Props for OS 55AX

[arun 0]

Has anyone run an OS 55AX before and have prop recommendations? It's going in a WOT4 Mk2 ARF ad will probably run on 16% Bekra fuel.

 

Manual suggests 12x7, 12x7, 13x6, 13x7 but I've read mixed reports on RC Groups etc. about how well the engine performs on these with most suggesting that they are too much load - would be happy to hear all suggestions, esp. if backed up by experience! Perhaps an 11x7 or 11x8 might be good?

 

Cheers

[Trevor 25]

Hi Arun, I ran am os55ax in a chinn yak, an apc 13x6 seemed perfect for it. Also 13x4w Went well for 3d

 

Sent from my GT-I9195 using Tapatalk

[arun 0]

Thanks Trevor, good to hear that the recommendations in the manual do in fact make sense!! Some of the comments on RCGroups suggest binning the manual. Maybe this engine is very sensitive to altitude

[Peter Royall 47]

I run 11 x 7 on an evolution 53. Bigger props have ground clearance issues.

Would be careful not to much throttle when motor cold or running in.

Mine is flying a Horizon Saratoga

Sorry my pictures are too big to put on here!

 

TTFn,

Oily .

[Gerard 10]

Arun,

 

I also have a Chinn Yak fitted with an OS55AX.

 

Have always run it without any problems on the prop it came fitted with which is an APC 12.25 x 3.75

[arun 0]

Wow, that's a proper "fine pitch" 3D prop well suited to the Yak The spool up must be excellent for prop-hanging, but probably not ideal for the WOT 4 mine will be going on! I have heard the engine "comes alive" on lower load props than the 12x7 so perhaps an 11x7 will be equivalent-ish to a 12.25x3.75 for it. I will fly it a bit and see, it's the only way!

[Gerard 10]

If memory serves me correctly, Gary gave a method of calulating prop load on an engine in one of the club magazines a number of years ago.

 

I think it was the square of the prop diameter multiplied by the pitch to give you a number which can be compared with other prop parameters using the same calculation.

 

I suspect that the 55AX is lightly loaded by the 12.25 x 3.75 prop. The yak has endless vertical climb at quite a speed with this prop!

[arun 0]

Thanks Gerard. I've always used the attached PDF to compare props - I imagine is based on a similar methodology.

Comparative Propellor Loading Chart.pdf

[Martin 0]

Looking at the numbers in the table in the PDF, it does appear to be a table of the "diameter squared times pitch" relation.

[arun 0]

Any recommendations for nitro content on the OS 55AX. I tried it on OptiFuel 20% which seems to run fine but is clearly over-the-top. Thinking of bringing it down to 12%. Thoughts, what has worked for others?

[Gerard 10]

To date, I've run everthing including the 55AX on 5% nitro with no problems.

 

[arun 0]

Thanks

[Trevor 25]

I used 10% and 5% on my 55AX and couldn't tell any difference between them.

[arun 0]

I've ordered some 12% OptiMix. Will be interesting to see how much different that is from 20%. The manual suggests 5-20% and does note that the engines are factory tuned on 15% nitro. Probably none of this is going to be game changing

[Bravedan 106]

I used 10% and 5% on my 55AX and couldn't tell any difference between them.

 

During the tortured life of my SC91 2 stroke powered Panic, it has been used on Straight, 5%, 10%, 20% Heli fuel, 25% car fuel, 30% Optimax, and intermediate mixes of the lot!!

 

Power differences were noticable, but not as significant as per the adverts!! ( ) What WAS noticable was that the higher the nitro the better the throttle response, idle quality, and lower speed of idle that could be set.

 

Just don't assume that because an engine runs OK on 10% after being on 5% (for example) that it can be left untuned for that specific fuel.

[arun 0]

My understanding of the addition of nitromethane is that whilst it increases power, also makes the engine easier to tune due to increase in available oxygen and thus fuel available for a favourable mixture on intake stroke. I fully expect to re-tune my OS55 on the new 12%, hopefully I won't lose the really nice idle I currently have on a 12x6 APC

 

it has been used on Straight, 5%, 10%, 20% Heli fuel, 25% car fuel, 30% Optimax, and intermediate mixes of the lot!!

 

Dave - given you have tried a lot of nitro mixes - in your opinion is there a "tipping point" for 2-strokes where idle speed/throttle response becomes much better? E.g is 10% half as good as 20% (over exaggeration but you get the point!).

[Bravedan 106]

My understanding of the addition of nitromethane is that whilst it increases power, also makes the engine easier to tune due to increase in available oxygen and thus fuel available for a favourable mixture on intake stroke. I fully expect to re-tune my OS55 on the new 12%, hopefully I won't lose the really nice idle I currently have on a 12x6 APC

 

it has been used on Straight, 5%, 10%, 20% Heli fuel, 25% car fuel, 30% Optimax, and intermediate mixes of the lot!!

 

Dave - given you have tried a lot of nitro mixes - in your opinion is there a "tipping point" for 2-strokes where idle speed/throttle response becomes much better? E.g is 10% half as good as 20% (over exaggeration but you get the point!).

 

OK, your para one................ Add nitro percentage and you are as you say adding an oxygen liberator, not a fuel per se, so the fuel flow rate has to be higher to cater for the percentage of total volume lost to the nitro, not 1:1 but you get the point.

 

Your para two................. From my experimentation (and this applies to my mainstream use 2 St engines from 39 to 91) it depends on the port timing and compression as much as the nitro, not in isolation. For example, the TT Redline Heli engines are supplied with a compression reducing shim to use as the nitro content rises. They must know what they are doing (?), yet I have run four of these for several years now without shimming as they say they should be on 20%. They run perfectly well, start and idle well, and I have had no issues like knocking bearings out, so I'm left wondering!!

 

I was given some fairly ancient Enyas from 25 to 49. Now these date from days when "Straight" without synthetic was the norm, and 5% was considered exotic. I have run them on 20% heli fuel. Yes they needed carb setup tuning, but they were not set anyway, so thats not significant. They ran (and run) superbly well, lovely low idles, perfect throttle response. The benefactor said he thought they were probably scared not to run!

 

Of my frequently used engines, the Irvine 39 likes 10% Contest, it does not run as well either above or below that percentage, no matter how tuned. On 10% it runs superbly with an idle so reliable and slow you can almost count the bangs (OS No8).

Below 10% and its relatively gutless with more uncertain throttle response (though still a bullet proof idle). Above 10% it still runs OK but in flight apart from a marginally better throttle response to pull vertical out of prop hang, there is not very much to show for the increased cost. That I guess is therefore your tipping point level.

 

The SC91 Panic is an unusual case anyway, as with that much cc in an ultra light ARTF frame, lack of power even on straight is never going to be an issue. With that combo its a case of extracting loony power BUT being able to land it without having to resort to deadstick approaches, so a low and reliable idle is everything. I have found that 20% gives that best, but its more idle orientated than flat out power.

 

The SC52 and Irvine 53 are unfussy and while I tended to run 10% (more from the cost angle) a recent bulk purchase of 20% for less than shops pay wholesale means thats probably what they will get, and the extra (not to say excess power ) is just a nice to have.

 

I have no direct experience of the OS 55AX in a plane. The heli 55 (and Hyper 50) I tend now to avoid. Most of my experiences with Hyper 50s has been that they are setting sensitive and eat bearings. My two didn't, but maybe thats because I ditched them for the excellent TT Redline 53s before they could. Oddly the TT engine is cheaper, much more powerful, uses the same bearing sizes, and does not eat anything!!

 

However, as I'm almost the last engine powered heli flyer in a large club with purpose heli only field, they have all gone electric, there is no more wider experience of nitro esp in heli engines to be had!!!

 

I expect your 12% purchase will be fine for the 55, and is probably as high as would be sensible to go, just be sure you do not get a lean run with it, as they don't like it up 'em, Mr Mannering.........................

[Martin 0]

On a vaguely related note, I remember seeing an odd suggestion in a book on model engines (which I think must have dated from some time when nitromethane was much harder to obtain). This suggested using a small percentage of petrol instead of nitromethane (e.g. 75% methanol, 20% oil, 5% petrol).

 

This doesn't really make sense to me, since the point of nitro is to provide oxygen rather than significant calorific input, while petrol would do the opposite. Given that as Dave notes the most noticeable effect of nitro is on quality of idle, perhaps the petrol acts in some different way to keep the plug warm or similar? Or maybe putting in a small proportion of petrol has an effect on ignition timing since petrol generally wants a lower compression ratio? Or perhaps it's just voodoo and people convince themselves that it's better.

 

Has anyone here ever tried this? It seems it would be an interesting experiment. Certainly, petrol (cheap, clean) seems a more attractive substance to be putting in engines than nitromethane (expensive, corrosive).

 

Maybe I should write to the "Wizard of Oz" who writes the engine articles in RCM&E to suggest this - he wrote a big piece about using petrol instead of glow entirely in various small 2-strokes (requiring carburetor changes to get an appropriate fuel mixture).

 

Martin

[Bravedan 106]

Yes, a small amount of petrol helped keep the poorer designed plugs "back when" hot, or so I have been told numerous times over the years, but I never needed to try it!. Modern glow fuels and plugs IMO does away with any of the advantage petrol mix with glow offered.

 

A petrol percentage offered greater range per tank, useful in some tank size limited racing formulae. I mainly did Club 20 Pylon so that was not relevent.

 

There are no doubt LOADS of forum threads around for the finding.

[arun 0]

Dave - thanks.