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Old Jan 30, 2012, 10:58 AM
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2-Stroke vs 4-Stroke

I'm sure this has been talked about to death already but I had a few questions:

I have a lot of experience with 2-Stroke Nitro engines as well as 2-Stroke gas engines. I am planning my next plane and I was thinking of a Decathlon in the .75-.90 size. All of my current planes are 3D from parkflyers to giant scale and I want to do a somewhat scale aerobatic plane. I was thinking of going simple route and doing a 2-Stroke nitro or potentially a smaller gas engine but 4-strokes have always interested me. I'd like to have a more scale sound and grunt.

Are 4-strokes any more reliable than 2-strokes? Also how do they compare for tuning purposes. I'm looking into the .90 size for 4-strokes.
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Old Jan 30, 2012, 01:11 PM
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United States, SD
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I got a a magnum xl .91 with a used cub I got a couple weeks ago. I haven't really tuned it to yet, but I took it out in the yard and taxied it around and it seems like a good runner.
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Old Jan 30, 2012, 05:10 PM
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United States, LA, New Orleans
Joined Sep 2011
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this HAS been talked to death but the purpose of this forum is for people to be able to come, ask their questions, and then get answers to those questions.

i think you will be more than satisfied with a 2-stroke on that Decathlon, but a good 4-stroke will definitely add to the pleasure of flying it. a well maintained 4-stroke can be every bit as reliable as a 2-stroke of the same quality. they do have more parts and therefore require more maintenance and upkeep, but with one that has a proven design the maintenance thats required is minimal.

they also use roughly half the fuel of the equivalent displacement 2-stroke, but in practice this fuel savings is about 30% due to needing to use a larger 4-stroke than the required 2-stroke size. still a significant fuel savings however.

tuning is a little more difficult initially and will require you to use a tach, but ive found that once they are set they are less "fiddly" than 2-strokes and require less adjustment over time.

given two engines of equal displacement:

2-stroke Pros:
-turn smaller props
-higher RPM
-lighter (generally)
-more power to weight
-less moving parts
-less expensive (generally)

2-stroke cons:
-less fuel efficient (some by a large margin)
-louder
-higher idle
-less fine throttle control
-heavy mufflers on some
-smaller props @ higher RPM = less thrust (see note)

4-stroke Pros:
-bigger props! = more thrust @ lower RPM (see note)
-pleasant sound
-quieter
-can be significantly more fuel efficient
-lower idle RPM
-finer throttle control
-smoother transition through throttle range

4-stroke cons:
-generally heavier
-larger engine required to match HP of 2-stroke of similar size
-more moving parts
-requires regular maintenance
-more expensive (some significantly)

*note:
2-strokes usually turn higher RPMs than equivalent sized 4-strokes and can develop more HP for a given displacement. however, this isnt always significant. for example a .40 sized engine from one manufacturer may have a prop range of 10-6 to 11-6 and have a rated RPM range of 15000-2500 RPM. that may sound amazing, but in practice most .40s will be fitted with a 10-7 and will turn that 10-7 prop at 10,500-11,000 max and a 11-6 significantly slower. however, a 4-stroke of equivalent displacement can have a prop range slightly larger at 11-7 to 12-6 or so, and still turn that prop between 9500-10,000 RPM.

that may not seem like a big difference, but the larger prop will deliver more static thrust even though its turning at a lower RPM because it is "grabbing more air" over a larger arc. this can make a significant difference in how your plane actually handles in the air.

ive replaced almost all of the 2-stroke engines i own with 4-strokes not only because they sound better but i find that my planes fly better with them.
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Old Jan 30, 2012, 05:15 PM
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USA, FL, Winter Haven
Joined May 2006
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After flying 4 cycles for years I still can't tune one in by ear like I can a 2 cycle.
I have always used a tach to set the top end a few hundred rpm's below max.
They seems to be reliable for me or at least Saito is as that is all I have.
But your right for certain types of planes nothing beats the sound they make.
Good Luck.
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Old Jan 31, 2012, 09:00 AM
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Everything's already been said, and there's alot of good info here. I just wanted to reiterate my experience breifly, in that I've found tuning a 4-stroke to be essentially identical, with the exception of being able to tune by ear.. just as Merlin disclosed, a tachometer is a very important tool... once set with a tach though, very little tuning is needed after that... and what little is needed is only to compensate for atmospheric changes..IE weather/moisture/climate/elevation.. with only slight changes needed even for these situations.

I say go for it.. they are a fun little power-plant... and very worthy of any aerobatic plane.
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Old Jan 31, 2012, 09:39 AM
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i dont agree with the above 10*6 10*7 2-stroke glow.45 sized example above.
good SPORT (not specials) engines in the .45-53 sized will turn 10*7's 14.5k-15krpm before you start working on tuned pipes. Not a paltry 10k. only crappy low balled engines from the likes of OS, evolution, etc. tuned by people that don't know what they're doing turn 10*7's under 13,000rpm in .45/46/47 cased engines.
sheesh... half the SPORT engines on the market 15 years ago in that size were turning 10*7s at 14k and up. (good os, TT, ST, irvine, K&B, MDS, everything coming from the GMS plant regardless of the name, etc ad infinum im just too lazy to keep listing)



i digress. .40 cased 4-strokes have never been an overly popular size in R/C. the power output of .30 cased engines, which was a dead size held over from control line.


this guy
Quote:
Originally Posted by Toysrme View Post
either. every stereotype you've ever heard is true.
2-strokes
  • are lighter weight for the same displacement and power output
  • powerband allows turning smaller, higher pitched props faster
  • more power for a given weight & displacement
  • higher fuel consumption
  • vastly lower throttle response when bogged down on larger props
  • less expensive
4-strokes
  • are heavier at the same displacement
  • rpm does not vary as much due to inconsistencies in fuel supply or maneuvering (i.e. . 2-stroke vs 4-stroke in prop stability. a 4-stroke, especially a pumped 4 stroke on a large prop, will vary very little during engine orientation changes, fuel changes and changes in airspeed while manuvering)
  • have a powerband allowing the turning of larger props at less rpm
  • less power for a given weight/displacement
  • around 20% less fuel consumption for an equal power
  • you do need a crankcase breather (or oil catch can) to vent oil on some models
  • sound better
  • easier to hand start due to the higher compression. for example, most Saito's will fire on a hand start simply by flicking the prop backwards against it's own compression once primed.

i have many planes with many makes & sizes of both engine types. no preference of one over another. it simply depends on the airframe & what it's meant to do.

4-strokes are great matches for scale, or airframes that are tail heavy, require fast, always there throttle response and do not need the crazy high pitch speeds 2-strokes are capable of.
I will add to this... Follow the OEM's recommendation and your own common sense. 4-strokes are not well suited for planes that you want to be fast, and trying to upsize a 4-stroke 30-50% from the OEM recommendation will never fly as well as simply running the max recommendation 2-stroke on it. So many times have I seen people build say a sleek, .40 class warbird with a .90+ 4-stroke on it & wonder why it doesn't fly as well and is vastly slower than any of mine running the recommended .45/.46-.50 class 2-stroke.


GOOD 4-strokes (Saito, older OS) are amazing for scale, aerobatics and 3-D. If it's an airframe you want to move out, or a weight/balance sensitive airframe. Forget them entirely. By the time you start downsizing props & adding pitch you'd have wound up wasting time, money & performance a 2-stroke would have given you out of the box.
to the OP. most people skip .90 cased 4 strokes and go for 1.20's.
tuning is mostly the same. where 2-strokes respond very quickly to needle changes, 4-strokes take a few seconds. so dont rush too fast. give it a few seconds for the fuel to catch up. also, most people tune slightly richer on 4-strokes than 2-strokes. (where 2-strokes you lean until max rpm is found, then slightly richen. most 4-strokers will go slightly richer still). The classic "i can barely see a white vapor trail from a saito/OS" as it were. Tho you do not HAVE to do this.
they are more difficult to tune by ear. where SMALLER 2-strokes are easily ear tuned with their higher pitch, 4-strokes and their lower rpm operation & ability to run about the same on a much wider AFR. youll want to pull out a tach at least the first few times. say you dont load one down very much and it's turning 9500rpm. it may turn rpm's so close it doesn't change its pitch on a half dozen needle clicks. which is a wide margin hah.

most stock 4-stroke glow engines (regardless of size) are tuned & for max power in the 9-11k rpm range from the factory. most good prop choices will keep a 4-stroke at 8-9k. you can go under, and it was common 20-30 years ago when gas power was far heavier & less power output than today, but you'd probably be better now a days served looking at a gas or diesel engine if you're only wanting 5-7k on a huge prop.

reliability is pretty much the same. depending on how often you fly, youll want to adjust the rocker arms once a year, once a season. once a month on a competition engine/plane. w/e floats your boat. that's about it.


oh, and one more digression. once you get into the 1ci engines and above. 2-strokes really don't turn any more rpm than 4-strokes do in most applications. they're pretty much completely the same by 1.5ci.
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Old Jan 31, 2012, 04:31 PM
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Thanks for the help guys, that really cleared a lot of questions up that I had!

I'm looking forward to the world of 4-strokes, it sounds like they are definitely something I should try at least once.
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Old Jan 31, 2012, 04:46 PM
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Quote:
Originally Posted by 94'325is View Post
I'm looking forward to the world of 4-strokes, it sounds like they are definitely something I should try at least once.
exactly! its not really a problem until you've had one of everything. try to make it a problem!
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Old Jan 31, 2012, 07:25 PM
I'm a pilot... 100 yrs to late
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USA, FL, Palm Harbor
Joined Jan 2005
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I have a magnum 70 and 52 four stroke and they are quite affordable. Absolutely love them. Took about 5 tanks to get them broken in and they just turn over at low idle on 15%. I have the .70 in the GP 40 size clipped cub kit and it just puttered by at 1/2 throttle. (past tense )
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Old Jan 31, 2012, 07:54 PM
Zor
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Toysrme,

You wrote concerning 4 strokes

easier to hand start due to the higher compression. for example, most Saito's will fire on a hand start simply by flicking the prop backwards against it's own compression once primed.

Do you personally use this method ?

Why are you saying "most" ? Some do not ?
I wonder why some would act differently/

Zor
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Old Feb 01, 2012, 08:59 AM
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yes i do, on engines that will do it. to do it requires high compression & power output on the engine. not all engines have enough power to reverse their direction, go through the next intake & compression cycle by themselves. (for example most old enya 4-strokes will not. very low compression engines).

most os and saito 4-strokes will allow you to back start them. especially larger ones with big props. (no comment on ys)


here's a satio 1.80 being back started after some repairs. notice the chicken stick is on the fat part of the blade & the quick flick to spin it backwards.
Saito 1.80 Cranking (0 min 18 sec)
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Old Feb 01, 2012, 09:42 AM
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"-pleasant sound"

And I have found that to lead to a much more relaxed flying attitude (for me, anyway). Two strokes make me feel like I have a bee, or a chainsaw in my hands. Not so with a 4 stroke. Someone should be playing a waltz. I would never even consider going back.

toysrme. I wondered why the chicken stick. I always snapped mine backwards against the compression stroke, by hand. But, I was not flying APC props.

Les
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Old Feb 01, 2012, 10:00 AM
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99% of the time i do also on that particular engine as the props are very thick near the hub. i just happened to have the stick handy by the door so i grabbed it.
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Old Feb 01, 2012, 11:32 AM
Zor
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Quote:
Originally Posted by Toysrme View Post
yes i do, on engines that will do it. to do it requires high compression & power output on the engine. not all engines have enough power to reverse their direction, go through the next intake & compression cycle by themselves. (for example most old enya 4-strokes will not. very low compression engines).

most os and saito 4-strokes will allow you to back start them. especially larger ones with big props. (no comment on ys)


here's a satio 1.80 being back started after some repairs. notice the chicken stick is on the fat part of the blade & the quick flick to spin it backwards.
http://www.youtube.com/watch?v=5D8QACCcA-Q
Toysrme,

Your video is not very convincing.
In fact it seem to show that the engine does not start due to reverse compression.

Also it quickly quit perhaps due to reverse rotation that many fellows have reported in the past.

This is a subject I have considered in my mind for a long time. I just cannot see that reverse compression can give enough momentum to the prop to carry forward (proper direction of rotation) inertia to have the engine get sufficient compression in the proper rotation and reach the ignition point.

From what I see in your video using the chicken stick downward, I believe that you are passing the ignition point in reverse rotation compression and that reverse firing is what gives enough momentum to start the engine.

Nothing to do from reverse compression else than actually firing the engine. Not compression alone as many postings have led to believe.

Perhaps one day we may be able to put this matter to rest.

Zor
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Old Feb 01, 2012, 08:51 PM
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NM, I misread what I responded to... comment removed.
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