Balancing/CG on AP heli

[Goldenhour]

There's a lot of good willed people here. I doubt anyone is trying to tear anyone apart.

"Fun, learning, friendship and a mutual respect"

Just about every AP set up is a DIY thing.

[TowPilot]

Originally Posted by Crash_Expert

i kept my mouth quiet for a while but i had no idea he was talking about a pro setup.
i simply thought it was some kind of diy thing

sheesh people

one second they'll help you out and the other they tear you apart like a pack of wolves....

Please consider that you are not giving those participating in the thread, the courtesy of reading the thread, then not responding unless you have something positive to contribute or ask.

Just like the thread you responded to me in, not too long ago
Remember? You stated that you posted before reading the last two posts, assuming they had no content.
That lack of courtesy over and over is not going to give you the responses you desire.

Please, take a moment, before posting, consider the topic. You are getting responses that you don't like because you have not taken the time to read, absorb, then post. That is very discourteous of the other participants of the thread. Yet you expect them to be kind to you after your lack of attention.

Patience goes a long way.
You are among adults, behave like one and you will be treated like one.

This is yet another thread where you didn't take the time to find out what is going on before posting.

If you expect to be treated well, and apparently you do, then take the time to treat others well first.

You have been quite brash. Why would you expect anything less in return?

Be nice, it comes around.

In the AP forum, there are people making DIY setups using key fob cameras and small helis. There are also people using cameras, ground stations, RC aircraft and post production setups costing tens of thousands of dollars and they have been at this for years.

If you read a number of threads instead of simply popping in, posting the first thing off the top of your head, you might have noticed the context of the thread.

This isn't the Test Forum, so please, post in context. It is OK to be brash, off the cuff and bold there. But to other forums, please read, learn, be respectful or don't post.

Thanks in advance,

Mike

[rdlohr]

Originally Posted by pc3associate

We teater ours on a dowl. Putting the dowl under the landing gear. Getting cg so that dowl is just under main shaft with a hair bit of weight forward. Seems to be most precise method for us.

Very interesting approach that had never occurred to me. I've always hung the heli or held it sideways and used the main shaft and always found either way a bit cumbersome. Your way sounds less cumbersome. Are there any drawbacks to doing it this way?
(realize I have NO AP experience or AP helis, this is a general question)
Rick

[R_Lefebvre]

Originally Posted by Goldenhour

Wont just "trimming it out" wear out your swash bearing?
I made an eye bolt(bolt, washer, solder) for the brake disk bolt hole. I then hung it from my garage ceiling by some wire. I'll tell you, ever messed with a triple beam?

Interesting point about wearing out the swash.

How much is too much out of balance?

[pc3associate]

Originally Posted by rdlohr

Very interesting approach that had never occurred to me. I've always hung the heli or held it sideways and used the main shaft and always found either way a bit cumbersome. Your way sounds less cumbersome. Are there any drawbacks to doing it this way?
(realize I have NO AP experience or AP helis, this is a general question)
Rick

My honest opinion is this way is more accurate and easier to fine tune. But hey i coukd be wrong.

[makiedog]

Holding the blade grips perpendicular to the tail is the easiest and most accurate. A slightly nose heavy attitude may be good for forward flight, it will also cause the heli to wonder when you try to do a slow piro in a hover.

[jolyboy]

An autopilot can trim out imbalance, but its better to keep the CG relatively neutral because sudden changes in momentum means the time for the autopilot to correct creates conditions that can lead to shaking (bad for AP). This is because unlike a flybarred heli, an FBL neutral stick is untrimmed thereby leading to decaying of the servo position to level (ie. untrimmed) until an error signal is re-introduced from the unbalanced heli overshooting its set-point, leading to shuddering or shaking.

A CG too far forward causes issues like shaking (FBL) and unnecessary force on the servos until you get into fast forward flight. That's because the 90 degree phasing of the blades combined with the addition lift of the advancing blade crates a nose-up pitching effect. Therefore, the faster you go the further forward you should make your CG.

A CG forward of the main shaft is better for forward flight because it aids in countering pitch-up, while a CG aft of the main shaft causes major issues because the system has to counter the cg AND the pitching up.

For AP, make your CG dead on the center of the main shaft because you'd mostly be hovering. I tune my 450's cg slightly forward of the main shaft.

Like makiedog said, hold by blade-grips.

Joly

[Goldenhour]

Originally Posted by R_Lefebvre

Interesting point about wearing out the swash.

How much is too much out of balance?

I had a friend ruin/ wear out the bearings on his swash by relying on trim alone to keep his AP copter level.
Too much? I'd say more than a couple three clicks/ numbers on your transmitter "home page". If my copter doesn't stay level with all trim at zero I adjust something mechanically, not the transmitter trim.
Just my two cents, I'm still a beginner.

[R_Lefebvre]

This is because unlike a flybarred heli, an FBL neutral stick is untrimmed thereby leading to decaying of the servo position to level (ie. untrimmed) until an error signal is re-introduced from the unbalanced heli overshooting its set-point, leading to shuddering or shaking.

This isn't entirely true, at least for my system. The I-term will hold the trim to some degree during movements. This is actually a bit of an issue with how you tune.

A CG too far forward causes issues like shaking (FBL) and unnecessary force on the servos until you get into fast forward flight. That's because the 90 degree phasing of the blades combined with the addition lift of the advancing blade crates a nose-up pitching effect. Therefore, the faster you go the further forward you should make your CG.

This is an interesting idea. Might be the cause of a problem I have been fighting flying fast forward in Altitude Hold. My 450 is just a little tail heavy. In Stabilize Mode (meaning no altitude controller) it flies fast forward just fine. But in Alt_Hold mode, it starts oscillating badly vertically. I know that it's a control problem at heart and we're working on that, but maybe it's being exacerbated by the CG condition. (By high speed, I mean 60-100km/h on the 450).

I had a friend ruin/ wear out the bearings on his swash by relying on trim alone to keep his AP copter level.
Too much? I'd say more than a couple three clicks/ numbers on your transmitter "home page". If my copter doesn't stay level with all trim at zero I adjust something mechanically, not the transmitter trim.
Just my two cents, I'm still a beginner.

I was hoping for something more along the lines of:

"+/-2° when hanging from the blade grips" or "+/-5mm from the shaft centerline, measured at the skids". Because I don't use trims on our system, it's self trimming really, so that's not helpful.

I would say that, if you find it's out of trim, and you adjust the swash servo linkage, that would have the exact same effect (ie: wearing out) on the swashplate as your Tx trim do. It's nice to not have any radio trims, but it won't in fact benefit the swash plate mechanically.

[cbergen]

Just yesterday we had a customer here with a slight vibration issue using one of our Turbine Observers. They were very meticulous on putting the CG right on the Mainshaft.

I added another pound of ballast to the camera mount and the heli became very smooth. Keep in mind this is an aircraft weighing up to 50 Lbs RTF, with a camera mount capable of handling appx 10-12 Lbs of weight.

The little contour camera and go-pro just weren't heavy enough....

This particular design actually feels and flies best with a CG between 1-2" forward of the mainshaft. With the CG right on the Mainshaft, it tends to hover tail down and exhibit this minor vibe.

It never hurts to experiment a bit, adjust your CG, try different Rotor RPM's, different size blades, different profile blades, finding what works best with your setup and your flying style.

[jolyboy]

Originally Posted by R_Lefebvre

This is an interesting idea. Might be the cause of a problem I have been fighting flying fast forward in Altitude Hold. My 450 is just a little tail heavy. In Stabilize Mode (meaning no altitude controller) it flies fast forward just fine. But in Alt_Hold mode, it starts oscillating badly vertically. I know that it's a control problem at heart and we're working on that, but maybe it's being exacerbated by the CG condition. (By high speed, I mean 60-100km/h on the 450).

Yes, that would contribute to the problem you are facing. Try making your heli nose heavy, it will help. It might not solve the problem but it will sure help.

Flybars generate NEGATIVE lift in FFF, so the flybar pitches DOWN, countering the pitching up caused by the blades. If you have a Flybarred heli, pitching up isnt as much of an issue assuming the effect of the flybar is balance with the lift of the blades Read this http://w3mh.co.uk/articles/html/csm9-11.htm and this http://www.copters.com/aero/retreating.html

An interesting fact is that this is the reason 1:1 helis are rated with a "DO NOT EXCEED" speed. That is the speed at which the retreating blade stalls (RBS). Those heli's heads aren't phased at 90degrees, so when the retreating blade stalls, the heli rolls out of control. If we ever went fast enough to hit RBS our helis would pitch up violently, causing us to slow down enough to regain control

Originally Posted by R_Lefebvre

I was hoping for something more along the lines of:

"+/-2° when hanging from the blade grips" or "+/-5mm from the shaft centerline, measured at the skids". Because I don't use trims on our system, it's self trimming really, so that's not helpful.

The reason nobody here can give you a straight answer is because the Trex line of helis are designed for 3D, so the user has to get the CG as close to the mainshaft as possible. So if you were to ask Align directly, its likely they would give you as vague an answer as you'd get here You are like me in that you mostly fly your heli fast and smooth, meaning you should definitely at least go for a CG forward of the mainshaft.

Joly

[R_Lefebvre]

I made a new battery mounting plate to move it forward a bit. It definitely seemed to help stability a bit when I punch it up from a hover. Probably no surprise there.

Didn't cure the porpoising however.

I've have tried filtering it out, and gain-scheduling, but nothing works to stop it, just makes it slower or faster. The problem could actually be mechanical. Trying to fly 45° forward at full collective the swash hits it's limits. So trying to add collective actually pitches it back because only the front servos can move!

I'm not sure if I'm on the hard stops, but the pitch servo linkage is definitely not in a great place. The angles aren't helping.

[jolyboy]

If you are hitting limits with the rear servo, that could be a problem.

A cause for this could be high disk loading or low headspeed. If you increase headspeed you wont need as much collective or cyclic to fly at that speed.

What headspeed are you running? Does your motor bog down badly at max collective?

EDIT: Just had a thought, maybe the problem is vibration at full collective?

[R_Lefebvre]

Originally Posted by jolyboy

If you are hitting limits with the rear servo, that could be a problem.

A cause for this could be high disk loading or low headspeed. If you increase headspeed you wont need as much collective or cyclic to fly at that speed.

What headspeed are you running? Does your motor bog down badly at max collective?

EDIT: Just had a thought, maybe the problem is vibration at full collective?

Well... don't forget, that a 450 heli flying at 100 km/h, there's a lot of wind pushing down on that boom so it takes a lot to try and hold the pitch angle.

But yes, I do have a low-ish headspeed. I try to keep it low because I'm more interested in duration than aerobatics. In fact, I've got some asymmetric blades to throw on and will drop it even more.

Not sure on the headspeed now. I've never measured it. I typically just under-gear it a bit (1-2 teeth below normal) and set 80% on my governor and go with that.

[jolyboy]

My spin asym blades arrived yesterday

I was running 2200rpm for a while until it started annoying me at high speed. This is a video I took of the issues I was facing until I increased headspeed:



With your asym blades you wont need as high a headspeed to avoid the issue so maybe try it with the asy blades?

[R_Lefebvre]

That is similar to what I'm seeing, but it only happens in Alt Hold mode, not in Stabilize, so it's not a purely aerodynamic effect. It's a control loop problem.

It could also be that the altitude controller gives a uniform PWM increase to all 3 servos. However, the rear servo, since it is tipped up, is at a greater angle, so a given PWM increase results in less linear movement than the front two. Therefore, the swashplate is actually tipping back a bit. Just enough to cause a problem.

That's my current suspect.

[jolyboy]

What servos are you running? Is it possible that the elevator servo is hitting stall force?

[R_Lefebvre]

It could be that too, but I doubt it. I'm running Corona DS929HV servos on 2S direct. So they should be strong enough. 2.4kg-cm

?

I plan to get some TGY-306G-HV servos soon. My Coronas are getting fairly sloppy I think. That doesn't help.

I am working on a fix for the servo arm angle problem and will see if it helps. It won't hurt, that's for sure.

[jolyboy]

DISREGARD - I just re-read your post [Those servos sound strong enough.. but they could still stall if your bec is maxing out its current. Is your bec extrenal 5A?]

Assuming your mechanical setup is 100% good, its possible that you'll need to factor airspeed into the alt-hold algorthim. Maybe to increase elevator rate-P as a function of airspeed? Reason being that airspeed doesnt affect planes and quads on elevator like it does helis

[R_Lefebvre]

I don't know about increasing the Rate-P, but backing off the Rate Feed Forward might help.

Helicopters do actually fly a lot like an airplane at high speed. An airplane with the CG a little too far back...