ARF and motor mounts? Might be lose.

[Paul Taylor]

I have a ARF that has a LA 46. Engine has aluminum plates under the mounts and bolted on using lock nuts.
The engine will not needle on the plane. On a test stand it’s rock steady.
I’m guessing a few things:
1. Motor mounts / nose has delaminated due to age / fuel.
2. Air leak in the fuel system / tank.

The plane does seem to vibrate a lot when running. But I have been flying mostly electrics and may have lost the feel for IC. 😜

I think I have seen this in the past.
Am I headed in the right direction on trouble shooting? What am I missing?

Tanks have been a metal wedge and clunk.

Thanks

[Dan McEntee]

 Are you using the same prop for both? A bench stand can be pretty solid and dampen out a slight vibration, but an air frame, not so much. Check the balance of the prop, and check the hole in the prop if it's an APC. I'm not knocking APC but I have seen this in the past, and others have posted here about it. Stuff happens, and I believe APC will stand behind their product and replace a proven bad one. I have a Nobler ARF that I have added a forest of wood to the engine mounts on, and it tends to vibrate more than the other one, no matter what I do to the prop. Does it with other props also. This was with a Brodak .40 and then a OS .35S. What airplane do you have?
   Dan McEntee

[Brett Buck]

I have a ARF that has a LA 46. Engine has aluminum plates under the mounts and bolted on using lock nuts.
The engine will not needle on the plane. On a test stand it’s rock steady.
I’m guessing a few things:
1. Motor mounts / nose has delaminated due to age / fuel.
2. Air leak in the fuel system / tank.

The plane does seem to vibrate a lot when running. But I have been flying mostly electrics and may have lost the feel for IC. 😜

I think I have seen this in the past.
Am I headed in the right direction on trouble shooting? What am I missing?

      There are a lot of possibilities but, for sure, weak/flimsy/soft engine mounts can make it run poorly. I haven't made any CL ARFs but I helped a guy at work to help with his RC ARF. Firewall mount ARF trainer, K&B 4011. He had assembled and run it, but the flying site guys told him there was something wrong with the engine. I ran it on my stand, works fine, ran it in the airplane,all over the place, and I tached it and it was down 2500-3000 RPM.

     I started looking at the engine mount and it was a plywood mount with the pink hot-melt glue. I replaced the firewall with 1/4" aircraft ply, and then made maple triangular stock and reinforced the firewall to fuselage joint, all epoxied. Ran it again, picked up 3000 rpm and dead steady. He took it back to the RC site, all the guys asked him how he modified the engine, they never believed him when he told them the truth.

     Brett

[Paul Taylor]

Are you using the same prop for both? A bench stand can be pretty solid and dampen out a slight vibration, but an air frame, not so much. Check the balance of the prop, and check the hole in the prop if it's an APC. I'm not knocking APC but I have seen this in the past, and others have posted here about it. Stuff happens, and I believe APC will stand behind their product and replace a proven bad one. I have a Nobler ARF that I have added a forest of wood to the engine mounts on, and it tends to vibrate more than the other one, no matter what I do to the prop. Does it with other props also. This was with a Brodak .40 and then a OS .35S. What airplane do you have?
   Dan McEntee

Thanks Dan.
Yes same prop. Thunder Tiger 11x4.5.

[Paul Taylor]

      There are a lot of possibilities but, for sure, weak/flimsy/soft engine mounts can make it run poorly. I haven't made any CL ARFs but I helped a guy at work to help with his RC ARF. Firewall mount ARF trainer, K&B 4011. He had assembled and run it, but the flying site guys told him there was something wrong with the engine. I ran it on my stand, works fine, ran it in the airplane,all over the place, and I tached it and it was down 2500-3000 RPM.

     I started looking at the engine mount and it was a plywood mount with the pink hot-melt glue. I replaced the firewall with 1/4" aircraft ply, and then made maple triangular stock and reinforced the firewall to fuselage joint, all epoxied. Ran it again, picked up 3000 rpm and dead steady. He took it back to the RC site, all the guys asked him how he modified the engine, they never believed him when he told them the truth.

     Brett

I’m thinking pulling the monokote off and pumping CA in all the seems.
I’ll check the fuel system and try again.

[Dave Hull]

We had an 60-sized ARF stunter on the field Saturday. First flight--ran great. Second start--you could see the engine shift sideways when flipping. Definitely not loose bolts or rattling. But when twisted hard, something was giving way. Need to check this often if you are ARFing....

[Brett Buck]

I’m thinking pulling the monokote off and pumping CA in all the seems.
I’ll check the fuel system and try again.

    I would suggest more than that, like gussets, etc, before it gets too fuel-soaked to stick together. Lots of ARFs have problems beyond just the glue joints - questionable material selection and questionable mechanical design.

     Of course, also check the prop balance, etc.

     Brett

[Peter in Fairfax, VA]

My Brodak ARF Pathfinder had the "clamp" style ARF mount.  Engine lugs sandwiched between two aluminum plates.  The bearers broke due to fuel soaking at the clamp bolt penetrations..

Replaced the bearers with rock hard maple.  Also added a 1/2" balsa doubler block.  Pleased with the result.  Seems solid.  As others point out, imbalance, shaking, etc. all costs power.

Peter

[EricV]

I have a ARF that has a LA 46. Engine has aluminum plates under the mounts and bolted on using lock nuts.
The engine will not needle on the plane. On a test stand it’s rock steady.
I’m guessing a few things:
1. Motor mounts / nose has delaminated due to age / fuel.
2. Air leak in the fuel system / tank.

The plane does seem to vibrate a lot when running. But I have been flying mostly electrics and may have lost the feel for IC. 😜

I think I have seen this in the past.
Am I headed in the right direction on trouble shooting? What am I missing?

Tanks have been a metal wedge and clunk.

Thanks

Early on when I was running "thumpers" I was following Big Jim's advice at the time, as suggested by my friend Eddy R, to build a stout front end, even at the cost of some weight. At least the weight was working for me, and better than having to go add lead which is of no other use than ballast.

I used to run full length 1/2 x 1/2 maple mounts to the high point of the wing, 6/32 bolts, cross grain 1/2 between bearers, boxed in the tank compartment with the "V" diverter and the whole bit. People wondered how we got a ST 60 to swing a 13" prop with authority in a deep 4, and assumed we had hopped up motors... nope, just sturdy front ends. Motors were basically stock but gone over to make sure had a good ring, maybe a bigger venturi if we were lugging a porker, etc... but pretty much stock.

Randy Smith cured me of all that when I went to his PA's... smooth as silk. Still needed a sturdy front end, just didn't need to be bomb proof like for the ST60.

In my opinion, ARF front ends should all be beefed up before their first flight (before fuel ever touches the wood so you get good bonds). Every ARF/ARC I've owned needed something, and even with that, most still never came close to matching 1/5th the longevity of a home built.

EricV

[Brett Buck]

Early on when I was running "thumpers" I was following Big Jim's advice at the time, as suggested by my friend Eddy R, to build a stout front end, even at the cost of some weight. At least the weight was working for me, and better than having to go add lead which is of no other use than ballast.

    Exactly, that was Big Jim's biggest contribution, recognizing that the front end had to be stiff enough. Just like a lot of other things, even tiny improvements in stiffness in an already decent system can make a big difference, even on smooth-running engines.

    Brett