Stick shake

[Abid]

Hi Anthony and Fara,
Many thanks for the responses. Adjusted the tightness of the roll and pitch bolts yesterday to get a nice stiffness without excessive force when using the stick. Adjusted the tracking using plastic shims. Did test flights. Previously I could fly up to 115 mph without any vibrations or shudders. The VNE in my machine is 100 mph so the 115 mph was a rare accidental occurrence. Yesterday when I reached 90+mph straight and level, the whole machine shook violently. I immediately closed the throttle, centered the stick and the shaking subsided. I removed any shims added but the shaking returned at above 90 mph. Anybody experienced such violent shaking of the whole gyro at higher speeds and what did you do to fix it?
Many thanks for any suggestions and advice. John H.

Try loosening the roll and pitch block friction slightly and re-doing this test. If your gyro hops (meaning the body shakes but not the stick) you likely have a tracking issue. This requires changing the pitch of one of the blades by either using shims or if you have an old Magni rotorhead without new eccentric bushing, no adjustment is possible. Its their design. I would advise then bringing it up to the manufacturer. Shim material is placed between the hub bar and the teeter block. Cut a strip of say 2 thou and place it on one side and try flying. If that doesn't work you placed it on the wrong side so change and try. It will be trial and error because you don't have a dynamic balancer

 

[Mike G]

At the risk of repeating myself ad nauseum

I SAY AGAIN

There is no point asking any questions about how to reduce your vibration until you've answered the question
what is the frequency of the vibration that bothers you?

If not all you get is knee jerk reactions from well meaning people who solved their problem by:
"cleaning the blades"
"balancing the blades"
"static balancing the rotor with a water level"
"tracking the rotor"
"adding shims"
"removing shims"
"balancing the prop"
"doing the Indian rain dance"
All these are good if your problem happens to be the same as theirs, especially if you want it to rain.

BUT IF YOUR VIBRATION ISN'T 1/REV LEAVE THE F..CKING ROTOR ALONE.

Mike G

 

[Abid]

At the risk of repeating myself ad nauseum

I SAY AGAIN

There is no point asking any questions about how to reduce your vibration until you've answered the question
what is the frequency of the vibration that bothers you?

If not all you get is knee jerk reactions from well meaning people who solved their problem by:
"cleaning the blades"
"balancing the blades"
"static balancing the rotor with a water level"
"tracking the rotor"
"adding shims"
"removing shims"
"balancing the prop"
"doing the Indian rain dance"
All these are good if your problem happens to be the same as theirs, especially if you want it to rain.

BUT IF YOUR VIBRATION ISN'T 1/REV LEAVE THE F..CKING ROTOR ALONE.

Mike G

LOL.
Of course you are right Mike but some users just want their stuff to not shake but without any training on any equipment. Their only choice really is shot in the dark and trial and error. Also so many think stick shake means rotor balance and tracking is off. You and I know that relationship is not always the case either.

 

[Burrengyro]

At the risk of repeating myself ad nauseum

I SAY AGAIN

There is no point asking any questions about how to reduce your vibration until you've answered the question
what is the frequency of the vibration that bothers you?

If not all you get is knee jerk reactions from well meaning people who solved their problem by:
"cleaning the blades"
"balancing the blades"
"static balancing the rotor with a water level"
"tracking the rotor"
"adding shims"
"removing shims"
"balancing the prop"
"doing the Indian rain dance"
All these are good if your problem happens to be the same as theirs, especially if you want it to rain.

BUT IF YOUR VIBRATION ISN'T 1/REV LEAVE THE F..CKING ROTOR ALONE.

Mike G

Hi Fara and Mike G,
Many thanks for the feedback. My gyro is an ELA 07S made by ELA Aviacion in Spain. Mike, I don't know what the frequency of the vibration is or if it is 1/Rev or 2/Rev. The general stick vibration seems to be left and right with some fore and aft component. The whole ship started to vibrate so bad as I passed 90mph straight and level I thought that if the shaking continued a catastrophic failure of the airframe might ensue. Even after making incremental changes, the same thing happened about three times as I passed 90mph, straight and level at 2000ft AMSL. Unfortunately, I did not have my Gopro with me to record the event to get an estimate of the magnitude of the "shaking". I basically killed the throttle and pushed the stick forward of centre position to make sure the rotor blades were as far as possible from whacking the tail without going into a nonrecoverable dive. When the shaking subsided after a few seconds, I leveled off and returned to the airfield. Everything seemed normal again so I did a few sharp s-turns to speed up the rotors for a soft landing in the hot air.

I found a possible connection on this forum in a discussion about "Flight in Turbulence" (rwdreams, March 18, 2021) and changes to VNE due to local conditions and mast bumping. I did change my propeller from a Duc Windspoon to a Meglin 3 bladed scimitar propeller SL106-1750 which has performed well. Cruising at 90mph should not be pushing the VNE limits as our airfield is 100ft above sea level and beside the sea. Doing rapid sharp s-turns to rev up the rotor for a soft landing in hot weather conditions did not produce any unusual stick feedback at an airspeed of 60mph. The air was fairly smooth at 2000ft AMSL yesterday when I was doing the test flights. I only got one test flight done today due to rough air conditions and thunderstorms.

Am I off track in considering mast bumping due to a lowering of VNE due to weather conditions? Or is the shaking coming from vibration frequencies from the rotor, propeller, engine becoming additive at airspeeds above 90mph to create a larger vibration. I currently have the propeller pitch set to give 5800 engine rpm at WOT on my Rotax 914 turbo.

Fara and Mike, I appreciate the feedback and admit you are both correct in your responses. I would love to have the latest Smart Avionics version of the PB4. Meanwhile, I must work with the old hammer and shovel toolbox and the ol suck it and see methods until funds become available. Mike, no chance you are coming to the West of Ireland any time soon to demo the PB4 system on our ELA07S and Magni M16 at our Spanish Point 550m grass airfield surrounded by fabulous beaches, cliffs and unusual landscapes and great pubs? Any feedback appreciated. John H

 

[Vance]

Mast bumping is a helicopter problem.

Without careful analysis; stick shake will be difficult to cure.

I have flown and trained is some ELAs and the stick shake was acceptable.

I was not involved in the process of getting the rotor smooth.

I did not fly faster than seventy knots.

I don’t like stiff pitch and roll pivots.

Good luck.

 

[Sv.grainne]

What did you change to cause the onset of this vibration?

Why are you exceeding VNE??

In my case we replaced rotor blades due to a blade strike. There was no vibration on the old blades and had been told the new blades had been test flown and we're fine but it turned out that there was a noticeable 1 per rev shake. Had no option other than try shimming.

 

[Abid]

Hi Fara and Mike G,
Many thanks for the feedback. My gyro is an ELA 07S made by ELA Aviacion in Spain. Mike, I don't know what the frequency of the vibration is or if it is 1/Rev or 2/Rev. The general stick vibration seems to be left and right with some fore and aft component. The whole ship started to vibrate so bad as I passed 90mph straight and level I thought that if the shaking continued a catastrophic failure of the airframe might ensue. Even after making incremental changes, the same thing happened about three times as I passed 90mph, straight and level at 2000ft AMSL. Unfortunately, I did not have my Gopro with me to record the event to get an estimate of the magnitude of the "shaking". I basically killed the throttle and pushed the stick forward of centre position to make sure the rotor blades were as far as possible from whacking the tail without going into a nonrecoverable dive. When the shaking subsided after a few seconds, I leveled off and returned to the airfield. Everything seemed normal again so I did a few sharp s-turns to speed up the rotors for a soft landing in the hot air.

I found a possible connection on this forum in a discussion about "Flight in Turbulence" (rwdreams, March 18, 2021) and changes to VNE due to local conditions and mast bumping. I did change my propeller from a Duc Windspoon to a Meglin 3 bladed scimitar propeller SL106-1750 which has performed well. Cruising at 90mph should not be pushing the VNE limits as our airfield is 100ft above sea level and beside the sea. Doing rapid sharp s-turns to rev up the rotor for a soft landing in hot weather conditions did not produce any unusual stick feedback at an airspeed of 60mph. The air was fairly smooth at 2000ft AMSL yesterday when I was doing the test flights. I only got one test flight done today due to rough air conditions and thunderstorms.

Am I off track in considering mast bumping due to a lowering of VNE due to weather conditions? Or is the shaking coming from vibration frequencies from the rotor, propeller, engine becoming additive at airspeeds above 90mph to create a larger vibration. I currently have the propeller pitch set to give 5800 engine rpm at WOT on my Rotax 914 turbo.

Fara and Mike, I appreciate the feedback and admit you are both correct in your responses. I would love to have the latest Smart Avionics version of the PB4. Meanwhile, I must work with the old hammer and shovel toolbox and the ol suck it and see methods until funds become available. Mike, no chance you are coming to the West of Ireland any time soon to demo the PB4 system on our ELA07S and Magni M16 at our Spanish Point 550m grass airfield surrounded by fabulous beaches, cliffs and unusual landscapes and great pubs? Any feedback appreciated. John H

Your ELA 07 indicated airspeed is decently far off from Calibrated speed. When ELA reads 100 mph, new AR-1 reads only 85-87 mph. Just to give you a rough estimate. I have some time in ELA 07. I do not believe your VNe changes with turbulence but certainly in turbulence and rough air you should fly slower like 60 to 70 mph not because of VNe but due to Vra (Velocity for rough air).

I think you may have gotten in some kind of resonance or vortex shedding type of state but getting something that can measure the frequency of vibration would be a good idea. I think there are some smart phone apps that can do that. Mike can guide you on them.

 

[Burrengyro]

Hi Bobby and Vance,
Mast bumping can happen in gyros if the scope of teetering is exceeded. Vance, I am open to correction on this: VNE is specified by the manufacturer to ensure this does not happen under normal conditions. However, all conditions are not the same and sometimes the VNE may have to be reduced to match the weather conditions and altitude. The original propeller on my ELA07S was a DUC Windspoon. The originally specified VNE was 115mph which was later reduced to 100mph by ELA Aviacion. I recall at least twice running up to the 115mph limit by accident when taking off from a grass strip. No major issues or vibrations were noticed. I stuck more rigidly to the max speed of 70 to 80mph to avoid trouble after that. About 2 years ago I changed the propeller to a Meglin 3 bladed scimitar propeller SL106-1750 and set the pitch to give 5800rpm at WOT for the Rotax 91 engine. This setting worked well for good climb-outs. But maybe it is too fine for cruise speed. Setting the pitch accurately for the three blades is a challenge. I always had some stick shake and wanted to reduce it, first by balancing the carbs, then the propeller and finally the main rotor. As you can see from my earlier posts, this is a work in progress. What surprised me was the violent shuddering which occurred at 90mph straight and level at 2000ft AMSL when I had no problem passing 100mph previously without problems. Getting to the root cause of the change and violent shuddering at 90mph is my main focus right now. There have been some accidents lately which seemed to show evidence of tail strikes which might have been caused by rotor flap at high speed due to teeter stop bumping and an inappropriate pilot input to retrieve the situation. I am learning a lot from experts on this forum as I progress and don't mind being corrected if my assumptions are errant. Would love to get a PB4 as described by Mike G when funds allow. Meanwhile, tests and trials continue tomorrow.....Thanks, Bobby and Vance for reviewing my progress. John H

 

[Abid]

Hi Bobby and Vance,
Mast bumping can happen in gyros if the scope of teetering is exceeded. Vance, I am open to correction on this: VNE is specified by the manufacturer to ensure this does not happen under normal conditions. However, all conditions are not the same and sometimes the VNE may have to be reduced to match the weather conditions and altitude. The original propeller on my ELA07S was a DUC Windspoon. The originally specified VNE was 115mph which was later reduced to 100mph by ELA Aviacion. I recall at least twice running up to the 115mph limit by accident when taking off from a grass strip. No major issues or vibrations were noticed. I stuck more rigidly to the max speed of 70 to 80mph to avoid trouble after that. About 2 years ago I changed the propeller to a Meglin 3 bladed scimitar propeller SL106-1750 and set the pitch to give 5800rpm at WOT for the Rotax 91 engine. This setting worked well for good climb-outs. But maybe it is too fine for cruise speed. Setting the pitch accurately for the three blades is a challenge. I always had some stick shake and wanted to reduce it, first by balancing the carbs, then the propeller and finally the main rotor. As you can see from my earlier posts, this is a work in progress. What surprised me was the violent shuddering which occurred at 90mph straight and level at 2000ft AMSL when I had no problem passing 100mph previously without problems. Getting to the root cause of the change and violent shuddering at 90mph is my main focus right now. There have been some accidents lately which seemed to show evidence of tail strikes which might have been caused by rotor flap at high speed due to teeter stop bumping and an inappropriate pilot input to retrieve the situation. I am learning a lot from experts on this forum as I progress and don't mind being corrected if my assumptions are errant. Would love to get a PB4 as described by Mike G when funds allow. Meanwhile, tests and trials continue tomorrow.....Thanks, Bobby and Vance for reviewing my progress. John H

You would have to be hitting around 140 mph to get to aerodynamic limits of Vne. Your lower 100 mph Vne is due to other reasons not running out of teeter range to solve dis-symmetry of lift. I have heard of and seen a couple of ELA composite blades that had developed cracks. One was changed to an Averso blade set and another to a Brako set for safety. Both located in Florida.

You may want to do a close inspection on your blades just as a precaution. Also depending on year of manufacture ELA 07/08 has had tail boom cracking off accidents due to sugaring in welding on tail boom butt joints. Butt joints have to be done with purge welding to avoid sugaring which was not done that way. Those accidents were in Spain and France if I remember right. They later introduced doubler plates welded outside to solve that issue. Unfortunately you can't inspect for sugaring cracks from outside. You'd need a lighted borescope to check cracks that start and grow from inside out. Not trying to scare you but these are things that have happened. Checking these out and eliminating them is cheap and not that hard really in my opinion.

Once you know none of those are indicated, definitely get a frequency device to at least figure the vibration frequency. Your rudder cable tension on the ELA should also be checked with a tensionometer like this one

Kent-Moore Cable Gauge | Aircraft Spruce

Kent-Moore Cable Gauge Easy to use - just squeeze ball handle, put gauge on cable, release handle and read tension on rotation dial. No conversion charts necessary.

www.aircraftspruce.com

Should be in the 40 pound range I believe.

 

[Burrengyro]

Your ELA 07 indicated airspeed is decently far off from Calibrated speed. When ELA reads 100 mph, new AR-1 reads only 85-87 mph. Just to give you a rough estimate. I have some time in ELA 07. I do not believe your VNe changes with turbulence but certainly in turbulence and rough air you should fly slower like 60 to 70 mph not because of VNe but due to Vra (Velocity for rough air).

I think you may have gotten in some kind of resonance or vortex shedding type of state but getting something that can measure the frequency of vibration would be a good idea. I think there are some smart phone apps that can do that. Mike can guide you on them.

Hi Fara, many thanks for the info. I am not keen to relive the shaking I got on the three test flights earlier. My own feeling is the vibrations from all three components, rotor, engine and propeller are becoming critical at the higher airspeed of 90mph. The shuddering was so strong that I was concerned that the retreating blade was stalling and was going to flap into the tail.

What is the correct recovery technique in such a situation?

My ELA07S is more comfortable flying at 60 - 70 mph. But I like to ensure that it works well and safely at speeds near the published VNE should the need arise. Slowing down in turbulent conditions is good sense and has always served me well. I usually fly with another pilot who flies a Magni M16 and he seems to be able to fly much faster than I can in the same conditions. The ELA is a bit more bulky than the Magni. I am going to set up my Gopro to record the onset of any vibrations in the cockpit when I fly tomorrow at speeds approaching the 90mph mark.
Thanks again for sharing your knowledge and expertise. John H.

 

[Burrengyro]

You would have to be hitting around 140 mph to get to aerodynamic limits of Vne. Your lower 100 mph Vne is due to other reasons not running out of teeter range to solve dis-symmetry of lift. I have heard of and seen a couple of ELA composite blades that had developed cracks. One was changed to an Averso blade set and another to a Brako set for safety. Both located in Florida.

You may want to do a close inspection on your blades just as a precaution. Also depending on year of manufacture ELA 07/08 has had tail boom cracking off accidents due to sugaring in welding on tail boom butt joints. Butt joints have to be done with purge welding to avoid sugaring which was not done that way. Those accidents were in Spain and France if I remember right. They later introduced doubler plates welded outside to solve that issue. Unfortunately you can't inspect for sugaring cracks from outside. You'd need a lighted borescope to check cracks that start and grow from inside out. Not trying to scare you but these are things that have happened. Checking these out and eliminating them is cheap and not that hard really in my opinion.

Once you know none of those are indicated, definitely get a frequency device to at least figure the vibration frequency. Your rudder cable tension on the ELA should also be checked with a tensionometer like this one

Kent-Moore Cable Gauge | Aircraft Spruce

Kent-Moore Cable Gauge Easy to use - just squeeze ball handle, put gauge on cable, release handle and read tension on rotation dial. No conversion charts necessary.

www.aircraftspruce.com

Should be in the 40 pound range I believe.

Hi Fara, Thank you! These are exactly the reasons why the shaking concerns me so much. I had not considered rudder flutter as a potential source. Will check these issues out! Thanks again. John H

 

[DASBUR]

What about blade flutter??? who's blades and what is the chord balance.

Doug

 

[JETLAG03]

Your ELA 07 indicated airspeed is decently far off from Calibrated speed. When ELA reads 100 mph, new AR-1 reads only 85-87 mph. Just to give you a rough estimate. I have some time in ELA 07. I do not believe your VNe changes with turbulence but certainly in turbulence and rough air you should fly slower like 60 to 70 mph not because of VNe but due to Vra (Velocity for rough air).

I think you may have gotten in some kind of resonance or vortex shedding type of state but getting something that can measure the frequency of vibration would be a good idea. I think there are some smart phone apps that can do that. Mike can guide you on them.

app on Iphone = AMC MECANOCAUCHO

https://www.rotaryforum.com/threads/stick-shake.1146264/#post-1178613

phil

 

[Burrengyro]

What about blade flutter??? who's blades and what is the chord balance.

Doug

Hi Doug,
My rotor blades are standard composite blades made by ELA Aviacion. Not sure of the location of the chord balance. They are similar to the Magni M16 composite blades. Please explain your understanding of "blade flutter", how it happens and what are it's effects? Much appreciate any info shared. John H.

 

[DASBUR]

My experience is with helicopters. If the blades cg is after the 25% chord line under certain conditions they can become unstable and pitch up. From what I have read and experienced this can happen at higher flight speeds. painting the blades can sometimes cause this problem. you have 3 times more paint behind the CG than you forwards of it. another thing is blade weave this is where the blade is very flexible and has a lot of tip weight can become unstable at higher speeds. I know of instances on helicopters where the owner was doing track and balance and after that they got a violent shake at higher than normal flight speeds.

Just my opinion on what I have gathered over the years

Doug

 

[Martin W.]

YES to everything Doug said above ....

Automotive type paint weighs almost 1 oz per square foot (.92)
This is after the paint dries and solvents have evaporated

Counting both top and bottom surfaces
a 6" X 12' blade is 12 sq ft X 1 oz = 12 oz of paint
Like Doug said , 75% of that is behind the 25% cord balance spot
Which is about 9 oz weight added to the rear 75% of each blade

A 2 blade system would see 1 lb 8 oz total paint weight added
1 lb 2 oz of it behind proper balance point

.

 

[Burrengyro]

YES to everything Doug said above ....

Automotive type paint weighs almost 1 oz per square foot (.92)
This is after the paint dries and solvents have evaporated

Counting both top and bottom surfaces
a 6" X 12' blade is 12 sq ft X 1 oz = 12 oz of paint
Like Doug said , 75% of that is behind the 25% cord balance spot
Which is about 9 oz weight added to the rear 75% of each blade

A 2 blade system would see 1 lb 8 oz total paint weight added
1 lb 2 oz of it behind proper balance point

.

Hi Martin and Doug, very valid info on the effect of weight of paint. No paint on my blades except on the tip ends for ease of identifying which blade is tracking too high. Took the rotors off today and checked the teeter bearings and bushing and pitch and roll block bushings for wear. Greased all bolts. No problems. Blade alignment was perfect. Reset the chord wise position in the teeter tower. Removed slop by adding a new shim washer. Loosened the roll and pitch bolt tightness by a small fraction. Test flight had to be cut short due to the sudden arrival of sea fog. Waiting for good weather to continue testing. all comments / suggestions appreciated. John H

 

[Martin W.]

In a perfect world the absolute center of mass of the rotor blades should be perfectly aligned with the center of the teeter bolt , that way everything is in the same plane of rotation.

But in real life the blades cone upward several inches thus the teeter point is set higher ..... sometimes this teeter height is done by guess and by golly .... sometimes it is set by much testing by the manufacturer and sold as a reasonably balanced set.

But even then there are variables .... the cofg of the rotors can move up or down because rotor rpms go up and down depending on loading , and cone angles change with it.

It is a black art and even the modern day rotor experts pull their hair out and hit brick walls sometimes. So anyone in a sport gyro should be reasonably happy with a reasonably balanced rotor ... and anyone with a smooth rotor is entitled to bragging rights.

.

 

[Jean Claude]

the cofg of the rotors can move up or down because rotor rpms go up and down depending on loading , and cone angles change with it.

I just want to correct this point.
The load does not change the coning, because when the lift increases, the rpm increases at the same time.
But if lift can change the blade twist, then yes, the rpm changes.
And also, of course, when the atmospheric density changes (altitude)

 

[Abid]

....
The load does not change the coning, because when the lift increases, the rpm increases at the same time.
But if lift can change the blade twist, then yes, the rpm changes.
....

Jean:
These 2 sentences seem to be confusing to me
If lift increases the rotor RPM increases ... Got that
But then you say if blades twist due to lift increasing (that would be chord wise stiffness not being enough), the rotor RPM changes. In reality the rotor RPM increases with lift and in high load condition (like pulling 2 Gs), all real world blades will have some twist change outboard but within reasonable controlled limits

So you are saying in both cases rotor RPM changes. So what is the point you are trying to make. I did not understand. Merci