Parkzone u/m j3 cub:
 

I’ve had a Parkzone Ultra-Micro Cub for several weeks and it’s my 1st RC plane. It’s had a few knocks and I’ve replaced the tail assembly and the prop and prop shaft. I’ve installed a prop saver and used one of the props supplied which is fairly similar to the original. But the Cub is now a little erratic and unpredictable whereas previously , for a novice, I had quite good control except when caught out by sudden gusts. All the controls test fine and I think my repairs are well-aligned. But I didn’t get an alternative spinner for the prop save arrangement so it’s flying without one - would this make a significant difference?? When I installed the new prop shaft I tightened the plastic nut to give just a small amount of play and the motor sounds fine. I know the Cub is about 28 gms so obviously weight sensitive. But presumably the odd bit of tape and glue has little effect (although these are generally balanced up about the plane)??

I will appreciate any suggestions or ideas.

Two or three things that come to mind are alignment and weight and balance issues.
You said the repaired surfaces look like they are aligned ok so that covers one area, take a look at the fuselage as well to see if it had any compression damage in the crashes. if it appears bent sometimes that can be fixed with a gentle heat application.
Take a look at the motor alignment to see if it still has the same thrust angle as before typically a little down and right.

If you had to do a lot of repairs to the tail, the glue and tape may have added weight to the tail and offset the CG. Likewise the prop saver may have added weight to the nose affecting the CG. Your manual says the CG for this plane should be 22 mm from the leading edge of the wing measured at the wing root. Check that and correct it if needed, without adding weight if you can. If it is off youmight be able to fix it by moving the location of the battery

Good luck and welcome to WattFlyer

Hi - thanks for the advice and welcome plus a geography lesson (I looked you up - I've heard of Columbus, Ohio but couldn't place it exactly!). I'm inclined to think it might be the new prop and saver and possibly bad adjustment when I replaced shaft. Some "obvious" things are only obvious after you're told! When nut at end of shaft is tightened (how tight?), I saw info that said go by sound of motor which is probably a good guide? The tail is all new so there's no tape and only a smidge of glue. I'm surprised you didn't mention the spinner? - I presume this is for aerodynamics and I would imagine leaving it off makes some difference?? Fuselage is fine and I don't think motor has moved but goodness knows how you check motor with any accuracy (just by eye?). There's a defined velcro strip for battery so it can only be varied a few mm. In the past I've generally put it as far forward as poss. I might get an original prop replacement and try it without the saver (plus spinner). Thanks again.

Definitely check your CG. It should be right around 25% of the way back from the leading edge.

These little guys weigh such a small amount that any tiny change can make a big difference, especially when located at the extreme ends of the airframe. Could well be the prop saver as you say. I am having trouble picturing what prop saver you are using based on your description of the end nut etc.
At the speed these planes travel at I would not think the spinner or lack thereof makes much difference from an aerodynamic standpoint, it might impact the weight and balance but I suspect the prop saver set up more than makes up for that.
If the prop saver works well and you want to stay with it, you may find that you end up treating the velcro position "not as a rule, but more of a guideline or suggestion" . On my old micro SU-26 I ended up enlarging the battery compartment probably a good 1/2 inch rearward and added a little velcro to get the CG to a spot the plane flies well. On the micro P51 I opened the battery slot forward to get the 150maH batteries to fit. Had both out this evening and had a good time flying both along with the 4 Site) - small field and absolutely no wind. Only had a hour or so but was a great time:D

I assume you are in UK- where?

Sorry - I never saw your final question but better late than never?! I'm just outside London (Croydon), to the south and 40 miles from the south coast. Things have now moved on - I joined a local club and have bought several beginner planes and a couple I'm too scared to fly! You were right about the CG - an ace flyer at the club balanced the Cub with thin bands of bluetak stuck on the nose and it was just right after the 3rd band! Fairly recently I fitted a P51 motor which transformed it and, not least, I was able to remove the bluetak! I must have done something right because it flys well, climbs vertically and will loop forever!
Regards.

I nearly forgot - the prop saver is a saver in the loosest sense of the word but it seemed to work. Strangely, it was only available from one (US) source. Just a piece of plastic that screwed on the prop shaft - it had 2 very small pegs to hook on 2 tiny (approx. 5mm dia.) elastic bands then over the prop. The Cub originally had a 1mm dia. shaft which broke if you gave it a fierce look! - the P51 is 1.5mm dia. which seems to make all the difference. The nut referred to simply retained the shaft at the rear end of the gearbox.
Regards.

Funny about that little plane - with no damage at all I was never happy with the way mine flew. Meanwhile the Champ, just a bit bigger, could take all kinds of ridiculous repairs including carpenters glue, tape, construction paper, etc. and always flew great.

The Cub was the 1st plane I bought (last year) - not the best choice for a newby but I did quite well initially. Now, a year on and even with the lively Mustang motor, I'm quite confident with it. Plus I'm a bit more clued up about balance etc.
I also bought a Champ several months ago which was incredibly well-behaved and almost therapeutic. Until a few weeks back! On a great day and flying high I turned and boasted "look, no hands!" but I got back to it ok. Minutes later, I took my eye off it for literally 10 seconds to admire another plane but then I couldn't see/find mine!! We have cropped fields nearby and I never found it. I'm almost over the trauma but I intend to get another - as I say, I regarded it as therapy!
Regards.