The first term you need to learn is 'incidence'. The incidence is the angle of the wing relative to the model. For our models, it is convenient to reference the incidence to the body of the model. The wing can be mounted with either zero, positive, or negative incidence as illustrated as below.

These same terms can be used for the stabilizer as well, and when combined with the wing can result in several types of what I call 'trim types'. Here is how they can be combined and the combinations which do and do not work.

Of the combinations which do work, you will most likely find that the lower three are the ones which you will end up with. The zero-zero combination requires special trimming and a balance point which is much further aft than the .7" called out on the plan. You can pick up some duration flying close to a 'zero-zero' setting, however there are some tradeoffs to consider such as reduced stability which may be detrimental to your flying efforts. I will discuss the 'zero-zero' trim method later.

Since we built our model with the body and boom in a straight line, the tail has zero incidence. Because we balanced the model at .7" in front of the trailing edge of the wing, it will be necessary to add some positive incidence to the wing to properly trim the model. This is done by raising the the leading edge wingpost 1/8 of an inch higher than the rear wing post. This is the plus - zero trim setting.

When you fly your model for the first time you will want to watch out for certain things.

Stalling is when the model pitches up and down over and over again as it flies (see the drawing above).  Stalling can be cause by several things but the reason it happens is that the lift forces from the wing and the tail are not in balance. Like a see-saw, the model rotates as the forces try to find a point of equilibrium. The figure below shows a greatly simplified version of how the forces are trying to balance each other.

Lw = lift generated by the wing
Lt = lift generated by the tail
lw = distance from the balance (pivot) point to the lift force of the wing
lt = distance from the balance (pivot) point to the lift force of the tail

For stable flight the moment (lift force times the distance) of the wing must equal the moment (lift force times the distance) of the tail.

This can be expressed mathematically as:      (Lw * lw) = (Lt * lt)

if (Lw * lw) is greater than (Lt * lt) then the nose of the model will pitch upwards.
if (Lw * lw) is lower than (Lt *lt) then the nose of the model will pitch downwards.

So a stalling model is an indication that the nose up forces are winning the see-saw balance. So to counteract that, we have several options.

• Reduce the incidence in the wing. If you have plus 1/8" incidence, then reduce it to about 1/16" of plus incidence, and fly again. However, do not ever reduce the incidence to a point where the incidence is negative in the wing, you always want zero or positive incidence in the wing.
• Move the balance point of the model forward. By changing the balance point, you are moving the pivot point of the model to give the tail a longer lever in which to utilize. You can move the balance point forward by adding clay to the nose, moving the landing gear forward, or you can remount the wing sockets further back.
• Check for and remove, if necessary, negative incidence from the tail. You may think that there is no negative incidence in the tail, but sometimes the balsawood warps, and it is possible for the tail boom to warp and bow upwards which is the same as raising the trailing edge of the tail higher than the leading edge of the tail (negative incidence). Please note that negative incidence in the tail is not necessarily a bad thing, it is only bad in excess. We built the model with zero tail incidence, but if the boom did warp upwards and the bow is less than 1/8" then you should be OK. If it is more than 1/8" then you will have to either remount the tail boom with less incidence, or cut the leading edge of the stabilizer free from the boom and place a 1/16" shim between them and reglue it.

• Increase the wing incidence to about 1/8" positive. You can either raise the front wingpost or lower the rear wingpost, or do a combination of both.
• If you find that you have more than 1/8" positive incidence then you may want to add some negative incidence to the stabilizer. You can do this by cutting the trailing edge of the stabilizer free from the boom and adding a 1/16" or even a 1/8" high shim to raise the trailing edge of the stabilizer upwards.
• Move the balance point more aft. You can do this by moving the landing gear rearward, adding clay to the end of the tail boom, or by moving the wing forward. You can also scrape off some of the weight off of the propellor.

Your model should have a little bit of left tilt in the tail. If not then it needs to be corrected. A quick and easy method, which may seem extreme, is to grab the boom about 2" behind the rear hook with your left thumb and forefinger, and then grab the boom about 1" further back with your right thumb and forefinger. Then twist your hands in opposite directions from each other making sure that the hand closest to the tail is twisting the direction you want to tail to go. Be gentle when doing this, or you may over twist the boom and crack the wood. If you do not wish to use this method (I use it all of the time though) then you will need to remove the boom from the body and reglue it. Be careful no to overdo the tail tilt otherwise it can cause other flight problems. You should have a few degrees of tail tilt as shown in the diagram above next to "LEFT TILT".

Turning problems can also be cause by insufficient left thrust or left rudder. Check to see if the model has one (1) to three (3) degrees of left thrust as well as the 1/8" of boom/rudder offset which was built into it.

Other trim problems can come into play at other parts of the flight. A typical flight can be broken down into three segments. The first is the climb phase which is when the model climbs to the ceiling. Then as the rubber band winds down, then you enter the cruise phase which is when the model flies at about the same altitude for several laps. Finally there is the descent, which is when the model starts coming down from the ceiling and eventually lands on the floor.

Sometimes problems occur in one segment and not in the other which can be quite troublesome to try and figure out. A model may not circle at launch but then circles fine 20 seconds later, or it may try to dive at launch but then flies just fine moments later. Here are some frequent segment specific problems which may arise.

Model flies straight during launch:

• Add more side thrust to the model
• Add more left tail tilt
• Check tail tilt at launch, if it looks like you have right tail tilt instead of left tail tilt then you may have a weak body which is twisting from the rubber band torque, and will have to be replaced with stronger wood or wider wood.
• Check to make sure the body is not bowing to the right during launch. If it is then the body is too weak and will need to be replaced, or made from wider wood.

• Too much downthrust in prop assembly
• Motorstick is bending downwards causing either excessive downthrust or causing the tailboom to bend downwards which reduces it incidence. If the this is happening then the motorstick needs to be made from stronger wood.

• Not enough downthrust or model possibly has upthrust.
• Too much positive incidence in the wing or too much negative incidence in the tail. You could try moving the balance point further back and then reduce the incidence angle.

• This could be a symptom of not enough wing wash-in. Check to see if you have 1/8" wash-in, if necessary crack the spar by the wingpost and raise the leading edge slightly until you have 1/8".
• Too much left side thrust in the prop assembly. Check to see if you have 1 to 3 degrees of side thrust with the rubber band wound. If you have too much then reduce it a little.
• Too much rudder offset. The boom should be built with about 1/8" rudder offset. If you have more than this, then the rudder can overpower the model and cause it to dive.

• This is caused by an unbalanced prop. If one blade is heavier than the other it will cause the model to vibrate. Balance the heavier blade with some clay to even the weight out
• Check the pitch in both blades to verify they both have the same pitch angle in them. If one blade has more pitch than the other then is makes more lift and causes the model to shake since the lift is not equal and balanced.
• Check to make sure that the prop tracks well. If the prop shaft is bent then the prop will wobble when it is rotating cause shaking.

• This can be caused by not enough rudder offset in the boom. Add a little bit more rudder offset and see if it helps out.
• There is the possibility that the left wing (when looking at the model from the front) has wash-out in it. Wash-out is just the opposite of wash-in (which the right wing should have 1/8" of) and should not be present in the left wing or left wingtip. Wash-out is when the trailing edge is higher than the leading edge.
• If all else fails, place a small piece of clay (about 0.3 grams) on the wingtip to help it turn.

• Check to see if the rubber is bunching together by the rear hook. If this is happening then the balance point of the model is being moved backwards by the bunching of the rubber. To cure this, you will need to increase the height of the spacer on the rear hook.
• The other possibility is an aft balance point combined with downthrust in the prop. At the beginning of the flight the downthrust compensates for the aft balance point by holding the nose down down, and then towards the end of the flight the downthrust looses its effect and then the aft CG causes the model to start to stall. The cure for this is moving the balance point forward and removing the downthrust.

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