Resonance...?

XXavier

Member
Joined
Nov 13, 2006
Messages
1,481
Location
Madrid, Spain
Aircraft
ELA R-100 and Magni M24 autogyros
Total Flight Time
913 gyro (June 2023)
Hello all...

I'm currently flying an ELA gyro, fitted with a 912 engine and a three-blade DUC propeller. The machine has now only 30 hours, and the stick shake has been very strong from the start. I believed that it could perhaps be due to an imbalance of the rotor blades, but yesterday, while making very long, gliding finals, I realized that the shake wasn't present when the engine was idling, independently of the airspeed. I tried gliding at different speeds, from 40 to 85 mph, and the stick shake was always very light, coming back strongly only when I opened the throttle.

I'm now suspecting that the cause of the shake may be related to a resonance between the propeller and the rotor. I've noticed that it reaches a maximum when the engine revs are around 5500...

Any comments?
 
Xavier, The shakes could come from a flutter of rudder. Have you checked that your cables are taut enough?
 
Jean - Claude;n1122374 said:
Xavier, The shakes could come from a flutter of rudder. Have you checked that your cables are taut enough?

I believe they are OK, but I'll re-check today. Thanks for the hint...
 
Xavier,

Check the propeller/engine vibration. Track and balance the propeller, and check the engine silenblocks.
 
In any case, it should be under warranty yet, so ask to the factory.
 
Arco;n1122376 said:
Xavier,

Check the propeller/engine vibration. Track and balance the propeller, and check the engine silenblocks.

Thanks... On the ground, when revving up the engine, I don't feel anything wrong. When flying, the vibration is felt in the stick; hence, the vibration must come from the rotor. But the fact that is that the shake appears only when flying with the engine, and does not when diving with the engine idling... Thus, either the engine or the propeller, or both, must have something to do with the vibration. However, and as I don't feel any abnormal vibration when revving up the engine on the ground, the cause of the vibration probably lies in some sort of rotor/engine - propeller interaction that takes place only while in powered flight...

I'm not inclined to file a guarantee claim, if only for practical reasons, as the factory may always affirm that my gyro's shake level is the normal one to be expected...

Regards
 
Xavier
The 6th harmonic of the rotor (6 x 350 = 2100 rpm) can be very close to the prop rotating frequency (engine revs need to be around 5100 of course). I've noticed that on some gyros the 6th harmonic is higher than the fourth (never higher than the 2nd which is 2/rev) and often wondered if this was due to the two vibrations "getting together". I've never managed to prove anything but the next time I see this on a gyro I'll try your technique of gliding with the engine at idle.

Jean Claude's comment about the rudder cables is relevent in my opinion. I've also noticed that with the DUC windspoon prop there is a noticable effect on the engine noise and vibration with rudder input compared to the prop on my Magni (forgotten the name of that prop).

You can also try tightening the roll friction in the rear cockpit (see my videos). Don't overdo it, I found that when doing the recent video with the roll pivot and roll friction really tight (I was perhaps a bit too enthusiastic with the spanner) the controls were really wierd with what seemed like a delay between input and response. I wasn't happy flying that time.

It would be interesting to hear the manufacturers opinion, especially as there wouldn't be the usual language communication problem that some people complain about.

Mike G
 
Hi Xavier,
is your DUC propeller a Windspoon or a Flash 2 ?
The Windspoon is usually very smooth, but the Flash sometimes gives much vibration because it's not balanced... leading to engine mount cracks on Magni (and Magni thus prohibiting the Flash on their gyros !).
First check carburators balance.
Then prop balance (vibrex) or swap the prop with another one that you know is already smooth.
Regards
Arnaud
 
It's the windspoon... For some reason, in the 'Eclipse', ELA has switched to the 'Helix' three-blade prop. This afternoon, I'm going to tighten the roll friction, as Mike advises. And, within a few weeks, I'll try changing the propeller. i don't believe that the carburettors may need balancing, but I'll check that too...

Thanks to all...

Javier
 
If the vibration is highest at 5500 engine rpm, I doubt if it's the carb balance, that's usually much more of a problem at idle. At 5500 the throttles are just about wide open so any difference would be negligable.

As I keep saying to everyone here, you must get some sort of spectrum analyser on this to detect what frequency your vibration is at. There are analyser apps available for smartphones and they cost nothing. Otherwise its trial and error, suck it and see with lots of trial, lots of error, not much to see and it sucks.

Mike
 
I'm just back from the airfield, where I've been flying touch-and-goes. I have observed that, at takeoff, once aloft, and with the engine revs at 5400-5500, there is no shake at all while the rotor revs are rising from 290-300 to 320, and then the shake starts, sharply and precisely at 320... I have observed that many times this afternoon...

The rudder cables were OK. I checked the pitch of the prop blades, and one of them has 0,5º less pitch than the other two at 63% radius. I don't believe it's significant. However, I'll correct it as soon as I remember to take with me a torque wrench...

Anyway, and despite the vibration, flying the nimble ELA in a nice summer day is a real pleasure, shake or no shake...

Javier
 
Xavier,
5450 rpm gives 36.7 Hz on your propeller. If the shake are to much lower frequency, then it not comes from there.
Changing the load (passengers or continues turn) is still precisely 320 rrpm ?
 
Xavier
320 rpm seems a bit low to me compared to my ELA. What diameter rotor do you have and how heavy are you?
I've noticed that the analog rotor and engine tachometers on my ELA are pretty inaccurate when I compare them to the digital tacho on the PB4 and the engine tacho that I have from my fuel injection system so keep an open mind as to the validity of your numbers.

Mike
 
No, of course the RRPMs in s/l flight are not 320, but 340-350. I mentioned 320 because the gyro lifts off at 290, and the RRPMs then rise all the way to 340-350, where they usually stay, rising to 370 in hard turns, but -–in the course of the rotor revving up at takeoff- the vibration starts at 320, quite sharply...
My rotor is 8,5 meters. My weight is 85 kg... I am flying alone, because it's summer now, and my airfield is at 3100 ft ASL. Yesterday, I flew with little fuel, no more than 20 liters...

Javier
 
Javier, I suggest you correct any (even slight) pitch imbalance between the individual prop blades. Then balance the prop. That's MUCH easier than balancing the rotor and can be done in less than an hour if you know what you're doing.

Then go flying again and notice if it feels any different.

Start worrying only if you've done this part.

Greetings, -- Chris.
 
The experience with the ELA G8 flying here in the US is when flown solo with the resulting lower rotor rpm the vibration is worse than when flown two up with higher rotor rpm, the rotor just likes a heavier load.

It's also been found that for the smoothest operation a slight adjustment of the barrel that centers the rotor in the head is required when going from solo to two-up.
 
Javier,
You says rrpm in hard turns only increases by a factor of 1.07 (ie 370 / 345). This means your coning much varies.
In this case, the fixed undersling is adapted only to a given load. For other load, vibrations increases.
In order for the coning to remain fairly constant despite of the variable load, the blades must keep their pitch setting. Unfortunately they twists due to the underbalancing chord.
 
Jean - Claude;n1122495 said:
Javier,
You says rrpm in hard turns only increases by a factor of 1.07 (ie 370 / 345). This means your coning much varies.
In this case, the fixed undersling is adapted only to a given load. For other load, vibrations increases.
In order for the coning to remain fairly constant despite of the variable load, the blades must keep their pitch setting. Unfortunately they twists due to the underbalancing chord.

Concerning the 'hardness' of the turns I mentioned, they may be not that hard at all... I use to fly in a relaxed style...
If I understand well, you state that the undersling is factory-optimized for a given load, where vibration is at a minimum, and that vibration increases for different, higher or lower loads because the blades twist and the coning changes... Interesting... I remember, years ago, flying another ELA that shook seriously with one person on board, and was really smooth with two...

I thank you all for the valuable advice. I'll report back on this matter if things improve...
 
With no dynamic twisting of the blades, the rrpm increases like the square root of load. Thus, during a 45 ° inclined turn, the load is increased by a factor 1.41, and the rrpm increases by a factor 1.19 In other words: 345 rpm quietly increases to 410 rpm
In this case, the centrifugal force increases exactly in the same proportion as the load, and the coning does not changes.
Unfortunately, the blades builders under-balances the chord. Instead of aiming 25% , they are often satisfied of 30% This creates a stress that twists the blades and changes the coning.
So, they not find the undersling suitable to all load
 
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