Here is what I think I see. When my plane is upright it trims out nice - it flys straight and level. However, inverted it flys level but yaws to its right quite a bit.

Billy

Billy this Yaw can happen when you have issues with your right thrust. ..there could be other things I'll discuss down below. Assuming the flying surfaces are right on....the thrust offset alone can cause this issue.

Think about it....right thrust is there to keep the vertical lines good, Yet we tend to trim the model in yaw or rudder while upright....I do it flying straight away from myself.

Now roll the model inverted and your right thrust becomes left thrust, and the rudder trim you may be carrying to fly upright and true in yaw is not working against what ever else you have going on.

Mismatched elevators can cause this too. You trim the rudder thinking it needs some trim but reality is the elevators being off a touch will tend to make yaw rather than roll. Same with stab adjustments...if a stab is off compared to the other side this will do the same. Usually stabs and elevator make yaw problems not roll problems. And they have to be off a bit to cause roll issues or loop tracking issues in roll. But off by just a little can cause issues with yaw. And can interact with thrust settings too.

I would look to a change in the thrust line or look for the stab and elevator alignment..


OR.....Something else going on like a bent fuse. If the two fuse sides are not identical this can happen. I had a model manufacture red in Argentina, the fuse was straight but the fuse sides were not the same. When you shot a laser down the fuse centerline it was dead on....But shooting a laser down the surface of the side or using a straight edge it revealed that the fuse halves were not mirror images. And in fact one of the fuse sides had a bow in it.

This meant it had funny yaw conditions upright and inverted, it would even yaw differently on 45's upright or inverted compared to verticals and horizontal lines.

This all made for a very speed sensitive model.

A fin that is not perpendicular to the stab and wing can have similar effects too. Or a stab that is not parallel to the wing.

I'm not sure what you are flying but....it could be something simple.....like thrust or stab-elevator align or it could be molded in and you are stuck with it. Then the trick is to mess up something else like the stab-elevator alignment to make it screw up the other way and cancel the problem out. I did this on my bent fuse model. It worked pretty well but would change with airspeed. So I fixed it for 1/2 throttle then go busy in the mix department for idle and full power settings. It is possible to fix it with a combo of mechanical changes and electronic mixing....but it can get time consuming and complicated.

The best thing for the stabs is a dead flat table and some height gauge work to make sure the stab is at the incidence on both sides and the elevators...are dead on...Also eye ball the elevators. If they were sanded or molded and are not mirror images its the same as them having the trailing off....in these cases you have to shoot the middle. Its usually not going to be 1000% perfect but you can get really close to not feel the problem...with a 1/2 clevis turn or the likes.


Troy Newman

Billy,

Down-Elevator is the Master, Left-Rudder is the Slave, try 5%. I have had numerous models need some type of mix to get inverted flight "right". Basically, any time you are using down-elevator you will have the rudder coming in via the mix to counteract the thrust. The Abbra is the only plane I've had in a long time that doesn't need any mix in this regards. All my wide body-type planes needed something. If you need more than 7% of rudder mix, depending on the whole airplane, it may be better to fix it with like 3% rudder to down elevator, and some amount of aileron to down elevator - depends on how bad it is.

Try this after you do the stuff Troy suggests, like making sure the elevators are tracking, etc.
See you at the Tangerine,
Jim

So what happens when you fly inverted lines that don't need much if any down elevator like 45's up and down....?

Or what happens if you only need a touch of down elevator...Enough that mix doesn't really come in enough to keep the model flying straight while inverted but when you push negative corners the model yaws the other way.

Does this mean you use a curve mix?

I have never had a model that used this type of mix. Regardless of how wide body it was.

I guess different strokes for different folks.


Billy check your model for wing tip weight. Are you sure its just yawing and not rolling even a little? The best is the long vertical downlines and pull out and push out in hard corners. This will show the heavy wing very readily. The next thing is to trim it dead leevl upright....and flip it inverted and only apply the little down needed to keep the nose up. Many model will roll ever so slightly here and it means that the wing tip weight is off. Now this is something I find very common. I have only owned 2 models that needed absolutely zero wing tip weight. It is surprising how much a 4-5g weight on a tip can make a huge difference in the model flying.


Troy Newman

Beyond the "what'ifs", it is certainly a way forward if the tendancy is bad enough and Billy doesn't have the time or inclination to get the laser meter out, or mount the plane to a flat table. The idea of mixing (in this case) is to reduce the pilots workload.

If the model is yawing off in level inverted flight, it is probably not tracking straight either in negative pushes either (probably corkscrewing in negative loops), where in this case the mix is still valid- reducing pilot workload.
Jim W.

I would view adding mix as a last resort. I believe it's better to learn how to trim an aircraft up front instead of using mix to compensate for a bad design and/or setup. I think many people are too quick to jump to the mixes instead of figuring out what is aerodynamically wrong with the aircraft that causes undesirable results. I would say try what Troy has suggested first, then when all else fails, use the mix.