Control Surfaces

Mass and Hinge Moments

Pages 28 and 29 of the Flight and Service Manual (added by technical note 278-23, January 1979) give specifications and methods for correcting control surface hinge moments. These pages are reproduced below for owners who may have incomplete manuals. This is critical because control surfaces with excessive hinge moments may tend to flutter destructively as air speeds approach Vne.

Click for full size image

The following table summarizes the specifications in metric and English units.


Control Surface
Metric Units
Hinge Moment
5.00 (sn < 532)    0.175
(sn > 531)    0.120
7.10 .143
2.10 .096
English Units
Hinge Moment
11.02 (sn < 532)    15.49
(sn > 531)    10.62
15.65 12.66
4.63 8.50
Elevator mass includes the T-fitting or other attachment hardware.

English values were converted from metric using the following formulas:

Conversion Formulas
Mass 1 (kg) = 2.204623 (lb)
Hinge Moment 1 (m*daN) = 1 (39.37 in)*(10*0.2248089 lbf) = 88.507 in*lbf
lbf = pound force

Check the Control Surfaces First

It is a good idea to check the control surfaces before performing a weight and balance measurement on the glider. It would be a shame repeat the procedure after discovering that the tailplane or rudder needed ballast to bring them into specifications. The rudder is not hard to remove, so at least check the aft control surfaces where a small amount of additional ballast will have the greatest affect on the CG of the glider.

The Tailplane

T-fitting Attachment

The hinge moment is particularly easy to check on a T-fitting type tailplane.

Clamp the T-fitting snuggly in a table vice. Cushion the jaws of the vice to avoid damaging the T-fitting and take care not to over-tighten. Place a scale on the bench under the center trailing edge of the tailplane. The manual recommends using a letter or spring balance. A beam balance would likely have better calibration. Support the trailing edge on the vice so that the tailplane is level. This may be done by propping up the trailing edge with a nail or thin stick of wood. The prop should be placed at the aft-most possible location in order to minimize errors in the measurement. The end of the prop in contact with the tailplane should pointed.

Subtract the weight of the prop from the measured weight. This times the distance from the hinge point to the trailing edge support point must be no more than specified above. The major source of error will be determining the pivot point within the tailplane.

Page 29 of the manual, above, specifies how to add ballast to the leading edge of the tailplane if necessary. It will be tempting to place your ballast in the very tip of the leading edge, but the factory has already beat you to that location. Therefore they recommend placing your ballast 120 mm (4.7 in) off center (Manual, page 29, above).

It may turn out that the ballast needed to correct the hinge moment will bring the tailplane over it's maximum allowed mass. In that case, contact Schempp-Hirth and your controlling aviation agency for approval of the heavier tailplane.

One final note. Adding tailplane ballast throws the trim springs off. You need a stronger front spring and a weaker aft spring. You can fly ok without adjusting the springs, but there is less back-force on the trim knob if you get it right. Obviously the range of effectiveness of the trim control is also affected, but I haven't experienced any problem after adding 1.8 lbs of nose ballast to my tail plane.

Jim Hendrix

Modified Attachment

The easiest way to check the new-fitting tailplane for weight & balance is to get a piece of steel or aluminium rod, about 10mm diameter and long enough to fit through the mounting bracket and latch in the tailplane. (This rod is a "stand-in" for the tailplane pivot (Item "C" in the diagram) in the bracket in the fin) Attach some light cord to each end of the rod and use this cord to suspend the tailplane upside down from some suitable support.

Attach a loop of light string to the trailing edge of the tailplane at one of the corners near the fin cutout (use some adhesive tape), and use an accurate sensitive spring balance to lift the trailing edge until the tailplane is horizontal. The reading of the balance times the distance of the string attachment point from the centre of the rod gives the moment to be compared with the data in the POH.

You can also get the total weight of the tailplane by suspending it from a spring balance using the same support cords mentioned above, and then subtract the weight of the rod. Alternatively, the total weight may be measured by suspending the tailplane from a spring balance by means of the front bushed bracket (Item "A" in the diagram) which connects to the elevator drive arm when the tailplane is fitted to the glider. In this case you don't have to subtract anything, as the piece of rod is not needed.

John Giddy

The Rudder

Removing the rudder is not difficult if you have the right wrench. You need a 10 mm box wrench which you will use through the access hole at the base of the rudder. It's a tight fit and you may have to grind excess material from the perimeter of the wrench head. Once you have removed the rudder, you will have to rig up a way to support it freely from the hinge bushings. You might use a pair of nails secured horizontally by driving them into blocks of wood.

The actual hinge moment measurement is the same as for the elevator, described above. In this case, however, the distance from the hinge axis to the support point is easier to measure. Page 28 of the Manual (above) shows how to secure ballast to the rudder.

The Ailerons

The ailerons pivot on roll pins that must be removed carefully to avoid damaging the surrounding structure. The ailerons can be checked with the same jig used for the rudder. The manual does not tell how to ballast the ailerons. It only says to contact the factory. So, it is doubly important to avoid adding weight to the ailerons when refinishing them.

Movement Limits

Page 16 of the Manual contains illustrations depicting the specified limits to control surface movements. The following images were taken from that page:

The point of measurement is the inner-most trailing edge point on the aileron (see the illustration on the right). Defection distances are measured along an imaginary chord line on the arch traversed by the measurement point on the aileron. Stops for aileron movement are located at the base of the control stick under the seat pan. These are bolts secured by jam nuts. Note that the left and right stops are not symmetrically arranged.

The point of measurement is the inner-most trailing edge point on the tailplane, at the inboard end of the bevel (see the illustration on the right). Defection distances are measured along an imaginary chord line on the arch traversed by the measurement point on the tailplane. Stops for tailplane movement also are located at the base of the control stick under the seat pan.

The point of measurement is the lowest trailing edge point on the rudder (see the illustration on the right). Defection distances are measured along an imaginary chord line on the arch traversed by the measurement point on the rudder. Stops for rudder movement are located at the base of the rudder itself.