I was trimming my pattern plane and I found I have to give left aileron trim to fly level upright. I then flew it inverted to check if it continued to fly straight and level. I found the plane tends to roll to the left. I thought this would suggest that the right wing side is heavy. When I checked it, I found that the left wing side is heavier! Any one out there has come across this experience and found the cause and solution? Appreciate your help on this.

Sounds like you have a rudder trim problem, vertical stab alignment problem or too much right thrust on the engine.

Hi 8178,
Thanks for your insights. Will check this out.

Check for a wing warp. Also are you SURE one wing is heavier than the other? Are your elevators level with each other? Do they travel the same distance at all angles? Wing and stab level with each other? If it is a two piece wing are angle of attack the same on both sides? Don't you just love this crap???

Rick H.

Add wing tip weight.

you can't determine lateral balance on the ground. The model needs to be flying to determine it.

Check all the surface like the elevators and look for aileron warps and such...but I would bet dollars to doughnuts its wing tip weight issue. You will be amazed how a simple 7g mass will affect the situation.

If the right wing is heavy then it will require you to add left aileron trim to hold the wing level in upright flight. But if you roll the model inverted the right wing is now on the left side and gravity will make the model roll left. Your aileron trim is also making the model roll left so the result…Its the heavy wing is the culprit. See gravity is causing you to need the aileron trim not the wings aerodynamic forces. I'm not saying that a warp is not involved. Just that the first thing to stick in there is wing tip weight. What happens is we tune and tune the models until we see all the little warps and twists. The big stuff stands out first then we see all the little warps and twists. The sanding of shapes not the same and those little things. They are subtle. Usually a warp or a twist will be speed dependant not orientation dependent.

If the model was warped and or the rudder trim was off it would act the same upright and inverted. Example if the wing was warped and required aileron trim to hold the wings level.... this would be the same aileron trim for a given speed. You would see a roll tendency as the model flies at different speeds but not upright vs inverted. See aileron trim is only affective for a given speed. But it would not really make the model roll opposite direction unless the speeds are radically different.

If the rudder trim was way off say the model was yawing to the right( and inducing a little roll, that you took out with your aileron trim)...Now roll the model inverted and the aerodynamic forces will still be yawing the model to its right in the same direction relative to the model, So the same corrective aileron trim would be needed. Air flow wise the model doesn't know if it going upright or inverted, knife-edge, straight up of straight down.

Gravity is the constant in this situation. It is always pulling the mass of the model toward the center of the earth. SOOOOO

When you are flying on Knife edge the fuse must make the lift to overcome gravity. When you or going vertical up the engine is making the force to overcome gravity, Straight down the Gravity is pulling the model and no other forces are needed to go straight in this direction.

SO the trick in trimming a model is to get all the forces from the wing, the stab and the engine to balance each other out in such a way that gravity is the only constant. gravity can fool you. If you are tail heavy then gravity will make you add down elevator to raise the tail. A nose heavy model will require up elevator to pick up the nose. This gravity thing can play havoc with your relationships between wing engine and tail. By changing elevator trim all you are really doing is changing the stab incidence. Or to be specific for the experts relative incidence or effective incidence. If you have too much down thrust then a sign is a power off dive. If the stab is carrying un-needed trim to compensate for thrust this will show when you let gravity play the role and not the wings and stabs. If your wing, stab and engine is not properly aligned and flying in equilibrium then you may have a pitch component in vertical downline or upline. For a model to track true all the forces are in equilibrium. This way Aero takes over and the model flies where you tell it by changing the aero. That is all you are doing by moving the controls. Changing the shapes of the lifting surfaces. You are disturbing that equilibrium to make the modle roll, loop, snap or spin.

In your case here you have a wing panel that is heavy, just my opinion. You'll have to add weight to see. Gravity is pulling on the heavier wing and causing the roll. Your aileron trim setting is exaggerating this situation. If you were to add weight to the "lite" panel and fly it you would need to add aileron trim the other way. This would then give you wings level upright or inverted. Same as for CG. If your model was really nose heavy you need to add up elevator trim right? But now roll inverted and the up trim combined with the gravity pulling on the nose is really going to make the model pull toward earth. If you could magically get the CG back far enough you could roll the model inverted and have it stay flying perfectly level with the same elevator trim right? We don't fly that aft of CG because the model doesn't lock in and track. Some guys do...but they won't ever be in the money because the model just don’t track straight and consistent with the CG aft like this. The reason is the model becomes unstable in other axis. It becomes very "unbiased" and will float which ever way the air tells it to. So in the windy conditions the tail heavy model like this will be all over the sky. So we Bias the models based on gravity. We make the model then require some down to hold the nose up when inverted. We can handle this effect and remember when the model is inverted it needs down elevator. The one we can't handle is when the roll trim is affected by gravity. We can't remember the model rolls this way upright but that way inverted. So we need to trim this out.


To test this you need to trim for upright hands off flight all the way across the field. Then roll inverted and hold only elevator...make sure it is only elevator you are holding. Then watch the model for roll. After you add the weight make serval passes upright and make sure it is not rolling at all. Long runs from one end of the field to the other hands off to determine if its right upright. Then roll it on its back and do the same thing holding only down elevator.

I have had very few models that didn't need even a little weight on the tip. My Smaragd Z that I flew for lots and lots of flights carried (2) quarters on a wing tip. Yep about 12g on one tip. By the way this is about 1/3oz. it flew about 4years and well over 3000 flights with those 2 quarters on the wing tip. Weigh the wing panels and they were within just about 3 grams of each other without the ballast on the wing tip....But in the air it could tell the mass was distributed differently. One of my Supreme’s from 2004 didn’t have any wing tip weight but the other one used a single quarter 7g.

The old lateral CG pick-up the nose and the tail and see if a wing drops doesn't work. There is no way for you to truly pick dead center of the fuse. The prop shaft is not it. It has down thrust and right thrust in it. The tail is a similar guess action measurement. So flying it is the only way.

One of my Pinnacles has a single quarter on its tip. And the second and new Pinnacle has about 3-4g a ground down nickel on one tip.

There are a couple of ways to try this out. One is stick a nail in the balsa tip. Not elegant but works. You can tape coins to the wing tip. I use quarters (7g or 1/4oz) for good luck and clear tape. But Nickels (5cents) are the max weight for min investment. Another old timer idea is to use modeling clay. But it needs to be the heavy old time stuff that sticks. The clay allows you remove part of or add to it and it can semi contour to the wing. I had trouble finding clay that fit the bill a few years ago so I gave up on and started using coins. The clay I found was really light weight and didn't stick for spit. I guess it was green peace clay. I think it was guaranteed not to kill a squirrel if he ate it. Or maybe it was would not kill a bunny rabbit it your threw a blob and bounced it off the rabbits head. Then again I was in California looking for clay. Figured it was an artsy enough place....but it was all ecco friendly crap that worked good for making pots, but not suited to this application. So I chose good old US of A AMERICAN money. Coins, easy to replace not expensive, and anywhere you go in the USA you can find a plentiful supply. Lets face it you are adding a $10 roll of quarters to a wingtip you got problems.

For weight if a single coin is too much weight then dremel it. Or get some stick on weights and dremel them. Fishing weights work well too. Once you find the proper weight the model will probably loop track better as well. Once the weight is determined you can stick it into the wing tip and make it hidden. For this I would use small fishing weights and just match the ballast on a gram scale. I have found within a gram is very accurate to determine the amount needed.

Good luck with it.... remember when trimming gravity is your friend and will help you isolate imbalances in your model. Like the wing tip weight issue. Or it can even help you determine thrust issues and wing and stab incidence issues. Basically look at the trim tests. They place the model in tests that includes or eliminates gravity to determine is the aerodynamically the model is flying pure.

Hi Rick H, Tnewman,
Thank you both for your inputs. I will certainly check them out in my trimming process and let you know the outcome. I guess this is what makes pattern flying so challenging!
Thanks again.

Not true! A given yaw input reverses the right/left direction of the aircraft path when changing from upright to inverted. That is from the RC pilot’s perspective on the ground observing and controlling the aircraft. Trying to trim out a yaw problem caused by rudder or engine thrust with aileron input causes a nasty problem when flying inverted.

"A given yaw input reverses the right/left direction of the aircraft path when changing from upright to inverted. "

Sorry, but no, not if you're trying to correct a problem with roll.

Unless you've flown a plane that had only rudder, and no ailerons, you can easily make this mistake. If a model has proverse roll couple with rudder application when upright, it will still have proverse roll couple when inverted. Yes, it yaws the "wrong" way, but the model will still roll in the correct direction. On rudder/elevator/motor aircraft, you still used right rudder to make a right turn when inverted. The model would still roll right with right rudder.

That is correct in relation to the pilots perspective. But we are not talking about pilots perspective. The point to be made is that the model rolls in different directions upright vs inverted. What force changed to make the model roll the left wing down rather than right wing down. The only change is gravity is acting on the same wing panel. Warps and twist will go the same direction as far as the model is concerned.

Try holding right rudder and roll inverted. Yes the model will now go to the pilots left but it is still the right side of the model. Any corrective input (trim) that would be needed upright is the same for inverted flight. The air flow around the model doesn't know its inverted. Warps and twists, trim adjustments are all aero related. A model will not yaw left with the input of right rudder. It can only yaw right. Regardless of the model orientation right rudder creates yaw right with respect to the model.

I still think it is a Mass and gravity related force.

But then again I'm no expert.

Good luck with your problem.

Troy Newman

Interesting discussion. I thought mck was talking about a pattern aircraft and most that I have flown tend to yaw with rudder application and make a very, very slow wings level heading change in the direction of the rudder application although some I flown yaw to the right but actually made a very slow heading change to the left. At slow speed the rudder tends to have a larger impact on the yaw and the ability to change the heading with wings level. Inverted they yaw the opposite direction than when upright in relationship to the pilot on the ground. I wouldn’t want a pattern aircraft that causes much roll with the application of rudder. True, rudder roll coupling was used on the old rudder only aircraft with a lot of dihedral to turn but that would really mess up a pattern or sport aircraft when doing stall turns, knife edge using rudder on landing, take off, etc. Maybe my aircraft are unusual and I have a great thing going. My main point was recommending to mck that he should make sure his vertical stab and rudder are perfectly aligned and his engine thrust angle is OK before he started messing with the aileron trim.

Here is a general outline of what I do.

The first step in trimming a plane is for level fight upright. Once this is done, fly plane inverted (when inverted if right wing drops, add weight to left wing or vise-versa to balance the plane from wing panel to wing panel. The plane should fly inverted with little to no down elevator with out any aileron correction trim via what is used to fly the plane upright level flight. After weight is added (to which ever wing tip) you must re-trim for up right level flight and repeat the process again until problem is corrected.

Next step is to see if the engine thrust is set correctly. Pull plane gently into a vertical climb from level flight (not like entering a loop). If the plane flat turns to the left, it will need more right thrust. If it flat turns (yaw) to the right less right thrust is needed. Correct thrust angle if needed.

Correct CG, fly plane at full throttle into the wind. When airspeed is up, chop the throttle to idle. If the plane glide straight and gently looses altitude, CG is correct. If the plane looses altitude immediately after the throttle has been cut. The plane is nose heavy. If the plane gains altitude, the plane is tail heavy. Adjust as necessary.

This is where the rudder/elevator trimming comes into to play providing that the above has been sorted out.

Now... pull plane into a inside loop. If the plane wants to turn in or out while doing this. One of two things are causing this. Either the rudder or elevator halves are not properly trimmed (set) to work with each other.(Just because your pitch gage tells you your elevators halves are aligned side to side one the ground.... does not mean they are in the air.) If the plane wants turn out to the left. Add left rudder trim. If the plane want to turn out to the right, add right trim. Correct as necessary so plane will track straight through out a inside loop. IF the plane does not turn out either way... with very little to no rudder input correction... you are ready for the next step.

Now... perform a outside loop once the above has been corrected. If the plane plane wants to turn out right or left, then the elevator halves are not aligned. In the event of turning out to the left, the left elevator is to low. If it turns out to the right, the right elevator half is to low. Adjust at the clevis by turning in or out. Make one adjustment at a time. Re-trim the plane for level upright flight . Re-trim for true inside loops as described above. Then perform outside loops to see if the adjustment has been to little or to much. After each adjustment, it is important to repeat level flight trimming along with inside loop trimming before performing the outside loop to see if the amount of adjustment has been to little or to much. Once the Plane will fly inside, and outside loops with very little to no rudder correction. Your plane is trimmed for the general basics of maneuvers involved in pattern flying or any other style of flying for that matter.

I didn't mean to roll the model with rudder. I meant hold right rudder in it......And roll the ailerons till the model is inverted. The plane will still be yawing to the models right. The rudder doesn't reverse its effective direction when the model is inverted.

If the fin is twisted, crooked or the likes it would mean the model has a yaw trim issue. It would need some rudder trim to compensate for the twist or misalignment. It might even need elevator or aileron trim if it is off a bunch.

But this force will be constant for a given speed regardless of the model flying inverted or upright. If the model needs some rudder trim to fly straight in yaw. This rudder trim will be identical if the model is upright or inverted. The plane always responds to the planes reference axis. We don't care about the pilot and the ground. as reference points. Other than the we want to the model to fly in a "true" way with out continuous, roll, pitch or yaw. Basically we want to take our hands off the sticks and the model flys upright straight and level. Roll it inverted and it needs down elevator only. Roll on to knife edge and it needs rudder only to maintain the straight line.

I prefer to have the model need a little bit more than just a breathe of down elevator when inverted. They track better in the wind. My experience has been guys that are not as proficient at ruddering their rolls like tail heavy setups becasue they don't need to rudder the rolls as much.

I prefer a model that locks in and is biased to some degree. I know that when I roll it on any 45 degree line any horizontal line I need to input rudder and elevator. Vertical lines just aileron only.

But I learned a few years ago that ruddering every single roll in the sequence except the vertical ones made the precision and presentation of the flight much better.

But then again what do I know.
The advice is worth what you paid for it.

Troy Newman

At the risk of exposing my ignorance, I'm a little confused about the terms "turn in" and "turn out" during an inside loop---------which way is IN and which is OUT? Do you mean turn, (yaw) from the plane's perspective, and if so, why would it be corrected by giving MORE of the same, or do you mean introduce a rolling motion. Help me picture this

Another method I've been told to check for proper CG is by putting the plane in a gentle, (30 degree) climb, and see if it will maintain its heading without settling. If it wants to settle, it's nose heavy. Seems to work.

King, I should of been more specific. With the plane flying at you and in to the wind. Pull up into a loop so the plane is in front of you. You will see the bottom of the plane. That puts the plane's right wing on your left side. You are correct about input needed. So, from the plane's point of view...if plane wavers to the right. Trim the rudder to the left. Or if the plane wavers to the left, trim the rudder to the right.

The "turn in" and "turn out" is yaw. Inside means toward the center of the loop either left or right. Outside means away from the center of the loop, left or right. Basically a slow roll. Keep in mind.... in a "perfect world"...... a loop should only require elevator input with the stick pulled back to a certain position and that position should not change through out the loop with out any other input needed. This style of trimming is to get a certain plane to preform as close to perfect as it can in the "real world".

Bax,

Just to make sure my memory hadn’t gone bonkers on me I tested this at the field today with a pattern aircraft.

With the aircraft upright, application of rudder caused a skidding turn in the direction of the rudder input with very little or no roll. Flying Inverted, right rudder resulted in a left skidding turn and left rudder resulted in a right skidding turn. The right and left turn is from the RC pilot’s prospective on the ground and the right and left input on the TX. Thus when inverted opposite rudder must be used to make right and left turns, e.g. a right turn requires left rudder input on the TX. I also believe if you mistakenly correct a yaw problem with aileron to trim for level flight, e.g. left aileron correction for a right yaw problem you end up with a tendency to turn to the left when inverted. Aileron roll is always the same upright or inverted yaw is not.

Again, all of this is from the pilot’s perspective on the ground and that’s what makes RC flying so much fun. I’m also not a big fan of adding engine right thrust to compensate for propeller P factor because of the negative impact on P factor when flying inverted.

Hi Rick and all,
my problem was indeed caused by differential wing incidence between the two halves of wings. When I checked with incidence meter, I found there was a differential of about 1 degree. Righting this solved my problem and I now have 0 trim on my aileron upright and inverted. I thought I share this with you all and once again thanks for all the input. I am sure they will come in useful in other situations I am sure I will encounter!

8178,

I think we're talking about two different things....

I thought the question was about roll couple with rudder application. When upright, if right rudder produces right roll, then when inverted, right rudder will produce right roll. Sure, the model will yaw in the opposite direction, but the wings' dihedral causes the model to roll the way you want.

If the model has no roll couple with rudder application, then upright right rudder produces right yaw when seen from the ground. When inverted, right rudder will most certainly yaw the model to the left as seen from the ground.

As mck stated in the post just above, it turned out the wing wasn't straight. The first thing any airplane must be is absolutely straight. Insuring everything is absolutely aligned and straight is also one of the most challenging jobs when building a model. A lot easier to talk about than do. (ask me how I know!)