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Vic
Joined: 12 Aug 2008
Posts: 116
Location: Southern Bavaria
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 Posted: Tue May 19, 2009 1:25 pm Post subject: Engine or prop judder on 18 T |
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Hello everybody,
our 18 T has developed an engine or prop judder since last year that can be noticed mainly in high speed low passes at , say, 70 % engine speed and over 250 kph . We replaced all engine mounting rubbers, did a dynamic prop balancing on the ground with up to 90 % engine speed and the engine felt real smooth. But in the air at high speed dives the shaking of the engine/prop was still there - at a very reduced rate, but still noticeable . The setting of the blades was checked and seemed to be allright in the dynamic balancing . Could we have a problem with the prop hub in case the prop goes to fully coarse in the dive and the blade angles in this condition might be different for each blade ? We suspect some sort of backlash in the hub different for each side ? Has anybody an idea on that one ? Or could it be an aerodynamic effect specific for the 18 T ? We will dig deeper next weekend but any ideas would be appreciated.
Regards
Helga and Vic
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dsavarese0812(at)bellsout
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| Posted: Wed May 20, 2009 5:09 am Post subject: Engine or prop judder on 18 T |
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Vic,
When setting the prop blades (assuming it is a 2 blade Russian paddle prop), it is important the length of both blades as measured from the outside edge of the collar under the counter weight clamp, be equal. Then the blade angles should be set afterwards. Whenever a counter weight clamp is loosened, make absolutely certain the counter weight clamp is flush against the outer edge of the sleeve below it all the way around before tightening the clamp nut. If this is not done, there will be an imbalance.
If you have the typical Yak 18T spinner installed, try removing it completely including the backing plate and test fly again.
I'm just grabbing at straws here, but you might try opening and closing the cooling louvers to see if that changes anything. Louvers without the anti-rattle springs installed might cause a vibration because the louvers are "shaking" at higher speeds. Make sure that the cowl mounting bushings are not totally worn out and you can not shake the upper and/or lower cowl at the rear where the pins are located. Does the front of the upper and lower cowl fit snugly against the cowl ring anti-chafe ring? Check all the cowl mounting brackets for cracks, particularly where the flat sections of the brackets mate with the round tube. Make sure each bracket is secure on the cylinder and the ring. Check the security of all of the engine baffles and baffle pads. Check the security of each of the cooling ducts, particularly where it mounts to the cowl ring.
Check the carburetor intake and heat door assembly for security as well.
During flight and particularly on Yak 52's, the nose wheel spins during flight. If the nose wheel assembly is out of balance, which you typically can feel when landing because of the higher speeds, and the nose wheel tire and hub are rotating in flight at higher speeds, it could be a wheel vibration. Who knows? Anything is possible.
It will be very interesting to know what you try and if you figure out what is causing the vibration.
Dennis
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mark.bitterlich(at)navy.m
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| Posted: Fri May 22, 2009 8:41 am Post subject: Engine or prop judder on 18 T |
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This might sound especially stupid and I hesitate to even mention it, but..... I recently had a very interesting problem in my YAK-50. Along with it becoming very difficult to start when hot, it also developed a very noticeable shake at high speeds. Pretty much exactly as you described which is why I am about to go out on a limb and just mention what happened to me.
Anyway, after a full year of problems with the M-14 not wanting to start when hot, and after replacing every single piece of the starter ignition system, the only thing left was the spark plugs. I am using automotive plugs via Dennis's kit, which by the way is the best thing since sliced bread.
So, I replaced the plugs. The starting problem went away and all is now well. I really had a hard time believing it was the plugs. Still do in fact, but the fact is.... The problem is GONE!
ANYWAY ~ Funny thing. The high speed vibration went away too. Mag checks during all of this were always good. No indication of plug problems at all. Upon inspection, found that the plug gap was set too wide ... My fault for not double checking the A&P that was helping me put in the kit way back when. The plugs were at .032 or so. Dennis recommends .025 I have talked to other folks who claim .018 is about right, and that wider than .018 can cause "starting problems". I have mine now gapped at .025 per Dennis's recommendation and everything is working perfectly, so I will avoid .018, but it is worth keeping in the back of your head.
Anyway... To get to the point... It's a real long shot, but you might have a spark plug problem, or some other sort of ignition problem, especially if you are using the original ignition wiring and ring.
Mark Bitterlich
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viperdoc(at)mindspring.co
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| Posted: Fri May 22, 2009 10:17 am Post subject: Engine or prop judder on 18 T |
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A glow plug. Quite possible...did not think of that one. The exact way a
diesel engine or R/C model airplane engine runs...well used to before
turbines and 4 stroke chainsaw engines were introduced the modeling
communities.
Doc
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