Sorry, I've not been on here recently to give an update - but yes there is news!

Basically it flew, but not well. I pointed it down the runway and opened the throttle. It set off and the tail came up quickly, but it showed no sign of lifting off and I was running out of runway so I hauled it off with some up elevator and it climbed out quite gracefully. For a moment I thought 'oh that's nice' but I soon found it was a bit of a handful. It seemed very unstable in pitch, I think the thrustline and probably CG are wrong. It also seemed very neutral in turning, once it's banked over it stays there and needs opposite rudder to stop turning.

I had a couple of uncomfortable flights to see if I could work out what needed changing, and ended up with a bent undercarriage so called it a day. The wing is leaning against my desk here awaiting attention but I haven't got round to it yet.

Sorry to hear your maiden flight didn't go so well Dave,
Looks like your plane is rudder -elevator - throttle?
I added ailerons to mine when I originally built it - but most of the planes I have flown are RET,
Should fly RET OK as long as the rudder area is sufficient - and it looks like yours is.

Not sure why your plane seemed a bit unstable in pitch Dave.
What incidence angle to you have on your plane?
My plans show about 2 degs of incidence.
Also - what thrust angle do you have? Mine is 2 degrees up-thrust relative to the underside of the wing

What percentage of wing chord is your C/G?
I use between 25 an 28% chord. The further aft you go the less stable in the roll axis a plane becomes. Personally I wouldn't go beyond 30% of wing chord.

The result of stretching the bows 2 inches on my rebuild means I can now get the 1300 3S battery far enough forward that I don't need any extra ballast to balance it, This saves at least 80 grams in weight.

I did a couple of hops and a short circuit with mine today. First time airborne since rebuilding it.
Appeared quite stable and the glide ratio seemed very good with a much slower airspeed than originally

It really wasn't ideal conditions for flying a new plane today with a bit more breeze than I would have liked. So didn't take it up and get it trimmed out properly, However it seemed the trim is probably pretty close to where it should be.

I have found a number of model plane plans that show incorrect position of C/G and other errors.
I have a plan for a 48 inch WS semi-scale Cessna 170 that I bought in 1966 that has the indicated C/G about a full centimetre behind where the aft limit should be. No wonder when I flew it it rolled horribly until it crashed.
I had to add heaps of lead to the bows to get the C/G right - then it flew so fast due to the extra weight that it wasn't comfortable to fly.

There is no way this plane built the way the plans show would ever fly very well since the whole fuselage and tail feathers are all sheet balsa. This makes the plane tail heavy
I do plan on building this plane again - but this time the aft end will be built from balsa sticks and covered with fabric as I usually do to keep the tail end a lot lighter.

I agree that the CG is the likely culprit. Once it is up and you reduce the throttle, if it is still unstable in pitch then it's not the thrust line.

Also, if the CG is too far back it can affect turns. That's because when the CG is farther forward there is more side area aft of the CG.

I had an old Goldberg Skylark, original, RET, that wobbled coming out of turns. The tail area is marginal. But when I moved the CG forward it cured it. More side area behind the CG.

As to the incidence angles and thrust angle, those can affect take off. If the plane won't come "unglued" on take-off, increasing the wing incidence should help. Wing incidence also affects the angle at which it lands. The thrust angle can affect take-off since that is when power is high.

Incidence angles of wing and stab do not affect pitch stability because you have elevator control and elevator trim. It's probably not good to have to carry much down trim, but that's not going to cause a stability problem.

Yes it's RET, the plan was single channel rudder-only but I added elevators.

The CG was on the wing spar (from the plan I think) but measuring that it's in the 30+% chord region. I have some lead in 10mm dia rods so I've drilled a couple of holes in the nose block to glue some in, that gets the nose weight as far forward as possible and allows me to remove some lead from further back behind the noseblock. That puts the CG at about 25%.

The wing incidence is about +2 degrees, the motor thrustline is parallel to the wing so about the same. I can't quite get my head round thrustlines on a pusher, would it be better to be 0 degrees with the horizontal tail?

Dave.

If it's sticking to the ground and if you find that when trimmed for low power or even a glide that going to high power pushes the nose down or holds it down too near to level then you might want to angle the motor so the rear of the motor is pointed up a touch. For example the wing and motor are up at the front by 2°? Then leave the wing and put shims under the lower mounts to drop the thrust line to 0. And maybe even point it down a hair. And by down I mean at the front of the motor.

Although to be fair with the stabilizer up in the prop blast odd things can happen. Altering the thrust line this way technically should work. But under power the stabilizer gets a fair share of it's apparent incidence from the prop wash angle. So ... hard to say. Try a little of both ways.

Thanks Bruce, that makes sense. I'll try the new CG position first and the see how it behaves.

Current Covid restrictions don't allow me to travel to the club flying field with the hard runway where I flew it the last time. It would have to be a hand launch at the other field which I can go to. Plenty of long grass though, for emergencies!

Dave - I'd give it a go with C/G. at 25% chord..
2 deg incidence sounds good.

Unlike planes with tractor motors that have a thrust angle of about 2 - 3 deg down and right thrust - there doesn't seem to be any rule of thumb when it comes to thrust angles for pusher planes.
Planes like the Bixler for example have a lot of down-thrust - whereas something like a Supermarine Walrus has several degrees of up-thrust.

The plans for my CWJ show the thrust angle as 2 deg of down thrust relative to the underside of the wing. (ie - 4 degrees of up-thrust relative to the rolling axis of the plane).

I have two other sets of plans for the CWJ - both by different designers.
One has the thrust line parallel to the underside of the wing (ie 2 deg up-thrust relative to the rolling axis)
And the other 4 1/2 deg of up-thrust (2 1/2 deg up thrust relative to the underside of the wing - 4 1/2 deg relative to the rolling axis)

I came across an article about the full size CWJ that said the thrust angle was 4 deg of up-thrust relative to the rolling axis (that's 2 deg up-thrust relative to the underside of the wing)

So I set my motor thrust angle the same.

I have yet to take it up and check the trims out properly (been too windy) - but the brief flight I did have last week didn't show any abnormal behaviour - suggesting that I have set things about right.

Correct thrust angles are essential to getting a plane to fly right.

Only make one change at a time and try it

Dave - on reading your post again I see you are saying the plane was reluctant to lift off.
That suggests to me that you may need more up-thrust on your motor.
That will give more downwash from the prop onto the tail-plane, pushing it down and helping it to rotate.
Also the up-thrust force will help lift the plane from the ground.

My plane had no difficulty getting airborne and I have 2 deg more up-thrust than you have.

Down and up thrust angles sort of lose their meaning when we're looking at these types of model. I tend to think of them in terms of pitch up and pitch down angles.

Since it's reluctant to lift the nose I suggested some pitch up angle in my post above. But in some cases with the stabilizer up in the thrust blast we need to consider the prop blast on the horizontal tail and how that tail is further from the CG location than the tail surfaces. So one certainly needs to keep an open mind and be willing to change directions if one trial doesn't work out as we expect and hope.

All of this high thrust line discussion is reminding me that I've got a date to design and build a Porco Rosso flying boat at some point.... Love that movie!

I had a 36" w/s Sea BB a while back.
Like the Porco Rosso is has a high tractor thrust line too..

The Sea BB has several degrees of up-thrust and flew well

SeaBB (1 min 29 sec)

Hope it isn't too long before the weather will let me fly the CW Junior again

Took the CW Junior flying this morning.
My mate did a video - but most of it is way out of focus - not sure why?
If flew OK - but seemed pretty fast - yet it doesn't seem to be overweight at all.
Managed three , five minute flights before the wind started getting up.

Took the CW Junior to the club this morning and had four flights.
On first flight once I got altitude cut power and gave it as much up trim as the Tx has. No sign of stall on glide.

So adjusted the pushrod clevis one full turn. Then in next flight added about 4 clicks of up trim.
I also added a 50 cent coin to the bows. Flying slower than before.

On third flight added another 50 cent coin and a little more up trim.
Flying at a more comfortable speed now. Still not quite as stable in the roll axis as I would like - but certainly manageable and much better than it was on earlier flights..

I'll make up a piece of lead ballast and fit it permanently to the bows.
Might still need a little more up trim?

The sink rate power off is now more like that of a Bixler glider;

By the time I have done a few more flights I think It will begin feeling comfortable to fly.

Didn't have my cameraman with me today.
Unfortunately the club will close in Feb for about 6 months due to expansion of adjacent sewage treatment plant which involves laying a large pipe right through our grounds. - so may not be able to get any flying footage in the short term.

I can't fly at the moment due to coronavirus restrictions, so no further forward with my CW Junior.

Currently building a Yak-3 fighter from foam board (foamboard warbird or foambird warboard?) to keep me amused.

Bummer not being able to fly huh! Hope it is not too long before you can start flying again.
Fortunately here we only had one fairly short shutdown last year

Had 8 flights this morning on the CW Junior. Flying pretty well and gliding power off nicely.
Have added the lead ballast (25 grams) and is definitely flying slower than early flights.
I increased the up trim a bit more and now seems about right.

The plane could have been built with another degree of incidence as the plans only show about 1.5 deg

My last landing was not good. A case of dumb thumbs and the wingtip hit the ground first causing a cartwheel. Fortunately only minor damage which can be easily fixed.

Here is some footage of my CW Junior flying.
Not good quality unfortunately. Camera wasn't staying in focus.
But at least you can see it flies LOL
The motor is a Bixler replacement motor - so pretty high Kv,
Plenty of power and flies level a bit below half throttle

MVI 0623 Flying the Curtiss Wright Junior at my club field (5 min 47 sec)