Gray Beard

My Feedback: (-1)

Just an FYI but there is a glow engine forum and you can ask the service rep directly, in the case of OS that is Bill Baxter. You have to give him every little piece of information though so he has a good idea of what is going on with your engine. I must have missed it in one of the posts but this is the first I have read anything about a loose carb screw or a thread problem. One friend with a .55AX had his carb come loose, both screws fell out. He didn't notice it for a while, he was fighting the engine all day. He replaced the two screws with something he had at home and the engine was still running for crap. I just happened to have two factory screws from OS in my box and he replaced the screws with the correct size and length then the engine ran fine again. The ones he was using were just a little longer then the correct ones. Just something that simple cured a bunch of problems. You should really go ask Bax and see what he has to say. Leave no information out when asking him so he has a clear picture of what you may have going wrong. If I have a mystery problem Bill is who I ask for help. Most the time he is correct or helpful, it's hard to figure everything out without having the engine in hand so Bill isn't always 100% correct but he is usually more correct then not. Even if his answer doesn't fix my problem it points me into another direction I haven't thought of. Give Bill a try, it's better to ask someone who sees these engines day in and day out and has a better understanding of what can go wrong.

ameyam

Will give that a try

Ameyam

ArcticCatRider

My Feedback: (6)

I hate to say so...but I don't care what that manual says, or what Bax says...a 14x8 is too big a prop for a .75

A 12x7 or a 13x6 is a much more appropriate prop. In fact, a 14x8 is even asking a lot for a .75 4-stroke.

jimmyjames213

My Feedback: (2)

prop makes a HUGE DIFFERENCE try pulling a semi trailer with your car.....thats what you are doing to your plane engine....if a engine has too much load it overheats.....thats why we use trucks to pull trainers because they have bigger engines.....
thats wht a .75 engine should run on a 14x4 or 13x6...because it is in its powerband....like your car engine pulling your car and only your car.....
the sound you heard with the 14x4 was the engine getting into its powerband....not it overreveing (which you cant really do on a two stroke unless you have a 10x6 prop or somethine

ameyam

It was a high pitched whine that we heard with the 14x4 and so decided to change the prop. With the 14x8 the tone changed dramatically, it was much softer. So much so that the first time I had difficulty tuning her by ear.

I have a 14x6 (though its on another model) and I will try it out over the week end, I have also flushed the carb as I said and am brewing 0% nitro fuel. So I will change one variable at a time

Ameyam

opjose

Let me echo what he said above...

A 14x8 on a .75 is WAAY too much.

I fly my .75's with a 12x7 on 9lb planes... e.g. a Big Stik 60 and I am QUITE pleased with the performance.

Mr Cox

Huh? You ran it indoors at idle and you think that it is overheating?

This is just wrong on so many levels...
You should join an rc-club, they can help help to tune the engine and explain how everything works.

ameyam

Mr Cox,

you read it selectively. I said two things:

1) I can run it indoor only at idle else the neighbours will complain

2) When I run it rich at full throttle it gets hot

Anyway, I will try other prop and fuel combinations. Interestingly, as I remember from a couple of years back, we used everything from 14x4, 14x6, 14x8, 15x4 on this same engine and it never seemed to overheat or develop squishy compression then. The more I think about it the more I feel it is either a blockage in the carb or a fuel problem. Remember it continues to run even at 4-4.5 turns out of the HSN needle?

I am getting new alcohol to make the 0% nitro fuel tomorrow and I have already flushed out the carb like I wrote earlier. I will keep my fingers crossed for Sat / Sun.

By the way, I have just decided to but a DLE20 for another model. There will be no further new nitro engines for me after the 2x55ax, 1x75ax, 1x72a, 2xFS91SII, 1x120ax I already have (1x72a, 1xFS91SII, 1X120ax are still in box, both the 55ax have been shelved for the time being. Yes, there is one engine reserved for each airplane I have, but most are still in boxes)

Ameyam

opjose

So then why ask for advice?

You are convinced that the prop sizes are OK.

If you do the math you'll see they are not.

ArcticCatRider

My Feedback: (6)

Neighbors complaining is a small problem in comparison to the issues that can become when running an engine inside.

The high pitched whine you're referring to is the engine actually coming close to what it's capable of doing. It's not bogged down. A high pitched whine is very characterstic of our 2 stroke, glow motors. 2 stroke glow engines, without exception, can always be set by ear.

I would keep a couple, five percent nitro in your fuel blend. There's a reason so few gallons of FAI fuel are sold.

Please, do not run your engine inside.

ameyam

ArcticCatRider,

I appreciate your advice. I dont have an air-conditioned house, its very well ventilated even by modern standards. I work in the HVAC industry so I know what I am saying. None the less, runs at home ar for less than a minute- usually to finish off any fuel in the carb.

Opjose, relax. The point is I will need to test the engine out myself, just wanted to know whether what I was doing is right. Thats what the forum ios for, isint it. I am not prone to instrumentation for my airplanes. Bax from the OS support forum has also suggested to go for a smaller prop which I was quite surprised with and was unaware of. I will both tach and temp gauge it and report over the weekend

Ameyam

dredhea

My Feedback: (13)

When I first started in this hobby, I lived in a very drafty house. Curiosity got the better of me one day and I fired up a .15 size motor. It ran for less than 10 seconds at perhaps a high idle. The exhaust smoke traveled the length of the house, down the staircase and then back the full length of the house. It set off all 6 smoke and 3 carbon monoxide detectors along the way. Even short engine runs in the house can cause CO levels to become dangerous.

ameyam

When I ran my FS91SII in the house, the entire house filled up with smoke. It was set for rich so that was inevitable even though it was at idle.

When I run any other engine in the house, I always pump off all excess fuel in the tank that can be pumped out. Its only to burn off excess fuel in the carb and any that I can remove from the tank just like OS recommends. It usually runs for just a few seconds. Otherwise, if I have just setup a new throttle setting or installed a cowl then I run the engine indoors. It is always a test and only at idle and only for under a minute. It beats carrying all the tools to the field or missing a day's flying or crashing because you didnt setup something correctly. My house has large french windows in all directions and the room is just 180sq.ft

Not defending my actions here, just telling why it is necessary sometimes. Anyway, we are digressing from the main issue here

Ameyam

Sport_Pilot

I have found that when running the largest props recommended by the manufacture, you cannot run them with the same plug and fuel as with a smaller prop. To lug the largest prop around you will need to use a cooler plug, or lower nitro fuel, or both.  However you will be well below the power band of thie engine.  I would suggest the best solution is to use a smaller prop.  Since you want to hover I would chose a lower pitch over smaller diameter.

Gray Beard

My Feedback: (-1)

He did ask Bax, Bill gave him very good answers and right to the point. Bill also mentioned why OS gives such large prop listings in there instructions, something I have wondered about but never knew the reason for. Noise issues in Japan so they run bigger props to help quiet them down. Bill answered everything except the last question on a re-post about the stripped screw holding the carb down. That will effect the way the engine runs if the carb is loose or there is any air leak at all!! I'm set up to do most of my own work so I would be cutting new threads and installing a new screw one size bigger so I'm sure there is no air leak at all. After reading the article I sent there should be a better understanding of what glow plugs and nitro fuel are doing and give an idea of things you may try. Somehow I think it's an adjustment problem and trying to run an over sized four stroke prop. Props are easy to change, if you have an air leak in the carb/stripped screw there may be bigger issues. That should be reapired correctly and not with just a bandaid.

jimmyjames213

My Feedback: (2)

you are avioding the real problem, your prop is too big, unless you really really need to keep the noise down then you need to go back to the 14x4 and you will not have problems, when the engine makes alot of noise thats a good thing, means the engine is making more power (not always but in this case it is true).
the 0% nitro will not help that much, it may help some but not that much, it will have an even harder time turning those props
it is not a fuel or carb problem, please just run the 14x4 and see if it overheats, if you run it with that prop and it still overheats get back to us

ameyam

I went over the thought process we used to use at our field- in my trainer days, I started a 55ax with a 12x6 (I think) prop and it flew nice and everything. But when I went into aerobatics with that semi-symmetrical 64" trainer (yes, it was possible on that airplane), I broke the prop somehow and then was asked to put in a 11x8 by my mentor. Though we never used to use either synthetic or nitro in those days, we did not have any overheating problems with the engine. So may be you are right with the prop size issue

I went back to my mentor (he has since become a bit irregular due to health issues) and pestered him into giving me a 13x7 APC from his stockpile of modelling stuff. Actually, I was looking for a 13x8 but 13x7 is also good. All my props are MAS, I use them for their longetivity but APC was all he had in that size.

I am also hunting around for a tach and infrared gauge. I have obtained the latter, the tach is a problem- lets see. Saturday will be a problem as its a working weekend, may be I can coax my bosses to let me come in a bit late

I also obtained the alcohol, just a case of mixing now. I want to make this brew without nitro first. I can always add some nitro later (about 150ml for 3%) later but once added I cant take it out.

I also obtained a bottle of RTV Sealent- the same kind used on HVAC duct work to seal holes. Again, problem there is that to remove it has to be scored off with a blade which is really not desirable. Remember, the screw that has stripped is one of the two holding the carb in place. The other one is still intact and the carb o-ring is still good and the first one is still there too. I dont think there is an air leak, especially since the engine does cut-off when the carb baffle closes

I will work through the problem and revert

Ameyam

opjose

Still too big for a .75!!!

12x7 = OK

13x6 = OK

13x8 = NOT OK.

13x7 = Marginal

DenverJayhawk

i found the 14x4, 14x6, 14x8 and 15x6 okay for my OS 95 AX. But even with 14x8, i was barely getting 9k RPM. The manual even said i could go up to a 16" prop, which I though unlikely.

I'd say a 14" prop is a little much for your .75ax.

jimmyjames213

My Feedback: (2)

if the screw is stripped i would just use a tiny amount of epoxy or jb weld, i actually rethreaded a idle screw with epoxy and it still turns great (put oil on the screw before i put epoxy so it didnt stick)

as for the prop issue, ive noticed that reliablility when turning a big prop is not their, while it may work every now and again the engine is much more reliable with a smaller prop, not to menton you use alot less fuel and get a ton more power

Mr Cox

The infrared gauge is not needed, it doesn't really tell you anything, the engine will tell you when it gets too hot by dropping in rpm. The tach is useful though and can tell you more accurately if the engine can hold the rpm or not. You should also have a clear rpm response when you change the main needle setting.

opjose

In his case a temperature guage is highly useful.

He is running a pusher configuration, where doing what you say is inapplicable.

If anything with a pusher engine the temp guage is practically the only way to tell if you have the engine tuned properly for what will happen in flight.

Been there, done that, learned it here.

freakingfast

My Feedback: (3)

There is nothing magic about the OS AX series engines that allows them to run much greater prop loads than other engines with the same displacement. At most one pitch number greater...maybe. Follow the prop advice the guys have given you here if you want to keep it cool and want power. About your 55AX, run 12X5APC for3D or 11X7APC for sport MAX on stock muffler 15% &OS#8. From what I gather, the recommended props in the manual are rather extreme to keep tip speed down, hence the noise. APC props are about the quietest you will find. I will give OS one thing, with so many guys following the manual to the letter, they seem to be handling the heat well.

ameyam

I am not running a pusher config. Who gave you that impression. The Reactor has the prop in the front

Ameyam

opjose

Oops, sorry wrong thread...

Never-the-less use a smaller prop.