2/rev vibrations 2

[klyde]

I have a semi-rigid rotor two bladed helicopter that has a 2/rev vibration that is very low with full fuel. However as the fuel level nears 1/2, the vibration level increases to an uncomfortable level. At a hover, the 2/rev is essentially gone. The 1/rev.balance readings in hover both vertical and lateral are below .2 ips. There is no increase in 1/rev vertical ips with forward speed. I'm sure that the blade c.g. is aft of the pitch axis and I believe that the center of pressure is in front of the pitch axis. Any hints as to what to try to eliminate/reduce the level of vibration?

 

[Intermesher]

klyde,

As like you, I am currently looking into the causes and solutions to rotor induced vibration. The following questions and points may therefor be valid or absolutely dumb.

Is the 2P vibration coming from the rotor head or from the flight controls?

The conventional method is to have the blade's spanwise CG located slightly ahead of the pitch axis.

A 2-blade semi-rigid rotor is somewhat unique. Since it is not a teetering rotor and does not have undersling, the Coriolis effect may be the cause of your vibration. Variations in the GW will have an influence on the Coriolis, of course.

I would be happy to pursue this subject further with you and others.

 

[Sparweb]

Klyde,
Do you mind identifying what type of rotorcraft you're dealing with? It would keep the number of shots in the dark to a minimum, if you could.


STF

 

[QCboss]

Klyde,
Knowing the type of aircraft and rotor head would most certainly be helpful. The information you give is not enough to make a diagnosis BUT...go back to the basics. At hover there is no dissymetry of lift. Once you start moving forward, the advancing blade's speed exceeds the retreating blade's speed by the airspeed + the speed of any headwind. One blade (on an articulated rotor head wants to climb and therefore lag and one wants to dive and lead- you teter SOMEHOW). Is this a newly installed roror head??? (if so I'd say to check blade sweep if you sweep them at install). If not, is this a sudden appearance of symptoms (check blade tip caps and for other blade damage). Your semi-rigid rotor has movement in the pitch axis and the flap (tetering axis). Check your pitch link bearings, attachment trunnions any on the blade horn if any, as well as on the swashplate. Let's hear how you make out.

 

[klyde]

Intermesher.
I think the 2/ is coming from the head, but am not sure.
I assume that you meant chordwise c.g. ahead of the pitch axis. My condition is apparently buried in the blade design.
My semi-rigid rotor is very similar to the Bell 206. It does have one teetering axis and has undersling and a coning angle built in the spindle. I have not made tests at varying GW to see if the amplitude of the 2/ changes.

SparWeb:This is a Safari Kit helicopter

QCBOSS: I have essentially zeroed the sweep on both blades and have balanced the rotor disc chordwise by the addition of weights on the outside of the trunion. But I will certainly check again more closely for looseness in the control and drive paths.
BTW To all that have responded and are interested, most of the other Safaris that I'm aware of share this problem.

 

[QCboss]

Klyde,
I have no idea how you go about checking the track on your Safari (Strobe or what?) but remember that just because a helicopter is in track on the ground and or in a hover, doesn't mean that it stays in track in forward flight. Of course an out of track condition will ALWAYS present an accompanying lateral vibration, perceivable or not. Just more food for thought. Move your accelerometers or velometers (whatever) around and you may get clued to a higher ips reading elsewhere eg.transmission mounts etc.etc.
Bob

 

[QCboss]

While I'm at it lemme ask you what may be a "ridiculous" question. I've never even seen a Safari BUT is it possible that your fuel tank is loose? Ask a few older Astar jocks and they may remember that the fuel tank used to possibly be able to slide forward, jamb the flight controls and fuck up your day.

http://www.tipmaster.com/images/sadeyes.gif

 

[klyde]

QCGOSS. You can see my Safari at

http://www.vkss.com.

and

http://www.acehelicopter.com

The bubble is the same as the two seat Bell 47.
I've taken care of the tracking by both "sticking" the blades near hover and measuring the vertical 1/rev in fwd. flight to 80 mph. No increase in ips with fwd flight tells me there is no "climbing blade". Have double checked the tank mounting. One guy drained the fuel down til the 2/ started and then added the equivalent weight at the mid point of the tanks and the 2/ went away.
Currently measuring the lateral 2/ just above the trans mounting and just below the swash plate. and the vertical 2/ out as far fwd as I can in the passenger area.
Just did another 1/ measurement and both lateral and vertical. Got good readings down below 0.2ips. Ship felt smooth. Fuel level was slightly higher than 3/4.
appreciate your interest and inputs.

 

[QCboss]

Klyde...I promise not to make a career out of your 2 per. BUT OHHHHhhhhhh...that's a Safari! Reminds me of ye olde 47G2/G4A. After this, I quit I promise. Fuel tanks are kinda sorta mounted on the TAILBOOM which a blade passes over twice per revolution. Move a vertical accelerometer back about halfway thataway to keep tail rotor junk out of the picture. When you get totally frustrated...jack it up and check the skids and skid mounting tubes. Adios and good luck.
Bob

 

[Intermesher]

klyde,

You're correct. 'Chordwise' was meant.

If you are saying that the 2P is not noticeable during hover, and that it varies in forward flight depending upon the crafts GW, then the following might be the reason.

The manufacturer has set the amount of undersling to suit the 'mean' coning angle of the blades. The actual coning angle will vary during flight of course due to changes in the GW and the varying loading on the disk. During forward flight, if the teetering hinge is not in the plane of the blade masses a 2/ oscillating dynamic load will be transferred to the mast.

The Bell 47 hub had two hinges at 90-degrees to each other. Perhaps this was done to reduce, or eliminate, this source of vibration.

 

[gugu]

You have a trim tab or lack of one used for forward flight track is it the old style head and blade or new style,?
if its old style add chord weight it stablize the blade aerodinamic ballance then track with nutral tab and check with a test flight using tab for fwd flight track,It sounds like the tab and p/c link adjustments are fighting each other...

 

[klyde]

gugu: I have the new style blade and head. I do not remember having the 2/rev problem with the old style blade and head. Further, I believe that all current versions of the Safari share this problem to a certain level. I did add chord weight to stabilize the aerodynamic balance in blade pitch using a technique similar to the Bell 47. It did neutralize the collective pressures, but nothing to change the 2/rev. Further, if I had a climbing blade I would expect to see an increase in the vertical 1/rev. I do not. The vertical 1/rev is low and increases only very slightly to 80mph fwd flight. The only thing that seems to affect the 2/rev is fuel load. Note to get 2/ readings I have to add a 2nd. reflective strip to the swash plate. The real rotor rpm is half of the rpm readings in the list. 360 degree of the listed phase angle is 180 degree blade rotation. Zero phase angle now has some ambiguity. The below are believed to be 2/rev vibration readings. The lateral is measured just below the swashplate and the vertical just forward of the passenger's pedals. Please excuse the screwed up format. It looks alright when I edit but it changes when I preview. I give up.

Fuel level= ½ tank.
Lateral RPM IPS ANGLE
974 0.43 322 HOVER
962 2.51 257 Translation
Lateral 952 2.79 028 High Speed (50mph)

Vertical 971 0.63 044 HOVER
975 1.68 114 TRANSLATION
945 1.65 156 High Speed (50mph)
Still rough ride…..No problem detecting vibration by watching instruments also feels rough.
Full Fuel
Vertical 951 0.58 278 High speed (50mph)

Lateral Lost readings but was smooth.. So smooth that I forgot to call in the readings and just went flying.

 

[aerovibe]

Kyle,

You have obviously checked out your blade pitching moments, be careful though that by adding chord weights that you are not introducing Lateral vibrations?? I'm in favour of adjusting pre installed chordwise weights, but not adding or subtracting them. Also, Semi rigid rotors, pain in the rear end for N/Rev vibrations, You may well be correct in that Fuel load and high forward speed are introducing the N/Rev vibration, in which case you will find that perhaps a solution is not to reduce 1 / rev vertical, but to raise it, to say 1.2 - 1.5 i.p.s. this will mask the 2 / rev. This is common practice on Helo's with your problem. As previously stated by others, check out sweep, bearing play etc etc.

How certain are you that you have the pitching moments under control? what type of kit are you using, strobe or linescan camera?

 

[Schuyler]

I am trying to understand engine control theory (gain and phase margin) as it relates to helicopter engine airframe capatability. Can anyone recommend an easy to read resource on this issue (i.e., rotorcraft engine controls for dummies).

 

[klyde]

I now have satisfied myself that all two bladed rotor systems will exhibit both lift and drag forces, that in forward flight will vary at twice the blade rotationg rate. (I've published an article in Experimental Helo magazine delving into the derivation in more detail). But basically,it is the forward flight velocity term that involves sine(theta) where theta is the azimuthal position of the blade as it rotates. The lift forces are proportional to the square of the velocity and squaring the sine term leads to a cos(2*theta) term and ergo the 2/rev forces. It is true however, that if the frame doesn't have some resonance at this frequency, the pilot won't know it is occurring. I now have to find which portions of my airframe are resonating at that frequency. One technique is to suspend the helo off the ground, mount numerous straing gages and apply impulse like forces to each frame member and see which one likes the 2/rev frequency. This is not an inexpensive process tho...

 

[Ameolive]

For two-bladed helicopters only a 2/rev vibrations or any multiple of 2/rev are transmitted from the blades to the airframe ( for refeence you may consult Ch 24 of Even More Helicopter Aerodynamics by R.W. Prouty ).

The reason for that resides in the physics of the pulsating forces acting on a helicopter rotor.
The fact that you are experiencing this vibrations is on my opinion related to the fact that the airframe of your helicopter is resonant at the 2/rev.
As you probably know the resonant frequency of any system ( including an helicopter fuselage ) roughly depends on the square root of the elastic stiffness/mass ratio.

This explains also why under specific weight configuration you don't feel any vibration.( the mass change and so the frequency ) .

What you are experiencing is a problem already faced by the designers of some famous helicopters ( including the Bell 214, EH101 and the EC135 ).

Under these circumstances for the passengers ( and the fuselage... ) comfort, the designers use detuning systems that may be either passive ( inertial masses and damper ) or active ( much more complex but basically consisting in actuators inducing in the airframe a force at such a frequency the cancel out the vibrations coming from the rotors ).

As per evaluating the resonant frequncies of your airfarme, I absolutely agree that a GVT ( ground vibration test ) is the best way to have an accurate picture of the phenomenon, however if I can say it is not so inexpensive and easy as you are suggesting, if I were you before embarking in such test I would try to do some analysis ( FEM or others ) and determine which modes of your helicopter are mostly affected by the 2rev frequencies. Once you identified your modes you may introduced some changes and check if you still have this problem.

Hoping of having been of help

Ameolive

 

[klyde]

ameolive: Thanks for the response. Listening to someone in the trade, gives me confidence that I'm headed in the right direction. Shortly after I posted the previous, I encountered a CalTech Phd. EE who had been working for Pratt&Whitney and Hamilton std. doing vibration measurments using a laser with a heterodyne receiver connected to a spectrum analyzer. He said that after awhile, with just earphones on he could give some pretty accurate vibration frequency analysis on some of the items he worked on. He owned his own consulting business for a number of years and still has some of the laser equipment. He is going to try and find the stuff and we are going to explore using his lasers in lieu of the strain gage method. It promises to be faster and should have even higher resolution? I also have a friend who did his M.S. in mechanical vibrations and he is interested in being involved. If all of this works out, I will have a smoother helicopter and an article for the Experimental Helo magazine. At worst I'm going to learn something about the laser vibration measurement techniques. Doesn't seem like I can lose.
Thanks again for your response. BTW I not only have Prouty's book, I've met him several times and enjoyed a good conversation about his career and writings.

 

[Ameolive]

Woh, I wish I had the same possibility of knowing Prouty as you had !

Although I have been working in the field of the certification of civil helicopter for 13 years I still have to find a text written in more plain and accurate way than Prouty's articles and books.

Getting back to your 2rev problem, here in Italy we still rely on the exciters/accelerometers ( not strain-gauge ) procedure as we considered it a more reliable and accurate method in particular for the dynamic characterization of a new airframe.

Strain gauge are generally used for stress/loads acquisition during the load survey to determine the fatigue loads.

As per the laser methodology to be honest I'm not very familiar if you wish I can make some inquire on the subject

Best Regards
Ameolive