Propeller wobble

[_deltaV]

I've been using the flaring ikara propellers cut down to meet the rules but the ones I use seem to cause the entire plane to shake while in flight, sometimes majorly. I 3D print prop hangers and the propeller nub, almost identical to the ikara ones, make my own shafts out of piano wire, and use the small glass beads identical to those that come stock with the ikara propellers. I've made sure the hook on the back is centered, that the prop nub fits very tight around the shaft, that the propeller blades are balanced by sanding them down, which seems to help but the wobble still persists. Can one of you veteran flyers please help?

[bernard]

Wind up the rubber and run the propeller once while you're holding the plane. Does it wobble still? Or is the wobble only in flight? It's possible the propeller is not mounted to the motor stick at the right angle.

[Less_Incidence]

There are 2 main possibilites.

The first is that your prop shaft is bent. I would check to make sure that the bend in your prop shaft is at an exact 90 degree angle.

The second is that the pitch is unbalanced between the two blades. I would use a pitch gauge to check if your blades are at the same angle.

[bjt4888]

dfaris,

You have already been given some very good responses to your question and I would agree with Less Incidence that shaft bend or uneven prop pitch should be analyzed. A couple of other things to check would be:

1. Maybe the fully wound rubber motor not staying centered on the prop hook. A diamond shaped hook or reverse-s hook will center the motor well. A round hook, not so well. If the motor is off center on the prop hook at launch you will get a slight "cam" action that will usually cause vibration and poor performance for part or all of the flight.

2. Something is loose. Probably you have already checked this, but if the nose bearing is not firmly bonded to the stick, you will get vibration.

3. Wing spars are too flexible. If you have this issue, it is usually pretty obvious as the wing might be fluttering. This would usually only be an issue at higher speeds and at the beginning of the flight. And, as you are probably discovering, the ballasted version of this year's airplane has to fly quite fast.

If this info and the other responses don't help, you could reply and attach pictures of your nose-bearing and shaft arrangement and of your wing and wing/wingpost connection.

Good luck,

Brian T.

[_deltaV]

Thank you for all the responses so far. Less_incidence, I suspected a bent prop shaft at first, but I've taken great care to keep the shaft straight while I make it and keep the bend at 90 degrees, so I don't believe this is the problem. Bernard, I run the propeller on just the motorstick with a tailboom and it still seems to wobble as well as during flight. bjt4888, I use a diamond shaped hook and my wing spars are quite stiff, to the point where I can run with the wings pointed up down a hallway with very little flex (this is how I test the stiffness of the spars). What do you mean by bonding the nose bearing to the stick? The only other thought that occurs to me is the glass bead being slightly uneven, but I tried using some very very thin plastic washers from some flight store and those didn't seem to help. I will have to check the pitch of the propellers, but they were stock before they were cut and haven't been exposed to any heat or cold to warp them.

[_deltaV]

What would be the best way to go about checking prop pitch? I'm limited by time at the moment as well as money, so is there a quick DIY way to measure the pitch of each propeller?

[DoctaDave]

Go on Ray Harlan's website indoorspecialties.com and go to the articles section. There is a pretty good article on pitch and how to make a simple pitch gauge. Links don't work from his site so I can't post a direct link to the article.

[calgoddard]

dfaris -

You have gotten some very good assistance from DoctaDave, Brian T, Less Incidence and Bernard.

You indicated that you cut down an Ikara propeller to meet the rules. Make sure that the blades of your propeller have equal area and identical shape. Otherwise your prop will generate asymmetric thrust, usually evidenced by shaking or vibration of the stab. Similar vibration of the air frame can also be caused by dynamic imbalance (blades have different weight) and unequal angling of the blades, aka unequal pitch, as already noted.

You seem to have ruled out a bent prop shaft. Make sure you use flexible O-rings to connect the rubber motor to the prop hook and the rear end hook. 90 durometer is a good hardness for these O-rings. Purchase the Science Olympiad size of O-rings from http://www.indoorspecialties.com. Hard Nylon O-rings can get locked on the prop shaft hook in a skewed orientation. Asymmetric coupling between the rubber motor and the prop hook can also cause wobbling. Hard O-rings can also ride off the prop hook during a flight. Flexible O-rings will not exhibit these problems.