What is the cause of this?

[Nigel Heather]

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 Cotsford]

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

[Josip Vrandecic -Mes]

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 Heather]

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?

[IanR]

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 8]

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 - Moderator]

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 Heather]

 

 

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 R]

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.

[PatMc]

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 Heather]

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 R]

General convention is that you've applied down trim.

Otherwise everyone will be confused!

[PatMc]

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 Stuff]

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 Carpenter]

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 8]

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.

[PatMc]

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 Walsh]

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 Heather]

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 Fry]

 

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

[The Wright Stuff]

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 - Moderator]

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 Fry]

We are complicating this. In theory. As BEB says, do the flying tests. Establish the problem.

[John Tee]

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

john

[Ron Gray]

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