TT 54 propeller thread is anticlockwise to unscrew!

[Steve Hargreaves - Moderator]

A method I always use with my 4 strokes (& I know others do too but don't think it has been mentioned yet ) is to turn the propellor until the piston has just gone past TDC on the power stroke before applying the starter (WARNING: don't connect the glow until after you have turned the motor over & are ready to apply the starter OR keep a very firm grip of the prop when turning the engine over....if it kicks it can start & your fingers will be the first to find out.......)

 

Using this method means that the piston has 3 strokes & the prop gets 1.5 revolutions before meeting serious resistance (compression stroke) it kind of gives the motor a "run up".

 

On the whole though I agree with other posters......tighten yer nuts properly & you won't have a problem.....I have yet to see a stripped crankshaft thread.......

[Jim Burton 1]

PB my apologies - due to forshortening and distorted perspective on the exploded diagram together with my poor memory, I can see that I was completely wrong and that the Female nut is the largest, and this, the one with the recess, is one you advise putting first after the prop washer i.e. the opposite way to the official TT order.

 

A photo of my engine using the official layout makes it clear which nut is the biggest!

 

 

 

I can see how this might well produce the effect that you describe, and unreservedly withdraw my query/comments in the posting above.

[Peter Beeney]

Jim,

I’m very sorry for any confusion, I will try and sort out some bits and pieces, take some photos, and come back here later, to explain what I mean. It’s simply that that I’ve never seen that locknut arrangement assembled like that before. But I also might not have seen this particular engine before, either.

With the greatest respect, and please believe that I’m not in any way trying to question any of your actions, I’m only trying to help, but in your OP you said the starter was consistently winding the prop off. Again I’ve never seen anything like this, to this extent. Sure, I’ve seen props wound off once, many times, but in two minutes flat the problem is cured!

So to analyse your little predicament, is there a problem with the prop nut arrangement, or the there a fault deeper in the engine?. If someone asked me this question I would say that in my very humble opinion it would be the nut arrangement, solely because I would be able to to do the nut up tight enough to withstand the starter being brought to an abrupt stop even against a hydraulic lock. Even using my good quality starter and a 12V car battery in a good state of charge. I say this only because I’ve been doing this for a very long time now…….

I will try and clear and clear up this conundrum but it might take a while.

PB

[Jim Burton 1]

Steve, that's good advice - in fact TT say much the same thing in their flimsy instruction sheet (I paraphrase) "apply starter for 5-6 seconds or until you see fuel squirting out of carb, with WOT and no glow igniter; then apply 25% throttle and hand wind back clockwise until you feel the compression stroke: then attach igniter and fire up engine with starter".

 

Problem (for me) was the "apply starter for 5-6 seconds..." bit. This was precisely when the prop would come loose.

 

All good advice here - I'm off to the tool shop to get another close fitting 12mm spanner, making two (non-adjustables). And then some serious torque on those locknuts

[Steve Hargreaves - Moderator]

H'mmm...I think that the TT instructions are setting you up for a bit of a fall here (after all what do they know...they only designed the engine...the REAL knowledge is here on this very forum...)

 

Spinning the motor over on such a rich setting is almost bound to flood the thing & one problem with a 4 stroke is that once flooded there is no where for the liquid fuel to go. Unlike a 2 stroke it cannot run out through the ports.

 

It is worth noting that a 4 stroke is much better at filling its cylinder with fuel than a 2 stroke....they draw fuel up much better. When I start my 4 strokes I merely ensure that fuel has reached the carb.....a quick spin on the starter....turn over compression once by hand (as noted previously) to check for hydraulic lock, apply the glow & apply the starter.....almost invariably I get a first or second time start....

[Peter Beeney]

You’ve rather beaten me to it , Jim, just put the nuts on in the reverse order and give it a try. There’s nothing to lose. Tighten everything, Martin’s ‘eye bulging’ limit is about right, my work colleagues might have used a rather more succinct expression but that’s maybe not suitable for a family forum…

Having now said all this, I’m beginning to wonder… in theory I’m not sure there should be much difference here anyway. I’ll have to have a closer look…

PB

[Steve Hargreaves - Moderator]

It looks a fairly unusual arrangement to me too (the nuts that is!!) My ASP 4 strokes all have a large nut with a shoulder on them that locates into the large washer fitted against the prop boss & is then tightened up firmly......the lock nut (which looks much like the "splined" nut in the picture goes on next & the "splined" part tightens against a tapered surface in the aformentioned nut.

 

My understanding is that by tightening this locknut the taper causes the splines to press inwards onto the crankshaft thread thus helping it to grip better & resist the "unscrewing" action of a backfire.....

 

Now I'm just going to tip a little bit of sand into the bearings here & say that personally I don't think the nuts should be too tight.....tight enough to prevent them unscrewing at the slightest bit of a backfire yes but personally I rather like the idea of the nuts coming loose as a kind of "weak link".....better for the nut to come undone than for the con rod to bend. Lets face it if you do get a backfire or hydraulic lock whilst you're applying the starter you've got a classic irresistable force versus immovable object & something has to give.....bent conrods & broken gudgeon pins this way lie!!!

 

So how tight should the prop nuts be...? Ah, Grasshopper when you know this you will have reached a state of true enlightenment.....

[Peter Beeney]

Steve,

Very strange that you should mention the ‘not too tight’ prop nut. Been there, a long time ago, but it was a very short consideration. Just how tight is tight? Just enough so that the prop can spin on the crankshaft, rather an taking the nut right off? Very difficult indeed to be able to judge this degree of pressure required; I don’t think you’d every get it right, unless you did a great deal of experimenting, and always used a torque wrench.

However, the angle on this that really blows it out of the water for me is the one of Safety. When a prop leaves an engine it is potentially very dangerous. This does happen with two stokes, when being turned over by an exuberant starter, only recently a prop spun off a 60 powered Panic in the pits, whizzed up in the air, for indeed several seconds, and came down very close because it went vertically through the wing of my model, which was right next door, and stuck in the ground; and with some considerable force, because it was halfway up one blade. I never thought quickly enough at the time, I should have taken a photo! I’ve seen this occur before, really far too often for my liking. I’m astounded that someone has not been hurt or injured by one of these ‘loose canon’ props; I’m sure that somewhere this must have happened.

The prop spinning off a four stroke, when it detonates, is again really something else! As I said before, the engine can only detonate when it is going at full speed, and having witnessed this I think it’s almost possible that the prop would just ignore anyone that happened to be standing in front of it and just keep going.

Indeed, there was at least one classic case of this, back in the days when the BMFA News was a spreadsheet and there was a Reader’s Letters section. Someone had wrote in, saying that this had happened to him, and some detail of his injuries. His only concern seemed to be making sure that other people were not allowed to go flying on their own. So that whilst you are lying there unconscious and bleeding to death there is someone on hand to give you some assistance. I wrote to the BMFA at the time, asking why they didn’t comment; and also suggesting that there were at least four things this gentleman had got a bit wrong, and if they didn’t care to inform him what they were would they like to tell me? Needless to say, as always, the answer was the inevitable very stony silence!

So, for me at least, Safety is Golden! What price a bent con rod against possibly a spell in hospital? This has happened with running engines, we’ve seen it in these threads and I make no apologies for repeating this yet again. I’m always telling myself this could easily happen to me, and I really wouldn’t like it if it did.

So, all in all, I think I’ve decided to carry on giving my prop nuts the strong arm treatment. I can’t remember the last time I had a prop go AWOL, or a hydraulic lock, or a detonation, (never had one of those!), or any other misdemeanour. I always try and start engines on low throttle, when starting a cowled fourstroke as I apply the starter I just place a finger very momentary over the silencer outlet. For about one one or two starter turns, with practise you can get an instant start. Also another little secret, I think anyway, is having a really good 2 volt glow supply.

Engines are always interesting to talk about, and certainly something I’ve always like to tinker with!

PB

[Jim Burton 1]

As a rider on the male-on-the-bottom, female-on-top debate - the makers of the famous industrial HardLock nuts, very similar in design to the TT jobbys and used commercially worldwide on Wind Farm props, railway bridges etc., pretty much confirm this configuration as shown on these two excerpts from their pdf diagrams, although it is hard to see why either way would not be acceptable just as PB postulates above, especially as Hardlock nuts can apparently be used either seated to a surface or free-standing on a screwed shaft.

 

 

 

 

So...Glasshopper shall try both ways. (With safety goggles and gardening gloves.)

[Steve Hargreaves - Moderator]

Hi Peter....I agree that safety should be paramount & don't get me wrong I'm not advocating that the prop nut should only be nipped up......props can & do fly loose. Indeed one came off a model at our patch some time ago now but it flew across the pits & struck a member standing about 20 feet away just behind the ear drawing a fair bit of blood....the engine was an SC30 fourstroke fitted with a 9x6 prop. I shuffer to think what would have happened had it been a 120 with a 16" prop.....

 

I have to say though (& this is just a personal opinion based on my experience) that I think it unlikely a prop secured by two nuts would actually fly off...come loose, yes but actually unscrew both nuts & fly through the air....unlikely. The above engine did not have a factory locknut fitted & this leads us into an interesting area......spinners.....

 

Like many I prefer the aluminium spinners & these usually have some form of adaptor nut that screws onto the end of the crankshaft. I have yet to see a 4 stroke crankshaft that is long enough to accomodate the spinner backplate, prop, prop washer, retaining nut & lock nut & leave enough thread for the adaptor so something has to be left off. For my own models I leave off the lock nut & place a plain washer between the retaining nut & the threaded adaptor to protect the tapered section where the locknut would fit.

 

So now I am effectively using the spinner adaptor as a locknut. However it doesn't have the compressible "splined" feature of the standard locknut which leads me to believe that this method is less good than the "approved" factory version......it does retain the propellor in most situations however.

 

I think many of us have seen props come loose.....I have seen very few actually fly through the air though. The one unknown in all of these situations is how tight the nuts were actually tightened by their operators......this is one of those areas where the actions of others can compromise the safety of those around them....being hit by a prop hurts...even if it does belong to someone else....

 

But yes I take your point that safety is paramount....personally I would rather have a bent conrod than a missing finger ot a prop blade stuck in my thigh or worse....

 

[Steve Hargreaves - Moderator]

Just out of interest I've just had a quick look at the Just Engines website (cos they have good pictures of their engines) & all of their 4 strokes bar the 30 have lock nuts but none of their 2 strokes do except the 91....even the 1.08 & 1.20 2 strokes have single nuts (if you'll pardon the expression....)

 

Make of that what you will......

[Steve Hargreaves - Moderator]

PS I'm not getting into the female on top/male underneath debate......some things are best kept private

[Martin Harris - Moderator]

Best protection against damage from hydraulic locks is to turn the prop over by hand immediately prior to applying the glow and starter.

 

I can't say that I've noticed much tendency toward them with 4 strokes and it's inverted 2 strokes that seem to suffer worst as they definitely have no way of getting fuel out other than turning the whole model on its side and winding the engine backwards (exhaust port down)

 

A 4 stroke usually has an inch or more of inlet tract above the carb and even if it's inverted fuel can only enter if the engine is positioned on a part of the induction stroke during fuelling so there's a much smaller chance of it happening.

 

When drawing fuel in, I usually just open the throttle and (assuming a pressurised tank) put my finger on the exhaust outlet until I either see fuel at the carb, dripping from the carb or the engine feels wet, which gives a lot more feel than a specified time with a starter. Of course, with a 4 stroke there's a hole which opens at regular intervals to let excessive fuel into the exhaust port.

 

P.S. Re. the nuts, I don't think it makes any difference and it's probably just that we're all used to the "normal" OS and clones arrangement. TT just seem to have redesigned the system slightly.

Edited By Martin Harris on 09/09/2011 15:31:50

[Peter Beeney]

Steve,

I have to say straight away that a detonating four stroke can be of sufficient power for the prop to take the main nut and it’s associated locknut straight off. I believe that others have witnessed this; and I know definitely this has happened twice, once to a friend that uses four stokes a lot, and is well acquainted with the idiocycronies of them and their locknuts; and indeed, just to what degree they should be tightened. It happened as he was running up in the pits, I didn’t see this at the time, but I arrived at the site just after. He was a bit shaken, he couldn’t believe that it had happened. It seems the motor stopped and the prop instantly left, making the (usual) unusual whirring noise. Up until then, I think he had been slightly dubious that this phenomenon was a hard fact. Our pits is a measured regulation 30 metres from the strip and he’d recovered the prop from well beyond that, perhaps 3 times as far. This was an OS official main nut and locknut.

So I think that no system, at the moment, can be guaranteed 100% to keep the prop on.

Perhaps here might also be a good opportunity to make the point that with some awareness of the reasons for detonation you can take steps to try and prevent it. Then the locknut would never have to attempt to save a searching party, at the very least; and, in part, would be redundant. I’m sure that many owners run their four stokes a very long time, maybe for life, without ever experiencing this. Again I don’t think you could say with 100% certainty it won’t happen, but you can say it’s unlikely.

Re the Just Engines website, the 2 and 4 stroke locknuts, this may be for the reasons I stated @ 21:27:41, a previous post. It’s simply the reason why you need (or don't need) the locknut in the first instance.

Now, just to keep the pot boiling, what about the ‘four stroke running backward’ debate? That might be a whole new can of worms, or at least beans, (castor?), it’s something that crops up from time to time.

PB

[Brian Parker]

All interesting stuff.
But, as I understand it.

Detonation in an IC engine occurs after the normal ignition point, creating localised uncontrolled high pressure pockets around the combustion chamber, the piston is already on its downward journey.

This causes the engine to ‘ knock ‘ (this is actually the engine resonating or ‘ringing’ but will not stop the engine unless it is so severe as to destroy the engine.

An engine at high rpm will not detonate simply because there is insufficient time for it to take place.

Pre-ignition is the ignition of the fuel before the normal ignition point and can take place if the engine combustion chamber is severely overheated.

This will cause the engine to run even hotter and could stop the engine and possibly to even run backwards instead of stopping .

Edited By Brian Parker on 09/09/2011 19:46:26

[Peter Beeney]

Brian,

Yes, I think I would largely agree with that. Maybe one of the confusing issues here is that many of these conditions are just lumped together under one heading. One very eminent source describes the combustion process, under certain conditions, as speeding up until it becomes an explosion, the flame front then reaching the speed of sound which causes the knocking sound. The ‘pinking’ noise, when running on low grade petrol, is also down to shock waves. All this is in a normal vehicle petrol engine, of course. However, I think there might be a unique set of circumstances which applies to our model four stroke engines running on methanol. We can call it whatever you like, I just call it detonation because that’s what I’ve always known it as.

As we’ve previously described, the common effect is that the engine stops suddenly at full throttle and throws the prop off.

So this would be my thoughts on the subject.

I think there are a number of conditions in a model four stoke engine which can affect it’s susceptibility to detonation. The type of fuel used, the compression ratio, the shape of the combustion chamber, the fuel/air mixture and even perhaps the glow plug and it’s associated ignition timing, these are all things that can contribute to the condition. Generally speaking, these are all more all more or less fixed, but we can change the glow plug, and much more easily the fuel/air ratio, just by altering the needle valve.

High volumetric efficiency is the ratio of the compression in the cylinder at it’s lowest, that’s at tickover, to it’s highest, at full throttle. Because the carburettor barrel is fully open here, the cylinder is being filled to it’s maximum at this point, hence we have the compression ratio at it’s maximum.

So now we have our engine running at full throttle. We decide to increase the speed and power by winding the needle valve in, thus making the mixture leaner. This leaner mixture burns faster and hotter, raising the combustion pressure quicker and higher. The piston is now really legging it up and down in the cylinder and the prop is causing a considerable draft. But, we think we can get jut a little bit more, so another couple of clicks in. Everything goes up a little bit, especially the heat. Because of the speed the ignition timing is at it’s maximum advance before top dead centre.

This continues for a while, but at some point a combination of the advanced ignition and the resultant fast moving flame front in the hot combustion chamber, together with the piston travelling at it’s maximum speed, causes the pressure to rise in the cylinder so fast that a spontaneous secondary ignition point is caused on top of the piston. Now we have two very rapidly expanding flame fronts, one in the combustion chamber, and one on top of the piston, which is in turn is being pushed upwards at some considerable pace. When these two meet, perhaps at say a point about four fifths up the compression stroke, it just becomes the original immovable object, totally incompressible, so the piston gives in easily and simply stops. Instantly. In virtually no time at all. But maybe the prop on the front doesn’t really want to play these games at that moment and using it’s considerable kinetic energy just unwinds the prop nut and continues merrily on it’s way. That can be to a very long way in front of the model; and as I’ve said, if determined enough it can also completely remove the cylinder from the crankcase. I think this was a case when the prop actually stayed on, and all that stored up energy just translated to a force/shock wave powerful enough to break the metal.

I would consider this is detonation in a model engine, and as I’ve said, I’ve not personally seen too many cases of this, so I’m pretty sure it only happens infrequently.

We can also see from this why we need the high compression ratio at full throttle for it to happen, and why it happens at maximum speed and also why it’s very important not to get it too lean. Adding nitro to the four stroke fuel might also be beneficial here, I’ve found they they certainly run better generally, especially Saitos.

I think it’s a plan to tamper with engines at any opportunity, practise makes perfect. Use a tacho, and carefully wind the needle valve in. The revs gradually increase but at some point they cease to rise and the engine takes on a slightly ‘hard’ note. This is the danger area, it’s getting too lean, back off the needle valve slightly. I’ve found our ears are quite sensitive, they might not be able to read the steady revs, but they can detect a very small change in pitch as it happens. I’ve also found that when I’ve found the optimum point for the needle valve, if I move it for some reason, and that’s not often, I always return exactly to it. This is the four stroke technique.

Rightly or wrongly, that is how I see it all going down. It works for me.

PB

[Brian Parker]

Peter,
In full sized engines, the ignition process is influenced (smoothed) by slower moving (cooler) boundary-layer gasses.

Under certain rare conditions are these boundary layers absent from small glow engines

Also at maximum rpm a degree of valve bounce is probably influencing the process.

[Peter Beeney]

 

Brian,

I’m afraid I’m not an expert on engines, or anything else for that matter, I tend to make it up as I go along, so I’m not able to really comment on these boundary layers. However, I’ve just found this little piece on full size multi-cylinder detonation, by V.A.W. Hillier, 1966.

Combustion knock, commonly called detonation, is a form of extremely rapid combustion which is likely to occur under certain conditions. It is harmful and must be avoided. It occurs if the mixture is heated and compressed to an excessive amount during combustion. It is also influenced by the quality of the fuel.

I also remember reading somewhere that detonation, in extremis, could be a bit more determined, such as punching holes in pistons etc. Which after all this stuff, now perhaps seems a mite plausible!

Even back in those days it seems that little credence or much attention was given to these conditions, I’m sure they were all being largely overcome in any event and thus were quite rare. Today, with modern fuels and engine management systems and all the other paraphernalia they must be completely unheard of.

It might be possible that valve bounce may have some effect, but I’m not entirely sure about that either. Valve bounce might be the ultimate limiting mechanism on the maximum revs but I’m not sure we always operate in this area. Which brings me to another old speculation of mine, the maximum power output verses the maximum revs…

I’m still convinced that the answer to the model engine problem lies in the mixture control; and when you think about this, giving the operator this control was probably about the worst thing the original designer could do! Of course, he didn’t really have any choice at all; but think of what chaos we would produce if we could adjust the mixture at will on all internal combustion engines!

Re-reading my previous post, it does seem a bit like a lot of words about very little; and I think my statement ‘about four fifths up the compression stroke’ was a bit too optimistic. I’ve no idea of these timings but again thinking about it this it must happen probably a lot closer to top dead centre, maybe in the last 5% of the piston travel, but I’m convinced the nett result is the same as if the piston has clouted the cylinder head!

I’m also not sure how important all this is anyway. Jim’s OP was concerned with something else, which hopefully he’s now cured, and these little ‘prop coming off’ episodes that happen infrequently to other modellers are never reported anyway.

When Mr Hillier said that detonation was harmful I guess there is at least one gentleman that agrees with him; see my post @ 09/09/2011 14:29:49.

PB

[Jim Burton 1]

Peter, your thoughts have made for interesting reading, whether slightly off-topic or not. I for one have enjoyed hearing of your extensive experiences in the field and in the shed!

However, this thread is getting quite lengthy, and I don't know if I should start a new one, as my original 'prop spinning off' problem is now completely sorted thanks to you and your generous-with-their-time colleagues.Two 12mm spanners now have permanent residence in my field box!

 

In the absence of any intervention from forum-master, I shall continue with this thread as it follows directly on. Is it not true that in modelling, as in so many other aspect of life, y'solve one problem only to reveal another.

Here goes.

 

BEB has opined a liking for aluminium spinners and Steve threw this into the pot. (I've inserted my own photos as illustration...

Steve said "Like many I prefer the aluminium spinners & these usually have some form of adaptor nut that screws onto the end of the crankshaft. I have yet to see a 4 stroke crankshaft that is long enough to accomodate the spinner backplate, prop, prop washer, retaining nut & lock nut"

& leave enough thread for the adaptor so something has to be left off. For my own models I leave off the lock nut & place a plain washer between the retaining nut & the threaded adaptor to protect the tapered section where the locknut would fit.

 

So now I am effectively using the spinner adaptor as a locknut. However it doesn't have the compressible "splined" feature of the standard locknut which leads me to believe that this method is less good than the "approved" factory version......it does retain the propellor in most situations however. "

 

Hmm, so the choice is

1) an 'official double locknut and no spinner or

2) no locknut but a spinner held on with the spinner adapter nut replacing the locknut.

 

I'm not thrilled with the latter configuration, having spent last week avoiding it.

 

So what I now have is a succesfully locked prop, sans spinner, which forces me to apply the starter to the naked propeller, centreing it by eye and, I have observed, leaving a considerable residue of rubber unevenly stuck to the front face of my otherwise carefully balanced prop. Not good.

 

I have a proposed 'fix' that I would like to run by you.

 

The spinner adapter nut could, perhaps. be 'drilled out' to the same radius and depth as the recess in the upper (female) TT locknut, which could then be discarded. The space thus saved would then provide the best of both worlds: a locked prop and an ali spinner.

 

I wonder if those of you with metalworking experience might like to give me some feedback on this idea.

[Martin Harris - Moderator]

I don't see any reason why you couldn't and the TT version of the locking nuts lend themselves to the idea better than the "normal" version.

 

I've made my own split, splined or whatever nuts to suit ali spinners in the past but a mild steel one gave me an awful job to get it off after it it took on a permanent deformation.

[Brian Parker]

Peter,
Yes, I was not denying the harmful effects of detonation and have indeed seen pistons from petrol engines with holes in their crowns.

[Biggles' Elder Brother - Moderator]

Hi Jim,

 

we'll leave Brian and Peter to chat about the theoretical aspects of detonation, carry on lads fasinating stuff . Meanwhile we'll get back to your model.

 

If you don't have a lathe - or access to one - then the simplest solution to your problem lies with "Just Engines". They make a range of adapters which fullfill a dual purpose, they are both the lock nut and the fixing for the aluminium spinner. So then you don't use TT lock nut you use this device instead. You have plenty of thread left over for one of them. You find details on this page.

 

BEB

 

PS Remember to tell them what engine its for as they make different ones for different engines.

Edited By Biggles' Elder Brother - Moderator on 10/09/2011 19:34:23

[Peter Beeney]

Jim,

Thanks for your answer, hopefully there is something of interest in there somewhere.

I have to say, surely I’m not the only person to have noticed that props can spin off four strokes at full chat? If I am that’s one hell of a coincidence.

I would say regarding the spinner that the most important thing is to consider that the prop can fly off at any time, irrespective of however you lock it on. So if you always bear this in mind it shouldn’t be any problem, if it comes off it’s never going to do any damage. I would always fit a spinner, in fact I would not allow any of my junior newcomers fly without, starting as you are could lead to a nasty situation. I’d just fit it on, I have used exactly those ali spinners, and maybe some one will come up with a practical solution that would enable you to fit the locknut.

I’m afraid I’ve not had any extensive experiences, at least not in the shed. I would not under any circumstances start an engine in the shed, but my very safety conscious friend did this and in an instant found he was holding one hand together with the other. Fortunately no real harm was done, the prop did not hit anything really vital, but he had a spell in hospital. However, he would never get his plane out of the car at the field until there were others present, although he had previously run up his models in his garage; he was entirely on his own at his house. I’m just the opposite, I never run an engine at home, but a very large part of my flying time has been lone-hand, at many different sites. Apart from the odd running in, in controlled conditions also at the field, if I start my engine I’m going to fly my plane.

All that detonation stuff is largely calculated supposition, I’ve had no real experience of this. But I like tinkering with engines and I’ve read quite a bit in the past. As I said I think the detonation is down to the mixture, don’t make it too lean and it should be ok. I often get involved in friendly discussions and fault finding at the field, which is all good experience.

We regard to your dilemma, I would personally go for - 2) no locknut but a spinner held on with the spinner adapter nut replacing the locknut. Having said that, unless it’s a special scale model I’d simply use a plastic spinner anyway. I’d agree with Martin, drill out the spinner adapter nut and give it a try, you’ve got nothing to lose.

Good Luck.

PS It looks as though BEB has now come up with the ideal answer. He makes it look easy, and that must be a very uncomplicated solution. They’re always the best!

PB

[Steve Hargreaves - Moderator]

Jim....although props can & do fly off it is a fairly rare occurance in my experience....I have only seen the one in more years flying than I care to remember. I have seen & experienced many props come loose on the starter but this is usually down to the nut not being sufficiently tightened.. (Guilty as charged m'lud....)

 

I have replaced the "official" locknut with a spinner adaptor on many models over many years & again I have never had a prop actually fly off so, whilst I accept that this can happen, I for one think it fairly unlikely & in the great scheme of things I think it is pretty low risk.

 

I would also highlight Martins point about using the correct material for any adaptors or home made locknuts you might contemplate. Typical spinner adaptors are often made in brass or even aluminium. These are very soft materials & if tightened with some enthusiasm into a taper recess will probably compress onto the crankshaft thread & give you a heck of a lot of grief trying to get them off again.....the crankshaft & retaining nuts of your engine will be made in a pretty robust steel....probably high tensile steel....to withstand these loads.... It is an oft forgotten fact that most manufacturers do actually know something about the products they manufacture....

 

Now, just to tip a small cup of cold water into the vat of boiling oil, one thing that hasn't been mentioned so far in much of the learned discourse above is the effect of heat on our propellors.....much has been written above about detonation & pre-ignition & I think we are all fairly agreed that lean mixtures are more likely to cause this. Another well known feature of lean mixtures is that they run hot......with our poor little engine now revving away fit to burst & getting very hot into the bargain might not some of this heat transfer to the propellor which, being plastic will soften slightly? Might not this slight softening lead to the retaining nut becoming slightly looser & more prone to coming undone??

 

(Ducks behind parapet once more!!!!! )

[Jim Burton 1]

BEB, PB and Steve. Thanks for your thoughts. Although JustEngines locknut solution looks expensive at a tenner plus PnP, if it is exactly the precise size for the TT54 crankshaft and assembled spinner hardware, I might have to give it a go.

 

Steve's highlighting of Martin's previous caution focuses on just the situation in which I find myself. The spinner nut is indeed chrome plated brass, and was relatively easy to drill out to the size of the official TT lock-nut recess. A little hacksawing to shorten the overall length and the spinner fits precisely.

 

Why my unease? When screwing on my 'remanufactured' spinner locknut, it was apparent that the 'give' of the brass was such that it was scraping noisily (squeak/groan noise) as I tightened it. I'll bet when I remove it to mount the engine in the plane, there will be brass shavings apparent. Further, because by thinning down the 'walls' of the brass nut I must inherently have weakened it, I can visualise it splitting under stress and then where would we be? Spinner and chunks of brass randomly scattered at high velocity like some miniature warzone. Not for me - safe, responsible flying does not include flying a known potential hazard of my own making.