|Once upon a time we were living in a happy situation. Control line stunt models were built and then flown without spending much thought about trim - this term was only known among our free flight friends. From a certain point on we agreed to make our life a little more complicated. Since then we have learned a lot. As in all areas of life, increased knowledge is the reason for the loss of paradise.|
we have to trim our airplanes. With the introduction of the word “precision
aerobatics”, requirements for high performance have risen considerably,
and the trim process is taking an expanding part in flight preparation.
Much has been written about this topic, and all authors agree that trimming
takes a lot of time. I’d like to add: it needs an enormous number
of flights . While the basic trim may be reached after some 20 or 30 flights,
those subtle modifications and tiny improvements never seem to come to an
end. The American Al Rabe - when asked when he stops trimming his airplanes
- replied “ when I hang it up and retire”!
Trimming should be divided into two parts. One goal is to have an aerodynamically clean airplane without big mistakes and bad habits. This is the objective part. In this part we set things like basic centre of gravity location, thrust line, control surface adjustment and deflection, and flight attitude. At this stage the airplane is able to do a competent schedule in the hands of every competent pilot. The other trim part is necessary to tune the airplane to the specific requirements and preferences of our own hands. Since no two pilots are exactly the same it is a very subjective procedure; nevertheless equally important and very difficult to achieve when high performance is desired.
|When talking about trimming it is presupposed that the airplane is built clean and straight. Crooked crows cannot be turned into a good flying model, and this story is not meant to offer methods to correct building errors. Also for ease of explanation it is supposed that all trimming devices are installed. Features like adjustable line guide, tip weight, pushrod length, and surface deflections ( independently from each other ) are an essential help. If these features do not exist many hints cannot be followed. In this case we have to give up or to consider some extensive workshop sessions with the balsa knife. However in order to cover the whole trim process these features must be mentioned.|
helpful gadget is the adjustable handle. Some handles allow for changing
the overhang, line length, line spacing, even individual vertical line setting.
For taming a stubborn beast some of these features might be necessary. My
own handles don’t have this luxury, but I like handles with variable
line length adjustable in flight.
Before starting to work with a new stunter I prefer flying with one of my better airplanes. I want to get the right feel of how a good model performs and to get an idea about what to look for. I always try to do it alone without other flyers waiting for their turn, so that I’m not in a hurry and can fully concentrate on each flight. Of course a competent friend would be a great help if one is around. Before entering the flying field it’s quite reasonable that we give the model a good push to see how it rolls on slick surface. It should roll straight ahead, neither to the left nor to the right. To the left could mean an early death. To the right means oblique jerky rolling with high drag resulting in a bad jumpy take off, coupled with a rocking tail end. We simply bend the tail skid to assure the desired roll direction.
|Now the lines can be attached to the model and adjusted carefully. We grab the handle and - without looking at the model - move our wrist up and down several times. We stop in a position when our hand feels comfortable and neutral ( this is how we hold our hand during level flight ). Only now we look at the elevator. It should be exactly in the neutral position. We can repeat this process several times to be sure that we have found the optimum hand position. If necessary we should adjust line length ( at the handle ) now.|
I keep trying until I “blindly” find neutral elevator position.
From this point line travel ( equals wrist movement ) should be the same
for up and down. This depends much on the individual’s own wrist respectively
its ability to move it up and down. No matter what the neutral hand setting
is - it has to be exactly the middle between the extreme up and down settings.
It is very important to find this “neutral wrist position”, since our ability to fly equal sized inside and outside loops without thinking depends on it. We cannot trim or find out trim errors unless we feel comfortable with the handle, and we have to find a point from where to start. With the handle adjusted we can start flying. Further adjustments may be necessary later though.
Now let’s start flying. At first we do some level laps with some wavy climbs and dives and some zig-zag corners thrown in to get a feel for the airplane. We can also try some inverted laps. I usually do this at about 60 degree elevation. In case the engine doesn’t cooperate there’s enough room below to easily recover into level upright flight and land safely. In most cases the engine setting is not perfect, so we make another flight with the same manoeuvres. This time we might fly the level laps at the correct height of 5 feet. These first flights are used to examine basic attitudes of our airplane. We mainly watch
1) whether the wing is level,
2) whether the airplane turns well and symmetrical
3) whether the airplane yaws.
These three tests will tell us enough about the basic behaviour of our airplane. An observer may be helpful to check the horizontal position of the wing. If we notice some really bad things we can roughly correct these and adjust accordingly. On the next flights we should try some manoeuvres. It’s not necessary to perform these perfectly. Instead we carefully concentrate on watching the airplane’s behaviour in several places of the manoeuvres. This should give us some better information for the trim work. From now on we do every change separately and one at a time only. Remember that any change in a trim setting will not only cause a change in that particular area. It will result in a change in another area as well; sometimes in several other areas! Also there’s a general rule: all trim measures are made at a minimum, as much as necessary but as little as possible. Everything beyond that point may solve a problem but will probably cause a new one; even in other places! With this in mind we can start trimming finally.
|The first task is to set the wing really dead horizontally. It can be banked inward or outward. The solution for this problem depends on the reason for this error. Look at sketches A and B ; these will explain things better than words can ( sketches C and D show how the pilot will see his airplane in flight).|
|Problem 1): the wing is warped or the flaps are misaligned. Inner wing is low in upright flight and high inverted. There’s a difference in Angle of Attack ( AoA ), thus lift. If the wing is warped there’s not much we can do. We can try to bend the flaps: inner flap down or outer flap up or both.|
|Problem 2): the opposite case: inner wing is high in upright, low in inverted flight. Of course the cure is vice versa: inner flap up, outer flap down, or both.|
When explaining the wing position I’m always talking about the inner
wing panel. That’s for one reason only. If our airplane flies with
the wing banked, we usually see the inner wing better. Sometimes we see
this panel only since the outer wing half is hidden by the fuselage. So
I mention the inner wing only.)
Problem 3): inner wing low in upright AND inverted. This is clearly a case of not enough tip weight.
Problem 4): inner wing high in upright and inverted. This is the opposite case: clearly the result of too much tip weight ( sketches E and F ).
|We can easily see: looking at the inner wing half and it’s position only is not enough. The same condition can be caused by two different reasons. To find out the true reason we simply go inverted and watch the wing now. This will help us to apply the right cure: bending flaps or changing tip weight (G and H ).|
|Recommending the bending of flaps on our fragile airplanes may sound rather brutal. Actually it’s not that hard. I have prepared four small plywood plates, large enough to cover the area where the horn wire reaches into the flap. With these little plates held tight above and below the flap on each flap half it’s possible to twist the flaps pretty hard without danger to break the horn wire out of the wood.|
the wing is the first trim measure because safe and precise execution of
manoeuvres depends on this adjustment. Flying manoeuvres with a banked wing
can be difficult or even impossible. So let’s turn our attention to
the pitch axis now. We’ve all heard those remarks about what to do
to get the desired sensitivity in manoeuvres. If we fly, say, a Ringmaster
there’s not much that can be done. Increasing or decreasing elevator
deflection is possible only when we have an adjustable elevator horn, and
we usually don’t. So we’re left with only one means and this
is adding lead to the model’s nose or tail. However we are talking
about an airplane with adjustable gadgets here. These gadgets allow to solve
the problem with aerodynamic measures. Let’s suppose the Centre of
Gravity ( CG ) is in the right place. This may have been decided by the
designer or we have determined a certain spot. Where ever it is: because
the CG is largely responsible for flight characteristics it should be left
where it was intended to be. Except for extreme deviations ( which were
caused by wrong design or building errors ) we shouldn’t employ the
lead cure. In all other cases trimming is done with aerodynamic means. This
is not a very simple process, and we need to understand what influence elevator
deflection and flap deflection have on our airplane. To put it as short
as possible: more elevator deflection gives better turn. More flap deflection
will increase lift, but makes for a smoother corner. Finding the sweet spot
is a difficult art, requires lots of practice flights, and depends on our
personal preference of course. This might actually result in an airplane
with less than optimum flight performance, but handling qualities are exactly
as we like them.
I’ll try to explain the effect of flaps as simple as possible. Sketches I and K can do this better than words. Gravity works on the CG and is shown as the downward arrow. Lift works on the Centre of Lift ( CL ) represented by the upward arrow. There’s a distance D between CG and CL.ift will apply a force, thereby creating a moment M trying to turn the model nose. If flaps are deflected, creation of lift is increased AND changed somewhat, and the CL moves backwards. Since distance D is bigger now, moment M has increased, too, with a stronger rotation around the CG now. Alas this force works in exactly the opposite direction as the deflected elevator does. That’s why flaps tend to smooth our corners.
|Remember that we’re talking about fully adjustable models here. In this case we can adjust the flaps independently from the elevator. Usually we have decided for a certain flap to elevator ratio. Changing the connection point of the flap pushrod will not change this ratio. However if we change elevator deflection we do. This will make things much more complicated. It’s wise to make changes in small increments only and to watch our airplane very carefully on the next flight. Usually we try some hard corners. We don’t watch for the turning ability only, but also for the end of and exit out of the corner. Depending on the outcome we might want to have more or less elevator movement. Remember that when cranking in more elevator, we proportionally reduce flap deflection and vice versa. Not enough flap is easily detected: the corners may stay crisp, but the exit is difficult to control, and the airplane might even stall in sharp corners ( especially when it’s heavy ). Too much flap deflection will soften the corner, create lots of drag, thereby reducing speed after corners. Also the execution of manoeuvres is made more difficult to control. Since sometimes there is more lift than necessary the airplane no longer rotates around the CG. Instead it appears to rotate around a point outside of itself. This is very difficult to see and requires lots of test flights. Since we don’t want more lift than necessary we try to avoid this situation.|
|There are models which when in level flight slightly wander up and down ( I’m not talking about a rearward CG location which makes for overall too sensitive an airplane ). This wandering is felt like a continuous low amplitude. I have even flown an airplane of another pilot with a steady fast oscillation ( around 5 impulses per second, tiny, and|
|hardly visible). I prefer to contribute this to the airfoil and not to the flaps. But if wandering ever occurs it might be clever to look at the flap deflection. I think we are better off if we have an airplane which flies VERY steady around the neutral position. This will cost us less concentration during level laps and help to regain attention for the next manoeuvre. An interview with the international modelling press during flight shouldn't cause more than 10 cm ( = 4" ) deviation from the perfect level flight height.|
is of prime importance that the airplane can fly smooth and steady at a
5 feet level, with exactly the same hand position just as we prefer, upright
and inverted. At first glance it seems to be an easy means to adjust this
by changing line length at the handle ( one line adjusted to the other ).
Except for lines which are connected totally wrong this is not the right
approach. We may get the desired handle position, but we fully destroy the
symmetric turning trim of the airplane. After having fixed upright and inverted
level flight, equal turning in inside and outside loops is the most difficult
and time consuming trim process. Those multiple loop manoeuvres like the
Eights simply cannot be flown well as long as the airplane is not trimmed
for fully symmetrical turn rates. This requires extensive playing around
with control surface setting and deflection. At this point I’d like
to mention the popular 0-0-0 trim setting. We should understand this rule
as a base from where to start the trim procedure. After arriving at the
final stage we may well discover that we have some subtle deviations from
these ideal numbers. There’s nothing wrong with that.
third trim aspect is yaw. In the past we flew fast and furious. Line tension
was great and yaw had never been heard of. But since we have discovered
the advantages of flying slowly airplane rotation around the vertical axis
deserves some thoughts. In order to establish line tension ( which we are
totally dependent on ) we mainly rely on
A ) engine side thrust
B ) line guide position
C ) rudder deflection
D ) CG location ( see sketch L )
|For all these aspects we should apply that basic rule again: as much as necessary, as little as possible. Alas the engine thrust line cannot be changed on our Super Orchid where the spinner blends beautifully into the fuselage nose shape. On airplanes without a nicely shaped front end ( like, say, a Barnstormer type model ) this shouldn’t be a problem. As was mentioned above CG location again shouldn’t be changed if it’s in the right place. This leaves the line guide and the rudder deflection for trim measures. Experts recommend using no or very little engine side thrust and right rudder. Our goal is to let the airplane fly on a tangent to the flight circle. This is not a problem in level flight. Problems can arise when we do gymnastics somewhere up in our hemisphere. A common place to install the leadouts is around 3 degrees back from a line through the CG, no matter where the bellcrank is mounted. In most flying conditions this angle is pretty much the same amount as our ( rearward bent ) lines enter the wing tip. However this figure "3" is correct for the flying airplane; that means: for a filled tank. With some of our big and thursty engines fuel weight may move the CG forward a few millimeters. So for a "dry" check we might get off better with an angle around 1,5 degrees. An easy check is to watch our airplane in level flight and - even more important - during manoeuvres. Provided the undercarriage is built precisely we shouldn’t see the outboard wheel. Can we see it slightly or fully? Then something is wrong. If we can see it “behind” the inboard wheel we probably have a line guide location too far aft.|
|Checking my airplane hanging at the leadouts in my hangar really surprised me. It's hanging exactly as shown in the photo - and the rake angle is close to 1,5 degrees! Credit for the 1,5 figure goes to Peter Germann. And - yes, the fuselage side is exactly paralell to the longitudinal axis.|
|A good means to find an acceptable angle is to fly a wingover as slowly as possible. With reduced line tension our airplane can rotate easily around the yaw axis. By the way, it can rotate around all three axis, thereby showing trim mistakes which usually cannot be seen at level flight with high line tension. I like to do the following test. The airplane is flown very slowly. I begin the wingover with a bent elbow. Near the top of the wingover I quickly stretch my arm to full length. At the zenith of the wingover line tension is gone and the airplane flies free for a short moment. During this moment all bad habits of the airplane can be seen clearly. We need not be afraid of a crash: our airplane will instantly be on the dive, the wind will push it to the downwind path, all controls are at neutral, and line tension is quickly recovered again in the dive ( sketch M ).|
rearward line guide location will improve line tension, yes, in level flight.
But it will cause all kinds of bad behaviour, like rocking after corners,
mushy manoeuvres, and severe loss of speed in the climb. Moving the line
guide forward will noticeably improve the climb and avoid wobbles after
corners. We shift it only a little bit every time, maybe about 5 millimeter,
until line tension starts to disappear. Then we go back the last step. The
same things can be said about the rudder. If it’s adjustable we should
start with a touch of right rudder, just to be sure we do not run into problems.
During test flights we simply reduce deflection up to the point where line
tension starts getting too weak.
By this time we’ve made quite a number of flights. Our airplane flies without any bad habits and we have become familiar with it’s flight characteristics. We have tuned it to our desire and requirements ( or ourselves to the airplane ?), and we can concentrate on fine tuning now.
Let’s get back to tip weight again. I like to use a generous amount of tip weight - it helps line tension. On the other hand it causes the outboard wing to dip after corners. That means: at the bottom of a loop the wing tip will drop, at the top of the loop it will rise. This is extremely undesirable, since the airplane will come in now. Bill Draper said that a tip weight of 1 ounce at 60 mph flying speed in a 8 feet radius turn will produce a force of two pounds, trying to pull the wing tip out of the corner ( off the loop centre ). Now there’s a way to compensate for a higher tip weight. We can simply increase the lift of the outer wing panel while manoeuvering. If we don’t already have a larger outboard flap we can reduce the area of the inboard flap. It can be done either by cutting off a narrow area at the trailing edge, or by cutting off one or two inches of the flap span at the wing tip. We just glue this part to the wing. We can even use this part as a trim tab and fix it accordingly.
There are more methods to make an airplane fly well if it refuses to do what we want. I have built new flaps with bigger area to cure a bad stall at the first corner of the triangle or the hourglass. People have installed new tailplanes at different AoA. Others have lengthened the fuselage nose. But I don’t consider this a trim method. This belongs definitely to the rebuild department, so I will not include such topics here. However there are a few gadgets left which shouldn’t be forgotten: the lines, the handle, and the prop.
|A certain line length is usually chosen for a given airplane and it is changed very seldom. Most pilots prefer to fly with maximum line length: it allows more room for manoeuvres. Nevertheless length is tuned to fulfil the requirements of the airplane as well as the preference of the pilot. In windy weather we prefer to use shorter lines to keep line tension.|
there are control line circles which are smaller than required for full
length lines. We cannot reduce airspeed as much as we want since our airplane
is most happy with one certain speed only and we have trimmed it for this
speed. Then a little bit of tail weight should help us to get on with the
limited space. On the other hand I’ve never had much problems when
changing to longer lines. Especially heavy airplanes respond quite pleasantly
when allowed to fly bigger bends. There’s only one thing to remember:
longer lines have more drag. It might be necessary to move the line guide
rearward just a little bit.
The handle is a completely different topic. Since most of us pilots build control systems with the same dimensions into all of our high performance airplanes, one handle should be sufficient to control all of them. However now and then we leave the serious path and relax with a fun model, an oldtimer, or some mad creation. Such a design might be just outside common rails and we cannot tame it with all the usual methods. The adjustable handle might be the only solution to save our model from the trash can. There are three features which should help to avoid the worst case scenario.
1) Wider line spacing makes the model feel much more sensitive, and vice versa.
2) More overhang gives the impression of more line tension. This can be dangerous when we have changed from a handle with short overhang. We feel that we have more line pull when we actually have not. More overhang makes the model much less sensitive; up to the point that we can no longer fly it. And vice versa.
3) Moving one line up or down can compensate for a model which just refuses to turn equally on up or down. Example: moving the top line up makes the model turn better on insides. We should never use this method to trim an airplane. It’s only a last resort for hopeless cases.
this leaves us with the last trimming measure: the propeller. Much advice
has been given in this area, many rules have been established. Some came
from highly experienced flyers, some from world top pilots. The number of
advices is wide and varied. Examples:
“Big diameter makes for a smoother airplane.”
“Two blade props help for better acceleration after corners.”
“Heavy props make a smoother corner.”
“Rounded blade tips have a better climb.”
Etc. etc. etc.
Actually I have no doubts that a single pilot has experienced exactly one of these things. And I have absolutely no doubt that we can change the flight characteristics of our airplanes with another propeller choice. Quite the contrary: I’m fully convinced that the propeller has considerable - I’d better say tremendous influence on the way our airplanes fly. I’ve had this experience more than once, and with dramatic results. We can make a lively model out of a lame dog, and the opposite is true as well. Alas I do no longer believe in fixed rules. There are so many brands and there are so many variations. We cannot even believe in those nominal numbers printed on the prop blades. We just have to try. But it’s worth to consider props as a trimming tool. . I usually start trying props when I’ve come beyond the basic trimming stage. It doesn’t make sense to begin fine trimming before we have found a suitable propeller. It can change our trim so much that all the work done before prop selection can be obsolete.
course our engine wants to have its voice heard before we finally make our
decision. But once we’ve made this the final trim work can commence.
Progress doesn’t stand still. We always want our airplanes to fly better next time, especially if we suffer from competition orientation. We know that improving our skills and our airplanes takes an unbelievably amount of effort and time. This is increasing with every day. In the future I can see a day coming when time is just sufficient to allow trimming of my airplanes. There will be no time left for contest flying.