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What is the cause of this?

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The Wright Stuff14/09/2018 15:27:19
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Posted by Don Fry on 14/09/2018 15:17:12:

Edit. A nose heavy plane does resist the nose coming up. That's why you need some up trim to keep it up. And the up trim gets more uppy when the power comes on, or it increases speed because of there is more wind, and force acting on the elevator

Since no up-trim has been added in order to produce level flight, in fact quite the opposite, a bucket load of down-trim has been added to ensure straight and level flight, I would agree with Nigel that it is unlikely to be the CoG too far forward.

Biggles' Elder Brother - Moderator14/09/2018 15:46:26
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Posted by The Wright Stuff on 14/09/2018 13:09:35:

It can still be useful to separate the effects of CoG / elevator trim versus thrust lines. If you want to check the CoG independently of knowledge of whether the thrust line is correct, you need to build up speed without using the throttle...

...a dive, of course, is quite a convenient way to do this. Of course, it doesn't have to be vertical, a 'dive' can be really quite shallow, provided it produces the required increase in speed...

I agree, that is precisely why I suggested the power-off dive test. Assuming the CoG is basically OK, it allows us to accelerate the aircraft without using the motor. In that way we eliminate thrust line effects and any "bad behaviour" is basically down to incidence. If no bad bad behaviour is seen - then it would look more like thrust line problems. All this assumes of course that the CoG is basically in the right place - ish!

BEB

Don Fry14/09/2018 16:06:28
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We are complicating this. In theory. As BEB says, do the flying tests. Establish the problem.

John Tee14/09/2018 16:21:12
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nobody has picked up on "14 clicks of trim". Seems a hell of a lot of trim -up or down

john

Ron Gray14/09/2018 16:25:23
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I recently completed a build of an ARTF balsa leccy  Wot4, vastly overpowered using 5 or 6s. C of G was checked by inverted 45 degree climb under half throttle and battery position adjusted accordingly. In the end I put 15g weight just in front tail wheel before I was happy. However straight and level flight required up elevator which I was not happy with so did the power off dive test the result of which was to raise the wing LE by about 2mm. Now flies level with no elevator trim and climbs slightly under power which I’m happy with. Basically this backs up what BEN has said about flight tests are the only real way of checking things out.

Edited By Ron Gray on 14/09/2018 16:25:57

Ikura14/09/2018 17:02:31
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186 forum posts

14 clicks of up elevator trim? Something is seriously wrong with the set up if you need that much. I have never heard of a model needing so much up trim.

As others have said, find the problem and then start from there or you will always be flying a model that is not set up correctly and never really get the best out of it.

My guess is the model is nose heavy, and the recommended CG may not be correct. Move the battery back a bit at a time and I bet you solve the problem.

Martin Harris14/09/2018 17:43:54
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Posted by John Tee on 14/09/2018 16:21:12:

nobody has picked up on "14 clicks of trim". Seems a hell of a lot of trim -up or down

john

It does rather depend on whether there was (a) much total travel set, (b) trim click increments and (c) starting position, although it is likely there is an abnormal factor at play.

onetenor14/09/2018 20:53:58
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I know if I have a nose he..avy glider it will dive without input. It's nose heavy Move the CofG back bit by bit

Biggles' Elder Brother - Moderator14/09/2018 21:37:31
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Whoa! We ae in danger of getting very confused here!

I know the OP says he added "up elevator trim" but he subsequently explained that by that he means he moved the trim button up! So to the rest of us - that's down elevator trim!

This aeroplane doesn't dive, it climbs excessively when power is applied. So its a power-on nose-up tendancy we have to deal with. Now as we've said (and I think agree) that could be due to three things; firstly it could be an incorrect CoG (but the OP says the aircraft generally flies well and the CoG is spot on the recommended point, which might be conservative but is unlikely to be way out), secondly it could be a misaligned thrust line, or finally it could be an incorrect relationship between the incidence of the wing and tailplane. Puting the CoG to one side for now the other two can be separated by the power-off dive test - probably!

BEB

PatMc14/09/2018 22:55:34
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If the down trim that the OP has applied has cured the excessive climb on high power & the model also generaly flies well at any power setting with this trim then I've misunderstood the problem.
I don't see that carrying out the dive test or any other flying checks would serve any purpose.

In fact the only problem would seem to be aesthetic otherwise the model could be flown with the down trim as a permanent feature with no great ill effect. From an aesthetic POV the cure would be to cut away the tailplane then re-mount it at a reduced difference in incidence wrt the wings [probably 0-0 would be best] but I'm not sure if the cure would be worth the effort or indeed practical with this model.

Nigel Heather15/09/2018 07:16:55
53 forum posts
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Thanks for the replies and apologies for the confusion caused for the way I described the trim applied. So in summary

With all surfaces set to neutral, the plane climbs under power, shallow climb with minimal power, gets steeper with more power, but not ballooning.

Applied 14 clicks of down trim (now using correct terminology) and that has resolved the climbing and the plane flies very nicely (although not tried inverted or any aerobatics yet).

So I could just leave it at that. But as I said in my first post, intrigued to know the cause and resolve it if possible.

I will try the dive tests.

And fair point about the 14 clicks - is that a lot - depends on the radio I guess - will look at the plane today and report back with the actual deflection of the elevator.

Cheers,

Nigel

Denis Watkins15/09/2018 07:36:26
2988 forum posts
141 photos

Clicks can be subjective Nigel,

Modern equipment clicks, can be minute, and 14 could amount to half a mm

IanR15/09/2018 08:11:39
753 forum posts
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Nigel, this could be an incidence problem. Have a look at Stevo's thread here **LINK**

Ian

Frank Skilbeck16/09/2018 11:23:05
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Posted by Nigel Heather on 15/09/2018 07:16:55:

And fair point about the 14 clicks - is that a lot - depends on the radio I guess - will look at the plane today and report back with the actual deflection of the elevator.

Cheers,

Nigel

Or setting on the radio, a couple of mine have the ability to adjust the trim step, if it's set to a very small step then you'll need a lot of clicks smiley to see much effect.

Martin Harris16/09/2018 11:45:22
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Posted by Nigel Heather on 15/09/2018 07:16:55:

Thanks for the replies and apologies for the confusion caused for the way I described the trim applied. So in summary

With all surfaces set to neutral, the plane climbs under power, shallow climb with minimal power, gets steeper with more power, but not ballooning.

Applied 14 clicks of down trim (now using correct terminology) and that has resolved the climbing and the plane flies very nicely (although not tried inverted or any aerobatics yet).

So I could just leave it at that. But as I said in my first post, intrigued to know the cause and resolve it if possible.

I will try the dive tests.

And fair point about the 14 clicks - is that a lot - depends on the radio I guess - will look at the plane today and report back with the actual deflection of the elevator.

Cheers,

Nigel

Some interesting information here - most trainers and intermediate models could follow this description of behaviour... (I don't recall flying a foamy Acrowot so I don't know if it resembles the original Acrowot's flying characteristics very closely).

I'd be looking for a decent trainer/advanced trainer to fly straight and level at mid throttle, to adopt a positive climb at full throttle and a steady descent when throttled back, in order to teach good throttle management from the start. Too many people trim for level flight at full throttle and only touch the left stick* when they think about landing!

I've advised people to add downthrust for excessive power change behaviour, but this can typically be when a model trimmed for level flight can be looped using throttle alone!

 

*If they fly "proper" mode... cheeky

Edited By Martin Harris on 16/09/2018 11:54:35

Nigel Heather16/09/2018 11:53:08
53 forum posts
7 photos
Checked the actual down trim - although 14 clicks it is only between 1.5mm and 2.0mm deflection in real terms.
Biggles' Elder Brother - Moderator16/09/2018 15:58:13
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If you decided to do the power-off dive test remember to put the trim back at neutral first or your won't get a true result!

BEB

PatMc16/09/2018 16:10:51
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Posted by Biggles' Elder Brother - Moderator on 16/09/2018 15:58:13:

If you decided to do the power-off dive test remember to put the trim back at neutral first or your won't get a true result!

BEB

No!
The trim should be where the model flies S&L at cruise power.
But I thought we'd established the model is fine with a permanent 2mm down trim or if the tailplane was re-set to give the same, which makes the dive test unnecessary.

It would be helpfull to find out how the much down pressure is needed to fly S&L inverted with the new (2mm down) trim.

Nigel Heather16/09/2018 18:00:31
53 forum posts
7 photos

Bit of a set back.

Took it down to the field and flew even though I thought the wind conditions were a bit gusty. Sure enough the plane flew okay but was getting blown around a bit.

So brought it into the land and on approach a gust pushed it down early into the uncut grass. Thought it had landed okay but had ripped the undercarriage mount out of the foam. Also broke the prop.

So needs a bit of gluing but nothing too serious - replace the prop which is no bad thing because the one in the kit flexes a lot.

Cheers,

Nigel

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