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Trimming Help

Old 08-14-2009, 03:30 PM
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Buck Rogers
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Default Trimming Help

I need a bit of help trimming my Ripmax Spit. There are a few problems. First off I know one side (the right side) of the wing is heavier than the other by about 10 grams, this could be part of the problem.


Anyway, if I trim the plane to fly straight at about 60% throttle and then cut the throttle it dips the left wing. This is kind of counter intuative because I would expect the plane to dip the left wing because of the torque from the motor - usually if it is a thrust line issue I would expect the left wing to go up then cutting the throttle.

Also due to the extra weight I have to dial in a bit of left aileron so it seems this be becoming more effective when the throttle is cut.

So - could the problem be with the thrust line or could it be something else (rudder maybe). Please help.
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Old 08-14-2009, 04:19 PM
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Sabrehawk
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You must get that lateral balance corrected. It's always been a no brainer to me that if any aileron trim is required then something isnt right, either the wing is warped, or there is a lateral inbalance. And any right or left trim put in will only correct the flightpath at one particular throttle setting. As soon as its reduced, or increased you again have to re-trim.

Bottom line is, both the plane's longitudal, and lateral balance must be right for the plane to fly right.
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Old 08-14-2009, 04:35 PM
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Buck Rogers
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Yeah, I was assuming I would have to correct the problem with the extra weight on one side of the wing. I was wondering if there was something else wrong as well.
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Old 08-14-2009, 04:58 PM
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JetPlaneFlyer
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Originally Posted by Buck Rogers View Post
Anyway, if I trim the plane to fly straight at about 60% throttle and then cut the throttle it dips the left wing. This is kind of counter intuative because I would expect the plane to dip the left wing because of the torque from the motor -
Buck,
I think you may have got mixed up here? You say it dips the left wing but then you go on to say say this is 'counter intuitive' because you expected the left wing to dip.. i.e. if it did what you expected then why is it counter intuitive?

So which wing dips when you chop the throttle?

Anyway....First thing would be to sort out the lateral ballance, whichever wing is 'dipping'. This may not be the direct cause but you need to eliminate it anyway. Next is check that all flying surfaces are warp free and 'true' and there is no excessive slop in the linkages.

Thrust line could well be part of the problem. If after the lateral ballance is fixed and everthing is checked to be straight and true it still drops the left wing when you cut the throttle then I'd suspect too much right thrust (is there any right thrust?)

Steve
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Old 08-14-2009, 05:08 PM
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One more point...
When you cut the throttle you are not letting the speed get low or pulling back on the stick so that it stalls and drops a wing are you (a.k.a. tip-stall)? The elliptical wing shape on Spits can mean that they have a quite nasty wing drop. I'm not sure if the Ripmax version suffers from this but many Spit models do.

If the wing drop is sudden and quite violent then this would indicate a tip stall. If it's more of a gentle controlable turning effect then that could be thrust line or lateral balance issues.

There is not a lot you can do about tip stall except avoid pulling back so hard on the elevator and or letting the model get too slow.. Unless you could twist in some washout?


Steve
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Old 08-14-2009, 05:08 PM
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Sky Sharkster
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Default Trimming For Straight flight

Hello Buck,
Sabrehawk is right. If your plane isn't correctly balanced, you will never get it to fly straight. Control surface response is airspeed-related, an example is the slow-flying 3D models with 50% elevator movement. It will do a loop within a very tight radius, but wouldn't even fly if that deflection were made at high speed.
So, the left aileron works to counterbalance the weight differential at normal powered speed, but is too much at a different speed.
Balancing the aircraft in the lateral axis is just as important as balancing it in the longitudinal axis. Only then will you be able to correctly trim the model.
Good Luck!
Ron
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Old 08-14-2009, 11:13 PM
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Buck Rogers
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Originally Posted by JetPlaneFlyer View Post
Buck,
I think you may have got mixed up here? You say it dips the left wing but then you go on to say say this is 'counter intuitive' because you expected the left wing to dip.. i.e. if it did what you expected then why is it counter intuitive?

So which wing dips when you chop the throttle?

Anyway....First thing would be to sort out the lateral ballance, whichever wing is 'dipping'. This may not be the direct cause but you need to eliminate it anyway. Next is check that all flying surfaces are warp free and 'true' and there is no excessive slop in the linkages.

Thrust line could well be part of the problem. If after the lateral ballance is fixed and everthing is checked to be straight and true it still drops the left wing when you cut the throttle then I'd suspect too much right thrust (is there any right thrust?)

Steve
Yes, what I was trying to say is that normally (if no right thrust was set) the torque of the motor will cause the plane to dip the left wing. Therefore if there was no right thrust set and the plane was trimmed to counter the motors torque I would expect the right wing to dip when the motor was cut. On my spit it is the opposite - the left wing dips when the motor is cut.


I have now added weight to the left wing so the plane is balanced now - I will see how it goes but it seems to me that this could have made the problem worse.


jetplaneflyer - its definitely not tip stall, if I leave the right stick centred and cut the throttle the left wing dips slightly and the plane goes into a turn. It isn't a violent turn and the plane still has enough speed. Perhaps there is too much right thrust, there is some right thrust built into the airframe so I'll have a look at that.
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Old 08-14-2009, 11:43 PM
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Sabrehawk
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Well I'll stick to my guns and say correct the lateral balance. And as far as torque is concerned, this is something that mainly is present at take off under full power, and generally goes away once flying speed is acheived. So its really only an issue during the take off roll, and the initial lift off.
It can of course affect the plane if you stall it, and power is on too but rarely do we suffer power on stalls.

And I told myself I wouldnt, but..............."Tip Stall" doesnt exist. It's a myth. Dont belive me, look it up in real aviation, and you wont find it. Google it, and it only shows up in R/C forums.

As for thrust line, this is something that should be "built in" much like angle of incidence and we as pilots need not mess with it. That is unless its a plane of our own design. Any of today's ARF's and RTF's have this all worked out and built in to the firewall and or motor mount.
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Old 08-17-2009, 08:15 AM
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Larry3215
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Our planes do a lot of things full scale planes cant or dont do - like harrier or hover or any of the post stall maneuvers.

They also fly at completely different renyolds numbers and wing loadings so the rules of full scale flight dont necesarily ALL apply to us.

As far as "tip stall" goes, rc planes do something that sure looks exactly like a wing tip is stalling. The cure is generally to add some washout to the wing tip - which will make it stall later and which generally cures the problem. This is a common building practice.

My very first plane was a Gentle Lady glider with a polyhedral wing. I did NOT add the recomended washout when I covered it. It "tip stalled" like crazy when it slowed down enough to land. When my instructor saw what it was doing he looked at the wings and told me to go home and add the wash out.

No more "tip stall". It would now fly much slower than before and still be in controll.

If it walks like a duck why not call it a duck?

I agree with the others -

1)fix the latteral balance
2)look for warped wings or warped aileron controll surfaces or sloppy linkages
3)check thrust line
4)be sure your rudder is straight and 90 degrees to the wing and your elevator is straight and is perfectly paralell to the wing.

All of those items can contribute to inducing roll and require trim to correct.
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