Tail Servo (now engine) Question

[The Mechanic]

So on my 700N gas I've got everything pretty much the way I want but the tail is... as somebody here put it; a little unsettled. It's not a tail wag it's more of spontaneous little twitches that spoil perfection and trigger my OCD. Pitch pumps are fine. I'm using the RC Zenoah with a heavy flywheel so I wouldn't think an occasional miss of the engine would be a big deal.

I'm using a ProModeler HV brushless tail servo. It should be as good as a tail servo can get but on more than one occasion I've installed an MKS DS95i (or DS-8910A+) tail servo on my smaller electric helis and seen dramatic improvement.

So for next season I'm seriously considering installing an MKS HBL880 tail servo just to see if I can get a rock solid tail.

My question simply; am I expecting too much from a high vibration gas heli?

[Colin Broughton]

That tail twitch means your motor is tuned to maximum performance

[The Mechanic]

Thanks for the reply; so everybody's gas heli tail is a little twitchy?

[Dawiev]

Hi, Yes, I would think an " occasional miss of the engine" would be a big deal in terms of tail holding.

I would only judge the tail servo to be a problem, if you've used straight throttle curves - no governor, engine tuning is complete and running smoothly.

[pilotmotor]

Its probably a little rich on which ever needle it is pulling from when it misses.(varies by carb and gear ratio )
If it hovers on the low needle (normal mode) and that is where the miss is then lean the low out a touch and retest.
The low needle always feeds , the high only when its slot is exposed ( part throttle) . An occasional miss in normal mode is ok but should be gone in idle up 1. It will pull from the high circuit in idle up 1 or 2 so adj accordingly

[The Mechanic]

Hi, thanks for the replies. So you're saying you don't have any "twitchiness" with the tail on your gas heli?

The engine is running as smooth as it's going to get. Like the other gas engines I've observed at the field they're not perfect except at wide open throttle which is where they're designed to run best.

But mostly I guess I'm asking what other peoples experience is getting a solid tail on a gasser is.

[pilotmotor]

post a video of the tail , we can compare.
this is a youtube video of how stable mine tail is (although its not my heli ) I have a trm270 in mineT-REX 700 Gasser - HELIBUG HB7 - Zenoah G290RC/ 29CC - Torpedo Slim - 17T (7 min 22 sec)

[The Mechanic]

This isn't a very good video but sadly, for now, it's the best I have. I tried making a better video a few weeks ago but I failed; had the manual focus out of focus and the zoom too wide of an angle. I'll try again soon I hope. The main problem is I can't run the camera and fly the helicopter at the same time.

https://www.helifreak.com/showthread...=2#post6667458

Even in the video you shared it's hard to really see good; not like being there anyway.

[The Mechanic]

OK; that was a good idea I didn't think of; just look at YouTube videos. This video is blurry as heck but I can tell that is the steadiness I'm looking for.

T-Rex 700 gasser (5 min 30 sec)

Makes me want to order a new MKS tail servo. I think the servo I'm using is worse than it was last year when I installed it. Doesn't make any sense but there it is.

Thank you for the help.

[GREYEAGLE]

The Carbi behavior doesn't behave that way either > Why the big advent of carbi swaps.

Looking for the perfect transition : To eliminate the puddling or Blurb or twitch.

CAN you get it near perfect ? Yes and its not in the tail servo.

3 things you must be aware of >


The desired Blade RPM { Tach em } for your sweet spot for that particular set
Normal base line around 1800 < Kinda Low > But near perfect for cruising - Leaves room for the top.

The Carbi Opening : Most develop Max Ventury at around 70 % butter Fly Opening / Max Power : Right in the middle of the transistion circuit > The Puddle or Blurb or Twitch

Then they must shake hands with a a Non -Liniear throttle curve { Must stay in the 70% opening mechanical { Unless you experiment with carb's trying to get a perfect transition}

Getting around it is known as the Malorie Mechanical Expo using a servo wheel .
" Lot of servo wheel rotation" for little butter fly opening - Leaves a Plateau in the curve - with 30 % REMAINING . Carbi really can't apply it do to ventury effect { Maybe the New ones can ? With out a dwell in power.

You can get them near perfect . RPM / Curve / and the Mechanical Expo of the throttle opening .

Then it's on to your Needle Adjustments - & Don't get into the bad habit of chasing them - just get em correct and stay on the Fat Side .

You also have throttle pitch and throttle hover adjustments on your trims on the radio for ambient conditions.
Essentially your fine tuning the RPM Sweet spot

Each trim will usually give you 3-4 point on + or - of that particular curve from the face of the radio .
By fine tuning on that particular day : Temp & Humidity : It's a Beautiful thing !

Examine all three adjustments as a singularity that must work in unison

[FrankFuss]

I have a Goblin 700 gasser with a Spirit FBL, and I get a slight tail wag hovering that I have never been able to tune out. It does not affect how the heli flies however. I have tried different tail servos (BK BLS-8005HV and MKS HBL990) and different tail sliders (currently using the KDE tail slider). My tail linkage is perfectly free, and the tail slider can move on its own with the pushrod unhooked from the servo. Playing with the tail gain in the Spirit FBL has not helped. It is something that I have learned to live with because the heli flies very well.

[The Mechanic]

Thanks Frank,

That is very helpful and I won't feel bad now if I buy that MKS servo and nothing changes. I too am very happy with how my heli flies. Perfection would be better though.

[pilotmotor]

head speed sounds low from the video , the head speed needs to be in the range the engine was tuned for . My TRM270 should run 13K-14,5K , 7 to 1 gear ratio =1850-2070 or so at the head as per the instructions that came with the engine. My 990 carb is a little fat at 1-1/4 on the high 80+ deg temps. Low is around 1-1/8 I think.

[The Mechanic]

Well it was a great day for flying; perfect weather and I had the field to myself so I didn't feel rushed to take my turn and get off the field. Sadly no video since I was by myself. 94 degrees and light wind.

I can't imagine why I haven't noticed this before; probably because I just wasn't looking for it; but for sure the tail of the heli follows the tone of the engine. It's for sure an engine issue and not a tail servo issue.

My carb has been 1 L and 1-1/4 H. It's been there for the last 2 years. My head speed in a hover is 2000ish with a 6.58 gear ratio; so about 13,160 on the engine.

I tried a little leaner and a little richer with both screws independently. I tried richer with the L and leaner with the H. And vice versa. No change. I then flew about half an hour with 7/8 L and 1-1/8 H. Spark plug temp 200 degrees. When it's good and warm it's a little better; not bad really; but a little annoying if I think about it. The L screw is probably pretty good because I can check that with the engine running (of course). Anything above 1 turn and the RPM drops rapidly. Between 1 and 7/8 it seems happy (with some burbling). I didn't go any lower than that because it's so far out of spec.

So the first thing I'm going to try is different fuel. I'm running Amoco Ultimate premium with 32:1 Husqvarna synthetic or synthetic blend. The reason I buy premium gas for the power tools, and the heli, is I think it has the best chance of having the best additive package in it. Fuel stabilizers and detergents. The higher octane should run cooler (and produce less power) in a low compression engine too.

I've been happy with 7-Eleven regular gas in my car so I'm going to try that in my heli. Same oil. It might make a difference. I hope so because if it doesn't it would mean either there is an air leak somewhere or this is as good as it gets with this stock engine. Right?

When I slowly transition from hover or just gliding along to faster forward flight when the stick gets to about 75% (by feel, I didn't look at the channel monitor) the engine totally smooths out and the tail settles to rock solid. Best of all at full collective/full throttle it continues to run perfect. This is my favorite part anyway.

I always fly in idle up with a 100-70-50-45-50-70-100 big U shaped curve. I tried my V shaped idle up 2, which I never use, 100-50-100 and it didn't change anything in hover.

Edit: As if I haven't rambled on long enough already one other thing I was wondering is whether changing to a pipe shaped muffler from the stock "can" would make a difference. I like the small can shape muffler but if a long pipe would make a difference I'd be willing to try that.

Edit Again: Oh snap! I just checked the user manual on the Zenoah; they recommend premium level octane. I guess "regular" gas isn't going to be a troubleshooting option.

[Dawiev]

I have very little experience with flying on throttle curves, so this might be useless.

With your throttle curve that ranges so wide, where do you set and maximize your tail gain ?

So that it holds proper at around 50% throttle ? Then it will wag fast at higher headspeeds ?
So that it holds proper at high headspeed and throttle ? Then it will be "loose" and wag slowly at hover ?

IMHO, such a steep v-curve will also put a very big premium on engine tune. If it is not bordering on lean and able to accelerate fast and clean, it will bog and the tail will be loose ?

Ps. One of the advantages of a governor would then be the ability to add precomp for the tail and keep it more locked in during the variance in headspeed.

[pilotmotor]

what engine are you running? I know its an R/C Zenoah but stock , modified by who? , size ect...

[The Mechanic]

Engine is a stock Zenoah G290RC.

Adding a governor would only add another variable. With a set throttle curve the throttle doesn't change unless the TX stick moves.

The rudder gain is as high as I can get it without tail wag in FFF.

[Dawiev]

Originally Posted by The Mechanic

.....The rudder gain is as high as I can get it without tail wag in FFF.

This is what I was trying to get at : Your rudder gain is set in FFF, which requires 70% TH and gives a headspeed of x,xxx ?

So, if you then come back into a hover and you are now at 55% TH and a lower headspeed you also have less tail authority - slow wag.

[rbort]

Don't make it so high, the servo will be overworked and overheated.

Set a gain as low as possible to get what you need to get done done, not as high as possible before wag. In other words, when you give rudder and let go, the stop should be crisp but not overly aggressive.

But all bets are off with your machine, because you're using a stock 29, shakes the most. I would get it balanced you would be much better off and less headaches, believe me. In the meantime you need to keep the rpms down like you have it in the video, so you don't shake the machine apart...

-=>Raja.

[The Mechanic]

The rudder gain is only something I recently adjusted. When I first set up the heli, Spirit gyro, I set the rudder gain to about 60 or 65 and left it there until a few weeks ago. It seemed fine until I started getting picky. I found I could turn it up to 107 before the tail started to wag. The gain channel in the TX is now at 78 with a 1.3x multiplier in the Spirit software. That makes it 101. I can turn it down; it doesn't change anything because it's the occasional missing of the cylinder at hover throttle that is causing the tail to wiggle with the tone of the engine. 75% (approx.) throttle and higher it's fine. It's not vibrating (excessively) it's just not getting the correct mixture at less than 75% throttle. This is common with gas engines that use a carburetor.

I thought (from memory) the Zenoah would take regular gas but I was running premium anyway for reasons I stated earlier. But now that I look it up premium is in fact the recommended fuel. So I was going to try regular to see if that made a difference but that isn't an option.

So now I'm wondering about the oil. I'm using 4oz of synthetic per gallon. Maybe that is changing the mixture enough to cause less than perfect mixture below 75% throttle. I'm considering dropping to 3.2oz of ashless mineral oil per gallon. Zenoah says you can go as low as 40:1 based on the oil manufacturer's recommendations. 3.2oz would be about lowest amount of recommended oil per gallon.

Maybe pure gas without ethanol. I don't know where I would get it but Zenoah says 10% ethanol is OK.

I'm also wondering if a different exhaust would make a difference or maybe there is an air leak in the carburetor or the crankcase. Or maybe this is just one of those carbureted engines that isn't going to run like it has electronic fuel injection no mater what I do. That's why they invented electronic fuel injection.

One thing is for sure; it will fly 20 minutes without stopping which is what my primary goal was when I built the helicopter. So to repeat, I'm being picky. I'm just trying to get it as good as it can be.