MartyH

My Feedback: (16)

I have a need for a 90 size 2 stroke glow engine for an airplane project and have an OS91 SX-H blue head heli engine on hand. Any reason I can;t use that in an airplane application as long as the shaft is long enough?

earlwb

No reason you couldn't use it. If you can get a prop on it, it'll work.
Now the carb is designed to be a little on the rich side through the mid-range. With heli's they use the mid-range throttle settings most of the time.
So worst case is you might have to change the carb out for a airplane carb, if it is too rich for you in the mid-range throttle settings.
But usually that isnt a problem.

NikolayTT

We had a discussion on this sometme ago, maybe you can find it with my name on RCU, where we came to
the conlcusion (briefly here) that it is OK if you are going to run the engine on ts higher end of the rpm range
because the porting of the heli engine is designed to deliver max power at highiest rpm while the airplane
engines porting is designed for broader range rpm in the upper half of the useful range. I.e. if you need
and engine for speedy plane (racing) then a heli engine might be a good idea. Mechanically, check when
you tight the propeller if you can still rotate freely the crank - that might be not(!!!) possible for just any heli engine
and then you need to add in specific place(s) a well designed washer; othewirse ball bearing(s) might go
in just couple minutes off...
Cheers,
Nick

MartyH

My Feedback: (16)

Found a heli guy who recently flew his OS91 heli engine on an airplane. He was turning 13,000rpm on a 12 X 8. Sounds like I'm in business! I'm going to order a prop drive washer and Pitts muffler designed for the OS91FX. The muffler dimensions match and I'm simply guessing that the drive washer will.

NikolayTT

The drive washer might be the problem; it is better maybe to check arround with someone who has
an .91FX. I believe you should place a thin (how thin, that is the issue) washer behind the front
ballbearing in oder to fix the propeller. Have a look at the assembly of .91FX: it has in front of
the engine and behind drive washer another one washer, black one which fits the inner ring of
the ballbearing so when you tight the propeller the effort is taken there... Of course everything
depends how is made the crankshaft of your heli engine. Anyway, the carb of the heli engine
is better than the one on .FX. As for mufller - yes it mathes OK. As for the propeller, 12x8
seems a bit too small for those engines, maybe better is 13x8 or even 13x10 and run it on
resonance pipe or open ehxaust with 10% nitro; should get at least 13000 on 13x8 on the
ground which will "translate" into about maybe 15000 in the air, while with 12x8 you might
overkill the engine by going to even near 17000 in the air...that is a way too much I think.

freakingfast

My Feedback: (3)

The drive hub OS part number 27708010 will fit the OS 61FX, 91FX, as well as 70 and 91 size OS heli engines. It also comes with the key (dont drop it, it's small)[:@] You will need the thrust washer that should have come with the heli engine.

freakingfast

My Feedback: (3)

You may find that you will need to close the mid range screw on the carb all the way in when you run a prop. You will lose a lot of potential power with a Pitts muffler.
If you can run a tuned pipe, do so. A Jett tuned muffler would be a good second choice.

Be aware they make an ungodly amount of power but there's no free lunch, it'll drink at least twice what a 91FX uses at full power.
On a clean plane, I'd try an APC 11X12 pattern that should be 15K or so, on a tuned pipe.

MartyH

My Feedback: (16)

I have the thrust washer on the engine and the prop drive washer is on the way. Thanks for confirming that it will fit!

Can you expand and maybe refine your comment that this engine will use TWICE the fuel that an airplane 91 engine will? I dont see how thats possible. This engine is slated for the new Great Planes Revolver so it shouldnt see a lot of full throttle use.

When you say the Pitts will cause a loss of power, are you saying it will lose the typical power a Pitts muffler causes or are you saying it will lose MORE power because it's a heli engine?

For prop selection, I'm going to load it with the appropriate diameter with an 8 pitch to start. I don't want to fly this thing as fast as a pattern plane unless it proves necessary to draw clean lines.

asmund

I`m doing it myself these days. I just built a World Model Fun World EP, and instead of electric power I installed an ASP .36 heli engine. It runs strong and I think the only difference to the plane version is the head, but timing could also be different, I don`t know??
This was a simple and very straight forward istallation, with no issues with the mounting of the prop. I think that as long as you get the prop mounted already it will be fine to use your OS heli engine in a plane. I actually use the OS 60L carb from a OS Hyper .50 heli engine in my .91 FX and that carb is fantastic on my aeroplane engine

Kweasel

My Feedback: (29)

Fuel draw will be almost nonexistent below 15000 rpm. Find a carb with a 9mm bore or use a pump, good luck with the mid range adjustments.

freakingfast

My Feedback: (3)

With a tuned pipe on 15%, a 91FX will max out at about 2.4 hp regardless of factory specs.
With a tuned pipe on 30% a 91 heli will make about 3.8hp. Fuel consumption is not linear to the power produced from the same displacement but two different timed engines. The higher nitro example wont help either. Yes it will run just fine even on 10% with some power loss.
If your not going to use full throttle, you may fined the carb may be rich at part throttle. Usually someone that stuffs a heli engine in a plane is a sick speed freak like me and flies at two speeds, full or idle. The heli is heavy, thirsty and hard on airframes, so you will need to beef up that Revolver in the right places or it will self disassemble in flight.
Are you sure you wouldn't be happier with the new Evolution 60NX?


Both. Pitts are not tuned. Some heli mufflers maker have striven to tune or reduce back-pressure.


Than try the 12X8 you mentioned. And 13K would be conservative even with a Pitts. FYI, It would be about 15200rpm with a tuned pipe.

NikolayTT

91FX gives even more than 2.4bhp, but it is more essential to check the thurst I believe.
If you use the right pipe & propeller, which in other words means to settle with the right RPM
which is not maybe that high as 15000, after all 91FX is not that high-rpm machine as some
other .91 are. I heavily hesitate the sense of using propeller on .91FX or .91Heli which is smaller
than 13". Surely the engine will screem like the End-of-the-World is comming at 16-18 k RPM
but I wander how much this is uselful on this Particular Airframe; after all it might even not take
off easily, or even if takes off the induced drag of the frame might be too big for
such small diameter propeller so the speed-and-flight will be quite likely disspointing
because the thurst even at 16000 might be not enough; i.e. we are just talking again
and again abut how the Engine+Propeller match the Airframe...

As for .91FX here is what I get steadily repeated performance (nearly 3.1 bhp)
http://www.mvvs.nl/prop-power-calculator.xls

(a copy from
http://www.rcuniverse.com/forum/m_92...tm.htm#9253171

Engines: two OS.91FX just broken in, and two because preparing for a fast Twin.
Fuel: 11% Nitro, 3% Aerosave oil(synth, Fuchs), 17% Castor oil, rest methanol,
Temperature 24 deg.C, 45% hummidity. Static thurst by Pe's Calculator and
by Fish-Scale were giving very much same results (+/- 50 g). I used the
fish-scale because wandered what coefficient to place for G-sonic and it
seems that 1.2 as for ordinary APC is the right one. The RPMs are the
max-possible measured continuously within couple minutes while playing
with the needle.
1. OPEN EXHAUST:
APC 13x10 gives 10750 rpm, Static thurst = 5.7kg
G-Sonic 13x10 gives 11300 rpm. Static thurst = 6.4kg
2. RESONANCE PIPE(Macs) after being tuned for each(!) propeller:
APC 13x10 gives 11200 rpm. Static thurst = 6.2kg
G-Sonic 13x10 gives 12050 rpm. Static thurst = 7.15kg
The difference between the engines was in the range of less
than 50 rpm on the same propeller and varied while
playing with the carb needle to get to the max RPM possible.

Cheers,
Nick

MartyH

My Feedback: (16)

This thread is starting to read like a bunch of academic theory. I'm going to go forward with this and see how it works out and will report back in the Spring when the airplane will be flying. There is no question now that I can get the components necessary to properly mount a propeller and spinner on this engine. There is equally no doubt that even an ill tuned, sloppy mid-range OS91SZ-H 2 stroke will pull the sub-10# Great Planes Revolver around. What remains to be seen is how well it can be tuned at partial power I guess.

freakingfast

My Feedback: (3)

Nikolay

Ive done exhaustive testing on the 91FX and it is a very good engine and I see you are familiar with it as well.
I will use your best example: APC 13X10 @ 11200, using Pe's calculator that would be 2.55 hp IF it was a sport prop, the problem is that prop is a pattern prop so you must use a lower constant than 1.2.

I've had higher HP numbers with Master Airscrew's S-2 however, when I check them with a pitch meter they are all over the place(low), so I wont use the numbers.
Do you know the constant for the G-Sonic props?

The props I listed for the 91heli for the given exhaust should be well in the range for ground and un-load. With a tuned pipe the 91 should be 14200 to 14800 on the ground. The over square prop wont unload much.

A 70 heli should be about 15200 to 15500 on the ground.

Marty, not theory:

asmund

Why? As I said I use the 60L carb from OS .50 Hyper (10mm).
That helicarb is working extremely well in my .91 FX at "normal" rpm range and I use this engine only on 3-D planes and I really depend on great throttle control and clean running when down on the deck where hovering is done at midrange.
Maybe the carb on the OS91 SX-H is even bigger than 10mm? I really don`t know. if not I see no reason why it shouldn`t work fine for him too at well below 15K

ChrisAttebery

My Feedback: (15)

I used the 91 heli carb on a Rossi 45. They draw just fine.

asmund

Then go for it! I`m looking forward to hear what rpm numbers you get on the heli engine and compare them to my own .91 FX. That will give us an idea of how much stronger than aeroplane engines the heli versions really are[8D]

NikolayTT

The G-sonic gives me the very same thurst on the fishing scale as the Pe's calculator predicts
with contast very near to 1.2, meaning, I varied the constant in the calculator untill the thurst
is calculated to be the same as the fishing scale shows. Of course an error of +/-5% is there
due to vibrations etc, but in multiple measurements the average is very near to 1.2.
As for the APC, the constants are APC-N = 0.90, APC-W = 1.5, and thus I assume that the
"regular" one is the aerobatic one you mentioned to be APC = 1.2. Isn't it or mean something
else ?

freakingfast

My Feedback: (3)

I'm at a loss at how you may interpolate thrust into the the constant or how you arrived at 3.1 hp repeatedly. I'll will error on the conservative or on the average results of number of known value props. If I call the pattern prop a 1.2 constant I get 2.56hp with a 13X9 APC @ 11700 that I tested, very close to the hp number for the 13X10 you gave, so our engines are running very close to the same output. http://www.rcuniverse.com/forum/fb.asp?m=8373854

At any rate, those are respectable numbers for that low an RPM.

Kweasel

My Feedback: (29)

There many variables when searching for the ideal venturi size for suction carbs. The effects of engine displacement, RPM, %nitro, and tank size/location are considerable. Try to peak out the needle with an empty tank while pointing straight up. Then fill the tank and see if it goes too rich when pointed down. If it does then reduce the venturi or increase RPM, more nitro will reduce the amount of mixture change. A high rpm .46 typically uses the same size carb as a low rpm 2.10 I once used a Webra P5 75 heli on a plane, when propped for 12000 the tank had to be held a foot above the engine with the needle about to fall out. It became a nice engine with a smaller carb.

freakingfast

My Feedback: (3)

Kweasel

The main problem with the OS heli carbs when used in fixed wing is they tend to run rich in the mid range and low speed. I closed the mid completely and the low speed cam-ed screw is near lowest position. If flown at part throttle for a while it loads up a bit but not to the point it quits unless you stab it.
I figure less muffler pressure(small bleed) would alleviate the problem but I never spent enough time at part throttle to justify a fix.

I don't know about the Webra heli carbs.

Several guys were running the YS 91 pumped heli in planes and I didn't hear them complain....... except when their planes came apart at critical speed.
One guy lost a rudder, but landed it.

NikolayTT

Maybe I did not explain in clear maner. Lets try again. Assume we have propeller whose Diameter and Pitch
are known but we wander what would be the constant. Then turn it at some rpm near the range of interest
with what ever engine (or El-motor - there you can also double check the Watts i.e. the HP pretty well if the
efficiency of the controller and motor are known well) and measure the exact rpm and kg-thurst.
Then insert those numbers in the Calculator and change ("play with") the propeller constant +/- several times
until get the Measured-thurst to be almost the same as the Calculated one. Then the last constant number
has the right value. I believe this way the errors are quite small, and I do not assume anything except that
the fishing scale has been not "fishing" on me, just play with the constant couple times iteractively, isn't it ?