The easy way to Dynamically Balance the Rotorsystem

DennisFetters

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Recently I have had more than a few people contact me about balancing a helicopters rotorsystem. I figured since I took the time to answer their emails, I might as well post my answer here on the forum in case it may help someone else;

Hello XXX,

As for the dynamic balancing procedure I came up with for helicopters, it is explained in the PDF attachment for the Mini-500.

When I hired the people to teach me about helicopter dynamic balancing, they said that you could balance out vibrations using rotorblade pitch to counter out-of-balance, and vice versa. I never agreed to that, knowing that the vibrations were still causing forces, except that one force was masking the effect of a different force from being felt by the occupants, but the forces were still acting on the airframe. Like taking two hammers of equal size and banging them together; the force is not canceled, but only masked, and still destructive. Also, any adjustments between hover and forward flight effect each other, and only work in a compromise situation, meaning you have to put up with some vibration in one or both modes of flight. For the people being paid for the service, this is a perfect system for them because it is much more complicated and requires more time they can be paid, as well as more visits required over time.

My system is more customer friendly and better on the airframe. I look at the rotorsystem in two different balance requirements only; Hover mechanical vibration balance and forward flight aerodynamic balance.

First, like a wheel on a car, the rotorsystem must be in perfect balance and perfect track in the hover mode. Rotorblade tracking greatly effects balance, but balance only slightly effects tracking, so it is of the utmost importance that the rotorsystem is ALWAYS in perfect track. If it is out of track, you are wasting your time trying to balance. As you balance, you need to check the track using the pitch-links only, (the trim-tabs need to both be set equally in the starting position of +6 degrees, do not ever make adjustments to the trim-tabs in hover mode), and if it goes out of track at any time during the hover balancing, stop and re-track using only the pitch-links and start over with the hover balancing again. ONLY after you have achieved perfect tracking with perfect balance in hover can you move on to forward flight balance. The calmer the wind conditions in hover, the more accurate your results.

As the helicopter fly's forward, there will be new vibrations, yet the mechanical system is still in balance as it was in a hover. This is because the rotorblades are now flying out of track. Why? Because it is impossible to ever build two rotors exactly the same, and one will twist differently under the forces of forward flight than the other, and fly out of track. That is why all rotorsystems need trim-tabs.

Remember; track greatly effects balance! The trim-tabs are placed on the rotorblade far enough inboard that if you adjust them for forward flight, they effect the hover tracking very little, but with the added airspeed of forward flight they will take effect, and with this you can adjust the trim-tabs to bring the blades back in the proper tracking as they were in hover mode. This will eliminate the forward flight vibrations.

Again, after you make the trim-tab adjustments, you should go back and check your hover tracking and balance, because the trim-tab adjustment may have affected the tracking a little. If so, again use the pitch-link and adjust the tracking to perfect, and re-balance in hover. Then again, go to forward flight and make final adjustments to the trim-tabs. Repeat until you have the desired results.

I hope the information is of help.

Most sincerely,

Dennis


Mini-500 Helicopter Rotorsystem Dynamic Balancing Procedure by RHCI

Hover-Flight Balancing Procedure

The reduction of rotor blade system vibration is, and always will be, the most difficult challenge to any helicopter manufacturer, especially within the kitbuilt market. For example, a certified helicopter is completely built, test flown and dynamically balanced by experienced personnel using sophisticated equipment and knowledge gained from years of experience working with the same model. The kit builder is always at a disadvantage, because they are not only in a learning process, but are also dealing with assembly, rigging, and part tolerance discrepancies. Certified manufacturers also deal with this, but they have the experience to better troubleshoot. On the average, it takes 10 to 15 hours of flight time to rig, track, and dynamically balance a new Robinson R-22, and that’s by experts! Therefore, be realistic on the time you think YOU could take.

In order for the dynamic balancing to be successful, it is imperative that your Mini-500 first be in its complete finished and flying form, CG (Center of Gravity) verified, and all systems functioning correctly. Any discrepancies, modifications, or omitted parts could prevent a successful dynamic balancing. If the rotor components and connecting linkages are not rigged and working properly within their design specification, it may be impossible to balance the rotor system for hover or forward-flight.

Dynamic balancing must be applied to the Mini-500 as often as needed, but at least once a year. If the rotor blades are removed, or trailered while on the aircraft, repaired or bumped, or basically anything that could change their shape or position, then the dynamic balance must be rechecked to ensure that the vibration levels are within limits. Do not operate the Mini-500 if the hover one-per-rev vibration level is above 0.25 IPS, or if the forward-flight one-per-rev is above 0.15 IPS.

Warning: Severe damage to aircraft and operator may result if variations to this procedure are implemented. Do not make or add your own procedures and expect to obtain optimum dynamic rotor balance.

Dynamic Balancing Equipment:

The following instructions and balance charts are specific to the “Micro-Balancer” #S0028, sold by RHCI, but can be used with other balancing equipment.

Hover-Flight Balancing Procedure:

Use these directions in conjunction with the Mini-500 Assembly Manual.

1. Install the accelerometer on the nose of the aircraft in a vertical position and the RPM/Phase sensor in a position to indicate “A” blade over the nose, as explained in the Mini-500 Assembly Manual, fig. 25 and 26, section 7.

2. Set the main rotor blades to a -0.5 to -1.00 degree negative pitch to allow for autorotation. This is best done with the collective fully down to the stop on the rotor head. Use a level on the bottom of the #0023 Control Transfer Plate as a reference to set the negative blade pitch. The blades should be parallel with the tail boom. Measure laterally and level the plate with cyclic movement. Have an assistant monitor this level measurement and measure the blade pitch at a point two inches inboard from the tip. Adjust with the Pitch Change Links. Adjust the Blade Trim Tabs to .006" TEU I/A/W fig. 23 of this manual.

3. Tracking is best done by using two different color of crayons. Mark a 1/4 inch line in the center outside tip of each blade, one color for each blade. A simple special tool to use here consists of the center cardboard from a roll of paper towels taped to the end of a broomstick. Wrap white paper around the roll and secure with tape. Run up rotor system to 104% RPM, and lift collective until helicopter is light on the skids. Have assistant hold paper roll perpendicular to tip path plane and slowly move towards rotor tip until brief light contact is made. Examine marks on paper. Adjust PC Links until colors are overlapping. Adjust both blades simultaneously, one up and the other down. This keeps overall rotor system pitch at an appropriate degree for autorotation. One flat on a PC link is about 3/16" shift of its corresponding blade tip.

4. Now collect dynamic data at a hover to obtain an accurate one-per-rev vibration level and phase angle. Collect all data at 104% rotor RPM, aircraft at a stable hover, nose into the wind. Using the new “Hover-Flight Balancing Chart,” plot and determine a solution.

There are three adjustment planes provided on the Hover-Flight Balancing Chart. Tip-weight move line, Head-Shift move line and Pitch-Link move line. The Pitch-Link move line should not be used in Hover balancing, and is only provided as a reference to see what affect its move will have on hover balancing if used in forward-flight corrections.

5. Apply the solution given in the Hover-Flight Balancing Chart, and repeat the track/dynamic balance process until your one-per-revs are below 0.10 IPS and blades are in perfect track. Adding weight affects the rotor track negligibly but a head shift may require re-tracking.

NOTE: If you need to add weight to a blade, we have found that applying “2 inch duct-tape” about 6 inches from the end of the light blade works best. This will significantly reduce the time needed to balance the aircraft. When using the tape for weight, apply tape starting from trailing edge — around leading edge — and back to trailing edge. If smaller amounts are needed, then apply it across the leading edge. Tape seam must always be to trailing edge so as not to peel off. After completing the entire dynamic balancing procedure in both hover-flight and forward-flight, go to step 4 of the Forward-Flight section.

6. Dynamic balance the tail-rotor according to the procedures in the Mini-500 Assembly Manual.

7. Before performing any extensive forward flight track and balance procedures, check and set your autorotational RPM. Refer to section 7, page 8 of the Mini-500 assembly manual.


Forward-Flight Balancing Procedure

There are three adjustment planes provided on the Forward-Flight Balancing Chart. Trim-Tab move line, Pitch-Link move line and Head Shift move line. Use the Trim-tab adjustments first, because this will affect hover track and balance least. Pitch Change Links are second, while Head Shift in forward flight is rarely required and least desired. By comparing both hover and forward flight charts, you can see what the affect will be to hover when you make forward flight adjustments by using sensitivity and move line differences. You should be able to achieve vibration levels in both hover and forward flight below 0.100 ips in this manner.

1. Aircraft should be within proper CG limits (Center of Gravity). The wind must be as light as possible, but should never be above 10 mph at any time during the data collection process to obtain accurate results. Data collected during turbulence will also affect the results. The smoother the conditions, the better the results. Balance runs should be performed into the wind.

2. Fly the aircraft at 75 mph indicated airspeed, straight and level, rotor rpm 104%, and collect data to obtain an accurate one-per-rev vibration level and phase angle. Using the new “Forward-Flight Balancing Chart,” plot and apply solution. Only use trim-tab and pitch-link to adjust. If called for in the solution, first adjust trim-tab, then, if called for in the solution, adjust the pitch-link. Make only one adjustment at a time. Note that trim tabs are adjusted opposite each other by equal amounts. Adjust until the one-per vibration level is 0.10 ips or less.

3. Recheck hover balance to assure that your in-flight adjustments have not excessively changed the hover ips reading. Note that PC link adjustments and to a lesser extent Trim-Tab adjustments will change the blade track and thus dynamic balance at a hover. This is normal so long as in-flight adjustments do not push your hover reading beyond .25 ips.

4. Remove any duct-tape from the blade and weigh it. Drill a hole in the top of the main rotor blade about 6 inches from the tip and about 2.5 inches from the leading edge. Drill the hole only large and deep enough to install lead balance weights that equal the weight of the tape (minus a few tenths of a gram for glue), then glue in place and touch-up with paint.

NOTE: Do not mark on the dynamic balancing sheet provided. Always use it as a master and make copies.

NOTE: No two helicopters even of the same make and model fly exactly the same. Any balance chart provided is always for use on a fleet in general, and may vary some in both phase angle and sensitivity. It may be necessary for you to interpolate the solutions to best match your Mini-500's specific characteristics.

WARNING: If you are having trouble dynamically balancing your Mini-500, do NOT continue flying it! Stop, and contact our RHCI technical advisers for help at (816) 637-2800.

Note: Your helicopter will only fly as well as YOU assemble an d maintain it!
 

Attachments

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Jincamty

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Thanks Dennis,
I always enjoy reading about dynamic balancing, It's a fascinating science.

Cheers Cam.
 

Mike G

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Dennis
I found your post interesting thanks. Your technique is similar in many ways to what I have developed with the PB3 balancer for gyros. It's interesting that we both came to similar methods independently.
I would appreciate a copy of your polar charts, I don't get to balance many helicopters and would like to see your charts.
Mike G
 

DennisFetters

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Dennis
I found your post interesting thanks. Your technique is similar in many ways to what I have developed with the PB3 balancer for gyros. It's interesting that we both came to similar methods independently.
I would appreciate a copy of your polar charts, I don't get to balance many helicopters and would like to see your charts.
Mike G

Mike, you are very welcome to download the one I posted above, or I can send you a high resolution by email, just give me your address.
 

bryancobb

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Mike,

Keep in mind, polar charts are specific for only one particular rotor system and that specific system`s features, with a specific set of corrective actions for each out of balance condition.

Once Dennis` FINAL version of the Mini-500 rotor system (I.E. with Mast Support, Cast Pitch Arms, and Trim Tabs) was set in concrete, he then deliberately made chanqes to one variable balancinq feature at a time, creatinq an out of balance condition that revealed the `move line` direction for each and the maqnitude of the effect caused by the chanqe made.

Forexample: It was determined experimentally, that in forward fliqht at 75MPh and Rotor RPM at 104%, bendinq the trim tabs up/down causes the vibration data point to move almost perfectly alonq the lonqitudinal axis of the fuselaqe and the AMOUNT of bend creates/cancels that vibration at the rate of 0.2 IPS per 0.008`` of bend on BOTh trim tabs (one up/one down).

Second Example: It was determined experimentally, that in forward fliqht at 75MPh and Rotor RPM at 104%, lenqtheninq/shorteninq the pitch links causes the vibration data point to move alonq a line approximately 45 deqrees to the riqht of the nose (047.5/227.5) and the AMOUNT lenqth chanqe creates/cancels that vibration at the rate of 0.2 IPS per 1/2 of a flat (0.2 IPS per 0.006`` in lenqth) on ONE pitch lenqth. The adjustinq barrels are 1/4-28 threads and each one has 6 flats. 1/2 flat equals 30 deqrees. Turninq the barrel 30 deqrees lenqthens/shortens the pitch lenqth (2)*[(1/28)/12] or 0.006.`` (one rod end has Rh threads and one has Lh threads so rotatinq the barrel has 2x the effect)

As I said, all this was determined experimentally by many hours of deliberately causinq vibrations by doinq different thinqs and plottinq the data.

The polar charts are only accurate at ONE ROTOR RPM and on that particular rotor system.

Since a qyro`s RPM is constantly varyinq, it is almost impossible to qet it smooth in all fliqht reqimes. If a qyro was flown like a fixed winq and only had 3 confiqurations for fliqht, Takeoff/Climb -- Cruise -- Descent/Land...with a correspondinq Rotor RPM for each...you could develop charts for each of those three RPM`s and sort of blend the three toqether into a compromise.

Your qyro`s 320 RPM move lines will probably differ a lot from your 275 RPM move lines.
 

Jincamty

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Has anyone done a correlation chart between fixed rotor speed and teeter height?
Just curious.

Cheers Cam
 

Mike G

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Dennis
Thank you, the attachment is fine, I didn't see it on your original post.

Brian
Thanks for your explanation, I already had a pretty good handle on how polar charts work.

Cam
Not sure I understand your question. JC Debreyer has posted some theoretical curves showing the relationship between teeter height (undersling) and vibration but that is for 2/rev vibration.

Mike G
 
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