Max Thrust Yak 54 Problems

I'm hoping that someone on here can help me diagnose the problems I'm still having with my Max Thrust Yak 54 please, so here's a description of what's occurring. With a neutral trim she drops her nose when on 1/2 to full power, and re-trimmed for level flight her nose rises when on low or no power I performed a few dive tests, with her tucking her nose under each time, which I believe indicates she's tail heavy. I added 20gms of lead to the front of the cowl, flew and did a dive test (neutral trim), which made no difference. I added a further 20 grams and repeated the test, and again no difference. The initial CofG was within the range in the idiots guide, which I still need to re-check with this added weight, but is there anything else I should be looking for before I start adding a further ton of church roof e.g. tail plane incidence, motor thrust angle etc? which all look ok to my Mk1 eyeball.

The elevator servos have been replaced, with ones of a similar size and weight, though I must confess I didn't do a precise weight comparison there should only be a few grams difference. Stig the pilot sits slightly forward of the CofG so he shouldn't make a difference, unless the idiots guide is way out, in which case I'll be looking to use bigger lipos in place of church roof as a long term solution.

Thanks in advance for any advice you can give guys and gal(s) this is the first model to ever give me this much trouble.

My first question would be has he model been like this from day one and if not, what major changes have been made. Normally if a model tucks its nose under on full power , it means that the downthrust is excessive, born out by you saying that the nose rises when you go to to low power, which indicates that the motor is pulling the model down when on full power. That would be the first thing I would look at .

Eric types faster than me

Garry

Never done a snap in the forum

Garry

I had the opposite with my first brushless it took off and went straight up over and did a loop so I had to add down thrust easy on a multiplex motor mount

The dive test is done at approx 45 degrees with the power off, and the prop freewheeling, and the plane should either maintain travel, or the nose should very gradually lift to indicate good CofG. It's been like that since her maiden flight, though the cr@p dual elevator servos didn't help, travelling at different speeds and distance, but these cheap digitals seem to have cured at least that problem. I'll check the current CoG soon, just need to unload the car first

It might be worth checking the actual position of the elevator when it is trimmed for level flight at full power. Unless it is truly in line with the tail plane your problem could be more than a CofG issue.

Off the top of my head, pitch instability can be caused by:

- C of G position (already mentioned)
- wing and tail incidence.
- engine thrustline.(also already mentioned)
- inadvertent movement of control surfaces: too much backlash or poor servo response, say.
- Poorly seated flying surfaces moving in flight.
- Poorly fixed engine moving in flight.

That should do for starters.
Andy

I'll put my money on too much downthrust.

Thanks for your help so far, I'll have another fettle this aft.