This is very strange. Plane, 4*40 with a Magnum .52 4 stroke. Radio gear Futaba 4EXA with standard 3004 servos and reciever. R168DF


A couple of times after starting the engine and picking it up to head out to the runway, the throttle will go wide open. It mainly does this if I hold the nose of the plane up. Radio in my left hand and plane in the right hand. And its the throttle servo opening, not some sort of lean condition. It will only do this while the engine is running and NEVER on the pit table. It happened once in the air when the plane was very close and today it happened just before take off ending the day with a broken spinner where it met a concrete ledge.

There are no other glitches and the range check is fine, in fact, it wont do it when the plane and radio are pretty far apart, but if Im holding the nose up with the engine running and get the radio close, itll rev wide open allmost every time.

My thoughts are that I might be getting some sort of antennea refraction. I had this problem once with a CB antennea mounted too close to a TV antennea. When I keyed up it would open my neighbors garage door. LOL

Since I cant duplicate the glitch on the bench, I figured I would ask yall before I just try re-wiring everything.

Thanks in advance.

The first thing I would try is replacing the throttle servo. There may be a vibration related problem within the servo. I have had similar problems that I traced down to a bad connection on the servo circuit board.

If the problem persists with a different servo, then you may need to send your receiver and transmitter in for check out and repair.

Lots of guys have reported issues with a TX too close to the plane.

Can you experiment by having someone watch your plane; taking the TX out to the field box; then moving the plane next?

Or, leave it on the ground securely tied....and move the TX closer and farther to see if the glitch re-surfaces? Rich

I have an extra servo and reciever and might give that a try. I somehow dont think its a vibration issue.

Thanks

Thats what it seems like Rich, when the radio is close it is worse. Think Ill try what JPMacG suggested and replace the servo and while Im there, swap out the receiver. A couple of us will be heading to Scobee in the morning if you would like to come out and fly with us.

Scobee too far. But thanks. I belong to the Houston Sport Flyers and the field is only 15 minutes from my house. Rich

Cool. I'm 4 miles from Scobee.

Gary, I think the problem may be that the tx is swamping the rx when they are too close together. This can happen with FM PPM systems. I have one plane where if the tx is too close (2-4 feet) the rudder starts jigging but by moving away it stops and does not occure in flight. The distance is also dependent on whether the antenna is extended. It is worse with the antenna extended.

Bruce

How close was the transmitter to the receiver when this was happening??? were the batteries on your radio fully charged? things like this can happen when the transmitter is sitting right next receiver, especially with fully charged batteries. The transmitter is simply overpowering the receiver and causing it to go crazy. If it doesn't happen after you a few feet away from the plane then you should be ok. But if you are holding the transmitter in one hand and holding the plane as well you could be overpowering it.

Ken

I will try leaving the TX antenea down untill I'm ready for take off. Thanks Bruce.

Thanks Ken, thats most likely what it is. Being an old electric rc car racer, my batteries are not only charged, I peak them! I have had these electronics in another plane and never had this problem though. I'm a little worried that this could be a safety issue and do you think I should change out the RX and servo, and maybe re-route the RX antennea anyway?

Thanks for yalls help. I allmost ran the plane into myself today. I can replace the plane, I don't want anybody getting hurt.

Edit: BTW, it only happens when the engine is running. It will not do it without the engine running. Does this mean something?

Yes, this is very important!

This often happens when metal is aloud to rub against metal, and I think this is the real cause for the problems rather than saturating the RX with a too strong signal.

My advise is to replace the throttle pushrod (or metalwire in this case) to a plastic one!

I just tried something else and I have no idea if it means anything. Engine not running. No matter what I do, I cant get a glitch unless I touch the TX antennea to either the engine or the throttle cable from the servo.

I thought that was only a problem with gas/spark plugs?

Gary, if you throttle cable is metal, your clevis is metal, and your throttle arm on the engine is metal, then even with a glow engin you can generate static. Is this the situation with your setup? If it is the problem could be a combination of swamping the receiver and generating static when the engine is running.

Bruce

In the original post he said it happened in the air and while taking of. I'd go back to a vibration problem in either the RX or servo.
Rich

Yes, but the servo horn is plastic! Shouldn't that be an insulator between the engine and the servo though?

And yes, the throttle arm is metal and I'm using an EZ connector on the throttle arm, which is probably not helping huh?

Hummm. This is finally sinking into my old arse brain and making sense thanks to you guys. And now, I have no idea what to do besides pulling the tank, motor and trying to figure out a new way for throttle control.

The strange thing about that is, it only happened twice after 40+ flights that seemed to be an intermittent issue and couldn't be duplicated. Holding the plane nose up while the motor is running, with TX and plane in hand is when it happens the most often.

Yea, weird I know!

It could be everything suggested all in one shot. I brought a "Challenge" didn't I? [:@]

Gary, when I said clevis, I was refering to the connector from the cable to the engine's throttle arm. Looking at your picture with a metal ez-link attached to the metal throttle arm, that could be the cause or a contributing factor. What you might do is solder a DuBro threaded solder stud to the cabel and use a nylon clevis to connect it to the throttle arm. This will eliminate metal on metal. I use an ez-link at the servo end of the cabel to allow the connection to be adjusted for length.

Bruce

The servo is probably sensitive to vibration. A ny-rod pushrod connection can help isolate the servo better.

To me that picture shows the real problem.

You have metal rubbing against metal there and the metal wire also acts as an antenna for that. This type of problem will only show up when the engine is running. It caused by vibrations and will happen for any engine (gas, glow or diesel). A plastic clevis and preferably also a plastic pushrod should solve the problem.

Garry, It looks like your receiver antenna is running right next to and parallel to your battery wires to the receiver for a few inches. That is a very bad layout. That will lead to a lot of coupling you don't want. Get them separated by as much as possible. Dan.

Check your TX ant. Just may be looseing contact.
Do a running range check

Cean your TX ant. and get any oil or fuel of it. I have seen a ant. intermitent when it is extended by loss of contact at joints. Will be OK when down.

That was the first thing I thought of and that is something I can do quickly. Replacing the throttle set up wont be easy.

Well, I did some wire re-routing and will see tomarrow if that helped. If not Ill being into heavy surgery replacing the throttle linkage, RX and servo.

Thanks for the help guys. We learn something new everyday.