Yesterday I returned back to fly my old dazzler, I made 2 modifications in order to improve plane behavior:
1)Added downtrust to the engine using two washers.
2)Move cg back 5 mm.
Flying with these modifications the model behaved even worse:
1)At any throttle the plane kept gaining altitude rapidly.
2)At inverted flight the plane dived rapidly.
3)At 90 degrees climbing the plane fell to the left and to the canopy.
The modification came after I had the same problem described above but now they seem to become even worse.

Why?


thanks

If your plane is built and balanced to plans, you have your ailerons set to low for neutral. Raise each one, one clevis turn...fly...repeat. It wont take long and you will have a sweet flying airplane.

Ummmm....

In that list of things you don't mention "Retrimmed elevator for level flight at 3/4 throttle".

Sorry if this seems too obvious but sometimes it's the simple things that get us.

You may want to try using dive test method to set the balance like the glider guys do. Perform the testing at idle so it's the decalage and balance setup rather than the thrust alignment that you're testing.

Do a google.com on "glider dive test" and you'll get lots of sites that explain it in differenty ways.

The plane recovered a crash and since then it has 3 bad major tendencies (before last modifications)
1) just a bit tendency to gain height.
2) 90 deg climbing - falls rapidly toward canopy.
3) invert flight - falls rapidly toward ground.

why flying upward caused just a bit of climbing and inverted flight caused a rapid fall.

thanks

rcFP

Your problem in inverted flight may be the result of the increased downthrust requiring 'up' trim on elevator - which becomes down elevator when inverted, of course.

With any model the first thing to get right is the CG. First, and easiest, check for this, asuming that the model is not excessive twitchy on elevator input, whihc indicates a CG too far\back, is to climb to height, throttle right back, and put the model into a vertical dive. As soon as the vertical down line is established return the elevator to neutral and observe. For a general sports aerobat you are looking for a straight dive of around 200 feet before the model starts to gently pull out of the dive. A rapid pull out indicates either a too forward CG or excessive difference in the rigging angles of thewing and tail (too much positive on the wing or negative on the tail - same effect). A model which starts to tuck under - going past the vertical - has the CG too far abck. Adjust accordingly.
Once the CG is demonstrably in the ball park, then is the time to start playing around with the thrust line.

Mike

On the "dive test"... don't go vertical the first time. It could be the last time!
A moderate pushover and release works just fine.

don't go vertical the first time. It could be the last time!


Paul


Why do you say that - I was careful to emphasise the need to start high. The power off vertical is a lot easier to interpret that a lesser dive, and easy interpretation is what is needed.

Mike

cg was checked using greatplanes balance machine and it's just a bit forward but yet it seems that plane acts as if it has aft cg.
it was quite a windy day but at any direction the plane kept climbing quite rapidly (almost at any throttle position).
as i said before inverted flight caused rapid diving.

Have you checked the incidence of the wing in relation to the stabilizer.?

.
Mike a plane that will diverge downward can show that property safer when it's not plummeting.
If such a plane is permitted to get too fast, it can become uncontrollable/unrecoverable. Any incipient tuck is more easily seen in a moderate dive, and stopped before it gets too far.
I've had a glider that was unrecoverable due to tuck from a moderate dive. 3 times!
Last time too much damage to bother with.
I look for just a bit of tuck on some fast aerobatic slopers.. I use it as a speed limit. The recovery can place a severe stress on the wings when a plane tucks.
Otherwise a slight amount of nose-up from the moderate dive is what I look for.
.
My buddy Mac and I were flying a Cox Lazy Bee ARF... the one with the zero pitch prop. Made lots of noise, but very little airspeed.
I coaxed it to a reasonable altitude and pushed over to get some speed for an Immelmann.. moderate dive. Pulled it up and rolled out at the top.. and then gave the xmtr to Mac for him to try.
To Mac, it's straight and level, or straight up/straight down.
He dove to pick up speed. Waaaaay too much speed, even for a LzB. It tucked! He pulled full elevator, and the wingtips touched, then the wings left.
We laughed all the way down... (It wasn't our airplane.)
We said... "Hey, Bob, your airplane is crashing!"...

Here's Martin Simon's graphic of the dive test.
BTW, he's doesn't recommend doing it on something slippery like a fast glider which can easily accelerate to "tuck" speed and structural collapse.

Paul

Thanks for the reply. I don't fly fast gliders so bow to your experience of them. I've used the vertical dive test on many, many IC models without problems - but each to their own

Mike

Mike, don't bother trying to trim out a Dazzler, it's a lost cause mate. See it fo what it is, a fun/junk plane...a bit of a dog actually.
Just miss use it on a regular basis and all is well. Spend time trying to make it half decent and it'll never get there.