NebulaDDS

I know there are many posts and articles about the differences between 2-stroke and 4-stroke engines, and I'm familiar with the differences.

4-stroke burns less fuel and produce less maximum horsepower. 4-strokes, however, get most of their power in the lower RPM ranges (i.e. they produce more torque at lower RPMs than do 2-stroke), and therefore it is easier to use larger props on them....although I don't understand why. 4-strokes can be tougher to tune and maintain. They weigh more. And, they produce a more realistic sound.

But what I REALLY want to know is this: why would someone choose a 4-stroke over a 2-stroke for an airplane?

Are there situations where there is an actual in-flight performance advantage using a 4-stroke that cannot be achieved with a 2-stroke?

da Rock

If you lock the comparison to displacement, then the answer is no. 2strokes of the same displacement are going to easily achieve what a 4stroke can do. They might do it with a different prop at a different rpm, but they'll do it.

If you base the comparison on weight, which with airplanes is a more sensible basis, then the answer is yes. But there are very, very few 4strokes of any weight that can't be matched by almost every 2stroke of that weight.

In-flight performance leaves out a number of things. And most guys buy 4strokes for those things. Like sound. It's probably the most often quoted reason for using 4strokes.

daveopam

My Feedback: (9)

I said this in another post but I think it fits this post also. "Forget sound,fuel consumption, weight,maintanence and all that. When your ruddder is 2" from the ground and you nail it the 4C will move first."
That is why I use them on 3D type planes. It is not a huge difference and a person can get use to anything. After flying mostly 4C engines a 2C feels a little soft to me. There is just that split second of hesitation before it comes to life.

David

MinnFlyer

My Feedback: (4)

Another big issue is mess.

I use mostly 4-strokes, but I will admit that there are a few planes that I prefer using a 2-stroke in.

But when I fly those planes, I am always amazed (if not slightly grossed out) by the amount of gunk that the 2-stroke has spit all over my plane!

With my 4-strokes, it's not unusual for me to throw it in the back of the car at the end of the day without ever cleaning it. Sure it has a little oil and grass on it, but not so much that I am afraid of messing up the car.

The next thing to consider is fuel consumption. Run a 61-size 2-stroke for a summer and see how much fuel you go through. Now do the same with a 91 4-stroke.

Here's the math:

.61ci @ 11,000 RPM = 6,710 Cubic Inches of Air/Fuel Mixture burned per minute

So in a 10 minute flight, 67,100 Cubic Inches of Air/Fuel Mixture are burned

If you fly 3 flights, 3 times a week, that comes to 603,900 Cubic Inches of Air/Fuel Mixture burned per week

Times 4 weeks over 5 months = 12,078,000 Cubic Inches of Air/Fuel Mixture burned total

Now let's look at a 91 4-stroke:

.91ci @ 9500 RPM (Divided by 2 since the cylendar is only filled on every other stroke)= 4,322.5 Cubic Inches of Air/Fuel Mixture burned per minute

So in a 10 minute flight, 43,225 Cubic Inches of Air/Fuel Mixture are burned

If you fly 3 flights, 3 times a week, that comes to 389,025 Cubic Inches of Air/Fuel Mixture burned per week

Times 4 weeks over 5 months = 7,780,500 Cubic Inches of Air/Fuel Mixture burned total

12,078,000 - 7,780,500 = 4,297,500 or roughly a savings of 1/3 the fuel

So if you're paying $15/gal for fuel, it's like paying $10/gal instead

TFF

If you built scale planes with big round cowls the 4 stroke has an advantage by getting more useable blade in the airstream. In 4S the larger props come from being able to turn the burn into force more efficiently. The expanding gasses are contained until the valve opens, which is a stroke in its self; where as in the 2S lets good unused energy escape as the piston opens the exaust port. The 2S has to share the intake and exaust in the same stroke.

NebulaDDS

MinnFlyer,

Excellent post! I have a lot of respect for someone (like you) who produces numbers like that. It shows that you take the time to think-through your answers rather than spewing out information that you heard someone else say some time ago. I do appreciate it.

Of course, somehow I doubt that estimating relative fuel consumption of two-stroke vs four-stroke engines is as simple as multiplying displacement by RPMs. Two-strokes, if I'm not mistaken, are less efficient, therefore they actually waste more fuel per cycle. I believe this is because the incoming mixture of fuel/air can actually mix with the outgoing exhaust, which allows unburnt fuel to be expelled....unlike 4-strokes, where the intake of fuel/air and explusion of exhaust are carried out in two separate strokes of the piston.

Not only that, I'm not really sure why you (and virtually everyone else) compares 2-stroke engines with 4-strokes of larger displacement. True, an airplane that requires a 60-size 2-stroke probably can't fly worth s.h.i.t. with a 60-sized 4-stroke, but nonetheless, when comparing the two types of engines, it seems that it'd be more appropriate to compare engines of equal displacement.

MinnFlyer

My Feedback: (4)

It is true that a certain amount of fuel/air is wasted with a 2-stroke (Even with a tuned pipe), but I didn't even account for that due to its inconsistent nature.

And I compared a 61 2-stroke with a 91 4-stroke because fair is fair - a 60 size plane (2-stroke) would require a 90ish 4-stroke.

But it just goes to show that even the larger displacement engine will burn a considerably less amount of fuel.

Tall Paul

Why?
Because it pleases me to do so.

NebulaDDS

"If you built scale planes with big round cowls the 4 stroke has an advantage by getting more useable blade in the airstream. In 4S the larger props come from being able to turn the burn into force more efficiently. The expanding gasses are contained until the valve opens, which is a stroke in its self; where as in the 2S lets good unused energy escape as the piston opens the exaust port. The 2S has to share the intake and exaust in the same stroke."

So what you're saying is that a 4-stroke can swing a larger prop than a 2-stroke.....which is not true. What IS true is that a 4-stroke produces its maximum torque at a lower RPM than a 2-stroke which, for some reason I cannot understand, is better for using larger props.

carlosponti

My Feedback: (1)

my opinion on this subject is this;

i like they way they sound

they seem easier to run

seem cleaner that the equivalent 2 stroke

they seem to use less fuel

they swing a bigger prop

and if you get a saito they just look cool with separate rocker heads.

most of the reason i like 4 strokes is just personal prejudice for them over 2 strokes.

k3 valley flyer

My Feedback: (18)

Carlosponti I agree with all your reasons, and I think they are quieter adn easier to listen to.

B.L.E.

Do four stroke engines really use the entire piston stroke to make power? The power stroke pretty much is over once the exhaust valve opens.

Here are the valve timing numbers out of the shop manual for a motorcyle that I have.

Intake open 35 degrees before top dead center
intake close 55 degrees after bottom dead center
duration 270 degrees

Exhaust open 45 degrees before bottom dead center
exhaust close 25 degrees after top dead center
duration 250 degrees

Note, this is not a race engine but a engine that's tuned for low and midrange power.

B.L.E.

Yep, so quiet that they don't even need mufflers.

khodges

My Feedback: (1)

This is true but 2-strokes tuned for max power at high rpm (like most glow engines) still have a higher "overlap" due to how the ports are cut, and less of the total power stroke is under compression, and less of the compression stroke is devoted to actually compressing the mixture. A 2-strokes ports can be cut for better low-end grunt (more torque) but it then causes power losses at high rpms. Then the difference in displacement would make a lot more difference when comparing 2- and 4-stroke glow engines, and 4-strokes would make close to equal power for equal displacement throughout the entire powerband.

2-strokes are more sensitive to exhaust scavenging, and most of your standard glow mufflers don't scavenge well. Put a tuned pipe on, though, and see how much more you can get out of it, but usually within a different rpm range.

We used to slot the camshaft sprocket screwholes so we could change the valve timing and overlap on our racebikes to tailor the powerband for the track we'd be at.. More low end grunt on tight courses, more top end power on tracks with a lot of straightaways.
I imagine the powerband on 4-stroke glow engine could be changed the same way but overlap changes would be more limited because most 4-stroke glow engines have a single camshaft, and the opening/closing of each valve can't be changed respective of each other like they can on a twin cam engine.

Which is all well and good, doesn't really address the theme of the post. I like 4-strokes for the sound and appearance. Tuning isn't that much more difficult, (if at all), and the maintenance is fun. I love messing with machinery.

B.L.E.

Back to the original question, "Why would you put a 4-stroke on your airplane? "

The short answer to this question is that this is a hobby.

Here we are at the dawn of the 21st century and people still ride horses, shoot flintlock rifles, sail wooden boats powered by sails, write with fountain pens, keep old steam trains running, hunt with bows and arrows, and fly old WWI airplanes. It has nothing to do with better or cheaper. It's a hobby and better or cheaper are irrelevant.

saramos

I use a particular 4-stroke for two features that I personally value. It fits in a narrow cowl, and it turns a huge prop. Both these features fit with my interest in scale warbirds, allowing for a close to scale 4 blade propeller, and a more intact scale cowl.
This is the RCV SP series engine. These 4-stroke engines are an unconventional configuration. First, the engine is internally geared 2:1 which allows for the large props. Second, with the piston traveling back and forth, and not up and down, it fits in narrow cowls. It also uses a rotary valve instead of poppet valves eliminating the need for valve adjustments. The engine, like other 4-strokes, uses less fuel. But unlike other 4-strokes, it does produce a lot of slime out it's crankcase breather.

Here's a couple of photos of the RCV 90SP (.90ci) on my TopFlite 1/7 scale Spitfire with it's 15.5" x 12" four blade APC flying prop.

Scott

mvallyman

My Feedback: (32)

Dont always assume that 4 strokes are heavier. The Saito 1.00 weighs 2 oz less than the OS .61 FX, There are many Saito's that weigh less than an OS 2 stroke of comparable power. Also, the 4 strokes have a more even (usable) power band than a 2 stroke engine. 2 stroke engines provide max power within a narrow rpm range, 4 strokes have a wide rpm power range giving you more consistant power through climbing manuvers. I dont see how they require more maintenance, adjusting valves once a year is no big deal.

But they do cost more.

CustomPC

This is a timely post.

Here are my own observations from testing and comparing a 2-stroke OS .75AX and and 4-stroke OS .91 Surpass II on a Great Planes Super Skybolt.

Both motors will spin the APC 14x6 at close to 10,000rpm. A calculated 1.6hp which is well below the specc'd figures for the .75AX and on par with the figures for the FS.91.
However the .75AX will spin smaller 13x6-8 props at rpms between 10,900 and 12,000rpm which is a calculated 2.2hp and on par with the specs.

This tells me it's torque and not hp which turns the props and the torque curve for the 4-stroke is flatter with more of the torque available throughout the rev range, hence thier reputation for instant throttle response.

In this instance, the four stroke is actually lighter by more than 4oz. The motors are roughly the same weight but the weight of their individual mufflers makes the difference. The FS.91's muffler is considerably lighter.

In terms of flying quality. With both motors spinning the same prop at the same rpm there's not a lot of difference at WOT. They both offer spirited performance in the Skybolt. The difference is felt in the lower rev range. I spend a lot of time around 1/2 throttle and i do notice that the .75AX will gurgle and sputter after prolonged cruising. At this point it will load up a bit as the throttle is increased. The FS.91 is happy to chug along at low throttle the whole flight.

I haven't detirmined which motor consumes more fuel. I fly for about 10 minutes and there's always plenty of fuel in the tank with both motors.

Noise and sound is the last issue. The .75AX only slightly louder with the stock muffler but almost unbearable with a pitts muffler. The 4 stroke sounds underpowered in comparison but the prop is still turning at the same rpm. To each there own i gues.

RVman

"Why would you put a 4-stroke on your airplane? "

I wouldn't. I tried them and they vibrate and shake and the mufflers fall off to much. As for prop sizes my webra 91 2s is quite happy with a 15-12 APC at around 7000rpm. JMO.

BillyGoat

To add to the list of reasons… I find four strokes are much less finicky than two’s. I can fly my four strokes day after day and never need to touch the needle valve, while my two stroke needs adjustment just about every flight.

Also more reliable. I had not noticed this before but a buddy pointed out to me, that two strokes go dead stick about 10 to 1 vs. a four stroke. Once I started taking notice of this at the field, I found he was right if not a little low on the ratio.

MinnFlyer

My Feedback: (4)

I look at it this way:

If 2-strokes were so good, why don't we see more Harleys using them?

rc-sport

My Feedback: (12)

I don't like 2 strokes because:
a. the noise they make is like finger nails on a blackboard
b. I hate those monster size mufflers
c. I like to run my motors inverted

B.L.E.

Are you sure that this isn't because four stroke flyers tend to be more experienced modelers who aren't learning how to run engines?

I have owned some very reliable two stroke engines.

jeffk464

Don't forget 4 strokes come with much smaller mufflers that can be swiveled into almost any position. This makes them much easier to install and improves the overall looks of the plane, in my opinion.

Also the hardware that holds the muffler onto the engine is more substantial and doesn't seem to want to shake loose like on 2 strokes.

Hill202

My Feedback: (24)

Small mufflers on 4 strokes? What happened to this one?