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Author Topic: Trim problem - Turn rates  (Read 1341 times)

Offline PJ Rowland

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Trim problem - Turn rates
« on: April 06, 2011, 02:56:37 AM »
So this one is perplexing me a little.

I'm trying to get my new model to track the same inside loops vs Outside loops.

I would normally solve this with mixing in a little elevator.

What Im finding is :

I have good turn rate inverted - sits and tracks well - with the correct amount of turn: however the model wants to sit tail down when inverted level flight. - but tracks well - no hunting.. - Infact no hunting PERIOD..

Upright : It sits level : But the inside loops are more open than inverted.

When I alter the elevator ratio - I find i will loose one way or the other -
I can go the opposite -  having a good turn rate inside loops - but it sits tail down .. and is a bit touching in terms of its tracking. sensitive.


So is this a CG issue ? - Or an elevator mixing issue ? Any idea's would be appreciated.

I want to have the model track the same stance inside / inverted ; and be able to adjust and trim the pitching / rotating of inside / outside turn rates.

I guess the core of the issue is :
When I have the model sitting correctly in level flight - the inside loops are to large - eg it swoops into the corner..
When the model is inverted - it tracks slight tail down - however the outside loops are perfect... rotational wise.




If you always put limit on everything you do, physical or anything else. It will spread into your work and into your life. There are no limits. There are only plateaus, and you must not stay there, you must go beyond them. - Bruce Lee.

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I Yearn for a world where chickens can cross the road without having their motives questioned.


Offline Matt Spencer

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Re: Trim problem - Turn rates
« Reply #1 on: April 06, 2011, 04:06:00 AM »
You can wind in flap till the feel is the tails following the plane , or the feel is through the wing , you cant feel the elevator .

Presumably this might be a maximum flap ratio.
Anyway , its getting the wing to do the work .

              --------------------

Some persons mention of downthrust .
Presumably in certain cases its valid .

The old spiral airflow scenario .
Worth a test to validate .
               --------------------

Presuming the incedances are sot on ,
Drag in the wheels will cause this. . . .
Check or trim with the relative incedance of the  wheel fairings,
or if theres a turbulance issue in the wheel openings ,

test by blowing ciggarette smoke through drinking straw from 4 or 6 inches forward of it , in line with oppening.
see if it gets caught and rolls at junction wheels / pants . This is why they use pants liners (around wheel )
as theres a pressure differential otherwise , too . ( take four very deep breaths before Ea. blow , to avert
health issues , chokeing , and turning blue ! ) y1 %^@ b1
This could effect insides , and be irrelevant or assit outsides.

                  -------------------

Having used 3 1/4 ballon wheels , the frontal hight of the center of resistance was apparent.
Combined with the high thrust line.
Offsets in forces and resistance .
Cranking in a few inches of diheadral solved it in this case.got the corners even.
even with the big wheels, though working hard the tailplane would thump through the turbulance wakes from the wheels once .
previously it'd kick three times going through it .

                       ==========

 :!

Offline Rafael Gonzalez

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Re: Trim problem - Turn rates
« Reply #2 on: April 06, 2011, 04:39:38 AM »
It is not CG. It would behave the same both ways. If we eliminate all things that could have a symmetrical effect, I only find that engine thrust, wing/elevator incidence and flap alignment/throw could have that behaviour. I would recheck those with an incidence meter. A plane with flaps offset a degree or two (down), would fly tail down in level flight. The same if it had negative incidence on wing or a thrust issue. It would also be very sensitive one way and not the other.

Offline PJ Rowland

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Re: Trim problem - Turn rates
« Reply #3 on: April 06, 2011, 05:32:23 AM »
I would say its a little more sensitive inverted - but tracking both ways level flight isnt a problem.

Its more the alignment of how it sits - and relative pitching moment from insides to outsides. - I havent experienced this before - where it sat tail down yet fine upright. - There is no downthrust on the engine - or incidence on any surface.
If you always put limit on everything you do, physical or anything else. It will spread into your work and into your life. There are no limits. There are only plateaus, and you must not stay there, you must go beyond them. - Bruce Lee.

...

I Yearn for a world where chickens can cross the road without having their motives questioned.

Offline PJ Rowland

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Re: Trim problem - Turn rates
« Reply #4 on: April 06, 2011, 06:28:23 AM »
Thinking about this dilemma a little further and trawling through hundreds of old posts..

I'l throw something out there - could it be that its simply not setup level yet ? - Model only has 4 flights on it so far which were today.


I was thinking it was sitting level upright - and tail down inverted.. but possibly its sitting tail up - upright, and tail down inverted..

This would explain the turn rates being slow for inside loops and fast for outside loops - simply geometry of the setup.

Possibly what I need to do -is move the flap / elev so the tail starts to sit down a little further during level flight and sligtly higher inverted, then match the turn rate with handle adjustment.. ??

Possible ?

Someone suggested downthrust also as a solution - its all 0 - 0 - 0 as my baseline, but in all my top stunt ships I run some downthrust and I've never experienced this before. Its quite strange 0however the model is still brand new.
If you always put limit on everything you do, physical or anything else. It will spread into your work and into your life. There are no limits. There are only plateaus, and you must not stay there, you must go beyond them. - Bruce Lee.

...

I Yearn for a world where chickens can cross the road without having their motives questioned.

Online EddyR

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Re: Trim problem - Turn rates
« Reply #5 on: April 06, 2011, 06:45:23 AM »
I have seen this a lot on the ARF Noblers. One of mine did it. It all came back to alignment.Motor wing tail. I know you are a good builder but these things do happen.
I cut the tail out of the nobler and reset it. Before doing that I had tried adjusting the elevator up and down and it would not correct it,
Ed
Locust NC 40 miles from the Huntersville field

Offline Alan Resinger

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Re: Trim problem - Turn rates
« Reply #6 on: April 06, 2011, 08:04:09 AM »
P.J. 
When you are talking about elevator mixing, are you talking about just changing the ratio of elevator to flap?  Sounds like you need to change the length of the elevator pushrod.  You need to lengthen the pushrod to give more up elevator in relation to the flaps.  This will increase the inside turn radius while slightly negating the outside turn.
Alan Resinger

Offline Ted Fancher

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Re: Trim problem - Turn rates
« Reply #7 on: April 06, 2011, 08:23:27 AM »
P.J.

Are your hinge lines sealed?

Ted

Offline Ted Fancher

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Re: Trim problem - Turn rates
« Reply #8 on: April 06, 2011, 08:48:43 AM »
First, I'm assuming you're talking about your new Nobler, right?

If so, with the exception of the Rolls Merlin in the front end, is the configuration still pretty much stock Nobler?...and where is the CG located aft of the leading edge at the average chord?

In general...

If the hinge lines aren't sealed, seal 'em up and try again.

If they are sealed but there is no noticeable improvement lengthen the flap to elevator pushrod 1/2 turn at a time and test fly after each adjustment.

Two things are telling me one of these is the likely solution.

First, failing to seal hinge lines is essentially asking for inconsistent response rates.  Not always, of course, but the odds of having perfectly matched hinge lines on all four surfaces thus avoiding inconsistent lift production (all trim problems are issues of lift distribution and/or CG of the airplane) is little better than 50/50, if that.

Second, two of your symptoms point to a need to refine the flap/elevator neutral in the same direction.  The fact the tail noticeably "drags" in inverted flight begs for a little "up" elevator to raise it level.  Remember, to fly level the wing's angle of attack must be just enough to support the one G weight of the airplane...a very tiny angle on a lightly loaded stunter that is almost undetectable upright and inverted if the airplane is in trim.  If the airplane is flying at the same altitude ("level") but with the tail low it is developing exactly the same amount of lift as it is upright (one G's worth) but the pitch attitude of the airplane must be different to produce it.  It is "out of trim" only with respect to our ability to maneuver with it.  IOW, it won't respond the same way in both positive and negative G  maneuvers.

The fact that the ship turns more aggressively in outsides and "swoops" into insides is symptomatic of the same disease.  If the problem is the result of variations in lift due to inconsistent bleed of high pressure air through the hinge lines sealing them should help the problem.  If not, what you call "mixing" of the flap/elevator neutrals (with respect to one another) is the simplest approach that doesn't required knives, glue and paint, etc.  FWIW, I can't recall any airplane of my own that didn't require exactly this kind of refinement to achieve satisfactory trim.

Finally, I know you're a great builder and this is "just in case" stuff but of course you've checked all the controls to assure there isn't binding or rubbing or other sorts of "hardware" related issues to control inputs...right?

Ted

Edited P.s.  Don't forget that every time you adjust the flap/elevator "mix" you'll need to adjust your handle neutral.  I'd suggest two flights minimum with each adjustment.  The first to experience the effect and the second to adapt your handle (the man/machine interface) with respect to the results of the change.  If you don't do this you'll almost certainly let your assessment of the value of the change be affected by the fact that it didn't "feel" right.  Remember, you're changing "stuff" on the airplane to make it "happy".  The way to make the pilot "happy" is to adapt that "interface" to work in harmony with "happy" airplane.  It is a subtle but very real difference.
« Last Edit: April 06, 2011, 01:33:48 PM by Ted Fancher »

Offline Jim Thomerson

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Re: Trim problem - Turn rates
« Reply #9 on: April 06, 2011, 04:24:26 PM »
All 0-0-0 stunt ships fly slightly tail low to make lift.  It is easier to see when airplanes are inverted because many airplanes have a bottom outline parallel to the 0-0-0, but the top outline is broken up.  I have had experience with an airplane doing greatly unequal turns, and cured it with a little upthrust.  A 1 degree shim on the engine mount would be a simple first thing to try.  If that doesn't fix it, re-read Ted's expert advice and follow it. 

Offline PJ Rowland

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Re: Trim problem - Turn rates
« Reply #10 on: April 06, 2011, 06:05:14 PM »
Hi Guys & Ted.

I always 100% without fail seal the hingelines Stab/Flap prior to the 1st test flight.

As RJ says : Its a waste of fuel otherwise. - I should have mentioned that.

2nd..... Yep its the New Nobler, and yes other than engine its as stock to plan as possible. - The CG location is EXACTLY where its shown on the plans.

3rd : Im using all Ball links - Rocket #87 and dubro pinhinges hardware + 30 mins on my " Flap looseing maching " which moves the flaps up and down continuesly for 30 mins - simulates approx 500 flights worth of movement . - So all control are very free.


Now that the formalities are out of the way..


I should preface that I only managed 6 flights or so .. So it was early days but I usually get things close after a few flights or get it going in the right direction.

I will confess : Don't forget that every time you adjust the flap/elevator "mix" you'll need to adjust your handle neutral.  I'd suggest two flights minimum with each adjustment.  - I didnt do that - thinking back to yesterday's flight perhaps I need to try that as an approach...

What I found when mixing the flaps/elv ratio was that I got the turn rate closer to being equal but I had the model sitting down either one way or the other. - For example I had it sitting right inverted and close to right upright but the turn rate was different. -

That is where this issue all started, off the deck it flew darn close to ideal upright & inverted tracking was pretty close to good.. - 1st flight did squares, loops vert's ect.. and was VEry pelased with its performance. However the insiders didnt turn as fast as outsides..( or was it the other way - I cant recall.. ) so thats when I started to play with the ratiomix - trying to equalise the turn rates.... getting it closer but then I started to dial in this tail down inverted and nose down upright feel to it.

I couldnt get the turn rate the same and have equal sitting upright and inverted.

I do think I have'nt given it enough time on trimming - As I mentioned to someone else yesterday - Im trying to compare a model thats 6 flights old, agaisnt one that had close to 2000 flights and was in perfect trim.

I need a full day - and I will try the handle adjustment and some more elev mixing.



 
If you always put limit on everything you do, physical or anything else. It will spread into your work and into your life. There are no limits. There are only plateaus, and you must not stay there, you must go beyond them. - Bruce Lee.

...

I Yearn for a world where chickens can cross the road without having their motives questioned.

Offline Alan Resinger

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Re: Trim problem - Turn rates
« Reply #11 on: April 06, 2011, 06:39:13 PM »
P.J.
You keep talking about mixing.  Forgive me but are you just changing the flap elevator ratio (moving the elevators either more or less with respect to the flaps)?  You've not said anything about changing the elevator pushrod length which is what Ted and I both mentioned.  Lengthing the pushrod will give you more up elevator with respect to the flaps as you know making the inside turns better than they are now.
Alan

Offline PJ Rowland

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Re: Trim problem - Turn rates
« Reply #12 on: April 06, 2011, 06:54:35 PM »
perhaps I should have been a little clearer with what I was doing.

When I say " Mixing " I mean adjusting the pushrod length in respect to the Flap / Elevator ratio.

This is seperate to adjusting the Elevator throw by moving the slider horn up and down Vertically.

I call it mixing becasue you are keeping the flaps the same and mixing in a little up or Down elevator. ( Its an RC term I guess )


I did Adjust that length - this has been the only adjustment I've been playing around with - that is how you solve the problem of turn rates, ive had this happen before - but what I haven't experienced is the attitude alteration with repect to upright & inverted.


So The consensus is lengthen the Rod, pushing the elev Upward, - I did try this first - but I didnt readjust my handle neatural, and I think thats part of the problem.
If you always put limit on everything you do, physical or anything else. It will spread into your work and into your life. There are no limits. There are only plateaus, and you must not stay there, you must go beyond them. - Bruce Lee.

...

I Yearn for a world where chickens can cross the road without having their motives questioned.


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