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Author Topic: Putting things in strange places  (Read 2119 times)

Offline Ken Culbertson

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Putting things in strange places
« on: May 07, 2019, 01:35:41 PM »
I have read as much as I can find on placing the parts of an electric power plants but all of them are really more about cooling than weight distribution.  Is there any reason not to place things like the ESC and Timer further back in the fuselage or even in the wing?  Assuming they are properly cooled does it matter where they are?  Does the length of the wires matter (except for weight)?

I am converting Tom Niebuhr's Top Hat to electric and that huge pipe tunnel is just begging me to put the ESC & timer under the wing so I can build it really light.

This should probably go in the Stupid Questions forum but all my life I have been computing wing area as span x average chord less something for tip shape and the part in the fuselage.  The FM plans for the Top Hat list it at 58" span and 710 Sq.  So I measured my wing that I got from Tom and it is 58" with a center chord of 13.5 and tips at 10.25.  No way that gives you 710.  So am I missing something or are the FM plans just wrong?

ken
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Offline phil c

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Re: Putting things in strange places
« Reply #1 on: May 07, 2019, 03:27:45 PM »
Not an expert, but: the power wires should be as short as possible, or have a diameter large enough to practically minimize resistance losses.  The ESC should be located as close to the motor as you can for the same reasons.  The ESC also gets information about the motor about the electrical performance so it can minimize losses and maximize power and efficiency.
If you want to experiment it's very unlikely to cause any harm to the equipment by mounting the ESC back behind the wing.

On the Top Hat- 710 square inches implies an average chord about  0.75in. longer.   Or some combination of span and chord to get the extra inches.  If your measured wing flew well go with that.  If you have to rebuild the fuselage and tail from the plans check that the other dimensions are the actually scaled to the existing wing.
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Offline Ken Culbertson

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Re: Putting things in strange places
« Reply #2 on: May 07, 2019, 04:16:20 PM »
Not an expert, but: the power wires should be as short as possible, or have a diameter large enough to practically minimize resistance losses.  The ESC should be located as close to the motor as you can for the same reasons.  The ESC also gets information about the motor about the electrical performance so it can minimize losses and maximize power and efficiency.
If you want to experiment it's very unlikely to cause any harm to the equipment by mounting the ESC back behind the wing.

On the Top Hat- 710 square inches implies an average chord about  0.75in. longer.   Or some combination of span and chord to get the extra inches.  If your measured wing flew well go with that.  If you have to rebuild the fuselage and tail from the plans check that the other dimensions are the actually scaled to the existing wing.
The ESC would be under the wing, roughly at the CG and that would require about a 3" extension to the motor wires.  I think I will try it.

The wing matches the plans.  I am going to assume that it really in somewhere around 675 and the plans are wrong.  I am shooting to come out under 13oz @ '2.  More area, more weight I can cary.

Ken
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Offline James Mills

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Re: Putting things in strange places
« Reply #3 on: May 07, 2019, 04:35:05 PM »
I have read as much as I can find on placing the parts of an electric power plants but all of them are really more about cooling than weight distribution.  Is there any reason not to place things like the ESC and Timer further back in the fuselage or even in the wing?  Assuming they are properly cooled does it matter where they are?  Does the length of the wires matter (except for weight)?

I am converting Tom Niebuhr's Top Hat to electric and that huge pipe tunnel is just begging me to put the ESC & timer under the wing so I can build it really light.

This should probably go in the Stupid Questions forum but all my life I have been computing wing area as span x average chord less something for tip shape and the part in the fuselage.  The FM plans for the Top Hat list it at 58" span and 710 Sq.  So I measured my wing that I got from Tom and it is 58" with a center chord of 13.5 and tips at 10.25.  No way that gives you 710.  So am I missing something or are the FM plans just wrong?

ken
This might be a silly question but does the wing area measurement include the flaps?
As far as converting it over to electric look at any of the planes that are specifically designed for electric for nose construction.  I personally would try not put the ESC that far back.

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Offline Ken Culbertson

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Re: Putting things in strange places
« Reply #4 on: May 07, 2019, 06:16:27 PM »
This might be a silly question but does the wing area measurement include the flaps?
As far as converting it over to electric look at any of the planes that are specifically designed for electric for nose construction.  I personally would try not put the ESC that far back.

James
Yes, it includes flaps.  When I look at some of the RC ships and draw on my own experience with RC I remember putting things all over the place to achieve the right CG with the least amount of weight.  My whole fleet, save one, is electric and they all have one thing in common - everything is crammed into the nose.  It would be simple to just eliminate the pipe tunnel and vent through holes in the bottom like everyone else does but if I could build a lighter tail by offsetting the weight by moving the ESC back - why not.  If it is truly a bad idea, (like a bunch of people tried it and it sucked) then I won't do it.

If it were not for the fact that the wing is already built, I would move the Bell Crank mount back 1/2" and redesign the wing LE to give me an extra inch to move the battery back.  I wish they made long flat batteries.  It wouldn't be hard to do with the 4 & 6 cells.

Ken
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Offline Tim Wescott

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Re: Putting things in strange places
« Reply #5 on: May 07, 2019, 06:33:54 PM »
There are sound electrical reasons for not lengthening the battery wires (it's complicated -- just trust me, I've designed motor drivers before, and besides, Igor Berger told me).  But those don't apply to the motor wires.  So you can make the motor wires longer at the cost of a bit more resistive losses.  If you make the battery wires longer you need more capacitance at the ESC.
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Offline Ken Culbertson

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Re: Putting things in strange places
« Reply #6 on: May 07, 2019, 07:43:13 PM »
There are sound electrical reasons for not lengthening the battery wires (it's complicated -- just trust me, I've designed motor drivers before, and besides, Igor Berger told me).  But those don't apply to the motor wires.  So you can make the motor wires longer at the cost of a bit more resistive losses.  If you make the battery wires longer you need more capacitance at the ESC.
I am using a Castle 75 and I would only have to lengthen the motor wires to pull it off.  I really want to put both the ESC and timer in that pipe tunnel where it will get all get glorious fresh air it can stand.  Motor case to ESC Case would be about 9".  I think the wires they come with are that long when added together.  I know I cut about 3" off of the ESC to get it to fit in the last one.

You are encouraging me!

Ken
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Offline Randy Powell

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Re: Putting things in strange places
« Reply #7 on: May 08, 2019, 02:59:25 PM »
I would think that the wire length should be as short as possible.
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Offline Tim Wescott

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Re: Putting things in strange places
« Reply #8 on: May 08, 2019, 04:56:25 PM »
I would think that the wire length should be as short as possible.

All else being equal, yes.  But as long as Ken isn't wrapping them around the wing he'll be OK: a couple of extra inches won't cause problems at all.

Given what I know about motor drives if I were building a twin I'd be strongly tempted to put the battery in the nose, the ESCs just behind the battery, and then run motor wires in the leading edges to the nacelles.  That's way more wire than Ken's running.
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Offline Ken Culbertson

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Re: Putting things in strange places
« Reply #9 on: May 08, 2019, 06:50:19 PM »
All else being equal, yes.  But as long as Ken isn't wrapping them around the wing he'll be OK: a couple of extra inches won't cause problems at all.

Given what I know about motor drives if I were building a twin I'd be strongly tempted to put the battery in the nose, the ESCs just behind the battery, and then run motor wires in the leading edges to the nacelles.  That's way more wire than Ken's running.
Thanks, you are confirming what I am thinking.

Ken
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Offline Ken Culbertson

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Re: Putting things in strange places
« Reply #10 on: May 09, 2019, 08:22:03 AM »
I built a 92% Thundergazer, eliminated the pipe tunnel and just centered the wing vertically in the fuselage but kept the same offsets for thrust line and stab height. All the E stuff is cleverly positioned in the nose. Plane flys great.


Motorman
I have plenty of room in the nose.  The issue is weight distribution.  I am keeping the pipe tunnel shape and using it to vent the electronics bay.  Putting the esc and timer back under the wing LE lets me keep the tail feathers light and not work the motor as hard.

Ken
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Offline Ken Culbertson

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Re: Putting things in strange places
« Reply #11 on: May 09, 2019, 10:47:28 AM »
The timer and ESC don't weight too much, you need to get the battery back. I have heard it said that you can make a box into the front of the wing back to the bellcrank so you can position the battery anywhere you want and you could shorten the nose too if you need to.


Motorman 8)
On a fresh build I would do it that way but the wing is already finished.  No way to add the extra spars to compensate for not having a LE.  As it turns out, after I switched to a rear mount, not to mention dropping about 1oz from not having all that collet crap,  I picked up about 2" on the motor leads.  Now the esc is going to be within 1/2" of where I wanted to put it without having to extend the wires any.  I am using a Castle Phoenix Lite 75 ESC. (My bad on that one but there is an explanation).  Moving 3 oz back 4" more is that much more weight I can trim off of the tail and still keep the CG.  Battery is already as far back as I can get it.

I didn't pay attention to weight on my last one and it came out ridiculously overweight (75oz). ~^  On this one I am trying to get it so light that I have to pump helium into the outboard wing for tip weight. ;D

Now I am interviewing pilot figures, lightest one wins.

Ken
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Offline Howard Rush

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Re: Putting things in strange places
« Reply #12 on: May 09, 2019, 02:42:10 PM »
Too late to move the motor back or use a lighter one?
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Offline Ken Culbertson

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Re: Putting things in strange places
« Reply #13 on: May 09, 2019, 05:55:05 PM »
Too late to move the motor back or use a lighter one?
Not really, I could shorten the nose but I have never liked short noses.  I have already lengthened the tail movement from 17 to 20.  History tells me I should have lengthened the nose an inch at the same time.
We are all, well most of us, educated in the IC world where everything has it's place and there really aren't any alternatives.  Electric opens the door to try different ideas.  Some will be good, most will suck.

Ken

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Online John Rist

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Re: Putting things in strange places
« Reply #14 on: May 10, 2019, 09:22:05 AM »
Make sure ESC and timer are easy to access.  Part of the charm of electric is the ability to program certain things such as RPMs, run time, and start delay.     D>K
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Offline Ken Culbertson

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Re: Putting things in strange places
« Reply #15 on: May 10, 2019, 01:22:56 PM »
Make sure ESC and timer are easy to access.  Part of the charm of electric is the ability to program certain things such as RPMs, run time, and start delay.     D>K
That is one of the main reasons I want to play with alternate locations.  I am tired of having to cram everything into the same space.  I keep being told that the ESC needs to be close to the motor then I see them 12-14" away on RC ships...and the difference is??  My other reason is that this stuff is heavy.
In theory the closer you can put the concentrated weight to the CG the lighter you should be able to build the plane.  You don't ballast a glider by adding weight to the nose and tail, you put it all at the CG.


IMHO the ultimate configuration has the ESC behind the motor with the battery well into the wing and the timer uncluttered and exposed when you open the battery hatch.  Someday batteries will be lighter and recharge quicker, maybe even to the point that we can have a charging plug and leave the battery in.

Ken
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Offline Paul Walker

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Re: Putting things in strange places
« Reply #16 on: May 16, 2019, 08:18:29 PM »
I have plenty of room in the nose.  The issue is weight distribution.  I am keeping the pipe tunnel shape and using it to vent the electronics bay.  Putting the esc and timer back under the wing LE lets me keep the tail feathers light and not work the motor as hard.

Ken

Where are you placing the pitch CG?

Offline Ken Culbertson

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Re: Putting things in strange places
« Reply #17 on: May 16, 2019, 10:08:35 PM »
Where are you placing the pitch CG?
No idea if this will be clear enough.  By keeping the pipe tunnel, well most of it, I can put the esc and timer anywhere from the nose to the TE of the wing.  It is a Castle 75 Lite so it is a bit heavy.  The C/G marked is only the horizontal position.  The vertical is just above the CL of the wing.  For reference it is going to be a slightly modified "Top Hat" converted to electric.  Tom was from our club and a wing he had built came my way when he passed.  I decided to finish it as his design.

Ken
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Offline Paul Walker

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Re: Putting things in strange places
« Reply #18 on: May 17, 2019, 08:59:56 AM »
I was curious about your CG location, and your picture is clear enough.

It is my personal experience after flying electric planes for 12 years now that that CG is way too far aft for electric power. Like you, I started out putting the CG in the same place as the IC counterparts, only to discover that my nose is too short.

The planes I fly now are Impact basic geometry. The CG is about 1.5 inches farther forward than the IC definition without fuel.

From your picture, the nose looks very short and will likely require more weight up front to make it fly well.

Just my comments from 12 years of flying electric.

Offline Ken Culbertson

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Re: Putting things in strange places
« Reply #19 on: May 17, 2019, 10:30:04 AM »
I was curious about your CG location, and your picture is clear enough.

It is my personal experience after flying electric planes for 12 years now that that CG is way too far aft for electric power. Like you, I started out putting the CG in the same place as the IC counterparts, only to discover that my nose is too short.

The planes I fly now are Impact basic geometry. The CG is about 1.5 inches farther forward than the IC definition without fuel.

From your picture, the nose looks very short and will likely require more weight up front to make it fly well.

Just my comments from 12 years of flying electric.
Interesting that you should bring up the nose length.  About the only mods I am making to Tom's design (other than the obvious E_Stuff)  is to lengthen the tale movement from 17" to 20" and to make the pipe tunnel a bit thinner.  The plans call for a 10 1/2" nose.  I don't know why I decided to keep it at 10 1/2.  Probably because I was more concerned with total weight and where to put a 15oz battery than overall design not taking into account that it was replacing a piped lawnmower engine and needed to be short.  Since that part is still only on paper I am going to add an inch.  I have never liked the visual from the center of the circle that a short nose gives me and when I put the plans on the wall and stepped back - it looked short  :-[.  Thanks for pointing that out - you just gave me the room I needed to make a fully adjustable battery rack.

As for the C/G.  What is on the "plans" is Tom's positioning for IC.  Every electric I have built ended up trimming nose heavy by IC standards.  The 1 1/2" is about right. Logic suggests that we should trim a bit tail heavy compared to a full tank IC to average out the shifting CG.   I do know that I can go way more nose heavy before corners become erratic than I can with IC and that confuses me as to why there is any difference.  I can't speak for anybody else, but I fly the verticals and overhead stuff much better when the ship is not tail heavy anymore and who would have thought that you could effortlessly penetrate the ground effect on landing!  Yes, I like electric.

Ken
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Offline FLOYD CARTER

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Re: Putting things in strange places
« Reply #20 on: May 17, 2019, 11:54:52 AM »
As far as wire length goes, the only critical wiring is between motor and ESC and between ESC and battery. These wires are normally heavy for a reason.  Very high current does create a voltage drop, which will rob the motor of power if wires are too long.  Remember also that motor power is adjustable by increasing the pulse width to the ESC,  That could compensate for wire losses, and all would be back to normal.

If absolutely necessary, wires can be extended if wire size is next larger size than originally provided.  Very heavy flexible wires are sold just for this purpose.

Since the ESC isn't normally a weight problem, then only the battery leads need extending to put the battery in proper location for CG issue. 
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Offline Ken Culbertson

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Re: Putting things in strange places
« Reply #21 on: May 17, 2019, 01:12:05 PM »
As far as wire length goes, the only critical wiring is between motor and ESC and between ESC and battery. These wires are normally heavy for a reason.  Very high current does create a voltage drop, which will rob the motor of power if wires are too long.  Remember also that motor power is adjustable by increasing the pulse width to the ESC,  That could compensate for wire losses, and all would be back to normal.

If absolutely necessary, wires can be extended if wire size is next larger size than originally provided.  Very heavy flexible wires are sold just for this purpose.

Since the ESC isn't normally a weight problem, then only the battery leads need extending to put the battery in proper location for CG issue.
IMHO the biggest culprits in wire loss are the connectors.  The bullet connectors I got with the motor and esc both used a slotted "bullet" that compresses to make a "tight" fit.  The total surface area making contact can't be much more that half the area of the wire.  I hope I am wrong since everybody uses them but I have had one come apart in flight so they can't be that tight.  The arming plug in another source of loss in a 6s.  Even a "T" or Deans connector does not have the same surface area as the wires it is connecting.  I purposely went to 75 size ESC because it had wire sizes greater than the 3520 motor.

Having said all of that, maybe what I need to know is just how long "long" is.  My motor has 4" wires to the connectors.  It came with 6".  My ESC has 5" motor leads.  It came with 8", I cut them down.  I agree that the ESC to Motor is the critical connection.  So, how long is too long?  If I use what the factoriy provided we have 14".  After my cuts we have 9"

My goal in all of this is to design a light ship that has the bulk of it's heavy items at or near the CG then be able to add/remove ballast to achieve the optimum weight for current conditions.  Similar to what I would do if I were flying sailplanes.

Ken

 
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Offline Paul Walker

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Re: Putting things in strange places
« Reply #22 on: May 17, 2019, 01:36:57 PM »
And, you don't need to use a 15 ounce battery.
I use a Thunder Power 6S-2800, and is sufficient for a Predator, and weighs just over 13 ounces.

The amount that comes out of the battery is significantly influenced by the prop being used.  Be aware!

Offline Ken Culbertson

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Re: Putting things in strange places
« Reply #23 on: May 17, 2019, 02:37:50 PM »
And, you don't need to use a 15 ounce battery.
I use a Thunder Power 6S-2800, and is sufficient for a Predator, and weighs just over 13 ounces.

The amount that comes out of the battery is significantly influenced by the prop being used.  Be aware!
Waiting on TP's to go on sale for just that reason.  I think they are shorter so I can move it FWD and still keep the CG.  Your post last night got me thinking about movements.  You were right that the nose was short at 10.5 but is the tail too long at 20?  Do you mind me asking what the movements are on the Impact or Predator?  I thought I could find it searching but Google seems to have added an algorithm that suppresses whatever it is I am looking for HB~>

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Offline Paul Walker

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Re: Putting things in strange places
« Reply #24 on: May 17, 2019, 03:08:03 PM »
They are all a little different, but...in general..

Nose length from wing LE at root, 10.5 inches
Tail length from flap hinge line to stab/elevator hinge line  18.5".

Offline Howard Rush

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Re: Putting things in strange places
« Reply #25 on: May 17, 2019, 06:48:42 PM »
Ken, I think your intuition and some of the advice you're getting are letting you down.  A little calculation could bring some insight.  Here are some numbers for an airplane the size of Paul's and the weight of yours.  Guys, check my assumptions and ciphering:

Assume power losses from ESC, motor, battery and wiring = 20%
Assume prop efficiency = 70%
Average battery voltage = 22.8
Battery usage for 6-minute flight = 2 Amp-hours

Average power leaving the battery = 456 W
Average power dissipated by ESC, motor, battery and wiring = 91.2 W
Average power going to the prop = 364.8 W

IMHO the biggest culprits in wire loss are the connectors.

Maybe so, but if 10% of the power went to a connector, it would be on fire.  You can measure the power loss.  Put a voltmeter across the connector and an ammeter in series with it.  Multiply the readings.

As far as wire length goes, the only critical wiring is between motor and ESC and between ESC and battery. These wires are normally heavy for a reason.  Very high current does create a voltage drop, which will rob the motor of power if wires are too long.  Remember also that motor power is adjustable by increasing the pulse width to the ESC,  That could compensate for wire losses, and all would be back to normal.

If absolutely necessary, wires can be extended if wire size is next larger size than originally provided.  Very heavy flexible wires are sold just for this purpose.


Mind you, this is coming from a EE.  I don't believe it.  20-gauge wire has a resistance of .01 ohm per foot.  At 20 amps, that's 4 watts / foot.  We aren't using fat wire to minimize resistance losses.  There may be some Eli-the-Ice-Man reason for fat wire.  Tim can explain it.  I use fat wire because I need the nose weight anyhow.  If I didn't, I'd look into the reason for it. 

The amount that comes out of the battery is significantly influenced by the prop being used.  Be aware!

Be aware indeed.  Propeller efficiency, which you can calculate if you want to, and which you can do something about if you use one of the available prop design programs, can vary a lot: probably from 50% to 80%.  I assumed a prop efficiency of 70%. I use 2,000 milliamp-hours of battery per flight.  If my prop were 50% efficient, I'd use all 2,800.  Prop power dissipation is a couple of orders of magnitude more important than wire dissipation.

My goal in all of this is to design a light ship that has the bulk of it's heavy items at or near the CG then be able to add/remove ballast to achieve the optimum weight for current conditions.  Similar to what I would do if I were flying sailplanes.

There is some discretion about where to put some of the masses in electric stunters. The battery is the big one.  However, the minimum moment of inertia (the barbell effect) is with all that stuff at the same place.  This is contrary to folks's intuition, and I've posted a calculator to demonstrate how it works.  The optimal mass distribution is with the battery, ESC, TUT, BICAS, wire, and switches close together and wherever they need to be to get the CG to come out right, which is way forward of where the IC airplane's CG comes out with or without fuel.   

That said, there may be an aerodynamic reason for having the motor farther forward than it would be for purely mass reasons, and moment of inertia is not that big a deal.  It's not worth having a short nose with a bunch of ballast in it as I've done. 

I don't think the sailplane analogy holds.  The design task, hence the reason for ballast in either case, is different.  I can imagine no need for ballast in a stunt plane with a control system having sufficient leverage over hinge moment. 
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Offline Ken Culbertson

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Re: Putting things in strange places
« Reply #26 on: May 17, 2019, 07:21:51 PM »
Howard, I am with you on all but the last point.  I think a stunter can be too light to fly well.  The purpose of ballast in a sailplane is wind penetration.  You don't have a motor to pull you through it.  My purpose in proposing having a place for ballast is to be able to build light then bring the ship up to optimal weight.  It's a whole lot easier to make them heavier after they are finished than it is to make them lighter and you don't know where that sweet spot is till you fly them.  It is just one more arrow in the trim quiver.

Ken
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Offline Howard Rush

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Re: Putting things in strange places
« Reply #27 on: May 17, 2019, 07:41:03 PM »
I can give you a bellcrank that will give that 75-ouncer a sweet spot that requires ballast to attain.
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Online Brent Williams

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    • Fancher Handles - Presented by Brent Williams
Re: Putting things in strange places
« Reply #28 on: May 18, 2019, 01:21:54 PM »
I can give you a bellcrank that will give that 75-ouncer a sweet spot that requires ballast to attain.

An adaptation of the Igor control system or something new?  I would be interested in reading more about your design specs. 
Laser-cut, "Ted Fancher Precision-Pro" Hard Point Handle Kits are available again.  PM for info.
https://stunthanger.com/smf/brent-williams'-fancher-handles-and-cl-parts/ted-fancher's-precision-pro-handle-kit-by-brent-williams-information/

Offline FLOYD CARTER

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Re: Putting things in strange places
« Reply #29 on: May 19, 2019, 11:05:40 AM »
Perhaps I should have added a summary conclusion to my suggestions, in order to make it simple and understandable by lay-persons.

Voltage drop due to excess wiring is probably small, and can easily be overcome simply by increasing the pulse width presented to the ESC.  This is accomplished with the "RPM" control featured on the timer module.  The battery will lose a small amount of charge in heating of the wiring.  If you can tolerate this, then the effect will be superficial.
89 years, but still going (sort of)
AMA #796  SAM #188  LSF #020

Offline Ken Culbertson

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Re: Putting things in strange places
« Reply #30 on: May 19, 2019, 11:59:16 AM »
Perhaps I should have added a summary conclusion to my suggestions, in order to make it simple and understandable by lay-persons.

Voltage drop due to excess wiring is probably small, and can easily be overcome simply by increasing the pulse width presented to the ESC.  This is accomplished with the "RPM" control featured on the timer module.  The battery will lose a small amount of charge in heating of the wiring.  If you can tolerate this, then the effect will be superficial.
Thanks.  I think your summary answers my question.  I am pretty sure that the difference in 9" and 12" of wire is not going to make much of a difference based on what I am hearing.

Ken
AMA 15382
If it is not broke you are not trying hard enough.
USAF 1968-1974 TAC


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