Twin Otter Trim / Balance Issues

[Waterbug]

I can't get my just finished Twin Otter - with lightweight styrofoam floats - to fly or glide normally. I played with the COG (multiple size batteries and positions) and subtrims to no avail. Counterintuitively, it climbs rapidly with power and dives without - a lot of down elevator to fly and up elevator to glide. It's a handful. Everything is "stock" except the floats (Seaplane Supply), which I have on multiple planes (that fly well) and the propellers (Master Airscrew 3-blade 9x7 - also used on my Sea Duck w/ same setup). I'm wondering if the motors may be angled upward? But they look fine. What do I try next?

FWIW: I also built the FT Legacy a couple of years ago - similar airplane - and it flies perfectly well (but it's no acrobat).

My favorite FT airplane BY FAR is the Sea Duck - I'm on my 3rd (or 4th?) - all of which have been a blast to fly off the water (4S). I only wish I had a foamy or balsa version that wouldn't warp or delaminate.

Thanks for helping ...

 

[Merv]

...a lot of down elevator to fly and up elevator to glide..

To me, is sounds like a thrust angle issue, not trim or CG. Try adding some down thrust.

But first, let's get the CG right. You have a rectangular wing, measure the cord and make the plane balance at 25% of the cord from the leading edge. Once you get the plane flying right, then move the CG to your liking. Now take the plane for a fly, trim the plane to fly straight & level at full throttle. Once it is fling straight & level, take it up to about 3-400 feet. cut the throttle to zero & dive straight down. Now watch what the plane does. It may pull out of the dive right side up or or may pull out on it's back, upside down. How quickly it pulls out & which way it pulls out will tell you how to adjust the thrust angle. If it pulls out right side up, you need less down thrust. If it's upside down, more down thrust. If it happens fast, in the first 25-50 feet of the dive, try 2-3 degrees. If it's slower say 100 feet or so, try 1 degree. If is goes more or less straight down for 150-200 feet, the thrust angel is about right.

Now lets talk about the why. When you trim your plane straight & level at full throttle, you are trimming out any "error in thrust angle". When you dive straight down with no throttle. You have removed the thrust but are keeping up the speed. Which way it pulls out is telling you which way you trimmed. How far the plane dives, tells you how much you trimmed. After you make an adjustment you must retrim the plane. Keep repeating this until you are happy, trim, dive, adjust. I've never gotten one perfect, you are looking for good enough.

 

[Mr Man]

Sounds like you’ve got a case of upthrust.

 

[bisco]

looks great, nonetheless!

 

[kingfieldbill]

I have the same issue with my otter. Im going to try Merv's advice this weekend but Im fairly certain it's not CG. I have flown mine with 1 2200 batt, 2 2200 with one tucked under the wing, and with a single 4000 all the way in the nose with very similar flight characteristics. They all take considerable down trim to keep it from climbing with throttle. If dive testing doesn't reveal something I'm going to add some down thrust angle.
I will say, though, with some down trim it flies beautifully.

 

[AIRFORGE]

I have the same issue with my otter. Im going to try Merv's advice this weekend but Im fairly certain it's not CG. I have flown mine with 1 2200 batt, 2 2200 with one tucked under the wing, and with a single 4000 all the way in the nose with very similar flight characteristics. They all take considerable down trim to keep it from climbing with throttle. If dive testing doesn't reveal something I'm going to add some down thrust angle.
I will say, though, with some down trim it flies beautifully.

Have you attempted to fly it without the floats?

 

[Waterbug]

Thanks for all of the advice and sorry for the late update - I didn't have time to dive back into it. So, first of all, the elevator was warped so I replaced it, straightened out the tail assembly, and recalibrated and rebalanced. It flies much better - doesn't go nose-up on throttle and nose-down on glide - but it still needs more work. It's constant correction flying, but least there's hope. On the other hand, in two weeks the wings have dropped significantly and the fuselage ripped open on the right side behind the rear landing strut. I should have added a carbon fiber tube to the wing assembly - like I did on my 3rd Sea Duck to keep the wings straight - and I should have reinforced the fuselage along the bottom and sides - it's a disappointingly weak design.

To Merv: it's not to the full throttle straight and level point yet, let alone a straight dive down! You obviously have more nerve (and skill) than I do, but thanks.

To Airforge: nope, floats only, but I often wonder how my float planes would fly with wheels ...