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Dlg elevator trim


Mark Evans
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Mark Evans

Hi guys,

So, flying the Auri and the spec sheet cg recommendation is 63-64mm. I am currently at 65 but I can visibly see some up trim in my elevator. A dive test shows it just pulls up slightly, not as much as I would have imagined though with the up trim I can see by eye.
 

Flying along I find it flies hands off ok but does nose up, even flying downwind and I have to push on the stick a bit to stop it stalling. But I can park it upwind and it will sit there for a while pretty steady. Maybe I’m just catching the lift up going downwind and nosing up into it? (I find this highly unlikely though) 
 

At this point would you change anything? I thought about trimming the elevator dead flat and getting a level flight via nose or tail weight but I think this would put the cg quite far back from what’s suggested.

When would you shim the elevator to correct any possible decalage issue? 
 

Thanks in advance. 

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44 minutes ago, Mark Evans said:

......it flies hands off ok but does nose up......

If it noses up hands off you need a down-elevator trim change.

44 minutes ago, Mark Evans said:

even flying downwind and I have to push on the stick a bit to stop it stalling.

Most people tend to stall going downwind - you get the impression that the model is flying ' too fast' so you pull back on the stick.

44 minutes ago, Mark Evans said:

but I can visibly see some up trim in my elevator. A dive test shows it just pulls up slightly, not as much as I would have imagined though with the up trim I can see by eye.

Once the model is trimmed for a steady glide, the amount it pulls up in a dive test will depend on the CG position.

The angle of the elevator relative to the fixed part of the tail, in a given trim state, depends on the angle at which the fixed tail is mounted on the fuselage.

If you can see that the elevator doesn’t line up with the tailplane when the model is trimmed for a steady glide you could shim the fixed part of the tail until all looks lined up but don't forget this won't be as much as it seems to need now.  As you rotate the fixed part LE down, TE up, you'll need less upward deflection of the elevator relative to the tail.

If you can see that the elevator doesn’t line up with the tailplane when the model is trimmed for a steady glide you could shim the fixed part of the tail until all looks lined up but don't forget this won't be as much as it seems to need now.  As you rotate the fixed part LE down, TE up, you'll need less upward deflection of the elevator relative to the tail.

50 minutes ago, Mark Evans said:
49 minutes ago, Mark Evans said:

 I thought about trimming the elevator dead flat and getting a level flight via nose or tail weight but I think this would put the cg quite far back from what’s suggested.

If the model is trimmed correctly for a steady glide now, and you move the elevator TE down to line up with the tailplane, you will need to move the CG back a bit.   This  will reduce pitch stability from your current condition.  It will then pull up more slowly from the dive test.
 

 

 

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The above came out slightly muddled with a repeat section sorry - for some reason I can't edit it without the lot disappearing!

I was going to add that, in general, if you have several flap positions set on your model for different flight conditions (variable camber) the elevator will only line up with the tail in one of them.  The drag due to the elevator being slightly deflected is negligible.

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Mark Evans

Thanks Mike, you came across clear even with the duplication.

When flying downwind it’s pretty much hands off, it will nose up by itself, but I find it noses up more by itself down wind than up. 
 

since I’ve never had anyone else fly my planes, how would you describe a more rearward cg/neutral plane? Will it just be very pitch sensitive or will it really be wandering on roll also and require constant corrections?

I may already be just on the side of stable and up the up trim just looks worse than it actually is, especially since it’s not aggressively pulling up from a dive test, just barely reaches level from starting a 45 degree shallow dive at 20-25m height (all I dared with a loose vert yesterday) I like how it flies, just I was re-watching my Bruce Davidson videos and when he talks of trim I wondered if I could really improve mine. 
 

Or I fly my planes a bit fast and slowing it down some more would should the true extent of my up trim. I get it’s a balancing act and the dive test is more to show the state of your trim and not necessarily the stability and cg of the plane.

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If you like how it is flying, then leave it alone.  The elevator trim will cause next to no additional drag.   You generally need different trim for different camber settings, so you are not going to have the elevator always lined up with the tail anyway. 
 

Just a note, from the perspective of the glider, there is no upwind or downwind, only from your ground based perspective.   So there is no trim difference required. Hence MikeF’s comment about people often stalling when flying downwind pulling up to ‘slow’ it down. 
 

 

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Mark Evans

Thanks Mike, currently I’m leaving it, I find it flies hands off a lot yet even I can still spot lift with it so I feel I shouldn’t mess with it anymore. 
 

 

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3 minutes ago, Mark Evans said:

I may already be just on the side of stable and up the up trim just looks worse than it actually is, especially since it’s not aggressively pulling up from a dive test, just barely reaches level from starting a 45 degree shallow dive at 20-25m height (all I dared with a loose vert yesterday) I like how it flies,

That seems to be pulling up quite a lot, if your estimates are correct. You may have too much up trim. It should not be pulling up on its own in level flight. It should continue for a fair distance maintaining its attitude and speed.   You may need a relatively calm day or at least one with constant wind speed to check this properly. Gusts will effectively change the flying speed momentarily and affect attitude. 
 

Remember, the dive test is pointless unless the glider is trimmed properly  for level flight. You can have a rearward cg, but have too much up trim and in a dive test it will pull up. Does  that mean it is stable/forward CG? No, of course not. 

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Mark Evans

I have wondered if I trimmed to fly too fast, and slowing the plane down will just show how much up trim I actually need to carry. On a calm day, just how slow do you trim your plane to maintain level flight? 
 

The thing that concerns me is I feel once I have finished that my cg will be far from the suggested range. 

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10 minutes ago, Mark Evans said:

The thing that concerns me is I feel once I have finished that my cg will be far from the suggested range. 

Don't worry about that.  There are no points for where your cg is and I have never heard of manufacturers coming around to check...  If it suits your flying style it's right.

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Mark Evans

I’ve clearly got dodgy eyes, another look at it today, and putting a credit card across the hinge shows I’m carrying only the slightest amount of up trim in my cruise mode. Without the card it actually looks more than it is, probably no wonder I haven’t been able to glue a vert on straight in the past without a square lol. 
 

The dive test from 20-25m to check elevator trim has it only just about pulling up before it would hit the ground. And my Zoom mode only takes a very few clicks of down to climb straight after launch so I think I’m pretty close.

As for it nosing up now and then, I’m probably just expecting it to fly hands off for longer than it should and the air wasn’t exactly smooth when I last flew. I like the way if flies so going to stop messing.

 

Thank you for the help guys. 

 

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StraightEdge

Looks like you're pretty close Mark but this windy weather isn't going to help fine-tuning anything.  Wait now till next week when we're due some more calmer weather! 🙂

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Mark Evans

While I started the topic on trim....

So since I am flying Taranis and have a 'zoom' mode, I don't need my Speed mode to also function as Zoom.

My Auri wing is molded in thermal, ailerons flush with tab is my cruise mode and trimmed to fly nice and level and a nice steady speed. Tests today even with a gently breeze shows it will fly the length of my field with very little corrections. If it matters then a dive test produces a very gently pull up in this mode.

How would you now trim speed mode? First I thought about trimming it so it doesn't pull out the dive test, but switching from cruise to speed produced a sharp nose down attitude so I wasn't sure if I was correct doing this. Or do I trim so in level flight it's just noticeably faster over cruise, even if this still results in a slight pull up in the dive test?

Or maybe the above description of my current trim of my modes suggests I can still lose a touch of nose weight?. 

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Hi Marc, I’ll chime in with two pennorth , you only need to do the dive test to set your cg. Normally this is done in cruise , ie with the bottom of the wing flat, you don’t do it in ALL modes. 
 

when that’s hands of level and barely pulls up in a dive test then that’s about all you with the cg. (That’s not doesn’t hit the ground pulls up but has a slight upward curve. If I put mine into a 45deg dive  from 60m it would plough into the ground at about 40deg)

speed mode is used mainly for clearing sink where you need speed at the expense of altitude. ( I use my cruise and lean on the down elevator a bit)

I think mike said it clearly earlier, thermal is max lift at the expense of speed so you trim your elevator for that best speed, if you put it into a dive it WILL pull up.

hope that’s clear ish ??? Cg set in cruise, dive test in cruise not the others

dave h

 

ps just been doing this with Falcon and found out why I’ve struggled these years with a plane that noses up when turning into wind. Too forward a cg. 

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Mark Evans

Thanks Dave, today ive been doing the same, using my cruise mode for general flying around and climbing in lift if the air is too active for my thermal mode. To come back I have just been leaning on the stick a bit.

I can’t throw over 40m (35 average) but from 60m my plane would be level by the time it got pretty low so I think I can loose a touch more nose weight. This would then require less down in my speed mode for going fast. I have started  switching into speed just for a bit of acceleration then going back into cruise 

Thanks Dave.

 

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17 hours ago, Dave H said:

 

ps just been doing this with Falcon and found out why I’ve struggled these years with a plane that noses up when turning into wind. Too forward a cg. 

This troubles me.   The glider doesn't 'know' which way the wind is blowing.  When flying in wind, the glider is drifting with the 'river' of wind.  The effect you notice is probably due to your incorrect perception of the air speed of the glider as the ground/perceived speed changes and how you operate the elevator .   If changing the CG helps you, then that is great, but try to be aware of what is actually going on.   It is easy to suffer this incorrect perception, I still do, but I try to work on it.   Gust of wind can of course have an effect on the glider, but that is not what you are describing there.   

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19 hours ago, Mark Evans said:

My Auri wing is molded in thermal, ailerons flush with tab is my cruise mode...

I am confused by this.  If ailerons flush with tab is cruise position, then surely the wing is molded in cruise as the tab is still part of the aileron when in the mold?  

 

19 hours ago, Mark Evans said:

How would you now trim speed mode?

After have set you CG position, you can then work on the trim in all other modes.  How to do this?  Put it in the mode you want to trim, fly it, adjust the trim until it does what you want.  For me, this would usually be flying pretty much straight and level.  

 

17 hours ago, Dave H said:

Hi Marc, I’ll chime in with two pennorth , you only need to do the dive test to set your cg. Normally this is done in cruise , ie with the bottom of the wing flat, you don’t do it in ALL modes. 

Mostly correct.  But as far as I know, bottom of wing flat is usually speed mode.  This would put the ailerons slightly up from the central tabs as most DLG wings are moulded in cruise position, which has a small amount of positive camber.

 

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People use the term 'cruise mode' - I'm not sure what they mean or if they all mean the same thing!

In my understanding, the terms 'flight modes' (or flight phases) describes several sets of positions for the flapperons and elevator.  Each mode is then selectable in flight by a switch or switches on the transmitter.

I use various modes or phases - this is what they are intended to achieve.

Pitch up.  An efficient pull up at high speed to the required climbing angle

Climb.  Straight climb at a steep angle

Glide at best glide angle (best L/D, best range)  I lean on forward stick in this mode to escape sink.

Glide at minimum sink (slower than best L/D)

Even slower glide to fly in strong steady lift

Which one do you guys call 'cruise'.

If I had a 'speed mode' it would be the best glide angle mode plus a tad of down elevator and a smidge of up flapperon.

StraightEdge posted above while I was typing - looks like 'cruise' is best L/D in that case - and the wing is 'moulded in min sink' ?

 

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I dive test In my best L/D mode.  In this mode, my pitch trim indicator is set to zero by moving the servo centre.  All the other modes are then relative to this.  Once the model is initially trimmed, I do not alter trim in any other mode independently.  Day to day elevator variations are sorted In the best L/D mode by changing the servo centre and this sorts all the other modes at the same time.

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