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Written by Gordon Buckland
RC Soaring
Column
As seen in the May 2016 issue of
Model Aviation.


Have you seen a beautiful sailplane on tow suddenly break apart? The model probably appeared to be perfect and had been launched dozens of times before without any issues. This time, it seemed to disintegrate on tow.

We sometimes hear a sailplane make an unmistakable machine gun-like fluttering noise as the pilot puts it through the “bucket.” The modeler is able to save the dying airframe by heroically landing it on flaps minus ailerons or elevator control (or both). The model sometimes only suffers wiper damage, aileron servo failure, or broken control horns. Sadly, we have all likely seen models destroyed during or after one of these towing incidents, and watched as the aircraft crashed back to terra firma.

I have heard pilots attempt to blame the manufacturer, saying, “The model wasn’t built right,” or “the stabilizers weren’t made strong enough,” etc. The truth is that competition has driven manufacturers to make models lighter, while maintaining the necessary strength to endure hundreds of highly stressful launches and bone-jarring spot landings.

Sometimes, on very rare occasions, models do blow up because of a manufacturing anomaly or mistake. Occasionally, insufficient resin might be used when the molds are closed, causing the spar cap to separate from the skin, or a leading edge could open up through insufficient resin when assembled. This can happen, but in my experience it is rare. The culprit responsible for your model fluttering and disintegrating is generally not the airframe manufacturer.

The root cause can also be attributed to wear and tear and lack of maintenance. Sometimes it is simply poor building techniques and lack of attention to the details. That is what this month’s column is about.


Attention to Details

Most of us are meticulous when we build a model and first fly it. We take great care to ensure that the installation of the electronics is perfect and we prepare and fly our new airplanes carefully during that all-important maiden flight.

As time goes by and the airframe becomes another member of our hangar with many hours of flying, we can become lax in maintaining the model as it was when it was new. This lack of maintenance is often the root cause behind the reason why many aircraft flutter or disintegrate during the high speed and stressful phase of winch towing, especially during the dive and zoom of “the bucket” at the top.

Close attention must be paid to slop in the control surfaces during the initial building and throughout a model’s life. Slop is easily the greatest contributor to flutter when models are flown at high speed. This control surface slop is especially important with ailerons and elevator (or stabilator).

Slop can be attributed to six main causes, all of which should be addressed during the construction and maintenance of older models.

1. Servo gear slop is a huge problem with our models. Much of the angst caused by control surface looseness can be overcome by using quality metal-gear servos with ball bearings. Plastic gears are virtually of no value in today’s sailplanes because they quickly wear down the teeth in the reduction gears and become sloppy.

Some servo manufacturers have invested heavily in our needs to create special wing servos that have increased metal-gear sizes, almost zero gear clearance, extremely strong motors, and quality ball bearings.

The MKS Servos brand is one such servo. This company is heavily invested in improving its product for our specific use. The latest wing servos are the HV6130 and HV6130H, which have many features designed to address the issues we face with our sailplane control surfaces.




This MKS servo is new and has been specifically developed and designed for composite model sailplane aileron and flap control. It has many features not included with other servos available today. Photo used with permission from MKS Servos USA.


The unique layout of the design incorporates a larger, longer, and more powerful motor than the previous HV6100 servo, and has a longer output spline that is supported with three ball bearings.

This servo is 2S LiPo battery capable, programmable, and has a new built-in failsafe system in the circuit that does not allow a stalled servo to burn out the wiring in a model. This is an example of how manufacturers have addressed our issues with products perfectly designed for our application.

Another good plan is to oversize the elevator servo for strength and longer gear service life. All servos gradually develop slop over time. When maintaining a model, you need to either replace old sloppy and worn aileron and elevator servos, or replace the gear set in them.

2. Keep servo horns as short as possible and replace them regularly. The use of longer servo horns accentuates any gear slop in the servo at the control surface. (Servo horns need to be replaced often because they can develop slop in the spline.)

3. Clevis pin to control horn and servo horn slop can be eliminated with a new installation by making sure that the hole in the control horn is sized exactly to suit the clevis pin. The correct drill size for most clevises should be checked by first predrilling an old horn and checking that the clevis is tight to push through the hole.

I then use the correct-size drill to wear the drill bit diameter down slightly by drilling holes in steel until there is a smaller hole in plastic than when the bit was new. Keep testing the hole it creates until the clevis pin is relatively tight.

Drill the exact-size hole with zero slop in the servo and control surface horns of your model. As time goes by and many hours of flying are put on an airframe, the holes in these horns will wear.

This slop can be eliminated by carefully putting a drop of thin Zap CA glue between the arms of the clevis horn and the clevis pin, allowing it to flow into the space between the clevis pin and horn left by the wear. (You will find that by using plastic or metal horns and clevises, the Zap CA will not glue the clevis to the horn permanently, but will effectively take up the slop.)

4. Servo movement in wing installations is an easy problem to resolve—whether using trays or gluing servos directly to the skin of a composite model.

When your servo installation is complete, tested, and working correctly, add a piece of hard balsa across the top of the servo, connecting the edges of the access hole in the lower skin of the model to the exposed surface of the servo. Glue this piece of balsa in place with a drop or two of thin Zap CA or use Shoe Goo.

It can easily be removed to take out the servo if required. It works like magic for removing slop and flex and stopping flutter in aileron and flap installs.

5. Make sure that your elevator pushrod is running through guide tubes or a sheath that is as straight as possible and without too much clearance. If you repair a broken fuselage, pay attention to the outer sheath and replace it if necessary. Make sure that the outer guide is supported at the end as close as possible to the servo using Shoe Goo.

6. Hinge wear is a problem on composite models, particularly with flaps because of the damage inflicted during landings, etc. The Kevlar material is only supposed to be flexing where the resin was scored during manufacturing.

The adjacent resin can become flexible or soft, or the material can delaminate from the wing and require repair. This repair and maintenance can be done with medium Zap glue, but be careful to use the minimum necessary to “stiffen” the softened hinge.

I fly with Jody Miller in Florida. If you get a chance to check out the control surfaces on one of his models, you will be amazed at how slop-free it is.
Pay attention to these six areas on your models and you might eliminate the death flutter. These are simple, but effective solutions.

Go downwind and soar.

-Gordon Buckland
gordon@gatorf3b.com


Sources:

League of Silent Flight (LSF)
www.silentflight.org

MKS Servos USA
(832) 287-2371
www.mksservosusa.com

Zap
www.zapglue.com






1 comments

I have eliminated flutter with counter weights. Some lead (solder) wound around a Popsicle stick, positioned forward of the hinge line. then glued to the control surface behind the hinge line. Put them on the top so they don't hit the ground on landing. Mount them so they don't quite touch the wing (or stab) at full up travel. The stick should be mounted with the thin cross section vertically, so they don't catch the wind. Very effective at dampening out severe flutter.

If ever you hear flutter, immediately pull full up elevator this will slow the plane, and you might save it!

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