Hello everyone,

When trimming out a high wing trainer type plane with lots of dihedral how can I tell if the plane needs rudder trim or aileron trim. It seems that small rudder inputs cause the plane to roll the same as small aileron inputs. Thanks.

Scott

I usually leave the rudder centered and adjust the aileron trim so it flies level.

Tom

Lots of dihedral is what makes the rudder effective in turning the plane and is common in trainers. Ther bad news is that the high angle of the wing makes the plane easier to get pushed around in windy conditions. Higher performance planes have little or no dihedral and can handle the wind better. Planes with no dihedral can not be turned with just the rudder. The idea of trimming a plane is to have the plane fly straight and level without needing to move the sticks. Bear in mind that on a trainer you will want to check this at 1/2 throttle. Reason being that trainers are designed to climb at full throttle, fly level at 1/2 throttle and lose altitude at under 1/2 throttle. If the plane gains or loses altitude at 1/2 throttle then you will need to adjust for that as well as if the plane turns left or right. [8D]

You can fix any trim with engine offset or angle of attack on the wing. Trainers by design have a high lifting airfoil so usually zero incidence is required .If you need to add right trim in order for your plane to fly straight . You can fix this by adding right thrust to your engine. Trim to the left and add engine thrust to the left. Add down thrust if trimming down. And upthrust if trimming up. I put washers behind the mount on one side or the other to adjust the thrust. All of this is not necessary in order to fly but I like to tweak my airplanes until they fly with no trim required. I first make sure that the wing and horizontal stab are perpendicular to the thrust line { Zero degrees} and also check the wing incidence and balance point. These have to be correct in order to analyze what is needed to make adjustments. By no means am I implicating that this is necessary in order to learn how to fly but these techniques are useful as you step up into more aerobatic airplanes. I am curious if their are other pilots out there that do the same. When I started learning to fly I had a seasoned RC pilot to help me at first with this, and now after 20yrs and lots of planes its kinda second nature to fix any unwanted trim problems. hope this helps

Engine thrust angles are usually preset on most planes for optimum performance. Changing the thrust angle to compensate for trim corrections would seem to open up a whole new set of problems. Ground looping, poor vertical, and differing tendencies directly related to throttle setting are a few that come to mind. I could be dead wrong but I have never heard of trimming a plane by changing thrust angles.

The trainer I had wanted to climb out. No amount of elevator trim would help. Come to find out it was tail heavy. I balanced it and never needed anymore elevator trim.

Try looping directly into the wind and notice if it "screws" one way or the other and trim the rudder to counter the screwing effect. of course trim straight and level before and after any adjustments and evaluate as you make changes. be aware that lateral balance can also effect tracking in the loop and will be worse the tighter the loop is.

Changing thrust angles is done in very slight increments I am not talking about a severe amount that would cause ground looping, and throttle settings is something to consider when tweaking out a airplane to fly straight. Also wing incidence and balance point also have a factor . If your plane needs trim to fly straight what is causing it ? is the wing warped ? is the balance correct ? is the wing incidence correct? and is the thrust angle correct? all these things can be changed slightly until you have your airplane performing at an optimal level for you . Very important in pattern flying. when performing loops and rolls snap rolls and spins and any combo thereof you don't want your plane to veer off heading because of a trim problem that can be corrected. and yes some people do adjust their planes until it is flying suitable for them. I am probably putting to much emphasis on correcting trim with engine thrust. But i know from experience that small changes in incidence, balance, and thrust
will eventually make your plane fly straight with zero trim at any throttle setting. considering of course wing warpage and making sure your wing and stabs are set up correct.

I don't doubt that slight engine thrust angle changes will have a trim effect on some planes but in most cases having the correct CG and lateral balance along with taking the time when building to make sure that the wing is level and straight with the stab and the stab is 90 degrees to the the fin will usually then require very little surface adjustments. Let's face it, a plane doesn't have to be perfect to fly or none of them would. I'm not knocking your method, but I personally don't know anyone that trims a plane with engine thrust angle, even the old timers in our club.

I agree with Dalolyn, minor engine thrust adjustments can very often greatly improve flight characteristics. If your CG is properly placed and measured incidence on wing and stab are correct, then the next obviouse adjustment is thrust line. Many of the ARF trainers are not well set up as to thrust lines and most can be greatly improved with careful adjustments.

OK guys, I agree with what you are saying BUT, for most beginners it is much easier and quite effective to simply use the trim buttons.

Because engine and prop specs are the only varying factor after the airplane is built according to the plans, the downward forces created by downthrust are also going to vary. Thats why what Rodney and Dalolyn said is very important and necesary specially on a trainer aircraft. Since the airfoil will create different amount of lift at different airspeeds if the downthrust is not compensated properly the trainer will tend to balloon on every change of airspeed. Therefore the beginner will be overcontrolling the airplane at all times.......trying to keep it straight and level. That defies the purpose of having a trainer. A little work doing the downthrust early and the rest of the time is just for enjoyment of your plane not to constantly keep trimming everytime you go flying.

Wow you guys are fast. I leave for a day and when I get back all this good info. Thanks for all the quick and informative responses.

Scott

You guys are making this seem way too complicated for a beginner. Most high wing trainers will tend to climb a little when the throttle is opened because of positive wing incidence, their made that way on purpose. I have helped lots of new comers at our field with new trainers and have ALWAYS been able to trim the plane in-flight to where it handles just fine. Most "ballooning" problems on a trainer are caused by too much elevator travel and over controlling.

the troll

gave a decent answer to the original question. Can someone else give a try at answering the original question because I have been wondering the same thing?

I don't think all that jive about engine thrust is really practical. I will agree that it is probably all true, but I would much rather hit the trim button a couple times in the air and be done with it rather than having to land and shim the engine. That is rediculous,lol.

But I have been wondering about that question. It seems with my LT-40 that the rudder makes it turn and go nose down, and slighly roll in the direction of the turn. The ailerons roll and make it go nose down (with no elevator in put). The difference is distinct when trying manuevors. For examples you hold elevator long enough, it will roll, hold ruddy long enough your spiral. However it is not so clear when moving just small amounts such as trimming it. When trimming the rudder or the aileron I have a difficult time telling the difference. I too just leave the rudder centered but how do you know for sure to trim with ailerons or rudder?

Wings,

wings,
Here is the way I usually approach this. When building the plane or ARF, take the time to make sure the vertical fin is EXACTLY 90 degrees to the stab, and of course the stab needs to be level with the wing. You also need to make sure that the vertical fin is as perfectly straight with the fuse that is humanly possible. Most ARF's need some tweaking in both of these areas before gluing them in. If the fin lines up straight with the fuse, set the rudder at 0 and leave it. Balance the plane laterally and then trim any banking tendencies with aileron. On the other hand, if the vertical fin is noticeably cocked to one side, you may have to trim the rudder a bit in the opposite direction.

jammer you are probably right about this being all to complicated for a beginner. Any one can simply make a plane fly straight by adding a few clicks of trim. But to comment that it is not necessary or useful for a beginner . Shouldn't a new-be Know all aspects of RC flight . You could easily say they don't need to know anything about tuning a engine. Why wouldn't you want to "tune" your plane for optimal flight characteristic, as well as the engine. I was taught by an old timer and do not know anyone in our club that hasn't tweaked there airplanes at one time or another. After a person has soloed and wants to advance you can make a trainer more maneuverable by shifting the weight and messing with the wing incidence. I have scratch built planes and built lots of kits, I try to make everything straight as possible when building . But I still change things around until the plane flys stright without any trim needed, and I was taught to do this as a beginer. so is it necessary? obviously thas is an opinionated Question that will not ever be answered as a fact. But in my opinion why wouldn't you want to learn how to work on and tune your engine as well as work on and tune your airframe for optimal performance as a beginner?

A student driver is taking one of his first classes on his dads car. The instructor notice that one of the tires is kinda low and tells him that it would interfere with the way the vehicle drives. So he tells the student driver..."empty some air from the other tires to make it even" [sm=bananahead.gif]

Just to answer the origional question, when I'm at the field, and a beginner's plane is up in the air, and going to one side, here is what I do so we can get some training done.

I get the plane straight and level, then go hands-off. If I think I see a yaw-then-roll, I trim more rudder, if I see more roll-then-yaw, I trim ailerons. If I'm not sure, I frequently split the difference and just do a little of both.

The way I see it, I've got about 15min of fuel on board. The more time I spend messing with the trims, the less time the student is learning to fly. And we only have so much daylight during a day or evening at the field, so the less time we spend with the airplane taken apart, the more time the student is spending learning to fly.

Once the student learns how to loop and roll and fly basic acrobatics, we can talk about ways to trim for better acrobatics.

I think most beginners (as I was) are so excited about flying they will do the fix that takes the least time so they can fly! Later on you can tinker, but the first thing every one wants to do is fly! That is my opinion.

Wings,

kirk and wings ;
You are "right on" I think back to when I was starting It was just a big enough task to feel around for the trim and move it. Yet alone worrie about changing things around. I quess thats where my oldtimer instructer came in and he helped me with tweaking the airframe when I had time. I cant count the times when I was nervous and cranked the trim to far or the wrong way. I could always hear him in the background "make sure your two mistakes high". good advice.

Very intrested in "downthrust" issuse.
I have a Kadet Lt-40, Leeps off the ground in about 20 feet with a .46 La. I have over 1/8" down Elv in it just to keep it from climbing at 1/4 to 1/3 throttle. When I cut the throttle, she nose dives to 45 deg and picks-up lots of speed. and comes in quick and fast.
I belive the landing problem is because of all the down ELV trim for somewhat level flight.
This is a ARF
Any help??????
goplaneman

To get back to the original question... I'm not an expert, but I think this is the answer:

In practice, with a plane built reasonably straight, it doesn't matter. Just trim the ailerons and fly. Personally, I trim the rudder with a straightedge on the ground, and never trim the rudder in the air.

In theory though, it is still an interesting question. I think the answer is that if you use the rudder to trim to straight level flight when you should have used the ailerons, the plane will be crabbing (i.e. the tail is not exactly on the line of flight), and you could probably notice it from the ground. In other words, the ailerons are off, but you compensated with the rudder, which would work in terms of leveling the wings, but it would throw the tail off whack.

Also, Fastsky, could you elaborate on your statement that, "Planes with no dihedral can not be turned with just the rudder." I fly planes with no dihedral, and I'm pretty sure if I use the rudder the plane is going to turn (other things might happen too, but it seems the plane will turn).

Mike in DC - Thanks!
That's exactly what I was going to say. On my trainer, I made sure the rudder is straight, and trim the aelerons. I have had to change the incidence of the engine for ground handling, (My Avistar would drive the nose wheel into the ground until I took out 3* of downforce.) but not for the flying part. This may not be the right way, but it worked for me.

It should turn with just rudder, regardless of Dihedral, dihedral will enhance it though. a plane turns on its rudder because when the rudder is deflected it pivots the aircraft, speeding up the relative wind over one wing, this causes more lift over that wing, which in turn causes it to rise up. the other wing will have a lower relative velocity, and thus will generate less lift, causing it to drop. The combination of these two factors causes the plane to bank, which in turn accelerates the turn.