Jump to content

What Would Cause A Plane To Want To Pitch Up


Recommended Posts

I have an Acrowot Foam-E that has never been quite right.  Recently, I had a crash which did some repairable damage but not the characteristic is even worse.

 

The problem, even from new, is that it would pitch up and climb even at low throttle as if elevator was being applied.  It needed a large amount of down trim to get it flying level.  Once trim it would fly pretty well.

 

Since the crash, the characteristic is even worse, the pitching up was so extreme that the plane was actually looping, I was very lucky that I had an assistant to dial in the down trim while I fought the controls to stop it from crashing.

 

What I have checked.  The control surfaces all start off perfectly aligned.  With the down trim the elevator is very noticeably deflected at centre stick.  The CoG is good, in fact I have even moved it forwards and that makes no difference.

 

The wings and tail plane are seated properly.

 

What could cause this, the only thing I can think is wing incidence or motor thrust line.  If this were only happening since the crash I would put it down to crash damage but this has been happening since new - although it is more pronounced since the crash.

 

Appreciate any ideas before I give up with it.

 

Cheers,

 

Nigel

 

 

Link to comment
Share on other sites

Posted (edited)
10 minutes ago, Brian Cooper said:

Check the tailplane incidence. 

It sounds like it isn't in line with the centre line of the fuselage. 


Thanks, will do that.  Checked it was seated properly and it is.  So if the incidence is wrong (and that does sound like a possibility) then  the error is built into the foam moulding and will need some after market mods (add shims).

 

Trying to get my head round this - so have I got this right - looping would be caused by the leading edge being lower than the trailing edge so I would want to wedge up the leading edge ?  Is that correct?

 

To be honest, I don’t have any means to accurately or reliably measure the incidence so I feel I will have to assume that is the cause and add a shim under the tail plane and see what happens.

Edited by Nigel Heather
Link to comment
Share on other sites

Posted (edited)

1. Dial in whatever elevator trim you need for level cruising flight.

2. From cruise, push gently forward on the stick into a shallow dive.

3. Release the stick and observe behaviour.

 

If, as I expect, the model pulls out of the dive and zooms up, then move the cg back a bit and try again. You're aiming to have the model level out slowly from the dive without zooming up.

 

There's no point fiddling with thrust lines until the cg is right.

 

Unless the model is flying in a distinct nose-down attitude, there's no need to change the wing incidence.

 

If the elevator ends up with visible down trim in it, so be it. 

 

Good luck,

 

Trevor

Edited by Trevor
Link to comment
Share on other sites

Does it pitch up even with power off?  If so, and the balance is correct, then it's either tailplane or wing incidence that needs fixing...

 

If it's when power is on, it's possibly a thrust line issue and you may need to add downthrust (this could easily be exacerbated by the crash as that could have resulted in more up thrust - assuming the model bashed it's nose on landing). IIRC, the motor mount is a sort of U shaped metal, which is relatively soft.

 

In answer to your question, yes, you need to raise the tailplane leading edge.

Link to comment
Share on other sites

Posted (edited)

Thanks,

 

I don’t think it is CoG.  I balanced it according to the instructions and had to add some lead to the tail.  It is unusually to have to add lead to an out of the box foamy but that’s I had to do to get the stated CoG.  I thought that maybe it was tail heavy ( although my balancer said otherwise) so I removed the lead bit my bit pushing the CoG forward.  None of this changed the pitching up.

 

The motor thrust line could be wrong, especially after the crash which put a slight bend in the prop adaptor (since replaced with a new part).  The problem with the AcroWot Foam-E is that the motor is essentially mounted onto foam and not that securely either.  It is notorious for not having a good motor mount and there are stories of it going out over time just through normal uneventful flying.

 

I can’t see how you could secure it better and besides the documentation doesn’t give any idea of what the correct thrust line should be.

 

Besides it has been bad out of the box but particularly bad now.  I’m going with tail incidence for now - the looping was extreme yesterday, as if I were continually pulling back on the stick - I’m not convinced the motor thrust line would do that - besides it can’t be out by much because the spinner backplate sits about right.

 

Yesterday it was looping (or attempting to) at all throttle settings - of course at the lower throttle there wasn’t enough power for it to complete the loop, so it would pitch up violently and then stall, recover, rinse and repeat.

 

If my friend wasn’t there to dial in the trim I would have lost it for sure.

 

Cheers,

 

Nigel

Edited by Nigel Heather
Link to comment
Share on other sites

(If a flat tail) How about taping a thin carbon rod to the tail to get an indication of its lie compared to the wing and motor thrust line?

Link to comment
Share on other sites

Nigel,

 

Don't jump to conclusions as to what is causing the issue, and certainly don't start adjusting your machine on a hunch.

 

Do as Trevor says and do a dive test to confirm the CoG is correct in the first place, if not adjust accordingly.

 

Remember the manufacturers recommended CoG is only a start point as are the control throws, you need to adjust to your own taste.

If you read enough threads you will realise that some manufacturers can't seem to get these right or there are typos in the instructions.

 

If you have added weight to the tail, then that is likely to exacerbate the issue you have now and have had from new.

Link to comment
Share on other sites

3 hours ago, Ace said:

(If a flat tail) How about taping a thin carbon rod to the tail to get an indication of its lie compared to the wing and motor thrust line?

 

This a foam model.  As I have said before this is a foam model with a well-known badly designed motor mount.  If you grasp the spinner and gently apply pressure the motor will move.  They are like this new out of the box and it is well documented that the thrust line moved over time.  Plus none of the documentation indicates what the thrust line should be.  All I can say is that based on the way the spinner backplate is sitting in relation to the fuselage it looks about right.

Link to comment
Share on other sites

2 hours ago, Andy Gates said:

Nigel,

 

Don't jump to conclusions as to what is causing the issue, and certainly don't start adjusting your machine on a hunch.

 

Do as Trevor says and do a dive test to confirm the CoG is correct in the first place, if not adjust accordingly.

 

Remember the manufacturers recommended CoG is only a start point as are the control throws, you need to adjust to your own taste.

If you read enough threads you will realise that some manufacturers can't seem to get these right or there are typos in the instructions.

 

If you have added weight to the tail, then that is likely to exacerbate the issue you have now and have had from new.

 

 

I will try that - unfortunately not allowed to use the flying field this weekend so will have to wait until some time next week.

 

All I can say is that the recommended CoG is 75mm from leading edge.  I have set it to that which needs a fair chunk of lead at the back.  I have also tried it without the lead which pitches the CoG well forward.

 

Whether the CoG is forward or backward I get the same pitching up behaviour and when I trim it out (large amount of elevator trim) it behaves very nicely  - so in my mind it doesn't seem to be a CoG issue.  But I will try what you say - but given that when it is trimmed it flies pretty much hands off I think it is going to level out nicely from a gentle dive.

 

Cheers,

 

nigel

 

 

Link to comment
Share on other sites

5 minutes ago, john stones 1 - Moderator said:

Reads to me as you know the issue Nigel, the motor mount, any way to make it rigid n add a bit of down thrust ?

I had the same on my Wot4 foam-e. Removing the cowl showed a gap between the foam and rear of the motor mount. With the model on it's side I used Gorilla PU, warmed up it flows nicely into the gap. Once dried I test flew and added washers to adjust the down thrust.

 

Steve

Link to comment
Share on other sites

8 minutes ago, john stones 1 - Moderator said:

Reads to me as you know the issue Nigel, the motor mount, any way to make it rigid n add a bit of down thrust ?

 

Thing is they are all like that - it is a 'feature' of the foamy Wots - build a brand new one out of the box and it will be like that.  Look at the motor shaft it does have down and right thrust already.

 

I'm not denying it might move a little, there are foamy Wot threads on a number of forums all saying that, but what I'm not convinced is that would be enough to induce a tight loop as if I had yanked back on the elevator.

 

Cheers,

 

Nigel

Link to comment
Share on other sites

2 minutes ago, EarlyBird said:

I had the same on my Wot4 foam-e. Removing the cowl showed a gap between the foam and rear of the motor mount. With the model on it's side I used Gorilla PU, warmed up it flows nicely into the gap. Once dried I test flew and added washers to adjust the down thrust.

 

Steve

 

Lot easier on the Wot 4 because it has a cowling that you can remove.  The Acrowot isn't like that, there is really limited access to the motor and mount.

 

Cheers,

 

Nigel

Link to comment
Share on other sites

20 minutes ago, john stones 1 - Moderator said:

All I can suggest without seeing it is, get someone to hold it down whilst you do throttle checks, is the mount moving a lot, is the elevator servo misbehaving as you throttle, is the control horn loose, broken hinge etc.

 

P.S Sounds a bit much for incidence to be causing a tight loop to be honest.

 

Hi,

 

Although you can move the motor my hand when it spins throughout the rev range it stays very axial.  I suppose you can think of it as like one of those flexible mounts - if you push spinner the motor will move slightly but when it spins up it stays aligned, it doesn't move around.

 

With this type of foamy there are no hinges as such the tail plane and elevator are a single moulding with a thin toughened interface between the two so it is 'hinged' along the whole length.

 

The control rods and horn are fine, as are the servos.

  • Like 1
Link to comment
Share on other sites

Well if you ever find the reason I would be also interested Nigel as I have the opposite problem with the model pitching down. I have started removing nose weight but not convinced the model is nose heavy as it has flown ok in the past. Possible my issues are the flying speed as the model is marginal on engine power and I have been trying different props to get the best out of an RCV58CD.

 

Assume you have checked the motor mount to see if the thrust line is flexing as you change throttle setting. Soft elevator push rods could be another reason.

Link to comment
Share on other sites

4 hours ago, Nigel Heather said:

. . . when I trim it out (large amount of elevator trim) it behaves very nicely  . . . . when it is trimmed it flies pretty much hands off 

 

So I’m not clear what the problem is! If it is just that the visible down elevator trim offends your eye, then adjust the tailplane incidence accordingly. Personally though, as long as you have enough physical down elevator travel for inverted flight and negative g manoeuvres, I really wouldn’t bother.

  • Like 1
Link to comment
Share on other sites

Mine did that.   I suspect there’s some slop in the elevator linkage which exacerbates issues with incidence and/or c of g.   Plus if it’s been crashed the fus may be distorted, introducing flaws in incidence and thrust lines.

 

”Sticktion”?   I recall this expression being used to refer to a control surface which won’t return to neutral.

 

Keep us posted!

 

BTC

Link to comment
Share on other sites

Posted (edited)
9 minutes ago, Martin McIntosh said:

I take it that you are using a 3S 2200, not 4S which would cause your problem.

Even - quote "Yesterday it was looping (or attempting to) at all throttle settings - of course at the lower throttle there wasn’t enough power for it to complete the loop, so it would pitch up violently and then stall, recover, rinse and repeat."   ? 

Edited by PatMc
Link to comment
Share on other sites

On 07/05/2022 at 21:01, J D 8 said:

              Elastic airframe?

I am not familiar with this model but reading the instructions it says ‘it has an innovative internal structure adding strength where needed and locking the components together’, to paraphrase.

If this internal structure (carbon tubes?) is not properly bonded to the foam (insufficient glue) and/or the model has had a crash, it is likely any stiffness that should be added by the reinforcement will be lost. If you grip the tail boom and try and bend it in pitch it should be quite rigid despite being made from EPO. If it flexes more than a couple of mm the inflight loads will be sufficient to cause the tail plane to adopt a negative A of A and a strong positive pitching moment. = loops! Hence you find a lot of nose down trim is required. 
Just a theory?.

Link to comment
Share on other sites

Join the conversation

You can post now and register later. If you have an account, sign in now to post with your account.

Guest
Reply to this topic...

×   Pasted as rich text.   Paste as plain text instead

  Only 75 emoji are allowed.

×   Your link has been automatically embedded.   Display as a link instead

×   Your previous content has been restored.   Clear editor

×   You cannot paste images directly. Upload or insert images from URL.

 Share

×
×
  • Create New...