Lots of UP elevator required?

[Jason Hyland]

Hi,

I am hoping for a little help trimming my WOT trainer. A bit of background

I've recently got back into the hobby after a few years away and so decided to revert back to my WOT trainer as I was feeling a bit rusty. Lots has moved on in the past few years so I decided that I would adapt my WOT trainer for electric flight, and give that a go.

I opted for a HobbyKing G46 670kv motor given its supposed to equal a .46 glow. It has a Turnigy Plush 60a ESC, along with a 4A 4S Turnigy battery. I replaced the motor mount using 4 m4 standoffs, and I believe I have maintained the original firewall thrust line (which looks like no down, and a bit or right). It's being pulled around with a APC 12x8 original prop (not one of the "electic" ones). I have checked suggested CG of 89mm and it's pretty much plumb with everuthing loaded up.

I ran it through a watt meter and I was getting approx 550w from it with an all up weight of about 2.9kg (6.4lbs), so about 85w/lbs - reasonable from what I've read for a trainer - although the motor was only pulling about 32a, so I could probably upgrade to a 12x10 if I need a bit more.

Anyway, finally to my problem. On the maiden flight it required a lot of up elevator to keep straight. It was also pretty twitchy, which if I recall would often point to being tail heavy - again according to the manual and my measurements it looks fine. I'm wondering if I have inadvertently dialled out some downthrust ? When the model is balanced on my balance stand, the prop looks exactly 90 degs to the thrust line.

Anyway, could it be under powered, over weight ? Or have I just measured something wrong and it's tail heavy? ANY help would be most appreciated!

Regards, Jason

[Martin Harris - Moderator]

Thrust line errors usually show up with trim changes with power alterations. A typical trainer will have its wing incidences and thrust line designed to give a gentle pitch up with added power and down with reduced power from the "cruise" power setting.

In other words, if you find it pitching up as you reduce power then you have too much downthrust and if you get a rapid pitch up with power you need to add downthrust.

I wonder if you're flying faster than before due to the electric power train being heavier than the IC and/or the motor revving higher than your old engine (12 x 8 is a big prop for a 46 engine) - did you do comparative measurements? That could make the controls more responsive...

Edited By Martin Harris on 07/05/2016 22:22:07

[Biggles' Elder Brother - Moderator]

If the thrustline was out - particularly if you have lost some intended downthrust - the effect would almost certianly be the opposite to what you describe - ie the model would need excessive down elevator.

So, if not that then what could cause this?

1. Its very nose heavy - but you say not?

2. There has been a shift in either the wing or the tailplane incidence. Could the wing be moving on is seating? Could the tailplane have become unseated or its seating have changed (fuselage warped in storage?)

Finally - how much is "a lot of up elevator"? Is it possible to trim the model with the elevator trim switch so it flys S&L on say 2/3 throttle? If so is there a highly visible amount of up-elevator in when you do that?

BEB

[i12fly]

I know it isn't the same model but it is similar. My Wot 4 (balsa ARTF IC spec) I built up converted to electric. The CoG was a few mm forward of the design point. It turned out to be very nose heavy in flight and needed full available up trim to fly almost level. I had to add 2 oz of lead near the tail to make it perform correctly in the dive test for CoG. Bit disappointed I can't move the battery back.....

[Former Member]

[This posting has been removed]

[Martin Harris - Moderator]

You serious, Percy - one of the hot topics less than a month ago!

[Jason Hyland]

Posted by Martin Harris on 07/05/2016 22:16:47:

I wonder if you're flying faster than before due to the electric power train being heavier than the IC and/or the motor revving higher than your old engine (12 x 8 is a big prop for a 46 engine) - did you do comparative measurements? That could make the controls more responsive...

Hi Martin, many thanks for the suggestion. I'm pretty sure when I flew this with an OS46 it would have been prop'd with a 10x6 or similar. The prop selection was really driven from the motor selection which recommend a 12x8 to a 14x10 - given prop clearance I stuck with a 12" prop, and I only really had a 12x8" available. My understanding is that the electric motor rpm is quite a bit slower?

It certainly doesn't feel "fast", if anything I wondered if it was under powered - hence my check on the watt meter. I was a bit gung ho on the conversion

[Dave Bran]

Seems more likely to be either thrustline or wing/tail incidence angles.

Have you done anything or over time could anything have happened to the wing seating or tail seating?

[Jason Hyland]

Posted by Biggles' Elder Brother - Moderator on 07/05/2016 22:32:33:

If the thrustline was out - particularly if you have lost some intended downthrust - the effect would almost certianly be the opposite to what you describe - ie the model would need excessive down elevator.

So, if not that then what could cause this?

1. Its very nose heavy - but you say not?

2. There has been a shift in either the wing or the tailplane incidence. Could the wing be moving on is seating? Could the tailplane have become unseated or its seating have changed (fuselage warped in storage?)

Finally - how much is "a lot of up elevator"? Is it possible to trim the model with the elevator trim switch so it flys S&L on say 2/3 throttle? If so is there a highly visible amount of up-elevator in when you do that?

BEB

BEB, Thanks for the suggestions. I've read a bit online about the thrust line and as you say, I'd be seeing the reverse if it was out in this was. I will post some images shortly which I hope might spark some ideas.

I remeasured the CG today. I used this manual for the reading which suggest 89mm. It suggests

"The Centre of Gravity (C/G or Balance Point) should be 89mm (3.5" back from the leading edge of the wing at the root, this being the centreline of the wing joining tube. This should be measured with the fuel tank empty. Support the completed model under the wing either side of the fuselage at this point and add weight or adjust the position of the radio battery in its bay as necessary to achieve a slightly nose down attitude. "

Given its now electric, it was measured with the battery installed which actually site forward of the CG - I wondered if this is throwing it off?

I don't think the wings or elevator have shifted, although I guess it's possible I didn't tighten the wings down enough - pretty unlikely to be honest.

As for how much is up, I'd say on the suggest throws of up to 10mm, it was about 4 or 5 mm - I didn't do a brilliant job of trimming it to S&L as I just wanted to get it down in one piece

[Jason Hyland]

Posted by i12fly on 07/05/2016 23:41:41:

I know it isn't the same model but it is similar. My Wot 4 (balsa ARTF IC spec) I built up converted to electric. The CoG was a few mm forward of the design point. It turned out to be very nose heavy in flight and needed full available up trim to fly almost level. I had to add 2 oz of lead near the tail to make it perform correctly in the dive test for CoG. Bit disappointed I can't move the battery back.....

Thanks i12fly - The CoG given in the manual was for glow, and with an empty fuel tank. I wonder if this means that the front would be lighter when balancing. I have an idea....

[Jason Hyland]

Some images that might help?

 

 

Edited By Jason Hyland on 08/05/2016 11:05:33

[John Muir]

If you need up elevator now, and assuming nothing else has changed, it would sound like it is now nose heavy really.

I wonder if the twitchy nature of the controls is related to the change from a small high revving prop to a bigger, slow revving one. There will be a lot of air moving over the tail surfaces at a much lower velocity and with much more in the way of a spiral slipstream effect over a larger frontal area so I wouldn't be surprised if the elevator and the rudder both had a big increase in authority. The other thing that might be happening is that the plane might simply be flying slower so you might need some up elevator trim to compensate for that. Certainly with that prop, Kv and battery combination you are going to be down on power and thrust compared to a .46 flat out on a 10x6.

I remember changing the 11x5 on my Wot 4 to a 13x4 and experiencing something similar. The plane was slower but had great acceleration and improved responses at low speed. Liked the 11x5 better as it happened.

[Rob43]

Just a thought. I see from your photo Jason, that you have made up a motor standoff from some quite long bolts without any tubes etc. How secure and rigid is this mounting? Is it twisting and moving under torque or loading and possibly influencing the behaviour of the model? Like i say, just a thought maybe. . .

[Jason Hyland]

Posted by John Muir on 08/05/2016 11:14:00:

If you need up elevator now, and assuming nothing else has changed, it would sound like it is now nose heavy really.

I wonder if the twitchy nature of the controls is related to the change from a small high revving prop to a bigger, slow revving one. There will be a lot of air moving over the tail surfaces at a much lower velocity and with much more in the way of a spiral slipstream effect over a larger frontal area so I wouldn't be surprised if the elevator and the rudder both had a big increase in authority. The other thing that might be happening is that the plane might simply be flying slower so you might need some up elevator trim to compensate for that. Certainly with that prop, Kv and battery combination you are going to be down on power and thrust compared to a .46 flat out on a 10x6.

I remember changing the 11x5 on my Wot 4 to a 13x4 and experiencing something similar. The plane was slower but had great acceleration and improved responses at low speed. Liked the 11x5 better as it happened.

I'm starting to think the same - the CG setting supposed to be measured was with empty fuel tank. The empty tank is about 80g, full it's about 300g. The battery weighs in at 480g. Not quite sure how to work out how far it should be back, I guess I'm probably able to move the battery back a bit until it improves.

I'm ordering a 12x10 and maybe a 13x9 to experiment a bit.

Thanks everyone for the suggestions - all very much appreciated!

[Jason Hyland]

Posted by Rob43 on 08/05/2016 12:53:28:
Just a thought. I see from your photo Jason, that you have made up a motor standoff from some quite long bolts without any tubes etc. How secure and rigid is this mounting? Is it twisting and moving under torque or loading and possibly influencing the behaviour of the model? Like i say, just a thought maybe. . .

Rob, I don't *think* so - I ran it at full pelt on my watt meter and didn't seen any twisting. However, I'm a belt and braces kind of guy, so would you suggest some brass tubing cut to length ?

[Rob43]

Jason,for me either with tubing or a ply built standoff. Either way will allow you to maintain your thrust line as per original. It looks really difficult (from the photo) to measure your thrustline and if your tubes are all exactly equal length then it will be the same as the bulkhead (assuming your old motor was mounted square to this beforehand) equally if you built a ply box this can be made exactly square negating the need for the long bolts and taking out the doubt. Maybe its not responible for all the problems but maybe a contribution. Anyone else agree with this?

[Former Member]

[This posting has been removed]

[Biggles' Elder Brother - Moderator]

Hi Jason,

fuel tank content weight is immaterial really! The reason why you balance with the tank empty and not full is simply to ensure that, assuming the tank is not located spot on the CoG, when landing (and the tank will be low or empty) the CoG is still within acceptable (controllable) limits.

If you have balanced it in the same position electric or IC then it shouldn't make any difference. True one may be overall heavier than the other, but that effects mainly flying speed not so much trim.

OK, how brave do you feel? Ready to fly it again - as it is? If so try this:

1. Fly at say 2/3 throttle and note how much "up" you need to add to achieve S&L

2. Fly at full throttle and note the same.

3. Finally fly the model in the glide - power off - how steep is the glide curve? Steeper than its uncorrected dive angle at 2/3 throttle - or less steep - or about the same?

The results from this will first enable us to eliminate thrust line effects (I don't think it is that, but this should prove that). Second it will tell us something about the CoG position - not much but its a start!

BEB

PS 4mm is a enormous offset!

[ken anderson.]

Posted by Martin Harris on 08/05/2016 09:34:55:

You serious, Percy - one of the hot topics less than a month ago!

.........................................................................................................................................................

arghhhhhhhhhhhhhhhhh .... not the dive test debate again..... ..

ken anderson.....ne....1......... pass the paracetamol dept

[Denis Watkins]

Really Jason, as Rob43 said,, your motor mount has no rigidity at all, the thrust will easily alter at high torque, and the whole system will loosen without any doubt over time as the bulkhead compresses at the fixing points. Rebuild this or buy a mount, as the WOT trainer is a dream to fly with no vices

[Martin McIntosh]

I am not familiar with the model but 6.4lb sounds a bit much. If you are only drawing 32A you could stay airborne most of the day with that large battery. An APC `E` prop would be much better to start with and to me it sounds as though you have a park fly motor rather than one with a bit of oomph in it. The Turnigy propdrive series is very good from my experience. A 50/50 would be far too much maybe but say a 35/48 would drive a 10x6 prop at a very respectable speed and I believe that there is also a 42mm dia. which may be your answer since it seems like you are having to drag the model into the air with lots of up.

Edited By Martin McIntosh on 08/05/2016 19:35:51

[Chris Bott - Moderator]

Posted by Denis Watkins on 08/05/2016 19:09:02:

Really Jason, as Rob43 said,, your motor mount has no rigidity at all, the thrust will easily alter at high torque, and the whole system will loosen without any doubt over time as the bulkhead compresses at the fixing points. Rebuild this or buy a mount, as the WOT trainer is a dream to fly with no vices

4x 4mm studs in this arrangement, with all the nuts tightened up is usually very rigid indeed. Plenty for this application, in my experience.

[Biggles' Elder Brother - Moderator]

Posted by Denis Watkins on 08/05/2016 19:09:02:

Really Jason, as Rob43 said,, your motor mount has no rigidity at all, the thrust will easily alter at high torque, and the whole system will loosen without any doubt over time as the bulkhead compresses at the fixing points. Rebuild this or buy a mount, as the WOT trainer is a dream to fly with no vices

Sorry Denis but but generally speaking I have to disagree here - very politely and with respect of course! While I obviously can't vouch for Jason's particular mounting, the general principle is completely sound - indeed I (and many others) have used this sort of mounting for many years now with complete sucess. The system is very rigid whilst at the same time being very easy to adjust.

The nuts used on one side need to be either nylocs or fitted with lock nuts that's all.

I would advise Jason that you get a pair of inside calipers (or the like) and check that the motor spider really is parallel to the firewall. This is not because I fear lossening or crushing of the firewall - its just that a steel rule isn't really an accurate enough way of doing this. You only need a small error to introduce significant unintentional up, down or side thrust.

BEB

PS Chris types faster than I do!

Edited By Biggles' Elder Brother - Moderator on 08/05/2016 19:46:41

[Martin McIntosh]

I use stand offs quite a bit to get leccy motors in the right position, mainly hexagonal M3`s with no problem. Many large petrol motors use albeit larger ones as standard.

[onetenor]

Deffo nose heavy. That motor could go a long way back which would bring the CoG back if you cannot move the battery back. Obviously you would need a shaft extension.A support bearing might be needed but that wouldn't be very heavy . In my experience CoG is usually about 30 to 33 % of wing chord on average for normal layout planes. There are of course exceptions .I remember one plane I had with the trailing edge as the correct position,.Increasing wing incidence would be the quicker /easier option or decreasing tailplane incidence