up/down play in main gear

	Wah Wah problem
Back to Main Page	1. results of Jeff Anthoney - main gear up/down slack 2. results of Klaus Thane 3. results of Chuck Smith 4. results of Carl-Olof Sandberg

9-25-1 A fellow Raptor pilot, Tony, emailed me with the part number of an OS engine gasket that makes for a good shim in case you can't make your own. The OS CV15 engine head gasket part number OSMG6261.

7-15-1 I read a message from a person that tried the shim and it worked for a short time then the wah-wahs returned. I know some say they have the engine adjusted right, but something to be aware of is each time you have a lean run, the coil in the glow plug can be damaged. It can change it's performance so that while it works fine in the high end, it does not get hot enough for the low end. If the low end is too rich, then the engine won't transition well when it comes off the high end needle into the low end range. It's important to determine that the engine is not the cause when trying to figure out the problem.

As a note to the two fixes listed below, if you try them and still have the problem, please let me know. I also suggest examining the problem from a resonance point of view. It's possible that the two things talked about here do change the resonant frequency. Whatever you find to fix the problem let me know so we can share the info with everyone.
Note:7-4-1 Jon (referred to in Klaus' results) said the slipper clutch had a tight fit with no play. This further supports Jeff's idea of shimming the main gear to remove slop. If you do this, add the shim above the main gear to avoid wear on the shim during auto's.

These are the result of Jeff Anthoney

I went to the trouble of putting my wah-wah'ing R30 on a test stand this sunday. The problem sets in on my heli when unloaded - at low or 0" pitch, which I tend to increasingly fly since I've started crudely imitating aero's and scaring it with inverted flight.
The heli has a TT36 v0 with an updated low speed needle/10% nitro/Weston pipe. Apart from the pipe everything is v.stock, and if I say so myself in pretty good mechanical shape.
Mounted and warmed up I could readily reproduce the wah-wah's by holding around 0" pitch when switched across to idleup (throttle v-curve). Dropping the headspeed less than around 1400 (unfortunately guessing) would stop it, going higher would increase the intensity - more noise and violence. The main drive clutch was fully engaged in all cases by the way, on my heli it starts to grip just above idle.
During the wah-wah's I noticed the maingear assembly moving up and down in time with the transitions. Everything else, at least to my eye, looked like it was staying in position, ie. throttle arm, servo arms, washout assembly, blade tracking, clutch assembly, tail belt.
Having been a student of wah-wah recommendations to "richen up the low end" (Oh how sick I am of hearing that) I richened it up plenty (the engine was already running at high revs so I could afford to be generous), I had to increase the throttle to bring the headspeed up - sure enough regardless of the amount of raw fuel blowing out the exhaust the heli was wah-wah'ing away.
Next I disconnected the tail belt (I read a theory about this at some stage) - the wah-wah's were still there but at a much higher frequency.
The maingear moving up and down had my attention. The heli only wah-wah'ed when the transmission system was under minimal load - near zero pitch. The wah-wah's could be stopped by adding either positive or negative pitch, regardless of headspeed. The maingear can move vertically because of the play between the oneway clutch and the retaining circlips.
I pulled the maingear and made up a shim which was deliberately tight to lock up the clutch and remove the freeplay, if it worked I planned to thin the shim out to narrow the problem down to either the clutch or the freeplay.
Anyway I had time to run it up with the locked up clutch and sure enough no wah-wah's. I tried a range of headspeeds and really worked the collective with no sign of their reoccurence. Unfortunately I didn't get time to adjust the shim and run the heli with the clutch free. Hopefully over the next few days.
So, it looked to me like a drive train resonance. I can't get my mind around why just a maingear moving up and down would cause it, perhaps just a symptom. My uneducated guess is that there is some interaction between the maingear and perhaps elasticity in the pinion/clutch assembly. Anyway I hopefully have a more definitive answer on how to fix the symptoms (if not the cause), at least on my heli, soon.

Part 2:
I have had a chance to reach some conclusions about the cause of the wah-wah's that have afflicted my R30. In summary a drive train problem centred on the auto one-way clutch.
As I said in the original post my R30 has wah-wah'd at zero pitch or whenever the rotor system is otherwise unloaded probably for the 18 months I've owned it, my advancing flying skills have increased the aggravation factor with the heli now flying anything but scalelike most of the time.
After mounting it on a test platform and wah-wah'ing away the only thing I saw was the maingear moving up and down and the tail flicking to the left in time with the wah's.
The maingear, hard mounted to the outside part of the oneway clutch, can move up and down between the retaining circlips. I made up a shim which removed this play and was tight enough to also lock up the clutch. With this in place I couldn't reproduce the wah's.
Last night I thinned out the shim to let the oneway work again and still no wah's, despite really winding up the headspeed and working through collective transitions that used to send the heli mad.
I haven't flown the heli yet, but given how easy it was to get the heli to wah-wah on the test bed before, something has changed and I have reasonable confidence that the fix will stay (at least for a while).
As to the actual cause, I suspect the one way clutch. When it was wah-wah'ing the outside of the one way bearing was able to move up and down on the centre shaft of the clutch - as evident from the maingear moving. The clutch should be locked up, drag from the mainblades and the tail system should be enough to keep the clutch locked. Nevertheless the clutch must have been coming unlocked and maybe even slipping given how the tail kicks left, probably due to mainblades losing momentary torque. There could well be some effect which starts the process off, like overdriving the mainblades during a descent or throttling off, but once the wah's are established they set into a stable cycle.
Playing with mixtures or throttle/pitch curves may have some effect because the engine would have to be involved in the feedback loop, but I can say for certain that it won't work in all cases, I have tested this pretty thoroughly including flying and on a test bed.
Of course keeping the headspeed low, or engine throttled back, when the blades are unloaded prevents the problem, but this is very limiting once you move the intermediate type flight.
Removing the one-way clutch play between the circlips seems to have had some effect. I have also introduced a slight amount of drag, due to having a close fit. Either no play or a slight drag on the clutch has controlled the problem on my heli which previously had a bad case of the wah's.
Jeff

Part 3
Flew the heli for 5 tanks on the weekend - no wah's. Even did the unthinkable of leaning the mixture out, took off trembling knowing that I was insulting the heli gods, but - no wah's.
No wah's when I wound the headspeed up to 1800 at the bottom of the v-curve (zero pitch) and did some quick descents.

Have I already said - it didn't wah.
I haven't quite got over the happy feeling yet.
Jeff

Part 4
I have thinned down my shim to where there is no noticeable drag on the one way clutch and this works fine. Just removing the play seems to fix the problem with the standard bearing. BTW, Jerry posted on the Ace site that he put the shim above the maingear to avoid wear on the shim during auto's (wish I thought of that).

Klaus Thane from NW Florida emailed me with a fix for the wah wah's. He has an interesting write up, hopefully this will help others with the problem. Thanks Klaus for passing on the info :)
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Today at the field we were determined to get rid of our buddy James' wah-wahs. Now this guy has had the problem almost a year. The guy can hardly make a descent without his engine going into convulsions. We tried richening the low end to the point where the engine had to be started at half throttle and practically hand launched. It did absolutely nothing. This raptor was sending smoke signals. Each wah had big puff of smoke with it. Then another guy, Jon, said that he had read on your site that some guy had a theory about the up and down play in the main gear causing the problem. Then he said that someone else had wrote that shimming it just put a load on the engine at zero pitch. Jon had a split gear constant drive tail on his rappy, but didn't like it. He said it bleeds too much head speed in autos. My thought was that if you just have to put a little drag on auto hub, why not try the constant drive tail, that will force the engine to drive the tail on the way down. Jon was wheeling and dealing. He put the constant drive tail on James' helo. James flew around, did some descents. No wah-wah. You could tell the engine wanted to, but couldn't. So then he really tried. He really got the head wound up and still could not get the thing to wah-wah to save his life. So they made the trade, I guess Jon thought that it must have been a combination of hub and the Hatori pipe James was running. Jon put James' main gear on his Raptor. First descent he made, the Raptor that had never wahed in all its life went into convulsions. He landed it, still wah-wah-ing. He looked over at James' helo. James said "Don't even think about it." Jon finally managed to get his gear back. James has had a slipper clutch laying in his field box for about 3 months. I suggested putting the slipper clutch on. He flew again; no wah-wah. His wahs are gone for good.
Klaus
NW Florida gang

Here's another possible cause of the wah-wahs. This was emailed to me by Chuck Smith:
--Well I changed the bell and put the Webra back in with a new clutch. No more Wa Wa's. My guess is the clutch BELL was mis shaped and causeing the Wa's. (in fact when I compared the two it was out of round) In the end it is my theory that the engine alignment, clutch and bell are the root of the Wa Wa's. You must have the correct clearence as well as a really good fit of the clutch in the bell. Not canted in any way.

Here is some ideas on how to cure the Wah-Wah´s on a Raptor 30/TT36 :

During this spring I had terrible problems with wah-wah until I replaced the thottle servo(I am not sure why it worked , but it did). But at some of the last wekeends the Wah-Wah came back again in full storm. Whenever I offloaded the engine in Stallturns, Descends, Dives etc. the heli started the Wah-Wah dance. Increasing the main needle didn't help and the low-end was rich up to the limit.
But then I did a desperate thing! I moved the total pitch curve up with + 5% on the pitch subtrim(center) and closed the main needle 2 clicks. And all wonders, the Wah-Wah problem was gone!

My hypothesis (to why this measure helped) is that when you increase the hover pitch and lean-out the main needle you will actually hover at a lower throttle position. By increasing the pitch you need to reduce the stick to come back to the initial pitch level. The reduced stick will give a lower throttle, but that is compensated by the leaner mixture giving a more powerful engine. This is a way to move the throttle position at hover more and more into the working area of low-end needle. If you then have a very rich low-end needle , the engine will rapidly loose power when you reduce the stick below hoverposition. The result will be that the engine will not increase revs (or at least not doing it fast) when you reduce the engine load with lower pitch.
If you have the reversed with rich main needle and leaner low-end needle , you will hover with a rich mixture at a high throttle position and as soon as you reduce the throttle/pitch the engine will go into a leaner mixture and rapidly increase revs due to higher power and lower load. This is when the Wah-Wah oscillation can start.
You should therefore try to have a hover throttle that is lower than 1/2 (50%) and probably even lower. When you then reduce the throttle/pitch the engine will then fast go into the low-end needle´s most rich position that normally is somewere between 30-40% throttle.
Comment: The reason why the OS 32 engines is not knowned for going into Wah-Wah could be the characteriscs of that engines carb. To my knowledges the (not drilled) OS 32 carb has a problem with a lean midrange (hover) and very rich idle and rich top end. This will result in a low throttle position at hover and that the engine will go into a rich mixture when you reduce the stick.

And here is some thinking about oneway bearings impact on Wah-Wah oscillations.
The oneway bearing must be involved in the Waah-Waah oscillations (a continous low frequency change in engine rpm´s) My idea is that when you reduce the throttel/pitch and the engine rpm drops, the oneway bearing releases(due to lower engine rpm than rotor rpm). The needles in the oneway bearing will go out due to the centrifugal forces and want to stay there. It is only the friction between the needles and the bearings inner ring surface that makes the needles go into the locking position. e.i. The oneway bearing needs longer time to go into locking position at higher rpm´s.
A "thick" lubricant in the bearing will delay the needles to go into locking position! A too good lubricant will also delay the "locking" due to the lower friction! When increasing belt tension you will preload the bearing with increased friction and reduced distance needles/inner ring. This could be the explanation why some people say that a harder belt tension helps.
Carl-Olof Sandberg