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HomeAircraftHelicopterFlybarless Rotor Head SystemsV-Bar › Vbar’d Trex 500 pitches up?
04-06-2010 10:04 PM  8 years agoPost 1
stevehof

rrApprentice

Oceanside, CA, USA

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My Trex 500 pitches up at the last 10-15% of collective on full power punch outs. The pitching is pretty aggressive. I’d say that if I didn’t back off of collective or correct with cyclic the ship would be inverted in less than a couple of seconds. It doesn’t slowly go nose up as you would suspect in the case of natural boom drag. I’ve tried elevator comp of up to 30 with no change other than it tips a bit nose down as it climbs until the collective is about 90%. At that point the dang thang wants to pitch over backwards. Naturally, this is more than a bit disconcerting. Other than this weirdness my Vbar’s flight characteristics are fine in every other area. When I check my swash in setup mode there is absolutely no interaction anywhere. I did the swash leveling at both top and bottom stick. The only thing noticeable was when I had the elevator comp up around 30 you could see the swash tilt forward a bit when I pushed from zero to max pitch, which is what I would expect to see with that much elevator comp

I am using a Futaba 10CHG, Hitec HS5065MG for cyclic, a Hacker A40-10s on 13T pinion on 6s 40C lipos. I will include a couple of photos. I would add my .vbr file but I'm not sure you can attach files in RR??? I am using BobOD’s I2 RC FBL head. HELP…!

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04-07-2010 02:08 AM  8 years agoPost 2
mikejr718

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omaha.ne usa

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Look closely at your servos. I dont think those hitec mg servos are a good flybarless servo. They dont center very well at all and endpoints are not equal. The gears are notorious for getting sloppy and causing all kinds of problems. Remember with flybarless the servo in effect has a direct link with the blade grip, unlike a normall flybarred machine that mixes the servo inputs thru the flybar for stabilization. All it takes is one servo with a slightly different endpoint and the swash tilts at full collective. I have fought this battle before. Do yourself a favor and get 3 good digital servos. The align ones are decent for the money, I think the new spektrum servos are actually rebadged aligns.

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04-07-2010 02:16 AM  8 years agoPost 3
Justin Stuart (RIP)

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Plano, Texas

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Could it be that your swash is binding at the end of the travel.

Avant RC
Scorpion Power Systems
Thunder Power RC
Kontronik Drives

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04-07-2010 02:39 AM  8 years agoPost 4
stevehof

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Oceanside, CA, USA

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Of course I can't know what the servos are doing under dynamic load. The total flights on these servos is less than 40. I put about 15 on them with an Align 3G and other than the well known 3G wobbles, there was no pitching issues. I then put the fly bar'd head back on for another 10-15 flights with no issues at all. This only started with Vbar. I'm talking about a near straight up punch out so there is almost no cyclic. Checking this on the bench there is no binding and the swash rises perfectly level from zero to top pitch...

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04-07-2010 06:43 AM  8 years agoPost 5
MrMel

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Gotland

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Either attach your setup file to the gallery, or PM me for email address and I can have a look at it to make sure its all Ok in the setup file.

CoG could cause it if its totally off.

Another consideration is vibrations, severe vibrations would cause it (most common reason), elevator precomp should not be needed to be adjusted.

You have checked "Level Swash" at min max (expert on the Level swash setting in setup mode)

Gone fishing..or hunting..or something
My site: http://heli.dacsa.net - VBar videos and more

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04-07-2010 09:25 AM  8 years agoPost 6
Stolla

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Port elizabeth South Africa

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As mr MEl states the collective has to stay level throughout the range which is something you have to do in setupmode so servos with different endpoints should not matter.
But what i've done in the past on a friends 500 with a booming issue is to set up the atv's(endpoints) to give a slight down(forward) elevator at max and vice versa at min collective (setup step 5 doctors hat) this seemed to do the trick, very easy, so worth a try if your cg setup etc is correct

Common sense may not be common after all

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04-07-2010 07:03 PM  8 years agoPost 7
stevehof

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Oceanside, CA, USA

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Thanks to all! If nothing else turns up I'll try the end point trick. BTW, the heli is smooth as silk at hover and looks good on high speed low passes. CG is dead on but I might try it a wee bit nose heavy to see if that affects the climb out pitching.

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04-10-2010 12:59 AM  8 years agoPost 8
stevehof

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Oceanside, CA, USA

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FIXED
Moving the battery around a bit to alter CG did little to nothing. Using the fine adjustment for leveling the swash at top pitch did the trick. However, the swash is now obviously quite 'unlevel' at the top in setup mode and when pushing it up when the gyros are active. It took about 5 to 7 degrees down to remove the upward pitching at the last 10-15% of throttle/collective. Seems mighty weird but the ship seems to fly just fine now. I didn't identify any other interactions with the few flights I put on it this morning...Gahhhhggg...I'm not too sure I'm ready for another electronic flybar...I've tried the 3G and that was a bust and this Vbar on my Trex 500 is not exactly playing by the rules either. Mechanical stuff....Gooooood....electrons...very hard to see...

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04-10-2010 03:02 PM  8 years agoPost 9
Stolla

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Port elizabeth South Africa

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Thing is if you put vbar on a logo its perfect as geometry is designed for vbar, or the other way round if you want. When you use it on another heli you have to more often than not tweak to get it perfect. Sometimes, as in your case,there is perhaps a geometry-setup issue which can be very difficult to fix except if you have a intimate knowledge of all the parameters involved, thats when it gets tricky.
Sometimes pitching up can be cured by loosening the blades in the grips a little or trying different blades other times it calls for more serious intervention but when you get it right the satisfaction is guaranteed, and you cant go back!

Common sense may not be common after all

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04-15-2010 08:07 PM  8 years agoPost 10
BobOD

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New York- USA

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Well, the problem is, there is no "perfect geometry" for a v-bar on a 425 bird...they don't make one. Actually, I consider the "perfect geometry" thing over-rated anyway. I try all kinds of geometry changes on my T500 and my Logo 600 and there are plus and minus effects with all...depending on your likes and dislikes, flying style...and a whole lot of other factors. Of course a Mikado Gyro on a Mikado heli with a setup file for that heli certainly should work. If you want to mix, unfortunately, you'll have to adjust....thus you want something that is easy to adjust.
All that aside, it's my experience that the bigger the bird, the easier it is to tune in. But, you shouldn't get discouraged. The end result is way less interaction and speed and agility than you ever get with a flybar.

OH, BTW, this is my best guess as to the trouble you were having. To me, it sounds like your tracking was off a little (or going off under high loading) and thus causing vibration that fouled the gyro. What blades are you using?

Team POP Secret

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04-15-2010 08:54 PM  8 years agoPost 11
jones007

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Monterey, CA - USA

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Not having spent much time with a V-bar system, can someone clarify - is this in a purely damping mode or a heading-hold mode? In a damping mode, non-linearities in the swash travel will manifest as un-commanded movement of the heli, but if it's in a heading-hold mode, all non-linearities or asymmetric swash travel "should" be compensated for by the electronics. as long as no cyclic command was given, the heli should stay in the same attitude as before.I suppose if the gain is too low, then the system might allow drift.

I guess I was under the assumption that the 3-axis systems worked like a HH tail gyro, where the stick now commands a roll or pitch-rate rather then a swash displacement, but perhaps I was wrong.

--Kevin

Trex 700N, Trex 600ESP, Trex 500, Trex 450Pro

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04-15-2010 09:02 PM  8 years agoPost 12
MrMel

rrProfessor

Gotland

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It's a HH mixed mode, different ratios etc depending on what you do.

(it's using Integral which is HH mode)

Gone fishing..or hunting..or something
My site: http://heli.dacsa.net - VBar videos and more

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04-15-2010 09:16 PM  8 years agoPost 13
jones007

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Monterey, CA - USA

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Controls aren't my background, but I work with control systems a lot, and frankly I can't see why the V-bar system should care if all the cyclic servos moved the same. Does a heading-hold tail gyro care if the tail servo is non-linear, not-trimmed (except for drift if the gain is too small) or has uneven end-points? Not a chance. In the end, it's not the servo pot that closes the loop here, it's the 3-axis sensor head, and with a pure collective input it should send whatever command is necessary to each of the cyclic servos to make this happen.

The only other explanation I can think of is if some of the servos become saturated (hit the end of their travel) and perhaps the V-bar puts more priority into collective than elevator, so it tries to add that last bit of pitch even though one or more servos cannot, and it lets the elevator command pass through in an effort to achieve the desired climb rate. Does the V-bar also have 3-axis accelerometers? If not, I don't think that it can have a control loop that involves collective, so I would think this option would be eliminated.

One last option is that the sensor head moves w.r.t. the airframe under heavy loads, in this case with an elevator-type movement. What kind of mount is used to anchor the sensor head to the airframe?

--Kevin

Trex 700N, Trex 600ESP, Trex 500, Trex 450Pro

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04-15-2010 09:27 PM  8 years agoPost 14
Stolla

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Port elizabeth South Africa

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"Well, the problem is, there is no "perfect geometry" for a v-bar on a 425"
Off course nothing is ever perfect but we're talking technology developed for a specific heli over a period so let's say it's "perfect" for now, till they find a more perfect solution tomorrow!

"Actually, I consider the "perfect geometry" thing over-rated anyway."

I can assure you having 9 vbar helis of all types and sizes and having done many more conversions, geometry plays a major role in vbar performance, thats why mikado released the online geometry tool.
If geometry is not correct yout heli will not fly well thats a guarantee.

Common sense may not be common after all

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04-15-2010 09:27 PM  8 years agoPost 15
rotaryguy

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Florida

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Its your heli paying you back for that wiring job..

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04-15-2010 09:42 PM  8 years agoPost 16
BobOD

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New York- USA

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Does a heading-hold tail gyro care if the tail servo is non-linear, not-trimmed (except for drift if the gain is too small) or has uneven end-points? Not a chance.
Partly true but not completely. If the output is non-linear, then the effective gain would be altered. For example, if there is less response in a certain region of the servo travel, then the gain (commanded response as a function of measured error) when in that area will be reduced. The control will say, "this much error requires a response of x" but x is reduced by the lack of linearity.
This would cause more subtle quirkiness though...I do not beleive the condition described here has anything to do with linearity.

I'm with those that suggested vibration on this one.

Team POP Secret

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04-15-2010 09:53 PM  8 years agoPost 17
BobOD

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New York- USA

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Its your heli paying you back for that wiring job..
Now that's just mean. True...Accurate...but mean.

Actually, I don't think this is it either. If the heli could see the wiring job, it would have just done a nose dive into the ground.

Sorry Steve...just having a bit of fun.

Team POP Secret

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04-15-2010 10:07 PM  8 years agoPost 18
jones007

rrApprentice

Monterey, CA - USA

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I guess V-bar systems (and probably others) are not straight PID loops like typical tail gyros, as I don't think the swash integrates off to a limit if you hold the stick with a small displacement when the heli's sitting on the bench (although I've heard that they are really integrating an attitude internally, so this will kill you if you mess around with the sticks pre-spool-up). I'm curious what the control system looks like. It would be awesome if there was an open-source platform, so we could try loading our own control laws in. Or maybe there is - there's an open-source autopilot for UAVs.

--Kevin

Trex 700N, Trex 600ESP, Trex 500, Trex 450Pro

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04-15-2010 10:46 PM  8 years agoPost 19
stevehof

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Oceanside, CA, USA

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Hah, for those of you who don't like the wiring, remember that is a Futaba system and therefore, I think, a few more wires than a Spektrum? In any case, I never did care for burying my wires inside the frame. That's just too much work on a small heli for what I consider a minor cosmetic improvement... Besides, when I decide I have the Vbar performance maxed I'm going to move it under the receiver tray...there's no way I can do that now and still have access to the USB port

If the ship has a vibration I sure can't see it so I don't know how I'd troubleshoot it. The blades have been balanced and the ship has never hit the ground before. I have 3 other T500's so I'm not a newbie with this airframe.

In any case, I have a set of Edge flybarless blades that just arrived today. I'm going to try them tomorrow and see if they behave better at the top end of a pitch pump than the 425D's that are on there...

The bottom line is that I can live with the current performance of the Trex + Vbar and Bob's I2 head. Although it would be nice if I didn't have to jury rig adjust the swash top level position. While I don't much care about the gob of wiring, an out of spec swash trim adjustment to correct a weird electromechanical interaction bothers my Teutonic bloodlines...

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04-15-2010 10:56 PM  8 years agoPost 20
rotaryguy

rrApprentice

Florida

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Hitec...full size RX with bank switching =

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