind in the SolidWorks we can get, obtain numbers for weight and moments of inertia of the engine, and find an engineering house to run stresses in the airframe acceleration load cases, combined gyroscopic load cases, steady state vibe over the range of engine speed and during start accels and stop decels.

Monkey-see, monkee-do works too. Does anyone else have your engine running with your prop and your mission?

Billski
 

oriol

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Hi Billski, thanks for your assistance!

I am now going through an iterative process, to see if I can design something that could fit under either the sub microlight 70kgs category, or the sub 100kg primary glider category. The best pick for that would be a Pollini thor, or perhaps even two rc engines, à la Cri Cri. It is very challenging but also very fun to work out a possible design.

In any case, regarding solid works. I think I still have it on an old computer. Since I remember you could visualize the Von Misses distortion in your solid. But something as complex as a structure with multiple elements can be more tricky?

In any case, I would like to come back to Solid works... but mainly for drawing ribs and laser cut them, cutting ribs by hand is hell. So that I can test concept RC airplanes, and see if it is promising enough to make it into a full scale.

I am not engineer, I did not finished my studies. However as a first approach, I will try to run the numbers, and then discuss them with an engineer, instead of using Solid Works. Computers are of great help, like calculators, but I prefer talking with humans about technical details.

I was slowly reading about structures, but multiple commitments have went through. I am still optimistc about being able to carry on.

Cheers,

Oriol
 

BJC

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I wish that I had taken a photo of Leo Loudenslager's Laser 200 engine mount. The fuselage bushing that accepted the 3/8” bolt probably had 1/5 th metal / weight of a typical set up. And Leo routinely flew it to high g and gyroscopic loads.


BJC
 

Pops

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Monkey see-- Monkey do. On the plans for my Falconar F-12, it states that if the engine mount is built to plans with the reinforcement straps , it is rated at +9/-6 G's. Same as the airframe.
Don't look at the engine mounts on the Cessna 120/140's. So small in diameter, I don't see how they hold up to much of anything.
When my Cessna 172 was shipped in a container to Germany the container was dropped hard enough to buckle the 2 lower engine mount tubes.
 

wsimpso1

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I just created a new thread concerned with strut wing attachment and shifted notes on that topic from here. New thread is:

 
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