Hi David

A nose that is a blur is not going to be helped by rotating a prop, You need to beef up the nose, maybe add a stiff cheek cowling

Randy

Hi David

I would then use a light wood prop, maybe a 10 x 5, That is what I used to run on my shoestrings and busters
Does the FOX have an ABC or PL  piston/sleeve setup in it? Stock or hemi heads?
The light wood props may shake less than the heavier ones , you can try both. It could be a heavier prop can change things if it is a resonance problem,The 10 x 5 should get the RPMs down and change the resonance of the engine at that RPM.

Also I would use a filter and make sure there is a thin layer of foam behind the fuel tank.
If you have to try an external balance, use the light blade of the prop on the ascending blade when you flip the motor..this is the one you touch to flip it thru compression, that will put more weight on the crankweight side
let us know what affect this has

If the motors are fairly new they will vibrate less many times as they run in


Randy

Thanks Randy!
 What I sort of remembered through the fog of time was 180 off.

Not ABC 100% stock. I take 4 or 5 P&L sets and mix and match until I get 2 or 3 I like (tight)and then taper the sleeve and fit em up lapping in hot running water until tight when cold and loosen up when hot.
Left overs are spares.
Edit: piston upside down for first75%-95% or so of  lapping.

Dropping the revs through prop or rich mix is where the vibration( resonance?) sets in.

The 10x6 makes it fly a little awkward, Like you can feel the gyro working.
The prop that seems best so far is the old paddle blade TF 9X6.  Best part is there is a box of 30-40 of these in the shop.

How do you feel about lowering the compression?
That helped some on a ST.35s although not enough to be worth shaking the Chipmunk apart. Worn out linkage in less than 10 flights.      O.S. is good

Now to go find an un balanced prop, My luck lately would be every thing I touch would be balanced!
Will try it in AM and post results.

Thanks again for the help Randy  especially after an already hard day ( I tried to be good today!)



David

Just as suspected, after having to balance every prop for months that it took longer to find 3 out of balance than it did to mill a counter bore in the front of prop hub

This am set up to fly with light blade inboard against compression.
This seemed worse but having slept, well give it a try.
For sure was not better.

Whoops no fuel left in can. Back to the house for fuel.
Turned prop the other way with heavy blade inboard.
Start/ pull batt/ dies repeat 5-6 times dies.
OK Plug.
Box of plugs apparently at the house,  HB~>But here is box of suspect plugs....
First one bad....2nd choice K&B1s R/C. well that should drop compression some.

Fire it up and much better. Vibrates like I remember a Fox from child hood.

In flight speed much more reasonable and only rich on outsides and now next to zero vibration compared to before.

Was it Balance? was it compression? Both changed at same time so not sure.

Made 4 more flights with decent runs only going rich on out side.

Next is to raise tank a little more and add a head gasket.

If wind doesn't come up much more should be back in the air in hour or so.

We'll see
David

I've been flying 11 x 5's on Fox stunt 35's for years.  Used to run a Zinger 11 x 5 wide, but am now real happy with a Taipan 11 x 5.  An APC 11 x 5 is too much prop, I think.  Weird, huh?

I'm curious why you say the shoestring is your best choice for a pa stunter?
There are other models that had doublers added to make the nose stiffer.
The coyote Comes to mind. Adding a 1/2 doubler to reduce vibration I think could be done to almost any model.

Pete
The stuffer did more good than anything. Apparently the higher pressure and velocity helps scavenge the case better and keeps fuel from accumulating and then massive amounts finally go upstairs, Repeat.
Makes one want to cork the piston to finish the cure!
It still has a trace of burp on outside entry but very livable now.

I suspect the uneven boost mixture was also causing the worst of the vibration.

The short plug and off balance prop, one or both did help some and will try to do more with that later.

For now I have something that flies more than good enough for practice!

Start clock, start engine, novice pattern with 5-7 laps after overhead roll to a stop at 3:20 to 3:30 on 2oz fuel.

Thanks guys!
Best help on the Net is right here in the Hanger.

David

Why?
Because of no interest in new engines and a truck load of old (new/used)engines....

My preferred choice would have been O.S. 35s But that just will not pass as a Foxberg.

Considering that the crank needs to be balanced it now runs fairly well with the case stuffed.

Looks like this winter 4 Foxes and 1 ST.35s get to be balanced.

Thanks Floyd
Heavy blade close as possible to counter weight CL and still have decent prop setting for starting did some good.
As long as revs do not drop into deep 4cycle range things are smooth enough for now. Before just a light 4cycle would vibrate like crazy.

Re-balancing the crank will come latter.
Or since then illegal for fox racing.....wonder if a counter weighted spinner would do the trick?(safety?)

I should think that a counterbalancing spinner (or backplate) would make it easier to experiment with and/or replace broken props.  Get the spinner right once, and only put well-balanced props on the thing.

Just a caution..I would really stay away from adding much weight onto the shaft at the front, The FOX 35 has a tendency to shoot parts of the crank out the frontend if it gets too heavy and is run hard

Randy

          I had a Fox.35 on a Ringmaster which shook so bad you couldn't hold the plane. The fuse was new due to a crash built with the beams running far back into the fuse with quality ply doublers which were epoxied with T-88 epoxy. I did everything I knew of to reduce vibration. I thought I made it too stiff. I fought with this for over a year. I finally took the engine apart only to discover there was a bind as the piston crosses the exhaust port. I found this to be true with several Fox's I own. After several attempts, I lapped the piston to the sleeve with the piston upside down while doing it. It turned out to be one of my best starting Fox's nowadays. I can run it extremely hard without sagging or seizing. This took not just a little of the shakes away, but all of them. Nobody believed it was the same engine. I also was led to believe that the piston liner fit caused the extreme vibes due to the largest parts that need to break in together. I found while installing a new case on one that the crankshaft fit is equally as important. I did a lot of sanding and polishing to the crank which helped tremedously. The area of concern is also where the crank web meets the inside of the case. I had to do a little deburring and polishing there as well. It really doesn't take much to cause a slight bind which in turn leads to bad vibes. The funny thing is, I own a Shoestring that shakes horribly. I just can't get it to stop so I gave up on it. I tried virtually all the tricks. I think you just get some wood /motormount material that just doesn't absorb the vibes. Ken

Just a caution..I would really stay away from adding much weight onto the shaft at the front, The FOX 35 has a tendency to shoot parts of the crank out the frontend if it gets too heavy and is run hard

I thought that parts falling off of engines was an essential part of racing .