Tony vibration found

LAST EDITED ON Jul-23-04 AT 07:42 PM (CDT)
 
Doug,Brad,Al

I ran the 82 and the 72 Saito`s today and the compression on the 82 is back.Checked for vibrations on the 72 and the 82.Big differance. The 82 was almost twice as bad as the 72,but with alot more power.Jim Rhoades came out today and help.We look at the Tony for along time and then went to lunch.When we came back Jim noticed a paint crack at the front of the motor mount next to the ply doubler.So I put my knife to it and low and behold the glue joint had broken loose between the mount and the doubler.So inspecting it closer found that the inboard mount when trying to pry it apart it too broke loose.It took about five minutes and bingo both mounts were out ready for the new ones.And thanks to Doug,since the nose ring was broken and has to be replace it will have the mounts extended into and supported with the new nose ring.Going to try to have Glen Dye Balance the 82 better to see if we can make it run smoother.I think the added vibrations broke the glue joint loose and thats why it was getting worse as more flights were put on a already failing structure.Not realizing it wasn`t the engine getting worse,but it was trying to tear the mounts out.And it was doing a good job of that.

Thanks guys for the imput.It makes one want to do more investigating before tearing into the repairs.

Gordy

Gordy

Gordy what type of glue or epoxy did you use on the mounts??

I tried a differant type of epoxy because it was thinner than the regular 30 minute which is what I normally use. I used the finishing epoxy.Won`t use that again.What a dummy.

Gordy

LAST EDITED ON Jul-23-04 AT 10:01 PM (CDT)
 
>I tried a differant type of epoxy because it was thinner
>than the regular 30 minute which is what I normally use. I
>used the finishing epoxy. Won`t use that again.What a dummy.
>
>Gordy

Thanks Gordy,
Yes, finishing epoxy is not of the strength of regular epoxy. Finishing epoxy is for the outside only. Do not use polyester resin on your profile ply doublers and motor mounts. Ask me how I know......

so the structure held, but the glue joint didn't. Interesting. Still think the nose construction is good though. At least, the inner structure is in one piece.

Hope Glen can smooth the engine out some. That banger was really banging in there.

I'm wondering why everything is spinning around?

LAST EDITED ON Jul-24-04 AT 00:09 AM (CDT)
 
Gordy,

Been there, done that! The first photos shows the original structure of the BBQB Bearcat. You can see that the engine bearers stop short of the front bulkhead "A". In service, after about 50 flights, the maple engine bearers began to split apart from supporting structure. I caught it very quickly because I paint all of the joints and intersections in the engine compartment white to show up cracks. These white stripes in the corners look odd in an otherwise black interior. By making frequent inspections, I caught the damage quickly before the nose structure became oil soaked. I was able to add structure to the front of the mount to carry the excess and unanticipated, vibration and torsional loads out to the bulkheads, longerons and fuselage skins. I picked up the problem before any increased vibration became apparent. I did not, and do not, consider Saito 72 vibration particularly excessive, I just didn't allow enough load paths to accommodate. After repairs, an additional 300 flights have caused no additional problems.

When I designed the Critical Mass, which has almost identical bulkhead sizes and spacing, I modified the structure by making bulkhead "A" plywood and screwing it onto the ends of the two engine bearers. This is shown in the second photo. The plans and laser cut parts for both the BBQB Bearcats and Critical Mass reflect this revised, improved design. you might notice both the BBQB and Critical Mass use 9 longerons under the molded skins. You might also notice two vertical, triangular pieces of plywood which farther tie the engine bearers to the back of bulkhead "A" and out to longerons and skins. My noses are big but designed with very robust structure to tolerate four stroke loads and vibration.

Gordy,

I am glad to see you found some spots that were letting go. It just sucks that it had to happen after the plane broke.

I like the way Al has his front bulk head srewed to the Beams. I might try this on my next stunter. Of course the it will be counter sunk to keep it from splitting.

Keep us updated on the things you find out.

You said it did vibrate more. A whole bunch more than the 72 or just some?

I think it is just a really large displacement for that size of a case. Brad still swears there is something wrong with motors.

Balancing it would really teach us about it. Too bad I dont know how to do it...

Doug and Al,
Thanks for the pics Al. A picture is worth a thousand words.At least for me.Doug I`m going to do as you suggest and add a new nose ring that helps support the front of the mounts as did Al. With the 4 strokes they need all the help and strength you can build in them. This was my first 4 stroke plane.It`s been a learning curve for me.The sad part is this has been my very best flying plane that I`ve had in 40 plus years. Hope it isn`t the last though.The vibration on the 82 was alot more than the 72 on the test stand.But,it did have a lot more power too.I hope Glen can balance it better to try to get the vibs down. You were right about the something coming apart which was making it vibrate more.I think the vibration was the same from day one on the engine.And as the glue joint failed the vibs got worse not realizing it was getting worse do to the glue joint failing and not the engine get worse.

Gordy

I just looked up the spec's for the 72 and 82 Saito. They are bores =1.063" and 1.135", and strokes 0.811" and 0.802" respectively (yes the 82 has a shorter stroke than the 72). What this means to me is that everything else being equal (like both getting a full fuel charge is that the pulse of the 82 firing on the larger diameter piston will be 14% stronger than the 72 (this is assuming the pressure due to combustion is the same, but the force scales as the area of the piston --or as the square of the bore). Saito claims the 82 is 20% more powerful than a"typical" 72 (the Saito 72??), so I seem to be underestimating the force of the power stroke.

Ok, what I am getting at is that the 82 gives more power than the 72, but with a slightly smaller stroke and larger bore, so one might expect the "vibration" to be larger. I am not sure how one balances out this force, except to make the engine heavier or beefing up the front end of the plane so that it becomes the effective mass for the recoil of the piston. I guess one could try to push in some tungsten slugs into the counterweight, but I am not sure if this helps a lot or at all, since the counterweight is usually determined by balancing off the reciprocating weight of the piston and part of the rod, and I have never seen the engine power enter the argument.

I think in general this is why you don't see too many single cylinder car or full size plane engines. I am curious how the Saito 90 radial (three 30 size cylinders) would work. I think it is a lot heavier than the 82 and also less powerful --not to mention more costly, but I bet it is pretty smooth!