rcbence

Hello. I recently got my escapade mx/gp in the air the other week for first time in at least 3 years. Probably been even longer. Now I remember not having much time on the new engine before I put it away a few years ago. Don't remember exactly how much. Its an OS .55ax

When I tested it on the ground at first I apparently didn't check the mid-range very good. I started with a 10x8 apc prop and flew with that. My first flight lasted a bit and I could tell midrange was suffering rich. It finally dead sticked and I landed safely although a bit hairy. What I noticed is that I was about 2 turns out on HS needle and it was fine at WOT. But when at mid throttle for just a bit and then opening it up it cuts out from being rich. I leaned it more and more until it just started to clear out from being at half throttle for a bit without dying and choking. I flew like that just fine on the second flight. It bore a hole in the sky with very fast unlimited vertical with the 10x8 prop. My HS setting was about 1.5 turns out total and about .5 out from where it lean sagged past peak setting. At about 1 turn out it lean sags past peak. Now the WOT rpm is the same from there all the way to about 2 turns out. I checked with tach that confirmed what I was hearing. It takes all the way to 2+ turns out before rpm drops 200-300 rpms. Almost 1 full turn of movement with no change in WOT rpm.....but..... definitely changing the mid-range. Definitely getting more fuel but at WOT it seems to take it with no effect. So my second flight I tuned it based on mid range response and not WOT rpms....since a 200 rpm drop at WOT leaves me with a midrange that instantly chokes it from being there for 2 seconds and throttling out of it. Actually it will cut out when at half throttle with no drop in RPM at WOT. At 1.75 turns out the RPM at WOT is the same RPM and as high as it will go without a 200 RPM drop, and it still cuts out in the mid-range at that setting when throttling out of it.

Is this what OS means by taking a few flights before it "takes on a peak needle setting" in their manual? I'm not remembering this ever before but maybe I forgot or never noticed years ago. If I base my tune on 300 RPMs less than peak its not flyable unless I'm literally at WOT the whole flight or snap back to idle and snap back to WOT bypassing mid throttle completely. I know the high speed needle controls the total fuel flow throughout the whole range. But especially from 1/4 and up.

It's able to take tons of fuel at WOT. Is this engine behavior the symptom of an engine that's still tight?

I ran an 11x6 just on the ground today with same results as 10x8 for how it tunes. No change in top end RPM for almost 1 full turn on HS needle. Highest I saw was 13000 rpms with 11x6. I think the 10x8 spun noticably faster but my tach reading was questionable the other week. That 13k was without baffle and with a hobbico deflector. I did a quick check without the deflector and it seemed to actually drop roughly 200 rpms. So it seems to like the deflector a little more right now. Also I am able to choke it down at WOT as I go past 2 turns out. Fuel flow isn't restricted.

I'm thinking there is even more in the engine if this is just a symptom of a tight engine still.

10% nitro 20% castor/synthetic
OS #8 plug
A touch above sea level.

In short, if I richen it enough to drop RPMs by 200-300......
(2 turns out) The mid-range will load up so quick and flood instantly while opening the throttle after being at half throttle for 2 seconds. But at WOT it sounds normal and no RPM drop or change and it takes that rich setting at WOT. Only way to fly it is to tune it leaner until it doesn't choke at half throttle so I don't dead stick. (1.5 turns out)That puts me almost .5 turns out from the lean/sag setting. (About 1 turn out)

​​​​​​If my tach readings were correct I got it to a reliable idle just under 2,000 RPM. But 2000 was about average with 11x6 apc. I wasn't facing the sun but I was pointing it slightly upward toward the sky. Both brand tachs read the same.

My reference point from the closed point on the needle might be a little off I'm not sure. But the numbers should be really close to reality.

Thanks for any help or confirming what I'm guessing this is.

A. J. Clark

Balance your prop and check for air in the line at high speed.

Propworn

My Feedback: (3)

You may have lost your initial setting moving the needles back and forth. Try this, you will need a short piece of fuel tubing, a 1/16th drill bit and your screwdriver of course. First back the low speed needle out (counter clockwise) a few turns. Carefully turn the high speed in until it stops. Now turn the high speed needle out 2 turns. Insert the drill bit down the carb and carefully close the barrel of the carb gently until it just traps and holds the drill bit. This replicates the approximate idle position of the barrel of the carb. Place the fuel tubing on the fuel inlet on the carb and blow through it. You should hear/feel air passing through the carb. While blowing air thrugh the tube slowly turn the low speed in until the air passing through stops. Slowly back the low speed out while trying to blow air though the carb until you can just detect air passing. This closely replicates the idle position of the barrel of the carb and the amount of fuel at the idle position. Reconect the fuel line and start the engine. Adjust the high speed needle at wide open throttle then adjust the low speed for best transition and idle.

Hope this helps

tedsander

After the above, do the pinch tests:

At high speed, use whatever will allow you to safely pinch shut the fuel line going to the carb.
If it dies immediately, you are too lean. If it keeps running and starts to pick up rpm before dying, you are too rich. Ideal when fully broken in is no gain in rpm, but a slight bit when newer is OK. But not dying immediately.
Do the same, with it at the lowest reliable idle you can get. Same adjustments. You don't want it to pick up any rpm, yet not die immediately.
After, see if you can lower the idle, and test the low again. Work your way down until it gives a satisfactory slow idle. When not fully broken in, that may be higher than you can get later after full break-in.

rcbence

I have it reset blowing air in the fuel line with my low speed starting a good bit leaner than where it was. I lowered the idle gap some too. I will check for bubbles in the fuel line also while running. Hopefully tomorrow I can retest it on the ground if it isn't raining all day. Prop is balanced. Thanks for the suggestions. Having such a wide tuning rage for the high-speed needle definitely threw me off. Even though I flew it once already, I'm not sure I'm comfortable not having the high-speed needle to where I know it's lowering the RPMs just a touch to make sure it's rich enough. I never noticed any sagging or a power loss.

the Wasp

My Feedback: (1)

Dude. an APC 11x6 is too small for the OS 55. not enough pitch. you need more pitch to load the engine. move up to a 11x9 or 11x10. or a 12x6, 12x7. I say an 11x9 or larger because the older OS 50sx loves the APC 11x8 and 12x6, some people run a 13" prop on their old 50sx.. so for the 55 move up.
note that I am talking only about APC props in this post.

remember, there is no flywheel on these engines to load the engine, the prop is the flywheel. know that more pitch ONLY loads the engine more. while a longer prop produces more flywheel effect. the longer 12" props will smooth-out your engine. so if you have-to use an 11" prop rase the pitch above 7 to load the engine more.

Jim

1200SportsterRider

Thia from the instructions:
10
BASIC ENGINE PARTS Cylinder head
Carburetor Type 40J
Crankshaft
Propeller nut
Propeller washer
Drive Hub
Crankcase
Cover Plate
Glowplug
Beam Mount
11
Tools, accessories, etc. The following items
are necessary for operating the engine.
BEFORE STARTING
The choice of propeller depends on the design
and weight of the aircraft and the type of flying
in which you will be engaged.
Determine the best size and type after
practical experimentation. As a starting point,
refer to the props listed in the accompanying
table. Slightly larger, or even slightly smaller,
Propellers
12x7-8, 13x6-7Sport
props than those shown in the table may be
used, but remember that speed the propeller
noise will increase if blade tip is raised, due to
higher rpm or if a larg

rcbence

Well I got it running and my balance for the needles was off. My low speed was too rich and I had to compensate by leaning out the high speed. Hopefully I didn't hurt anything on the flight and some run time on the ground I had with the previous setting.

I was able to lean out the low speed and start dropping the idle and then tune the high speed until there was a little RPM drop. Now it's clearing out from half throttle very nicely.
I noticed it tuned differently with and without the deflector. With the deflector it hit my 12700 (11x6) peak quickly as i leaned it to peak. Then leaning it any further the RPM just stays the same until the sag point which is pretty far away from where it peaks out with the deflector.

Without the deflector I had to lean the needle more to peak out. It was just shy of 12700 most times. Over a quarter turn leaner than with deflector to hit peak. Without the deflector the peak point was closer to the lean sag point.

I was able to fine tune the mid-range with or without deflector. I think I like how it runs with the deflector a little bit better. It throttled up after long idling nicely. With the deflector on after it hits peak I could lean it quite a bit and the RPM just stays exactly the same until the sag point. Without the deflector small RPM changes were noticed by moving the needle quite a bit until it hit close to 12700. With the deflector once it's too rich smaller movement of the needle makes a bigger difference until suddenly it just peaks sharply within a much smaller range of movement. Something different going on with the pressure and I'm not sure exactly how that's working. But engine is running great

Thanks for the help.

the Wasp

My Feedback: (1)

what glow plug are you using??

Jim

jaka54

Hi!
As The WASP said! 11x6 is too small for an OS 55 for most sport airplanes.

A. J. Clark

Works best to adjust the high speed needle first. If the HS/LS are far apart on adjustment then you'll have to work the two back and forth a couple times.

rcbence

Os # 8 in first post.

When I flew it with the needles not balanced I used a 10x8 apc and it definitely wasn't turning 16000 rpms with that prop. It flew fast with unlimited fast vertical. No lack in power. I believe RPMs would be somewhere in 14k range or so.

It was not quite 13000 with an 11x6 apc. Why would I want to load the 2 stroke down to 9-10k rpms? What gain will it be over a prop thats in the power band making close to peak power? A 13 seems huge in my own mind. Not to mention much less ground clearance on a somewhat bumpy field.

I've read many say that the OS recommendations are too big overall also. I suppose its going to be personal preference.

What are the actual downsides not having RPM even reaching 16,000 RPM where it's rated for its power with a smaller prop? Less heat and load? Is the port timing any different than a .46? From some searching it seems it would be under 10k rpms with a 13 inch prop. Isn't that pushing it for load and making it run hotter?

​​​​​​I have 11x7 also available to try. I think I have one 12x4.

It runs really well on the ground now with a very low steady idle.

Propworn

My Feedback: (3)

I was with the university of windsors sae aerodesign team for 9 years as the advisor and pilot. During that time we had use of the small wind tunnel to do dynamic testing on different engine and prop combinations. We found that the numbers given by most 2 stroke manufacturers was unrealistic. Unless the engine was purpose built for high rpm like the Jetts we used in the final years I was involved max usable and dependable rpm was indeed in the vacinity of around 11,000 to 12,000 static on the ground. Some seemed more comfortable in the 10,000 to 11,000 band static (on the ground). These may unload in flight increasing as much as 1000 additional rpm.

We found the higher or lower you forced the engine the harder to tune and get to run reliably.

This is just my personal findings using APC props, balanced, sharp edges sanded smooth. Gave best all around performance.

the Wasp

My Feedback: (1)

first of all. an engine is most efficient at it's peak torque point (not it's peak RPM and not it's peak HP point). and you are flying sport. not racing. so, over spinning a 11x6 at 13000 or higher is wasting power and fuel.

OS RPM rating:
if your OS manual tells you, that your engine will run up to 16000 that number is a suggested peak number for that engine for those people using a Tuned Pipe and Racing Fuel of 30% nitro and higher.

another ting. as I said before, more pitch, and or longer length will load your engine more, this will help you to have a wider tuning envelope, making it easier for you to find your Idles sweet spot.
do yourself a favor and put an APC 11x9 on that engine!

BTW, I hope after you find you engine's peak RPM on the grown you richen it up 3 or 4 clicks before you fly. not doing so will allow your engine to unload in the air and lean out. this will damage your engine.

I will ask again "what glow plug are you using?"

and what Nitro percentage?

Jim

the Wasp

My Feedback: (1)

thank you Propworm.

1QwkSport2.5r

A .55AX is a bored out .46 which was a bored out .40. They don’t like to scream, but they sure as hell are not times to run 9,000rpm with a 13” prop. An 11x6 is probably as big of a prop as you’d want to use load-wise. A 12x5 would probably work fine as well - shoot for a 12-13k ground peak. For a little more speed, shoot for a 14k ground peak.

On carburetion: set the high speed first. If you have a tach, it’s easy. Find the max number, then open the needle so it has a noticeable drop in rpm but still runs clean. I’m of the belief that 400-500rpm will yield a happier and longer lasting engine than just a 200rpm bump rich. YMMV. Once high speed set, set the low speed for a solid idle and clean transmit ion after idling for 10-15 seconds or so. Sub-2,000rpm idle speeds are not fast enough to get a proper idle mixture setting. Set your low speed mixture with an idle rpm of 2,500 to 2,700rpm. The airspeed inside the carb at sub 2,000rpm isn’t fast enough to get an accurate setting.

the Wasp

My Feedback: (1)

not so^,
the 55ax has a longer stroke and a larger bore.

the 46fx and 46ax have a stroke of 19.6mm. and some OS 40s had a shorter stroke.
OSMG0548-Manual-EN.pdf (horizonhobby.com)
OS Max 46FX (sceptreflight.com)

the OS 50sx and Hyper 50 have a stroke of 21.5mm. that is very much close (1.9) to a 2mm difference. their bores are 22mm.
OS Max 50SX (sceptreflight.com)

the 55AX has the same stroke as the 50sx, 21.5mm. page 44. it's bore is 23mm. so the 55 is a bored-out 50.
OSMG0557-Manual-EN.pdf (horizonhobby.com)

Jim

1QwkSport2.5r

If you want to nitpick the minute details, fine. But what I meant by my post, nitpicks aside, is the crankcase is the same size or minutely different from a .40/.46 case. Just because it’s 1cc smaller in displacement than a .61 doesn’t make it capable of swinging the same lumber a .61 will swing. The .50 engines are no different. Larger bore (bored out) and longer stroke will certainly yield more displacement, but the crankcase is no bigger. But go on. Let’s continue to reason why it’s a good idea to put oversize props on an engine that shouldn’t really have said props used despite the manual stating as such. Let’s see a good needle setting and proper throttling characteristics from an engine that is overloaded and running outside of the power band it should be run in. Remember, those elephant size props are only recommended for noise constraints, not for getting the best, or rather, proper performance from the engine.

the Wasp

My Feedback: (1)

dude, I was not talking about case size, you weren't ether in your first post. why are you now????
the 50 has almost a square bore/stroke combo, the 50 is a bigger engine, ever think it could have a higher port timing than the 40/46, read the test.? the Hyper 50 uses the same liner/piston/rod and combustion chamar as the sx, with a lager carb. I have a Hyper 50 in my P-40 with the sx head, and stock Hyper 60L carb and stock type muffler. it turns an APC 11x8 at 12,500, tuned at 12,000 for flight, burning 15% nitro. rather 5% or 15% the 11x8 is not too big for the 50sx. go back and read the repost in my link. the test suggests the larger 50sx has 1/3 more power than the 40/46.

OS Max 50SX

I'm sorry you think I'm a nick picking jerk.

1QwkSport2.5r

Yes, I was talking case size. How many .50/.55 engines are in a 10cc case?! A .40 cased engine, regardless of physical displacement, cannot breathe nearly as well as a 10cc case can. The point I was trying to make is a bored out (and stroked since we are being detailed) .40 case engine will not swing the bigger props a 10cc would normally turn mentioned previously. When you over-prop an engine, the needle setting gets incredibly sensitive. It happened just a week or two ago; a fellow was having a hard time getting (IIRC) an Enya CX tuned. The midrange was all sloppy. With a 13x6 prop on a .45-.50 engine, the idle speed was too low and top rpm too low to get a proper needle setting. If the prop one uses gives reliable and adequate performance, then the variables are all “right” for the application. If a good standard OS engine cannot be adjusted (especially a new(er) engine), there is something wrong; whether it be the fuel system, the prop load, or exhaust system leaks/inadequacies. First thing I’d be doing is change the prop load. A lighter load will almost always clear up midrange issues in OS engines. Proof the dingleberries that wrote the manuals weren’t the folks that designed the engines. This is all assumed that the proper length glow plug is being used and of the proper heat range for the fuel being used.

A 13x6 or equivalent has no business on a 7.5cc case engine. That’s a .75-.91 size prop.

the Wasp

My Feedback: (1)

I agree a 13x6 is too big for a 40 size engine. but a hotter plug most likely would have fixed that^ (45,50) problem by burning off more fuel.
BTW, the largest prop OS recommends for their 35 is a 12x6.

jaka54

What nitro content do you use if you cannot reach a good idle using a 13x6 prop on a .55 OS engine.

1QwkSport2.5r

A 13x6 is too big for a .55 engine (.40 sized case) regardless of fuel used unless it’s a diesel.

Propworn

My Feedback: (3)

I am 72 and have been fooling around with IC engines since I was 15 years old and electrics before there were brushless. I have no degrees or any other qualifications than time in the hobby.

Firstly I have found that in most cases the average nitro engine for your average plane likes to run in the rpm range of 10,000 to 11,000 on the ground or static. It will unload and rev a bit higher in the air. Some of the nitro engines like for racing etc will like a higher rpm and the larger ones same as gas will like a lower rpm. Listen and observe heat generation and the ability to tune as a guideline to the rpm that makes the engine sound good. You should with a bit of practice be able to recognize an engine that is loaded and working too hard or one that is too lightly loaded, and the rpm is way up there. These two situations usually shorten the engine life.

As has been mentioned picking the right glo plug/spark plug should be your first concern. I have tried many with varying degrees of success. A lot of the glo plugs are produced by single companies and branded for sale under a different banner. I have reduced my choices for nitro two strokes to the McCoy MC14 and the OS F plug for four strokes. I have a bunch of others including the Mirical Plug even OS #8 etc, but I usually offer them at the field when someone burns out a plug and has no replacement. I know some of you will tell me the McCoy plug is a heli plug however it is one that works for me in all two strokes. Give excellent transition, low and high speed. I use the OS F plug for my four strokes and have a whole card of YS plugs that are OS F plugs rebranded for YS engines. I bought these instead of the OS F because they were a few dollars cheaper when sold under the YS brand. Spark plugs well I throw away the Chinese plugs that come with most motors and replace them with Champion as they are the easiest for me to buy. The bigger plug will be a RCJ6Y and the smaller one is a Y82. I have a few spark plugs with a 1/4 32 thread I cannot remember who makes them though I think it's also Champion. I do not have issues with glo plugs or spark plugs.

I use a good quality fuel 15% nitro and 18 % pure synthetic oil in all my bearing engines. In my older bushed engines I will buy fuel with a mixture of caster/synthetic mix and add a few ounces of caster. In my gas motors I run pure synthetic or regular petroleum base 2 stroke oil as recommended by the manufacturer with regular gas. All of my carbs have had the gaskets replaced with the newer versions that are alcohol resistant. I have even tuned some of these engines to run on Colman camp fuel (white gas) for some who have allergies to the smell of gasoline.

Unless I have a special purpose like speed or high static thrust I use either a 6 or 7 pitch prop and adjust the diameter of the prop, so I run 10,000 or 11,000 rpm at wide open throttle. I find this provides very good static thrust and good high speed even for war birds. Large nitro and gas I usually run 8 to 10 pitch and adjust the diameter for the recommended RPM.

If I am running electrics I figure out what I would run if it were an internal combustion model, prop pitch and diameter then pick an electric motor that will turn the same prop. If a trainer with a .46 nitro motor turns an 11X6 and flies the plane well then an electric combo of battery and motor that turns an 11X6 the same rpm should perform the same or very close.

Like I said this works for me and I have been flying all sorts of models over the years. If this helps anyone then all is good give it a try to see for yourself.

Propworn

My Feedback: (3)

This is an OS .46 with an 11X5 prop