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Feb 26, 2016, 11:58 PM
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Blade Theory Question


I have a somewhat odd situation with my RPG and could use some expert advice. Sorry for the long post but I'm trying to cover all the bases in the first post.

I built my RPG V2 blades (the new wider ones) per the plans, using basswood as the hardwood leading edge. I shaped and sanded to the plans profile, balanced spanwise, etc. and marked and drilled the mounting holes on the chordwise balance line. However, I did this before I covered them. Don't really know why.

The RPG flies fairly well, but it is quite pitchy, especially in windy conditions. I moved the CG quite far forward (18 degrees plus hang angle) and reduced throws, added expo, etc. On calm days it flew fairly well unless I even slightly over-controlled. Then it would pitch and/or roll in an oversensitive way. Windy days it was still overly sensitive. A good description would be pitch and roll "divergent." I finally guessed that maybe my blade mounting bolt holes were not on the chordwise balance point. Sure enough they were not and the blades are "tail heavy."

I stuck pins in the ends of the blade along the bolt hole chord line. placed them on two straight edges, and added weights to the leading edge to balance them. It took 8 grams! I duct taped these weights onto the leading edge at the existing chordwise balance point so as to minimize the tip weight "effect." The flying improved dramatically. The pitchiness and over-sensitivity was gone to the point that it almost seems a little sluggish. I only have three flights in this mode so far, all in windy conditions, and still need to decrease the large amount of expo I had previously added. I think this will probably improve the response to what I am looking for.

The bolt hole now is at about the 30% chordline. I understand that the center of lift is around the 25% chordline on a Clark Y type airfoil.

The blades weigh 58 grams without the weight, and 66 grams with the weight. Again, I mounted the weights at the existing spanwise CG so they are not in effect "tip weights." My theory is that this location just makes them "heavier blades."

So here are the questions:

1) Should I take the weights off and just plug and re-drill the mounting hole at the true chordwise CG, even though that is probably around 35-40%? This would have me around 58 grams.

2) Is 66 grams (the new weight after adding the balancing weights) an ok weight for the 24" RPG blades and I should give preference to correcting the blade chordwise CG to 25-30%?

3) If I permanently add the weights, would it help to move them inboard some more to reduce the tip weight effect? Or would this move the spanwise CG too far inward?

4) Whatever you guys think that I haven't thought of?

Thanks as always,

Bryan
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Feb 27, 2016, 12:58 AM
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Hi Bryan,

I have been through he same ordeal, in my experience (very little), the tips are the best place and have a better effect for desensitizing controls. Here is what I think, and by no means am I correct, just my thoughts and trial(s) by fire....

Weight reduces coning, coning can be bad. Placed at the fast tips of the blade provides the most centripetal force which makes the least amount of weight more effective to reduce it.

Weighted tips can also reduce flap up of the rotor during forward flight, or in essence stabilize the amount of flap up with different forward speeds.

I think it also regulates fast rotor rpm changes by having more mass, I have found that fast rotor rpm changes have done some weird things with pitch sometimes.

And lastly, as the blades get larger / longer I have had trouble with tip speed and flutter. Essentially, what I think is happening is the blade is twisting in flight and causes it to go out of pitch / track and slow down. I have solved this on one set of blades by tip weighting excessively as far out towards the leading edge as possible or beyond in one case.

Thats just my thoughts, and this really only applies to two bladers with longer blades. I haven't had any of these troubles with shorter blades and especially the three bladers. I assume the length vs. size of the blade makes them stiff enough to resist some of the forces.

So to answer, I would tip weight the blades towards the leading edge to correct your chordwise CofG. I don't worry too much about blade mass, it just always appears to be a benefit with my two bladers with more mass.

Another thing to check is motor thrust line, very important on a pusher. I have found that changes in blade mass affects the thrust line and also the tilt back of the rotor can introduce more drag which affects the thrust line of the pusher motor relative to the vertical C of G. So when your putting in more hang the rotor has to tilt back to compensate, more drag introduced, higher the motor goes.

Another thing that comes to mind is the horizontal stab, just be sure its not introducing pitch changes with varying amounts of thrust or different speeds in forward flight. Same goes for your motor thrust if you have that angled up or down.

Hopefully someone with more experience will chime in

Chad
Last edited by chadandrews; Feb 27, 2016 at 01:12 AM.
Feb 27, 2016, 02:44 AM
Keep it simple
alfoot's Avatar
I had similar pitch-up issues but it was cured by using a more flexible head plate.

Have a look at this page from the main RPG thread, in particular post #2291

https://www.rcgroups.com/forums/show...68853&page=153

Al
Feb 28, 2016, 03:11 PM
I'm not as bad as they say.
Quote:
Originally Posted by bdoxey
1) Should I take the weights off and just plug and re-drill the mounting hole at the true chordwise CG, even though that is probably around 35-40%? This would have me around 58 grams.

Bryan
No


Quote:
Originally Posted by bdoxey

2) Is 66 grams (the new weight after adding the balancing weights) an ok weight for the 24" RPG blades and I should give preference to correcting the blade chordwise CG to 25-30%?
I think correcting the CG is a far better option than the slight weight increase.

Quote:
Originally Posted by bdoxey


3) If I permanently add the weights, would it help to move them inboard some more to reduce the tip weight effect? Or would this move the spanwise CG too far inward?
Outward, and you can probably use less.

Quote:
Originally Posted by bdoxey

4) Whatever you guys think that I haven't thought of?

Thanks as always,

Bryan
My procedure was to hang the blades at the bolt hole (@25%) and add leading edge tip weight until they hang straight. This is a good starting point and makes sure that the blades aren't leading too much and aren't trying to pivot around a too far aft position.
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Mar 02, 2016, 11:20 AM
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Thread OP
Thanks guys for the replies.

After some "scientific" duct taped weight tests I settled on adding about 6 grams at the spanwise cg leading edge. Mainly because it flew really well and I didn't want to change anything. My math or weights must have been a little off prior, as when I buried the weights in the blades permanently the total blade weight was 63 grams.

I got one test flight in with the fully completed blades just before a trip. It was quite windy, but it flew very stable in conditions that previously would have thrown it all over the sky. Did my first RPG loop even. Cool!

I'm still thinking about the weight location from what I read here. I put the weights at the spanwise CG with the thought (not necessarily correct) of "statically" balancing the chordwise CG. However, the idea of hanging the blade and adding less leading edge weight but at the tip to get the blade hanging vertical seems to make sense also as it is "dynamically" balancing the blade chordwise?

I had tried tip weights on my DAG way back when I was having issues and really didn't like the handling. I was trying to avoid that situation by adding the weights at the existing spanwise CG on my RPG. In this case it worked and the blades came out weighing less than I was estimating.

Bryan
Mar 02, 2016, 11:42 AM
BigTradioman's Avatar
RPG loop?!?!?! If it was intentional that's a very exclusive club indeed.
Mar 02, 2016, 12:28 PM
Registered User
Hi Bryan,

Little confused...Are you saying you put the weights at the CG of the blade measured length ways? I don't think length ways C of G is a factor, just have to be the same in each blade wherever it may be for dynamic balance. The C of G on most of mine (after adding tip weight) are actually two thirds out the length of the blade from the root, not on the C of G of the blade itself before weighting. Make sense?
Mar 02, 2016, 02:08 PM
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Thread OP
My RPG was flying ok but was overly twitchy in pitch and roll. I was trying to move the chordwise CG forward only, as I discovered that the blade mounting bolt was well in front of the balance point. When adding the weights I didn't want to change the spanwise CG so I added the weights at the existing chordwise CG. Not really sure why, but a great bit of thought with only a little bit of knowledge went into it. I lucked out this time I think.

When I had early trouble with my DAG, I tried tip weights to make it more stable. It really flew weird. I didn't want to end up with that result so that's why I kept the weights inboard. As it turned out, proper hang angle and downthrust solved all the DAG problems. Note to self: Read the plans, dummy.

My RPG is turning into a test bed so I will probably try a few other things with it. It really is flying nice right now though.

Bryan
Mar 02, 2016, 02:32 PM
Registered User
Well sounds like you have done something that fixed it!! Thats great!!

Say, is that a two blader or three blader?
Mar 02, 2016, 09:42 PM
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Thread OP
Yes, I've done five or six loops with it now. It didn't quite have the power I wanted so I put a new motor on it today. I flew it twice today in windy conditions. I did one loop, but with the crosswind I was flying in a different orientation at our field and didn't really position it properly first. I ended up doing it more end-on so really couldn't see how it did. It did survive though!

Right now it is a three-blader with the blades built as close to plans as I could. I have already received my Aerobalsa blades to make up a two-blade set. I want to get the three blade setup finalized first. Then I'm going to set up a totally different model on my transmitter for the two blade so I can switch back and forth by just bolting on whichever head and choosing the corresponding transmitter model.

I have completed Rich's Masters Degree Course in pusher autogyro theory, otherwise known as reading the "My Pusher Designs" thread multiple times. I have the post with his two blade head drawing and several posts on how he weighted the blades so I should have a good start on the process. Hopefully after my requisite mucking it up first it should work out pretty well.

Bryan
Mar 03, 2016, 11:56 AM
Registered User
Ahhh, mine is a teetering 2 blader. This is one that I had the most fooling around with to get it where it is today. The three bladers I have have no weights in them and they work fine. I have experimented with different weights through different methods of blade construction. Still working on some different ideas. I too have read that thread front to back a few times over the past year. A lot of great information there to absorb.

Nice idea on setting up two different models in the TX, would be great for quick in field testing for sure.
Mar 25, 2016, 03:02 AM
BigTradioman's Avatar
Quote:
Originally Posted by bdoxey
Yes, I've done five or six loops with it now. It didn't quite have the power I wanted so I put a new motor on it today. I flew it twice today in windy conditions. I did one loop, but with the crosswind I was flying in a different orientation at our field and didn't really position it properly first. I ended up doing it more end-on so really couldn't see how it did. It did survive though!



Right now it is a three-blader with the blades built as close to plans as I could. I have already received my Aerobalsa blades to make up a two-blade set. I want to get the three blade setup finalized first. Then I'm going to set up a totally different model on my transmitter for the two blade so I can switch back and forth by just bolting on whichever head and choosing the corresponding transmitter model.



I have completed Rich's Masters Degree Course in pusher autogyro theory, otherwise known as reading the "My Pusher Designs" thread multiple times. I have the post with his two blade head drawing and several posts on how he weighted the blades so I should have a good start on the process. Hopefully after my requisite mucking it up first it should work out pretty well.



Bryan

After much bullying from my Gyro chums I adventured into the 2 blade RPG conversion closely following all the advice from the 2 Riches and Steve and Chas etc etc. They made it sound so simple, just add more nose weight and more back tilt at neutral elevator.......
That's why I had to build a new RPG! Never made it to the first turn, actually never got off the ground!
Mar 25, 2016, 10:18 AM
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106flyer's Avatar
Not sure what went wrong as i made the blades to Rich E spec, took out the left tilt, added 120g extra nose weight, reduced roll rate to 40%, added a few degrees extra rear tilt...

Flew like a dream and now I like it better than the other 3 blade RPG...

Steve
Mar 25, 2016, 05:27 PM
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Thread OP
Steve,

I thought I knew most of the changes for 2-blade but I think I missed something.

I should take out all of the left tilt?

You had to add 120 grams noseweight? Were you balanced towards the tailheavy side already in your 3-blade configuration?

I am still playing with adjustments on my 3-blade setup. My CG is set now so that if I go to idle my RPG holds pretty much the same attitude/to a slight nose drop in a descent all the way to the runway. It feels like any more tail weight would have it pitching up and stopping if I chopped the power. I am assuming this is close to optimum CG.

You think from here that 120 grams when converting to 2-blade would be about right?

Thanks,

Bryan
Mar 26, 2016, 05:35 AM
BigTradioman's Avatar

Blade Theory Question


Quote:
Originally Posted by 106flyer
Not sure what went wrong as i made the blades to Rich E spec, took out the left tilt, added 120g extra nose weight, reduced roll rate to 40%, added a few degrees extra rear tilt...

Flew like a dream and now I like it better than the other 3 blade RPG...

Steve


I don't know either, I did the conversion as above but used the AJ blades same as Rich E used on one of his conversions. The first issue was definitely my fault as I transposed the length measurement and the blades where too long. After adjusting the length I tried again but I couldn't control the roll or pitch on lift off, cart wheeled on the first hop and busted the tailboom. After repair I reduced the throws, increased hang angle to make it a bit more docile ( my RPG s always tended to be a bit nose up in flight) tried again and got poor spin up. Increased shim, got better spin up, on lift off it went into a nose up right roll and went in upside down. Game over.


So I reckon 2 blades are for the super experts with a really docile gyro. Not worth busting a model for IMHO.


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