The three head positions, vertical CG and matching batteries

[Gregor]

I’ve always like the look of a lower head position, like the TDR. When setting up my E7 I noticed that there were three head positions and originally I had planned on the lower position.  I incorrectly assumed the rotor being closer to the center of mass would produce tighter, more axial rolls and flips.   What I did not consider is that the rotor head itself is part of the weight of the helicopter and changing its position relative to the boom would change the vertical CG.   Matt discusses this in the following post regarding the N5c.

I designed the N5c so that the vertical CG would be right on the boom with the head in the stock position. Moving the head up and down does change the vertical CG

http://synergyrchelicopters.com/forum/index.php?topic=306.msg2198#msg2198

The head position alters the vertical CG on one end of the model.  At the other end of the model are the batteries.  This means that the stock head position puts the vertical CG on the boom for a specific pack weight, the stock pack weight (more on that in a minute). Using a heavier or lighter pack would also affect the vertical CG. To correct for this, the adjustable head positions can be used to compensate for differing pack sizes.
 
So what are the pack sizes at weights for each head position?  The only one we can be sure about is the center position matched to the “stock battery size”.  I will take guess that the stock size would be Matt’s favorite pack for the E7, the TP 4400 65c.  The rest are a just a guesses on how the CG might line up with a heavier or lighter pack.  Matt, can you run some heavier and lighter pack weights through Solid Works to see which pack sizes provide the correct vertical CG for each of the three head positions?  If there is interest, it might nice to also see these numbers for the E6 with the lighter 606 blades.

E7 with Rail 696 blades

Head Position                        Pack Weight (grams)
Low (top hole)                       522 each (1044 total) NanoTech 3300 35c    
Middle (stock)                       705 each (1410 total) TP 4400 65c
High (bottom hole)                804 each (1608) TP 5000 65c

With a FBL controller, this may all be academic since the controller will correct for minor CG issues.  But as there are three positions to choose from, it would be nice to match the head position to pack weight so the mechanical balance is closer to perfect.

[stevehof]

I'd like to know the advantages, if any, of a lower vertical center of gravity. The heli rolls and flips on axis with the blades. The blade plane is the pivot point. The helicopter does not rotate around the vertical CG. It always rotates around the head. So, what are the advantages of a higher head and/or lower vertical CG?

Lower vertical CG just takes more cyclic pitch and collective pitch and motor power usage to rotate the helicopter body any given amount. Or in today's jargon more collective management skill to produce an 'axial looking' roll. This is nothing more than basic mechanical leverage.

As an example, my Compass 6HV required less collective finesse to produce an axial roll than any ship I've owned. I attribute that to correct head geometry, a very low head position and a high vertical center of gravity. The very light disk loading didn't hurt either. OK, that's my opinion, what's yours....;^)

[Gregor]

an 'axial looking' roll.

  IMO an "axial looking roll" is when the model rotates on the boom not the rotor head.   The CG on the boom would make it easier for the model to naturally rotate on that axis.  It could have a negative impact on flips which tend more to rotate on the head, rather than the center of mass.   

It would be interesting to see where the vertical CG is on the Compass 6hv as well as others. With 6HV's high mounted batteries, I would not be surprised if the vertical CG was above the boom.  The TDR design has the battery below the boom.  So there's still a good chance the vertical CG is on the boom.  While we all know of the importance of horizontal CG, it didn't occur to me that model designers were concerned about the veritical CG until I saw Matt's post.  More comments would be appreciated.

[stevehof]

an 'axial looking' roll.

  IMO an "axial looking roll" is when the model rotates on the boom not the rotor head.   The CG on the boom would make it easier for the model to naturally rotate on that axis.  It could have a negative impact on flips which tend more to rotate on the head, rather than the center of mass.

The model always rotates and flips on the head axis. If it "looks" like it is rotating on the boom during a roll that is because the person on the sticks is exercising good collective management, ie; stick timing and movement volume. If the geometry and fore/aft CG are good and there are no interactions and the pilot pushes the stick exactly straight over, the boom will stay straight during a roll. The rest is up to collective management.

The blades are the lever, the power on the lever is lift and the fulcrum is on the line where the main shaft and spindle would meet. The lever (blades) has to move/lift the entire weight of the ship at the fulcrum point. So, I still can't see how a lower vertical CG helps with any type of maneuver? However, I'm open to be educated by someone who can 'show me the math'...

[QuantumPSI]

Unfortunately, I can't show you the math via text, but the original concept is the correct one.  When a torque is applied to an object in free space, it ALWAYS rotates about its center of gravity. 

So yes, moving the head up and down will change the location of the CG and thus the pivot point. 

What you say is correct that collective management can make any heli roll on or off axis.  However, to see if your CG is truly spot on.  Enter in a dive, zero pitch, and apply aileron... I assure you the helicopter does not rotate about the head (even though that's where the torque is coming from), but rather an axis within the body.  This axis will be in line with the CG.

You can do the same thing on the elevator axis by doing a death spiral.  Again, the point of rotation will not be the head.

[stevehof]

Unfortunately, I can't show you the math via text, but the original concept is the correct one.  When a torque is applied to an object in free space, it ALWAYS rotates about its center of gravity.

OK, I'm now educated....;^) I'm going to assume this phenomenon is covered by a physics equation? How would I search this on Google so that I could learn more? Got a link handy?....Thanks!

[nwmtech]

Give this guy a shout. http://synergyrchelicopters.com/forum/index.php?topic=57.0 I guarantee you he'd be the one to answer this question with accuracy. If you asked him I'm sure he'll answer.

[QuantumPSI]

If you want more information on it, just google torque, rotational inertia, rotational kinematics/dynamics.  That should give you MORE than enough reading material.

Yes, the guy Nick speaks of would be able to answer as well.  I'm not in the propulsion industry but I am aerospace engineer as well and did my masters thesis on scramjet propulsion.

[stevehof]

Thanks for the additional information. I'll check it out later. OK back to Gregor's original post regarding battery weight, CG and head position. Just for grins me and a couple of friends did our best to find the vertical GG on my E7. My head is mounted in the lowest position. My batteries are Nano 4500 mah 35c @ 1380 grams. The CG was about 1/4 to 3/8 inch above the top of the boom. So, the ship was a bit top heavy if the goal is to place the CG on the boom line. I didn't have any heavier batteries to try. I did fly with my Nano 3300 35c, which weigh about 1050 grams. Naturally, I adjusted the fore/aft CG for this flight. I sure like the flight characteristics with the light batteries. I really believe it was easier to get a nice looking roll with the 3300's. I realize that doesn't jive with the science discussed here so it's only my opinion. So unless I made significant error in my CG measurement you'd need some fairly heavy batteries to get the CG on the boom line in the 'stock' or middle hole of the head block.

I'm sure there was room for error in our CG check method. I had the two friends support most of the weight of the ship, without canopy, while I used my thumb and forefinger as a pivot point along longitudinal axis under the mast. Clumsy as this sounds I was surprised to see how little movement of my fingers caused the ship to tip one way or the other...

So, how about it guys. Someone else up to finding their vertical CG and reporting back? I'd be very curious to see your findings. How about a better method of measuring the vertical CG?

[Gregor]

How about a better method of measuring the vertical CG?

  I'm not sure if this is better, but here was the plan I was cooking up.  Start with a long thin rod like one of the extra boom supports and lay it on a table.  Then get a thick book and lay it on the rod.  Next lay the model (canopy, batteries and all) on its side, on the book with the boom in line with the rod under the book.  Finaly roll the book forward and backwards until it teeters.  If the skids are hitting the table, use a larger book, or hang the skids off the side of the table.

Between work and Thanksgiving day preparations, I have not been able to test this.

[stevehof]

I'm not sure if this is better, but here was the plan I was cooking up.  Start with a long thin rod like one of the extra boom supports and lay it on a table.  Then get a thick book and lay it on the rod.  Next lay the model (canopy, batteries and all) on its side, on the book with the boom in line with the rod under the book.  Finaly roll the book forward and backwards until it teeters.  If the skids are hitting the table, use a larger book, or hang the skids off the side of the table.

Between work and Thanksgiving day preparations, I have not been able to test this.

It looked good enough for me to try it. The problem I quickly ran into was that you have to keep the CG of the book centered over the rod or your ship's CG will be skewed by the weight of the book. I used the 600 size boom under a book. When you start rolling the book around the CG of the book also moves. I gave up on getting an accurate CG with this method after a few minutes. It's nearly impossible to keep the ship, book and rod all aligned properly.

[stevehof]

OK, um...er....I so hate to post misinformation and it seems I've done just about nothing but that in this thread... I decided to try to balance the ship by myself with just thumb and forefinger on the belt drive spindle on the mast. This keeps the fore/aft CG good so you just need reasonably strong fingers. Using this method I was a bit surprised to see that the CG was just on the bottom of that belt spindle with my 1380 gram batteries. Almost exactly on the boom line.

The guys who were helping me with my first attempt to measure the vertical CG called the position of my fingers out as far as high low goes. I really couldn't see down the frame at that time. So I don't know where we went wrong. However, three guys makes for 3 times the chance of errors I guess....

I CG'd the canopy separately by just resting it on the 600 boom. It was within 1/2 inch of the boom line too. So I don't think the canopy weight would skew the complete CG by much if any.

[Gregor]

It looked good enough for me to try it. The problem I quickly ran into was that you have to keep the CG of the book centered over the rod or your ship's CG will be skewed by the weight of the book.

 

Steve, thanks for testing and reporting the results.  I forgot about the CG of the book.  It looks like the finger method is more reliable.  So it sounds like 1380 grams is nicely balanced with the lowest head position.  It would be intesting to retest the different head positions with heavier packs.  Or better yet, it would be good to have Matt report the results from the software modeling tool, Solid Works.

[stevehof]

Steve, thanks for testing and reporting the results.  I forgot about the CG of the book.  It looks like the finger method is more reliable. 

The thumb and forefinger balance method works OK for me. It might not work for everyone. Of course you do have to keep your 'weak' hand loosely griping or guiding one of the skids to keep the ship from completely tipping over when you pass the balance point. The blades should probably be aligned with the boom too otherwise you might get some pendulum effect which would make finding the CG a bit harder. Lastly, if your CG is out of range to use the belt pully as your resting place, at least you'll know it's not on or very near the boom center line...