Posted Saturday, January 22, 2011 @ 09:28 AM
Hi again Guys,
More information on my Aeropower 1750 engine saga...
I have worreid about the excess friction in this engine for over six months, so as soon as I arrived back in Autralia, I took Eugenio's advice and pushed and pulled hard on the propeller, but could not hear any clunk-clunk.
So, I removed the engine, removed the magneto, fitted a dial test gauge (DTG) and measured the crankshaft end play.
I did this by first hitting the centre of the prop flange several times quite hard with a rubber hammer. Then I put the end of the DTG probe against the flange and set the dial to Zero.
Then I pulled the probe away from the flange, went around to the back of the engine, hit the magneto drive with the rubber hammer and then went back to the front and replaced the DTG probe against the flange.
I did this four times. Every time the DTG measured exactly 0.9mm, which is perfect.
So the friction is not caused by the crankshaft end float.
I can now quantify the friction. With the spark plugs removed, it takes a pull of 8 pounds (3.5kg) on a spring balance to move the propeller, when the spring balance is fitted to the propeller at 20 inches (50 cm) from the centre of the hub.
As a comparison, measuring the force to move the propeller on the old Rectimo takes 2 pounds (0.9kg) at the same diameter. So there is about four times as much friction as there should be. This makes it extremely hard for me to swing the propeller enough to get the engine started. I can get one 'pop' from any cylinder, but to get the engine to run, I need to have the throttle nearly half-way open, which is very risky. Also I think the engine will not turn over for a 'windmill start' in the air if it stops during aerobatics.
Other suggestions I have been told include that maybe the line boring was not perfect, but the guy who did it is a real expert in VW line boring, a very precise machinist and and he was carefully supervised by Mike Munninger, the Aeropopwer man (and formerly his boss). It may be wrong, but I doubt it.
Mike insists that the friction is caused by the nikasyl cylinders, saying it might take 50 hours for them to bed-in and the friction to reduce (I have only flown 3 hours so far).
Other suggestions are that one of the main bearing shells may have moved off its dowel, or that the front (dynamo) bearing, which has two holes -- one for its dowel and one for the oil-way -- may be fitted the wrong way around. Again, I doubt this, since Mike's small company specialised only in VW engines, and I doubt if he would make such a mistake.
I have a photo of the engine, that I took during its assembly, and this shows that the main bearing shells are held in place by red stuff (Hermetite or similar?) so I doubt that they have shifted.
I suspect it may be something to do with an inadvertent overheat on the dynamometer. For the last, full-power run, we forgot to turn on the big electric cooling fan until after we had gone to full power, but then we did turn it on and left it on for a while, so this may or may not have had an affect on the nikasyl cylinders.
And of course, after that I completely filled the engine with oil and then left it on a fuel drum in my hangar for four years, so there may be some internal corrosion or rust soemwhere, but I would have thought this would have gone after 3 hours of flight.
I think my next step is to remove one head and a pair of cylinders, to identify whether the friction is in the pistons & cylinders, or whether it is in the crankshaft & bearings. If it is in the crankshaft, I guess the I will just have to open up the engine, but as you can imagine, I am very reluctant to do this.
Does anybody else have any ideas?
Yours, Bob
[Edit by Bob Grimstead on Saturday, January 22, 2011 @ 10:08 AM]
[Edit by Bob Grimstead on Saturday, January 22, 2011 @ 10:53 AM]
