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All Forums >> RC Airplanes >> Beginners >> Trimming problem
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Trimming problem - 9/21/2008 8:42:34 PM   
mc1982


 

Posts: 76
Joined: 12/14/2007
From: limassol, CYPRUS
Status: offline 		Hi,

I have an oxalys parkflyer, i leveled out the ailerons and elevator to follow the wings, flew then trimmed it, ailerons did not need any trimming but when i added throttle it would climb steeply and when i was inverted not matter what thottle settings the plane would dive towards the floor. The CG is fine, i have added alot of downthrust to the point where its silly and still, it climbs when i add throttle and dives inverted. is there any way i can trim this out from the radio? If i move the ailerons a few degrees down would it sort this out?
       Post #: 1

RE: Trimming problem - 9/21/2008 8:44:08 PM   
mc1982


 

Posts: 76
Joined: 12/14/2007
From: limassol, CYPRUS
Status: offline 		Ah yes also no matter if i trim it with down elivator it will still do the same thing.

(in reply to mc1982)
       Post #: 2

RE: Trimming problem - 9/21/2008 9:02:30 PM   
aerowoof


 

Posts: 1948
Joined: 6/11/2006
From: pembroke, NH, USA
Status: offline 		my guess is the wing incidence is set wrong in relation to the stab the downthrust made things worse

(in reply to mc1982)
       Post #: 3

RE: Trimming problem - 9/21/2008 9:05:45 PM   
mc1982


 

Posts: 76
Joined: 12/14/2007
From: limassol, CYPRUS
Status: offline 		You mean the tailwing has an angle that gives natural elevator? How would that be possible if the parts are laser cut ?

(in reply to aerowoof)
       Post #: 4

RE: Trimming problem - 9/22/2008 3:25:26 AM   
Charlie P.



Posts: 2285
Joined: 2/25/2003
From: High Above Upstate, NY, USA
Status: offline 		I am assuming it is a symmetrical airfoil?

From your description it could be main wing incedence, horizontal stab incedence or up-thrust built into the firewall/engine mount (or not enough down-thrust built in if it calls for some.).

Lazer cutting has nothing to do with how a model is assembled. That just gives you nice crisp pieces with burned edges. You still have to check alignment frequently and carefully when assembling.

_____________________________

Charlie P.
"Gravity is weak but it is persistant" AMA 747089/IMAA 30723

Oh Lord, your earth is so hard and my planes are so frail.

(in reply to mc1982)
       Post #: 5

RE: Trimming problem - 9/22/2008 4:12:58 AM   
jetmech05


 

Posts: 1810
Joined: 7/12/2005
From: Jacksonville, FL, USA
Status: online 		when you say you leveled out the ailerons to follow the wings......do you mean the top of the wings or the bottom of the wings? If the ailerons all not correct (a little down) you will have the exact problem you describe...is the aircraft sensitive in pitch?

(in reply to Charlie P.)
       Post #: 6

RE: Trimming problem - 9/22/2008 5:40:52 AM   
opjose



Posts: 6207
Joined: 11/26/2005
From: Poolesville, MD, USA
Status: offline
quote:

ORIGINAL: mc1982
when i added throttle it would climb steeply and when i was inverted no matter what thottle settings the plane would dive towards the floor.


On a new mid wing, symetrical winged plane, with the C.G. set properly, e.g. a pattern or sport plane, at 60-75% throttle the nose may drop slowly both upright and inverted. ( This assumes that at aileron neutral, the ailerons align with the wing chord... ).

If the plane is built right and the C.G. is where it belongs the amount of drop, or rather the rate of change, should be the same both upright and inverted.

Normally we then tend to adjust the trim for hands free flight at mid to high throttle, which acts to drop the nose more quickly in inverted flight... you've done the latter, but it sounds as if the C.G. is too far forward to begine with...


quote:

ORIGINAL: mc1982

The CG is fine,


Don't assume this is true! Even if you have the C.G. EXACTLY at the recommended point, it still may be way off for "neutral" flying chacteristics, which you seem to be hunting for.

Instead, adjust the trim so that the rate of change into a dive, is the same both upright and inverted... land the plane and start moving the C.G. back by moving say the battery pack back... do not more than 1/2" at a time maximum...

Fly it again.

When you notice that you only need a small amount of up elevator at 75%-80% throttle to keep the plane level, you've found a good "working" C.G. point...

Flip it over, and you'll also find it much easier to hold it inverted.


quote:

ORIGINAL: mc1982

i have added alot of downthrust to the point where its silly and still, it climbs when i add throttle and dives inverted. is there any way i can trim this out from the radio? If i move the ailerons a few degrees down would it sort this out?


This is NOT why and how you adjust downthrust.

If the plane is trimmed for level flight, when you are flying it at 80% throttle, then if at the moment you pull off throttle to idle, the plane climbs, you need MORE upthrust ( NOT downthrust!!! ). If the instead when you pull off throttle the plane DIVES you need more DOWNTHRUST!

Whaaat?????

Yes, This is completely counter-intuitive... the idea is to get the plane's attitude NOT to change when power is pulled off.. that the thrust line is in the plane of forward flight...

If in turn the plane still dives suddenly when you pull off throttle ( and the trims are neutral ), then you have either a C.G. or incidence issue... since incidence tends to be defined by the manufacturer's build... C.G. is the most likely culprit...






_____________________________

-
Build em'', fly em'', crash em'' and build more.

(in reply to mc1982)
       Post #: 7

RE: Trimming problem - 9/23/2008 4:55:24 PM   
mc1982


 

Posts: 76
Joined: 12/14/2007
From: limassol, CYPRUS
Status: offline 		k will try and get back with results. Thanks for info.

(in reply to opjose)
       Post #: 8

RE: Trimming problem - 9/23/2008 7:15:08 PM   
opjose



Posts: 6207
Joined: 11/26/2005
From: Poolesville, MD, USA
Status: offline 		Remember that the point of up thrust and down thrust angles, are to fix problems when power is applied or remove.

In effect if you pulled OFF from full throttle, and the plane were to continue moving forward at the same speed, it's attitude should NEVER change assuming no control inputs. ( Conversely pushing full throttle, when at normal flight speeds, should have almost the same result( s ). )

This is what Up-Thrust or Down-Thrust fixes if there is a change.

In reality the plane starts to slow down, so typically the plane holds the same attitude briefly, then begins to start changing attitude as speed decreases.

With proper up or down thrust your plane may still dive all too quickly, but not more or less so than when at throttle.

An excessive tendancy to dive or climb must be seperately addressed.

< Message edited by opjose -- 9/23/2008 7:20:53 PM >

_____________________________

-
Build em'', fly em'', crash em'' and build more.

(in reply to mc1982)
       Post #: 9

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