SEN 97-14 - 23 Sep 1997

News and Reports - 1997 SEN 97-14 23 September 1997

Scat Electronic News - F1A Bunter Trimmimg

This is a little experiment. the following article, orginally by Dietrich
Sauter has a graphic image added by Ernesto Busnelli. This is attached
to the mail message. Note that Juno users may not be able to view this. I
would be interested in feedback if you were able to read it ..

How do I trim an F1A for bunting?
Originally by: Dietrich Sauter

One frequently sees wonderfully crafted gliders lacking the right trim. I
always try to get the maximum performance out of my airplanes, focusing
my effort on the trim right after the ship has been finished. I will delineate
a technique that I would like to share through this article.

I start trimming in the workshop.The first thing to adjust is the stabilizer
and wing decalage. The values should be the following: the gliding
position should be 2.5degrees ; the towing position should be 3.5degrees
and the stabilizer should drop 14 mm. during bunting.

The next item in order of importance should be the wing warps: both
central panels must be perfectly straight. The outer wingtip should be
warped -2.5 mm. and the inner tip of the circle should be warped -0.5 mm.

My philosophy is to use minimum possible warps in the wings (since an
increase in warps diminishes performance.)

The next step is assembling the wings onto the fuselage to make sure that
they are perfectly straight. Then, I connect the wing wiggler, and
consequently the trailing edge of the inboard wing should drop 0.7 mm.

The next item to check out is the rudder. The straight towing position
should have a bit of deflection in the opposite direction of the gliding
circle.Usually the circle towing position will be 3 mm. tighter than the
gliding circle. After I assemble the model completely, I make sure that the
C.G. is at 54%; which is adequate for most airfoils. The towhook is
placed using a template.The hook should be placed 18degrees
ahead of the C.G., which is usually the optimum position.

This method of using the template has been very useful for Stefan Rumpp
and for me. The mentioned template has the position of other gliders that
tow correctly. We find this technique to be a quick way to place the
towhook effectively.

The last thing to adjust is the timer. The tailplane should drop for 0.8 of a
second after the release and go to gliding position 1.6 seconds after the
release. The D.T. time for the first test flights is about 45 seconds.
The timer setup was the end of the workshop task and big part of the

In the field I hand launch the plane to make sure I am not too far off the
correct glide. Then I hook up the towline and latch it, but without
connecting the first motion of the bunting mechanism(**). I adjust the glide
circle at about 30 seconds per circle and tow pitch until the plane clims
vertically after released.

After the basic adjustments are done I concentrate on the fine trimming. I
consider the towing a very important part of the flying for getting good
results. If the model zig zags during the straight tow, I know that I need to
readjust the towhook position aft until the airplane follows me reliably. If
while I am towing I cannot correct a deviation of the model by pulling of
the line, I know that I need to move the towhook forward to increase the
lever distance between the tow ring and the C.G.

To fine-trim the positioning of the wings, I release the plane as fast as I
can and make it stall. The model should fall into the turning side and
recover quickly. If I do not get that result I know I need to increase the
inboard panel wash-in until after the stall the plane turns inward cleanly
and clearly.

I like a tight tow circle because I believe it is easier to set the glider for
the release and control it during the tow.

Now it is time to test everything, so the next test will be with all the
mechanisms connected. It is crucial that the timer is always in the exact
same position. As I mentioned during the workshop timer setup, each
function must last 0.8 of a second. It is also very important to always
release the model in the same position and with the same cable tension. To
achieve all this requires a great deal of practice at the beginning, but the
results will pay off in the future. A good example of methodic work
leading to good results is Stefan Rumpp who is very consistent with his
towing and releases. We need to assure that when the bunting mechanism
kicks in, the airplane is climbing at an approximate vertical angle. The
towing decalage is going to fine- tune this and if we can not sprint enough
to acquire good tension, we will need to increase the tow pitch by
adjusting the buntroller hight. The over or under bunting will be corrected
with the timer’s settings.

After all this, we finally arrive to the gliding attitude. Perhaps the most
important factor is the positioning of the wings. The plane should turn
immediately after released, if not you will miss the thermal. Two very
important hints that will tell us if everything has the best position are:

1- The rudder must be off the fuselage axial just a bit, enough to
compensate the inboard wash in warp.

2- After a bad release the plane must dive into a clean circle and glide
instead of stalling to the ground.

I do not like to squeeze out that last bit of performance from my airplanes
with a “too close to stall speed.” That would surely give me the best flying
time but also a glide too sensitive to turbulence. I like seeing my planes
with that extra bit of speed so it would not be caught in “downers.”

This method followed step by step is a reliable technique for good
results. After 10 years of aeromodelling I have never felt frustrated
following this trimming procedure.

(**) Note from the translator (J.H. Abad): I always recommend the first
tests of a new plane with the hook unlatched. After I know the glider will
tow straight and the gliding circle is close to the desirable diameter, surely
the tow circle rudder position will be around +3 millimeters tighter than
the glide circle position.

Graphic: The template made out of Plexiglas is for the towhook position.
The vertical line must intersect the CG,and the other line with the angle
range from 17degrees to 20degrees will determine the ring position.

Translated of Termiksense 1/96 by Javier Hernández-Abad- to spanish
and then Ernesto Busnelli to english .. !