Plywood crutches not proven for Saito 72 and 82

I have been developing a fully laser cut engine crutch for my designs for a few years. I have had failures in the past with the Saito 72 and Saito 82 (poor runs/high vibration). This also led to several broken aluminum engine pads. The broken pads were solved with drilled through engine holes replacing the drilled and tapped version. Most of the vibration went away with this change.

The only failures I have had are with the crutch delaminating from the side wall (this was also determined to be the cause of the failure of GSO's Tony crutch which was maple BTW). I feel this is due to the heat of the 4 stroke case and the constant oil drenching the glue joint (Saito's leak a lot). I would recommend high quality epoxy such as West systems or an equivalent and a 3/4 oz glass fillet on the inside for any crutch assembly to be used with a 4 stroke.

The same crutch that is only marginally effective with the 4 stroke appears to be total overkill with a RO Jett. I use the 1/4" 6/32 bolts to secure the engine to the pads (the pads are drilled and tapped). I suspect that the RO Jett will go 1000's of filghts on the plywood crutch without failure, but this is all speculation. I plan on laminating carbon into the next crutch just to be sure.

So, in conclusion the plywood crutch needs further development, and is in no way a success story, but instead of just giving up I will trudge on. That being said, I do not think I have ever said anything to the contrary.

But in defense of my crutch, I have probably had more runs on the Saito 72 except for Paul Walker. My Shear Panic was a plywood engine crutch plane, and I do not think anyone said anything negative about my runs at the 2002 Nats, quite to the contrary. I did not have any problems with the nose on that plane until the crutch began delaminating from the sidewall. My Zone had a plywood crutch, and only had problems after experiencing the same problem. The Mr. Hyde had delam only after I went the high RPM setup.

Long story short the big 4 strokes shake. Some more than others. Be prepared to spend some extra weight in the nose for a 4 stroke plane. I might even recomend going to a nylon RC mount on a .25" PLYWOOD firewall, to let the engine flex around a little.

Good enough?

Have you ever tried phenolic? We have a place here in St Louis that will cut 1/2 x 3/8 by 3 foot long. These are strong enough to mount a motorcycle engine to. Cant spell sorry.
Just a thought.

Is that 'phenolic' you are referring to?

Isn't it a fairly brittle material?

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"Man does not live by words alone, despite the fact that he sometimes has to eat them." -Adlai Stevenson

>Is that 'phenolic' you are referring to?
>
>Isn't it a fairly brittle material?

Hard as a rock! I don't think that the Saito would break it. They use it a isolators on Harley-Davidsons. They vibrate a bunch!The nice thing about it is a resin based product. Epoxy sticks great to it.

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>
>"Man does not live by words alone, despite the fact that he
>sometimes has to eat them." -Adlai Stevenson

You're right - phenolic is a tough laminate construction of woven materials itself as I recall.

I was thinking of bakelite - which IS brittle, and wouldn't be suitable at all..

Sorry!

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"She jerked away from me like a startled fawn might, if I had a startled fawn and it jerked away from me." -Raymond Chandler

>You're right - phenolic is a tough laminate construction of
>woven materials itself as I recall.
>
>I was thinking of bakelite - which IS brittle, and wouldn't
>be suitable at all..

Phenolic is not just one thing - there are a whole lot of different varieties, with different "filler" materials, ranging from kevlar and graphite to *sawdust* and very fine *straw*. Bakelite (or some related thermosetting plastic) is to phenolic what epoxy is to graphite/epoxy composite - a matrix to hold the filler together. Different fillers of course result in different characteristics, so phenolic can have range of characteristics.

>Have you ever tried phenolic? We have a place here in St
>Louis that will cut 1/2 x 3/8 by 3 foot long. These are
>strong enough to mount a motorcycle engine to. Cant spell
>sorry.
> Just a thought.

What about the material that Tom Morris uses to make belcranks. Isn't that some kind of phenolic?

I just laminated my latest crutch. It has .2 oz carbon mat between 4 layers of 1/8" birch plywood to make a total thickness of 1/2". The open holes in the middle have been plugged with cross grain balsa laser cut to shape (bottom 1/4" only). The fiber was laminated to this cross grain filler material also.

I do not understand this idea that maple or any other hardwood is as strong a a birch laminate. All of the new engineered homes are manufactured out of laminate. The wood material is really irrelavant as the resin does the bulk of the work.

By far, the stiffest rifle stocks are laminated birch, except maybe for the carbon super expensive ones. I have had stocks of maple and walnut, and neither are as stiff as birch laminate.

I would submit, however, that the material uses to laminate the birch and the number of laminates is important. That is why I went from 2 layers of 1/4" 5 ply to 4 layers of 3 ply. I have more control over the laminating process.

>>Have you ever tried phenolic? We have a place here in St
>>Louis that will cut 1/2 x 3/8 by 3 foot long. These are
>>strong enough to mount a motorcycle engine to. Cant spell
>>sorry.
>> Just a thought.
>
>What about the material that Tom Morris uses to make
>belcranks. Isn't that some kind of phenolic?
Yes it is phenolic and what I am talking about is a solid piece 3/8 x 1/2. In place of maple. You can machine it to shape. Hollow it. Tap it.

I'd like to try some.

Where do I get it?

>I'd like to try some.
>
>Where do I get it?

>Bigs

I will call Cope Plastics here in St.Louis tomorrow and see if they will cut me some. If so I'll send you enough for motor mounts. I used it 10 years ago (before I wet on a layoff). I had forgotten about it till this thread. Works great! You have to be careful to machine it. It does not weight a ton. It is heavy.

>Yes it is phenolic and what I am talking about is a solid
>piece 3/8 x 1/2. In place of maple. You can machine it to
>shape. Hollow it. Tap it.

I am not sure that making a stiffer crutch is the answer. Isolating the vibration would be better, rather than just transmitting it further aft.

I know this doesn't work for the 2 strokes, but what about a soft mount for the 4 bangers? I'm sure somebody already tried that and failed, but I thought I would ask....?

>By far, the stiffest rifle stocks are laminated birch,
>except maybe for the carbon super expensive ones. I have
>had stocks of maple and walnut, and neither are as stiff as
>birch laminate.

That's a thought. Maybe if one contacted Bishop or Fajen, they might be able to supply small waste cut-off pieces from their gun stock blanks. Might be plenty big enough for motor mounts and other uses (carved props, anyone?).

>I do not understand this idea that maple or any other
>hardwood is as strong a a birch laminate. All of the new
>engineered homes are manufactured out of laminate. The wood
>material is really irrelavant as the resin does the bulk of
>the work.

In the small size of our crutch I feel maple is stronger than a couple of peices of plywood.

Your comparison of engineered beams in new home contruction as opposed to motor mounts is apples and oranges.

You should also notice that these new engineered beams are much larger to get the same strength their one peice brothers afforded many years before. These engineered beams you talk about are much cheaper also. And the homes I inspect that are made from these very componenets you talk about aren't holding up worth a crap either. EVERY, yes EVERY SINGLE ONE, new home inspection I have done within the homes first year has horrible cracks in them that I dont see in homes that are pushing 60. Heck my home is 41 and it was built for $16500 and it is in way better shape structural wise than many of your $250000 homes you see today(here in dallas TX anyway). Cost is your main objective with these beams. Beams can be made alot cheaper than hunting down the lumber needed for these new large homes. Are they stronger? From what I have personally witnessed in my profession, I think not. Are they as strong? Well they have to be, according to the engineers, or they cant sell them for the use of building homes.

Still it is apples and oranges.

>
>I would submit, however, that the material uses to laminate
>the birch and the number of laminates is important. That is
>why I went from 2 layers of 1/4" 5 ply to 4 layers of 3 ply.
> I have more control over the laminating process.

Number of lams is VERY important. In the 80s when I was a huge skater the 5ply boards stunk and they couldnt take hardly any abuse and were found in the Wal-Mart stores. Go to the skate shop and fork out some dough for a lighter board with 7 lams and you had a real piece of lumber. I even had a couple with 9 lams. They were the best by far.

> The
>broken pads were solved with drilled through engine holes
>replacing the drilled and tapped version. Most of the
>vibration went away with this change.

Oh, my goodness! You were using the pads? On a big stunt motor? No offense intended, but *wow*. I guess I saw someone using these the other day, but at least that was on a 25. I can't imagine that it's a very good idea - it's like building in a tuning fork.

> Oh, my goodness! You were using the pads? On a big
>stunt motor? No offense intended, but *wow*. I guess I saw
>someone using these the other day, but at least that was on
>a 25. I can't imagine that it's a very good idea - it's
>like building in a tuning fork.

It seems fine for pipe motors. The Moons have been using them for years, and the RO Jett does not mind it a bit.

BTW this is what I found with the Saito. So, I agree.

>> Oh, my goodness! You were using the pads? On a big
>>stunt motor? No offense intended, but *wow*. I guess I saw
>>someone using these the other day, but at least that was on
>>a 25. I can't imagine that it's a very good idea - it's
>>like building in a tuning fork.
>
>It seems fine for pipe motors. The Moons have been using
>them for years, and the RO Jett does not mind it a bit.
>
>BTW this is what I found with the Saito. So, I agree.

We all seem to be living charmed lives when it comes to vibration issues, I think. I can't say that I have ever really analyzed the structural flex characteristics of an airplane nose - just made it as rigid as possible and as damped as possible.

The problem, of course, is that aluminum has very little structural damping compared to wood. If the combination of the mass properties of the engine, the thickness of the pad, and the distance between the hold-down bolts happens to equal the running frequency of the engine at some point, there next to nothing to stop it from large-amplitude motions. With an all-wood structure, you still may get close to or pass over the natural frequency of the mount, but there's to much damping that it won't necessarily hurt anything.

I really haven't had any problems, but I think that's almost certainly just luck instead of any intentional plan.

Brad,
as you know, the 56 doesn't shake mearly as much (if any) as the 72 and up. I think that's because they (Saito engineers) wanted to squeeze more displacement in a small, 56-compatible crankcase. The piston got bigger, but the small crankcase kept them from increasing the size of crankshaft's counterbalance cheek. Hence the vibration.
Instead of trying to beef up the motor mounts, see if you can balance the engine better.
There are a couple of ways of doing that.
First, remove extra material from the piston (mostly skirt, below wrist pin holes). Second, if space allows, solder on a blob of regular (high-led, pereferably) solder to the crankshaft cheek. You will need a pretty agressive flux to get the solder to stick, but it's been done.
I've balanced a Fox 35 that way, and a couple of other iron-piston engines.
Let me know if you'd like to learn more about balancing..

Oh, the 56 shakes plenty. I tried resolving this with a carbon fiber/ epoxy one piece crutch system. The bulkheads and mounts are all CF and laminated together with a direct tie in to the fuse side aft of the wing and to the main spar and leading edge in the wing. The thing still vibrates, but I haven't noticed any problems with stress cracking or erratic engine runs. but the plane only has about 150 fights on it, so stuff may show up later.

>> The
>>broken pads were solved with drilled through engine holes
>>replacing the drilled and tapped version. Most of the
>>vibration went away with this change.
>
>
> Oh, my goodness! You were using the pads? On a big
>stunt motor? No offense intended, but *wow*. I guess I saw
>someone using these the other day, but at least that was on
>a 25. I can't imagine that it's a very good idea - it's
>like building in a tuning fork.
>
>
> Brett
>
>

Yep,

Been on tapped pads for a long time. Bob Gieseke taught me to use pads in 1997. He is the king of changing engines and blind buts cant take the beating. Pipe motor doesnt seem to be bothering the pads at all. At least they have been working for me. I cant speak for everyone but for me they work good. Makes for an easy way to change engine brands and get a good fit.

Hey Brad,

Maybe the opposite is true. Your crutch is too strong. It delivers too much vibration to the sides and the glue joint fails.

PW found this to be true with carbon mounts on the B-17.

The stiffer something is, the higher its resonant frequency..

As Brett said, if you end up "tuned" to a natural frequency of the driver (motor), it's going to buzz..

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"Knowing is not enough; we must apply. Willing is not enough; we must do." -Goethe

Winfred has a mold for a CF crutch....pretty nifty, because it may be drilled and tapped and provides about 1/2" of "meat" for screw purchase.
They seemed a bit heavy, but there is lots of material that could be trimmed away, I imagine.

See Ya

Brad,

First, if I were going to build a crutch front end for a large motor, 2 or 4 cycle, I'm not sure that I'd feel any necessity for exotic adhesives or materials. I consider maple motor mounts sufficiently strong. My BBQB has 3/8 x 1/2" maple bearers laminated to a 1/8" lite ply floor. The adhesive is CA. The assembly is attached to the front bulkhead with CA and wood screws. It penetrates the next two bulkheads back to the wing leading edge. The whole assembly absorbs loads and vibration and carefully routs them out to the rather large fuselage surface. I don't notice any particularly objectionable vibration and this fairly light construct has survived crack free for more than 350 flights. In fact the BBQB doesn't seem to have any more vibration than my PA 65 powered Classic Bearcat which I've been flying a lot lately. What vibration I've experienced seems to be coming from the props. My props are all hand carved and very carefully balanced but can still be improved with field modification. I used to carry a bottle of CA to the field and would wipe a coat of CA on one blade of a propeller and then trying the propeller with first one, then the other blade coming up on compression. When I was competing, I found this an effective way to minimize perceived vibration.

I am not at all displeased with the vibration characteristics of the Saito 72. I don't think it is any more noticeable than many other engines I've run. It certainly hasn't been a concern. If I had any doubts at all I wouldn't have spent a year building the 72 powered Critical Mass.

My point is that I think the geometry of the structure is more important than the materials or adhesives, also, that it is wise to back up adhesives joints in critical locations with mechanical fasteners, wood screws. It doesn't hurt to use balsa fillets on critical structure to spread the loads and increase contact area. Again, I think design is more important than material.

Al

PS Threaded pads for a Saito 72/82? You gotta be kidding. Weak engine mounting by itself can go a long way to contributing to problems with loads and vibration and may very well be responsible for your problems and Gordy's with his Tony.

>Brad,
>
>First, if I were going to build a crutch front end for a
>large motor, 2 or 4 cycle, I'm not sure that I'd feel any
>necessity for exotic adhesives or materials. I consider
>maple motor mounts sufficiently strong. My BBQB has 3/8 x
>1/2" maple bearers laminated to a 1/8" lite ply floor. The
>adhesive is CA. The assembly is attached to the front
>bulkhead with CA and wood screws. It penetrates the next
>two bulkheads back to the wing leading edge. The whole
>assembly absorbs loads and vibration and carefully routs
>them out to the rather large fuselage surface. I don't
>notice any particularly objectionable vibration and this
>fairly light construct has survived crack free for more than
>350 flights. In fact the BBQB doesn't seem to have any more
>vibration than my PA 65 powered Classic Bearcat which I've
>been flying a lot lately. What vibration I've experienced
>seems to be coming from the props. My props are all hand
>carved and very carefully balanced but can still be improved
>with field modification. I used to carry a bottle of CA to
>the field and would wipe a coat of CA on one blade of a
>propeller and then trying the propeller with first one, then
>the other blade coming up on compression. When I was
>competing, I found this an effective way to minimize
>perceived vibration.
>
>I am not at all displeased with the vibration
>characteristics of the Saito 72. I don't think it is any
>more noticeable than many other engines I've run. It
>certainly hasn't been a concern. If I had any doubts at all
>I wouldn't have spent a year building the 72 powered
>Critical Mass.
>
>My point is that I think the geometry of the structure is
>more important than the materials or adhesives, also, that
>it is wise to back up adhesives joints in critical locations
>with mechanical fasteners, wood screws. It doesn't hurt to
>use balsa fillets on critical structure to spread the loads
>and increase contact area. Again, I think design is more
>important than material.
>
>Al
>
>PS Threaded pads for a Saito 72/82? You gotta be kidding.
>Weak engine mounting by itself can go a long way to
>contributing to problems with loads and vibration and may
>very well be responsible for your problems and Gordy's with
>his Tony.
>
>Al

I agree with everything you said. Gordy did not use pads, that was me. His problems might could have been solved with screws as you suggest (if he knew he had a problem). This is the modification I made to Shear Panic, I simply ran a line of 4-40 wood screws down the side of the nose at the beams. It never delaminated again. Gordy should consider doing this, as his primary sidewall of his fuselage is hidden anyway. That together with a good adhesive and maybe some fillets (wood or glaas ) should last forver.

You also have the preferable setup in that you nose is big around and short.

>Hey Brad,
>
>Maybe the opposite is true. Your crutch is too strong. It
>delivers too much vibration to the sides and the glue joint
>fails.
>
>PW found this to be true with carbon mounts on the B-17.

I don't know, but I would stress test any of my crutches against a maple version, especially one laminated in such a way as the new one I just did last night (12 layers of birch 4 layers of carbon mat).

Your skateboard analogy is a good one. All of my boards were birch laminate. All of the best boards were. I only remember the first Dogtowns being too thin and weak (before your time I bet). We abused our boards. I was a big kid too.

I think the next 4 stroker I do will have a firewall nylon RC mount. I think this would be the best compromise. Instead of stiffening everything back to the wing spar and translating the vibration there, I would like to isolate it at the motor and reduce the length of the "tuning fork". This type of mount also allows changing to different engines easily, like the pads do. This would also simply be easy and cheap and good for the builder of all levels, it would not require any special building techniques.

Balancing the Saito, especialy the Saito 82 is a tremendous idea, GSO is already in the process of having the crank balanced to the piston properly. He informed me on the phone that the cranks of the Saito 72 and Saito 82 appear to have EXACTLY THE SAME COUNTERWEIGHT (starting to believe me now?). That would be bad for vibration I bet, huh?

For us to assume that an OEM engine, especially a new one, is balanced to the level that we as perfectionist stunt fliers would like is well, a bad assumption. We use much smaller, lighter, and yes weaker structures to build our planes that the average RC flier.

Even Randy Smith has improved the balance of the winning PA line over the years with changes to the crank. He made a new change recently in fact. The Fox 35 has been improved exponentially with several design changes both OEM and aftermarket.

I still believe the biggest change to improve the Saito mounting would be made to the Saito. I believe it could be further balanced. I know that another motor that I have tested it against of the same size and weight is WAAAAAAAAAYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYY smoother and DOES NOT LEAK OIL out of the crankcase!

Doug:

I believe the reason the glue joint failed is that is was done with 5 minute epoxy. This epoxy will not stand up to the heat of the 4 stroke crankcase or the constant crankcase oil bath from the Saito.

BTW I had to stop using 5 minute epoxy to put together golf clubs with ferrules because they would simply pop off the ferrules if I left my clubs in the trunk of the car in Dallas. The torch barely needs to hit a clubhead to pop it off if it is put together with standard hobby 5 minute epoxy.

Brad,

I don't think much of firewall mounted motor mounts for large engines in stunt models. I have used them but consider them too heavy when the necessary stiffness of structure is obtained. Both Sea Furys had radial mounts on 1/4" plywood firewalls, large, heavy firewalls which did effectively carry loads out to large fuselage shells and longerons. It worked, but was inefficient in that elements were overstrength and overweight to obtain the necessary load and vibration resistance of the nose structure. The Critical Mass model is a modified Sea Fury as is the real Critical Mass a modified Sea Fury. The noses have similar dimensions but the Critical Mass reverts to a more efficient BBQB like maple based assembly which is certainly lighter and stronger.

The firewall of a stunt ship tends to be too Small to counteract the leverage of prop loads and vibration. Farther, it concentrates all of the loads and vibration onto a small structure which is difficult and heavy to construct with the necessary strength. I would consider this application poor design. There are better ways to distribute and absorb loads and vibration into the nose structure by spreading, rather than concentrating, those loads. Spreading the loads, rather than concentrating them will generally give a stronger, lighter structure and impose less stress on joints, materials and adhesives.

For 21 years I have been using carbon blocks glued to .5" x .1" carbon sticks for combat-airplane motor mounts. These assemblies replace the maple in conventional engine mounts. The .5" faces of the sticks are glued to a .5"-thick balsa block, as we do with maple mounts. (An aside here: speaking of maple, today we had a maple tree cut down in our yard. The guy who did it was Dan Maple of Maple's Tree Specialist in Maple Valley, Washington. I'm not making this up.) I might post a picture of these motor mounts if I can find one. This thread brings to mind three stories:

1. I thought I'd save some carbon composite material by replacing the carbon blocks with phenolic blocks. I made a batch of Rushpuppies like this and took them to a Lake Charles Nats. The phenolic didn't stick to the carbon sticks. I was practicing at the Nats when my best Fox flew off into the bayou. I started after it when I encountered the sheriff. "They's cottonmouths in there, boy," he advised. I was convinced. "Tell you what. I'll have some convicts out here tomorrow cleanin' up the place. I'll have one of 'em fetch your motor." He did and they did. I gave the sheriff a fifty. "Now I don't get none of this," said the sheriff, pocketing the money. "It goes to these fellas. That's their cigarette money."

2. A local guy had a Nelson .15 on an FAI combat plane. It didn't go very fast. We took his plane and mine out to diagnose the problem. After matching prop, fuel, and everything else we could think of, his engine ran 3,000 RPM less than mine. His was mounted on maple mounts. We put his engine on my airplane and recoverd the missing 3,000 RPM.

3. Another local guy was building a stunter powered by wimpy OS.15s. I told him story #2 and volunteered to make some of my standard carbon mounts so's he'd get his money's worth out of his engines. He installed them and was horrified to find a 1/2"-amplitude vibration at the prop nut at an RPM pretty close to where he planned to operate the engines. I made some elaborate carbon gussets to stiffen the mounts. No good. He whacked the mounts off at the firewall and screwed some nylon radial mounts to the firewall. They worked fine. In this case, he had some high-powered structural dynamics advice to go by. For the rest of us, I agree with Brett that not hitting a structural resonance is luck.