Bax

My Feedback: (11)

When cylinder head bolts loosen up in flight, or are noticed to be loose after a flight, the engine has flat-out overheated. Overheating will expand the case and head, but not the bolts. The bolts will be stretched somewhat. When the engine cools, they become loose. If one or more bolts become loose enough to allow leakage between the head and cylinder liner, the gasket will become damaged.

Most times, just replacing the gasket, doing a little cleaning on the head, and reinstalling everything properly will restore the engine to its original operating status.

Bigger engines need better airflow. That's because they have more mass to cool, and the area of the cooling fins is smaller in relation to the volume of the engine. That's why smaller engines react so well to higher-nitro fuels...they have a large amount of cooling area in relation to their volume, which means they tend to over-cool.

We've seen this on several engines sent to us. The fix has been simple, and test runs showed no harm to the engine.

alvarogn

My Feedback: (4)

Thank you I will check on the recomended props.

kochj

xeor good prop\?? I think that is how you spell..... da has this brand

F-4

Bill,

I have a 1.20 AX in an Excelleron and couldn't be more
pleased with performance. However, I am always concerned with engine
overheating. I think I understand the requirement to ensure air passes over
the head fins. But, I am not sure I understand what you wrote about not
seeing the exit holes when looking through the entrance holes. Would you
please elaborate on this. Also, is it necessary to get air to the back of
the engine for crankcase cooling? Thank you.

John

DarZeelon

John,


I am not Bill, but I have also what to say about cooling.

I believe Bill meant you should see the path of the cooling air, from the intake to the outlet, going through the head cooling fins.

If the flow just bypasses the head fins, your engine will experience over-heating.

In many cases it is necessary to fabricate baffling inside the cowl for the engine to receive sufficient cooling.

...That is, if you want your engine to last.

Bax

My Feedback: (11)

Basically, when you look into the air inlet holes of your airplane's cowling, you should not be able to see a large passage that goes directly to the air outlet. If you do, then that's the way the air will travel...the path of least resistance. You must make sure that any air entering the cowling actually passes through the engine's fins.

If you have enough airflow to keep the engine properly-cooled, you won't need to worry about airflow around the case...it will be kept cool because the cylinder is kept cool. Aluminum is a metal with excellent heat-transference properties, so if you keep the cylinder cool, the case will tend to not run particularly hot, either.

fastmachine

hi, can anybody tell me sizes of 3 blade would suit os 120 ax,
thanks...

DarZeelon

FastMachine,


I would say a typical OS1.20AX would spin an APC pattern 14x14 prop, at 9,200 RPM...

This would equate to a real output of 2.55 HP.

Now, changing to the prop constant of MA 3-blade props sees a 14x12 spun at a bit more RPM.
Also, a 14x8.5 Mejzlik would be similar, as would...

...Download the PropPower from [link=http://mvvs.nl/]Pé Reivers' web site[/link]Pé Reivers' web site (->Propellers->calculator) and do it yourself.
If you don't have an electronic spread-sheet software on your PC, you can do it directly on the web site.

Flyer95

My Feedback: (1)

Dar,

I think APC 14x14 is actually slightly too much prop for this engine and the stock muffler, but on a tuned pipe maybe 9200rpm is possible?.
When running mine on the stock muffler it would turn a APC 14x12 @ 8800-9000rpm. Now I use a quiet tuned pipe and can peak it at 10000rpm.

DarZeelon

Thank you for the numbers, Amir.

I thought using a tuned exhaust system, would be a great benefit for this engine.

76.ta

My Feedback: (39)

I have a 17 x 5 and it turns 9900 RPM on the ground. I love this engine. It seems to take a some time to idle down . Move the stick to idle and it runs a few 100 RPMs above idle for about 5 sec. then it will go on down to idle.
(Any ideas?)
Thanks,
Roy

Kweasel

My Feedback: (29)

All of the guys having problems are making the same mistake, they are overpropping and trying to hover, probably with a large tank mounted too far away. The 120AX needs close to 10,000rpm to draw fuel well from a properly sized and placed tank.

DarZeelon

Ken,


This 1.20 engine is rated at only 9,000 RPM...

It is not designed to spin over 9,500 and whatever 17x5 prop Roy is using would be, if anything, under-propping it.


Using this engine with a remotely placed tank, in a 3-D model, would get lousy fuel-draw at any RPM... unless a Perry/Iron-Bay/Cline device is incorporated into the setup.

76.ta

My Feedback: (39)

I have the tank up against the engine wall so fuel-draw is not a problem. Once again it runs great but just take some time to idle down.
Thanks,
Roy

DarZeelon

Roy,


Try leaning the low-speed mixture setting.

...If like many beginning R/Cists, you have not yet touched the low-speed needle yet...


All OS engines I have ever encountered, came from the factory with this primary fuel control, set significantly too rich.

RoysterDW

Just recently installed the 120AX into a 25% Yak54 and got maidened yesterday but went deadstick twice after about 20secs of flight.

Tested on the ground and held the nose up, all seemed fine. Waited 30secs on idle and hit full throttle and was fine. I have since lifted the fuel tanks as it was sitting under the carb.
The motor has been run in on another plane which ran very well with 20% nitro but went down to 10% and have had problems ever since. I say its not a heating prob as 15-20secs of flight is not enough for to over heat. Also when my son maidened it I told him on the 2nd flight to pin it at full throttle then I aske dhim to back off slightly but then the engine died.
Is it too rich?

Prop size is 16x8 but looking to go to a APC 16x6 or the Bolly 15x11.

DarZeelon

Roy,


You must be very well-off to use 20% nitro in AU, in an engine that should do 'very well, thank you' on just 5%.

Nitro does not solve all problems and you seem to be having one with fuel supply...

20 seconds ARE enough to produce overheating at full throttle.
Is your engine cowled-in and if so, is there sufficient baffling to direct cooling air-flow at the cylinder and the head?
Is there a large enough exit opening, located where air would naturally flow after going through the cooling fins?


Since I cannot see your setup, I don't know what the exact problem is, but you will have to sort it out.


Please let me remind you that there are two fuel controls; the high-speed needle only controls the very top of the range and the low-speed needle controls the 0-80% fuel curve. If the low-speed needle is too closed, the very top could well be affected...

Many modelers refrain from 'tampering' with the low-speed... Regarding it as a tamper-proof idle needle, in an old emission controlled car... This is wrong and even if the setting was right on the previous plane, doesn't make it right for this one too.
This needle must always be checked for correct adjustment!


Make sure your fuel-tank center-line is at the same level as the carburettor jet (with the model in level flight attitude) and as close as possible to the engine!
In a tail-dragging Yak-54, the initial mixture settings will need to cater both for the lower initial tank level, before the tail is raised on the take-off run and for forward acceleration during the TO run.

RoysterDW

Thanks for the reply!

I have 20% as I use it on my 4 strokes such as the YS110 & Saito 62 but I mainly use 10% on 2 strokes, but hey if the 120AX will run on 5% with no ill effects then I may make the change again.

I will see if I can post up some pics to show you the install. One other thing I would like to try is flying it with out the cowl.

So the OS manual says 2.5 turns for first start, where can I set the bottom end to get a starting point?

I was told you need 4 times exit space of entry space for proper cooling.

Yeah nitro is not cheap here hence my son and I fly gas as well as electrics.

DarZeelon

Roy,


It is not the mere size of the exit hole(s) in the cowl...
If you have a good baffling setup, 1.5 times as large is quite enough.

If air heats from from 20ºC to 140ºC (which is unreasonable... It would typically rise in temperature by 30-40ºC, while passing through the cooling fins), it would only expand by 45%. The widely accepted '3 times the intake' rule is nonsensical, unless (even if) that air just 'loiters' inside the cowl...

The cooling air does not 'know' where it should go... It must have a clear route, where it is forced to go by the subsequent incoming flow and forced to leave by it and by lower pressure areas outside the cowl.

RoysterDW

Heres some pics of the Yak!!

Going to tune and fly it without the cowl. Which is what I should have done in the first place.

I still dont buy it was over heating. Its winter here and theres no way the engine over heated in that short amount of time. My guess its tuning and or cowl pressure in flight.

UltimateFlyer120

My Feedback: (5)

My AX120 is on an OMP Yak 55 90 size. I run 15% cool power with 2 oz's of castor added. Master Air Screw 16x6 Scimitar. I hover on the deck without a wory in the world. Great motor, OS Standard 2 stroke quality.

RoysterDW

Just spoke to fellow flyer whos been at it many more years than me and says the deadsticks I had were due to the low fuel tank install. This week end will tell, fingers crossed thats all it was. Will fly it wih the cowl off first and go from there. Changed prop from 16x8 to 16x6 for more thrust (not speed)

Also ordred a Bolly 15x11 prop for the 120AX. Saw this prop in action on a pattern plane with a 120AX (in Bundaberg) and it had great vertical for nice long uplines!

DarZeelon

Quote:

ORIGINAL: RoysterDW

I still don't buy it was overheating...

Roy,


Maybe it was and maybe it wasn't...
If you look at your own photos; all the air entering at the right-side of the photo (left of the cowl) does absolutely nothing to cool the engine. Perhaps some of it is cooling the Pitts muffler...

Air entering at the cylinder side will tend to take the easy way out and go back and down, bypassing the cylinder and the head altogether...


Put a 'fake radial engine' in the cowl and cut a hole in it, only in front of the cylinder and the head.

Also, build a balsa (or other material) baffle just behind this 'entry hole', that would prevent incoming air from going down (forcing it to go toward the cylinder and the head).

Put another, flat/rounded baffle, angled down just behind the cylinder and the head, promoting the flow to continue down, closely following the contour of the cylinder and head, around their rear side.

Air flowing more than 2 mm from the fins cools absolutely nothing...


Go to your nearest regional airport and look closely at the Lycoming/Continental engines in light planes.
See to what elaborate lengths, the makers of these engines went, to make sure a relatively small amount of air effectively cools the cylinders.

Huge cooling intakes and exist holes; without effective baffling, is like quenching your thirst, sipping water from a lake with a very thin straw...

anuthabubba

My Feedback: (102)

I'm agreeing with Dar on the cowl/engine airflow. Looks like (in your pics) the cowl exit is smaller than the inlet. That would cause back pressure that would stagnate the flow through the cowl and actually limit the amount of airflow entering the cowl. The blanking/baffling advice would surely help promote a flow through that large, cavernous cowl.

Terry in LP

RoysterDW

Thanks for the input guys

I may have to make something simple and angle a piece of ply from the front of the cowl angled towards the carb/head. Never made a baffle so I will try and keep it real simple

I will test the plane as I mentioned earlier by flying it without the cowl and if all is good I'll add the cowl and see what happens.