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Old Sep 23, 2011, 09:21 PM
VOLTS > AMPS
stgdz's Avatar
United States, MN, Buffalo
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Fighting CG, thrust angle, and trim. Should a super decathlon be this difficult?

Been really fighting with this one, I have a hobbyking decathlon that has been giving me nothing but trouble. Its current problem is two fold.

-It dives
-It snaps randomly

I have followed this page , http://www.scootworks.com/rdrc/trim.html ,to try to get everthing correct focusing on CG and thrust angle. I first attacked the diving (at 3/4 and 2/3 it still dives, hell at full throttle it still dives) with all the controls and trims zero'd by adjusting the CG, pushing the battery back would result in porposing. After figuring it out that it was the thrust angle I added a couple of washers to try and push the thrust angle up a bit.

After trying it out tonight it still dives a bit but not so much (I didn't want to add a lot of washers and make a drastic change to it). So my next problem is when i put it into a stall it drops the right wing and will start to spin down. I am able to recover it but it will snap a lot.


After balancing it out level tonight I went flying again. I went into a tight loop tonight and it snapped out of it at the top of the loop and went into a full spin, I was able to recover from the spin thanks to scott stoops wonderful book but I have noticed that it is snappy for me and spins a lot. I have went vertical with it a couple of times and it has snapped on me.

I am going to add an additional washer so that I can get it hands off at 3/4 throttle but the snappiness of this plane is something that am having trouble figuring out, I mean its a high wing plane not a cap232 and at times it feels like I am flying my cap232.
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Old Sep 23, 2011, 09:40 PM
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As far as the snap on a tight loop goes, you have to much elevator throw. Cut it down some. When I set mine up I adjust the elevator just to the point of " not " snapping at the top of a tight loop. That might also help with some of your other issues. ENJOY !!! RED
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Old Sep 23, 2011, 09:55 PM
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Did you actually try moving the CG forward? Sounds like you only adjusted it backwards.
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Old Sep 23, 2011, 10:16 PM
VOLTS > AMPS
stgdz's Avatar
United States, MN, Buffalo
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Quote:
Originally Posted by ChillPhatCat View Post
Did you actually try moving the CG forward? Sounds like you only adjusted it backwards.
All the way till it touched the motor and then I moved it slightly back, putting it in a 45 degree dive would dive down even more so I started to push the battery back.
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Old Sep 24, 2011, 10:04 AM
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About the diving.. why didn't you just add a few clicks of up-trim on the elevator

Until you have the model flying level 'hands-off' then doing the 45 deg dive test is pointless.

A Super Decathlon really should not snap. It could be that you are simply pulling back too hard and/or the model is a bit heavy and you are not keeping adequate flying speed, but it's worth checking the the wings aren't twisted as it can cause a snap if the tips are twisted to a higher angle of attack than the root..
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Old Sep 24, 2011, 10:15 AM
flying since 2000
Winterthur ZH, Switzerland
Joined Mar 2009
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check the decalage (in german: Einstellwinkeldifferenz, don't know if decalage is the exact translation). My Hobbyking Decathlon (the .46) had a completely wrong decalage, I corrected it to 1, but after this correction I still didn't get lucky with this plane... there somewhere has to be another thing that's wrong with it.
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Old Sep 24, 2011, 10:37 AM
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Quote:
Originally Posted by JetPlaneFlyer View Post
About the diving.. why didn't you just add a few clicks of up-trim on the elevator

Until you have the model flying level 'hands-off' then doing the 45 deg dive test is pointless.
Until you get the down thrust right the S&L trim will change with the throttle. Using elevator trim to compensate for too much or too little down thrust can result in disaster, with a dive into the ground or a nose up and stall when the throttle is closed.
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Old Sep 24, 2011, 11:57 AM
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Quote:
Originally Posted by RobC(UK) View Post
Until you get the down thrust right the S&L trim will change with the throttle. Using elevator trim to compensate for too much or too little down thrust can result in disaster, with a dive into the ground or a nose up and stall when the throttle is closed.
But the OP appears to be saying that the plane dives all the time.. which would indicate up trim required. I'd agree if it flies level at low throttle but dives at high throttle then that's a thrust line problem (too much downthrust) but that's not what i read in the OP's post.

Stgdz, can you clarify; is the plane diving all the time or only at high throttle settings?
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Old Sep 24, 2011, 04:53 PM
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Remember that the Thrust angle must be down, in your first post it seems you used washers to put the thrust up.
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Old Sep 24, 2011, 09:35 PM
VOLTS > AMPS
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Took it out again, it does dive at almost all throttle levels. I tried to trim it out but it would still want to dive. Putting it in a 45 degree with no throttle would result in a near vertical dive with the plane wanting to pitch down a bit more.


this was with the washers on the top to add some up thrust and it looks like that was wrong so I am going to add the down thrust on the bottom of the mount but won't that just cause the plane to dive even more? Its really tough though with the snapiness of this thing, putting it inverted results in a snap, some near vertical climbs and it snaps out, its just really touchy. I am trying to get the diving corrected first and then go after the snapiness.



I am pretty much done with these cheap planes though after this and my cap 232. I got that back up in the air tonight and it flew level and it wasn't snappy at all, well the reason it wasn't snappy was due to the fact I had no elevator control. It would just barely move up and down. I was able to get it back to the strip but landing with no elevator control it was a bit difficult to flare and it took out the landing gear. Everything else is fine but upon further inspection the cheap skates who built this plane used a balsa stock to connect the two so you could easily flex the non horn side keeping it level while the horn side was at full deflection. Its the first time I have seen something like this and I am pissed. I am 1-3 on the cheap kits(the kinetic 800 is still going strong but the cap and the decath have given me nothing but problems) and 2-2 on my Horizon Hobby kits (T-28 and radian pro have been wonderful).
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Old Sep 25, 2011, 01:16 AM
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Quote:
Originally Posted by stgdz View Post
All the way till it touched the motor and then I moved it slightly back, putting it in a 45 degree dive would dive down even more so I started to push the battery back.
If all else were correct then an increasing dive would mean the plane was tail heavy, less stable.
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Old Sep 25, 2011, 02:34 AM
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Quote:
Originally Posted by stgdz View Post

this was with the washers on the top to add some up thrust and it looks like that was wrong so I am going to add the down thrust on the bottom of the mount but won't that just cause the plane to dive even more?
Packing out the top of the motor mount so the shaft points down is down thrust and will make the plane nose down when power is applied.
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Old Sep 25, 2011, 03:32 AM
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If it dives at all throttle settings then you simply need more up trim (and check that you are not very nose heavy). The acid test is to see how it glides; take it to height and cut the power. Hands off does it glide at a nice steady speed in a gentle descent or does it dive steeply? if it dives in the glide it's up elevator you need, not thrust adjustment.
If you run out of trim on the Rx then adjust the elevator pushrod... basic stuff. If lots of up elevator is needed to get it to fly level then I'd suspect that you are very nose heavy. You should be able to get the CG very close by using one of the online CG calculators to work out the balance poing:http://adamone.rchomepage.com/cg_calc.htm On a conventional plane like this you will most likely end up around 30% wing chord for the CG location so if you are much forward of that it backs up the nose heavy diagnosis.
It's pointless doing the 45 Deg dive test until you have got the model trimmed for straight and level flight 'hands off'.

If washers on top or bottom give downthrust depends on how the motor is mounted. On a motor mounted inside a motor box so mounted by its front face then washers at the top give up-thrust, the opposite is true for a motor mounted by it's rear end to a firewall.
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Old Sep 25, 2011, 07:37 AM
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Quote:
Originally Posted by stgdz View Post
Took it out again, it does dive at almost all throttle levels. I tried to trim it out but it would still want to dive. Putting it in a 45 degree with no throttle would result in a near vertical dive with the plane wanting to pitch down a bit more.
That looks like it is tail heavy to me.

I have just looked at the trimming guide that the OP has been following and it tells you that in a power off 45 degree dive a nose heavy plane will steepen the dive or tuck under.

Maybe this would be a better one to follow.

http://www.gbrcaa.org/acrobatfiles/Trim.pdf
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Old Sep 25, 2011, 07:47 AM
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Rob,
Agree 100% that guide is the very opposite of how to use the dive test to set CG. It just goes to show how careful you have to be with some of the garbage that's posted on line. However as I've said a few times already if the plane wont fly straight and level 'hands off' then there is no point doing the dive test as it will give misleading results.

Note the very first line of the (excellent) trimming guide you linked to:
Quote:
Trim your model for straight and level flight with the engine set at just over half throttle
Failure to follow that critical step will invalidate all the tests.

Steve

PS.. The OP's linked guide also has the downthrust test completely wrong!
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Last edited by JetPlaneFlyer; Sep 25, 2011 at 07:53 AM.
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