I have been fussing over my flap setup on my Skymaster L-39, and I'm at a standstill. No matter what the orientation of the servo arm I still get servo buzzing at the 'up'/flaps retracted position. There is no buzzing at mid (1/2) and full-flap extension. I'm using a JR x9303 TX, with the 3-position toggle for Norm/Mid/Land, and each flap servo is a Futaba S9155.

The flap control horn has only one hole. Due to the mounting in the wing, only the end hole of the servo arm protrudes enough from the bottom wing skin to be useable. Both the servo arm and control horn are on the bottom of the wing, with the servo arm moving toward the nose as the flaps are 'pulled' down. Right now I've got the servo arm positioned where at full flaps the servo arm/linkage clevis just barely clear to wing skin, to try and get the servo arm, spindle, and linkage as close to "in line" as possible. This puts the servo control arm a bit less than perpendicular to the linkage rod.

When I physically reposition the servo arm on the spindle to have it more in-line when the flaps are up, the buzzing is still there, but seems less intense. However, this puts the arm at 90 degress to the linkage when the flaps are full down. Seems like this will cause the servo to work hard to hold the flaps in the full-down position.

Should I widen the slot where the servo arm protrudes so that I can use a hole closer to the spindle? I hate the thought of this, since the slot would be kind of ugly if wide enough to allow the clevis end to go down below skin-level.

I'm a turbine newbie...maybe I'm making too much fuss over this.

Thanks,
Sluggo

Are you using clevis on both ends ? if your control horn and servo arm are not in line this could put some strain on the extreme end of servo travel. If this is the problem using a ball end one side could solve your problem.

Chaitanya.

Yes, both ends are clevises...hadn't considered a ball end.

I was also wondering if I have the neutral/zero point wrong. Right now, the servo arm is zeroed at the flaps-up position (the arm slightly forward and a bit less than perpendicular to the linkage rod), with mid- and full-flap 25% and 50% of travel (these numbers give the 'book' extension distances).

Thanks for your help!

Sluggo,

I'd suggest putting one of the Hangar 9 inline current monitor meters on each of the flap servos so you can see how much current is actually being drawn. Some high-performance servos will buzz but not pull tons of current .. that's really what you need to know - a little buzzing with low current draw is no problem. The H9 meter is a very good investment for setting up jets in case you don't already have one...

Also, I usually set the mechanical mid-point of my servos on flaps to the mid-flap position to be able to get a good mechanical setup at either full-up or full-down .. but this depends a lot on the gemoetry of your flap horns which you may not be able to do much about. Can you post some pics?

Dave McQ

I have the exact same problem on my L39, I have spent ages trying to reduce the buzzing (current draw).
So any comments gratefully accepted
Regards
Ross

Use a ball on one end, the servo should center at the middle of the hole, and make the linkage at 1/2 flap from there. In other words zero should be a little more than takeoff position. The rest of the adjustments can easily be done in the radio.

Get an ammeter and use it when setting up flaps, so you will be able to nail the lowest current flow exactly. just plug it into the receiver between the battery and receiver and minimize current. Also if you have them separate channels, you will easily be able to match them. IF on Y cord, it takes more mechanical matching.

I use matchbox's alot on my flap setups. they make it really easy. IF it binds on both ends and you cannot enlongate the hole without having the clevis go inside the skin, then get a longer servo arm (use HD) arm. almost any good metal servo can handle flap loads if you dont put them down at 120mph..

Dave,

Good idea. I'll get a meter. As you mentioned, I initially had the servo mechanical midpoint set with the arm vertical, flap linkage adjusted for mid/takeoff, and MID on TX at zero. Then I adjusted the throw +/- for NORM and LAND as required. The buzzing at NORM/flaps-up seemed a little less with this setup.

Then, falling squarely under the "a little knowledge can be dangerous" description, I read where the flap servo spindle, arm, and linkage, should all be in a line to take the load off of the servo when the flaps are full down. That REALLY got me chasing my tail.

So, I plan to go back to the MID-zero setup. The question is where to orient the servo arm? Do I put it angled forward so that at full flap the servo arm at full-flaps is as in-line with the linkage as possible given the physical geometry of the jet? Or do I go back where at MID the arm is vertical, then angled back for NORM/flaps-up and forward for LAND/full-down?

Ross, not that misery loves company, knowing that you've had the same issue at least tells me this may have as much to do with this L-39 as anything else. In any case, I'm definitely going to get that H9 current meter so I know what's going on.

Thanks!

Randy

Goose,

I'll definitely change to a ball end. Now, I hate to sound dumb, when you say the servo should center in the middle of the hole, I assume that means the slot in the wing skin that the servo arm moves back and forth in, right?

I'm also using a Matchbox on the flaps, so matching the flaps up is pretty easy.

Thanks,
Sluggo

I'll post some pics of my set up later tonight, I had very little problems with setting up the flaps on my L-39, I am also useing 9303 radio, so I will copy my radio info and post that too......


Mark

here are a few pics of my flap set up. I actually have too much flap in it now as you can see in the pics. the pics were taken with the plane up side down..... I do not have any buzz in the flaps when in either any one of the 3 positions on my 9303 radio. the one pic shows full flaps.

It was easy setting up flaps, had one hell of a time setting up the gear doors as I have never had doors on any other aircraft before, but they work well too.....

Hope this helps.

Mark

more pics

last one............the rods I used were left overs from the Boomerang XL I built a year ago, I used different rods and had these left over and they fit real nice.

Mark,

Yes, that helps. The alignment/position of the servo arms and linkages are like what I had before I read the stuff about lining up the arm and linkage. WhileI have never been free of buzz, it did seem less when set up like you show in the pics.

Took the wife out to dinner tonight, but somehow ended up on the way stopping by the LHS for some ball-end fittings. She sniffed out right away the ulterior motive to my dinner suggestion!

Thanks for the help.

Sluggo

Randy

As others have suggested, get the amp meter and use it on all servo linkages to make sure you have no binding anywhere.

If you lift the flap sightly does the buzzing stop? Is this not just the weight of the flap putting a small load on the servo? As the flap lowers the load decreases. It will all be completely different once airborne. I really wouldn't worry about it. Unless it's that strained sort of buzzing if you know what I mean.

Alex

S,

Regarding the "servo arm straight at full flap". Yes, I know some people favor this setup .. to do it, you arrange the servo arm to be exactly parallel to the pushrod when the flaps are down .. then any force from the blow-back on the flap pushes on the arm in line with the pivot point of the arm and thus provides (theoretically) no torque on the servo. The only drawback to this technique is that small movements in the servo arm position at flap-down provide essentially no motion of the flap (that's the point, right!)

The side effect of this is that you cannot use the radio or a matchbox to trim the flap down position .. you can only use mechanical adjustments. This can make it a pain to trim any slight turning tendency with full flap, or do vary the amount of flap deployed at the full flap position, again since all adjustments have to be done by turning ball links or clevises.

Since we now have servos and arms that can easily handle the forces involved, I typically do not use this setup, but it's a matter of personal preference.

Dave

I've probably been overly concerned about the in-line setup, so I won't worry about that anymore...sort of. I will, however, get an amp meter. As suggested by Alex, this buzz may be totally due to the weight of the flap at rest loading the servo, since I can alter the buzz by lifting the flap slightly. I still like to know how many amps this buzz is costing me, though.

Now, my original servo arm alignment was like that shown in Mark's photos, where the flaps are full down when the arm is almost 90 degrees to the linkage. This was the position that had me worried, as this gives the flap max mechanical leverage against the servo, and a corresponding higher effort/current draw by the servo to hold the flap in position. So I shouldn't sweat this?

.

I'd say the 90 degree setup is fine, let us know what you find with the current draw measurement.

Oli (Olnico) may chime in here since he uses a modelling tool to compute torques and forces of jet linkages...

Dave

Will do. Had to order the H9 current meter, though. In the meantime, I'll put on a ball-end fitting and re-rig the arm/linkage.

RC

RC,

Just to be clear, if your servo arm and flap control horn are in-line and the servo arm is moving in that same plane (ideally, 90 degrees to the flap hinge line) then the ball end is not really necessary and may actually not be desirable. With the ball end mounted to one side of the control horn and/or servo arm, a twisting moment is created on the horn/arm, making the linkage less solid.

If the servo arm and control horn are not reasonably in alignment, then the ball link is an acceptable solution to resolve the geometry mismatch and potential load on the servo.

My best guess is that your servo buzz is just the resistance of the flap seating in the full retract position and not necessarily a linkage problem. The current meter will tell the story and it may be an acceptable draw or else just a click or two of throw off full up to minimize the draw/buzz.

Craig

Hey Craig,

I'll take a straight edge and check the plane of motion between the arm and horn. Just eyeballing it, they sure look to move in-plane. A ball link may introduce an offset, as you suggest.

I think you guys are right about the buzz being related to the resting load of the flap on the servo, especially since I can reduce or even eliminate the buzz when hold a little 'up' pressure to help the servo out, so to speak. I do want to know this amp draw, but it will be a few days before my meter shows up. Even if it turns out the buzz is nothing to worry about, it sure is annoying. I end up dropping the flaps to MID just to shut 'em up while I'm tinkering with other things

RC

=========== EDIT =============

Ok, here's the latest with what I found: I carefully measured the servo arm travel plane and the flap control horn plane, and they are not in-line, or even parallel. Then, using the horn as a baseline, the offset of the horn rotation plane from the servo arm grew/diverged at the arm as the servo/flap moved to flaps full-down. After some head scratching, I decided to try a ball link at the servo arm.

Well, the servo buzzing is much less pronounced, or even sometimes none at all. I definitely think what buzzing there is now is due to the weight of the flap itself. It seems clear now that some of the buzzing was from the servo working against the resistance in the clevis/linkage due to the misalignment between the horn and servo arm. When the amp meter gets here, I'll know for sure if the draw is minor now and not a worry.

Speaking of that, what is a "minor" current draw? What amount would be cause for concern?

Thanks.