4 stroke throwing prop

[Tony Read]

Hi I've got an OS40 Surpass in a model I acquired at the Nats. It came with a 10x6 prop and running it up to full throttle it coughed and threw the prop. I never did find the prop washer and domed nut.

Anyway, one new washer, nut, 11x6 prop and new glow plug I have run it up again. Similar result, it threw the prop but much quicker and had a 'clacking' noise as it approached full power. I'm using a Model Technics fuel with 10% nitro.

Is this some sort of timing problem? Any idea as to a fix would be much appreciated - its getting expensive in props, nuts and washers!!! Maybe I should stick to electric!

Thanks!

[Hangar 9 nut]

I would of said it is to lean,you could also check the tappets also wooden props are sometimes to light for four strokes but you don't say what type of prop

[Shaunie]

1. No wooden props.

2. Do not lean off the mixture too much.

3. Check the plug, an OS four stroke plug would be best.

4. Go to a lower nitro fuel.

5. Use a lock nut, nylok nut or a spinner (if you choose a bright coloured spinner you should be able to find it with the nut and washer inside it!).

Shaunie.

[Robin Etherton]

Hi

As one who cut his eye in half and spent six months off work when starting an early os four stroke I can only urge you to be extremely careful.

In those days there was no lock nut or split pin retention and when they threw a prop it was likely to go up as likely as down.

It didn't half hurt.

Robin

[Mr.B.]

May I add, 6. Check for an air leak which would give an eratic mixture.

I'm sure there are at leat 2 prop nuts in low orbit over my house.

[cymaz]

 

 

Found this for you....DUCK!

Edited By cymaz on 16/11/2014 19:21:57

[Tony Read]

Thanks guys,

It was a Graupner GF prop and its the correct OS four stroke plug. I turned out the needle 3 full turns, so it should not be too lean (I would have thought).

The spinner idea is good until I find the problem, but that still means £5.00 a prop! For about a 30 second motor run that's quite expensive! I stand well behind the model when running up having seen the velocity of prop going in one direction and the nut in the other.

This may be a stupid idea, but how about adding some castor oil to the fuel?

[Tony Read]

Thanks Cymaz,

I found something similar, that's why I went for the larger prop. Maybe try the 11x7 next. Still don't know why the prop got thrown quicker on the larger prop as compared with the smaller 10x6. I'll make the fuel mix a bit richer and see what happens.

[Adrian Smith 1]

One thing I found useful when starting four strokes is make sure the throttle is on the lowest possible setting enough to start it. Also wait for it to cool down before starting it again. hope that helps.

[Steve Hargreaves - Moderator]

All good advice from the guys so far......the "clacking" noise sounds like pre-ignition (ie the mixture is firing before it should & trying to stop the piston going over TDC thus kicking the prop loose). This is often caused by a lean mixture as the others have noted or by using too much nitro (which advances the ignition point of the fuel) although I would have thought that 10% nitro would be fine in this engine. It might be worth cadging a tank full of 5% or even straight fuel off a fellow flyer. My concern with very low or zero nitro fuel is how the engine will then idle.....

I suspect you are simply running it too lean.....very easy on a 4 stroke as you can wind the needle in & hear the engine pick up....the engine will reach max revs & you can often turn the needle a good bit further (making the mixture too lean) & not notice any difference in engine speed, unlike a 2 stroke which will lose power & slow down almost the instant it goes lean. The trick is to get the engine to peak revs & then when you spot you are able to close the needle & the engine revs don't increase you should then slowly open the needle until you hear a very slight drop in revs....this means the mixture is now a tiny bit rich & this is A Good Thing...! Hopefully that will stop the prop flying off......

Maybe you could get hold of a "modern" nut & locknut set up off a more modern engine....I would guess that the OS will have the same crankshaft thread as a modern engine.....it won't stop the engine kicking the prop loose but it should make it less likely & should certainly stop the prop flying off...

Good luck....

[cymaz]

Get yourself a tachometer. Very useful. Even if you don't hear a note change on the engine you will see the rpm change.

[Jon H]

I disagree with cymaz. You should never tune with a tacho as engines loose rpm when hot so will naturally loose a few revs when they warm up. This can lead to the dreaded rpm chasing if you check it when the engine has just started and then 'repeak' it.

Most of the suggestions here are right on. Check plug, valve clearances and use 5% nitro fuel. remove the needle and make sure its all clean. Make sure the airbleed screw on the carb is at the 50% closed/open position. when leaning the engine off use the 2 clicks and wait trick. ie, lean by 2 clicks wait for a response, another 2 etc etc. If you hear that clacking noise then go back 2 clicks.

I would use a 10x6 prop and not expect more than 10000 and most likely not more than 9000. The 40 surpass is not a powerhouse so don't expect too much from it.

[Jez Harris 1]

Posted by Jon Harper on 17/11/2014 08:51:10:

I disagree with cymaz. You should never tune with a tacho as engines loose rpm when hot so will naturally loose a few revs when they warm up. This can lead to the dreaded rpm chasing if you check it when the engine has just started and then 'repeak' it.

Most of the suggestions here are right on. Check plug, valve clearances and use 5% nitro fuel. remove the needle and make sure its all clean. Make sure the airbleed screw on the carb is at the 50% closed/open position. when leaning the engine off use the 2 clicks and wait trick. ie, lean by 2 clicks wait for a response, another 2 etc etc. If you hear that clacking noise then go back 2 clicks.

I would use a 10x6 prop and not expect more than 10000 and most likely not more than 9000. The 40 surpass is not a powerhouse so don't expect too much from it.

Errrr..But how is he going to know if he is getting 9000 - 10,000 rpm if he isn't using a tacho?

I think Cymaz's advice is spot on - a tacho will always show quite clearly when your engine is tuned and just listening to it doesn't tell you whether you are on the lean side.

Jez.

[john stones 1 - Moderator]

Yep me too a tacho is a useful tool, also no problem with 10% or more nitro..read the manual !! O.S made them they know their stuff

John

[Martin Harris - Moderator]

I have to say that I agree with Steve and Jon on this one. Despite the perceived wisdom that a tacho is "essential" for a 4 stroke, I find that the pitch change with rpm is easily discernible and just as accurate. I have a tacho somewhere but only use it very occasionally and usually for "academic" purposes.

The idea of propping up to reduce prop throwing is a bit of a non starter and Jon's recommendation for a 10 x 6 seems about right to me. Pre-ignition (the clacking noise) is worse with increased load and needs the timing retarding to cure it - either reducing the compression ratio, changing the plug to a colder one, reducing the load or reducing the nitro content of the fuel will help to stop it.

One other possibility is that the combustion chamber is badly carboned up - especially likely if castor oil has been used in the past. Once running, the carbon can glow red hot and ignite the fuel early.

Lightweight (e.g.wooden) props lack inertia and allow the piston to slow excessively when the fuel charge is ignited (in any engine with halfway decent performance, ignition starts well before the piston reaches the top of the compression stroke) and tends to shock the prop nut loose.

Edited By Martin Harris on 17/11/2014 12:39:38

[will -0]

Also check that the exhaust pipe is clear and not sludged up with carbon. I have an oS20 FS that had it's exhaust restricted by about 50%

[Tony Read 2]

Thanks guys!

i've got some more props and a lock nut on order. The pre-ignition noise was worse with the larger prop (increased load), so hopefully I can get some weaker nitro fuel. I'll also make the air/fuel mix a bit richer and see what happens.

[Jon H]

Posted by Jez Harris 1 on 17/11/2014 12:01:37:

Posted by Jon Harper on 17/11/2014 08:51:10:

I disagree with cymaz. You should never tune with a tacho as engines loose rpm when hot so will naturally loose a few revs when they warm up. This can lead to the dreaded rpm chasing if you check it when the engine has just started and then 'repeak' it.

Most of the suggestions here are right on. Check plug, valve clearances and use 5% nitro fuel. remove the needle and make sure its all clean. Make sure the airbleed screw on the carb is at the 50% closed/open position. when leaning the engine off use the 2 clicks and wait trick. ie, lean by 2 clicks wait for a response, another 2 etc etc. If you hear that clacking noise then go back 2 clicks.

I would use a 10x6 prop and not expect more than 10000 and most likely not more than 9000. The 40 surpass is not a powerhouse so don't expect too much from it.

Errrr..But how is he going to know if he is getting 9000 - 10,000 rpm if he isn't using a tacho?

I think Cymaz's advice is spot on - a tacho will always show quite clearly when your engine is tuned and just listening to it doesn't tell you whether you are on the lean side.

 

Jez.

Clearly you can use a tacho to check the revs, but don't tune it while staring at the screen. Tune it up by ear, which is easy enough to do with a 4 stroke and no harder than a 2 stroke.

A tacho will never show you if you are 2 clicks lean, it cant as the engine will most likely be doing the same revs but will sound hard. Also as I said before, engines slow down a touch as they warm up, so if you peak it just after you start it you might see (for example) 9000rpm but that will slow down to 8800 when the engine is warm. So if you try and reclaim your 9000 by leaning off it will be too lean and then you will spend an afternoon searching for your wayward propnut. You might also get 9000 one day but only 8500 another day due to a change in atmospheric conditions. Again you do not want to chase that 9000rpm as it can lead to being too lean.

The same is true of twins. Never tune a twin with a tacho, just tune each engine until its happy and fly. Tachos are great tools but should be used for information only and not as a guide.

Numbers are awful things as they are absolutes, and people get carried away trying to match numbers of revs or numbers of turns on needles etc. A much better plan is to just set it up the way it wants and then see what numbers you get afterwards!

 

Edited By Jon Harper on 17/11/2014 15:21:39

[Engine Doctor]

Hello you got it right about the timing . The os 40 surpass could be a so and so for throwing props and were some times critical on fuel settings . I say sometimes as they seemed to vary a lot between engines . Set the main needle slightly rich and run the engine at full throttle slowly turning the main needle in until it will run without the glow connected, It should still be misfiring slightly with plenty of smoke. Continue to weaken the mixture until it runs cleanly . Listen to the exhaust note . If it sounds as if it starts labouring slightly with a hard sound then close the throttle slightly before it throws the prop then richen the mixture a few clicks and start again . Another trick is to fit a Taylor short reach idle bar plug if you can find one or put a second copper washer on the existing plug. In extreme cases a head shim will be needed as last resort . The shorter plug or second washer reduces the comp ratio slightly as will a head shim so retarding the timing . You may loose a few rpm but not enough to worry about and getting the last few rpm may mean the difference of a reliable engine or a prop thrower. Once you get it running ok at full throttle close the throttle slowly as they can throw a prop if you close the throttle and the bottom end mix is to rich. Once bottom and top end are adjusted correctly it should be a sweet engine .Whatever you do stay behind the prop out of harms way .

[Stevo]

I had one of these - best engine I ever had!

I cant really add anymore, other than possibly far too lean, richen her up or maybe a head shim. ED has a tried and trusted method!

[Luke Lane]

Hi Tony.

Several years ago I had the very same problems with the same engine that came with a Cub I bought. After many attempts to get it to run nicely, I decided to strip it down, only to find that the head gasket / shim was missing. This was making the compression ratio to high, causing pre-ignition and throwing the prop all around my garage. I gave the engine a complete overhaul, with new and gaskets. It runs like a dream. Must put it in an airframe some day.

[Peter Beeney]

I think there is a specific reason for these four stroke engines suddenly coming to an abrupt standstill and then throwing the prop off, it’s called detonation. Most manufactures can suffer from this to a greater or lesser degree, the earlier OS perhaps slightly more so, particularly the 40. Although this might well be because there may more of them around.

One explanation of detonation, at least in my view, goes something like this: There are a number of things that can lead to this, to a greater or lesser degree, the shape of the combustion chamber, compression ratio, ignition timing, type of fuel and ratio of fuel to air etc. In general term, the only item that the modeller can easily fiddle with is the fuel to air ratio, or in other words, the mixture. Detonation only happens at a state of high volumetric efficiency, which is just a long winded way of saying when the throttle is fairly well open; quite often it happens at full throttle. Consider such a motor running on a mixture that is a trifle lean, this tends to run hot anyway, so the combustion chamber is already getting nicely warmed up. The glow motor is very successful at automatically adjusting the ignition point to suit all it’s varying conditions; when the engine is running fast the ignition will be advanced. So at some point when running on a lean mixture because of the extra heat the ignition is even slightly further advanced. The combustion flame starts to travel quickly though the mixture, the higher the pressure rises the faster this movement becomes until it suddenly is sufficiently high enough to cause a spontaneous second ignition point on the hotter than usual piston top. Now we have two flame fronts rushing to meet each other, one also being aided and abetted by the piston pushing it along as it’s legging it up the cylinder bore at a rate of knots. When they do get together the pressure area becomes as an immovable object, so the piston has no option but to stop short in it’s tracks right there, with the result that the propeller unwinds itself off the crankshaft and continues on it’s merry way for a fairly short but all engrossing distance.

This situation can be quite potent, I saw one OS 40FS, that had done much flying in a Magnatilla suddenly stop with a loud bang in mid flight. This time the prop had stayed on, but the cylinder had been smashed completely off the crankcase. The result of sufficient kinetic energy being dissipated in a short time. Also I suspect a locknut might help prevent the prop departing, but they can take this off as well on occasions. Wedge shaped combustion chambers also do prevent this, perhaps why Laser’s don’t suffer from detonation, but the wedge may not be quite so efficient in other ways.

There is also another definition of detonation, I believe, concerned with larger multi-cylinder engines. When, for whatever reason, the pressure rise in the cylinder is fast enough due the faster than usual flame travel, the flame front speed can reach that of sound, thus causing the characteristic knocking noise.

One thing I think is fairly predicable, though, anyone who has witnessed an OS 91FS throw a prop whilst running at full chat will never ever be tempted to stand in front of a running model engine! At least for no longer than three milliseconds, anyhow…

PB

[Shaunie]

As Pete said!

What many don't appreciate is that the combustion happens as a "flame front" burning progressively from the ignition source towards the edge of the combustion chamber where it burns out. In extreme conditions the pressure rises so sharply when combustion starts that the rest of the mixture bulk burns almost instantly throughout the combustion chamber, causing the knocking sound with the abrupt pressure rise stopping the piston dead in many cases.

Shaunie.

[GrahamWh]

I would add when you check that there is not a build up of carbon in the head, also check that the cylinder head shim is still there. If it isn't replace it as the compression would have been too high.

[balsabilda]

One thing that can throw out the timing, check the tappets clearances. it doesn't have to be done very often but as you acquired the motor second hand its worth giving a thorough overhaul, and make sure any locknut is tight use a dab of med Loctite for extra insurance, I use wooden props on 4s motors (91's and 120's) make sure to balance them and you shouldn't have problems