Rotax 618 Rave valves.

[Rattler 1]

I've heard a lot of negative comments on the 618 scoring pistons and RAVE valve issues. (sticking) . I purchased a Sport Copter with a new rebuilt 618. The RAVE valve is supposed to open at 6200 RPM to give it more airflow out the exhaust at high RPM and close below 6200 for higher compression for better low end torque. My theory is that if the RAVE valve sticks closed the compression gets too high above 6200 RPM causing pistons to score. Especially when in lean conditions. I adjusted the prop on my Sport Copter for a high idle of 6500 RPM. Test flew it and could on.y reach a true ground speed of around 50 MPH. I then took and installed washers to hold the RAVE valves full open all the time. I test flew it and the high RPM went to about 6900. I actually increased HP at high RPM. I re-pitched the prop to achieve 6500 rpm and now have a true ground speed of about 70 MPH. I don't like to run it wide open but prefer to run about 6000 at cruise. I filled the fuel tank and marked the level. I then flew it for exactly 1/2 hour at 6000 rpm (60 MPH). I measured the fuel I then added to bring the level to the exact mark and it burned exactly 2 1/2 gal. That's 5 gal per hour.

Does anyone have any input on what I did here. I like the outcome and plan to keep it this way. I have been concerned about those RAVE valves sticking. I figure I only lost low end torque and ensured full HP at high RPM. I don't need the low end torque in an aircraft like you do in a snowmobile.

 

[Jazzenjohn]

Interesting experiment! If the rave valve is for low rpm power plus high rpm HP, it suggests that for us, if the valves were stuck down it would limit max rpm and if they were stuck up it would show up as difficulty spinning up the prop from low rpm, slower throttle response. Did you notice anything like that? You may very well be on to something Rattler!

 

[Vance]

The last Sport Copter Vortex I flew with a Rotax 582 and the enclosure would easily reach 80kts (92 miles per hour) at a density altitude of approximately 950 feet.

I suspect it would have gone faster; I was not testing the top speed of the client’s aircraft.

In my opinion with a 618 it should be faster.

I suspect that for some reason your engine is not producing full power.

I know nothing about the 618 Rave valves so I made an inquire and this is what I found from Ultralight news. I have seen similar devices on two stroke motorcycles.

Rotax Rave valve maintenance and cleaning.

The Rotax 618 uses a system called RAVE to adjust the power output of the 618 engine at lower engine rpm., and like anything that moves up an down in a channel, which is effected by heat, and oil it requires maintenance and cleaning to work properly.

The 618 Rotax uses the spent exhaust gasses to open and close the RAVE valve, which is a simple guillotine style of valve. A small hole in the exhaust port applies pressure to a set of bellows, under load or at high revs the back pressure pulls the guillotines up and down adjusting the back pressure of the exhaust system. At lower rpm a spring is used to keep the valves closed.

If the valves are stuck open or closed they can effect the 618 in lower or higher rpm settings. If your engine revs fine at full power but is sluggish at the lower end then your valves are sticking open - not giving enough back pressure to the system.

If your engine is not putting out full power at top end but run fine from at an idle they there is a good chance your valves are sticking in a closed position, or failing to open all the way up.

Cause:

The major causes of sticking RAVE valves on the 618 Rotax engine are the type of oil used and the pitch of the propeller.

Oil - The RAVE valve system requires oils with a "high flash point," which refers to the temperature that an oil ignites at. Higher flash point oils are less prone to leaving oily residue on the guillotines, residue that builds up, hardens, and prevents proper valve action.

Too much oil is another problem. The 618 uses an oil injection system, to help prevent excessive oil input into the engine it is essential that the oil pump be set up properly.

Storage: Many of our craft get stored for periods of time. During these storage periods it is essential that the engines be stored properly. With fuels drained and engines lubricated.

Propeller:

The next major contributor to oil build up is the propeller. Too much pitch in a prop will lug the engine down, causing excessive build up in the RAVE valve system. The 618 likes to rev and Rotax recommends a setting of 7200 rpm full power straight and level flight. This can usually be set up by achieving 6800 tied down on the ground.

Cleaning: The RAVE valve system is not complicated, but attention must be paid when disassembling and reassembling the unit. A parts manual, and Polaroid pictures during disassembly are recommended.

To remove the RAVE valve unsnap the retaining spring (43), remove the cap (42), being careful not to loose the compression spring (40) use a 10 wrench or socket, to remove the nut (41) on top of the rave valve stem (21), remove bellows, (34, 36, 37) use an allen wrench to remove two allen bolts, remove housing with valve assembly.

When you have the slides out inspect them for carbon deposits. If present remove them with a find wire brush and cleaning solvent.

 

[Rattler 1]

I don't really notice low rpm power loss. But with a Gyro, I don't go to full RPM at once. I gradually increase to spin the rotor up on take off. So I don't think it would be noticeable. That said, like Vance stated, I do think it should go faster. I will need to call Sport Copter and ask what they suggest. One thing I'm not sure of and I should know this is the size of the propeller. I will measure it next time I go out. This could also be an issue. Not sure. It's a 3 blade Powerfin.

 

[Doug Riley]

Gyros, like any device that uses a propeller, can be optimized either for high or low forward speeds. You can't optimize for both at once. Choosing prop pitch is analogous to choosing which gear to use in a manual-transmission car. Shallow prop pitch = stump-puller mode; high pitch = overdrive.

If you set your prop pitch for fast cruise, your takeoff and climb will be relatively poor. The prop is apt to be partly stalled until the aircraft is travelling at a few dozen mph.

Two-stroke engines are "peaky" by nature, and it's actually possible for a 2-stroke to make insufficient torque to get prop RPM past a certain mid-range figure when the aircraft's airspeed is low. The high-pitched prop simply presents too much of a load. Sometimes, you can get off the ground at this low engine RPM, but can only get the engine up to redline by diving past a certain airspeed (diving unloads the prop, allowing it to spool up). This phenomenon is more likely with the RAVE valves locked open, reducing low-RPM horsepower and making the engine even more "peaky" than it already is.

Also, with high cruising pitch, you may find that the gyro surges ahead once you get past some certain airspeed (I had one that hit its stride above 70, but was a total pig at 45).

I'd try to get the RAVEs working as designed, and then set the prop pitch to whatever compromise meets your preferences. Some people have rigged a manual RAVE opener, to guarantee that they can get the valves open if the automatic system is balky.

 

[MikeBoyette]

I have rave valves on my 670 as well. I used Amsoil Interceptor and have never had a problem with carbon. I use avgas so it does a very good job with the detergent in it. Had the engine apart several times in 200 hrs. Never had any carbon in the engine at all.

 

[gyrojake]

I have rave valves on my 670 as well. I used Amsoil Interceptor and have never had a problem with carbon. I use avgas so it does a very good job with the detergent in it. Had the engine apart several times in 200 hrs. Never had any carbon in the engine at all.

Amsoil Interceptor is the best oil for RAVE Valve engines, It's great for all rotax engines

 

[Rattler 1]

Amsoil Intercepter is also the only oil I use. I had the high idle rpm set at 6500 rpm. Once I locked the RAVE valves to stay wide open the RPM went to 6900 which tells me I gained HP by doing so. I don't notice any issues going through mid range. I readjusted the prop to again achieve 6500 and I gained airspeed. So now I'm trying to fine tune for the speed advertised. I spoke to Sport Copter and the next step is to see what the LBS of pull it has static tied down. I do have larger flotation tires on it and they told me I could loose 10 mph with them. I checked the prop today. It is a 66 inch diameter adjusted to 13 deg pitch which is close to what they recommend. I may have low HP but don't know. will know once I get the get the LBS of pull test.
More to come.

 

[gyrowoody]

On another problem with the Rotax 618 : does anyone know the solution for a starting problem ? My factory overhauled 618 starts well when cold , but does not want to start when warm ( after a previous flight, for instance).
I have intake silencers with the carbs set up as per the manual, I get the correct EGT’s , the plugs shows the correct colour so I know the mixture is correctly set for each cylinder, and it has a manual starter. Whether I use choke or not on hot starts does not make any difference. It just does not want to start. Start from cold is not a problem however. How can this problem be rectified and what is the cause ? Anyone ?