I have started to use the OS 61SF's. On my Intruder's, the World Models, ARF's, which everyone is aware of, I am running 15% and APC - 12 X 8's. No problems, all fine, vertical's out of sight, hauls butt. Is this combination OK ? Should I go to the "pattern" props, 11X10 or what fit's for this ? Do what is working fine, don't change ? Right now I am SPA Novice. Any comments appreciated.
SPA/BPA contest coming up this weekend, two days, and one in Orlando first week of December, SPA/BPA. I have two of these Intruder's both equipped the same.

Crank

Hi Crank:
Prop choice sounds ok to me. However, find out what the max torque RPM is and what the max HP RPM is, select the prop that will give yoiu a good combination of the two curves. These curves may be obtainable from the manufacturer. I would not go to the 10 pitch as I believe that may be to much of a load for the engine.
If you want to slow down, increase pitch by 1/2" with same diameter, but remember you'll have to richen the needle somewhat for the engine to carry the extra load.
You need to get a reliable tach and start tuning from there. Try and fly with different prop combinations and see which gives you the best result. Then that wil be the right selection.

Crank, I will not be able to go the Miami contest, as my plane is not finished, almost but not quite. I still have to paint and set the tuned pipe header length for the engine.
Crashlander

Hey Crank...

I think you ought to prop that engine to run about 11000 RPM's...From what I have read the sweet spot should be between 10500-11500...I'm trying to keep all mine well under 11k (static) and figuring the engine will turn up a bit more when it's in the air...

I have yet to run any of my SF engine's with a stock muffler...I'd imagine a 12x8 with 15% nitro should put you right in the ballpark...Maybe some of the guys who are running that same set-up can advise better...

All I've run so far have been rear exhaust pumped versions of that same engine (RF-P)...I'm also using PowerMaster YS 20/20 fuel and tuned pipes...I doubt much of my info will apply to your particular set up...I'm turning about 10500 on a APC 12x10 prop on the test stand...I'm almost happy with that but I'd really like to get the static RPM's down a hair more...I may try a 12x10w prop to see if that helps...(w=wide)

My advice would be to borrow/buy a tachometer and set your engine up to turn 10500 RPM's or less on the ground...(I'd almost bet your current set-up is spot on)

Keep in mind these are Long Stroke engines and they produce their power at lower RPM"s...They don't need to "wind up" or turn the kind of RPM's other engines do...I'm actually planning to keep my engines running on the low end of tolerance as an effort to preserve them...I'll try not abuse them by squeezing every last RPM out of 'em...

Have Fun and enjoy those Great Engines!! They are really nice...

Chuck

crank,

Good advice to use a tach to help compare different props. And of course to fly them to find out which ones make the plane behave like you want it to. I like Chuck's idea about trying to make these OOP engines last. I'm gonna try some 13 inch props on my SF/RFs. Maybe tomorrow on the test stand. My <7lb. monowheel Calypso flies great with its OS FS90 turning an APC 13~8 near, IIRC, 10K.

Terry in LP

Just reaffirming what others have said. I was told by Bax, the OS rep, that the mid 11's is the range to aim for.

I also agree about using a tach. But, in addition, you should download Pe Reivers' thrust/hp/speed calculator:

http://mvvs.nl/prop-power-calculator.xls

It's the only one I've found that factors in prop pitch. The others, like ThrustHP, give the same data for an 11x4 as they do for an 11x10, which doesn't help much. Here's a partial screen shot of Pe's spreadsheet.

David

Thanks to all about the tach, etc. I never used a tach in the past, and now is a good time to get one. Question: If the engine tachs 11000 on a 11 X 7.5, on the ground, then airborne, it unloads to 12000, can I put on a 11 X 9 or 11 X 10, get maybe 1500 RPM less, but have more torque ? Is RPM's necessarily the answer, if the long stroke SF's can do the job at less RPM ? I see the term, "Pattern Props", for the 11X10, and other sizes like this. Where are these props being used ?

Hope I am making sense. I have many TF 11 X 7.75 and 11 X 7.50 Super M Power Props.

Thanks,

Vince

Not edited.

As a reference point, my OS 61SF ABC-P engine with an APC 12 -10 W I’m getting about 10,400 on the ground using Omega 15% with a pipe. The engine has a Nelson liner/piston and is mounted on a solid glass filled mount. It unloads some in the air but I have no way to know how much. It’s fast but that’s the goal!!!

Vince,

Just got through running the SF I got from you with a few different props. Used Omega 10% and the stock muffler.

Zinger 12~8 tached 10K

Zinger 11~10 was 10K

APC 13~6 tached 11K

APC 13~8 tached 10.1K

All the props were balanced on my High Point and they all tried to fly the concrete block the test stand was C-clamped to. Zingers are the old style. Not the blunt edge, double-drip dipped in plastic boards they are now. Didn't have an APC 12~8 and my glow driver wore out (hadn't been charged in a while) by the time I was gonna try a 12~7 APC. Hope to do some comparisons with a TP next week when I'm off work.

Terry in LP

Terry, those numbers seem a bit low.

Stock muffler; 10% Powermaster; APC props:

12x8 - 10,780

12x6 - 12, 140

11x10 - 10,860

11x11 - 9,780

Mac's long stroke quiet pipe; 5% Powermaster; APC props:

11x10 - 11,860

12x8 - 11,910

David

11x7.5 isn't going to be enough prop for your engine...

I wouldn't change anything right away...Take a little more systematic approach to making these changes...Get a base line where you are right now and then make small adjustments to tweak...Little steps at a time...

The program raindave mentions looks really handy but will take some time to figure it out (for me anyway)...So lets look at Propeller Load Factor (PLF) to simplify things...

Propeller Load Factor is simply the diameter of the prop squared multiplied by the pitch...For example a 12x10 prop: 12x12x10= 1440 PLF

You mentioned in your initial post that your are currently running a 12x8 APC prop...(I'm thinking if you had a tach you would find that your probably turning somewhere in the ballpark of 10800 static RPM...Just a guess but I would be pretty surprised if I was wayyy off...)

So the first thing you need to do (in my opinion) is get a tach and take a reading with your current set-up...That number will be your baseline RPM...

Now we know for sure according to BAX the OS 61 SF is designed to be most efficient in the "mid 11000" RPM range...So you can call that your target maximum RPM...

I can not tell you exactly how much an engine "unloads" in the air...**I personally** shoot for static RPM's to be 1000 lower than my target maximum RPM...I'm not sure the engine really unloads 1000 RPM's in the air...If not that will be OK and I will stay on the conservative side which is my goal anyhow...

Your current PLF is 12x12x8= 1152 (That is your baseline PLF)

-SO-

If you find your engine is turning 10500 static RPM on your 12x8 APC...Then you know your in really good shape...You can use your current PLF of 1152 for any changes you wish to make to your prop...(Whatever diameter prop you wish to run...Choose a pitch that will give you a PLF close to 1152)

If you find your engine is turning significantly higher than 10500 static RPM's then you need to raise the PLF...Switching to a 12x9 APC would give you a PLF of 1296... A 12x10 would be 1440 PLF...You will need to experiment with what prop will put you engine in the correct static RPM range...

Once you have your correct PLF figured out...Then you can consider changing diameter of the prop to give you the flying qualities you desire, you'll just need to experiment...


As with anything there are variables...using the Propeller Load Factor concept isn't really an exacting science...There will still be experimenting required to fine tune your airplane to suite you and your needs...It's just a baseline for you to tune from...

Propeller design is a consideration and different manufacturers props will perform differently...Atmospheric conditions have an effect performance so all of us may get different results even using the same set-up due to or respective environments...Of course fuel selection will have an effect too...Tons of little variables which can all add up...It really boils down to each fellow doing some tinkering with his chosen engine airframe combo to dial it in to his preference..

Hope I didn't make this too complicated??

Just my opinions...

Chuck

Chuck, Dave, Terry and all of you, thanks very much. I am getting more interested in this side of it. You all make sense, I will follow what you have said. In Intruder #1, I am using the OS61 SF ringed, and #2 has the 61 SF - ABC. 15% Omega, OS #8 plug. These baby''s start on the first flip, do the instructions idle screw, etc., that's it. Sometime on the ABC, I am having a little problem on transition. Both 7L carb's.

I don't know how much faster I am going to go with these engines. They are great, and now I wonder how many I missed to buy bfore you guy's and some other's turned me on to them. Anyone comment on the 61 RF ? Thanks.

Vince (Crank)

David,

This seems to be a tired old engine kinda low on compression. My main focus was on the differences between the props. Also remember that the 11" and 12" props were Zinger wood props which will usually turn less rpm than an APC of the same labeled diameter/pitch. I'd expect the APC 12~7 would turn 11,000+ if I coulda got the engine started. Maybe I'll try again tomorrow after charging the plug driver overnight.

Terry in LP

The 61 RF is the rear exhaust version of the SF...All that is different is the Case (because the exhaust routes out the back not the side), Piston and Wrist Pin...Everything else is the same...Then there is also the RF-P which would be like the SF-P (pumped engine)...

The RF engines would be used in an airframe which required a rear exhaust configuration such as Atlanta, Aurora and so on...You are pretty much gonna be using a tuned pipe exhaust system on a RF engine...

I have several RF's because most of the airplanes I like are designed for rear exhaust engines...All mine are the pumped versions...

There is also the OS 61 RF-P "Hanno Special" variant which is a little bit different than the standard RF-P...It's designed to run in the 9500 RPM range and was brought about to help lower the noise BITD...The engine with the read cylander head is the Hanno...

Don't forget there are others out there who need engines too....

Chuck

Thanks for the info Chuck! Sounds like my engine is putting out some serious HP to turn a APC 12 X 10 W at 10400.

They are strong running engines that is for sure!! I believe the pumped engines are suppose to be rated about 2.0hp...Not sure that really means anything but sounds impressive...Who knows how they calculated HP?? I'm thinking everyone has their own "Formula" for doing so...he he

If this persist after you have run it a while you may consider changing out the carburetor o-rings...

Even thought you may have a NIB engine...Keep in mind it's probably working on 20 years old...he he

One other important word of caution:
Never use any petroleum based products with these engines (ie: after run oil ect)...The petroleum products will disintegrate to silicone o-rings...Only use synthetic after run/storage oils!! (Ultra Oil from P'Spec -or- Mobil 1 pure synthetic engine oil 0-40 or lowest viscosity available)
http://www.pspec.com/details.asp?ProdID=40&category=7

Some very knowledgeable fellows who I respect very much recommend not using an "after run" oil...The claim is today's modern fuels have "built in" after run features in their blend...Adding oil is still recommended for storage purposes...

I'm still using Ultra Oil as an after run oil but am thinking about backing off that when I get to fly more frequently...Right now my engines are technically being stored more than run... [:@]

21281800:
http://www3.towerhobbies.com/cgi-bin/wti0001p?&I=LXCS00
24881824:
http://www3.towerhobbies.com/cgi-bin...&I=LXCS01&P=OW
24981837:
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29015019:
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The #7L is probably my all-time favorite carb. I have also purchased those O-rings from Tower that Chuck listed. Remember, like a Perry, you only turn the low speed adjustment a TINY amount at a time. 1/32" is an effective adjustment. Start with the screw slot inline with the axis of the carb throat. Bring the engine to idle. Pinch the fuel tubing. If it runs for a while, gradually increasing in rpms before dying, turn the screw to the right a tiny bit. If it dies more quickly without picking up when the tubing is pinched, turn it a bit to the left. Restart and check your transition. It doesn't take much to get this carb working great.

David