Hey guys, I've got an acrobatic plane in the works that calls for a .46 2 stroke. I'd like to install a Saito 4 stroke, what would be one to use? Maybe the .56 size Saito, or bigger yet? The plane has a cowl so need to stay with in the .46 2 stroke size.
I seem to like more than enough power, but everything has is limits. Most ARFs have a engine recommendation for either 2s or 4s. This one does not. For more than enough power I will usually step up a knotch from the recommended size, with no qualms about fabrications for fitting.
I guess what I'm looking for is a "Rule of Thumb" for changing a 2s engine for a 4s, I know I need to go bigger, But how much?

Thanks, Mando...........

A rough rule of thumb is to go up one or two sizes (46 2 stroke becomes a 56 or 62 4 stroke).

I also like "more than enough" power. so for most of my 46 size planes, I use Saito 72's or 82's. Haven't found it to be a problem yet. The difference in weight and physical size between say a 56 and an 82 is minimal.

if a 46 fits in there well the next move is alway a 91 4stroke on my book

Hi,
I had similar problem. The easiest power improvement is to go to two stroke .50 - 0.52 engine from OS(ringed), Webra, MVVS, SuperTiger(rigned) etc. Then you do not need any(!) modifications - it fits just 1:1 in the place where .46 was, and
you usually gain power almost as ABC type of the .61 engines. Alternatively there isn't much you would gain from
the 4 stroke, maybe sounds a bit funny like old barnstormer ... and some bit of fuel efficiency and maybe a lot
more ... difficult to maintain engine, expensive too. It looks to me the facination of the 4-stroke has gone or it
is going to away at least for the small engines below 1.0; well, let see what the others will say. Of course you
get lower RPM more stable and higher torque at low RPM. But all that would require you to buy new type of
glow plugs and new propellers, while moving from 0.47 to 0.50 you do not have to change almost any thing,
you will just gain like 10-30% higher rpm and about 15-20% power increase too; i.e. 4-stroke is a bit of "waste
of money show" (Well, some one might be right to say that the whole RC is that type "business"). At least there
is one thing for sure: in all RC packages you get the reccomendation to use bigger 4-stroke engine in sense of
displacement, that needs bigger size sleeve and piston and also valves-related mechanics and that is why
Saito had to do special alloys to keep the weight comparable to 2-stroke, and then prices are not that low.
Cheers,
Nick

Mando, If you want speed, go with the 2 stroke. If you want power go with a four stroke. I've flow a 40 size trainer with a O.S. 52 four stroke, and a 12 1/4x 3 .34 APC sport prop. It pulls like a tractor, but it isn't real fast. I've got a Sig 4* 60 with a Magnum 91 four stroke, and again it pulls like a tractor. It has more than enough power for any aerobatics you want. Straight up as far as you want it to go, and as slow as you want. I sure wouldn't replace a 46 witha 91fs though, it will way over power the plane. One other thing to watch is prop size. If the plane is setup for a 10" prop, you may have trouble with a bigger four stroke as they like big props. I crank a 14-6 with the 91 on my 4* and am tempted to go to a 15-4. Just some things to consider.

Don

The old rule of thumb is to multiply the two stroke displacement by 1.5 so a .46 becomes .69 or .7 cubic inches. But modern four stokes are more powerfull today so you may get by on less.

I don't know a rule of thumb. However a Saito 72 or 82 will be just right.

David

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I'm with you, David. The .82 would be perfect in my book. The .72 is a bit too light in the power department for me. You're carrying nearly the same weight and size in a .72 as you are with the .82. You may as well enjoy the larger engine. I'm surprised that Saito is still making the .72.

To a previous gentleman; You don't have to run large diameter props on four-strokes. You do need to run enough load on the engine to put it in its desired rpm range. There is nothing wrong with running an 11x10 or 11 on an .82 instead of a 14 or 15 something or other. Yes, prop efficiency goes to hell in a hand basket, just like it does on a .61 two-stroke running the same prop. But sometimes it is the best that one can do under a particular set of circumstances.


Ed Cregger

Well, you would have to buy new props if you changed from a .46 to a .61. Some 2 stroke engines run fine on Saito SS 4 stroke plugs and they are cheaper than the popular OS #8.

I have both, but prefer 4 strokes. I like the instant power instead of waiting for a 2 stroke to wind up. If you are going to do anything like loops or other things that need vertical power you may end up liking 4 strokes better. I like to fly pattern, so that's important to my kind of flying.

The fuel savings are real. My jug empties much faster on the days that I'm flying a 2 stroke engine.

Most 4 strokes stay tuned longer than 2 strokes.

8*)

I replaced an OS 46AX with a Saito 82 on a Funtana X50, and I was VERY happy with it. Unfortunately, I snapped the wing off of the Funtana, so I don't have that plane any more, but I don't feel that it was the engine's fault. Would the same thing have happened with the .46? Probably not, because it didn't have nearly as much power as the Saito did, but I pushed that plane to a point BEYOND it's limitations.

The 82 Saito powered that plane into snap rolls plane faster than I could count the revolutions - no kidding! It sure was fun while it lasted. If I had backed off of the throttle, just a LITTLE, perhaps I wouldn't have lost that wing though. So, I guess what I am saying is - go for the 82 Saito, but just use a little judgement on the throttle. These planes, just like real ones, DO have limitations!

BTW, the Saito survived the Funtana's demise just fine, and it is now on a Showtime 50. I haven't flown the Showtime - yet!

All good Feedback guys, with lots of good variables such as props and power with a small amount of weight difs between engine sizes.......
We'll be look'n for power over speed on this acrobatic plane, and won't want to wait for the RPM to build on the 2 stroke. Flying weight will be about 6 lb. with a wing area of 531 sq. in. Looks like the .72 and .82 Saito are the same size for mounting, with the .82 1oz heavier. Compared to the .46 2 stroke they are only 25 mm longer and 6 mm wider.
I'm running a Saito 100 on a 10.5 lb warbird with great results with power and fuel savings.

Mando.........

If you are using a smaller diameter prop at the same RPM, even with a higher pitch, prop efficiency will go up not down. You are confusing static thrust with thrust while in motion. The static thrust would be greater for the larger low pitch prop, but at speed the thrust will be greater for the smaller high pitched prop.

Too many people undersell the fourstrokes power because they don't grasp the difference between HP and flying power. That difference is far closer than 1.5 to one.

Sport Pilot- You have that reversed. You will gain prop efficiency with a larger diameter.

Wrong. Not if the same RPM is maintained. The efficiency is thrust HP over BrakeHP. Since the higher pitched prop will have more thrust it will have greater efficiency. You might be right if we are talking about a static condition, but at speed the lower pitched large prop will be near its max and have very little thrust thus little efficiency. I think you are confusing using smaller props of near the same pitch, where the smaller diameter allows the engine to speed up and thus has more friction loss and little improvement in angle of attack. But in this case we are keeping the RPM the same and the higher pitched prop will have more thrust.

No, I'm not confusing anything. It has to do with differential pressures, regardless of pitch, etc. Also, it does have to do with both airspeed or static speeds as you have to put those into the equasion to figure either 'ideal efficiency' or Froude efficiency. In a nutshell, the less pressure differential across a span the greater the prop efficiency. A smaller blade has a short span which increases the differential, and smaller blades generally turn faster with higher differentials. Both bring the efficiency down for the prop.

This is actually on the subject for this thread because 4 strokes are often chosen to swing larger props.

One should specify the efficiency you are discussing. If one was talking about propulsive efficiency, as one would expect when talking flying aircraft, one would need to determine the power output to get a specific airspeed. Then we could discuss it. Otherwise we are guessing. Theoretically a higher pitch prop one a clean airplane would go faster. but a lower pitch prop on a draggy aircraft could be more efficient. If you don't have the thrust, power, and drag plot of the aircraft and prop you don't know.

Now, BLW is correct that for a fixed POWER the larger prop will be more efficient in any case. At fixed RPM and POWER as we have with an engine you have to deal with what I just mentioned. In electric models you can usually gear to whatever RPM you want and generally used as large a prop as possible. I have flown an Ultra Stick Lite with a 19x14 prop at 4500 RPM on electric.

I am going by prop efficiency Thrust HP over Brake HP. Even on a draggy plane if you limit RPM such as by increasing pitch to prevent overspeed or with redline limits on full scale, then the higher pitch will have the most thrust ergo it will have more thrust thus more thrust HP. Even on a draggy plane like say a Cessna 172 the higher pitched slightly smaller diameter cruise prop will result in higher speed (well by a few knots anyway) with the same fuel used. So either way both fuel and prop efficiency is attained with a smaller higher pitched prop. However the larger lower pitched prop may be more effective for aerobatics. People confuse effective with efficiency. Obviously when you are cutting circles in the sky with a model airplane burning nitro fuel, efficiency is not anywhere near your objective. So forget about using the most efficient prop.

I suppose methanol and nitro are cheap so efficiency may not be important to some. Posts above indicate that a modeler might spend a good amount of money for improved fuel efficiency of a four stroke engine, why not take it to the prop? Propulsive efficiency is power in the fuel/power output. That includes prop and engine. So if you spend hundreds extra on an engine, what is a few bucks trying a handful of props to get the performance you desire? Optimized you would pay for the props pretty quickly in reduced fuel comsumption.

Prop efficiency was very important in electric model prior to LiPo. That's where I'm coming from.

I wouldn't consider a Cessna to be a draggy model aircraft. I was thinking in the model realm of a WWI biplane or triplane compared to virtually any sport model. You raely see a prop with a P/D of greater than 50% on a WWI model.

Yes but if that were important we would be using true Diesel engines with #2 Diesel fuel not alcohol and nitro. But we want more power not efficiency, so we use a fuel that packs the power in with the expense of efficient. It's more effective, not more efficient. I suppose if one is doing 3D then the larger prop is more efficient in a hover. But not sure, both props would have the same thrust, the weight of the airplane. So the larger prop would be turning slower so less drag, but not sure which one would use more engine torque, though I suspect the smaller one would have less, but more power due to higher revs.

Yes, it's important to some.

I have an FS-48 diesel that burns 12oz an hour at WOT.

If you don't like what I'm saying just say so. If it's wrong prove it.

What's wrong with what you say?

The .72 is 8 grams heavier than the .82, according to Saito's specs! Click on the link below:
[link=http://photos.imageevent.com/ken6ppc/scannedos120manual/large/Scan20010.TIF.jpg]Specifications in Saito Brochure[/link]

Odd how specs change from one site to another

Specs

Type: 4-stroke
Displacement: .72 cu in (11.80 cc)
Bore: 1.06 in (27.0 mm)
Stroke: 0.81 in (20.60 mm)
Cylinders: Single - Chrome Plated
Total Weight: 16.6 oz
Engine (Only) Weight: 16.0 oz
Muffler Weight: 0.6 oz
Crankshaft Threads: M7 x 1mm
Benchmark Prop: 13x8 APC @ 9,800
Prop Range: 12x8 - 14x8
RPM Range: 2,000 - 12,000
Fuel: 10%-30% Synthetic
Mounting Dimensions: 104 x 56 x 111 mm
Muffler Type: Cast
Cylinder Type: AAC
Carb Type: Barrel, 2 Needle valve
Crank Type: Ball Bearing


Specs

Type: 4-stroke
Displacement: .82 cu in (13.80 cc)
Bore: 1.14 in (29.0 mm)
Stroke: 0.80 in (20.40 mm)
Cylinders: Single - Chrome Plated
Total Weight: 17.6 oz
Engine (Only) Weight: 16.0 oz
Muffler Weight: 1.6 oz
Crankshaft Threads: M7x1mm
Benchmark Prop: 13 x 8 APC
Prop Range: 12 x 8 - 15 x 4
RPM Range: 2,000 - 12,000
Fuel: 10% - 30% Synthetic
Mounting Dimensions: 104 x 56 x 111 mm
Muffler Type: Cast
Cylinder Type: AAC
Carb Type: Barrel, 2 Needle Valve
Crank Type: Ball Bearing


Mando..............

I was talking about fixed pitch props, like we use on models, at varying RPMs and in flight. The best pitch and/or diameter to use is based upon figuring out the efficiency based upon a formula that includes airspeed. You can't have large diameter and a large pitch for most engines, so you have to make a choice. That would be what it is all about in the end. Getting the most out of an engine and airframe. You could call it calculating the pulling power of the prop, or whatever. A lot of people think that only the highest RPMs give the most power.

You can use a smaller diameter prop for faster RPMs and reach a point where it is the most efficient. If you get the tips near the speed of sound your performance will drop quite a bit. You can do the inverse and choose a larger diameter to reach peak prop efficiency. The large diameter wins out in efficiency if used on the same engine, same airframe, etc.