elevator offset, cg issue?
 

I've just got round to powering up my black horse decathlon after it's dead stick the wrong side of the tree line and noticed it has around 10º of up elevator, the trim on the Tx says the same so it's not a mechanical fault because of the crash.

There are 2 cg's listed in the manual, one with the standard diagram saying 100mm, and in the text it says 90mm. I set it to 95mm so that should be alright, certainly not enough to cause the elevator offset. I've had a look at the manual for the seagull decathlon which is a very similar plane and it has the cg at 80-90mm! [X(]


Any ideas what could be the cause? the only thing i could think of is the cg, but, if you believe the manual it's as good as spot on, and if you go by the seagull manual it's too far back [:'(]


so confused :D hehe

RE: elevator offset, cg issue?
 

enter the specs into the blank spots and see what it should be, this calculator is very close to right!!;)

http://sky.prohosting.com/air2/cg_calc.htm

RE: elevator offset, cg issue?
 

how does it fly inverted...that is how much down elevator to maintain straight and level.
The amount of elevator trim carried is usually a result of wing incidence...that is you need to increase it when you have "up" elevator.
Where do the instructions tell you to place the CG?
Wing incidence first THEN CG.
Only then move the CG a slight amount at a time.
JLK

RE: elevator offset, cg issue?
 

I just reread your original post.
10 degrees up elevator trim...is that right?!?!?
Something is terribly wrong.
Do you have an incidence meter.
I recommend you get ahold of one and try to figure out what's going on.
If you are in fact in the middle of the recommended CG range there is no way CG is causing 10 degrees up elevator trim.
That is as much TOTAL throw I am flying with a pattern plane I am just starting to trim.
Time to check, recheck and think about what the crash caused.
Good Luck.
JLK

RE: elevator offset, cg issue?
 

I'm reminded why i shouldn't post the wrong side of midnight [:-]

It's more like 3-5º, hehe, sorry.


Anyway, it's first and so far only flight was just long enough to set the trims and do a circuit then it decided to land in the next field. she did seem to glide rather like a brick, from experience a forward cg can do this, however the incidence theory sounds good

During construction i measured the incidence and it's within a degree of everything being inline, unfortunately adjusting the incidence of this model is no trivial task, otherwise i'd stick some balsa under the wing and try it :)

The mechanicals must be correct, visual inspection says this, as does the fact if i return the elevator trim to the center the elevator is level :)



and gjeffers, i'll have a play with that later, thanks [8D]

RE: elevator offset, cg issue?
 

I just bought one of these planes last week and my first flight had me on my toes. I leveled the elevator when setting up but when I took off I needed a mass of up trim for level flight. My instructions only gave a 100mm cg location but I worked that out to be 37% of the constant cord wing , so I set mine on the spar about 80mm back.
Landing was a task because it sank like a brick when I took the power off. I could tell this plane was very nose heavy. The stock tank that was supplied is too big and I did not need all this fuel for my asp 61 four stroke ( it would fly for well over half an hour) so I put in a small light tank. I also put the elevator servo in the tail and rebalanced, now I have 85- 90 mm back and the plane is much better and will land at walking pace. You can really pull back on the stick for a great three pointer.
Even though it flies well now I was amazed that I still needed all the up trim I had before, so I flipped it over inverted and to my surprise I only needed the slightest amount of down to maintain level inverted. This has to be an incorrect wing incidence, it pitches to the canopy badly ( I know this is usual for this type of plane) but if I alter the negative incidence I can take out all that up trim.

RE: elevator offset, cg issue?
 

Check out the thread on the Seagull Decath. I had to change incidence, build in thrust angles and add 2 degrees of washout ( which is a slow process on a finished wing ) Now my Decath with a Saito .91 is a dream. ;)