Hi I have just purchased a 91Fs 11 to put in my Seagull Funfly I had a 55 Ax in it but I couldn't get it to run right as the motor had to be put in inverted, anyway's this is my first four stroke and I want to do a lot of hovering with it but not sure what size prop would do the job?

Look at the list of recommended props in the manual and pick the one with the biggest diameter and lowest pitch for best hovering and vertical performance. I'd worry about your tank setup though if a 55 AX wouldn't run right.

tank was ok the 55ax just kept loading up never could get it to idle and power on right!

I have several of these engines they have loads of torque something in the 15/4 to 15/6 is going to give you good performance while not bogging the engine down,you can go with a 16" prop but in the 4 pitch range for 3D, on scale planes a 14/6-14/8 works well I have used 14" props with 3 blades up to a 10 pitch but they seem to bog the engine a little and I couldn't get much more than high 8,000's.

top RPM expectations are around 10,000 usually I spin it up to 10,300 and then roll the needle back to about 9,800 and they run like a sewing machine, it all depends on the prop your using as to what your max RPM is whatever you get in that range then back it off about 300-500RPM and it will run well and not over heat.

16 '' diameter ?

I really just want to hover with it !

yes 16 inch

then start with a 15/4 and then adjust from there

page 10 of the manual http://www.os-engines.co.jp/english/...e/sinindex.htm talks props briefly

Yes briefly is right ! Is there any specific prop you should use to brake in the 91?

The best person to talk to about your engine is in the glow engine factory support-OS. Bill Baxter. Bill will tell you everything you need to know that you can't find in your owners manual. I'm old school and don't pay a lot of attention to break in. I run the engine until it will idle and transition correctly then go fly with it. I do a lot of vertical maneuvers like BIG open sky loops and stall turns. The old way of thinking is the engine heats up on the up line and cools down on the down line. About 8 or 10 years ago people started getting deep into the break in. My OS and YS engines have been with me for ages and just keep running. I moved 7 years ago and no longer even have a run in stand at the new house. When I had one I used it for test running engines after a rebuild before they went into a plane. Only engines I break in are the SK two strokes but they are another story.
Prop selection has a lot to do with the plane and type of flying you do, one size does not fit all. I used a 16X4 on a OS .91 on a Kaos and it worked very well for how I fly but I liked it more with the 15X6. Some of my planes I used the 14 inch, either a 4, 6 or 8 pitch. Testing is the only way you will ever know what prop works best for you with your engine/plane combo.

Cheers Gray , yes I really just want it for hovering off the prop so 16x4 might be the go what rpms were you getting with the 16x4 ?



0

It turned in the 8s, I can't really recall, it's been a while but the big prop does drop the rpm quite a bit. I still have a couple of the OS 91s but I use the YS 110s instead most of the time these days. In the Kaos I started with an MDS 68, then went to a YS 110 then put in the OS 91. The OS ended up staying in the plane for a number of years, it turned out to be the best combo for the plane. I have a good tach but once I find the prop I like and have the engine in tune I keep using the same size prop and never touch the needles again except when our temps drop. From summer 115 degrees to winter mid 20s the high end changes about 4 clicks. When I find the props I like I buy about 4 of them, balance and put in my flight box. I only use the tach on the first tune just to get the engine set, about 400 rpm fat. I never write what it is down, doesn't mater to me as long as the engine runs perfectly.

Thanx for the advise

Thanx Tim I just ran the Fs 91 in the other day seemed to of got a bit hot was a warm day tho did ever thing by the book but I leaned it out maybe for to long while running her in, it is my first four stroke anyways I let her cool down and instead of playing with high end needle I just left it rich and opened the throttle off and on till a tank ran threw her seemed to be a better way?

Izzy it is quite common for folks new to four stroke engines after being successful with two strokes to push main needles far to lean. The reason is two fold first while the two stroke responds to tuning by ear far better than a four stroke and that sweet spot is very obvious that is why most four stroke operators always suggest tuning by tach with a four stroke.

Now the second reason is simply most all four strokes will operate at lower rpms than the two strokes formerly used by the fellows . This is a hard mental transition to make with the results that a lot of folks will start pushing the needles far to lean when working with their first four strokes.

Two strokes are far more responsive and obvious by ear and this is why I always recommend to beginners to start with two strokes. This is also why I with two strokes recommend not using a tachometer but instead tuning the main needle to the pinch method (something I have taken a lot of heat for in these forums but that's OK, it works even better than a tach).

Back to your four stroke all bets are off with the high speed pinch though and the tach come into its own for those engines.

Anyway in summation anytime your four stroke seems to run hot the the reason almost all the time is either to much propeller load (diameter and or pitch) or simply to lean on the needles.

John

Thanx John yes I think you are right I might of just left her lean abit to long when I was running her in was not like I cooked it but noticed some oil on the exhaust pipe discoloured that's why I asked

What size prop is that Tim that you are getting the rpm up at 10,300?

Start with an APC 15 x 4 W the W stands for wide blade and seems to work well on 3D models.

Tried a 15x4 on her today don't think I will be trying a 16'' prop as I only have about a inch clearance when I hold up the tail so 15'' is the max I can go, now I just gotta balance the prop and see what rpm's I am going to get,should I trie stay under the max rpms that the book sates 12.000 rpm???

thanx Tim, a 14x4 was to small as I found out today as she was getting up into the high 13.000 rpm so got a 15/4 see how that goes!

Izzy "high 13000 range" sure sounds out of the ballpark for any fourstroke glow engine, are you sure you did not have your tach set on the three blade prop setting? happens sometimes,

John

Good Question John, now you have me thinking lol !!!

If you select a propeller such that your O.S. FS-91 II will turn in the mid- to high-9,000's when leaned to just rich of peak RPM, you'll be running right in the best power band. This is where we tend to run our engines. Any prop that will give you that RPM and fly your model as desired is the "correct" propeller. Any idle 2,500 RPM or lower is considered "good".

tried the Apc 15x4 today had her running at 7800rpm and idle nicely at 3000

Just a guess but I'm thinking your tach may be off by about 1000rpm. I still use a couple of the 91s and mine run about 8900 and idle at just under 2000 with the same prop. That's off the top of my head though, haven't had the tach on them for a long while, doesn't mater though, it is what it is and it's hard to over rev the engine with that prop. As long as they fly the plane like you want it and it stays in it's working RPM range your OK. I found the engines ran great with the 14 inch prop but I like my planes to fly slower with more torque and the 15X4 gives me that. I have a three way switch on my radio I use for the throttle, high rpm for starting and warming it up, down a click for the low idle, mine idle at about 2000K or a bit lower for idling out to the flight linewith just adding a couple of clicks of throttle, as soon as it's in the air I click it back onto the high idle so I don't get any flam out during stunting. Low idle to land and the last click of the switch is to kill the engine. Works well for me but I have been doing it that way for a lot of years and it all happens naturally without thought.
Ask me sometime why I started rebuilding all my own engines and never send them in for service.

I have stripped down a OS 46AX as it done a rear bearing was my first ever attempt and had fun doing it with a friends help went to a bearing shop and paid a 1/4 of the price of what a hobby shop would charge you, was a great learning experience seeing what goes on in a little OS 2 stroke . So why do you rebuild your own engines and not get a service guy to do the job?