Discrepant Engine Mount

[ibjet]

I decided to show this as a new post because it is about having a Discrepant Engine Mount. My earlier related post was about Engine Removal. 

So, I discovered after removing my engine (with the engine mount) that the lower left engine mount tube was mis-located by nearly 1/4 inch (comparing the right and left sides). I took a picture before I removed the engine mount and later realized that there was a metal to metal condition at that mount (the SPACER 87 was resting on the flange inside the mount tube). I'll post a picture showing that mis-alignment. 

FD USA said they could not authorize any repair (I submitted 3 weld repair ideas). Here is where it gets weird: FD Europe would not say if it was usable or not, and they would not approve any weld repair, but THEY AUTHORIZED FD USA TO APPROVE AN MRA FOR REPAIR OF MY ENGINE MOUNT. That drove me nuts, but it also caused me to go back to the drawing board and think how I could compensate for the mis-alignment inside that mis-located tube. So, I did come up with a good idea and did a drawing yesterday. I'll attach a copy of my drawing as well as an excerpt from the FD drawing for comparison. 

I will also post a picture of the inspection station that I set up just to do a precision inspection of my mount. I needed a granite plate (surprisingly a 9" x 12" x 2" was only about $40) and a 24" digital height gauge (they are very expensive, but I made my own from a cheap 24" digital read out). This picture shows the mis-location of the lower left mount tube. The 0.1805 reading is the actual precision amount that it is mis-located above the lower right mount (the picture shows the front of the engine mount, left and right reversed). But, it turns out that the engine mount can only shift off nominal by about .12/.13" before you get a metal to metal condition.

So, just in case any of you see this kind of mis-alignment when you have your engine cowling off, there is a fix. But, you would need to do a precision inspection and you would need a milling machine and a boring head (to remove the 2 flanges inside the mis-located tube). Also I will custom make my shock rubber using the correct grade of 3/8" Neoprene rubber sheet and a cookie cutter technique.

Oh, I gotta show off the response I got from Tom at FD USA: 

Hi Ed.

          I think it looks like a very sensible solution and yes. I’d sign an MRA for that change. Airtime is fine with using the mount they have for repairing an airplane so there will be no hard feelings there either I’m sure. Please let us know when you come up with the final fix it’s very clever.     

 

Tom Peghiny

President, Flight Design USA

860-963-7272

 

Happy Flying all . . .

ET

[AGLyme]

Nice Aerospace-professional dimensional checking station... good work.  So, the solution now is to mill away some material ? No more welding ?

[ibjet]

AGLyme: Thanks for the compliment! Correct, no welding. To put it simple - My lower left engine mount tube is mis-located up .180", so I will move the center hole in the rubber shock discs down .180". If you look at my drawing, you can see there is about the same amount of mis-alignment externally, but there will be little or no mis-alignment inside the mount tube. So, the toughie for me, I have to find a suitable rubber and make 6 offset rubber shock discs. I will use a steel rule die technique, but I'll probably just get the correct diameter steel tubing and sharpen the end to a knife edge. Then I'll cut the rubber "cookie cutter" style but I'll make up a little press frame and use a hydraulic jack to shear the rubber. 

If anyone wanted to understand my madness more, ha ha, here is the description that I sent to Tom Peghiny: 

What I plan to do is make special rubber shock discs with the center hole offset .180". I decided to make this work I would need 3 shock discs on each side of the mount tube. I will also need to remove the 2 flanges inside the lower left mount tube (I will bore them out with a boring head on my mill, I will clean up the inside of the tube very precisely). And, to replace those 2 flanges, I will make and install 2 new flanges with the center hole shifted .180". And, instead of welding them in I will rig them into position and lock them in place with loctite 272, High Temperature/High Strength thread locker.

 

I plan to use .375 thick Neoprene sheet to make the shock discs. I found a source for it, but I will have to verify they can supply the right hardness and grade. I will cut the ID and OD with a home made steel rule die, I've done that before (not that thick, but I'm sure I can do it).   

 

The other details I will make from High Strength Low Alloy Steel. McMaster Carr sells it in small sheets in thin sections and tubing by the foot. I found out we have a good welder in Kingman (the local I.A. at the airport has used him many times). I may ask him about brazing in the new metal parts. Or, look into putting in a couple of high strength rivets to lock the metal parts in place (just as a fail safe). 

 

ET

[GrassStripFlyBoy]

Glad to see this settling in a good outcome, and simplicity compared to the more aggressive original ideas.  One suggestion, (and note I've not checked into the type of rubber used here, or know what sheet you'll fabricate the new discs from).  I'd recommend placing a larger diameter washer under the head of the bolt as well as between the last disc and mount location.  The thinner section of disc may want to compress easier than the wider side, additionally the open / exposed area of disc that is not as supported in this offset situation may slowly "cold flow" over time and loose compression.  A larger washer covering the entire surface would help maintain uniform clamp load.  Probably a minor point. 

[FlyingMonkey]

Glad you are getting things put together the way you want them. 

I do have to ask again, because I never saw an answer in the other thread:  Is this work a result of trying to fix an actual defect noted in flying, handling, or structure, or an attempt to correct a dimensional variance?

 

[AGLyme]

I’m not an engineer which is evident within my post history ; )...

in My blue collar terms; you are milling in a new hole position and building from scratch new rubber mounts.

And you already built your Manual “CMM” To measure before and after results.

Can’t wait to see your fixturing and drilling procedure.
 

Amazing stuff... you realize the A&P who handles the next rubber replacement is going to have a cow

[ibjet]

Andy - I pulled my engine because of a coolant leak in the back (turned out to be just the hose connection to the coolant elbow) and because my engine was sagging (the spinner was getting too close to the lower cowling). Then when I saw that the lower left engine mount tube was so mis-located (see picture above) I started measuring the hole pattern (1/4" different on the left side than on the right side) and started thinking about how to fix it. Once I realized that the lower left engine mount tube had a metal to metal condition I was sure that I could not put it back together that way. My airplane is extremely noisy in the cabin while flying and I didn't worry much because I bought 2 nice David Clark head phones with noise cancelling. But, now that I know about this situation, I suspect the extreme noise is due to the vibration being transmitted there. I hope to notice a big improvement. 

Darrel - Re: larger washers - brilliant suggestion , ha ha. I was thinking I should do that! Re: the type of rubber for the shock mounts, I started researching last night and most shock mounts are made of Neoprene. Some are made with Silicon rubber and I just found a specialty called Sorbothane. I also searched for a gauge to measure rubber hardness. I was pleased to find they are very reasonable. I bought one last night for $24.99 from Amazon. I read that the softer the rubber the more the vibration dampening. So, I will get rubber the same hardness as the existing discs. I'll post a copy of my MRA when I get it done. 

AGLyme - No, I am not correcting the location. If you compare my Mod drawing to the picture of the mis-location it is essentially the same. I just compensated for the mis-location inside the engine mount tube. The machining I will do is to carefully bore out the 2 flanges inside the engine mount tube. I'm doing that because I need to make room for a total of 6 shock discs instead of the original 4 (and that is just "gut feel" engineering). I probably will end up with this mount having less shock absorption than the others, but it will have to be way better than metal to metal. 

Thx folks!

ET

 

[AGLyme]

got it

[Tom Baker]

Just curious if there is no welding what will keep your new washers and tube spacer in place?

[ibjet]

Some kind of loctite + maybe a couple of high strength rivets as fail safe. I could have them brazed, but I think I don't want to do any heating. I found loctite 272, but I think I'll use a bushing retainer. Anyway, TBD, ha ha.

[Tom Baker]

Those 2 washer that you want to remove are carrying 25% of the load of the engine. I'm not sure I would trust Loctite to carry that load.

I assume your airplane is a SLSA, or you wouldn't be going to the trouble to get a MRA for repairing the mount. I know this is going to come across wrong, but who is going to sign off on all this when you are done?

[AGLyme]

Tom has a good point.  
If you want a perfect mount at the lowest “total cost” then you may want to buy the discounted mount offered by FD... you will gain more in a future sale vs fixing the old mount with a future rubber (mount) replacement challenge. 
I must say I admire your tenacity.  It’s been an educational post.

[ibjet]

Tom - I appreciate your critical eye, that's what we all need here.

I have not finished my total MRA package yet. The retention of my 2 new flanges and the spacer is a big concern to me. It is TBD ("to be determined"). I will have some type of a mechanical retention as a fail safe (but the weight of the engine is on the tube, not the flanges). Those flanges and the spacer will have to withstand 1/4 of the thrust of the prop. I confess - I'm not really sure what the force diagram would look like!   

As far as someone signing off on this work, I have a repairman license and Tom Peghiny will sign the MRA.

As far as the custom rubber isolation discs, I will have the tools to make them with and a source for the rubber to make them from.

And if you want to understand why I am doing this, I would rather make something than buy it.

The offer of a discount was if I could wait several months for FD USA's next big delivery and then I would have to pay for delivery from back East. If I wanted it delivered to me, it would be "about $3,200" (same as retail price).  

ET

[Tom Baker]

28 minutes ago, ibjet said:

As far as someone signing off on this work, I have a repairman license and Tom Peghiny will sign the MRA.

ET

Having a LSRM is not enough for you to be able to sign it off for return to service.

[ibjet]

I do have authority to sign it off for annual inspections and authorized repairs. Are you citing some regulation?  

[Tom Baker]

34 minutes ago, ibjet said:

I do have authority to sign it off for annual inspections and authorized repairs. Are you citing some regulation?  

Absolutely,

14 CFR 61.107 (c), (3) Only perform maintenance, preventive maintenance, and an alteration on a light-sport aircraft that is in the same class of light-sport aircraft for which the holder has completed the training specified in paragraph (a)(3)(ii) of this section. Before performing a major repair, the holder must complete additional training acceptable to the FAA and appropriate to the repair performed. 

14 CFR 61.107 (d) The holder of a repairman certificate (light-sport aircraft) with a maintenance rating may not approve for return to service any aircraft or part thereof unless that person has previously performed the work concerned satisfactorily. If that person has not previously performed that work, the person may show the ability to do the work by performing it to the satisfaction of the FAA, or by performing it under the direct supervision of a certificated and appropriately rated mechanic, or a certificated repairman, who has had previous experience in the specific operation concerned. The repairman may not exercise the privileges of the certificate unless the repairman understands the current instructions of the manufacturer and the maintenance manuals for the specific operation concerned.

I'm pretty sure what you are doing to the mount would be considered a major repair. Have you had the additional training for that repair that is acceptable to the FAA? 

Have you removed and reinstalled an aircraft engine before? From your earlier thread I suspect the answer is no.

[ibjet]

Tom - I will not consider this to be a major repair. Removing and replacing the engine is definitely not a major repair (but yes, I have done it before). I do have a very good IA in the hangar next to me. I have been showing him what I'm doing too. I think he will be very tickled that I am doing this without welding. Thank you for the info. 

ET

[Tom Baker]

What would you consider it if it isn't a major repair?

[ibjet]

Tom - Machining the flanges out of the engine mount and making the new details is a pretty difficult task. I will hire a specialist to do that (me, ha ha). But, once I have the parts made and clean up the bore inside the lower left engine mount tube, the inspection of the parts and assembly (which will be detailed in the MRA) is a minor repair. And, for good measure, I will have my hangar neighbor/resident IA sign off my repair. 

BTW: I found Loctite 620 which is for bearings and it is high temperature and high strength. It is rated 3,800 psi and rated for 450 degrees F. The mating surfaces of the flanges and the spacer will be over 4 square inches. And, for my fail safe retention, I'm now planning to make 2 custom washers which will be larger diameter than the tube and would keep the engine mount bolt from pulling out if the Loctite should fail. The washers and the custom flanges will be made from 1/16" thick 4130 steel condition N which is stronger than the ST37 steel used in the original mount. The bigger OD washers will keep the rubber from bulging out of the tube like they did before (you can see that in my picture with my first post above). I'm gonna hate machining the 4130 on my mini mill, but I ordered 5 carbide endmills so I can do it (and a 4" rotary table).  

ET

[FlyingMonkey]

I always thought that second bolded line Tom quoted to be perplexing.  You can't do work unless you have done it before, otherwise you need to get the FAA to come out and look over your work...that really makes the rating useless from a practical standpoint of somebody trying to make money or a living as a LSRM.  If I have to pay an A&P or IA to supervise everything I do that I have not previously done, I might as well pay them to "supervise" the work and sign it off without me having an LSRM... 

[Tom Baker]

31 minutes ago, FlyingMonkey said:

I always thought that second bolded line Tom quoted to be perplexing.  You can't do work unless you have done it before, otherwise you need to get the FAA to come out and look over your work...that really makes the rating useless from a practical standpoint of somebody trying to make money or a living as a LSRM.  If I have to pay an A&P or IA to supervise everything I do that I have not previously done, I might as well pay them to "supervise" the work and sign it off without me having an LSRM... 

Andy, first off this isn't just a LSR thing, it also applies to A&P mechanics. The regulations allow mechanics and repairmen to do a wide range of complex procedures on aircraft and their components. Do you think it is safe for someone to do a complex task without having any training, or at least someone looking over their shoulder the first time. The FAA does give a pretty wide latitude for compliance. Performing a similar task one one aircraft transfers to other aircraft even if the task is slightly different. If you have removed wings from a CT you can remove wings from a Carbon Cub. Lastly if you have an A&P supervise the work you don't have them sign off the work. You have them sign off that they have supervised you performing the work, and that it was satisfactory. This allows you to make the sign off for the work and provides documentation for the next time that you perform the task.

[ibjet]

Now we know . . .

 

[ibjet]

Set up my engine mount on my mini mill today and machined the first flange off up to the point of the weld bead (I thought it was brazed, would have been easy to machine). Anyway, it was very hard and it would have ruined my boring bit. I quit for the evening and shopped online for a way to grind the weld off. Ordered 2 sets of rotary bits. The second one is very promising: Dremel makes a mandrel and abrasive discs that are 1.5 dia! The tube ID is just under 1.5, so I can size the discs, then plunge cut to clean out the weld/remaining flange. I am enjoying this because of the challenge. I think I will do the MRA tomorrow, I love the drawing/technical writing too.   

[ibjet]

Poking along on this engine mount repair. I am really enjoying the challenges. I have bored out the 2 flanges in the mis-located tube and polished out the paint. I bought a hone to finish polishing it.

I made one trial rubber shock mount. I machined the OD and punched the hole (with a store bought punch, very reasonable).

I'm sharing a picture of my holding fixture for machining the chrome moly spacer (don't have the hardware to clamp it to the mini rotary table yet). This piece of chrome moly tube I am actually going to make into a punch to punch the OD of the remaining 5 rubber shock mounts. I'm amazed how well the punching works, it takes 3 or 4 hits with a mini sledge, but makes a very precise hole. This first rubber shock mount meets my repair drawing dimensions/tolerances, yay!

BTW: Got my MRA approval a few days ago. Big yay!  

[Runtoeat]

The isolator came out surprising well.  Is it better to make a quick strike as you're doing with the hammer or to slowly press the punchies thru the rubber with a vise or arbor press?