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Severe vibration when backing off the throttle at higher airspeed with 914

  • Red7
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1 week 6 days ago #22492 by Red7
I have a 914 on a gyroplane that is giving me fits. The fixed pitch Helix prop is the one supplied by the kit manufacturer and unfortunately is not adjustable in pitch but I confirmed that it is the correct prop that the manufacturer provides for the 914 (15° on a 68" prop which is the biggest prop that will fit this gyro). Since I got this flying, it has had a violent vibration when coming off the throttle to land and after doing extensive testing, I've realized that it only does it at certain RPM/airspeed combinations. I've synched the carbs several times and it runs perfectly smooth at idle and smooth above 4000 rpms. However, if I'm flying the pattern at 70 mph and back off the throttle, as soon as I get below about 3700 RPM the whole aircraft starts to shake and I'd say it's at it's worst about 3300 rpms and then gets fairly smooth again below 3000 rpms. Here's the kicker.....if I slow down to 30-40 mph before backing off the throttle, I can go from 4000 to idle and it is fairly smooth the entire way (I say fairly smooth as I wouldn't call this prop the smoothest I've felt so maybe a blade is not tracking but there is really no adjustment in this prop). So I did some testing where I set the rpm at 3300 and went from 70-30 and back to 70 and below 30 mph there is no vibration, at 40 mph the vibration starts, at 50 mph it much worse and by 70 the entire aircraft is shaking.

I've spoken to Lockwood numerous times and they thought it was carb related so I drove 5 hours each way and paid to have them go thru the carbs and make sure everything is fine but it still does it. I put in all new spark plugs and that didn't help. I've spent so many days trying to figure out this problem but can't. After doing my testing at various speeds, I spoke with Lockwood again and they think that it's my prop because at climb out, I'm only getting 5100 to maybe 5200 rpms rather than 5500. What they think is happening is that the prop is taking too much of a bite out of the air and when I back off the throttle and the rpms drop, it's too much pitch combined with the higher airspeed causing a vibration that then causes my floats to bounce around which in turn, unseats the needle causing the engine to run really rough. Another symptom I'm having is that I get fuel in the manifold pressure sensor tubing coming out of the crossover tube between the carbs. This is why Lockwood thinks the floats are bouncing around and unseating the needles which inturn is making it run even rougher because they've seen this same fuel in the MAP tubing when the engine is flooding.

Has anyone else experienced anything like this? I'm at my wits end because I have over 4000 hours with 912 engines and have never had any issues and this is my first 914 and it has been nothing but trouble between fuel pump issues and this vibration issue. I've spent so much time trying to figure this out and could really use some help. I've ordered a new prop (different brand) so I can adjust the pitch but it won't arrive for another week or so and I can't find anyone with another prop to try that will fit my gyro. I've thought about cutting the prop blades down in small increments until I get 5500 rpm at climb out but I'd hate to hack into a brand new prop if I'm not sure that is the issue. BTW- at WOT I do get up to 5500-5600 rpms when going over 100 mph but I have zero intention of flying around that fast so it does seem like I need to get less bite in this prop. I've thought about a constant speed prop but again, have no desire to fly around above 90 mph.

Hopefully someone here can help! Thanks!

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  • INFO@HANGAR36.COM
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1 week 1 day ago #22563 by INFO@HANGAR36.COM
I think getting a different prop that can be adjusted or is "less aggressive???" sounds like a good start.... I would like to know how the new prop works out. Thanks Jay

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  • Roger Lee
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1 week 1 day ago - 1 week 1 day ago #22564 by Roger Lee
You've addressed some of these, but there are more causes. Your 5100 on take off should be fine since you can get 5600 in level flight. Most of the LSA aircraft in the US is up along those numbers without any issues. How long is the prop that you have? Has it been dynamically balanced? How many hours on the engine. How old are your rubber engine mounts? The gearbox may just need an overhaul inspection and tightened up with some new belleville washers and shims.


www.rotax-owner.com/en/rotax-blog/item/57-vibration-help

Roger Lee
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Tucson, AZ Ryan Airfield (KRYN)
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Last edit: 1 week 1 day ago by Roger Lee.

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1 week 1 day ago - 1 week 1 day ago #22572 by garrett
Here is a product that may help. Balance master bolt on for 912 and 914 keeps prop balanced at all speeds.
Last edit: 1 week 1 day ago by garrett.

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6 days 14 hours ago #22581 by MesquiteTim
In m opinion you have a problem with your rotor blades, not the propeller or engine. Everything you describe in the first paragraph points to an airspeed related problem. If the prop or the engine was the issue, your airspeed would not matter. The propeller would cause the vibration at any airspeed, right? Try the test again at 4000 RPM to verify that it still happens. Also, what is "violenty vibrating"?. Are you experiencing stick shake? My stick will shake more than normal if I perform a descending turn while accelerating. You might be in the same envelope which is "normal". It would be helpful to know what gyro brand you own.

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6 days 3 hours ago #22584 by Gyrochook.
In most cases the vibration is generated by the propulsion system and the harmonics are fed into the mast and thus the rotors so in all cases the source of the vibration has to be identified BEFORE fiddling with the rotor system. I have just been through this same exercise with my ELA gyro where the engine is mounted directly to the mast. I had done the carby balance, 3 different propellors, engine mounts and electronic dynamic balancing which allowed us to get the prop balanced but we could still not get rid of the base vibration. Removal of the gearbox and a report back from the service centre in Victoria Australia confirmed that there was some damage in there which was not noted in the logbooks. A$3000 later and I am awaiting return delivery to test for the vibration. These are the risks that you face when purchasing used aircraft. Good luck Red7 and thanks for keeping us posted.

Regards............Chook.

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  • Red7
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6 days 1 hour ago #22585 by Red7
Thanks everyone for the input so far and I apologize for not replying sooner. When I posted this it said it needed to be approved by an administrator and I'd be notified once it posted. I checked and didn't see it the next day and just checked again and see that it posted.

Ok so after some more experimenting, I have some more information to share. First off, this is not a rotor vibration as it's much higher frequency and is definitely engine and prop related but I do think I have a couple things going on that may be playing off of each other.

1. In my original post I mentioned that at greater than 30mph I get a horrible vibration (entire aircraft) mostly between 3000 and 3700 rpm that Lockwood thought was prop related and causing enough vibration to unseat the needles and it appears they were right. I borrowed a 68" Sterna prop and swapped it out with the Helix that comes on my gyroplane and the really bad vibration went away. I can now back off the throttle below 4000 and down to idle without what appears to have been the carbs getting completely off their rocker.

2. However, though the violent vibration is gone, I still get a lesser, high frequency vibration that feels more like an aircraft that has really old motor mounts even though mine aren't. But today, I may have discovered a possible issue. I should have mentioned this in my first post but thought it was just caused by the needle unseating and causing the engine to run rich but after I got my carbs rebuilt by Lockwood, I put in new spark plugs and noticed that my 1, 3 and 4 cylinders all had perfectly tan plugs but my #2 cylinder had black soot on both plugs. I put in the new plugs and then put on the new prop and since that vibration went away, I didn't bother checking the new plugs until today. Same exact thing where all of the plugs look good except for both of them in #2 are black. What would cause cylinder 2 to be running rich when cylinder 4 shares the same carb? Would this likely be the source of my problem causing the vibration? Could it be a problem with the ring? Valves? I do not have a lot of engine experience and am wondering if it sounds like I need to fly it down to Lockwood to have them dig into this but are there anythings I should check/test first? Roger- given what you said about the gearbox, it's sounding like I probably need to get more experienced eyes on it. The engine only has 140 hours on it but clearly something is off.

I just hope I can get the vibration out of this aircraft as it's just not fun to fly and I spend all of my time trying to find the smoothest combination of airspeed/RPM to make it as comfortable as possible. Chook- I will be curious to hear your outcome as mine is an ELA as well and I'd been fearing that it may be the design of their motor mounts but now that I know that one of my cylinders is running rich, I'm holding out hope that once I get this resolved it will run smooth.

Thanks everyone for your help!

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6 days 1 hour ago #22586 by Red7

garrett wrote: Here is a product that may help. Balance master bolt on for 912 and 914 keeps prop balanced at all speeds.


Thanks! I did borrow one of those from a friend and tried it with my original prop and it made zero difference but it's very possible the vibration was just too much for it. It's possible it could help with the new prop so it may be worth buying one.

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6 days 1 hour ago #22587 by Red7

Roger Lee wrote: You've addressed some of these, but there are more causes. Your 5100 on take off should be fine since you can get 5600 in level flight. Most of the LSA aircraft in the US is up along those numbers without any issues. How long is the prop that you have? Has it been dynamically balanced? How many hours on the engine. How old are your rubber engine mounts? The gearbox may just need an overhaul inspection and tightened up with some new belleville washers and shims.


www.rotax-owner.com/en/rotax-blog/item/57-vibration-help


Thanks for that link Roger! I will check the gear box friction and magnetic plug.

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