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What is the cause of this?

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Nigel Heather14/09/2018 10:12:37
53 forum posts
7 photos

Firstly, let me say that I have the plane flying nicely, this is just a minor niggle that I would like to understand and sort out if possible.

This is an Acrowot Foam-e. Maidened yesterday, extremely well behaved, not as fast as I thought it might me.

the one issue, which I have addressed at the moment with 14 clicks of up elevator is that with neutral elevator it would climb under the slightest power - a steady climb not a ballooning.

So what might be the cause and what (if anything) can I do about it. My thoughts

Tail Heavy - CoG is at 75mm as per instructions. Besides it is very behaved otherwise. Only thing is that there is some online debate as UK (Ripmax) Acrowot specifies 75mm and the US (Flyzone) Acrowot specifies 62mm

Incidence - either wing or tailplane, but they are both properly seated.

Thrust line - the mount is embedded in the foam so not much I can do about that

Feature - maybe all acrowot foam-e models do this.

Are there any flight tests that I can do to identify the cause?

Cheers,

Nigel

Bob Cotsford14/09/2018 10:19:31
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7410 forum posts
419 photos

search for 'trimming' in the keyword field at the top of the page, one return here

Josip Vrandecic -Mes14/09/2018 10:23:08
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2900 forum posts
246 photos

Hi Nigel , it's always what looks impossible to change... and it's a thrust line. There must be a way and if you can not, call a colleague from the club ... someone will know how to set the motor to -2 dgrs down and 2 dgrs right ... aprox.

All the best and good flight.

Note:Of course this is just my opinion.

Nigel Heather14/09/2018 10:37:12
53 forum posts
7 photos

Posted by Josip Vrandecic -Mes on 14/09/2018 10:23:08:

Hi Nigel , it's always what looks impossible to change... and it's a thrust line. There must be a way and if you can not, call a colleague from the club ... someone will know how to set the motor to -2 dgrs down and 2 dgrs right ... aprox.

All the best and good flight.

Note:Of course this is just my opinion.

When I say the thrust line cannot be changed - what I mean is that the motor is attached to a metal bracket that is sandwiched between the two halves of the foam fuselage when it is assemble in China.

There is no way of getting at it other than cutting the fuselage in half.

I guess you could try and put washers between the mount and the motor but that would be pretty trucky to do.

I’ve also read that the thrust line wanders over time - so maybe I just have to accept that as a limitation of this type of foam design.

Looking at my model there is definite right thrust but no discernable up or down thrust.

Is there a way of testing whether it is thustline or CoG?

IanR14/09/2018 10:49:14
753 forum posts
4 photos

Nigel I had exactly the same problem - it was so bad that I gave up on it.

When you say you solved the issue with 14 clicks of up trim, did you really mean down trim?

Ian

J D 814/09/2018 10:55:00
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914 forum posts
57 photos

Hi Nigel, on a similar model I had I mixed in some down elevator in the TX as the power goes up,just a case of experimenting until you have the right amount. Was a better option than chopping the airframe about.

Biggles' Elder Brother - Moderator14/09/2018 10:56:01
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Two possible explianations:

1. Thrust line too high

2. Wing incidence too positive relative to the tailplane. Alternatively too much negative incidence on the tailplane - the decalage is wrong in other words!

The fact that it manifests itself under power suggests 1. But it doesn't rule out two!

A test you can perform. Go nice and high and put it in a power off steep dive. If the CoG is sorted it should track down reasonably straight, or show a slight tendancy to shallow or tuck. If it does track reasonably true the decalage would seem to be OK and its probably the thrustline. However if the dive shows a definite and strong tendancy to shallow - ie it wants to pull out more and more strongly as the dive acelerates - that would suggest an incidence problem.

BEB

Nigel Heather14/09/2018 11:37:29
53 forum posts
7 photos

 

 

Posted by IanR on 14/09/2018 10:49:14:

Nigel I had exactly the same problem - it was so bad that I gave up on it.

When you say you solved the issue with 14 clicks of up trim, did you really mean down trim?

Ian

Think just different terminology for the same thing. I’m expressing it as I have to move the stick up to level the flight, so I have to click the upper of the two trim buttons.

So I am moving the elevator stick (trim button) up to push the nose down.

Will do some more experiments over the weekend weather permitting.

Cheers,

Nigel

Edited By Nigel Heather on 14/09/2018 11:46:45

Edited By Nigel Heather on 14/09/2018 11:47:11

Edited By Nigel Heather on 14/09/2018 11:48:07

Nigel R14/09/2018 11:37:45
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1554 forum posts
336 photos

First port of call, dive test as BEB says.

That will tell you if the thrust line is badly out.

If it is, it'd be worth getting some washers in. Cyano can be very useful to keep them in place while you re-attach the motor!

Last thing I would say is that foamies are a bit of an approximate sort of device, somewhere between an aircraft and a bath sponge.

PatMc14/09/2018 11:52:52
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3801 forum posts
479 photos

Assuming that, as IanR mentioned, you mean you solved the problem with down trim then it sounds as if it's simply nose heavy. Try moving the cg back in small increments test flying, at each change, to check that elevator doesn't become too sensitive.
Or without moving the cg try flying inverted with & see what trim change is required to fly S&L as power is increased. If up (in the normal upright flying sense) then it would confirm that it's nose heavy.

It is quite possible that the problem can't be cured by moving the cg far enough back & still remain flyable in which case I'd try the Tx mix of down trim with increased power as suggested by JD8. If your Tx has the facility use a small time delay (about 0.25 - 0.5 second) between throttle opening & trim action or be ready for the down trim to take effect before the power increase does, especially on low passes.

Nigel Heather14/09/2018 11:59:16
53 forum posts
7 photos

Posted by PatMc on 14/09/2018 11:52:52:

Assuming that, as IanR mentioned, you mean you solved the problem with down trim then it sounds as if it's simply nose heavy. Try moving the cg back in small increments test flying, at each change, to check that elevator doesn't become too sensitive.
Or without moving the cg try flying inverted with & see what trim change is required to fly S&L as power is increased. If up (in the normal upright flying sense) then it would confirm that it's nose heavy.

It is quite possible that the problem can't be cured by moving the cg far enough back & still remain flyable in which case I'd try the Tx mix of down trim with increased power as suggested by JD8. If your Tx has the facility use a small time delay (about 0.25 - 0.5 second) between throttle opening & trim action or be ready for the down trim to take effect before the power increase does, especially on low passes.

Can’t see it being nose heavy, surely that would cause it to dive rather than climb. Think it is a terminology issue - when I said up trim, I meant that I pushed the uppermost trim button, like pushing the stick up. So when I said UP I’m referring to the stick input rather than what happens to the nose.

Cheers,

Nigel

Nigel R14/09/2018 12:13:00
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1554 forum posts
336 photos

General convention is that you've applied down trim.

Otherwise everyone will be confused!

PatMc14/09/2018 12:25:26
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3801 forum posts
479 photos
Posted by Nigel Heather on 14/09/2018 11:59:16:

Posted by PatMc on 14/09/2018 11:52:52:

Assuming that, as IanR mentioned, you mean you solved the problem with down trim then it sounds as if it's simply nose heavy. Try moving the cg back in small increments test flying, at each change, to check that elevator doesn't become too sensitive.
Or without moving the cg try flying inverted with & see what trim change is required to fly S&L as power is increased. If up (in the normal upright flying sense) then it would confirm that it's nose heavy.

It is quite possible that the problem can't be cured by moving the cg far enough back & still remain flyable in which case I'd try the Tx mix of down trim with increased power as suggested by JD8. If your Tx has the facility use a small time delay (about 0.25 - 0.5 second) between throttle opening & trim action or be ready for the down trim to take effect before the power increase does, especially on low passes.

Can’t see it being nose heavy, surely that would cause it to dive rather than climb. Think it is a terminology issue - when I said up trim, I meant that I pushed the uppermost trim button, like pushing the stick up. So when I said UP I’m referring to the stick input rather than what happens to the nose.

Cheers,

Nigel

If a model is trimmed to fly S&L when it's nose heavy it will have to have more difference in incidence between wing & elevator. This means that any increase in power/speed increase the lift causing the nose to rise.
This is the basis for the well known, but apparently not well understood, "dive test". Have a look at the "dive test" explanation about half way down this page.
The test was concieved with gliders in mind & it's not necessary to actually dive a power model as the only reason for the dive is to cause an increase in speed, increasing opening the throttle of a power model has the same effect.

The Wright Stuff14/09/2018 13:09:35
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1306 forum posts
225 photos
Posted by PatMc on 14/09/2018 12:25:26:

If a model is trimmed to fly S&L when it's nose heavy it will have to have more difference in incidence between wing & elevator. This means that any increase in power/speed increase the lift causing the nose to rise.
This is the basis for the well known, but apparently not well understood, "dive test". Have a look at the "dive test" explanation about half way down this page.
The test was concieved with gliders in mind & it's not necessary to actually dive a power model as the only reason for the dive is to cause an increase in speed, increasing opening the throttle of a power model has the same effect.

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...

Colin Carpenter14/09/2018 13:15:30
508 forum posts
34 photos

My original Acrowot had this tendency and I put washers on the motor mount to add down thrust . One washer ! The aircraft then flew straight and level when power was applied from low speeds. The second fuselage needed lead at the tail as well ! The foam slits the motor mount sits in all degraded after 100 or so flights ! Colin

J D 814/09/2018 13:57:29
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914 forum posts
57 photos
Posted by Patch on 14/09/2018 11:52:52

It is quite possible that the problem can't be cured by moving the cg far enough back & still remain flyable in which case I'd try the Tx mix of down trim with increased power as suggested by JD8. If your Tx has the facility use a small time delay (about 0.25 - 0.5 second) between throttle opening & trim action or be ready for the down trim to take effect before the power increase does, especially on low passes.

On my TX the down elevator mix comes in progressively as the throttle stick is moved up so there is no sudden change of trim. Also I can switch it out if I want. like Patch say's handy for those low passes when you want it to go up at the end of the run.

PatMc14/09/2018 14:04:32
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3801 forum posts
479 photos
Posted by The Wright Stuff on 14/09/2018 13:09:35:
Posted by PatMc on 14/09/2018 12:25:26:

If a model is trimmed to fly S&L when it's nose heavy it will have to have more difference in incidence between wing & elevator. This means that any increase in power/speed increase the lift causing the nose to rise.
This is the basis for the well known, but apparently not well understood, "dive test". Have a look at the "dive test" explanation about half way down this page.
The test was concieved with gliders in mind & it's not necessary to actually dive a power model as the only reason for the dive is to cause an increase in speed, increasing opening the throttle of a power model has the same effect.

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 with you.
Of course testing if a forward cg contributes to the problem independant of the thrust line is also achieved by flying inverted as per suggested in my first post.

Shaun Walsh14/09/2018 15:05:00
43 forum posts
A while ago I had reason to take the motor off my Riot following unplanned contact with terra firma which meant that I had to straighten out the motor mount. I noted that washers were used to give down thrust and right thrust and made sure that they went back in the same position. The Riot flies straight and level with no tendency to climb on powering up so I guess the designer got the angles right.
Nigel Heather14/09/2018 15:06:40
53 forum posts
7 photos

I don’t think CoG is an issue.  It is balanced at 75mm exactly as per the instructions and once the climb is trimmed out it behaves very nicely.

A nose heavy plane would resist climbing.  And it doesn’t have any of the nasty vices of a tail heavy plane.

Cheers,

Nigel

Edited By Nigel Heather on 14/09/2018 15:10:33

Don Fry14/09/2018 15:17:12
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2587 forum posts
30 photos

 

The position of a balance point is not written in stone, the instructions generally give a safe position, not the best position. It's very likely PatMc is spot on, a too far forward, but safe manufacturers position, giving a climb under power. Move balance back, the climb disappears.

But bottom line is, try the tests given above, no point in debating it until you establish what is wrong.

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

 

Edited By Don Fry on 14/09/2018 15:23:19

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