When hovering, my heli likes to try to shift to the left (ailron) slightly, not a lot, very little though. Is this because the rudder is creating a force of wind blowing to the right? If so, this is normal, right? Thanks

Ernie

Anyone?

Yes, it's normal, it's do to the force of the thrust from the tail rotor.

yup, people forget that on a frictionless vehicle the tail rotor actually gives some lateral thrust...

Thanks everyone.

So I really should try to trim this out, right?

Ernie

I wouldnt worry about mechanically trimming it, should only take a couple clicks of right trim on your transmitter to kill the drift. However you'll be faced with the problem I'm faced with now.

The problem I'm facing is how to cancel out the left bank tendency when I give it right rudder. Its very slight but still noticeable to me. Doesnt happen with left rudder because, I'm assuming, of the rotation of the main blades. But because it only happens in one direction I cant use channel mixing to fix it. any ideas or is this one of those things you have to live with?

Hmmmm,
I'm guessing by right rudder, we're talking about moving the nose right.
I'm, also thinking this is a clockwise rotating rotor.
All that being the case and thinking back to Physics 1300; it sounds like gyroscopic procession. Force applied to the rotating disc (rotor) will cause an output 90 degrees from the input. Since you're trying to rotate the entire thing to the right, the force gets translated 90 degrees and makes the whole thing roll left just a little. There is also a little bit of Mr. Newton's "for every action -- equal and opposite reaction" in effect here too. It only happens when going to the right because of the tail rotor and it's own procession? Not sure about that. I'm sure you could work up about 3 full pages of equations as to why it's happening!
This is all greatly simplified, and I believe that if the rotor turned CCW and you tried to turn left it would roll right.
I could be wrong about all of this, but it seems logical.
As for fixing it -- I'm too new to helis to have a clue. Less rotor RPM? I'd for sure double check for binding in that direction and make sure all your alieron c. rods are of equal length too.
OK, class is dismissed!
Y'all feel free to correct my guesses here!

Here is my theory...
I don't think it has anything to do with the rotation of the blades...but I still might be wrong...

Ernie

Oops, I didn't clarify!

fastawdtsi -- you're absolutely correct in your explanation for YOUR problem/question.
My little physics lesson was pertaining to Eric's problem of left roll induced by right rudder.
Sorry, I didn't specify that on the post!

Docster, I think its even simpler than that...

if the tail rotor is not exactly inline with the main rotor disk when you apply rudder then you effectively apply a force on the side of the helicopter causing it to bank....

its why a lot of agile helicopters use an upswept tail rotor it makes the whole tail exert the force in line with the torque..

Well that and gyroscopic precession occrus 90degrees ahead [b]but[b] the force to cause the precession would have to be applied in the vertical axis for the main rotor to cause precession in the longitudinal axis.

Doesn't the tail rotor exert force on the vertical axis?
Maybe I'm not getting my head wrapped around this correctly?

But I think I understand what you're saying, Eric. The Applied force would be translated 90 degrees ahead, so on a CW rotating rotor, to get a left bank, you'd have to apply vertical force to the horizontal plane (rotor) at the rear. That would then cause a dip on the left side of the rotor and cause a left bank. (Are we all confused now?)

What gailfrey is saying, makes sense -- effectively the tail rotor's thrust would apply a latteral force below the center of pressure and make the thing swing like a pendulem to the left. However, pure theory would lead you to believe that this would cause the nose to dip -- 90 degrees aheard, CW rotor, right?
Since, we all know that it ain't a perfect, pure theory world, air friction, rotor wash, etc all affect it. So, now were back to square one. I'm guesing it's a little bit of all of our theories working together.

What's it all mean?
Eric's Heli is still banking to the left and we don't have a clue as to exacly why!

Sheesh! My brain hurts now -- I'm going to get some coffee!

I know the theory (lots o' reading), but I lack the practical experience. I KNOW I've STILL got a LOT to learn!
Thanks for all the info guys! My coworkers think I'm nuts beacue I can convert mertic to us and have the conversions memorized. I'd HATE to share all this with them -- they'd have me committed!

The tail rotor has three main forces it exerts on the air frame. Torgue Tendency (like the main rotor wanting to spin the aircraft in the direction of it travel, hence the need for a tail rotor), this force would act on the lateral or pitch axis but is negligible. Gyroscopic Precession caused by control changes in the longitudinal or roll which would couple to the vertical or yaw axis, which is why in forward flight without a HH gyro engaged the tail rotor tends to drift in the opposite direction of bank giving the heli a semi co-ordinated turn. Finally there is a thrust vector component which couples to the vertical axis and nullifies the Torque Tendency of the main rotor. Are we all thouroughly confused now?

fastawdtsi's diagram is useful when explaing drift in a hover and hence why this discussion began. Use a couple clicks of trim to null it out in a hover.

I think he also inadvertantly explained the tendency to bank left when right rudder is applied as well. When you feed in right rudder, the tail blades "bite" more air, increasing the thrust thrust vector which overcomes the few trim clicks you have to nullify it in a hover. IE nothing you can do but live with it.

I think I notice these alot more because I am a newb to RC heli's, so every little twitch and bank and roll and pitch is a big deal in my mind. As time goes on I probably wont even register the left bank tendency because my brain will be trained to automatically kick in a tad of right cyclic with right rudder. Hell this problem will probably dissapear once I get into actual forward flight!

Ok no more physics brain hurts!

It's been a long time since college physics... but I can add that I read somewhere that even the real heli's pitch slightly forward and slightly right cyclic to maintain hover to counteract all the forces involved with spinning rotors...

The book I read prior to building my Quick .30 even said to set up the swash pitched forward and to the right. I did and seems to work fine. Not that I can't say I don't get drift in hover... since our RC helis don't weigh what the real ones do, they are not as stable.

Allow me to correct a few things I think are wrong

> fastawdtsi's diagram is useful when explaing drift in a hover and
> hence why this discussion began.

This is assuming he tries to hold the main disk LEVEL. To hover he would need to be a little bit rolled so that the RIGHT is a bit lower (assuming a CW rotor; CW as seen from the top of the heli; tail rotor is pulling to left as in fastawdtsi's diagram)

> Use a couple clicks of trim to null it out in a hover.

You can NOT trim this out. You FLY this out by shortly applying a little bit of right roll

About Eric's problem of left BANK problem. It seems to me the same effect but now with changing forces. If you turn right the tail has to be pulled to the left, hence the tail rotor pushes left harder, causing left bank. To correct it you would need to roll the heli to the right a bit. This becomes tricky because you can not just roll to the left with the same amount after the turn to become stable again -- your heli has turned so that what was a left roll after the turn appears to be maybe a little bit of downpitch.
To illustrate this, consider a sudden 90deg turn right while hovering. After the turn your heli will be accelerating (1) forward: the right side of the heli was down before the turn, so after the turn the front side of the heli is down (2) to the left: assuming the heli was not pitching down (with the nose) before the turn, after the turn the heli will not be rolled a little bit to the right anymore and the tail rotor will push the heli to the left.

Docster, about the gyroscopic precession effect of the left-pushing tail.. I think you're nearly right, only the nose will go UP with right turn instead of down. More tail force to the left will cause a force downwards on the right side of the rotor disk, and on CW rotors this results in pitch UP. This might partially correct for acceleration (1) mentioned above, if these forces are carefully balanced in the design.

EVSCOTT
> Hell this problem will probably
> dissapear once I get into actual forward flight!

With forward flight you will get additional problems due to different airspeeds of the forward going and retreating blade, causing increased force on the left side of the disk (CW rotor) resulting in pitch-up tendency. Some people try to trim this out - of course the problem then returns in the hover. Others claim that the flapping head is solving this - I dont see that right now...

ADAMB
> real heli's pitch slightly forward and
> slightly right cyclic to maintain hover to counteract all the forces
> involved with spinning rotors...

I dont think that forward cyclic is required for this. As far as I know real helis have their rotor slanted forward because of other reasons such as efficiency in forward flight.

BTW the original question was a beginner's issue, but this roll due to tail force certainly is not. For additional discussion on this I think the question general discussion forum would be the right place, probably with a link to this thread.