RC Airplanes

I was wondering if anyone on this site had any expierence with SPAD (simple plastic airplane design) RC airplanes? I have been making a gas powered airplane with a 18cc weedwacker engine on it. My problem is that on the ground it runs like a top but, when it gets into the air it goes all squirly. I am pretty sure that when it gets into the air the air currents going around the aluminum channel body create a vibration. The vibration causes a radio interferance with the reciver. I have tried butting rubber bushings on all metal to metal surfaces but, i am still having problems. Does anyone have any ideas on how i could solve this problem?
Thank you.

Re: RC Airplanes

I used to work with a guy who was serious into rc planes. He made a wind tunnel out of plexi and used a blower off a household central hvac unit. He made a tube out of the plexi bolted together with supports on the outside and put some louvers in the entrance to straighten out the air flow-- kind of looked like the end of one of those french fry cutters. He had flat sheet metal used for ductwork cut and formed into a plenum to atatch the blower to the tunnel with wing nuts. What he used for smoke or how he introduced it into the tunnel I don't know. The thing was about 20" across and 4 feet long but I assume one can build as large as needed if u can find a big enough blower. There have to be plans on the web for something like that. With an 18 cc motor that must be a large plane--- I wonder if you can do basically the same thing with a smooth air flow and smoke minus the tunnel and just move it around the plane. All the tunnel does is eliminate outside influences. I wonder if you could use for instance an electric leaf blower with some louvers on the discharge and blow that across the plane as it sits on the ground?

Re: RC Airplanes

that is a very good idea the only broblem is my wing is 72 inches accross. I would need a massive wind tunnel and i know i could build it i just don't know what i would use for a wind source. Thanks for the idea i minght be able to work something out.

Re: RC Airplanes

Built a lot of RC planes over the years. Never seen a mechanical vibration cause radio interference yet.

What do you mean by squirrely? I have heard that term used to describe a number of scenarios from servos going whacky to just plain impossible to maintain control due to design or set up problems with the airframe.

If the servos are going whacky, have you done a distance test with it on the ground? Some people just collapse their transmitter antenna for this test but that is not always a good indicator IMO. How did you route your antenna- especially improtant for an all metal airframe?

If it's flight control issues, what kind of behavior is it displaying while airborne?

Is this an established flying field? The reason I ask is that I found one great place to fly but consistently experienced interference when the plane got above 20ft off the ground. Never did pinpoint the source but there were radio towers and power lines in the vicinity...

Re: RC Airplanes

I agree with Mrbiggstuff. I flew a long time ago and we really need more information. Can you be much more specific about what squirly means? Is it not responding correctly to control inputs? If not how so? Does it seem to change course on it's own? How is the antenna routed? If it is external to the airframe (say from the top just behind the wing to the top of the vertical stabilizer) reception is probably not your problem. If it is internal the airframe may be causing a reception problem. Not generally a problem with wood or foam, but is with any type of metal. Have you checked all of your control surfaces for any slop. It may feel marginal to you, but when it starts getting airflow over it at speed it will flutter and make life really interesting. They should be just as tight as possible when connected to your servo's. To check my radio function I would collapse the antenna as far as it would go and then I walked away from the plane atleast a hundred yards. If it works reliably like that then the odds are pretty good that it will work in the air with the antenna at full extension. I have also experienced outside interference and it was not fun. I did manage to get the plane down in one piece. Give us all the details that you can and we will try to help you. Good luck and examine everything that might effect control.

Re: RC Airplanes

The servos on the plane begin to jitter at high engine rpm and the plane begins to go out of control untill rpm is reduced. The flying field is an a large hill that is in the middle of nowhere. The antana is routed down the frame just like any other plane, and all controll surfaces are good and tight. I have never had any problem with any of my other planes at this site and i have been flying for about 4 years in this spot.

Re: RC Airplanes

Two things I can think of to try.
1. Have you tried a resistor sparkplug? Most weedeaters wouldn't have one.
2. Try routing the antenna out in the air, from the cockpit to the top of the tail, with a spark ignition motor, it might be getting interferience from the metal framework.
I don't have any other ideas, my only experience is with glowplug motors. If it doesn't do it on the ground, it's probably because the motor runs at higher rpm when in flight. I've had weedeaters cause TV interference when they are reved up.

Re: RC Airplanes

First thing that I would try is relocating the antenna to the outside of the fuselage. Secondly would be looking online and seeing if there is forum for RC airplanes and find out if someone has experience with this particular plane or atleast some experience with gas powered. I have had very limited experience with gas and it was a fiberglass model with the antenna external to the fuselage. It never did anything that you are describing. After you relocate the antenna I would colapse the transmitter antenna and while having someone hold the plane walk about 100yds and run the engine up to full throttle and see what happens. If nothing else it might give you some insight as to what it is doing. Not sure what else to tell you. Good luck.

Re: RC Airplanes

Thanks guys i will try all listed ideas. I did come up with a rig to test the plane at high rpm on the ground and it did have some servo jitter but not as bad as in the air i will expirament more thanks.

Re: RC Airplanes

When you do your test on the ground, either walk far away from the plane or operate with your antenna all the way down to simulate the lower signal strength when it is further away from you.

Make sure your receiver is vibration isolated with foam.

Make sure the receiver crystal has a tight fit in its socket.

Make sure no metal control rods touch other pieces of metal and vibrate against each other.

Are you using AM or FM? AM is way more susceptible to noise.

Re: RC Airplanes

I would imagine one of the new Spektrum radios would solve some of the issues also...http://www.spektrumrc.com/ Spread Spectrum 2.4ghz control. also uses 2 antenna's to recieve the signal and do comparison to determine if signal received is legit...

Re: RC Airplanes

I ran the antanna out the wing and put some more vibration dampaning material on the reciever and it seems to have helped. It still isn't perfect but, i will try a few different things. I also think that i am going to give up on the aluminum body and swich to a 4" plastic fence post to reduce vibration. Thanks.

Re: RC Airplanes

"Built a lot of RC planes over the years. Never seen a mechanical vibration cause radio interference yet."

Metal clevis or rod to throttle with vibration can cause an electrical current of it's own.
AM is asking for trouble with a gas engine with CDI ignition.
FM with a PCM receiver is almost a required item to eliminate interference.
DO NOT mount the engine with any isolator type mounts...this can cause harmonics that can destroy anything.
I've got a 4.2 sachs with no isolators because it kept breaking flying wires on my Pitts. I've also got a 8.4 sachs on a 40% Laser that is solid mounted.
My Carden Cap 232 has a DA-150 and with this bird, the weaknesses showed up in std servos used for throttle and smoke.
I learned the hard way never to use a non-ball bearing servo on ANY control surface, even on a 90-120 size bird. Digital if you can swing it, will help your problem more than anything.
A good prop balancer is a must as well.
30 years of this hobby got me to many unlimited competitions and 1 shot at the TOC.....I can fly with'em, but I can't compete with them.
I do think you are getting some interference from your ignition system..or, it could be vibration is causing intermittent loss of contact with non gold-plated contacts....system signal is strong on the ground...if the transmitter is either not "tuned" to the receiver, or you have changed crystals that are more than 1 channel away from factory stock, you will have radio problems sooner or later.

Re: RC Airplanes

I would suggest doing a ground radio check first. Collapse the antenna and walk 100 feet away from the plane. The throttle should still be smooth in its transition from idle to WOT and back.

Re: RC Airplanes

I spent the last two nights changing all the equiptment over to the new plastic frame and it has eliminated the problem all together. Thanks for the help guys.

Re: RC Airplanes

Sounds like you owe yourself a pat on the back too!
These engines are great when set-up right...just make sure to eliminate any metal to metal such as a metal clevis or rod in the throttle arm...at certain harmonics, these items create an electrical charge by themselves.
3 mistakes high my friend, and good luck!
Atleast that's what Dave Patrick told me years ago as I tried to beat his inverted pass(Lower) and drug the rudder of a Goldberg Sukhoi in the grass and caught hard.....:redface:...Re-Kit as we now call it....