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Effects of out of trim control surfaces

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Effects of out of trim control surfaces

Old 03-18-2021, 09:55 AM
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AA5BY
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Default Effects of out of trim control surfaces

I've recently completed a build that incorporated independent incidence adjusters for each of the wing panels. It now as eleven flights with adjustments made between the flights. The maiden flight required several clicks and about 3/32 of up elevator trim for level flight. Knife edge had serious tuck to canopy making knife edge extremely difficult.

As the adjusters have been trimmed between flights to give positive incidence and reduce elevator trim, knife edge flight has increasingly gotten better with reduced amounts of canopy tuck. Now, nearly having reached neutral elevator, knife edge tuck is about gone, allowing easy knife edge horizon to horizon.

As well, in the past I've noted that significant elevator trim causes a variance in trim between power off and on. If the elevator is trimmed down, landing glide slope is generally too steep and conversely if the elevator is trimmed up, landing glide slope is too shallow. When the elevator is neutral, landing glide slope is generally good with landings requiring little pitch control

What are your observations/experiences regarding non neutral control surfaces? Do you sense them to be a problem? Do you try to neutralize them?
Old 03-18-2021, 11:57 AM
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daveopam
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As a aerobatic/3D pilot I do try and get things as close to center as possible. I've never tried to change incidence but have tinkered with thrust angles. I also try and get the elev neutral with CG. 25 yeas ago it was much easier as the stuff I built was straighter than most ARFs. I had a 50cc AW Extra once that I could not get the elev halves to zero out with each other. I leveled the fuse and put a Robart incidence meter on each one and was going to have to drill out the rotation pin and reset it to get it right. AW had great customer service and mailed me a pair that matched up. On that plane the stabs were on a small wing tube so it was pretty easy.
I'm getting off topic but yes it's a problem. I've also noticed on a lot of ARFs that they twist the control surfaces when heating the covering and that has to be fixed. It's a little extra work to get things right but makes for a MUCH better flying plane. I don't think a lot of guys realize they are making correction in flight that they would not have to do if the plane was right.

David
Old 03-18-2021, 07:12 PM
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speedracerntrixie
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Originally Posted by AA5BY View Post
I've recently completed a build that incorporated independent incidence adjusters for each of the wing panels. It now as eleven flights with adjustments made between the flights. The maiden flight required several clicks and about 3/32 of up elevator trim for level flight. Knife edge had serious tuck to canopy making knife edge extremely difficult.

As the adjusters have been trimmed between flights to give positive incidence and reduce elevator trim, knife edge flight has increasingly gotten better with reduced amounts of canopy tuck. Now, nearly having reached neutral elevator, knife edge tuck is about gone, allowing easy knife edge horizon to horizon.

As well, in the past I've noted that significant elevator trim causes a variance in trim between power off and on. If the elevator is trimmed down, landing glide slope is generally too steep and conversely if the elevator is trimmed up, landing glide slope is too shallow. When the elevator is neutral, landing glide slope is generally good with landings requiring little pitch control

What are your observations/experiences regarding non neutral control surfaces? Do you sense them to be a problem? Do you try to neutralize them?

You have just taken your first step down the yellow brick road. For more trimming " secrets " take a look at the CK Aero website. A few tidbits to be found.
Old 03-19-2021, 12:59 AM
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DGrant
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There can be several factors that affect non-neutral surfaces, but when a plane gets closer to trim and balance, if everything is straight, the surfaces are true/trim/neutral as well..

Incidence is definitely a part of it, and should be set accordingly, but it's adjustment isn't the whole picture. Balance(fore/aft/left/right), thrust angles(pos/neg/left/right) obviously have everything to do with trim as well. Balance will affect elevator trim as much as incidence might.

I try to get my planes trimmed so everything is at neutral. Even if the surfaces might be off a few thousandths', for the most part you can't notice visually that any surfaces are anything but true and neutral. If they're off any I don't see it, and the real test is in flying and testing.

There's a popular trim guide written by Peter Goldsmith that takes you through a series of manuevers, and explains what/how to go about getting a plane dialed in. I've found it invaluable and have used it for years. I suggest you Google it and see if you can find it and give it a read if you can.

I'm also going to take speedracer's advice myself and check out his recommendation.. You can never have too much information, and everything in this hobby is a learning experience.
Old 03-19-2021, 04:20 AM
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speedracerntrixie
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Just one word of caution, once you dive into the " complete trimming " and fly your first airplane that is 97% dialed in you will never be happy flying anything that has not been through the process. Not to be negitive but most airplanes are being flown with really bad setups and guys are perfectly happy flying them that way. For the most part I have stopped trying to teach guys trimming ( unless asked to do so ). Like anything else worthwhile, it takes time and is work. Most just want to fly blissfully unaware.
Old 03-19-2021, 04:44 AM
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ahicks
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I have to agree with SRT. After identifying the the need (hard pull to canopy on KE), and the time/research taken to fix it (whatever is required to get a neutral elevator), you are now spoiled.

Now, you not only have a neutral plane that's a pure joy to fly, you have a more capable plane. You sir, may not recognize it yet, but you've taken a giant step forward in your flying ability.

That said, adjustable incidence is nice, but on planes not equipped, most of the time you can use your CG to accomplish the same thing. Up elevator trim is trying to lift a too heavy nose. Down trim a too heavy tail.

If your neutral elevators allow nice straight flight in one direction, but not the other, a couple of percent rudder to elevator mixing (I've never needed more than 5 degrees) can fix that too!

The Goldsmith notes can be a blast used to verify something that doesn't feel right. You don't have to follow them to T. Just getting a plane trimmed to 95% can make all the difference in the world. You're never going to get it where it flies itself.... -Al
Old 03-19-2021, 04:48 AM
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All wise words. One early trimming article likened it to squeezing a balloon. Change one thing, and something pops out somewhere else. So a very structured approach (aka "the Goldsmith method") helps immensely in determining where to squeeze, and how much. It is never done, and never 100% perfect. All one can do is get it as well balanced as that airframe allows. And there will be a few that have a noticeable "defect" - such as surfaces not perfectly aligned. But if it flies well, then that is what it is....
But once you've experienced a very will trimmed plane, all will have to be well trimmed. And that can be a curse. I even put the time in on each stupid $2 foam board steamer combat plane I build. Even though it's life expectancy is very short......
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Old 03-19-2021, 09:05 AM
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AA5BY
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I enjoyed each of the replies. Much of my reason for the post is my recent giant scale build of what I call an Ein-stick-er (a cross between a stick and an Eindecker) uses the planform of the 30cc Ultrastick. The wing foil is the same and basic measurements and areas are similar with the main difference a thrust line mounted wing compared to shoulder.

The Ultrastick has a good bit of up trim in the elevators (1/8") and of course adjusting incidence of wing or stab to remove would be difficult given a glued in stab and wing sitting in a saddle. And... the plane flies well other than not knife edging worth a hoot, having both serious tuck and roll components. So, KE was simply removed from its flight profile.

When designing the Ein-sticker-er, the desire was to solve those issues hence the incidence adjusters which were easy to do in a giant scale scratch build effort. Also, the Ein-stick-er was given thrust line mounted wing panels to reduce one of the drag components. While tempted greatly to use a high stab to balance drag couples of stab and undercarriage, concern that the thick foiled wing would shadow a high stab when pulling the nose up during landing and create an elevator stall problem nagged... so stayed with the venerable low stab of a stick that has traditionally provided very good pitch control during landing.

The Ein-stick-er is the better of the two fliers. While it is 2oz heavier, it has five inches more wingspan so is slightly more agile. Notable is that for whatever reasons, the Ein-stick-er has no roll component from rudder control compared to the 30cc Ultrastick that has a good bit and that of course is a great plus in allowing the Ein-stick-er to knife edge well. I'm not smart enough to know why one has a roll component to rudder control and the other doesn't.

Three possibilities exist in my mind. 1st, the Ultrastick (an Arf) does not enjoy neutral ailerons whereas the Ein-stick-er of course does given the ability to trim the independent adjusters for neutral elevator and neutral ailerons. 2nd, the Ultrastick has a shoulder wing and the Ein-stick-er thrust line. 3rd, the stab is not quite as low on the Ein-stick-er due to curved boom on all four sides.

Flight trim is a fascinating dimension and I'm in agreement with the comments that once a good trimmed air frame is enjoyed, thereafter one can sense the difference and appreciate the better and even dislike the poorer. The old saying that its all good... some is just better than others, becomes harder to argue.
Old 03-20-2021, 06:15 PM
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speedracerntrixie
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As you can see from the lack of interest in this thread that trimming is somewhat a lost art. Those of us who still go through a thorough sequence step by step until an airplane is performing as well as it possibly can. I just ordered a couple of Smart Fly Equalizers at a cost of $80 so that I can get the 4 aileron servos matched up correctly. Would the airplane fly using Y connectors? Yes, but it would never be as good as it could be.



Old 03-21-2021, 06:56 AM
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AA5BY
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Forty years ago, the best builder and flyer in our local club (an engineer by trade) made a simple comment. If you really want a plane to fly well, provide it with the ability to adjust the stab. Probably like most, those words didn't affect my building of kits that didn't provide such means.

In recent years having embraced plan or self design builds of giant scale, the words were remembered, perhaps because of concern of putting a lot of time and effort into projects and wanting them to fly well.

We expected a kit to fly well and the designer was trusted to have done the work. When the design is our own or we've taken liberties with a design, or want the best possible trim, then the onus for a well flying plane is on us and giving a build the means to trim it, is a reasonable - even prudent thing to do.
Old 03-21-2021, 08:04 AM
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Not to sound too self serving, I question even what kit designers do especially if it is an older kit. They did what was known at the time. It leads to the occasional online disagreement as I frequently get " you know better then the guy that designed it " type comments. Sometimes the answer is " yes ". I have spent decades learning to design and trim model airplanes, I have zero issues sharing what I have learned. The airplane in my avatar is one of my designs and has proven very competitive in masters pattern competition. Not an easy nut to crack.

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