gsmith6879

My Feedback: (37)

This subject was discussed about a year ago but I need to reopen the issue.

We have 5 MVVS 26cc gassers in the club. 2 are the older style with the Vtech ignition and big plug and 3 are the newer style with the small spark plug. All were broken in per the manufacturer and Pe's instructions using Amsoil Saber mixed at 30:1 for break in and them changed to 40:1. These engines are on Ugly Stik type of ariplanes, have never been cowled, and have plenty of cooling.

We have never had any problems with the 2 older engines and they are on about thier 6th-7th gallon of 92 octane gas. One has the stock MVVS silencer and is running an 18-6 Top Fight prop. The other has the MVVS tuned pipe with an 18-8 Bambula prop. They are very strong engines.

The 3 newer engines however are experiencing some serious problems. On the older of the 3 newer engines the photos below show what appears to be a broken lower rod bearing and the crank shaft may have slipped back and hit the back plate on one of them. The photos show a very dark or black oily residue and we have always gotten this gunk out of the exhaust on all of these engines. This engine was fitted with the MVVS tuned pipe and an 18-8 Bambula prop and has about 4-5 gallons of gas through it.

The other 2 newer engines are showing some backplate rubbing and the lower rod bearing has some play but is not broken. They are also using the MVVS tuned pipe and an 18-8 Bambula and have about 2-3 gallons of gas through them.

Can anyone shed some light on this problem? Can we replace the rod bearing or do they need a complete overhaul? We don't want to give up our MVVS engines but sure would like them to run longer before requiring repair.

aeroparts

This is a known problem , we have had the same problem.
We had the problem already during running in.
The lower bearing is broken.
MVVS change the design to stronger one.
We got outstanding support with the problem with our engine from Pe and MVVS.
Here is some video of the engine performance.
http://www.junkshop.nu/flyg/imac_20080712/imac5_big.wmv
Lennart SWEDEN

gsmith6879

My Feedback: (37)

Thank you for the information. I'm hoping that Pe' can give us some advice on how to accomplish the repairs.

pe reivers

For direct support, you are invited to mail me direcly (Not PM), and I will take care of your needs as well as I can. RCU is not the place.

pe reivers

I have commented on this on numerous occasions, but it does not hurt to do it again.

The engine is very powerfull. That means that the generated amount of waste heat also is very large. This has consequences on plane design and engine application.
In plane design, open engine placement will do, as long as there is ample forward speed. Without forward plane speed (like in 3D) the cooling is very lacking and engine temperatures soar up into the range where oil films will break down. Only sae 40 grade or higher oils will cut the cake.
Most two stroke oils however are SAE 30 grade, and fall short of the requirements of the little mvvs gasser. I have no data on Amsoil products, especially the high temperature viscosity (SAE grading), so I cannot comment on the oil's suitability for the 26cc gasser. As far as I know, MVVS prescribes Mobil1 racing twostroke (fully synthetic) racing oils.
There are a few oils that fill the bill, like motul 800 and others, see listing down on this pages link:
[link=http://www.mvvs.nl/MVVS/main_bestanden/petrol.htm]Petrol engines, available oils[/link] scroll all the way down.
Only oils in bold fulfill MVVS requirements.
An added heat load in 3D flight is the plane's attitude (nose up). This causes extra rubbing on the big end bearing, and thus elevated temperatures on top of the average engine heat loading.

It was the suitability of this engine for 3D planes, and subsequent massive 3D application, that started to make clear that the engine had reached it's design limits when operated under less than favorable conditions (thin oil, wrong oil, lack of cooling, Lower rpm props (rpm <8000).
MVVS goes to great length to support their customers, even in those repairs where there would be no question about user error.
Very often, the user is not to blame, because he acts in ignorance of the limits, having previous experience with glow engines that run so much cooler in the same applications.

k_jitsu

Thank you for this information, very usefull in my opinion. I have realized that only those oils whose "100ΒΊ viscosity" is higer than 10 are recomended. My question is: Is the higer viscosity, the best the oil? or is it preferable a medium degree? (May be this is the reason why Mobil 1 is the recommended oil ...)
I think that a too high viscosity (21 for Castrol A747) could prevent oil to reach certain parts of the engine, like front bearing for instance ... or am I wrong?
What's your opinnion?

pe reivers

High viscosity in an oil has advantages and disadvantages. The high viscosity will stay thicker at elevated temperatures, but at the same time it will generate more heat if used in a fast spinning bearing. 8000 rpm in a 8mm bearing is not fast spinning, so there is only the lubricating film to worry about. That is why the all out racing oils have such high viscosity.The oils with 100C viscosity in the 8cSt range (SAE 30 grade) are more suited for light loads and medium temperatures. Would you run a 10W30 oil in your car engine in a hot tropical summer? the choiche would probably be a 20W50!

Thick or thin oil reaching bearing parts makes no difference. Thick oil will penetrate slower, but it will penetrate and do the intended job. Once there, the thicker oil will be harder to remove, and it takes longer for a loaded ball to make metal contact with the race.

k_jitsu

Understood.
Thakn you

pe reivers

You're welcome