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250 Class Electric Helicopters 250 Class Electric Helicopters manufactured by Align, Tarot, SYMA, Airhog, Chaos, HK and similar.


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Old 03-02-2014, 02:08 PM   #1 (permalink)
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Default T-Rex 250 pitch setting & servo calibration - newbie questions

Hi All,

This is my first post. As a first-time helicopter builder and flyer, I have some questions about setting up my Align T-Rex 250 Pro Super Combo w/ Flybar.

I've assembled the head, shaft, main body, and have just installed the links between the swash plate and the elevator, pitch, and aileron servos. I've done my research and figured out how to properly reverse the servos and set swash mix to get the desired directions for stick input.



I've watched a number of videos (Jasmine's, notably) and everyone seems to say I need to set the horns of the servos perpendicular to the shaft when at 50% throttle stick position - that seems to give 0 of pitch at the 50% stick position. However, reading the TREX 250 manual, for general flight they note 0 of pitch at the lowest stick position.



I'm not looking to do any acrobatics right now, and thus I don't see the current need for negative pitch like the manual notes for "Sport Flight" and "3D Flight". I just want to get it off the ground and learn to fly first.

Thus, what my engineering intuition tells me, is that I need to:

1) Set the stick at the 0% position with the radio and servos on
2) Install the horns perpendicular to the shaft - gives ~0 pitch for manual-specified link lengths
3) Install the links and set the subtrim to level the swash and get exactly 0 of pitch on the blades

Am I missing something?

Thank you for any help! I only recently took the plunge into the world of RC Helicopters, and already I'm addicted! Excited to be a part of the hobby.

Cheers,
Kevin
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Old 03-02-2014, 02:44 PM   #2 (permalink)
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Hey welcome to the best hobby in the world

I'm a newbie also but on my third build....the zero pitch at low stick your manual refers to is your pitch curve set within your transmitter, with zero degress pitch at mid stick you theoretically will have equal negative and positive pitch as the servo will have equal travel distance. It is advisable to have some negative pitch on your heli in case you fly on a windy day or catch an updraft or wind and the heli rises, without negative pitch you'll really struggle to bring the heli down.

Hope this helps and is clear....I'm absolutely positive that there's a load of other people on here that can explain this far better than me so if what I've stated doesn't make sense please post again and hopefully someone else will be able to explain better
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Old 03-02-2014, 02:46 PM   #3 (permalink)
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Also check finless bobs 450 build videos....bigger heli but the build process is the same....I've just built a trex 250 pro dfc also with the 3gx...your gonna love your heli....great flyer when setup good.
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Old 03-02-2014, 02:50 PM   #4 (permalink)
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I know of 2 reasons for having a little bit of negative pitch at low stick:
1. if it is windy and there is some updraft, you can keep it from climbing.
2. on landing you can "stick" it on the ground with a little bit of negative pitch, while it is spooling down.
I have seen suggested a normal pitch curve or 42,42,50,75,100. For hold, I like to set the low value at 50 so I can easily and accurately check the 0 degrees without trying to find mid stick, I just put on hold and have the stick at low.
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Old 03-02-2014, 02:53 PM   #5 (permalink)
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Mutna said it well, set up the mechanical stuff so it will have zero pitch at mid stick with 50% being the actual mid stick % pitch curve set up your cyclic pitch too with the swash mix.

Then you can dummy down the pitch curve. You still want it as leaner as possible tho so it's not all over the place. If I we're you I would pick 2 throttle maps, flat ones at that. Set one for 100% and the other 85-90%. You need a lot of head speed for this heli to fly right and may want to steer clear or a variable throttle as suggested by the manual. Constant head speed will be your friend and the higher the better on this heli.

Put your pitch gauge on and lock the flybar, set low stick -2 degrees , mid stick 0 and high stick like 10 degrees and try it out. Your going to want all the pitch you can get anyways on this heli because of the high disk loading. You will be all over the collective trying to do a stable hover without a mad yoyo effect. I run -12/+12 pitch and it's just right for test flights to tic tocs. Be careful if you choose to run low pitch on this lil guy.
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Old 03-03-2014, 10:28 PM   #6 (permalink)
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Thanks for all the replies!

I took the advice and set the mechanical pitch at 0 at 50% stick.




The video by Bill White was especially helpful for setting this all up; but he mentions in this video:

http://video.helifreak.com/?subpath=...CPM_Part_2.wmv

... a process in which he changes the rod lengths to set the travel of the swash plate... I only set mine to get exactly 0 pitch @ 50%, the remainder of mine in the kit are not adjustable, so I left 'em alone.

What exactly is "dumbing down" the pitch?
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Old 03-04-2014, 04:51 AM   #7 (permalink)
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Dumbing down the pitch I believe refers to adjusting the amount of overall pitch both collective and cyclic on your heli. For example having 12 degrees of cyclic pitch would mean a really responsive and fast moving heli, on the opposite side if you only had say 4 degrees of pitch the heli would move less and be "dumbed" down......so on the 12 degress at full pitch you would have 12 degrees and on the 4 degrees setup you would have 4 degrees max pitch, a third of what you have with the 12 degress setup. probably not explained this very well...post again if this doesn't help you.
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Old 03-07-2014, 09:18 PM   #8 (permalink)
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What I'm confused about is this:

If I have it mechanically set up for 0 @ 50%, how can I ever get a pitch/throttle curve like the one shown in the "General Flight" figure of the Align manual I posted above...?
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Old 03-08-2014, 03:17 AM   #9 (permalink)
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Once you have zero pitch at mid stick ie 50% throttle measure your full pitch range. So with your pitch gauge attached start at midsticj and the gauge should read 0 degrees or thereabouts seeing as yours is digital. At low stick measure what the gauge states then also at full stick. The readings should be equal ie low stick is negative 12 and full stick is postitve 12 or thereabouts. On your transmitter which looks like a spektrum you have an adjust list that has pitch curve on it . Access the adjust list you will see nor, and stunt and hold, highlight norm and you will see you can scroll through 5 points labelled L 2 3 4 H. Low stick being L and full stick being H with 234 equal distance points between L and H. Highlighting and selecting one of these you can see you can adjust a percentage number on the bottom of the screen and see the line on the graph change relative to what point you select be it L, 2, 3 or whichever one you want to change. If at full stick when you checked your pitch gauge you had positive 12 then having 100% selected at point H on the pitch curve would translate to 12degrees. Point 3 needs to be left at 50% as this is your zero degress pitch at midsticj upon which we build around. So if the manual wants 11 degress at full stick the easiest way for you to archive this is to put you gauge on and adjust point H on the pitch curve of your transmitter till you archive 11degress. The split the difference between point H percentage you adjusted to and mid sticks 50% ie 96% - 50% = 46 half that value ie 23% and add to 50% or deduct from 96% you get 76%. This is point 4 on the pitch curve. For point L pop the gauge on and adjust the percentage on the pitch curve till you read -5 like the manual recommends. Do the same sum above to get point 2. I know this is long winded, sorry but it's the only way I can explain it....someone else here will probably explain it a hellova a lot better so post if what I state doesn't make sense.

Also look at finless bobs 450 build videos, he does pirtch and throttle curves on those and explains it so well. The 450 might be bigger but the process is still the same for your heli.

Last edited by mutna; 03-08-2014 at 05:09 AM..
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Old 08-11-2020, 05:36 AM   #10 (permalink)
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Hi can anyone help I have a align trex 250 dfc I have set it up at middle stick 0 12 top 12 bottom but I takes full stick to take off and not nice to fly when it comes down I put it at full stick hits the floor hard it's like no power keep it up I running it full power I have many helicopters I setup all the time so it's not that u have any idea cheers
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