Variable Pitch Prop - How to? - Page 3 - RC Groups
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Sep 11, 2004, 10:45 AM
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Bill Mixon's Avatar
Hi, guys
Sorry for not posting sooner. I wanted my next post to include a picture, so I borrowed my brothers digital, and got some pictures of my current setup.
Quote:
Originally Posted by rroback
I think we simply need to first, come up with a reliable variable pitch motor system, then work on designing a control system that is not to one direction as todays systems
Rhett
The ideas that everyone has come up with are all things that have potential, but the post by Rhett pretty much sums it up. The prop needs to be worked out first.

Bill
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Sep 11, 2004, 11:31 AM
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Hi Bill,
Thats a great picture. I see how much moment arm you have to actuate the prop- I'm a little worried with my set up as far as that goes, but we shall see. I'm still waiting on the 72mm x 3mm prop shafts. Once they come I'll be able to see if it'll work. Thanks for the picture! Here's what I came up with so far. Please offer any input and advice you can Bill. Your system looks like it'll work better then the way I did it. I copied from those wesites from Germany.
thanks
Jon
Last edited by JLeyland; Sep 11, 2004 at 11:33 AM.
Sep 11, 2004, 12:05 PM
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Bill Mixon's Avatar
Hi, Jon

I'm not sure yet that the gear drive setup is any better than the direct drive outrunner. An outrunner might actually work better than what I'm using because the total range of pitch is a lot less than what I'm using on my geared setup. In other words where I'm going to around +8 and -7 the outrunner may only need 4 degrees in each direction.
I'm using the long arm for the precision of it. It would work just as easily if shortened up, but then the slight amount of play at the linkage/pivot points would cause some excess flop in the setup.
I like how you have been able to use lots of off the shelf components to make yours.
Here are a couple of things I see..
The DuBro links look like they would have a lot of friction on the shaft when trying to rotate them. How free are they? I tried something similar to start with but it was a PITA to get the plastic parts to rotate freely without excess flop. If you can keep them and the other parts from binding up you should be ok.
Are your blades attached where they can fold (pivot)?
This is an issue that I'm not quite sure what to do about. With mine I have to tighten the blade screws just so, to get the right tension. If I don't then it wants to vibrate. If they are locked down, then in a crash..... Well I think you know what can happen then.
Overall it looks good..
I'm curious to see exactly how you are going to control the pitch shaft. That's the biggest hurdle to clear it seems. I couldn't figure an easy way to turn the rotary action of the servo into linear motion at the actuator shaft, and still have a simple (low # of parts) setup behind the motor.
Even on my type of setup it was difficult. Starting at the servo it's a rotary action going to the straight linear wire linkage, to the lever which is back to a rotary action again, then that turns into linear action again moving the "slider" back and to on the shaft, and finally it's turned back into a rotary motion at the blade hubs. All those points and changes in the type of movement can add up to trouble if anything gets off, or is too tight.
Once you get everything working "free" then it's all downhill from there.

Bill
Sep 11, 2004, 01:51 PM
FNG
FNG
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FNG's Avatar
Just a crazy thought, how about a motor on each end of the A/C(pusher & puller), set props to be effective for pulling in the direction they are facing. This would give good thrust forwards and backwards. Airflow over ctrl surfaces foreward and backwards. Still with veriable pitch props, but it would solve the prop blade issue as well as the airflow over the flight control issue.

Have fun and keep us posted.
Sep 11, 2004, 02:33 PM
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"The DuBro links look like they would have a lot of friction on the shaft when trying to rotate them. How free are they? I tried something similar to start with but it was a PITA to get the plastic parts to rotate freely without excess flop. If you can keep them and the other parts from binding up you should be ok."

Hi Bill! They are 'pretty' free. It doesn't take much to move them. Do they have to be almost frictionless? Should they feel like they are on a bearing? I may have to redo that part of it. I worked them back and forth for awhile and now they can rotate very easily when you take the front white plastic clevis' off but there is still a bit of friction.

"Are your blades attached where they can fold (pivot)?"

They can pivot about 5 degrees back and forth. I'll have to move the holes out more - I forgot about when I crash what will happen!

"I'm curious to see exactly how you are going to control the pitch shaft. That's the biggest hurdle to clear it seems. I couldn't figure an easy way to turn the rotary action of the servo into linear motion at the actuator shaft, and still have a simple (low # of parts) setup behind the motor."

I've been talking with George H. a little bit about this - On the first unsuccessful attempt I made, (the earlier pic of that disasterous contraption)
I had a dubro wheel pant mount with two bearings inside it. The bearings were carefully soldered onto the shaft. I never figured it out past that but it seemed like it might offer a stable platform to drill into and attach a control to the servo. This was aft of the motor. On the current one I'm planning on a fixed shaft with a bearing pressed into the servo arm thats on the front. The bearing will be attached to the shaft. (not sure whats the best way to do this) Currently the bell (part with the magnets in it) will rotate on the shaft- it would be nice to fit a bearing into the bell, right now it just rotates on the prop shaft hole.
Since the shaft will be fixed I'm hoping it will take alot of complications out of the deal. I'll use the 3/32" micro control rod attached to the fixed prop shaft then wrapped with kevlar around the rear of the shaft. This will have a z bend in it to attach to the servo. If it doesn't work I'll have to prod George for more ideas.

Thanks for all the tips Bill - You've helped me understand alot more about this whole thing. I want to do one like yours also. What kind of servos are you using to actuate the prop? Have you found it requires a powerfull servo?

thanks Bill
Jon
Sep 11, 2004, 03:09 PM
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Bill Mixon's Avatar
Quote:
Do they have to be almost frictionless? Should they feel like they are on a bearing?
That would be best, but it's not something you have to have. Really it would work best to have ball bearing pivots in every moving part, but you know how impractical that is. That's what makes a heli tail rotor work so good. Everything is very precision, and typically ball bearing mounted.
Just try to make all your connections as free as possible without adding slop. Too free can be just as bad, or worse than too tight.
Quote:
I've been talking with George H. a little bit about this - On the first unsuccessful attempt I made, (the earlier pic of that disasterous contraption) I had a dubro wheel pant mount with two bearings inside it. The bearings were carefully soldered onto the shaft. I never figured it out past that but it seemed like it might offer a stable platform to drill into and attach a control to the servo. This was aft of the motor. On the current one I'm planning on a fixed shaft with a bearing pressed into the servo arm thats on the front. The bearing will be attached to the shaft. (not sure whats the best way to do this) Currently the bell (part with the magnets in it) will rotate on the shaft- it would be nice to fit a bearing into the bell, right now it just rotates on the prop shaft hole.
Ah, I see. That's where things get complicated. Either way will work great, but having the bearing at the rear behind the motor is very difficult to me it seems. The bearing in the front arm seems to be a better arangement, but then you have the shaft stationary while the motor is rotating around it like you mentioned.
BTW: Is Hicks working on a reversable prop as well??? I'll have to get in touch with him if he is. I'm sure he has some ideas.

I can't remember which servo this is that I'm using. The label is gone. It's a Cirrus 20,21, or maybe 22 I think. It's about the same size as a hitec HS 55, and I have also used a GWS pico BB that worked ok.
The most important thing is not having the blades folded as the motor is started. That can strip the servo very easily. I spent some time getting everything working really smooth, so there is very little load on the servo.

I forgot to mention that the slider was the most difficult part that I made for mine. It has a single ball bearing in it so that the inner aluminum tube doesn't rotate on the prop shaft. It turns with it, and the other white plastic ring is held still and controlled by a small pin through the aluminum control lever.
Here is a picture of the other side.
Bill
Sep 11, 2004, 08:51 PM
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I'm just checking your designs out, and trying to figure out what I was trying to say, when I previously posted. man, I gotta work on my proof reading. both your designs are nice, but then seem very fragile. I'm not digging the servo horns ( with the weight of the linkage etc) spinning so far from the shaft, thats a lot of torque going on to little nylon parts. also, just to remind or alert you guys, these systems are now ending some safety of a typical prop plane, in that, if the prop comes off, you dead stick, or the motor jams wor whatever. with these, one blade is more likely to get messed up, causing a hell of a shake. bill, is that white collar I see teflon? or is there a bearing in it? are those heli tail grips or custom made? your system seems the most simple, in that you do not need a hollow main shaft, but also requires more space then the other, hollow shaft, style. just getting some thoughts out

Rhett
Sep 11, 2004, 09:40 PM
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Bill Mixon's Avatar
Quote:
also, just to remind or alert you guys, these systems are now ending some safety of a typical prop plane, in that, if the prop comes off, you dead stick, or the motor jams wor whatever. with these, one blade is more likely to get messed up, causing a hell of a shake.
Rhett, with as many moving parts as there are, it does make the prop more prone to catastrophic failure. That's just the nature of something like this. Believe me I know all to well what can happen. On one flight I had a ball link to come off (it was messed up and too lose) which caused full pitch deflection in one direction which was opposite the other blade. It was a interesting flight.
As long as everything is in good condition though it's not that much of a concern. Helicopters both RC and full scale fly with a fair safety record when it comes to mechanical type failures like you mention.
Quote:
bill, is that white collar I see teflon? or is there a bearing in it? are those heli tail grips or custom made?
It's not Teflon, not sure what it is, just some scrap plastic material I had laying around. It's harder than Teflon. The bearing is pressed into the collar through the front. This was done after the aluminum hollow shaft had the bearing and arm both pressed and bonded to it. I guess a drawing would help better explain how it goes together. It looks big in the picture but it's pretty small, and was the hardest thing to make. Off the shelf heli parts that would fit would be a much more easier way to do it. I made all of the parts including the grips and the hub. Machined from 6061T6.
Both methods of pitch actuation have thier pros and cons. What I used is more complicated to get working.
Bill
Sep 11, 2004, 09:46 PM
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you should machine something to replace the servo horn. I'm just looking over your system. how does the little round white thing ( not sure what to call it) connect to the servo horn?

Rhett
Sep 11, 2004, 10:02 PM
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Bill Mixon's Avatar
Quote:
how does the little round white thing ( not sure what to call it) connect to the servo horn?
I asume your talking about the modified servo arm that the ball links are attached to? The ball bearing is what connects it to the collar on the slider assembly. Think about how this thing looks while it's turning. The modified arm, links, and prop which is attached to the aluminum tube, (barely visible in my pics) and the inner race of the ball bearing are all rotating together. The outer race of the bearing with collar sits still as it's held by the pitch lever.
Does that make sense?
Sep 11, 2004, 11:23 PM
Nickel what?
Phreakish's Avatar
is the servo horn 'keyed'?? I mean, how does it spin with the shaft? Is it only the force exerted by the ball links, or do you have a square section or something?

If its not 'keyed' or held on the shaft with something that gives positive anti-rotational holding force, I can see problems there - at least if you add a whole lot 'o power to the equation, haha.
Sep 11, 2004, 11:32 PM
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Bill Mixon's Avatar
I might need to post a drawing of how the slider goes together. The servo arm is plenty strong enough though as is. Even the larger glow rc helis use a much thinner section of plastic in most cases. I would like to machine another inner aluminum tube with the integrated arm just for the extra precision. The strength of the arm is not really an issue. There isn't or shouldn't be that much load on it.

Here is a site with full scale tail rotor pictures and information that might be useful. It shows both methods of pitch actuation (through the hollow shaft, and the slider)

Bill
Sep 11, 2004, 11:38 PM
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Bill Mixon's Avatar
Quote:
Originally Posted by Phreakish
is the servo horn 'keyed'?? I mean, how does it spin with the shaft? Is it only the force exerted by the ball links, or do you have a square section or something?
If its not 'keyed' or held on the shaft with something that gives positive anti-rotational holding force, I can see problems there - at least if you add a whole lot 'o power to the equation, haha.
It's not keyed. Your right, only the ball links flat surface on the bottom or back keep them 90 degress to the arm. You have to look at how low the load is here. There basically is no torque load applied to the links. Only the friction of a very good ball bearing.

Bill
Sep 11, 2004, 11:55 PM
Nickel what?
Phreakish's Avatar
oh, there's a bearing in the servo horn too? If so, then its not THAT bad, ha

I can see where there'd be almost no force involved there, but I'd be worried about a snag or 1/2 second bind that could possibly knock one of the ball joints off... Of course I'm also imagining this rig on a 300W setup, lmao. Good work, looks like one of the simplest and most effective designs.
Sep 12, 2004, 10:29 AM
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Bill Mixon's Avatar
Quote:
oh, there's a bearing in the servo horn too?
No, well not in the arm at least. I think we all might be getting confused as to what part is being discussed. I don't know what a servo horn is for example. I know what a servo arm is though, and I think that's what you and Rhett seem to think is so fragile. It's plenty strong as you can pick the plane up by it. And in normal operation it will only see grams of force in any direction.
The arm is pressed onto a piece of aluminum, and the bearing is on the otside of that part as I explained above. Remember the aluminum tube with all that is hooked to it rotates with the prop shaft. It's not spinning on the inside of it or anything so there is no load or torque being applied. I probably need to draw it out to make it more clear.
Quote:
Of course I'm also imagining this rig on a 300W setup, lmao.
Mine is operating at 80 watts and less.

Bill


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