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Author Topic: Balancing Stuff  (Read 289 times)

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Online Deerslayer

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Balancing Stuff
« on: December 14, 2017, 11:35:19 AM »
At least some folks make an effort to balance their props. Others just trust that things are OK as bought - count me in that sorry lot! I use APC props exclusively, unless something else was provided with the airplane I bought. I find them to be very high quality and well balanced. I have had a little, relatively unused, balancer thingy lying around in my flight box for "about a hundred years" (a FlyPaperism). Then, a couple of years ago I sprung for a more expensive and capable unit. Well, I had paid for it so I just had to try it on several of my APC props; I never could determine that I needed to do anything to improve them. Meanwhile, I got the Crack Beaver and its somewhat unusual props. When I checked them, they needed a LOT of balancing! So, perhaps buying the balancer was worthwhile, after all?

What about balancing electric motors? What about the balance of the whole motor+adapter+prop assembly? Well, I came across this very interesting article. He uses an iPhone app to do this. I have an Android tablet and phone, but, there are several apps available for Android which can do vibration or seismic analysis.

I hope someone else finds this interesting and perhaps even useful:

http://www.itsqv.com/QVM/index.php?title=How_To_-_Motor_Balancing
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Offline Dwayne

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Re: Balancing Stuff
« Reply #1 on: December 16, 2017, 09:55:00 PM »
I check the balance of every prop I buy, over the years that's hundreds of props and have only found a handful that balanced as bought, I find Xoar to be the best followed by APC, some of the worst are Master Airscrew, I had a 11X6 that was so bad I had to cut almost an 1/8th inch off one end and add a bunch of CA on the other...lol. So it's always good to check em. 

Never heard of balancing  a motor seems a bit over the top...lol  :P :laugh:
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Online Deerslayer

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Re: Balancing Stuff
« Reply #2 on: December 19, 2017, 09:26:47 AM »
I check the balance of every prop I buy, over the years that's hundreds of props and have only found a handful that balanced as bought, I find Xoar to be the best followed by APC, some of the worst are Master Airscrew, I had a 11X6 that was so bad I had to cut almost an 1/8th inch off one end and add a bunch of CA on the other...lol. So it's always good to check em. 

Never heard of balancing  a motor seems a bit over the top...lol  :P :laugh:


As for balancing props: I carry a heavy load of guilt for my slovenly practices! XOAR and APC have reputations as good as it gets. Some of the knock-offs and el cheapos are pretty poor for balance was well as being of poor efficiency, from what I have read.

As for balancing motors: I agree, it does seem excessive. The context was that the author is trying to make his FPV work as smooth as possible. That has, so far, been the least of my concerns, as all of the little variances and movements probably make any motor imbalance to be relatively insignificant. However, it is an interesting exercise, especially if you have a smart phone and are looking for entertainment other than watching cute cat videos!
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Online Deerslayer

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Re: Balancing Stuff
« Reply #3 on: December 26, 2017, 06:22:35 AM »
 Speaking of balancing stuff:

 As with a number of other aircraft I have designed and/or built, I often think up some different situations w.r.t. C of G concerns - before carving out material. For my new Penguin FPV plane, I decided to move the Elevator servo way back to the tail, rather than keeping it up front and having the long control rod. This is a common mod for the plane.

 Prior to the Penguin, I created a spreadsheet (available to anyone who asks) to investigate many configurations for a flying wing of my own design. I had flown it with a stabilizer for protection and determined where the C of G had to be in order for it to fly nicely, non-stabliized. In that case, I was about to add some FPV stuff onto the wing and needed to shift the LiPo sideways and rearward a bit to accomodate the camera, thus moving the C of G rearward unless I either added some nose weight (detestable!) or using, say, dual 1000 or 1300 mah batteries, or one 1800 mah battery rather than the single 2200 mah battery. There were a number of other possibilities.

 I ran the spreadsheet against about a dozen options and chose one of the best results. The plane flew as well as before the changes, with no parasitic weight addition. I have done this kind of paper work on balsa planes, where I wanted to achieve certain performance improvements (usually snappier!) without cutting into wood or stooping to adding weights somewhere.

 I may remove those atrocious nose weights from my Optera and compensate with larger or more LiPo, applying the same approach - measure twice, calculate, then cut once.


Now, for the Penguin example:

Given:
Required C of G location is the datum (everything measured from there)
Servo Weight = 20 grams
Standard Location = 10 cm aft of C of G, approximately (distance from datum to middle of servo)

Scenario:
Rear Servo Location = 65 cm aft of C of G (conservative estimate, could be slightly less)

Required Counterweight Location = 35 cm forward (conservative estimate, could be slightly more, your choice)

a) Initial Servo Moment Arm = 20*10 = 200 g-cm

b) Rear Servo Moment arm = 20*65 = 1300 g-cm

Moving of Servo from Standard to Rear Location: 200 - 1300 ~ -1100 g-cm required to compensate
Therefore, required Counterweight at 35 cm forward: 1100/35 ~ 32 grams

For the Penguin, this much compensation is not a big deal, probably well within what I will instead do by battery selection and location. I have a large number of 2200 mah batteries, this being my personal upper limit for my electrikery inventory. I plan to parallel a couple - or possibly even 3 - of these for this plane. To fit the servo in its standard location, I would have to hog out the lasered hole a bit anyway. I hot glue servos into foam aircraft, so rear mounting is simple and I can still install a couple of CF strips if I am really concerned about strength. Of course, my landings are always as soft as settling onto a pillow anyway
« Last Edit: December 26, 2017, 06:26:00 AM by Deerslayer »
My purpose in Life is to serve as a Warning to others