Out of trim trainer
 

Yesterday, a fairly new pilot had suffered another landing incident and made a minor repair and had another pilot trim his plane the first flight after the repair. The pilot complained that the plane had a lack of up elevator control when landing. The balance was checked and it was noted that the elevator had about 1/8 down trim for level flight. The response was to increase the up throw but it seemed a gross amount.

Because I'd built one of these trainers for my grandkids and have it flying well, I offered to fly the plane and offer a perspective. The plane is a Sig LT-25 Kadet. What was found is very typical of trainers such as the LT-25 that have a flat bottomed high lift wing. When the new pilot would get others to trim his plane for him, they would do so under full power and when cutting power for landing, the sink rate was typical to a 32 oz/ft war bird when it should have been gentle and land itself without any elevator control. No wonder he was having problems landing.

I trimmed the plane for level flight at about 1/3 throttle and let the plane come in for a landing and it glided in and touched down without a bit of elevator control. We examined the elevator and it was neutral. Of course what was going on was that he'd installed an OS 32 on a plane that Sig recommends to use a .25 and the extra speed was simply causing the flat bottomed wing to provide exponentially more lift that was being trimmed out by the elevator. I'd noted in the past that his plane flew probably twice as fast as the one I'd built powered with a Saito 30.

My first suggestion was that the upper throttle setting should be limited. At this, the other pilot helping baulked and suggested that the plane was all ready marginally powered. With all do respect, he was a 3D flier and used to and probably more comfortable with lots of power and he was not aware that the LT-25 is still flying on the wing at walking speed and in fairness, the LT-25 at over 5' wingspan just looks much to large to be powered by a .25 -.32 engine.

The other suggestion was to increase the engines down thrust to counter the need for down trim at faster speeds. I'd preferred the first recourse because it would have given the new pilot more reaction time but the down thrust should work if it can be dialed in.

I commend Sig who provides a distinct statement that a .25 is their recommended engine for the plane and I'm a little critical of the training pilots who trimmed the plane for level flight at full throttle leaving it diving like a brick at low throttle when landing.

RE: Out of trim trainer
 

I totally agree with you AND your assessment.

Way too many fliers not only over-power their planes, but they also tend to fly with the engine "ON' or "OFF" no in-between.

I have seen several 3D fliers who, when faced with a dead-stick, wind up crashing their plane due to the fact that they stall it all the way down to the ground - even when the dead-stick occurs at altitude - because they have no concept of basic aerodynamic principles. Without power, they simply do not know how to fly.

Now, it's easy to say, "That's their problem", but the problem really surfaces in a situation like yours, where some one who doesn't know how to do much more than point a guided missile tries to "Teach" a beginner.

RE: Out of trim trainer
 

Dittos!

Flying always at full throttle is likely the #2 problem of a lot of fliers and not always just trainers. The Number #1 is flying to far out.

Most genuine traners when forced to fly beyond their trim speed will be difficult to fly well. So folks who never reduce power after soloing will hit a flat spot in the learning curve with no improvement because of this.

John

RE: Out of trim trainer
 

Therein is another one of the reasons I like the Avistar for training. It's less sensitve to speed/trim changes due to the semi-semetrical wing. Not that its bad for someone to learn how to properly trim and fly a flat wing plane, but most people's second nitro plane is usually something closer to an Avistar's wing.

RE: Out of trim trainer
 

As a newbie I'm always looking at a lot of threads getting all the information I can. This one in particular really makes one to stop and think. I've only flown 2 times so far with an instructor but fly my G4.5 almost daily. Trimming a plane can sometimes be a little difficult for us new guys but this one really helps. Thanks guys.

RE: Out of trim trainer
 

I am flying a GP PT-60 with an OS 65LA.  If I have my idle just a little too high I cant land it.  I usually chop the throttle to half after reaching cruise altitude and only give it full throttle when I am going to pull into a loop or roll (the only 2 manuvers I know).  I find that half throttle gives me plenty of power to keep my nose up when turning.  And then there is the obvious fuel economy.  The PT 60 just wants to climb at full throttle.  </p>

RE: Out of trim trainer
 

This is also a reason that I like using the Hobbyzone Super Cub for true begginners. It's a slow flying, forgiving plane that even when way out of trim can still be flown successfully by a novice. I do most initial intruction and demonstration about trimming a plane on the Super Cub, then when the student has successfully flown out a few batteries on it, we move to the Avistar.

RE: Out of trim trainer
 

Hi Steve.... with no headwind, most trainers have enough lift that the idle can't be fast or they float a long way on landings... that is normal. The excessive climb can be dealt with by adding down thrust to the engine. Down thrust is when the angle of the motor shaft is tilted down from the airplane center line having the effect that at higher motor speeds that produce more lift and thus climb, the down angled motor counters that lift by pulling the plane down.

All high wing planes need down thrust to deal with the relationship of wing and motor positions and increasing speeds even for a symmetrical wing on a high wing plane would cause a greater angle of attack and generate more lift so increasing down thrust is needed for a motor that pulls a high wing plane faster. While it is more noticeable on a flat bottomed wing, it is nonetheless still true to a degree no matter what foil on a high wing plane.

The fix is to shim either the mount or the motor on the mount with washers to reposition the angle of the motor.

If it were not for another ill effect of trimming the elevator down when it really should have down thrust added is on takeoff, a mix could be done with the elevator at higher throttle settings. With the elevator trimmed down for level flight at high speed, the airplane needs a lot of up elevator to break ground and then as soon as it breaks with too much elevator it lurches upward rather than a smooth climb from rotation. So, its much better to shim the motor than do the mix. The goal should be to have the elevator very near neutral for level flight at the range of speeds that a particular motor flies the plane.

RE: Out of trim trainer
 

Speed sensitivity vs pitch is a trait of very nose heavy airplanes. Which is how almost all full size private airplanes are setup as well as most RC trainers. While dialing in downthrust will compensate for power changes somewhat during unloaded 1G flight, and changing incidence angles of the wing to tail can eleminate part of this, throttle management is the answer for the beginner.

However, to minimize the effect, move the balance of the airplane towards the back of the recommended CG range (on some airplanes, you can even move it behind that). As you move it back, you can also remove some of the incidence. What this does is decrease the pitch response to speed changes. It will also increase the sensitivity of the elevator, make landings easier, and allow more advanced flight manouvers like spins and inverted flight.

If you find the airplane still has too much lift and you want to do some interesting experiments. This will get you an idea of how more advanced airplanes with heavier wing loading will fly and land. You can add small spoilers to the top of the airfoil, starting with 3" strips taped about half way out on each wing panel. Out there is best since you avoid tripping the air that the tail flys in and you leave both wing tips flying. Start small, and work your way up to larger sizes.

RE: Out of trim trainer
 

“Trim the model of a new pilot and he will fly for a day. Teach a new pilot to trim his models, and he will fly for a lifetime.” Chinese Proverb:)

Training in proper trimming is frequently overlooked; however, it is very important.

Trimming from the Ground Up (Three parts article):

http://www.masportaviator.com/ah.asp?CatID=8&ID=141

RE: Out of trim trainer
 

I'm kind of astounded that no one has mentioned incidence yet.

My trainer was a Laniar explorer 40. It had the nasty habbit of not just climbing when the throttle was abovetrim speed, but trying to loop. The oposite on throttle below trim speed, it nosed over and dropped like a rock. I finally ended up with a stack of popsicle sticks under the TE of the wing, four sticks thick, and it really tamed the beast. It would climb at throttle, and it would sink if the throttle was cut. However none of the attempted loops or nose dives from before.

The "Give it more down thrust" doesn't make much sense to me. A little, 1 or 2 degrees is OK, some scale planes fly like that, but when a plane has so much down and right thrust that it looks like the nose was broke off and glued back on without much thought aboutalignment, something is just wrong. If you go out to the local airport and look at the down thrust of the real planes, it's really hard if not impossible to find one that looks like it has any down thrust. Why should our models be any different?

Don

RE: Out of trim trainer
 

Was I too succinct for comprehension?

RE: Out of trim trainer
 

Don, you won't see down thrust on a low or mid wing plane... it is something necessary to deal with the power/drag couple between the engine center line and a wing mounted much above that center line. The motor pulling forward and the wing drag pulling aft, lever the plane into a positive up pitch angle of attack and thus wing lift results. The faster the plane goes, the more drag on that upper wing and the more lever and the more lift forces are generated. Those must be countered and down thrust is the mechanism to do that.

As per our discussion, if down elevator were used to counter, then when the power is reduced, the trim is way out of whack and the plane noses down because of all that down trim and dives and is much more difficult to land than is a plane that is in trim for a landing glide slope.

The same is true for the wing incidence, if it were trimmed negative to deal with the wing/engine couple at power, as soon as power was reduced, the glide slope trim would be horrible.

You however do draw attention to the importance of incidence trim and Highplanes did point that out with good thought. To some extent my point is that the designer usually gets the incidence right but the designer has no control for example what motor one puts on his plane. In the case I mentioned, Sig goes out of their way to say, put a .25 on this plane and the owner puts a hot .32 that flies it considerably faster than the design parameters call for and the plane exhibits a lot more wing/engine couple than the down thrust in the firewall was set for and the plane is out of trim.... something his instructors should have caught.

In fairness to the instructors, they are probably flying their own planes with sink rates typical to that of the out of trim trainer and they don't notice or think its any big deal... but it is a big deal to the new pilot who should be flying a trainer with a gentle glide slope who would have an easier time with landings if he wasn't holding up elevator during the whole landing approach or diving at the ground too steep.

A word about incidence... if under low or no power the elevator is not in center trim then its time to check the incidence and that is why I pointed out that when the LT-25 was trimmed for level flight under one third power and then displayed a proper landing glide slope, the elevator was checked after landing and it was found to be neutral... that said the incidence was where it needed to be and that inadequate down thrust was the culprit.

If the plane is not flying with neutral elevator at low throttle settings and doesn't have a proper glide slope at neutral elevator then incidence should be checked. As Highplanes pointed out, if the plane floats with nose up on glide slope, trim the incidence negative and if it sinks too rapidly, trim the incidence positive. Very small amounts at a time.

And... all of this is based on CG being where it should be.

RE: Out of trim trainer
 

Dean Pappas wrote the 3-part article. Excellent read. Gives you a basic-to-advanced approach for trimming your plane.

Even if you have the information doesn't mean that it's easy to implement. As an example I'll use my own experience with a SIGLT-40 (bigger brother of the LT-25) that I recently built. It's powered by an Evolution 52 2-stroke. Mfg link:sigmfg.com/cgi-bin/dpsmart.exe/MainMenuFV4.html. <font face="Arial" size="2">It includes </font><font face="Arial" size="2">"</font><font face="Arial" size="2">Recommended
Engines</font> <font face="Arial" size="2">.30 - .40 in&sup3; (4.9 - 6.6cm) 2-Stroke; 40 - .50 in&sup3; (6.6 - 8.2cm) 4-stroke</font>". It's obvious that my engine choice is a bit "excessive". I live at altitude (5500') and adhere to the old adage to kick the displacement up to compensate for the loss of HP.

Editted: SIG's site uses an interesting approach - a single webpage whose content is reloaded with different content based on selection from the menu on the left side. To find the LT-40 -&gt; Aircraft -&gt; Aircraft R/C kits -&gt; Kits - Trainers -&gt; select the LT-40 ...

Straight-n-level flight is achieved at 1/3 throttle (sound familiar?). At full throttle this thing will darn near loop with no elevator input.

This is intended to be a club buddy-box trainer. Time to rethink the use of the Evo 52. Maybe it's time to toss an OS46 LA on the front and call it good. Leave the more agressive flying to something "more appropriate". Every hanger needs alternatives! http://www.rcuniverse.com/forum/js/f.../thumbs_up.gif

RE: Out of trim trainer
 

the simple solution to a not so complicated problem........get a different prop
maybe a 11x3 or 10x4. that should slow it down very easy and give it all the torque the pilot would need to get out of any problem.
it will also allow the plane to fly much much slower so he can get used to landing speed in the air.
i have seen two tower trainers with 10x6's on them that wouldnt slow down at all, poped a 11x5 on it and wow what a huge difference. not only will plane fly just above idle, but when brought to a slow idle the plane slows down very quickly. if you have a fast idle it will gently set down.

RE: Out of trim trainer
 

Time to pull out the prop selection and go to the field. Thx for the direction.

If you were Irish you'd be known as Seamus Seamus :D

RE: Out of trim trainer
 

hello again AA5BY.....KOOLKRABBER47 here......remember my post in the other forum? (hanger 9 ultra stick 40).....remember my comments about the "flying brick"???.....sounds to me like these are some young guys, up to some old tricks???.....sounds like the old days when everybody got a new plane, took it home and the first thing they did is install a motor 2-3 times the size of the manufacturers recommendations.....no problem, as long as you kept it moving balls out 100% of the time they were fine.......the minute you throttle down to land or what not, "instant flying brick".....that thing came in like a rock......for as long as this hobbies been around, you'd think people would've learned something by now......maybe they have and maybe it's just the guy's starting out and won't listen to anybody until they're running out of money......in case you haven't noticed, i tend to hang out in beginners area more than anything else helping, well you guessed it, "beginners"......somebody took the me under their wing and taught me the ropes, after 25+ years of flying i think it's my turn to give something back to this hobby..... i could hang out in the intermediate or advanced sections of these forums all day bragging about how big my planes are and how fast my planes are,etc,etc,etc, but i feel i really wouldn't be serving this hobby or this forum any good....there's all ready plenty of other people keeping those forums busy......i enjoy surfing the beginners section and trying to help the up and comers transition into this spectacular hobby, as effortlessly as possible......i say, try to help, because like you say, some people can't be helped or won't take our "seasoned advice"......it's like were a bunch of old farts trying to tell them what to do and yet day after day we log on to these forums and there's our newbie, whining and whining again about crashing another new plane....sound familiar???, oh! i don't know what i'm doing wrong.....flying a plane is not easy but, it's definately not as hard as as these "new" guys want to make it seem.....RC flying has been around for ever and has evolved so much, the days of crashing aimlessly into the ground should have been an after thought......follow a couple of "established" guidelines and everything should fall into place......we can only "try" to help and then stand back and let history take it's course......when they're done crashing planes, they'll come back, humblely, with ears wide open and ready to learn.....

RE: Out of trim trainer
 

JimmyJames.... Yes, that would help a lot. We did fly it again today after two washers were added to each side adding down thrust and all I can say is that it helped, reducing climb by about half and now in the reasonable range.

After the test, I encouraged the owner to leave the trim set for mid throttle and don't let the field instructor re-trim for full throttle and once he reaches altitude, accept the reality to reduce power. I will pass on your suggestion as well as I'd thought of a prop change as another help mechanism as well.

I suggested test mixing some down trim at high throttle settings but I didn't know how to do it quickly with his radio and my window at the field today wasn't very long. I'm concerned about what that does on takeoff but if it doesn't hurt there, it could help. If a prop change doesn't do it, maybe another washer... or two.

He has another issue that also has to be dealt with. He put a quick connector on the throttle arm and the hole is very sloppy... like in two clicks of throttle trim don't move the throttle barrel so that has to be corrected as well to establish a proper and consistent landing throttle setting.

RE: Out of trim trainer
 

Ball link on the throttle arm solves lots of problems.

RE: Out of trim trainer
 

stick around TomBates.... as a flyer with 25+ years flying, i can see this is gonna be a good one.....theres a lot of seasoned, educated flyers flyers debating here.....this is the most accurate forum i've seen so far.....just took one bad flying incident to fire up a good "debate".....KOOLKRABBER47......

RE: Out of trim trainer
 

Highplanes...yes, something with a nut to lock it in place.

RE: Out of trim trainer
 

AA5BY....ha! ha!, don't be too hard on him.....over-sizing the hole on the servo arm is something many of us were probably guilty of when we were newbies......to somebody that's been flying for awhile, these things would probably come across as common sense......to some one who's never looked at a plane, they probably meant well, but didn't know any better......like my dad used to say......everybody makes mistakes, making mistakes isn't wrong, it's when you make mistakes and don't learn from them, that's wrong.......by the way....if the flyer your helping hasn't said it already, thanx for hanging in there and taking this guy under your wing......i know people don't say it enough......this hobby and these forums need more people like you......KOOLKRABBER47......

RE: Out of trim trainer
 

The prop size accounts for alot, so Ive recently learned...I am flying a Hobbico Superstar with an Evolution .46NX. The engine, if ordered as part of the Evolution Trainer Power System, comes with a 10x4 three blade prop and the appropriate spinner. I was using a 10x6 wood top flite prop and the thing just flew like it was on crack...very fast, and would climb forever. Not to say that it was bad, but it just flew fast and didnt like to slow down. Being that this is a high wing trainer with about 19oz of wing loading, it took a long approach. I went back to the 10x4 three blade this morning after not having one for about three months, and the three blade prop acts like a brake when you decrease down to around idle rpm. Not so much to affect the flight charactaristics, but it really slowed the thing down! It will not fly as fast as it will with the two blade, but it flies alot cleaner and more gracefully with the three blade. I am about to rebuild the elevator to be larger, because on landing it will fly so slow that it has very little elevator authority just before touchdown. I like to land as slowly as possible, so I think by increasing the surface area of the elevator, I can get a little more flare just before touchdown. This would make a landing rollout of about 30' on asphalt. Its around 40' now, before it is at a dead stop. Not bad, but hitting the brakes faster helps sometimes.

Most of the LHSs will orderer you any prop you want.

TonyG

RE: Out of trim trainer
 

When I learned to fly my PT40 this is what I was taught.
"Cut the throttle to idle and try not to land the plane" lesson.
If I was coming in short to the strip I would slightly increase throttle temporarily to increase lift.
If I was too long to the strip I would go around and cut throttle sooner.
I was taught throttle management vs elevator input simply because with too much elevator throw the plane would stall before landing. Yes even a PT40 would tip stall.
Now with my biplanes and warbirds I come in faster, but still use throttle more than elevator for landings.
I still have that dumb left thumb though dang it!!


Tom

RE: Out of trim trainer
 

If you are running out of elevator authority when slowing for landing, just move the balance point back.

Really people, this is related to rocket science, but is not rocket science!
More like teeter totter 101