KA-50 RT&B

[159th_Falcon]

I am a helicopter mechanic myself and have some (pretty limited) experience in performing Rotor Track And Balance flights and adjustments.

 

On most helicopters its pretty straightforward, though im curious how this is

done in the KA-50. Sure, you can measure the unbalance in a similar way, but how do you determine which rotor (upper or lower) is causing an vibration?

 

Also, how are the rotors track controlled, i assume its an critical thing

since the rotors on the RH side of the helicopter get closer in flight.

Having wrong tabbing on the blades could have catastrophic consequences.

 

Anyone has some 1st hand experience in performing RT&B on an KA50 and cares to explain a thing or two about how it works?

(any other coaxial rotor design is also ok whit me cause the principal would be the same)

[isoul]

I think that you have more chances of finding someone with such experience in the russian section. Russian use co-axial rotor-crafts that western people, right?

 

If you want someone who has enginnering experiesnse on the Ka-50 specifically, I think you have to search a lot since Ka-50 are too few so I am guessing that its engineers will be few aswell.

 

Nice to see that we have someone with real knowledge of helicopter mechanics among us!

 

Good luck finding one!

[AlphaOneSix]

AirTito speaks English just fine, and has lots of experience with Ka-32's, so I'm sure he can answer your question. May be a good idea to just send him a message rather than hope he sees this thread, though.

 

I'm also a helicopter mechanic and I have a great deal of experience at tracking single-rotor helicopters, but that doesn't help with your question, so I'll keep my mouth shut. ;)

[isoul]

...I'm also a helicopter mechanic and I have a great deal of experience at tracking single-rotor helicopters, but that doesn't help with your question, so I'll keep my mouth shut. ;)

 

Glad to see that we have one more with real knowledge of helicopter mechanics!

 

PS: Note that the one who really knows keeps his mouth shut!

[159th_Falcon]

Thanx for the reply's, ill send AirTito an PM.

 

Also, i know how RT&B works on single rotors, well the basics that is.

And isoul, i'm fairly sure that AlphaOneSix and Me aren't the only helicopter

mechanics on this forum by far....

 

Ill ask AirTito to reply in this thread BTW, then other people interested also know the answer.

[RvETito]

Sorry for being late with the answer but I'm abroad on a work trip and internet access is limited.

 

Yes, I've done several times rotor balance and blades tracking on a Ka-32. I presume it shouldn't be any different on a Ka-50 but with the russian military you never know :)

 

So here's the deal. It is more complicated than a single rotor design but it isn't a rocket science. Once you do it you realize it's pretty straight forward. Assuming that the flight controls rigging is done (Kamov uses the term 'static adjustment' of the rotor) which means with cyclic and pedals rigged in neutral plus collective full down the swashplates are perpendicular to the rotor shaft and the blades pitch is within specified limits (measured at scale fitted to each blade's feathering hinge) you can go ahead with the blade tracking and rotor balance for which Kamov uses the common term ' dynamic rotor adjustment'.

 

Rotor weight balance.

 

Since the two rotors together with the rotor mast assy form a single dynamic system it is possible to install balance weights pretty much anywhere. On Ka-32 that place are the upper rotor blades roots - each upper blade has a couple of bolts where you attach a set of balance weight. The lower rotor has anti-vibration pendulums approx. 1.5m on the blade span which you just lubricate, nothing more. It is considered that the upper rotor weights are enough to balance the whole rotor. The procedure is:

- hover at 5-8m for 3-5min and record the vibrations amplitude without installing any weights on the upper rotor. For this purpose is used a mechanical vibrograph installed near the cockpit floor with the sensing element touching the floor. During the hover it draws a sinusoid line. You use a magnifying glass with a scaled ruler on it to measure the amplitude (the absolute distance between up and down peaks) in milimeters. Land and shut down the engines.

- install set of balance weights with total weight of 1kg on blade #1. Repeat the vibrations survey and record the amplitude.

- remove the weights from blade #1 and install them on lade #2.

- do the same for blade #3.

- depending on the results there are several ways to proceed according to the maintenance manual. In general the method is purely graphical - you draw a line proportional to the value of the vibrations amplitude of 'clean' rotor then you draw three circles with radius proportional to the amplitude of each blade. Depending on how the circles intersect with the line and with each other you define how much weight (represented as a line) you need to put on each blade.

 

Blades tracking

 

In russian terms this is also called rotor cone adjustment because you actually adjust how much the blade is flapping with centered and neutral controls. Here is the standard (as per the AMM) procedure which I have used.

 

- apply a layer of slowly drying paint with different color at each blade tip, for both rotors independently.

- position the standard mast with attached at it's upper end piece of thick paper (with rectangular shape) approx at the rotor diameter within pilot's FOV. Best is 10-11 o'clock.

- start both engines and at idle trim the controls in neutral - a green light on the overhead panel indicates cyclic in neutral, the pedals have a scale on the cockpit floor.

- carefully raise the mast until the lower rotor blades hit with their tips on the paper.

- lower the mast and look at the marks - write down the number of the blade with the corresponding color (for example next to a blue mark - blade #2 etc)

- remove the paper and attach it with the other (untouched) end toward the rotor.

- adjust the height of the mast for the upper rotor. It is actually two tubes one in the other with the inner having two pin holes - one for the lower and one for the upper rotor.

- carefully raise the mast until the upper rotor blades hit on the paper. Lower the mast and shut down the engines.

- measure the flapping of each blade (the distance in mm on the paper).

- adjust the length of the blade pitch links which are adjustable rods. One turn of the adjustable sleeve increases or decreases the blade track with 35mm. Usualy you adjust two of the blades to match with the one between them (on the paper). Lets say one blade is 35mm above the 'middle' one and the other is 70mm below. That means that you have to make 1 turn of the first to decrease it's flapping and two turns of the other to increase it respectively in order to match with the middle blade. One must keep in mind that the direction of turning the adjustable sleeve is mirrored for the two rotors because the rods are connecting from above for the lower rotor and from below for the upper rotor. So to increase the flapping of a lower rotor blade you have shorten the rod (pitch link) while for the upper you have to make it longer.

- once the adjustment is done install a new thick paper and check again the two rotors using the same procedure. Apply fresh paint if necessary, using the same colors.

- if blades tracking is satisfactory at idle tell the pilot to increase power to AUTO mode. Check the tracking again for both rotors.

- if further adjustment is necessary (if any it would be minor) shut down the engines and proceed with the fine 'tuning' of the blades flapping - in this case, after high power tracking you use the tabs at each blade trailing edge and bend them at the necessary degree - if you need the blade to flap more and bend the tab up and vice versa.

- once you're done with that you go ahead with the last (hopefully) check - rotors torque imbalance. Because there is now way of knowing how the torque between the rotors is distributed after the blade tracking adjustment (both rotors may be running at perfect cone surface but one could be at bigger collective than the other). For that initiate a hover at 5-8m and wait for pilot's report for yaw imbalance - if with pedals in neutral the helicopter tends to turn either direction the pilot compensates with pedal and trims it. The pedals have a pointer and like I mentioned above there is a scale attached to the floor in front of the pedals. So the pilot has to look at the pointer and report the needed pedal deflection (in degrees) to balance the yaw (torque) imbalance.

- land and shut down the engines. Adjust all the rods of one (or both) rotors at the same length - for example if the helicopter spins to the right that means that the lower rotor has bigger torque which means that you have to either adjust all of it's pitch links length with the same value (to decrease the pitch) or increase the pitch of the upper rotor blades at the same value or change both rotors pitch with half of the value. That depends on the rotor RPM at hover - if it's on the 90% mark you'd better adjust both rotors - increase the pitch of the upper rotor blades and decrease it on the lower rotor blades with the same amount. If it's stable at 88% just decrease the lower rotor pitch, if it's 92% increase the upper rotor pitch.

 

So, that's pretty much all. If you have to do all that from 0 than you'd be lucky if you manage in a day ;)

 

But once done properly you forget about it until blades or rotor mast removal.

 

Hope that helps.

[sobek]

Sounds like quite the mindjob with having to turn the rods in opposite directions :)

[RvETito]

You can say that again... One wrong move with a single rod and you have to start it all over. I've seen it happening. It sucks.

[AlphaOneSix]

I'm just happy we don't have to use flags. In fact, I've never used flags, even though our Mi-17 MM describes that method. I've always used strobes and cameras to find the track spread, and accelerometers to check vibrations.

[RvETito]

Unfortunately, I have only read about those...

[159th_Falcon]

Thank you very much for the reply AirTito.

 

To be honest, i have readed a very similar procedure for our S61N we have at work.

Though we use a chadwick whit a camera and accelerometers so ive never used the flag method. Ive hearth about it from some older technicians though.

 

I am wondering though, don't you check the RT&B in flight?

For example all our company's helicopters use the following regiments for RT&B

Flat Pitch on Ground

Hover

Slow Cruise

Cruise

 

and some use even more regiments, for example the AW139 requires you to also record the vibrations at VNE.

 

So, that's pretty much all. If you have to do all that from 0 than you'd be lucky if you manage in a day ;)

 

Just one day? sounds bloody fast, we usually take 2 to 3 days to perform RT&B, must be noted though that we don't have a crew available always so more then half of the time where waiting for a pilot so we can take the next set of readings.

 

Once again, thanx for taking the time to reply AirTito.

Edited February 14, 2010 by 159th_Falcon

[RvETito]

Kamov says it's enough to record vibrations at hover to judge for weight imbalance. Experience proves that right.

 

1 day is the worst case scenario. If you don't screw up somewhere (like turning of the 6 rods in the wrong direction) it can take even less.