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Member postings for John Cole

Here is a list of all the postings John Cole has made in our forums. Click on a thread name to jump to the thread.

Thread: LiPo battery packs
01/02/2009 17:19:31
Of course, but when this happens the 2*2250 cells are totally exhausted and the 3*2500 cells will be reverse-charging them and "using up" their own residual voltage (which will be about 10 Volts) by creating a back-Voltage.  So I don't think there will be enough net voltage from the two packs to drive the BEC.
 
What do other people think?  I've never tried to reverse-charge a completely-flat LiPo. and don't have a dead one to try (and measure the volts across it).
01/02/2009 17:02:20
Doug: yes, it has a connector-block for JST-XH connectors, each part of the block capable of taking one size (e.g. 4-way for 3S). The block "size" varies according to whether the ESC can handle 4S, 5S or 6S.  You must connect the balance-lead BEFORE the power leads, and of course  you must disconnect the balance lead after use (or the LiPo will eventually go flat and die).  See:
 
and

for a better photo -but take care: some of the text on this page is generic, but see:
 

for the manual, which tells you which A-rating covers which packs, and the BEC data.  The 40A plus models have the Switch mode BEC.
 
Posted again so Timbo can delete my naughty one
01/02/2009 16:57:35
Eric and Timbo: I've smacked my hand just in case, but Eric I'm not sure what you mean.
 
I compared this thread with yours about the Reebok thingy - which I chose NOT because it's yours but because right now it's had no replies and so ain't going to be screwed up by people like me.  Neither has any text on the RHS below the Biggin Ad.
 
Of course, I may have screwed up the entire Forum!
 
But I've posted it again with the links entered in the corrrect manner - but it took many tries to make it work.  Copied / pasted the old posting and deleted the links.  Then tried to put them in from my web-browser by copy - click the Chain.  Mostly just left blanks and did not enter the link.
01/02/2009 14:45:16
I did a few sums on the 2*2250 + 3*2500 mixed pack.
 
As I see it ,the real danger is that when the total pack voltage hits the motor-cut threshold (which I've assumed to be 2.85 volts) the 2250 cells will be essentially flat. When that happens the BEC will I think stop feeding the radio (Rx plus servos), with complete loss of control.  That's more serious than destroying the 2250 pack.
31/01/2009 10:59:47
Well, yes you can do it safely.  How? You need a special type of ESC, or a stand-alone circuit that does the same job.  What you need is an ESC which monitors the CELL voltages individually, not the overall pack voltage.  This will then cut motor power when the FIRST cell drops below the designated voltage, say 3.00 volts.  The HobbyWIng Guard series do this.  Pack monitoring gets less safe (compared with cell monitoring) as the sell-count rises.  The Guard series therefore has a switch-mode BEC, to cope with high cell-count.  Do these cost a lot? NO: 40A /5S about £20.
 
But is it a good idea to mix capacities, even if you can do it safely?  NO!  So this response is really aimed at the experienced readers who have not come across this technology, as it really makes sense to me - for a matched-cell battery if you e.g. put 2 2S in series - and of course for that you need a series-Y lead for the balance lead.
Thread: What makes models zoom
30/01/2009 17:58:26
Gemma: modern terminology is to describe the lift forces on a wing surface as a couple (or moment) plus lift, and not to talk about centre-of-pressure.  Symmetric-section aerofoils do not display this moment (this is equivalent to the CoP not moving at varying AoA).  One reason this terminology is preferred is that it is simpler, as for most aerofoils with a cambered  midline the two descriptors can be an AoA-related lift force running through the 25% point, and a couple which is independent of AoA (but both of these DO of course vary with airspeed: roughly in line with the square of the airspeed).
 
The couple for a cambered section is NOSE-DOWN.  See the appendices to the excellent paperback Model Aircraft Aerodynamic by Martin Simons.  As a plane dives and the speed increases, the nose-down couple increases.  So that's not the explanation.
 
A second reason that CoP fell out of favour derives from the two characteristics I've described.  As the AoA reduces the couple remains the same but (at constant airspeed) the lift decreases.  If you represent this by CoP movement, the CoP moves backwards as AoA decreases.  At very low AoA the "CoP" is aft of the trailing edge!  Unless you believe in magic (or action-at-a-distance) this all starts to sound a bit silly!
Thread: Which way to go?
30/01/2009 17:41:54
Just like to support Timbo's recommendation of the Flying Wings V-trainer.  Easy to put together and easy to fly, and bounces well.  But you do need rather more than the £50 kit.  As well as the radio gear (transmitter, receiver and transmitter-battery charger unless you are going to run the Tx on dry cells, which is not recommended for aircraft) plus two micro servos, a LiPo battery (preferrably two), a LiPo charger and (unless that's 240 Volt pwered) a 12 volt source. Plugs, soldering iron, ...
 
But if you get all this, will your wife be happy?  It will take one weekend to put it all together (wet day stuff) but then you can only fly it on nice days!
 
How about a micro (indoor) electric helicopter?  Ideal for bad weather!
Thread: LiPo battery packs
30/01/2009 17:29:07
Whether you go 4S or 5S you'll need to be careful about the BEC circuit and losing power to the Rx and servos.  BECs (battery eliminators) come in two kinds: Linear, which often run into problems if fed by more than 3S, and Switched, which are OK at 4S or 5S (some at 6S).  I have heard it said they can cause interference on 35 MHz (but not on 2.4 GHz).
Thread: Engine starting
30/01/2009 17:19:32
Electric motors start easily!
Thread: What makes models zoom
19/01/2009 15:56:28

In February 2009 RCM&E there are I think two references to models zooming: Peter Miller on page 46 in “To the Drawing Board” and Lindsay Todd on page 64 in “Zulu E” – though he actually refers to ballooning, which I think is the same thing.  In both cases, these flight characteristics are blamed on the use of flat-bottomed wing sections. I’ve heard this statement many times in the past, but I’m not convinced it’s true. In my opinion, flat-bottomed sections have only one thing in common: they have flat bottoms, and so can be built easily on a building-board. They have nothing in common aerodynamically. Flat bottom sections often have cambered centre-lines, but not always: flat-plate sections are flat-bottomed but symmetrical and the same applies to Eppler 472. As they’re symmetrical, they have flat tops too!

From an aerodynamic (as opposed to building-board) point of view, I see three main classes of aerofoils: symmetric, cambered and reflex. These characteristics all really refer to the shape of the mean-line, around which is drawn a “streamline” shape or body, giving the outer shape. The issue raised here is I think whether a cambered mean-line causes zooming (whist symmetric does not). I can think of no aerodynamic reason why it should, as the only two substantial effects of the camber are angle of attack for zero-lift (and minimum-drag) together with the nose-down couple (or, as older readers may know it, Centre of Pressure movement).

I suspect the zooming is more to do with C of G location (and the consequent longitudinal dihedral), but what do others think? Do any full-sizers know from their book-learning? And please don’t say semi-symmetric in your replies: it’s a contradiction in term. Things are either symmetric or not, there’s no half-way state.

Thread: Looking after your LiPo cells
17/01/2009 18:32:35
I have some more data.  Flightpower recommend storage at a 3.75 volts.  I have found a link which explains why LiPos deteriorate in storage and recommends storage at 60% discharged: about 3.80 volts:
http://www.buchmann.ca/article23-page1.asp
 
Here's the text giving the technical explanation and table:

 
I've taken the data from this US Army chart (top line of graph):
 
and put it in a spreadsheet:
 
 
These results are slightly different from Timbo's.  Both lots of data say that Tim Mackey (RCM&E Feb p. 85)has given the wrong voltage for safe storage.  He quotes 40% (i.e. 60% discharged) as 3.9 Volts.  3.9 Volts is 70% charged / 30% discharged.
 

Edited By David Ashby - RCME Administrator on 17/12/2009 20:35:16

Thread: Parallel charging multi Lipo packs
14/01/2009 11:36:00

I can see them OK.

 In My Forum, Forum Settings, for Forum Images I have Render Images selected (not Render as Link).

 Does that help?

Thread: Where should the motor go?
26/12/2008 17:50:00
Well, if you're going back that far I guess we can include the Wright Fliers!  But they, and I guess the one you mention, would be draggy old wire-braced biplanes, not the sleek aerodynamic item I'm after.  There's the Edgley EA-7 Optica of course, but that bulbous canopy (and the shrouded prop.)..
26/12/2008 10:23:00

Eric: I see what you mean but I think Simon's correct: each blade will pass through BOTH the airflow from the upper AND the lower wing surfaces twice in each revolution: if the blades are turning clockwise when looked at from behind the aircraft then each blade will (on the right) descend through the upper airlflow and then the lower airflow and, half-a-rev later ascend (on the left) through the lower and then the upper airflow.

But I think it may be one of the oddball layouts that I've got in mind.  What models can you think of that are like this?

24/12/2008 12:01:00

Thanks, Simon.  I had not thought of that.  But since your note I've done a few sums and think this is much more of a full-size issue than one for models.

In full-size the airspeed over the wing top is going to be similar to the flying airspeed (say 400 mph) and the downwash component will be just a few percent of this.  The airspeed over the prop tip will need to be subsonic  (say under 500 mph at 40,000 feet).  So at (say) the 80% point on the prop the two will not be that different.  So the prop will really feel the wing-generated downwash.

But with a small electric model flying say 10-15 times slower the ratio will be very different: a 10" prop turning at 9k has a tip speed of over 250 mph (half the full-size), and at the 80% point the airspeed ratios will be about 7:1.  So the effect will, I think, be much smaller.

And even for the B36 I think I read that the pusher config improved performance by more than 5%.

Thread: FMS flight sim.
20/12/2008 18:25:00

I gave the address of a good download site above.

Download.com is really for programs so probably not a good source.

The FMS homesite has only FMS-7 models (20) but if you look on the Forum you'll see a download section that covers both 7 and 8:

http://n.ethz.ch/student/mmoeller/fms/index_e.html

20/12/2008 10:53:00

I would suggest you try this first with an FMS-7 par file, and then move to FMS-8 PAR as a second step, assuming you are using FMS-8.  Edit your 7-par file with notepad. For the second step (8-par) you'll really need a FMS-8 PAR editor such as those found at

http://www.rc-sim.de/dl_engine_eng/index.php?subcat=85&PHPSESSID=a6079bba2720211e0a2646dd63f19aa7

You may need to start at http://www.rc-sim.de/dl_engine/ and navigate.

In 7 the PAR file is pretty straightforward, like the one I've listed above.  Look for a plane that's a bit similar in layout to the one you've designed (let's call that myplane) and then take a copy of that .PAR file, modify it to be the right size / weight / power and rename it as myplane.par and put it in the models folder.  FMS-7 planes just need a .wav file for sound, and .x file for how it looks and a .par file to describe how it flies.  Most also have a .jpg file for the decoration.

You don't say how you are building the (appearance of) your new planes.  If you are using Metasequoia (also a free download from this site) then my understanding is that this generates both a .x appearance file (used by DirectX) and a .mqo file which contains the dimensional data which can be translated into a FMS-8 .par file, but I've never done that myself: I'm more interested in how the planes fly than how they look.

Many FMS planes, in my opinion, fly very unrealistically.  This is mainly not because of the limitations of FMS (particularly FMS-8) but because of poorly-written .par files.

19/12/2008 16:56:00

My entries for the FMS 7 .PAR files have been deleted at my suggestion, because some of the character-strings were interpreted as smilies.  So here's my SuperCup again.  This time I've inserted a space after the colon on each line.  Seems neither FMS 7 nor 8 mind that.  You can open and edit both the 7 (small) and 8 (large) .PAR files with Notepad, but the 8 ones are pretty complicated and so better edited with the proper version-8 PAR editor downloadable from several of the FMS sites. 

0        1: Type(-)      Type[0=Plane] ParDesigner Ver0.8
8        2: Tmax(N)      Maximum Thrust
.3       3: Drmax(rad)   Maximum Rudder Angle
.08      4: Demax(rad)   Maximum Elevator Angle
.004     5: Damax(rad)   Maximum Aileron Angle
0.8      6: CLmax(-)     Maximum Lift Coefficient
-0.5     7: CLmin(-)     Minimum Lift Coefficient
6.2      8: CLa(/rad)    Lift Gradient
9        9: CLaSt(/rad)  Lift Gradient in Stall
.01      10: CDw(-)      Drag Coefficient of Wing
0.01     11: CDb(-)      Drag Coefficient of Fuselage
.2       12: mug(-)      Friction Coefficient of Wheels
1        13: dCDSt(-)    Rise of Drag Coefficient in Stall
0.02     14: CM(-)       Moment Coefficient
.005     15: alpha0(rad) Wing Angle
1.8      16: b(m)        Wing Span
0.3      17: c(m)        Wing Chord
.16      18: hce(m)      Equivalent Center of Gravity
2        19: m(Kg)       Mass
.5       20: Izz(Kg*m^2) Yaw Inertia Moment
.5       21: Iyy(Kg*m^2) Pitch Inertia Moment
.02      22: Ixx(Kg*m^2) Roll Inertia Moment
0.09     23: She(m^2)    Effective Area of Stab
.002     24: Sfe(m^2)    Effective Area of fin
1        25: Lt(m)       Tail Moment Arm
.0015    26: VForm(-)    Effect of Dihedral Angle

19/12/2008 12:23:00

Eric: as you prefer the Reality craft version then you probably won't be interested, but in case others are:

To load a version-8 "IC" model with the engine running you need to have the throttle set to a level that FMS recognises as at least tickover.  You can always do that if you open the throttle a few clicks.

If the engine's stopped when you load the model with Throttle stick back ans throttle-trim up then you just need to calibrate the control: Controls / Analog control ... / Joystick interface / Mapping-Calibration.

The trick then is to remember that when you move the throttle to MINIMUM you must also pull the trim back before you click Calibrate (and don't forget: when it tells you to Centre the sticks THIS INCLUDES THE THROTTLE!).  Then in use you set the throttle trim for a good tick-over: open the throttle a bit, Load (NOT Re-load) the model (engine sound will be heard) and reduce throttle but trim-up until the stick's right back.  THEN reduce throttle-trim for a nice but safe idle.  Once set for one model should work for all.

19/12/2008 12:04:00

Erfolg: these are the old version 7 models.   If you start up FMS and then click on Help and then About ..  a little window comes up and the contents scroll upwards.  Almost immediately you will see the version.

This will in your case I think say Version 2 Beta 7: the old FMS 7 which does not support version 8 models (and does not allow for wind and gusts).

The more modern version will say Version 2 Alpha 8.5 (just possibly 8. something else).  8.5 is the most modern version.

I recommend you de-install the old version SAVING a copy of the models first to a Temp folder, and then re-install the new version.  Then copy back the old models to the Models folder.  You CAN have both versions installed (I have) but this is not recommended, and it took me some work to make them live together: for instance, I think they use the same Registry values, so if you change models in 8 and then close it and start 7 you will be using the same model as you used in 8.  OK but not if it's a version 8 model, as it won't work!  And you'll think something's gone wrong.

Controls: I used the Microsoft Sidewinder 2 joystick with some success, but now use a USB interface (from China) that plugs into the Buddy port on my Futaba EXAP.  It's nice to have trims and to be able to set the progressive Exponential on the TX, rather than the fixed option built into FMS.  Just cost a few pounds.

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