Edge 540

How come there are so few 4 stroke helis? 2 stroke engines are geared heavily to produce lots of torque at lower RPM. Wouldn't a 4 stroke be a better choice in the first place?
simplicity of 2 strokes?
better spin up on 2 strokes?
smaller and lighter?


Just wondering.

Gorgok

Unless i'm completely off, i believe for small displacement engines, like RC ones, 2 strokes are far more efficient in a power to displacement ratio.

Kind of like motocross, where 500cc 4 strokes compete against 250cc 2 strokes on a even level.

Now if you could fit a .60 4 stoke in a .30 heli... maybe?

Shortymet55

Like for airplanes a 4 stroke can spin a larger blade , but at less RPM, so saying higher torque, less speed. So for helis RPM is very important, plus a 4 stroke could swing a larger blade to make the lift equal, but thats more weight. And you want high RPM for 3D flying because head speed is very important. Its also like a gyroing effect, that keeps the heli stable.

JRexA

For smaller engines you get a lot better power/weight ratio with 2 strokes than with 4 strokes, and for helis (and motorbikes) power/weight is very important,

The reason for that is simple, the 2 stroke ignites on every single rotation, the 4 stroke only on every second rotation. A 4 stroke has to be twice as efficient as the 2 stroke, in order to produce the same power, because of the non power producing rotation.
You typically go for high rpm with 2 strokes, as you can normally get more power out of the 2 stroke engine at high rpm than low rpm.

BarracudaHockey

My Feedback: (11)

They made a few, the one that comes to mind is the Nexus Leggato which was a 46 size heli with a 52 4 stroke.

You had to run high nitro and keep the engine on the lean side and it still could barely get out of its own way. They do sound cool though.

It could be done but it would be inefficent.

tnd2000

You can't say 4 stroke engine is 1/2 as efficient as a 2 stroke engine because it has more non-power producing cycle.
In fact a 4 stroke engine has 3 non-power producing 'strokes' (intake, compress, exhaust) so that would make it 1/3 as efficient compare to 2 stroke engine by your theory.

The same is true for rpm. Yes the HS is very important but that doesn't require the engine to have high rpm. Just like Edge pointed out, "heavily geared", you can simply gear a low rpm engine to produce the same high HS.

I do not know the answer to Edge's question but I always thought that 4-stroke engine runs on regular gasoline. 2 stroke runs on glow fuel which carries it's own oxygen for burning (think of glow as adding nitro to your car). Engine running on glow fuel is more powerful then on gasoline.

BarracudaHockey

My Feedback: (11)

It takes 2 revolutions for a 4 stroke to complete each action, a 2 stroke fires each revolution. The compression and combustion stroke occur on the same revolution just as the exhaust and intake occur during one revolution.

tippy

My SAITO engines (4 strokes) in my two planes ran on nitro fuel. I used Wildcat 2/4 stroke fuel so I didn't have to carry more than two types of fuel to the field.

So no ... not all 4-strokes run on gasoline.

Good Luck,
d.tipton

BarracudaHockey

My Feedback: (11)

I have a Saito 100 4 stroke in a Mustang, it runs on 30 percent nitro heli fuel.

A quick review of 4 stroke operation.

The intake valve opens while the piston is at TDC (top dead center)
The piston heads down ward creating an area of low pressure and from the intake valve flows the fuel/air mixture from the carb.
The piston reaches the bottom of its stroke and the intake valve closes. The piston heads upwards and compresses the aforementioned mixture. At the top its ignited by the glow plug which pushes the piston down on its power stroke. When the piston reaches the bottom the exhaust valve opens the piston heads back up pushing the exhaust gas out, the piston reaches the top, the exhuast valve closes and the intake valve opens and it starts all over again.

In a 2 stroke engines the exhaust and intake valves are replaced by ports.

The intake and compression occur on the way up, power and exhaust occur on the way down and repeats. (simplified explanation)

Hence 2 stroke engines are less complicated, have no valves and assocaited pushrod and timing cams so they weigh less.

Since each revolution produces power they are more efficent and produce their max power at higher rpms. While they do provide a power stroke twice as often, they don't truly produce twice as much power as the same size 4 stroke.