FlyBlueSkies

My Feedback: (2)

I had a very interesting thing happen Saturday. I had just landed my plane and was taxiing for the pits. All of the sudden, the plane started turning circles.

As I looked closer, I noticed the engine was on TOP of the fuselage! It was still ON. Now mind you, this is a .91 Four Stroke engine with a 14 x 6 prop. The throttle linkage is a cable and was in such a way that I could not turn the engine off nor slow it down.

I took a few seconds to think of my next move...the plane was slowly circling towards the pit area...I ran out to the plane and placed my transmitter close to me, but on the ground. I took my 'Crocodile Dundee' stance and waited for the proper moment...

...then I pounced on the tail of the plane. I was holding the plane to the ground while another flyer ran over and threw a rag at the spinning prop...

It simply chewed the towel up and spit it out! Although it did not stop the engine, it did rotate it 90 degrees. It was now in front of the plane, basically where it should be. The engine stayed in place and did not move...I thought about letting the gas simply run out, but it might take awhile. I decided to pinch the gas line. I slowly moved my hand in, fortunately, the four stroke has the gas line come in to the rear of the plane.

I was able to pinch and stop the engine.

I used a cast aluminum motor mount. It simply broke at both rear mounting holes. I had the engine off two weeks ago and there was NO indication of cracks or breakage.

Has anyone else experienced a cast aluminum motor mount breakage?

Fletch124

My Feedback: (7)

wow man you got lucky. never heard of an engine mount breaking really. hehe croocadille dundee stance, thats pretty funny. Sorry about the mount and i hope it didnt cause to much more damage.

wahoo

My Feedback: (59)

You threw something into a moving propeller ??

FlyBlueSkies

My Feedback: (2)

Actually, no I didn't throw anything into a moving prop.....another flyer threw a towell into the prop in an attempt to stop it....

I'm curious though, hind sight always 20-20, what would you have done?

smokingcrater

found a bigger towel! an engine doing that is not only a danger to itself, but also the plane and any spectators nearby. throwing in the towel [8D] isn't exactly the best for the engine, but at least it brings a safe end to the situation.

wahoo

My Feedback: (59)

All hindsight aside it really depends on the urgency of the situation. If you did in fact have a grasp on the plane then there was no danger and no need to throw anything into the spinning propeller.

FlyBlueSkies

My Feedback: (2)

Really, I was looking for more information on aluminum motor mounts breaking.

I was very lucky with this 'adventure'. It could have been much worse...

The motor mount could have broken in mid air, in which case the plane would have surely crashed.

The motor mount could have broke while I was starting it, in which case, I might have some severely crippled hands....

I'm really trying to find out...does the aluminum motor motor more regularly than I anticipated...or is this an 'out of the normal' failure?

How many of you have run into this type of failure?

What do you think led up to it?

JB

LearjetMech

Did you modify the mount in any way when you put the engine in (ie grind, or spead the mount legs)? If so you might have induced some localized fatigue just at the right spot! Not blaming you but just throwing out options. It could of been a bad mount too. Is it cast or machined?

rw Guinn

what we have here is a bad case of fatigue fracture, it seems. I have a couple of questions first:
1. How many hours total has this engine run on this mount?
2. Has the mount been used for any other engine/airframe combinations? For how many hours?
3. do you have soft mounts on either the engine side or firewall side?
4. What type of flying do you do? Full throttle all the time? mix? brief description, if you please.

Aluminum, unlike steel, has no fatigue limit. With steel, if the cyclic load is below a certain level, it will last forever. Not so aluminum.
Every cycle with aluminum lessens its lifetime. At some point, it will fail. That is why Rider scale airplanes eventually go to that airbase in Arizona. the alloy used in most of this stuff is generally "Beer Can", which is probably pretty close to 5052-0, which has a long fatigue life, but at a low yield stress.
IT may not even show a crack visibly-the first indication is "It Broke"
Can you post a close up of the busted part?

Roger

LearjetMech

.

Scar

My Feedback: (3)

I'm going to wager that this mount was drilled and tapped for threads. Thus you have (1) a bunch of sharp stress concentrators - the threads - and (2) a screw applying what amounts to a tensile load around the hole, wedging the screw into the aluminum and trying to pull it out by spreading the aluminum. Engine vibration just creates additional tensile stress on the thread location.

I drill my mounts and put the screw through with a lock nut on the other side to hold the engine in place. This applies compressive stress to the hole, much less likely to fatigue.

Just my 2 cents worth,
Dave Olson

rw Guinn

that's why I asked for a picture--where did it break? Concentrations around the holes is always a factor...

SkyDude

Could it be possible that the mount screws were over-tightened?

rw Guinn

Anything is possible! That's why I like to see pictures...or drawings...
Without more data, your guess is as good as mine...

3D-kid330

My Feedback: (1)

Wow good thing it didnt bite the plane or worse yet your hand! Four strokes like to vibrate a lot; mabey this caused fatigue?
Like he said pics would help.

FlyBlueSkies

My Feedback: (2)

Thanks for all the great suggestions and analysis. I'll try to get some pictures of the broken mount soon.

The mount is cast aluminum.

The engine mount holes were predrilled and are through holes.

Please see the attached picture.

The engine is clamped in place with a strap on both sides (left and right). The bolts go through the strap, clamp the engine in place, then go through the holes of the mount. Again, these are predrilled through holes. There are nuts on the bottom side. I did not modify the motor mount in any way. It is completely stock.

The break occured in the rear two mounting holes of the clamp.

It's certainly possible I over tightened...but, the bolt uses a philips head screw driver. I would think a philips head would lessen the amount of torque used to tighten, but I could be wrong.

The mount is hard mounted to the firewall.

The four stroke engine and mount were brand new this year. The (14 x 6) prop was balanced.

I probably had about 12 hours on the engine and mount.

My flying style is a mix, but is more full throttle than not.

Previous to this failure, the idle stop screw vibrated loose and was lost. The screw holding the carburetor barrel came loose.

Just two days prior to the motor mount failure, the pipe, which connects the muffler to the engine, cracked and broke. A low flyby revealed the muffler 'flapping' in the wind as it was connected via the rubber hose.

After this failure, I looked everything over before I flew. But, as you know, I missed the broken motor mount.

I think there was a great deal of vibration.

Just prior to this, a friend suggested 30% nitro. I ran probably 1/2 gallon of this through the engine before I started having idle stop screws, etc., fall out.

Using the higher content fuel, my high end rpm increased from 9400 to about 9800.

I didn't notice any 'extra' vibration until the screws started coming loose.

My 'gut' feeling, is the 30% nitro led to pre-ignition and more vibration. I know this may be controversial, but I believe this to be the case. At any rate, no more 30% nitro for this engine.

However, it could have been a bad motor mount, which led to the vibration. The 30% nitro might have simply quickened the failure.

At any rate, I must replace the mount. If I replace with the exact same one, I have a LOT LESS work to do. If I replace with any other mount, MORE WORK.

If this is an isolated failure, I want to use the same mount. However, if many other failures happen with this mount, the answer is obvious.

Again, thanks for all your insight and I'll try to get pictures of the failure up soon.

LuckyArmpit

That looks alot like a Hangar 9 mount. I had one of these in my Arrow and it mounted an OS .46. I never had problems with anything coming loose. I did however use lockwashers and for the nuts, I used ones that had nylon on the inside. Generally, they don't come loose but I check them every few flights because I don't trust anything that is moving.
Wahoo, notice your from NC PA. Thats where I'm originally from. Born and raised there. What club do you fly at?

Dave...

LearjetMech

A bad casting I bet. Or one other question. Has the plane ever been landed really hard. If not send it in and get a new one. Worth a shot right!

FlyBlueSkies

My Feedback: (2)

Ok,

Here are some pictures. They're a little dark, but you can see the break.

As far as landing's go, I have not had a crash landing nor a hard landing. The last landing was very smooth...


Anyway, if you have any thoughts, let me know.

FlyBlueSkies

My Feedback: (2)

Sorry for the double post.

Ok,

Here are some pictures. They're a little dark, but you can see the break.

As far as landing's go, I have not had a crash landing nor a hard landing. The last landing was very smooth...


Anyway, if you have any thoughts, let me know.

Gravityisnotmyfriend

My Feedback: (2)

I've had that engine mount break in the same place. Of course, this was AFTER the wings folded up and it came in like a lawn dart. It probably saved my engine. But, I have never had one of those mounts failed for no reason.

SkyDude

The right-side mount arm break looks a bit discolored. Could've been cracked by stress or a bad mold as said earlier.

Good thing no one was hurt. I probably would have done the same thing and tossed a rag into it to stop it, but only after I made sure no one was in the danger zone in case the prop came off. I wouldn't want my hand too close to a running engine that's loose, and I wouldn't want to wait to run out of gas.

carcraze

I had an aluminum mount let go across the threaded holes once. It was hard to determine the cause of the fracture but I speculated a prop blade shead in flight mind you the prop was in good condition and was nylon. It happened on a Ultra Sport 1000 with a 3500 BGX in it. The fuse was badly damaged and man getting a tail heavy plane to the ground in one piece is more than a chore. Took a while to find the engine as well.

MormonMike

My Feedback: (2)

Flyblueskies, Have you considered making a "Tilt-Rotor" type RC plane. Yes, I fully realise the potential danger you had on your hands at the time. But....... mm.

FlyBlueSkies

My Feedback: (2)

Everyone that's looked at this close up, noticed the same thing. As you look at the picture with the motor and the mount attached, the right hand mount DOES have some discoloration.

We speculate the darker portion broke earlier. Then the brighter area broke when I taxied back to the pits. The rest you know!

The left hand bracket is actually discolored all the way through. We speculate it was completely broke for a while. Bear in mind, I completely removed the motor two weeks prior to this problem. The left hand bracket was not broken...it would have dropped off!

Anyway, thanks for all your comments!

JB