OS 70 heli- recap
 

OS Max-70SZ-H heli motor on propeller.
Flown in a World Models 46 size P-51 Mustang with 16oz Sullivan cylinder style tank totally foam isolated and OS bubble-less clunk. The engine was chosen because it was the largest two stroke vs. wing area in my plane that the race class allowed.


The prop drive hub with woodruff key OS #2770810. The same hubs used on the OS 60-91 FX.
The spinner is a Dave Brown 3 ¼”. No bushings were needed as the back plate registered on the short 3/8” shoulder on the prop shaft.
A Tru Turn short shaft adaptor was used TT-0519-A.
In order to allow this Adapter Nut to protrude into the prop you must ream out the front of your prop just a little.

I used a Macs header (#2860 standard) on the test stand and a ( #2846 down under) header on the plane, a muffled pipe 10 cc. (#1260) and a 90* thin wall pressure fitting (#9051) mounted just to the rear of the pipes apex.



Air temp 100*
APC 10X10 prop 16050 rpm peaked 30% Wildcat Youngblood Heli fuel
15450 rpm peaked 15/18 Powermaster
14700 rpm peaked FAI Powermaster (used to flush out engine)

The 10X10 proved to be a bit too much prop in the air when the prop hooked up so I dropped down to a 10X9 at 16,600 on the ground with 30%, this unloaded well in the air.

Recently I tested another prop and it may be faster on the stang.
Air temp 45*, Wildcat Youngblood Heli fuel 30%
APC 11X8 15,500 rich for flight.
Header length was 4” over all length, measured with a square, ( not flange face, but the
flange end, past the screw hole). Work your own length up.
I may cut down and reshape a 11X9 to a 10.6X9 as that may be the ultimate prop.

It starts and engine runs beautifully inverted, VERY low idle, whip fast rev-up to peak and no hint of loading up at any speed in-between. Much more powerful than a OS 91 FX on a tuned pipe and approaching that of a Jett 90 LX.

Buy the time its ready to run with pipe the 70SZ-H will cost you a bit over $400. For reference, a Jett 90 LX side exhaust that comes with tuned muffler will run you $400.
The Jett 90 LX F.I.R.E (rear exhaust) costs $450, add a header and tuned pipe about figure another $120.

The four stroke engine that compares to this is the YS 110S at $365 is the cheapest and race class legal in my plane. It’s heavy at 730g vs. 553g OS and 565g for the Jett, but the YS may be more powerful and it swings a larger prop at slower more efficient rpm which is an advantage if you have an airframe with some drag like a mustang.

It looked very fast this weekend (gold class?) but I want the facts.
I will be getting a Sky Trace GPS and doing some racing with some new props before I come to my conclusions, I may be switching over to a YS 110S next season.

RE: OS 70 heli- recap
 

Who should we congratulate more, the designers of the YS 110 or the rule makers of the class you are running in?
I participated in an event for many years that had just 1 engine that ever showed up in the winners' circle. I used to look on with great interest at those who experimented with alternatives, to me those guys were the "real" hardcore hobbyists.

RE: OS 70 heli- recap
 

I think the rule makers weren’t thinking about a supercharged four stroke coming along.

RE: OS 70 heli- recap
 

They will after you buy one ;)

RE: OS 70 heli- recap
 

I was referring to the case supercharged YS 110’s that the herd is currently running in gold. YS did build a heck of a motor, so far ahead of the other four strokes in terms of power but I see some guys fighting to keep them running nice all the time. About 5 years ago I planted YS 91…..DEEEEP!!! When I lost a receiver.

RE: OS 70 heli- recap
 

Racing really reveals subtle differences, especially when you are in a heavily regulated class. I don't know what the .70 looks like inside, but there is always one area I look at for flow improvement and it is the pathway leading up the transfer port when the piston is near, at and slightly past BDC. Some engines will benefit from notching the counterweight, or the piston skirt to unshroud this channel for improved transfer. Some engines will show an explosive increase in power. Give it a look next time you've got the backplate off.

RE: OS 70 heli- recap
 

Good info CP, I’ve seen what your taking about.
Amazing what grinding can do if it’s done right.
On one of my OS 55AX’s, I did a bit of cleaning up on the inlet port on the crankshaft. I didn’t change the timing just radiused it all around. Than I radiused the bottom edge of the transfer ports on the outside of the sleeve. When I went to run it I found the engine was struggling and the power was down. What had happen was the engine had increased the volume of the air/fuel charge into the combustion chamber which raised the pressure.
I had three choices, raise the exhaust port, shim the head or drop the nitro content.
I went with the simplest, I tried FAI fuel and it worked great, the engine turns the same prop as it did on 15% now 500RPM higher on 0%.

Don’t try this unless you know what you are doing and are willing to replace what you may wreck.

RE: OS 70 heli- recap
 

There always seems to be a balance that can't be upset too much. I don't think there are any tricks that OS isn't aware of, just whether or not they can do detail work that is cost effective. For a big company like OS, having engines that are too powerful means having service related issues that they don't need.

The work you describe on the 55AX leads me to believe that turning a couple of different head buttons would be worth looking into. You can bet on it.

RE: OS 70 heli- recap
 

Good info on the 70H engine. Sounds like it is running very well :)

More than a few "retired" heli engines being used out there. Exspecially the "turbine" approach higher rpm engines that followed the "torque" engines that were the rage in the early 1990s.

Worth noting, the YS80H will perform along the same lines.

RE: OS 70 heli- recap
 

It’s been a while and quite a few gallons, so I thought I’d give you an update.

The engine just started making noises when the spinner was flipped briskly by hand off compression. It would make the noise twice as the shaft changed directions as it bounced off both sides of compression. It makes a squeal/chirp sound, like there’s an angry gerbil is inside.
There is no visible or felt play axially or radially of the prop shaft. The bearings feel smooth and not gritty.

I think what is happening is, there is a small clearance in the bearings now that allows the ball and cage assembly’s momentum/mass to momentarily not match speed or direction to the races and they squeal and bounce as it does. The sound can also briefly be heard as an electric starter is energized, than the engine sounds normal after that.
I just hope the bearing races aren’t spinning on the shaft or housing. I’ve had quite a few engines but never had one make this sound.
I think a bearing change is in order.

I tried a new prop today but first I tached it and the power was down a bit, but it could be the questionable fuel freshness, not my normal glow plug, the 94* heat or bearings???
My normal prop is an APC 11X8 at 15,500 and today it was at 15000 flight richened.

An 11X10 APC was installed and it tached 13750 flight richened, a bit lower than I expected. The surprise was a few moments after takeoff it got on the pipe hard. Holy Moley it was fast! The engine seemed to be unloading to almost the same rpm as the 11X8, I’m guessing about 16000 and didn’t sag in hard turns.

RE: OS 70 heli- recap
 

Hate to tear down an engine that's making good power, but squeals and gerbil sounds usually don't go away by magic, do they? Does it still make the sound if you remove all the prop drive stuff hanging off the front? You figure a helicopter engine would consider life in a pylon racer pretty special duty and not give you any trouble?

RE: OS 70 heli- recap
 

The biggest difference between fixed wing and helicopter engine use ...... is the prop and the shaft side loading.


On a helicopter, thin output shaft, smaller diameter flywheel, and no real crankshaft loads. (marine application is similar). Bearings handle the roational component mainly, with the rear bearing dealing with the power stroke.

Ducted fan - small diameter fan, low mass, bearings see some decent sideload due to gyroscopic force (because of the high rpm and energies)

On fixed wing prop - large diameter prop, HUGE gyroscopic forces bending the shaft sideways when you manuver. This translates to big forces being applied to the bearings (front bearing in particular).

If you look around the internet a bit, you can find a few good videos and stories of aerobatic planes (full size) losing the prop and crankshaft from doing extreme aerobatics. That is why the current generation use very-low-weight composite props to keep the spinning mass down.

Bob

RE: OS 70 heli- recap
 

the hyper 50 uses a plastic cage on the rear bearing for longevity, but i dont know about the 70, i bought a os sx-h WC new in the box last week for a lil fun with pipes etc, it has alot of potential, it has higher exhaust timing.
ing as well as more crank timing along with a 10MM bore two needle carb, today i will be running it in

RE: OS 70 heli- recap
 

FWIW i wanted to have the throttle arm closer to the carb like a fixed wing engine, and was going to use a nosegear steering arm, which seemed rather lame, well the carb throttle shaft is .256 in diameter, and the local LHS had some super tiger carb throttle arms in stock, the slide right over the shaft and fit perfectly, yay

RE: OS 70 heli- recap
 

Good to know, I did it the hard way, machined it back.

It does use a plastic ball cage on the rear bearing.

RE: OS 70 heli- recap
 

do you remember what thread diameter it was to retap the shaft? i have some metric taps 3MM etc

RE: OS 70 heli- recap
 

I flew it again today and several onlookers commented that it visually seemed a lot faster today, they didn't know about the prop change.
Remembered to install the K&B HP plug this time and used fresh 30%, it made an improvement.
Pulling the engine before the bearing lets go.

Now if I can just land the darned thing, had to do eight go-rounds, about a 10 mph cross wind and this thing has a narrow flight envelope in that situation.
Too fast and it'll shoot off the runway, too slow and the downwind wing will stall and the plane will snap roll just off the deck, been there-done that[:@]

I have the CG at factory and unless you grease the landing it'll bounce 8 or 9 times. I'm thinking of moving the batteries up to the front of the wing, more nose heavy instead of neutral.

It's one of the worst planes to land in a cross wind, makes a large wing area, high wing, tail dragger look easy, but it's not bad to land with a head wind.

RE: OS 70 heli- recap
 

Not off hand. The shaft was heatreated about 35-40 RC by the way. I like your way next time.

RE: OS 70 heli- recap
 

ran my 61 sx-h wc today, it has a bit of intake reversion at 1/3 throttle, but has a flawless idle, an the transition took about 7-8 clicks open on the middle needle, and 1-1/2 open on the high speed needle, i used a nelson ultra thrust for a muffler, 15% coolpower with castor added. i was given some wood master airscrew 11x7's so i used one on this first run, turns better than 14800 with brief top runs on first tank still rich and juicy, going to fly this ona trainer tomorrow to get some time on it, i regret not buying the 70 sz for 70 dollars on runryder, but this one was 70 NIB shipped. this engine ran very cool during this first run, which was assuring and no black or silver crud from exhaust either, plug was like new afterwards. i did find that dave shadel made a sleeve for this engine, it is spec'd with 165 degree exhaust timing, for heli use lol;) nice info for me