Off topic- Caution.

Oldenginerod Sun Jun 08, 2014 1:10 am

Hello folks.
A lazy holiday weekend here in the land of OZ so I decided to convert some nitro to noise & smoke. Due to reasons mentioned here before I don't often get airborne but I do like having a play with a few engines every now and then. There were no Cox or other 1/2A engines on the agenda, with a newly acquired Torpedo .40 the priority. Happy to say it fired up easily but had a few difficulties with the Perry carb so I swapped over a spare- not too shabby. Then on to another Torpedo .40 I made up a while back out of 2 junkers which has very poor compression. Once I got enough speed into it to make the Dykes ring work it also ran pretty well, with improved compression at the end. What else is sitting around? Ah, the Testors .19 series 21 (a boat anchor to some). Never had it going properly before. Yep, off she went. Next was a McCoy .35 Red Head. Just couldn't get it to play ball until I flooded the heck out of it and off she went. Man those engines are loud. Mad

All these engines were firmly clamped in my (well) home made wooden test stand, until something we all fear happened. Gents, beware of complacency. The final test was a McCoy .29 Red Head. Fired up really well and peaked out beautifully prior to me adjusting it back to a comfortable breaking 4 stroke. I thought I tightened the wing-nuts down firmly, but I noticed them vibrating loose. Do I tighten them? Do I throw a rag in the prop? (Don't do that). I know, I'll pull the fuel line off. By the time I made that decision it was too late. I was looking at the engine one second, and then it just vanished. Man, did it rev up well as it took off. Only about 6 feet to the wall of my shed and bang. followed by the tinkle tinkle of bits of prop landing around me. Fortunately no damage to me or the surroundings that I've noticed. I retrieve the engine from behind the bench to find that it only had a small scar on the front of one head fin and just the stub of a prop left.

So the moral of the story is, check everything's tight, then check again. It could have been far worse.

Rod.

roddie Sat Sep 13, 2014 9:24 am

A few of us have mentioned engines coming loose from their test-stand while running.. and not having enough time to take action. This got me to thinking about a safety measure that wouldn't be a PIA.. (like many safety/protection mods can be..) I think that any test-stand could be made safer by drilling two 1/4" thru-holes below the stand's mounts, to feed a zip-tie through. Thread the zip-tie through one hole on the back-side of the stand, out through the front and up around the engine's case in-between the cylinder and venturi.. around and down through the 2nd hole out the rear of the test-stand.. and "zip-lock" it tight.

All you need to do, is keep a bag of zip-ties and a small pair of "snips" with your engine test stand. Removing an engine afterwards only requires cutting the zip-tie off with the snips... which would only take a few seconds. One extra item "could" be added.. which is a short (2" long) piece of silicone tubing to thread the zip-tie through where it bears on the case, if there are concerns of "scratching" the case.. or heat melting the zip-tie. Zip-ties are pretty tough material though.. and the hottest engine temperatures are up higher in the cylinder.

If you are in the habit of just using C-clamps to hold your test-stand/fixture.. you could also drill a single 1/8" thru-hole through the stand's/fixture's "base".. for a deck-screw with fender-washer to screw down through the base and into the bench, as another extra added measure for safety.

pkrankow Sat Sep 13, 2014 11:20 am

My commercially made aluminum test stand has pins that engage the engine lugs, one per side. The engine has to be pretty loose to come out and hop the pins. If your stand lacks these it would be an easy modification to make.

I have a smaller stand than this, but it shows the pins well being photographed upside down.

http://www3.towerhobbies.com/cgi-bin/wti0001p?&I=LXR587&P=7

Phil

roddie Sat Sep 13, 2014 11:34 am

pkrankow wrote:My commercially made aluminum test stand has pins that engage the engine lugs, one per side. The engine has to be pretty loose to come out and hop the pins. If your stand lacks these it would be an easy modification to make.

I have a smaller stand than this, but it shows the pins well being photographed upside down.

http://www3.towerhobbies.com/cgi-bin/wti0001p?&I=LXR587&P=7

Phil

That looks like a really well-built stand Phil.

Oldenginerod Sat Sep 13, 2014 4:47 pm

My stand is a home-made wooden unit which I was very proud of when built. It has served me well and does have the pins that Phil mentioned. In may case I would suggest that I should have balanced the prop to reduce vibration. As the problem started to emerge I could see the wing-nuts securing the hold-down plates vibrating loose. Rather than risk damage I chose not to throw a rag in the prop but attempted to tighten the nuts. With an open exhaust and 9" nylon prop this proved difficult & dangerous. I didn't make it to the fuel line before it managed to jump the pins.

Roddie's idea is a good one, just for that little added security. I think I also need to get myself a prop balancer and make sure I minimise the risk of vibration causing a similar scenario.

Rod.

getback Sun Sep 14, 2014 7:35 am

Rod as I have had this happen to me but the engine didn't come off just scared the S$$$ out of me !! If you don't have a spring cup type washer under your wing nuts you should look into it , the first one with the serrated loc in this pic is really good stuff to keep the wing nut from backing off http://www.centuryspring.com/Products/discsprings.php?utm_source=msn&utm_medium=cpc&utm_campaign=brDisc ... Also prop balancing would greatly keep the vibrations down . I need to purchase or make one for my self Very Happy

Getback

roddie Sun Sep 14, 2014 8:24 am

I built a prop balancing fixture using a "Kavan" unit, a couple rare-earth magnets, two 5/16-18 x 1-1/4" bolts, some corner braces and scrap wood. It's based on the Top-Flite's magnetic balancer design.. where only ONE end of the balance-shaft actually contacts a magnet. the other end of the shaft "floats" in the opposing magnet's magnetic-field. If you check out these google images, you'll see that many people have custom-built this type.

https://www.google.com/search?q=magnetic+prop+balancer&source=lnms&tbm=isch&sa=X&ei=v5EVVPvjJonWigLks4GYBQ&ved=0CAoQ_AUoAw&biw=1366&bih=643

Here's mine. I glued the smaller diameter magnets to the tips of the bolts and drilled/tapped the vertical wood frames midway/on-center, to accept the bolts. This serves two functions; one being sensitivity adjustment for different prop-weights, and the second being that it also allows for adjustment if the vertical rails "bow" due to changes in humidity. Turning either of the bolts "outward"; just before the point where the weight of the prop causes the shaft to fall out of the magnetic-field, provides the best indication of the state of balance. It will accommodate up to a 16" prop.. providing the magnetic field will support the weight. It will support my largest; which is an APC 11 x 6.

Off topic- Caution. Prop_b10