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Author Topic: PAW 35 help  (Read 3809 times)

Offline Rob Roberts

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PAW 35 help
« on: March 10, 2012, 04:02:09 PM »
Can not get a good flight with my PAW 35. I have about 20 flights in my 47oz. re-kitted arf cardinal have tried 12x6, 12x6w, 12x7, 13x6 cut down to 12.5 props. Starts out ok but sages by the vertical 8s.

     Rob

Offline Tim Wescott

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Re: PAW 35 help
« Reply #1 on: March 10, 2012, 05:52:46 PM »
I can't say anything about PAWs (or any other diesel, I assume it's a diesel) specifically, but here's what pops to mind:

  • It's getting hot
  • The mixture is changing as the tank runs out -- is it a uniflow?  Is it leaking somewhere?
  • You're starting too rich, or too lean, or too compressed, or too undercompressed -- have you tried intentionally launching a bit rich and/or undercompressed, to see how it does?

I dunno what else to guess at -- hopefully this response is so lame that some expert will be motivated to give you the real story.
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Offline Steve Thomas

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Re: PAW 35 help
« Reply #2 on: March 11, 2012, 01:46:04 AM »
Dunno about being an expert, but I've been running a PAW 35TBR in a semi-scale Supermarine Spiteful with good results. I use an 11x6, 11x7 or 12x6 prop (all work well) with a uniflow tank.  Fuel is 25% castor, 33% ether, 42% kero, plus 1.5% ignition improver, although I've tried a few different blends and the engine doesn't seem to care much.

My guess is that your engine is either overheating or going overcompressed. Why this would be is hard to say. Some of the props you're using, while well within the recommended range, are bigger than I'd go for in this application (and FWIW I believe the guys flying stunt with the 40TBR still only tend to use an 11x6 or 11x7). Perhaps your engine doesn't like the additional load when manoeuvring.   Have you tried something like an 11x6?

Overcompression is an obvious possibility. These things need to be given a decent warmup before launch, and the compression adjusted if necessary. Both my 35 and a friend's need the comp wound in half a turn for starting; it then has to be backed off immediately once the engine's going.  I prefer not to mess with the compression of a running engine, but in this case it's unavoidable.

You didn't mention what fuel you're using. Too much ignition improver can cause an engine to overheat.

Hopefully there's something useful in all that and it hasn't just insulted your intelligence.  :)  

Cheers,
Steve
« Last Edit: March 11, 2012, 06:06:56 AM by Steve Thomas »

Offline Rob Roberts

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Re: PAW 35 help
« Reply #3 on: March 11, 2012, 08:20:02 AM »
Tim
 Those are all good suggestions, the plane is definitely going over compressed but starting out with less compression results in a long sputtering boring flight Thanks

Steve
The engine runs like it is over loaded but when I tried a narrow master airscrew 12x6 I got a bad vibration after the plane "un loaded" in the air. The engine has had allot of break in sense then so I will give a smaller prop a try.
Long warm up has not helped. Fuel, Davis Diesel with 3% added castor, & Dr Diesels fuel.Have tried  some "home brews" not as good as DR D.

Thanks for the help
Rob

Offline Tim Wescott

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Re: PAW 35 help
« Reply #4 on: March 11, 2012, 09:02:57 AM »
If a propeller change doesn't help, then you might try a more aggressive warm-up.  I do this on my OS 20-FP, which is a cold-blooded beast that doesn't seem to really warm up until it's been run a good long time.  I start it, then run half a tank through it on the ground, lean, then I top it off and fly.  I cheat; because it's a profile I can lean it out just by squeezing the muffler pressure tube, so I hold the tube until it starts sagging lean, then let go, the hold again while it goes through peak, etc.

I get much more consistent flights out of it that way.

If you're buying his fuel anyway, you might want to have a conversation with Mr. Clutton about your problem.  He'd be an expert, by all means!
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Offline Andrew Tinsley

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Re: PAW 35 help
« Reply #5 on: March 11, 2012, 09:55:41 AM »
Hello,
  I did send a PM to Kenneth and have just seen your post. It is easy to see if you are overcompressed. Look at the colour of the oil residue AFTER the flight, if it is black, you are overcompressed, if colourless then undercompressed. You should be looking for a light honey / straw colour for the correct compression. When you get there, then adjust your fuel needle. If it is burping along then you are probably too lean, richen up until the engine is running strongly.
Be very careful of your fuel. Keep it tightly stoppered in an air tight can unless you are fueling up the plane. It is very easy to lose HUGE amounts of ether if you don't do this. The engine then does lots of strange things, starting will become difficult for example.
DO exactly what the running instructions say.........it is important. Forget your accumulated glow knowledge, diesels are different. When you have it sorted, then beware that PAWs seem to dislike upright or inverted mounting. Go Sidewinder and all will be well! Otherwise you get Fox 35 burps on manouvers, for exactly the same reason.
  PM me if this doesn't help and we can look at other causes. But first get the above in place. Once you have got the correct settings, then you can enjoy constant speed aerobatics, just like a pipe without the extra expense.
  Just one last thought, if you have got a dodgy tank then this could be the cause. Usually it is simply unfamiliarity with diesels!

Reggards,

Andrew.
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Online kenneth cook

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Re: PAW 35 help
« Reply #6 on: March 11, 2012, 01:18:13 PM »
                     I hope you get it corrected Rob, that engine is just too nice to sit on a shelf. ken

Offline Steve Thomas

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Re: PAW 35 help
« Reply #7 on: March 11, 2012, 03:29:05 PM »
Unfortunately you can't use oil colour as a guide with PAW 35s (at least the couple I've used). Doesn't matter how you set them, the oil is dark. Might be something to do with the muffler, as all the other PAWs I've run unmuffled don't do it. I've even flown the 35 deliberately undercompressed to see what would happen, and the residual oil was still nearly black.  (Kero type can also influence the exhaust colour, but all my other diesels leave much lighter oil with the same kero). 

Offline Chris Wilson

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Re: PAW 35 help
« Reply #8 on: March 11, 2012, 03:50:13 PM »
Hi Rob,
    I have two PAW 40's and although they are yet to see a model I can relate to you about what I have been told about them for Tony Eifflaenders Gold Cup team mate and several good club fliers here in Oz.

Believe it or not the recommended prop for a PAW 40 is an 11x7 and aim for about 9 500 rpm in flight. I have seen incredulous looks when quoting such high revs for a cast iron pistoned large diesel but the original was flown at quite a rate of knots.

This was borne out the previous owner of my 2nd (2nd hand PAW 40), he actually made a smaller venturi than the asthmatically small original trying to keep the revs down to about 8 500 and failed to get a good run also quoting IPN% as problems.

Anyway, if the 35 is much like the 40 then I would be tempted to let it run free with less of a load and forget the hype of diesels being able lug along happily.

That, and as mentioned before let it warm up before backing off the comp to a run setting. It has after all a steel liner and cast iron piston with cast iron contra piston all waiting to come up to temperature before the compression and heat related ignition point can be set properly.

If you get a bad vibration when the engine gets its head then I would investigate prop balance (of course) and how your big diesel is mounted - firmly I hope!
Is by any chance your Cardinal  - a profile model?

Good luck!

P.S Oh, I nearly forgot, PAW does offer a piston lightening service as part of its tuning services and maybe the 35 needs help in this area?
MAAA AUS 73427

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Offline Chris Wilson

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Re: PAW 35 help
« Reply #9 on: March 11, 2012, 03:58:57 PM »
Unfortunately you can't use oil colour as a guide with PAW 35s (at least the couple I've used). Doesn't matter how you set them, the oil is dark. Might be something to do with the muffler, as all the other PAWs I've run unmuffled don't do it. I've even flown the 35 deliberately undercompressed to see what would happen, and the residual oil was still nearly black.  (Kero type can also influence the exhaust colour, but all my other diesels leave much lighter oil with the same kero). 

Hi Steve,
                the explanation that was fielded to me in regards to the dark colour of the exhaust residue was that the close proximity nature, and therefore heat of the collector ring re-cooks the castor oil beyond what the combustion process does.

I have seen similar in home made chambered mufflers used in glow engines for stunt, when ever the castor is trapped behind a baffle or collected in a ring (like PAW engines) it just gets blasted by the hot exhaust gases and you think that something is wrong internally with the engine due to the dark oil colour.
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Offline Steve Thomas

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Re: PAW 35 help
« Reply #10 on: March 11, 2012, 08:20:49 PM »
Rob, if you're using sensible fuel and not running overcompressed, there's one other suggestion I can offer.  While I've never had any overheating issues with the 35, I can't say the same for the 19.  Between us, a friend and I have 3 plain bearing 19s, a 19BR, and two 19TBRs.  Of those, the 19BR and one of the plain bearing engines have been very prone to overheating, regardless of settings and fuel.  I've been at my wits' end trying to solve this, and couldn't.  My final throw of the dice was based on the fact that PAWs tend to have a very sloppy fit between the cylinder and the fin assembly.  Some people are of the belief that oil is designed to fill the gap and thus transfer heat; I reckon that for proper heat transfer, it should just fit better in the first place.  (The other thing to bear in mind is that the 19 and 35 are bored-out versions of the 15 and 29 respectively, and actually have less fin area despite having more capacity.  Perhaps they are inherently more marginal for cooling?)  What I did was pull off the fins off the 19BR and wrap a couple of layers of Alfoil round the cylinder, so that the fin assembly fit nice and snugly when I replaced it - the idea being to have a good proper metal-to-metal fit.

I flew this for the first time this morning.  One flight was very slightly overcompressed (evidenced by slight slowing in manoeuvres); despite this, the setting never actually changed, and the engine didn't overheat.  The second flight was fast and slightly lean. Once again, it held the setting perfectly and showed no signs of overheating.  I'm not yet ready to believe that I've cracked the problem - it'll take quite a bit more flying yet, plus trying the same thing with the misbehaving 19DS - but it's looking promising.

The overheating symptoms I was experiencing were that the engine would initially have a good setting (even slightly undercompressed), but would load up after a few manoeuvres.  Vertical and overhead 8s were particularly prone to causing this.  The engine would sag and slow, and not pick up regardless of what the model was doing; generally I would land it and stop the engine to try and avoid damaging it.  The cylinder head would be sizzling hot.  Does any of that sound familiar?

Offline Chris Wilson

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Re: PAW 35 help
« Reply #11 on: March 11, 2012, 09:34:26 PM »
Very interesting Steve and I have heard of the foil trick before and the Eifflaenders stating that oil 'will' fill the gap but stopping short of saying that is what is actually intended.

The PAW 19 that I have never seemed to overheat but as you know, I used one of those underwhelming 9x4 props so again, perhaps letting it spin out a bit is an answer.
(Wonder if the vintage combat boys ever faced the heat issue with their smaller props too?)

Something here puzzles me though mate, if the engine was overheating due to poor heat transference to the cooling jacket, how could the head of it be sizzling hot?
Perhaps it IS working as intended and the issue is before that?
MAAA AUS 73427

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Offline Steve Thomas

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Re: PAW 35 help
« Reply #12 on: March 11, 2012, 10:57:43 PM »
Very interesting Steve and I have heard of the foil trick before and the Eifflaenders stating that oil 'will' fill the gap but stopping short of saying that is what is actually intended.

The PAW 19 that I have never seemed to overheat but as you know, I used one of those underwhelming 9x4 props so again, perhaps letting it spin out a bit is an answer.
(Wonder if the vintage combat boys ever faced the heat issue with their smaller props too?)

Something here puzzles me though mate, if the engine was overheating due to poor heat transference to the cooling jacket, how could the head of it be sizzling hot?
Perhaps it IS working as intended and the issue is before that?

That's interesting - I'd never heard of anyone trying the foil thing before.  Damn, thought I'd actually come up with an original idea..

Prop-wise, I've had overheating even with a narrow-blade 9x6, which is not much of a load for the 19. Blaming it on the prop doesn't explain why all the other 19s run quite happily.

As to how the head gets sizzling hot if there's poor heat transfer: I don't know.  Perhaps once the cylinder's hot enough, it expands sufficiently to transfer more heat to the fins, but by then it's too late.  Or the fins only get really hot when the model's on the ground and there's no airflow over them any more.  Who knows - we could fill the forums with this but it'd still be largely guesswork.  What matters more to me is that what we saw today was more encouraging than anything else we've tried in the last couple of years.  Still early days though - I'll keep at it and report back.

Offline Tim Wescott

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Re: PAW 35 help
« Reply #13 on: March 11, 2012, 11:06:57 PM »
What I did was pull off the fins off the 19BR and wrap a couple of layers of Alfoil round the cylinder, so that the fin assembly fit nice and snugly when I replaced it - the idea being to have a good proper metal-to-metal fit.
Thin shim brass might work better -- mostly because it's mechanically stronger than aluminum and comes in a wider selection of widths, so finding something that gives you a snug fit yet won't just get wadded or torn up when you try to insert the assembly may be easier, but also because I think that brass conducts heat better than aluminum.

But -- I'm just guessing.
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Offline Andrew Tinsley

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Re: PAW 35 help
« Reply #14 on: March 12, 2012, 03:37:48 AM »
Hello again,
I must admit that I do not run diesels with a muffler. So the problems with oil residue colour being different with a muffler is a new one to me. I run diesels for stunt and sport so I never get into the realms of high revs that the combat and team race guys regularly use. I find that an unmuffled diesel at moderate revs is quieter than a silenced glow.
  I have heard of the problems with heat transfer and using aluminium foil etc. I rather doubt that they are applicable here. We are talking of a moderately reving engine, not one of the stressed screamers that you find in combat or team race. I have never had the problem of severe overheating in normal sport / stunt usage. I am not saying that the people that advocate foil  to help with this are wrong, just that we are talking of a different application.
  I have some experience of the bigger PAW diesels, but I prefer to use MVVS diesels when you get to the 35 and above. The main reason being lighter weight and less vibration. If you are using a PAW35 in a profile model. Then I suspect that this might be the problem, if everything else has been sorted. The bigger PAWs do have a reputation for more than the usual vibration problems.
  Why not contact PAW directly if you are still having trouble, they are a very friendly and helpful group of people! paul@paw.ac should get you to Paul Eifflaender. He would pass on your query to Tony, who has flown his Freebird with a PAW 35 with great success.
  Thinking about the Freebird, this has a very sturdy bearer arrangement, deliberately designed to strengthen the front end. Rigidity could be your problem, if you have mastered the diesel art and eliminated the usual problems.

Regards,

Andrew.  
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Offline Chris Wilson

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Re: PAW 35 help
« Reply #15 on: March 12, 2012, 03:37:05 PM »

  I have some experience of the bigger PAW diesels, but I prefer to use MVVS diesels when you get to the 35 and above. The main reason being lighter weight and less vibration. If you are using a PAW35 in a profile model. Then I suspect that this might be the problem, if everything else has been sorted. The bigger PAWs do have a reputation f

Regards,

Andrew.  

I concur Andrew, fuel foaming caused by vibration could well lean out an engine and especially so when you reach the overheads with a partially empty tank.

I would be keen to hear if the 'sagging' is evident if the overhead stuff is done firstly instead of lastly.

Now the original Freebird/PAW 40 combination featured a chicken hopper tank with a very tight and small 'hopper' container that probably contained that possible foaming aspect quite well.

And Steve,
                 perhaps easier to get our friendly machinist mate to machine up a set of heat shrunk on fins or simply skim coat solder onto the outside of the steel liner for a real tight push fit.
But yes its odd that some out of a bunch suffer whilst others don't and perhaps the difference between engines is balance, and therefore foaming and lean runs rather than mechanical fits?

I might expect that the twin ball race engine that escapes the 'heat' issue will have the lightened piston and thus the better balance too.

The last thing that I can think of is that sometimes the exhaust timing can be out with these drop in liner engines and the goal to aim for is 140º  - combine an exhaust timing error with a tight fitting muffler and you could well have heat issues coming your way.

The plain bearing PAW 19DS I have has no crank web counter balance at all, how is the single ball race engine endowed?

Good luck!
MAAA AUS 73427

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 Nothing in life is so exhilarating as to be shot at without result.  It's not enough that we do our best; sometimes we have to do what's required

Offline Steve Thomas

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Re: PAW 35 help
« Reply #16 on: March 12, 2012, 04:42:54 PM »

Quote
I concur Andrew, fuel foaming caused by vibration could well lean out an engine and especially so when you reach the overheads with a partially empty tank.

I would be keen to hear if the 'sagging' is evident if the overhead stuff is done firstly instead of lastly.

Now the original Freebird/PAW 40 combination featured a chicken hopper tank with a very tight and small 'hopper' container that probably contained that possible foaming aspect quite well.


Yes, but we're talking about diesels here, not glows.  Any diesel I've ever used will misfire when lean, not sag.


Quote
And Steve, perhaps easier to get our friendly machinist mate to machine up a set of heat shrunk on fins or simply skim coat solder onto the outside of the steel liner for a real tight push fit.

What, easier than wrapping a bit of foil round the cylinder?!  (In any case, the machinist in question is busy making conrods for T2.5s, and I'd hate to disturb such important work!)

It's been a while since I had the back off the 19BR, so I can't remember if the crank's balanced or not.  I'd be very surprised if it was.  My 35 on the other hand, does have a balanced crank - just as well, as the piston is a heavy little lump of metal. 




Offline Chris Wilson

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Re: PAW 35 help
« Reply #17 on: March 12, 2012, 05:08:14 PM »

Yes, but we're talking about diesels here, not glows.  Any diesel I've ever used will misfire when lean, not sag.


What, easier than wrapping a bit of foil round the cylinder?!  (In any case, the machinist in question is busy making conrods for T2.5s, and I'd hate to disturb such important work!)

It's been a while since I had the back off the 19BR, so I can't remember if the crank's balanced or not.  I'd be very surprised if it was.  My 35 on the other hand, does have a balanced crank - just as well, as the piston is a heavy little lump of metal. 





Hi Steve,
                 (just for the audience, we know each other here) I am not convinced that an aerated fuel doesn't contribute to overheating though whatever the type of I.C. engine.
You can have the correct air/fuel ratio delivered in a less than homogeneous state.

The solder wipe, although not as easy as simply using a foil wrap, has to be more conductive to heat transfer as lead is far more malleable than aluminum.

(As to the 'machinist, ' I don't dare disturb him from his task either!)

My old PAW 19 DS full circle crank web suffered at the wheels of my grinder in an attempt to restore some semblance of counter balance - all 'machinist' approved of course!
MAAA AUS 73427

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 Nothing in life is so exhilarating as to be shot at without result.  It's not enough that we do our best; sometimes we have to do what's required

Offline Rob Roberts

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Re: PAW 35 help
« Reply #18 on: March 16, 2012, 03:55:03 PM »
Made 6 flights today with my PAW 35, did not have any 11" props so I cut down a Xoar 12x6 to 11.25 Flights were very consistant with no change from the 1st lap until 2 or 3 laps from the end. Power was lacking but it was a step in the right direction. I will experiment with more 11" props.

Thanks for all the help
  Rob

Offline L0U CRANE

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Re: PAW 35 help
« Reply #19 on: March 17, 2012, 01:28:17 AM »
I stumbled across another possible source of inconsistent running on a PAW 35BR. My venturii choke diameter was over 0.265". A friend's 35BR had a 0.250" choke ID as stock... so, I tried that.

Sleeved it with 1/4" ID Al tube, OD of which happened to fit the stock venturii. Due to the frustration of trying to get a reliable setting with that OEM choke diameter, I'd replaced the 35BR in the intended model with a different engine - no in-flight experience with the modded 35BR yet, but on the bench it starts and sets much more easily, and holds setting through a 4 fl oz tank run on 11-5 or 11-6 props.

Note on vibration: Given that the prop is carefully balanced and the model's front end is reasonably solid, you should find a setting combo (compression and needle) that has noticeably less vibration than others. For stunt/sport flying, this is usually close to what you want for RPM on the given prop.  Of course, all diesels shake quite a bit on initial warm-up from cold, and while spinning down after the fuel is gone.

(That 'sweet spot' setting seems to reduce the need for physically balancing the shaft - the compression and power impulses seem to synchronize with the balance - or lack of it - to give a net reduction in vibration. It might be best to study the 'sweet spot' effect on a bench, THEN try for it on a model. Ground setting that gives an in-air 'sweet spot' might not sound different, so it is worth some attention to do it consistently.)

Diesels do, however, hold heat for a minute or so after being shut down. In a contest situation, let the judges know you will start the engine briefly to warm it up, shut it down, THEN signal for start. ...Which should be first or second flip if you get the feel of the need for any choke priming - often just choked while lifting the prop several degrees into compression, NOT a full turn choked.

Another recent finding: #2 automotive diesel fuel at the filling stations seems to burn cleaner than other kerosenes I'd used. Campstove kerosenes can cost more than #2 Diesel, and are pretty dirty burning. Even the 'collector-ring and small tube muffler' PAWs spew clear or very light honey exhaust from a fuel built on #2 automotive diesel.
\BEST\LOU

Offline Chris Wilson

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Re: PAW 35 help
« Reply #20 on: March 19, 2012, 08:53:51 PM »
MAAA AUS 73427

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 Nothing in life is so exhilarating as to be shot at without result.  It's not enough that we do our best; sometimes we have to do what's required

Offline Dennis Toth

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Re: PAW 35 help
« Reply #21 on: April 06, 2012, 04:23:38 PM »
Rob,
One thing you could check with the PAW's is the motor mount. They really like a wood mount not one with hard aluminum pad that we use with motors with machined flat mounts. If you have it on AL pads and tighten it down it can stress the case and cause all kinds of run problems as it warms up. Also make sure the fuel has enough oil in it. The Davis fuel is really formulated for ABC or AAC engines not iron/steel. The Dr Diesel old English blend should be OK or something with 29% oil, 30% either, 30% kero and 1% ignition improver. The PAW's are like the Fox 35's they like lots of oil. Also don't over load them, they are a shorter stroke motor and like their "r" s. I ran mine with and 11x6 Zinger with no problem. When setting the motor first back off the compression until it starts to miss then adjust the needle to smooth as best you can then trim the compression and do a final needle a little on the rich side. Once the setting are found you can start a cold engine by going 1/2 turn down on compression and 1/4 turn open on the needle, once started back off the compression first then trim the needle and go fly. If is was sagging you had to much prop.

Best,           DennisT

Offline Chris Wilson

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Re: PAW 35 help
« Reply #22 on: April 09, 2012, 05:51:04 PM »
Rob,
One thing you could check with the PAW's is the motor mount. They really like a wood mount not one with hard aluminum pad that we use with motors with machined flat mounts. If you have it on AL pads and tighten it down it can stress the case and cause all kinds of run problems as it warms up. Also make sure the fuel has enough oil in it. The Davis fuel is really formulated for ABC or AAC engines not iron/steel. The Dr Diesel old English blend should be OK or something with 29% oil, 30% either, 30% kero and 1% ignition improver. The PAW's are like the Fox 35's they like lots of oil. Also don't over load them, they are a shorter stroke motor and like their "r" s. I ran mine with and 11x6 Zinger with no problem. When setting the motor first back off the compression until it starts to miss then adjust the needle to smooth as best you can then trim the compression and do a final needle a little on the rich side. Once the setting are found you can start a cold engine by going 1/2 turn down on compression and 1/4 turn open on the needle, once started back off the compression first then trim the needle and go fly. If is was sagging you had to much prop.

Best,           DennisT

Dennis, I am not sure about the metal mounting pad problem being a real issue mate.
Many Team Race PAW's mounted on alloy crutches do just fine!

Another 'piece of the puzzle' could well be the venturi size as supplied.
Just recently I was informed that my PAW 40's should be running a slightly smaller size than the factory standard when used for control line stunt and this news from Tony Eifflaender himself, so its not hard to believe that the 35 should be similarly treated.

The control line PAW's seem to be delivered for general CL use and being optimized for stunt use may well need a slightly smaller venturi.

One of my 40's is second hand and the previous owner said that it constantly suffered from heat problems that were incorrectly diagnosed as DII problems and eventually needed a new piston/liner set due to damage. So it was sent back to the factory to be 'set up' for stunt and this is where the news came from concerning the venturi size.
It would seem that PAW's (the 40 at the very least) need to be held back a bit to get the best out of them for stunt use, and doesn't this hold true for stunt engines in general?
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Offline Dennis Toth

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Re: PAW 35 help
« Reply #23 on: April 11, 2012, 08:27:25 PM »
Guys,
One thing to watch on the overheating is the level of ether in the fuel. If it drops to the low 20% level the engine can start running hot. Simple solution is to add some ether. Some have also tried to run the PAW's on Davis ABC fuel which is formulated to run hot to get the ABC's to proper operating temperature. Bob Davis has stated that the iron setups could be damaged by running the ABC fuel and recommends using his 1/2A blend that has higher oil and ether.

Best,           DennisT


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