My TD just got an attitude... and it was good

Oldenginerod wrote:

RknRusty wrote:I'm thinking removing a head shim might be the next thing to try.
See y'all later.
Rusty

I'd go with adding a shim Rusty.  It makes sense.  Not enough momentum in the shorter prop to carry it over high compression.  Slight reduction in compression, and the lighter prop might carry over TDC more readily.  If you remove a shim it will only fight harder to stop it spinning through compression.
Just a wild guess.  Won't hurt to try.

Rod.

I think Rod may be correct for the cure. Shims don't stay the same thickness forever and are capable of being compressed when tightening and retightening. Because of the material - some shims have a tendency to "work harden" and become less flexible.

Thanks, guys. Makes a lot more sense now than it did at 2:00 AM. That's what I'll do. Thanks, and stay tuned for my 1/2A Day flight report.
Rusty

Make sure the backplate is tight.

The pop is all you get when it's loose. You can add the shim, but if everything is the same as when you last played with it then something new is amiss.

Cribbs74 wrote:if everything is the same as when you last played with it then something new is amiss.

Not everything is the same. He tried a smaller prop than he usually runs. The engine still ran ok when a larger prop was re-fitted. Just a matter of fine tuning the engine to the prop. Obviously the compression is just on the limit for the original prop.

Good Luck on 1/2A day Rusty !! Is this one of your TDs that needed repowering (Pis/cyl. kit)? you had spoke of this some time back but I don't remember if ya did it .

This funny. I was going to post about the TD on my little Satan. Flew it about six months ago and it screamed. Took it down two weeks ago, didn't change a thing since the last flight, now it just bounces back and forth farting against compression. Maybe compressed shims? Sorry for the hijack rusty.

Jim

JPvelo wrote:This funny. I was going to post about the TD on my little Satan. Flew it about six months ago and it screamed. Took it down two weeks ago, didn't change a thing since the last flight, now it just bounces back and forth farting against compression. Maybe compressed shims? Sorry for the hijack rusty.

Jim

It can also be a build up of gum/varnish after a dormant period. It doesn't take much to change the characteristics of a small engine.

My COX engines are oscillating back and forth without passing the TDC if the current to the glow plug is too much and ignition is too advanced. Before giving up a good compression by adding a shim try reducing the glow current/temperature and it will start right away.

Oldenginerod wrote:

Cribbs74 wrote:if everything is the same as when you last played with it then something new is amiss.

Not everything is the same.  He tried a smaller prop than he usually runs.  The engine still ran ok when a larger prop was re-fitted.  Just a matter of fine tuning the engine to the prop.  Obviously the compression is just on the limit for the original prop.

Hey Rod, I fail to see how prop selection affects compression and starting characteristics. RPM's sure.

Please enlighten me as to how a prop changes compression. The engine has no idea what prop is on it until it revs up and experiences the load. 1/2" in prop length wouldn't even be noticable. Now if he used and 11" prop on it then sure, it would definitely notice a difference. Yes, I've done it.

Ron

Smaller props have low inertia to turn the crankshaft over especially if ignition is too advanced. This may be the result of either to few shims or too hot glow head or high nitro or their combination.

I usually run my TD-s on 5x4 props and the phenomenon described above occurred many times but always disappeared once I turned the glow current lower.

I edited my original post. Still not seeing how 1/2" of prop makes any difference on the first flip.

Ron - the propeller doesn't affect compression - it is the other way around. The smaller the propeller - the less inertia it can generate.

I notice this quite a bit on diesels. the engine fires but the mass of the propeller isn't enough to keep on taking passed top dead centre and running up to speed. It would work if you used an electric starter to spin it up but you can flick until you are blue in the face and the engine firing isn't enough to pick up where the flick left off.

It can also be a symptom of the fuel used and, as balogh says, pre-ignition or pinking.

It does seem a small amount - 1/2" or a 1/4" on each blade but that slight change in moving the c of g of each blade further out can make a difference. This is also dictated by the style of propeller.

Ok, Ian thanks, I can kinda see that, it's not like Rusty was running a prop way out of the realm of normalcy. These are Cox engines, not highly tuned masterpieces. Problems in my experience are always simple fixes, always... And I have run a ton of them, in flight and not on a bench. I am not an expert by any means, but I highly doubt his prop choice caused the issue.

His first go to, checking fuel flow, it's what I would have done. The fact that it fired on a 5.5x3 is pure coincidence. Remember he tried to electric start the thing with no luck so prop momentum doesn't play into it.

Cribbs74 wrote:

Oldenginerod wrote:

Cribbs74 wrote:if everything is the same as when you last played with it then something new is amiss.

Not everything is the same.  He tried a smaller prop than he usually runs.  The engine still ran ok when a larger prop was re-fitted.  Just a matter of fine tuning the engine to the prop.  Obviously the compression is just on the limit for the original prop.

Hey Rod, I fail to see how prop selection affects compression and starting characteristics.
Ron

No, never said it changes compression. Just meant that the prop size changes the characteristics of the compression that the engine currently has.
I think Balogh and Ian have pretty much covered what I was meaning.

1/2A day was great. I flew every plane I took, and the Gladiator reminded me how slow my fast regular planes really are. I will tell the stories in my flight report, they're pretty good.

I put the original 5.25x3 back on the Satan just for the sake of not extinguishing the fun-light too early. I used most of the last of my Fritz Glowplugboy 35 in it. This is the first flight since I fixed the elevator linkage which had been causing wild mysterious roller coaster flights. I hoped. It was fixed, and the plane was back under control. The TD cranked easily and for 5 minutes the plane burned a scorching hole in the air in a blur of screwing looping big grin and occasional sun-blinded fun. I actually got a standing ovation when I ran over to the grass edge of the pad(as I had promised) and put it down softly. The Tee Dee was running as fast and strong as a TD should. I'll try the 5" prop again when I have time to be patient.

There are a lot of thoughtful comments to my original post, and I thank you all for that. In addition to the flight report, I'll return here to address your input. Hey, at least I made you and me both think.

Btw, a good portion of the Gladiator flight was done with me standing with my back to the sun, arm straight over my head, and flying the plane in loops and 8s unseen directly behind me. I finally belly skidded it to a quick unplanned stop, but not before a few minutes of freakin' insane flight.

Stay tuned, there's more to come with video entertainment. Oh yeah, Bandit and I both won trophies. Wayne flew BFS-II
Rusty

Glad it all came together for you Rusty, sounds like you had a good time out there. Been doing some flying up here in KS. Couldn't make the Tulsa contest due to work. Been chasing problems after a lightning strike.

I don't know what caused your non-start issue, others here think it may have been your prop, not sold on the idea myself, but I have been wrong before. I understand the principle of the theory, just never came across it personally. If I have an over compressed engine that wobbles I will throw a bigger prop on to see what happens.

Ron

Yeah, Ron we had a good day, thanks for the reply. I'm about to start making some movies now.
I can pretty much rule out my prop choice with that non-starting episode, The Gray Cox 5x3 and the Tee Dee are made for each other. Saturday, I flew with both of my fuels too. That Cox Super Power fuel is good stuff, and I only had 25%. The Fritz Glowplugboy 35% runs scorching fast. I used it in the Satan and the Gladiator. But that speed might be partly due to the 17% 15/2 oil content. It's time to re-supply.

The Tee Dee .051 in BFS-I -the R,W,and B one, compression is completely shot. I mistakenly called it "II" in the above post. With a Merlin head and one shim, it still screams a beautiful song, hard and fast. But if you try to fly outsides, the plug drowns. It'll turn insides all day. With the outside Gs, I guess fuel rushes up the sides of the piston and into the combustion chamber, in addition to what pumps up through the bypasses. Maybe the point of excess entry is right near the tops of the bypasses where it's too loose to seal. I don't know. But after a Reed Speed gift from Cox International a couple of years ago, I just happen to have a brand new Tee Dee .049 P/C still in the bag. The crankcase is in great shape.I'll go make that movie now.
Later,
Rusty

Bravo !!!! Good report, and glad the cranky Tee Dee decided to straighten up and fly right !!!!