Mystery solved

Well, I may have solved a mystery. I'm a might slow, but I do figure things out everntually. Some of you in the Northwest may remember the Bill Simon's Shoestring I flew in classic a couple of years ago. The engine was quite erratic. One good engine run, 3 bads ones. I fought with the plane all season. Changing plugs, tanks, fuels, you name it. the problem just wouldn't go away. It ultimately resulted in the demise of the plane when the engine quite in a very bad spot during a windstorm (though I don't know if it would have mattered if it was dead calm).

Anyway, at the same time I was building the Shoestring, I built my elliptical design, Tango Red. I also had engine run problems with this plane. It was initally designed for a Saito 56 and the engine tended to run erratically. I swithed to a RO-Jett .65 but still had problems. The engine would quit at some very inconvienent times and run inconsistently. This problem has contined throughout it's life. I flew it a couple of weeks ago and got two fairly decent runs on it, but they weren't consistent. Though it did manage to run out a full tank for once.

Today, I was out flying with Bob Smiley (sorry about the Jamison, Bob---and the rudder on the Impact). I had planned to use the elliptical plane in the upcomming Stunt-A-Thon next weekend. I couldn't get the thing to stay running. It started easily enough, but would run erratically on the gound and quit before I could get it into the air. I switched fuel and the plug with little difference.

One the way home it occurred to me that it was having the same problems (almost) as the Shoestring I built about the same time. I started wondering what the two planes had in common. The only thing was that both had carbon fiber engine mounts. I had made up a mold to make CF mounting beams that look like the regular sort of maple motor mounts we all use. I think that the problem is, these mounts are very hard and must transmit vibration to the tank and airframe more than maple. Perhaps it causes the fuel to foam or shake excessively and makes for erratic runs. Probably the Shoestring had a few good runs because I managed to get the RPM adjustment just right on the ground so that it was sort of "in sync" with the harmonic vibrations of the plane to make for a decent run. I'm thinking it's the same thing with the elliptical plane.

I got home today and pulled the engine from Tango, put it on the test stand and ran it. It ran great on the same fuel and plug that was horrible with in the plane.

So, I'm guessing that using CF mounts qualifies as a "bad idea". I won't use them again, that's certain.

I am wondering if I padded the tank if it would help for the elliptical plane. Or found some way to stuff an 8 ounce clunker in there (the darned thing needs 6.5oz to complete a flight). Not sure if it would work, but I definately have the motivation to get the new plane trimmed out tomorrow.

In trying to compare notes, could you describe the way the engine stopped?

For instance did it slowly die on you?
Or did it just cut out ?
I am trying to figure a erratic running engine my self and I am not making progress. Mine behaves like it is seizing in mid flight.It also dies on the ground, when take off is not imminent .

By your own analysis : you could try a fuel defoaming agent .
This could consisty of car cockpit spray. Try a few puffs in quart of fuel and see what gives.
Suspending the tank could only be done sub-critical, i.e. hanging it in very weak spring substances.Its natural,own frequency should as far away from the engine revs as possible. It would be interesting to do clever sums on this problem, but since the tank mass varies rapidly in time there is no point in it yet.

Randy --

Sounds like you did some great detective work! How often do our "improvements" in construction and design to turn out to cause future problems? Often, I suspect.

If your analysis is correct, see if you can find the old thread that Leonard did on the "deck of cards" tank mounting. I can't find it. Maybe in 2001? Leonard's premise was that the tank should be mounted with a "deck" of thin plywood (or rubber?) above and below the tank. Moving one of the plies to the top or bottom raised or lowered the tank in micro increments, and kept the total height of the compartment constant. They would, in your case, also work as vibration dampeners.

Randy,

It is good to see you are looking deep into your analysis. Far too often, people will bad mouth an engine when it is not the engine that causes the problem. I have seen sales of one engine supplier hurt very badly, because of negative comments that had no basis. Unfortunately, the same thing happens with airplanes/kits also.

Larry,

The "deck of cards" tank shims are used by most of us. It is a great way to fine tune the tank height.

Although an excellent method of adjusting tank height, the "deck of cards" shims would do practically nothing to isolate the tank from vibration. Rubber sheet is almost as stiff as wood or metal in compression. Rubber gets it's flexibility in shear. If one uses foam rubber sheets some effect might be noticed.

I think that an isolation system for a tank would have to be so soft that the the load from G forces would bottom out the isolator to the outboard side.

It's better to try to isolate the engine, but then the g-forces would again come into play.

don Burke

Walker had the same problem with his Bomber.. Carbon mounts and bad engine runs.. Paul's fix was to cut the mounts off flush with the firewall and use RC mounts.

LAST EDITED ON Jun-11-06 AT 11:50 AM (CST)
 
I tried anti-foaming agents. No effect. With the Shoestring, I used one of Len's SSW OS35FP engines. I was very careful not to blame the engine. It's bad practice unless you can be absolutely sure of the problem and I never was. I just said I couldn't make it run in that plane (which was true) but that others had clearly had good luck with it. As it turns out, that engine came back to me and is now in the drawer, so it may end up getting a try in another plane.

The fix for the problem is clearly going to be doing a Gordon Delaney on it. You may remember his Tony.

edit to answer Phil

Phil,

I tried every imaginable fix on the Shoestring(and a few other came up with that I couldn't have imagined). New NVA, replaced all fuel line, 3 different tanks, additional vent holes, numerous plugs, fuels, props, etc. No effect. Tried a clunk tank in it and as I remember it was slightly better. Not much, but a bit. Not enough room in the nose for much padding, unfortunately.

Try running the plane using a small tank,say a 4 oz Sullivan, and mount it in a swath of foam rubber at least half an inch thick. That should take care of any vibration induced foaming. If that doesn't work, try adding de-foamer to the fuel. Use the old, add a drop, shake it up and see how it foams trick. Keep adding drops until it doesn't foam. If those two tests confirm that the plane is foaming the fuel, regroup and figure out how to fix it(if you want to spend the time). It may be as simple as using defoamer in the fuel. You may have to run a plastic tank(although don't count on them to eliminate foaming problems). Or possibly rebuild the whole front end. Since the plane doesn't work as it is, it's become a test mule. get as much out of it as possible.

don't forget to try a different brand of needle valve either. It may be leaking air or foaming the fuel there too.

Randy - A buddy had a similar problem with a set of extra hard (felt like Canadian rock maple) mounts. You could even feel it all the way out to the handle and it foamed the fuel like mad. Part of the problem was he tied the mounts to the LE of the wing. The solution we came up with; ended up cutting a 1/8" vertical slot through 80% of each mount as far as he could reach into the tank compartment with a Dremel cut off wheel, and jambing a spruce filler into each slot. It was enough to dampen out the vibes so the problem went away with the fix.

Another thought, starting with a question - do you have the motor sitting up on Aluminum pads, or is it down right on the C/F mounts? If your using 1/8" pads, get a set of phenolic pads from whoever took over Tom Morris's stuff and try running them instead. They're a little softer and might be worth a shot.

Hope it works out, keep us posted.

I suddenly have this inspiration:

the step of carving slots in beams and suspending the tank on soft foam , combined with softer engine pads might be emulated on a test rig, open to inspection and modification.
As I understand, a lot of R/C pilots suspend their engines on silent blocks in various stiffnesses. Just a thought,

I hope you're wrong. My new dog has carbon mounts.

Hey, tell us the story about the Jamison and Impact rudder.

LAST EDITED ON Jun-11-06 AT 11:55 PM (CST)
 
Howard,

If it's something else, I don't know what. This is the third engine in the plane with the same basic problem. And all run fine on the bench and have run fine in other planes. Literally every thing has been changed out. And this is the only non-standard element of construction.

Bob had his very, very nice Jamison Special come apart in the air. It was a take apart and the back screws that attach the wing went missing (apparently). It wasn't pretty.

I was stepping over Bob's Impact and just touched the rudder with my shoe. It broke the plywood ear on the fuselage for the rudder adjustment. Bob built it this way just so it would break rather than the rudder for just such instances, but it didn't make me feel any better. Sigh... Not a good day at the field. I flew my new plane and suffice it to say, it's needed some trimming. Some serious trimming. Flew it again today. Hey, at least the engine ran great.

oops, forgot Eric

Eric,

No, the engine bolts straight to the carbon. I suspect I'll just shelve it for now and maybe later I'll figure out a way to cut them out and replace them with maple...or just build another.

> maybe later I'll figure out a way
>to cut them out and replace them with maple...or just build
>another.

Can anyone please explain the reason for using maple engine bearers?

I always go for beech wood, which has proven to be very good. Just wondering what would make maple the better choice? Is it harder or more flexible than say spruce ?

Gerben,

Generally, maple is quite hard but has certain dampening qualities that make it a good choice. I've used beech, oak and ash in the past. Ash is, in my opinion, too brittle (watch what happens when a baseball player hits a ball off the end of an ash bat – Yeow!). But can work OK. I've tried an assortment of exotic woods, too. Some are too oily (rosewood and zebra wood being good examples) to take glue well. Others have poor grain qualities for this application like most mahogany and jacarandas. Billet wood (makore or aglaia) seems to work pretty well, though and has less malformations in the grain.

Maple is perhaps the best compromise between hardness, glue-ability and ability to absorb and dampen vibration. Noting wrong with beech except that it's a bit harder (generally, depending on species) and is probably slightly more susceptible to vibration, though not enough to worry about. Various oaks can work very well too, but are slightly more apt to deform and twist, so you have to be a bit more careful about grain issues.

All this was why I wanted to try carbon fiber. I suspect it had more to do with the way I laid it up than an inherent problem with the material. I used CF cloth and tow to put mine together and used West Systems epoxy that was post cured. Made for a very hard and "threadable" material. Very cool in that I could glue them in just like maple and just drill and tap for an engine. But in the process, I believe made them too hard. Perhaps if I would have machined them out (as suggested above) and filled in the slots with wood, the vibration would have been dampened a bit more. I did machine them out with slots through the middle (horizontally)to lighten them, but the voids stayed empty. Could be that this is causing the problem. Hard to tell.

So, it's back to maple and the inset mounting system I've used. I know that works. I'm not feeling very experimental these days anyway.

You are running a straight up "CSI" operation. That was some great detective work. Hearing you talk about Bill Simons Shoe String really brings back memories of Bill at Flushing Meadows with his beautiful red original. Bill was a fantastic painter and his Shoestring was a masterpiece. As you know, Bill placed 7th at the Nats with his. Erratic runs with the Saito 56 really caught my eye. Something must be definitely wrong. You have seemed to nail it. Can't wait to get my 56. Looking forward to comparing it to my 72. The 56 is like a good V6, the 72 is like a Hemi.

Pro,

I just figured, 3 bad engines in the same plane? Gotta be the plane.

Randy, have you thought about filling those slots in the carbon crutch with some wood inserts? Can't say I'd have a clue as to what kind of wood to use. I'd consider basswood, tho. Steve

Steve,

Due to the structure of the nose, it wouldn't be impossible, but just about. I doubt it would be worth it. I might as well replace the whole thing. I'm thinking about it. May cut them out or insert a new layout for the forward bulkhead and use an RC mount. I'm concentrating on the new plane right now and may tackle it later. Still could be a good back up plane.