I thank you Jess for showing me my shortcomings and lack of intellegence, to bad were not closer to each other so I could thank you upclose and personal. If you ever get up this way make sure to let me know I'd be more than happy to meet up with you somewhere.
Again thanks for keeping my well being in your best intrest, but I have a exhaust fan in my heated shop, which allows me to run engines year around if I so desire to. Some of us do live where the temps get rather cold at times, but im sure that dosent bother a pro like yourself.

I would love to meet you CW, but don't sell yourself short. I have great respect for your experience and intelligence even though we may not always share the same opinions. After all if we all thought the same these boards would be pretty dull. Sorry you took offense.

Good luck and enjoy the hobby.

jess

I have a not so enjoyable experience...

I recently bought a TT F-75s, which is my first 4 stroker. I started to run it, and the guys at the field wanted to deal with my needles....and took me as a newbie.... (I'm new at the field), anyway I had about 30-40 minutes of total run time with the engine, and it took very good compression over the time, and since Idling was very smooth and slow since the begining, and the engine was responding good, they all told me...go fly..what do you want...an hour of running in etc....go flying!....and I said....ok...I wasn't sure because I felt it was completely run-in, but was running very good (over 9600RPM with the New Graupner 13x6). I don't know if it's a fact of the four strokers that they don't react much to small adjustments in needle valves (I've read here that it took about 3-4 seconds to respond), so I let it...I found a confortably sweet spot...and the plan took of...it was O.K, I knew I wasn't able to get all the power, about 5 minutes later, I thought that the engine turned off... I had no throttle response.... so I tried a dead stick landing...when the plane was close, I heard a nice idling from the engine..so it was still runing....but I couldn't have throttle response... loose linkage I thought (which wasn't)... so after a though landing, the new plane was safe...

When I took the plane, the engine was very very hot, I know 4 strokers run hotter, but it wasn't normal, compression was still really good, so I took of the engine cowl, the engine was very hot... linkage was good and working, everything seemed ok... So I guess that the engine just didn't wanted to work more because of the overheating... Good that I was using fuel with 22% castor oil...

Is this kind of behavior to expect when a 4 stroke engine is overheated?, that it doesn't pick up RPM's ?

Anyway I've learn my lesson.... I should have more confidence with what I know about the engines (specially the one adquired here)...and second..... Don't fly your plane untill the engine is completely broken in!!

Saludos,

Jorge

Jorge,

I don't know this engine, and have only limited experience of 4-cycle engines.

Because of the valve train, 4-cycle engines have practically a built-in RPM limit. Too fast and valves float. Not good in a car, not good in a model engine... So, they do not speed up if the mixture goes leaner. I have heard that a safer way to set the mixture on a 4-cycle is with a tachometer. As the mixture is leaned from very rich you may find RPM increases at full-throttle. If so, you will see the RPM stop increasing at a certain value, even though you can make it leaner. My opinion: the richest mixture that comes close to that RPM should be safer than a leaner setting...

I've noticed that some engine manufacturers recommend less oil for their 4-cycle engines than for 2-cycles. Also, quite often, synthetic oils or blends of synthetic and castor are recommended. 22% castor seems a lot. What does the manufacturer recommend?

How did the engine feel after it cooled down? Have you checked the valve clearances since the very hot run? Did the carburetor drum (or plate or butterfly) actually turn as the linkage was operated? (The linkage arm could possibly slip...)

Does the cowl have enough opening for air to enter, AND a bigger opening for hot air to leave the cowl? I hope the slow running was due to the carburetor arm slipping, and not to binding of overheated parts inside the engine.

I hope your engine is still in fine condition, and that you have long and pleasant service from it.

Hi Lou

thanks for answering...,

I know about the tacho...but I don't have one here...some people at the field have radios with built in tachos, I'll ask them next time.

Im going to continue the running in process anyway, it hasn't run even a liter of fuel, so it needs more run in time.

The plane is a Black horse (phoenix) SU-31, the Cowl is very big, the cowl has a opening where the muffler goes, but it has no more openings.... I'm not pretty sure, but I haven't heard of overheating problems with the SU's, I think that the problem was due to overheating, but the engine itself seems to be in condition and didn't suffered any damage.

I've checked the linkage twice and it is ok.

I didn't started the engine after the flight.... but I will do it today or tomorrow after the work. I'll keep posted =)


Saludos

It seems funny that I have a Phoenix Su 31 that having a SK 50 2 strk.And several of the initial flights have given me same feeling that my engine doesnt respond my throttle commands for several times separately.I dont remember how it may solved but I remember very good of such a problem of first several flights.

Quote:

ORIGINAL: Lou Crane

Because of the valve train, 4-cycle engines have practically a built-in RPM limit. Too fast and valves float. Not good in a car, not good in a model engine... So, they do not speed up if the mixture goes leaner.

Lou,


What the needles do in a glow engine, is to adjust the effective ignition timing.

THIS is the reason RPM changes so drastically, when you change the mixture setting...

After all; from very rich to stoichiometric, virtually the same amount of fuel is burnt, which should result in a very similar amount of energy released...

But the power changes considerably because 'timing is everything'.


A richer mixture is cooler and its propensity to be ignited, is lower than that of a leaner mixture.
Also, the flame propagation is much more rapid in a leaner mixture.


Closing the needle in a glow engine, is the same as advancing the ignition timing, in a spark ignition engine...


At a certain point, the ignition will come so early and the flame will propagate so quickly, that very high combustion pressures will occur before the piston reaches TDC.

This will initially slow the piston, causing the RPM to decrease and overheating to set in. But at a certain point, detonation will occur and the piston will be kicked back, causing the engine to stop in a millisecond and the prop to be thrown...


Not only four-stroke engines are subject to this, but also two-strokes, where the 'lighter' form is called a 'lean run'...


The situation with Jorge's TT, however, seems to me to be simple, cowl-induced overheating.


Jorge,


You did not bother to baffle the cowl, to make sure your engine is cooled, did you?

Leaving it to chance and only following the intake/exhaust hole-size ratios, will not cause the cooling air to flow where you want it to...
Only proper baffling will force it to do that.

Hi Dar

I Think no....what I'm thinking ist that I will try to cut the cooling fins that come in the cowling.... I mean, those who are for esteticall purposes resembling the ones in the real Su-31... maybe this will help? Since I'm living here in Germany, I haven't got tools or something to make this kind of things...

But also, is it to expect this kind of behavior? I mean, that the engine just stay at Idle no matter what I do with the throttle?

Thank you!

Quote:

ORIGINAL: estradajae

But also, is it to expect this kind of behavior? I mean, that the engine just stays at Idle no matter what I do with the throttle?

Jorge,


I have not seen a TT .75 yet, but I have seen some engines do this 'trick', when they were extremely hot...

It even happened with some older, carburettor equipped car engines. I do not know why this happens.

What if I decide from the bench running I don't want that engine in that plane? What is so hard about retuning? You want to kill the fun? Also on the bench I can move the tank up and down and duplicate the position in the plane.

Only ABC engines will be reliable after just two tanks. And even with them I run a couple more just to be safe. Ringed engines may seem ok after a couple of tanks, they may even run right for a flight or two. But usually the compression will change and get worse, better, worse, etc before the rings seat and this could cause a dead stick. I don't think I have run any ringed engine on less than a half gallon before flying, and some have taken over a gallon.

Which is why it is better to bench run than to fly the plane. If it is running hot you can shut it down or richen the mixture. In the air you may not notice before the engine is fried. Also you would have to hold on the model to be as save as a bench. I do not trust the stabilizer to hold the model for more than a short run to adjust the mixture. I have seen them fail to see the model shoot off the end of the stand.

Extremly early ignition and extreme blow by is my guess. I have only seen it happen with four strokes. My guess is that if you have that much blow by in a two stroke you the intake charge is too contaminated to continue running.

I ran the engine again today... this time I tried a Graupner 12x7 (square tip)...the engine seemed 100% in condition!! was running super! , i just let it run 2 times, about 10 minutes each (a 8oz tank seems to last forever), it feels much better now. I think the engine may be well over 11k RPM when peaked, so a 12x7 seems to little prop for it, but the thrust...ohhhhh what a thrust !! =)

On the other hand, I find it hard to make a baffle, I don't have any means here.... I'll try cuting the cooling mesh from the cowl, those who look like shark gills, I'll post a picture of it.

Quote:

ORIGINAL: estradajae

On the other hand, I find it hard to make a baffle, I don't have any means here.... I'll try cutting the cooling mesh from the cowl, those who look like shark gills, I'll post a picture of it.

Jorge,


If you want to make effective baffles, to provide ample cooling, please read [link=http://www.supercoolprops.com/articles/cooling_largemodelengines.php]this article from Joe Supercool[/link].

It will give you a better understanding, of the flow of air within the cowl and how to alter it, so cooling will be maximized.
Having what you define as entry holes and exit holes in the cowl, just does not cut it...


That web site has a lot of other very interesting material too.

I agree. If it doesn't run well on the bench it won't run well in the aircraft.

Bill