ro347

My Feedback: (13)

When experiemtning with props....how do i know where i should be keeping my rpms at ull throttle? Does it matter? I have a saito 91. According to the manual the range is 2k -11k. Currently have a 4x8 on it running at about 8400 rpm at full throttle. Is this a problem?

Campgems

I know you don't mean a 4x8. A 14x8 may be taxing the engine. You should prop the engine so it will run at least 9500, otherwise you are placing to great a load on the engine. Your manual should give you a range of props also. I would stick in that range. For most of the planes you would use a 4 stroke in, top end isn't important, but pulling power is. That is what the 4 strokes are good at. In order to get that pulling power, you need to run lower gears, Ah Props. A good prop for maximum pull with that engine woudl be an APC 14x4W. That should turn up in the high 9k range to low 10K.

Don

broke_n_bummin

I ran a 15X4W and a 14X6 and had decent results. A 14X8 shouldn't have loaded the engine down that much. I was getting 9000 with a 15X8. It would probably handle a 16X4W or a 15X6 and keep the RPMs around 9500.

Charlie P.

It's not really a problem, per se. Engines are rated to a maximum horsepower (usually at an RPM you can't achieve with a sensible prop, anyway). You need to keep enough prop on to load the engine but not so much that it over taxes it. I like Broke n Bummin's 9,500 RPM range for a 4-stroke at high throttle.

Prop choice ultimately depends on your flying style and expectations.

da Rock

We really don't have good rpm information in our hobby. So you need to learn to pick a prop without that info.

With a 4stroke, a tach is really useful for setting the fuel mixture.

You tach the engine on the ground to set the needle valve to optimum. Run the engine WOT and adjust the needle to highest rpm, then back off some. How much to back off? You try something and fly to find out.

To find the best prop, you observe the flight performance. How does the prop pull over the top of consecutive Cuban 8s? How slowly will the plane fly with that prop, and how does it accelerate from that speed? How is the vertical?

Once you have your props worked out, record what rpms they do on the ground for highest rpm and best needle setting, and you've got info you won't find in the owner's manual for most engines, or printed by any prop mfg.

BTW, if you're lucky and are a reader, you might see your engine in a magazine review. If you have, and then checked your own engine with the props the magazine writer reported, you'll see clearly why engine manuals and prop mfg's don't publish rpm data. The info you want changes with temperature, altitude, and pressure. How much? Your tach is about the only way to find that out too.

carrellh

From: http://www.horizonhobby.com/Products...rodID=SAIE091S
Benchmark Prop: 14 x 6 Zinger @ 9,100

ro347

My Feedback: (13)

Well ive flown a 15x4...great thrust but to slow and had to keep the throttle up for speed - killer on fuel. Have the 14x8...flies nice but fast on landing(landing at my field is like landing on an aircraft carrier. If you can land here...you can land anywhere) I will try the 15x6 prop. Hopefully that will be in the middle. Hve to figure out if my Tru-turn wide spinner will fit on the 15x6. Oh well. Yet another trip to the LHS.

alfredbmor

My Saito .91 works fine with a 14 x 6 Master Airscrew Wooden prop.

mscic-RCU

My Feedback: (6)

I use either a 14X6 or a 13X8 on my Saito .91

Jester241

"I" would say that a 14x8 is a little heavy a prop for that engine and "I" would think your 8400 is right where that prop would put your engine just like stated.

I dont think that prop is WAY overboard,or 8400 is WAY too low....but your definetly heavier on the prop and lower on the rpms then I would run it. "I" also think that 9500 rpms is the MAX I would want my 4-stroke to run steady while at wide open throttle. I wouldnt want to go any higher then that becuase it usually means you leaning out your engine too much to obtain those high rpms. I generally try to find a prop that will put me around low 9000's when properly tuned on a 4-stroke. In fact,I've only hit the 10,000 mark one time with a 4 banger and IMMEDIATLY backed it off. I couldnt imagine running it for periods of time even close to that. I've said in other posts that I think most engine manufactures tend to tell us higher rpm ranges then we should be hearing for no other reason then to try and make their engine look more powerful then others.

I'd say put yourself a prop on there that will hold a steady rpm range of about between 9000-9400(it fluxuates a bit) and you'll have a good running engine. I like to run my engines a tad on the rich side because it not only is better for the motor,but it helps keep the motor from deadsticking(too lean) while flying long verticals.

...just my opinions

bkdavy

My Feedback: (1)

Don't forget that the prop will spin higher in the air as it unloads. The rated RPM of 11,000 is NOT what you're looking for on the ground. I have a Magnum 91 that turns about 9400 on the ground with a 14x6, and I thinks its just about perfectly propped.


Brad

ErikElvis

My Feedback: (2)

My OS 91 4stroke pulls around 95-9600 with a 13X9 prop. I may drop to a 12X10 or 11