Jetdesign

My Feedback: (8)

I borrowed an incidence meter and found that my rear stabilize has between 1.5 and 2 degrees of 'up' built into it; the wings are close to 0, and the engine is about 1.5 degrees down (supposed to be 2 degrees).

What should I change? The plane likes to fly UP, and uses lots of down trim to fly straight. I know I can add a washer under the top of the engine mount to add more down thrust, but not sure (haven't thought too hard about yet) how this would affect everything else. Obviously I can't change the incidence of the elevator as it's epoxied in place. The tail was able to be repaired with minimal amounts of CA and epoxy, so it's not going to be removed.

Thanks.

Steve Steinbring

GO SLOW DON'T JUMP TO CONCLUSIONS.

Before you get all crazy and start changing things it would be a good idea to find out what the design incidences are! What do the plans say about down thrust and such.

When I check incidence I set the fuselage in a jig where the horizontal stabilizer is at zero degrees. Then I get a reading on the wing incidence. I fabricated a set of jigs that allows me to set up everything very stable to get measurements, anything less is just guessing.

You said the airplane came through the crash quite well it is doubtful the wing saddle or the horizontal stab incidence has changed to any degree unless the fuselage was broken.

da Rock

What you're talking about, incidences, are values that are absolutely not important by themselves. They are important, but in relationship to other things. And you've not mentioned any of those important other things.

For example, the incidence of a cambered wing will quite naturally be different than a symmetrical wing. And a highwing airplane's stabilizer will usually have a different incidence than a low wing airplane's stabilizer. And the airfoil of the wing would have an affect on the stabilizer as well.

Also, what do you mean by the tail having "up built into it"? Do you mean it is angled down in front or angled up in front?

What the plane does should be good enough to work with. If you feel that it climbs when you wish it to fly level and that trimming the elevator with down trim gets it to fly the way you wish, work with that. Since you're not planning to change the stab's incidence, the change the wing's incidence. Raise the wing's TE some and test fly the airplane. Expect the elevator trim to be incorrect.

BTW, if you know that the engine is supposed to have more downthrust, then set it to whatever you know it should be and test fly. You'll learn right away if that engine thrust line mattered. And all it took was slapping some washers in place.

What is the airplane?

Jetdesign

My Feedback: (8)

Nothing moved in the crash. Trhe horiz stab is angled down in front two degrees. The plane climbs more than just trimming can fix, and an experienced pilot saw the amount of down trim needed and said it worried/bothered him. Ill try the engine thrust angle first since its only a washer to change it. I felt that changing the wing to match the tail would drastically change the aoa, messing with take off, etc. Ill see if i can find the incidences published somewhere.I dont mind using trim to fix problem but would like to reduce drag if possible.

Gray Beard

My Feedback: (-1)

I think he is speaking of his ARF Decathlon?? Was it A Phoenix??? Makes very little difference who makes it though.
You need to know where the datum line is before you can even guess about the incidence, if not then you need to know and trust the incidence that the builder has installed in the wing or stab so you can get the plane set up like steve said so it doesn't move around on you.
So many other things effect how the plane fly's and trims, CG and engine thrust for starters. These are things you can figure out before you borrow an incidence meter.
An incidence meter and incidence is something you really have to learn about before you dive right into it. Try reading up on it in A book or the instruction book if it's A Robart.
I have an old book titled Scale Aircraft by Gordon Whitehead from 1980, published in the UK. It's an old book but you should read it or one like it several times so you have A better understanding of what you are trying to do. I have also built from Mr. Whiteheads plans, the man knows his stuff!! MY copy was A gift and located on Ebay several years ago.
Books, instructions, learning, sort of like school!!

ChuckW

Compared to what? What did you use as a reference for checking the angle? How do you know it is incorrect?

This gets really fuzzy when dealing with an ARF. You have no information on incidences or thrust angles, no drawings, no plans, nothing. If you are lucky, you might find some information someplace but it is unlikely especially on some of the off-brand stuff like Phoenix.

I'd be careful on the engine thrust angle unless you are sure it needs it. One good way to tell is by flying at full throttle then quickly pulling back to idle. Does the plane quickly change in attitude? If so then thrust angle might be an issue. If it just keeps flying forward and slowly descends then there likely isn't a problem.

What about your CG? Are you sure it is right? Have you tried moving it forward some to see if the handling is better and if the plane doesn't want to climb so much? It might be worth a try.

Sure, it's possible something on your plane may be out of whack. It does happen on ARF's. Just be sure though. I'd try working on the trim first. There is a good trimming chart on the last few pages of this manual: http://manuals.hobbico.com/gpm/gpma0400-manual.pdf

da Rock

You don't change the wing to match the tail. The idea is to adjust the wing according to how you thought the plane was flying and how you wish it to fly.

Once the airplane has shown what it does in the air, you can respond to that. You know what it's doing and it's often rather simple to make changes and test them.

Do you know what the name of the model is?

Is it a highwing model? Is the wing rubberbanded on or bolted on?

Nathan King

Wait a minute. First of all, how did you have the airplane set up when checking the incidence? You can't just sit your airplane down on the ground and slap the meter on - you won't get a valid reading. Secondly, are you sure the airplane's climbing problem is even related to incidence? The way you're testing during flight does not isolate a potential problem. For example, if you suspect an incidence problem test it with the engine at idle. That way you won't have any thrust issues skewing your incidence test. Isolate, isolate, isolate. Next time you fly it, try this. Get nice and high up and enter a power off dive - straight down. Now let go of the controls. What does the airplane do?

It could also be a thrust problem. Fly straight and level and cut the throttle. Does the airplane abruptly make a pitch change? Changing the thrust line on the airplane requires a retest of many other things - especially incidence.

Now, what exactly do you mean by "requires a lot of down trim." When you have it trimmed out what does your elevator deflection look like (about how many degrees from neutral)?

Jetdesign

My Feedback: (8)

First offd i just wnt to point out that my instructor saw something in my plane and said i should check it, so he let me borrow his meter. What he saw was asymmetrical washout, and he thought this was causing the plane to stall to one side. When checking the plane (phoenix super decathlon, high wing) i saw that the stab was 2 degrees different from the wing. The directions on the meter said they should both be zero. I had the plane set up so it was level, the flat part on the bottom of the plane was level, what i expected to be the thrust line was level. The firewall was angled down 1.5 degrees from level (supposed to be 2) the wing at fuse was level, and the stab off by 2 degrees. The pplane needs 7 degrees of down elevator to fly straight. When i go into a dive the plane levlels off pretty much immediately. Havent had a chance to test too much else.

Nathan King

Okay, the leveling off immediately suggests an incidence problem. Leveling off gradually as the airplane picks up airspeed is, of course, normal. This would suggest to me that you may have insufficient wing or stab incidence. An airplane flying straight and level is a balancing act between the weight of the engine up front, the wing, and the stab. The climbing indicates that the stab is providing too much downforce, which is caused by an excessive negative incidence. Let me guess, the stab incidence was two degrees too far down (negative), right?

The twist of the wing is a different issue altogether. Like your instructor said, it can cause bizarre stall characteristics. These things happen from time to time, it's just a matter of adjusting everything out. This takes some time to do properly. Just remember, make one correction at a time, fly it, and let the airplane give you feedback. The airplane will tell you what's wrong if you can decode its language.

Steve Steinbring

CALL PHOENIX AND FIND OUT WHAT THE INCIDENCE SHOULD BE!

Eyeballing what is a perceived washout is just that Kentucky windage at best. Just because someone at the field tells you something doesn't necessarily make it right. Ask ten people at the flying field more than likely you will get almost ten different opinions. Do your homework!

Using the bottomside of the wing as a reference is an assumption that may or may not be correct for the thrustline or incidence datum for the airplane. The manufacturer will state what the incidence should be usually with the tail being zero and the wing at X angle of incidence. Its very common for a high wing airplane to have a couple degrees of positive incidence, what your airplane is supposed to be setup at WHO KNOWS except Phoenix.

Once you have the airplane squared up in a jig and checked for correct incidence then you can check for washout. Is it supposed to have washout? Could it be that the wing is warped. You can't assume anything until you have the correct information to use as a baseline. Samething for the thrustline!

Jetdesign

My Feedback: (8)

Ok, thanks, I'll try to find the correct incidence. I only found a variation of .5 degrees between the washout of the left wing and the right wing, so I'm not too concerned about that. I think the covering, stripes, and edges of the covering not being perfectly square were giving an illusion of improper washout. Both wings have close to 1 degree of washout.

I forget if the stab has positive or negative incidence; I guess it depends on which way you hold the meter? Either way it's the direction that causes the plane to climb, i.e. pushing the tail down when flying.

I am thinking that trimming the plane so it flies straight will do just that, let it fly straight. However the trim would not help as much when in a dive, and the plane climbs out, correct? I think this is the case with my Dolphin, too. It's crooked but trimmed out so it flies in a straight line. When you enter a loop or something the trim becomes either ineffective, or effective in a different way.

alfredbmor

If you have the possibilities to fix it up I honestly recommend you to do it. As you described before you could start with the engine (0 incidence) I am not really sure where is your primarily basis of your measurements, but I would not take the main wings because they always have incidence. Forget about the wash out by now. Luckily your stab could be at 0o incidence (It is in a great number of ARFs) and often it is built into the primary line of the plane which is 0o incidence. If so, then position your engine to 0o incidence and check it out, see how it flies, if that simple fix repaired the issue congratulations, if not, the you will have to shim the wing saddle of the main wings to achieve a desired incidence.
I had a Phoenix low wing Sonic and it had an incidence problem. A buddy of mine also bought another Phoenix model and had to do some changes to correct the incidence. Be aware that in the world of the ARFs, this kind of problems are very common, many people just use the trim and they fly acceptable, but if you really want to fix your plane and want to have the best of it go for it, you will learn more than a simple fix.

Jetdesign

My Feedback: (8)

Well Phoenix is now distributed by Great Planes/Hobbico, and they said the only way to get the incidence was for them to actually build the model and measure it.

The directions from the incidence meter said to have the wings and stab on same line, and check manual for engine thrust angle (which is 2 degrees down and 2 degrees to the side). I thought I remembered my instructor saying the stab should be on the same line as the engine, and if that's the case then it is, with a 1.5 degree wing incidence. I read in the Phoenix Decathlon thread that one or two people did change the incidence for improved flight, but also that the recommended CG may be too far back (100mm, 3/8" behind main spar).

This is too much for me to deal with right now; while this might be good practice for future (pattern) planes, it's taking away from my whole positive flying experience. I'm supposed to be just learning to fly. I'll play with the CG, and the engine thrust angle if need be, and save wing and stab incidences for later, at least for when I can find what they are supposed to be.

ChuckW

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How do the instructions included with the meter know what kind of plane you have and how it was deisigned? Incidences can vary on planes. Different airfoils, different designs and so on will dictate how it is set up. Sure, your plane COULD be designed to have the wing and stab exactly parallel but then again, maybe not. Look at various airplane plans and you'll see how these things can really vary. As has been suggested, the incidence should be altered only in response to flight characteristics, especially when the intended design is a complete unknown.

Personall, I think your CG is too far rearward. I could be wrong of course since I haven't actually seen or flown the plane.

To be honest all this talk of incidences and thrust angles is probably a little advanced for the beginners forum too.

If this Decathalon is too much trouble, I'd be tempted to shelve it myself. Take alook at a good handling plane such as the Tiger-2 or World Models T-34. Heck, even a World Models Mach-1 is a very aerobatic, great handling plane. Neither are really scale but they fly great. Plus, if your goal is to someday compete in pattern, you won't have a scale plane for that either. Both airplanes would do well in sportsman competition plus be appropriate for your experience level. They'll teach you a lot. The GP Cherokee might be another good option but it is a little on the porky side and can land hot if you aren't careful. The flaps help tremendously of course but just add another level of unnecessary complexity for a beginner in my opinion.

da Rock

It would be to your advantage to get a copy of "Basics of R/C Model Aircraft Design" by Andy Lennon. It's an excellent book for modelers. It explains the details of aerodynamics rather well for people who have no previous schooling in the science and who are interested in how it applies to our models. It's about $20 and is an excellent value.

You will discover that it's rather difficult to learn aerodynamics from what you hear at the field. For example, "the stab should be on the same line as the engine" is pretty awful for an aerodynamic truism.

That book is really worth studying. You know how people say you can learn to fly R/C with the pc simulators? Well, it is actually possible to learn aerodynamics from that $20 book.

ChuckW

I have that book and I agree that it is great. I personally think it might be a bit much for a beginner though. I think it might just confuse them further when they should be concentrating on learning other fundamentals.

Very true. There will be a whole bunch of "experts". A select few probably really are experts but the rest are usually full of sh**. The hard part is determining who to actually listen to. Unfortunately, many people listen only to the ones who are saying what they want to hear and ignore those telling the truth because they may not like the sound of it.

Jetdesign

My Feedback: (8)

I agree that this is beyond a beginner topic, as it's beyond me. I'm gonna give back the incidence meter and pretend I never had it. Will work on CG, and maybe play with a washer under the engine mount as that's easy to do and easy to remove. Changing the shape of the plane to change the flight characteristics is definitely beyond someone flying toy airplanes for less than 3 months.

I'll check out that book over the winter, sounds interesting. I'll have taken Fluid Dynamics by then so maybe it would make more sense after that class (I can't WAIT for fluids!)

Nathan King

Yes, Steve is right. I forgot to mention what he brought up. You don't have anything to go off of since you don't know what it should be. You can, however, make an educated guess from making some measurements, math, and flight testing. Anyway, setting up the bottom of the fuselage as zero may or may not be true zero. If you don't know this, you really can't get anything more than relative measurements between the wing and stab, but not take true measurements. The aerodynamics book will be of great help. If I wasn't an aviation major in college (and an aerodynamics junkie), I'd really be up a creek trying to figure it out.

Don't listen to everything at the field. There are a lot of old "veterans" that spout BS 60% of the time they open their mouth.

Villa

Hi gaRCfield
I thought I read some were you were just starting enginering college. When I graduated from engineering in 1960 I seem to recall taking Fluid Dynamics in the last year, or at least toward the end. Of course things have changed. The slide rule was THE computer then. Much better now.

Jetdesign

My Feedback: (8)

I didn't even know what a slide rule was until a couple of years ago, and saw my first one last semester! We take fluids start of junior year; just took 2 semesters of thermo and loved it, and I really like chemistry, so fluids should be a blast. Also taking fluids lab

Nathan King

In high school I had an old Calculus teacher that didn't allow "electronic calculators" in his class. It was miserable. I got the idea of bringing in a slide rule - after all, it wasn't an "electronic calculator" as he said. I brought it in and he stared at me as class started. He smirked and said, "if you know how to work that thing then you can use it."

Come to think of it, I was always kind of an ass. I was nice to everybody, but was always using my creativity and technicalities to get myself out of irritations like that. I can't help it, I'm left handed and creative.
One of those "lateral thinkers."

Jetdesign

My Feedback: (8)

A lefty, I knew it!!![&:]

beau0090_99

Don't get too excited about Fluids based on Chem and thermo. I also loved thermo and chem, but Fluids was not my cup of tea. Do they offer any Combustion classes at your school? They generally are taught by the chem department, but they are fun. I would also suggest taking a turbines class if you like Thermo. Mach theory is pretty freaking cool, then you can apply it by getting into jets!

Back to the subject though, was this the plane that you mounted the .55 on? Do you think that might be causing issues? Added weight, but added thrust will most likely make your plane change dynamically over a range of speeds. If this is the plane with the .55 and if they recommended a smaller motor (.40-.46), maybe you might want to try putting your .46 in it and seeing how it works. with the added thrust of the .55, that might explain why you might need to increase your down thrust of the motor. It might be pulling the nose up too much.

If this is not the case, then disregard. If it is the case, look at it this way. Think back to your statics class and think about it this way. If the motor shaft is not mounted inline with the wing chord and the stab (like a stik - they are closer than most planes), then you are creating a bending moment when you are creating thrust with the engine, because the drag of the planes wing (being high) will try to pull the plane back, creating up AOA. This is partially overcome by adding downthrust to the engine mount. The faster the airspeed the higher the drag (drag increases proportionally to the square of the velocity).

Don't give up, but like Nathan said, isolate, isolate, isolate. It's like deconstructing a problem in school, you can do it, just stay with it.
Regards,
Curtis
P.S. Of course this doesn't explain why it might fly different than before the crash. However, if you got the engine running better now, then more thrust.

Nathan King

I find the term "bending" to be a bit ambiguous and abstract. I usually think of it as adding a small horizontal component of thrust, very much like the horizontal and vertical components of lift in a turn.