Vibrations in mi-17 helicopter

[DennisFetters]

Originally Posted by okikuma

Even after following his method, and the blades still vibrate more than compared to US designed aircraft that you are use to, I suspect that is the best you will ever obtain.
Wayne

Yes, it does vibrate more than US designs, but in my experience they don't have to.

I had an opportunity to play with a Mi-8. They had the blades down on a bench and let me examine them. We did a twist-test and a resonance-test. I found that the blades resonated at 80 cycles per minute and were not near as rigid as I thought they could be.

I suggested to the Ruskie engineers that they should place another slightly larger trim-tab inward some distance from the original. It was my opinion that the original trim-tab was doing it's job, but the inner portion of the blade was not influenced by the corrections due to what I believe is too much blade-flexibility.

They shrugged their shoulders and said they thought the vibration levels were good enough. I asked them if they ever flew in an American helicopter.... they said no. So the problem is they have accepted a standard, not willing to try to set the bar to a higher standard.

If you ever seen any self-respecting card-packing Ruskie that ever flew in a Blackhawk, he will act like he just got off a flying saucer!

[okikuma]

Looser tolerances mean less time to produce and at a reduced cost. Lower standards are easier to obtain.

I suspect Ali is a US Army Aviation Mechanic or Civilian Contractor and working on the following or similar project.

Wayne

http://www.army.mil/article/56256/ar...qis-on-mi-17s/

[magilla]

As a UH-60 Blackhawk test pilot with extensive experience in rotor tracking, I can offer some thoughts...(same essential Sikorsky design with semi-articulated rotor head)

Recommended process to achieve successful track and balance:

A) Static balance: plot CG and weight of each blade, then arrange accordingly
B) Dynamic ground track FIRST; get all blades to fly within +/- .5" inch tip path plane
C) Dynamic ground balance (this will be "initial" ground balance)
D) Flight test 1 (Hover, 80, 120, 145 - use your typical autorotation airspeed, cruise, and max for MI-8)
E) Plot results on polar scale, by airspeed
F) Make initial PC link adjustments to bring polar graph into limits. I agree with Dennis here in that hover track is the most crucial. Use readings from track and balance computer for the first change.
G) Dynamic ground balance (add weights to hub to adjust for PC link changes)
H) Flight test 2 - same profiles
I) READ THE POLAR CHART TO SEE HOW THE CHANGES AFFECTED YOUR TRACK. Bounce against recommendations from computer to see if it says to make the same adjustments. my experience has been that the first correction by computer is 50% accurate in initial track correction - a lot of times it makes it worse and increases the error!!! Again, READ and INTERPOLATE what the polar graph is telling you. This is an art form. Most of the time, the computer, after making a "wrong" change, will send you down a bracketing path, much like field artillery. If you only follow the computer, be prepared to spend days as the computer brackets you within limits...
J) Make PC link changes to get hover into spec.
K) Make trim tab adjustments to adjust for auto, cruise, and max
L) Dynamic ground balance (to adjust for PC link changes)
M) Flight test 3
N) Read polar charts again, and make necessary changes
O) Dynamic ground balance
P) Repeat M,N,O until track is within spec.
Q) Final ground balance.

If you read the polar charts correctly and make the correct adjustments, and your blades are true, you can typically get a set of blades tracked and balanced within 2-3 flights.

To answer the OP's original question, and add some insight:

1) If the same vibration occurs on take-off going through 10-30 KIAS as on approach coming down from 30 to 10 KIAS, you are feeling the effects of ETL (Effective Translational Lift). You will NEVER eliminate this vibration completely, as the blades are experiencing different relative wind (clean air in front half vs. "dirty," induced flow air on aft half of rotor path plane). You then have to determine whether or not the vibration you are feeling is "normal" for your aircraft. Some shake like a banshee until you get through ETL, and then smooth out. I have no experience in the MI-8 or 17, but suspect that this vibration may be a "normal" characteristic of the MI aircraft. In the Blackhawk, "excessive" vibration felt in this flight envelope could be attributed to the harmonic balancer being "out of tune." There are weights in the nose and in the cabin suspended by torsion bars that vibrate harmonically to reduce the vibrations specifically when the acft is going through ETL. These are tuned based on 100% rotor RPM. Note: harmonic balancers being out of tune typically create a "buzz" in the pedals and the cyclic through most of the flight profiles. You feel more or less "buzz" depending on airspeed, but it is ALWAYS there. Good test pilots can "feel" this, and will know to increase or decrease rotor rpm to see when it goes away...(for us, we typically adjust between 96% and 104% Rotor RPM to find the "sweet spot"). To confirm seat of pants test, we hook up accelerometers to the two harmonic balancer plates and take separate balancer readings to measure IPS at different % Rotor RPM, and add/subtract weights to the nose and cabin until the vibration is less than 0.2 IPS at 100% Rotor RPM. I don't believe the MI-8 has this creature comfort, but it is a primary reason (along with blade design) that US helicopters fly more smoothly than Russian counterparts. However, we NEVER attack the harmonic balancer vibrations until we achieved rotor track and balance within limits. Lesson for ALL helicopters: Any variance from spec on track and balance will be magnified greatly going through ETL.


2) Track effects balance, balance does not affect track... NEVER FORGET THIS. I say again: Track affects balance, balance doesn't affect track...
Bottom line: you cannot dynamically ground balance the blades until the ground track is set within allowable limits. Any small adjustments in track will have a HUGE effect on balance. For example purposes, draw two horizontal lines straight out from the hub representing two blades, and then dot the cg of the blades in the center of each blade. Now draw a second line on one side, of the EXACT same length, at a 20 degree up angle, and then dot the cg in the center of the line again. Now draw a dotted line straight down from the elevated blade to the original blade, and you will see the point for cg moves inward!!! This is a gross exaggeration, but you can see that balance is greatly affected by track changes. How the blades fly will affect your balance. The absolute most important priciple in tracking and balancing rotor blades, in my opinion. In other words, you cannot balance a set blades UNTIl they are tracked correctly.

3) Track is FIRST adjusted on the ground by pitch-change links to get the blades within .5" track. Do ground runs to dynamically track the blades. Use the computer to tell you what and how much to adjust. This is assuming that the trim tabs were set to factory specs before you started. If you have a "rogue" blade that climbs during the ground run, read the computer, and make initial trim tab adjustments to get it to "fly right" on the ground. The reason: if you have a climber on the ground, it will "typically" (from my experience) continue to climb as you increase forward airspeed. Note that this is only an "initial" trim tab setting - you will most likely have to adjust later after flight profiles. After initial ground track, a dynamic ground balance reading is taken, and weights added to get you within limits. This is BEFORE any flight profile is taken.

4) The ONLY way to accurately measure in flight track is with two accelerometers (A and B) on the horizontal axis hooked to a track and balance computer that measures vibration in Inches Per Second (IPS), along with a blade index magnet to adjust the correct blade. A-B = balance, A+B = track. From my experience, if you get the PC links adjusted really close during the ground track, the trim tab adjustments only serve to fine tune the blades in different flight profiles. For us, we measure track and balance at five different locations: Ground, Hover, 80 KIAS, 120 KIAS, and 145 KIAS. (See steps above)

5) Sometimes, after going through numerous test flights, we end up with blades that just won't track right... We then look at the polar charts for where the vibrations are, and as a last resort, will "swap" blades from one axis to the other (on a 4-bladed system - I know you have 5). Same priciple will apply for you - look at the polar charts, and determine the blades that track the closest, and put them "opposite" of each other. In a lot of cases, even after we swap blades, we may still be out of track and/or balance. At this point, if the vibes are right on the limit, we make adjustments to get the smoothest flight at a chosen profile - for us it is usually at cruise - 120 KIAS. The test pilot has to make a command decision to allow for some vibes at 80 and hover say, to get the best profiles at 120. This only comes through experience. It is a compromise, as you'll never get 100% track and balance across all flight profiles.

6) FACT: EVERY blade flies differently. IIRC, the MI-17 and MI-8 blades have thick spars, and then the blade sections are inserted - excellent for battlefield repair, but horrible for fine tuning track and balance. The fact is, some blades or spars are "weaker' than others, and flex more at different flight profiles. These weak blades will "climb" or "dive" on you the faster you go, as the trim tabs have more effect than on a "stiffer" blade. If you plot the "climb" across different airspeeds (ground, hover, 80, 120, 145 for us) on a graph, you can interpolate the "average" flight profile for that blade. As you make trim tab adjustments, you may see that the blade climbs at 80, tapers down a bit at 120, and then flies "true" at 145. A classic "seagull wing" profile across the different airspeeds. Reason: The faster you go, the more the trim tabs have an effect, due to relative wind speed. The weak blade climbs at 80, but the trim tabs start having an effect at 120, and then overcome the "climb" at 145. After making the trim tab adjustments, replot the "average" flight profile of that blade, and then, counter-intuitively, you will make a Pitch-Change link adjustments to get all of the blades on the same "relative" path. For example, with the seagull blade above, the "average" profile is still an arc above a "straight" line. To get the blade "average" within limits, you make a corresponding adjustment to the PC links - in this case, you will PURPOSELY set the blade "low" on teh ground so that as the blade "climbs" the average path falls on the "average" line for all of the blades. You may have to make trim tab adjustments later on to get the profiles within spec as you bring the average flight profile closer to the tip path plane. Counter-intuitive, but you have to compromise to get the "average" flight path within limits. In some cases, the ground track will end up being outside of .5" on teh ground. However, as the blade climbs, it reaches the average track path and smoothes out in flight. Note: this is done ONLY WHEN you cannot track and balance your blades by following the steps A-Q above. Normally, getting ground track within .5", and then getting balance within spec gets you really close.

8) AFTER all tracking is done, we do a final dynamic ground balance to get the blades within limits on the ground.

9) A bit on blades... I am assuming that you have statically balanced each of the separate blades to determine their cg and weight. These figures are going to be crucial when determining initial track and balance. You are going to have to plot the cgs and the weights of each blade so that the blades statically balance out left to right . Draw a line down the center of a rotor circle, and number the blades. Since you have 5, put the "index" blade at the 12 o'clock position (choose a blade that is closest to the average in weight and CG of all blades for the index), and then place the other blades around the circle, and match the weights/cgs from across from each other left to right. You'll see as you "rotate" the blades, the weights and cgs will balance out as the index changes. If you do this, you will have a much higher success ratio in balancing the blades, and in my experience prevents you from having to do the "musical chairs" swapping routine with your blades. Swapping blades is a real pain, as you have to start from step B, as indicated above. Afterwards, start the dynamic track and balance adjustments.

Hope that helps, and sorry for the long explanations. Accurate track and balance is what "separates the men from the boys" in the test pilot arena

Good luck, and let us know the progress!

[Arnie Madsen]

The blade problem
http://www.rwas.com.au/blade-problem.html

Blade balancing
http://www.rwas.com.au/blade-balancing.html

Static - dynamic relationship
http://www.rwas.com.au/static-dynamic.html

Rotor track and balance
http://www.rwas.com.au/rtb.html

[okikuma]

Here's a photo of the Mexican Navy Mi-17 along with three videos about the Mi-17



Chopper Comeback: Legendary Soviet Mi-17 on Afghan frontline - YouTube

New Afghan Air Force Mi-17 Chopper Arrives - YouTube

Mi17 V5.mp4 - YouTube