Vibrations in mi-17 helicopter

[Arbabz]

It has been an extensively hard-fought journey for me and my team to look after the mi-17 helicopter. It has flown around 20 hours of test flights for rectification purposes. The troubleshoot includes:
1. The rigging of controls
2. The use of 8500+ kit and the solutions it gave
Once the above two did not respond, we went on with changing the hub and the rotary swash plate. Normally this is the last action and heli give satisfactory response. But on the other side, now during the approach for landing, the problem is still and the use of word "Excessive Vibrations" would not be an over-reaction. Anyone with any thoughts to share on this ?

[DennisFetters]

Originally Posted by Arbabz

It has been an extensively hard-fought journey for me and my team to look after the mi-17 helicopter. It has flown around 20 hours of test flights for rectification purposes. The troubleshoot includes:
1. The rigging of controls
2. The use of 8500+ kit and the solutions it gave
Once the above two did not respond, we went on with changing the hub and the rotary swash plate. Normally this is the last action and heli give satisfactory response. But on the other side, now during the approach for landing, the problem is still and the use of word "Excessive Vibrations" would not be an over-reaction. Anyone with any thoughts to share on this ?

You need to hover-balance the helicopter first.

Adjust all blade trim-tabs to neutral. (probably 6 degrees tab-up is normal)

Track your blades to perfect by using pitch-link adjustments only. (Adjust to full collective down with autorotation pitch angle first)

Do NOT try to adjust for vibration solutions in hover by using blade track. (Throw the old book away) Get perfect blade track nd balance dynamically only.... do NOT correct balance by slightly adjusting out blade track. If a dynamic adjustment makes the blades go out of track, then adjust blade track back to perfect with each adjustment.

Once you have achieved good hover balance with the blades in perfect track, then you can balance at cruse flight, but then only use trim-tab adjustments in forward flight. (Do not use dynamic adjustments using out-of-balance to counter-adjust for vibration in forward flight)

Use this new technique and things will go much easier with better and longer results.

[twistair]

http://www.pprune.org/rotorheads-23/ is more appropriate forum for your specific questions with lot of pros in those big toys there. The best way is to contact manufacturer/supplier for tech support though if this helo isn't gripped one.

[Arbabz]

well since the vibrations are felt in the forward flight, the trim tabs are active part of the rotor systems. So the configurations of the trim tabs are as stated:
-.8, 2,2,3.14,2 (Degrees). Ijust have a feeling that this position of the trim tab is a bit unorthodox.
Any experience ? Thoughts ?

[DennisFetters]

Originally Posted by Arbabz

well since the vibrations are felt in the forward flight, the trim tabs are active part of the rotor systems. So the configurations of the trim tabs are as stated:
-.8, 2,2,3.14,2 (Degrees). Ijust have a feeling that this position of the trim tab is a bit unorthodox.
Any experience ? Thoughts ?

Like I said above....

You are using trip-taps to counteract balance problems, and that is the old way. You need to correct your balance problems first, and that should only be done in hover, not forward flight.

You need to set all your trim-tabs back to whatever the factory said is on setup for neutral, then track your blades with pitch-link. Once balance is set in hover and your blade track is perfect, then do forward flight and adjust only trim-tabs.

It will make a world of difference doing it this way.

[Rehan K.Janjua]

Hello Ali

Good to have you on this forum.But we are all auto rotators.
Please check out the following papers it might help.

http://www.mi-helicopter.ru/eng/index.php?id=254
http://www.dtic.mil/cgi-bin/GetTRDoc?AD=ADA261809

And what is 503 doing at Qasim. We have lots of info and remedies here.

Good Luck
Rehan

PS....Thank you Dennis appreciate your valuable input.

[DennisFetters]

Originally Posted by Rehan K.Janjua

Hello Ali

Good to have you on this forum.But we are all auto rotators.
Please check out the following papers it might help.

http://www.mi-helicopter.ru/eng/index.php?id=254
http://www.dtic.mil/cgi-bin/GetTRDoc?AD=ADA261809

And what is 503 doing at Qasim. We have lots of info and remedies here.

Good Luck
Rehan

"We" are not all just auto-rotators. Some are both. ;-)

[brett s]

Originally Posted by DennisFetters

You need to set all your trim-tabs back to whatever the factory said is on setup for neutral, then track your blades with pitch-link. Once balance is set in hover and your blade track is perfect, then do forward flight and adjust only trim-tabs.

It will make a world of difference doing it this way.

Surprised anyone would even try doing it differently, it's the only way that makes any sense to me.

On CH-47's we weren't allowed to adjust the trim tabs from the factory setting, you ended up either finding a compromise between hover & fwd flight you could live with or trying different blade combinations. That was a pain!

[500e]

Originally Posted by DennisFetters

Like I said above....

You are using trip-taps to counteract balance problems, and that is the old way. You need to correct your balance problems first, and that should only be done in hover, not forward flight.

You need to set all your trim-tabs back to whatever the factory said is on setup for neutral, then track your blades with pitch-link. Once balance is set in hover and your blade track is perfect, then do forward flight and adjust only trim-tabs.

It will make a world of difference doing it this way.

Dennis I agree set tabs & links to datum but Don,t understand how track will help, if blades are different weights by more than a few grams you will never get smooth flight, phasing will help \ cure balance problems, but eventually screw the damper life.
So many blades are vastly different weights due to paint & manufacturers errors.
A lot of interesting reading on this site

http://www.rwas.com.au/rtb.html

[DennisFetters]

Originally Posted by 500e

Dennis I agree set tabs & links to datum but Don,t understand how track will help, if blades are different weights by more than a few grams you will never get smooth flight, phasing will help \ cure balance problems, but eventually screw the damper life.
So many blades are vastly different weights due to paint & manufacturers errors.
A lot of interesting reading on this site

http://www.rwas.com.au/rtb.html

Like I said, you have to dynamic balance them, so sure you may have to add weight.

First, you need to dynamically balance your blade system..... just like the tire on your car. The best way to do that is in hover, of course.

Forward-flight vibration will come from three things;

1. difference in damper friction

2. bad bearings

3. diffrence of stiffness and/or flight characteristics between the blades, and this shows up in forward flight.

If 1 and 2 are correct, then the most likely reason for vibration in forward flight will be blade difference, since no two blades will ever be the same.

Trim-tabs are what adjusts for the differences between each blade making them fly the same. Trim-tabs are placed on the blades where they will not effect so much hover, but the increased airspeed of the advancing blade will make them more effective in forward flight adjustments.

[500e]

Thanks Dennis
We have found that balance is critical before even trying to fly blades, if the static weights are out you will never get smooth run up down & flight the root & tip should be same over all blades,
Hope you agree with this, if you were not 1\2 a world away could prove it
Don't know about heavy metal but would think it followed same pattern

[bryancobb]

Not trying to speak for Dennis but he, and anyone else with a fair amount of helicopter experience WILL know that getting all blades to weigh the same and have the center of their' mass equidistant from the mast is the FIRST step in balancing.

That's the easy part. Then the challenging part starts, trying to trick them into thinking they are aerodynamic clones of each other.

Challenging and time-consuming. No one ever gets it 100% right. They always stop when it's "good enough." I flew Phil Colaco's FACTORY BRAND NEW R-44 when it only had ferry time on it from Torrence to Georgia on it. It was unacceptable if you ask me but it was "good enough" for the factory!

[jebthereb]

Originally Posted by DennisFetters

Trim-tabs are what adjusts for the differences between each blade making them fly the same. Trim-tabs are placed on the blades where they will not effect so much hover, but the increased airspeed of the advancing blade will make them more effective in forward flight adjustments.

Is this why the trim tabs are closer to the blade tip?

During the hover are you using less of the blade tip to generate lift? I thought I read that in the rotorcraft handbook.

[okikuma]

Hi Ali,

Playing with Mi-17s in Pakistan I presume. For those who don't know, the Mi-17 is an export version of the Mi-8 with some noticeable differences. One has to remember that the Mil series of helos are very robust and simple to maintain compared to US designed helos.

Dennis' suggestion is right on the money. 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.

How many hours are on the airframes? Is your unit the original owners of these helos, or are the airframes hand-me-downs from one or more previous military owners?

Wayne

[DennisFetters]

Originally Posted by jebthereb

Is this why the trim tabs are closer to the blade tip?

During the hover are you using less of the blade tip to generate lift? I thought I read that in the rotorcraft handbook.

Yes, but placement of the trim-tab depends on the stiffness of the blades design as well. You want them away as far as possible from the tip, so they will not effect hover so much, but far enough out where they will work in forward flight. All rotorblades need trim-tabs. Those that don't will fly smoother if they do.