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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: The August 2009 issue
01/07/2009 17:48:24
I DO like the new look, and particular Counterpoint: no more matching up numbers and pix.  I also like the way Editorial and Contents are gradually moving to the front (though I'm not sure if that's careful layout planning or tight purse-strings with the advertisers who want low-numbered pages!)  Me, I would like to see the contents pages as pages 2 and 3, so I can immediately find my way round on opening the mag
I was never a fan of the "tabs" at the top of e.g. Switch On. The page title told me where I was.  The tabs just duplicated that, and told me I was not on the page for Counterpoint, All Write, Going Places ....  Glad to see them gone and that you are using the space for more text and bigger pix.
Thread: Downthrust
01/07/2009 17:23:17
Well, here's what I've found: the down-component of the thrust on a pusher model is behind the CoG and so might be expected to push the nose up under power.  But it doesn't, at least on my model (with about 7 degrees of downthrust, more than you would usually find in a tractor model, I think).  In flight tests, the downthrust keeps the nose down under high power settings.  But why?
 
I did a static test, marking a vertical line on the fuse side  through the CoG, and marking a line on the fuse side representing the thrust line.  I then stuck pins in where the lines intersected, and balanced the model on those pins.  Static thrust from the pusher prop should then not pitch the model up or down
 
But it does!  It goes nose-down!
 
I think what's happening in the static test is that the airflow over the horizontal stabiliser pitches the plane. With no downthrust (and a quite-forward CoG, my starting point), the stabiliser is at negative incidence to the propwash and the plane pitches up (which is what I observed initially).  With substantial downthrust the stabiliser is at POSITIVE incidence to the propwash, and so the tail is pushed up (and nose down). And the flight test suggests this has a bigger effect than the nose-up moment of the behind-the-CoG downward component of the thrust.
 
I guess the same is true to an extent with a tractor config - with the two moments acting in the same sense, but with the propwash element weakened by greater prop-stabiliser separation and perhaps a flow-straightening effect from the wing
18/06/2009 10:17:59
Hi, Gemma.  Nice to hear from you again.
 
Timbo: yes, the CoG may have been a bit forward: spot on 25% as per the plan, with a cambered-centreline wing section (as you would expect for a powered glider).  I've moved it back to 30%.  You say the model flew S&L under power "because of the upthrust".  Sorry, I did not make clear but the starting point was a couple of degrees of downthrust, which my friend advised me to increase.
 
So, yes I've taken the CoG back and trimmed first for the glide - and I've put in quite a bit of downthrust and that seems to have reduced the power-on / power-off trim difference.  But I'm none the wiser on why the downthrust has made a difference (which I think it has).
 
Phil: I wondered the same as you, whether the location of the motor made a difference, but it seems to me that the fact that the thrustline is above the centre of (parasitic) drag would give a nose-down couple under power, but that would give a result exactly OPPOSITE to what I'm seeing.  And I can'tsee that the effect of downthrust links to that couple either.  But if you do then I would be most interested in your explanation (pm me if you wish).
 
So, thatnl to you all for the advice on trimming - but I'm still looking for comments on why downthrust helps!
 
Thread: Cheap cyanoacrylate glue
18/06/2009 09:55:02
Mowerman: I can't beat that!
Thread: Downthrust
17/06/2009 17:50:21
One of my models, trimmed to fly straight and level on half-throttle, glides at a very steep angle when power-off. An experienced modeller recommended several changes so that, when trimmed as above, the glide will be much shallower (all of which I've made).  One of them was to increase the downthrust considerably.  When I asked him about that change and why it would help he gave me an explanation, but I'm not sure he's right (in his explanation, I mean - I'm not disagreeing that downthrust helps).
 
So if anyone out there can tell me WHY downthrust helps in this situation I would be grateful for your comments.
 
In case it affects the answer, here's the model configuration: high-wing electric glider with motor mounted behind and slightly above the wing centre-section (a pusher).
Thread: Cheap cyanoacrylate glue
17/06/2009 17:40:38
I've just had a new kitchen put in and was surprised to see the fitter using CA to stick mitred MDF together.  He bought it from the local timber merchants, and it's available in quite big pack sizes: 20 ml and 50 ml, and there's a aerosol can of kicker / accelerator.  This is marked with a warning that the solvent can affect materials, but it didn't seem to affect white EPS when I tried it.
 
If you can't get big packs from your model shop, you might find this will save you money.  Here's the link to the manufacturer's website for the 20 ml pack:
 
Thread: V-Trainer
12/06/2009 18:13:28
I'm afraid I've come to this thread very late, but I've a couple of thoughts on putting the V-trainer together: too late for the post-starter but may be of help to others.
 
First let me say I'm really pleased with mine.
 
The main mod I made was to the LiPo mounting: I cut a horizontal slot right through the nose part of the fuselage, to hold the LiPo - sized to be a snug fit, and relieved at rear-right to let the cables out.  I then glued two strips one each side of the slot, near the front, to stop the LiPo coming out sideways.  I now slide the LiPo in from the rear, slightly angled.  Seems very secure.  I used Mylar strips, but thin ply would work too.  Doing it this way is easier and you get the LiPo further forward, too.
 
The lead shot supplied as noseweight: if you use this then it's hard to adjust after the glue has set.  Another time I would use lead strip at the front of my LiPo slot, bent round the Lipo, initially sizing it to get the recommended balance point, but then adjusting it so it flies nicely - in my opinion the recommended balance pointt is a little too far forward (a safety measure).
 
It took me some time to realise it's better to fit the motor to the mount after getting all the cables in place, and with the wing in place.  Note: screw the mount in place so the grub screws are horizontal; if they are vertical, the lower one is hard to tighten.  And yes, do tighten them HARD or they undo and fall out!
 
Don't test the motor-run direction with no prop: bad for the motor.  Test it in place, but connect it to the ESC using a screw-block connector.  Then solder up when you've worked out the correct leads-match.
 
Flying Wings recommended the very-small 2-pin JST connector.  I found it a LOT easier to use Deans T-connectors.
 
To stop the O-rings flying off, I tied them to (one of) the screw-heads with button-thread.
 
I fly mine using Futaba 2.4 GHz.  I drilled two holes at 45 degrees to the axis, "swept back" back out of the electronics space, and glued 2" lengths of drinking straw into them.  The twin aerials go one in each.
Thread: How far away is your plane?
12/06/2009 17:31:31
When I first posted this, I lamented that it was a shame I had just come up with this idea around the time I switched to 2.4 GHz, and the idea only works of course with  a long Tx aerial.  Well, I've NOW an idea how to estimate model distance if you've moved to 2.4 GHz.  It came to me this morning; I was flying a small (1 metre wingspan) electric with a small high-rev prop.
 
If the model is close to me and I flick from half- to full-throttle, the engine note rises instantly (if your doesn't perhaps you've got soft-start enabled on your ESC; I haven't).
 
I'm flying in a big field with trees about 400 metres away (judged from Google Earth, with the scale switched on).  My rule of thumb is that the model is twice as far away as you think!  So when I'm flying from e.g. left to right in front of the trees, my fear is that I'm about to fly right into them.  Sighs of relief every time I get past!
 
Now the speed of sound in air at 15 degrees C is about 340 metres per second.  So if I'm nearly 400 metres away and snap the throttle open, it should take MORE THAN A SECOND before I hear the change in engine note.  It didn't.  It took less than half a second, so my plane must have been about 150 metres distant, much closer than the trees.
 
If you have a similar distance to measure, work out your predicted delay and then try to learn to estimate it.  Then compare it with the delay you note when you flick open.  Petrolheads are probably better flicking closed (momentarily), because of the potential lag on throttle-open, but I've only tried it with electric.

Edited By John Cole on 12/06/2009 17:39:44

Thread: Fire Extinguisher
08/04/2009 18:11:18
From Thursday April 9 LIDL are selling 1kg powder fire extinguishers @ £6.35
Thread: 2.4ghz rx arials
08/02/2009 15:14:57
Timbo: for optimum radiation pattern with the module-equipped Futaba Tx should you not hold it the other way up (sticks underneath)?
Thread: What makes models zoom
08/02/2009 12:45:24
Gemma: you say the Moment of a cambered aerofoil produces a nose-up pitch.
 
I say the Moment Coefficient about the AC is nose down.  I gave a Wikipedia link for other readers to see, and an academic  book-reference which supports it. 
 
If you mean that I am wrong an you are right then only one of us can be correct.
 
The link you just pointed to correctly states that the CofP moves forward as alpha increases.  That fits exactly what I said when saying that CofP is a somewhat-strained term: that as alpha DECREASES  the CofP moves backwards, and in fact goes behind the wing TE at low alpha. That's why I prefer the use of CM, but my preference has nothing to do with whether the moment is nose-down or nose-up.
 
The AC is defined as the point where the Cm is invariant with alpha.  So if the Cof P moves forward so it is 50% closer to the AC then by definition the lift has doubled (and the moment stays the same).  But it (almost) always remains BEHIND the AC, giving a nose-down couple.
 
If you think I am wrong and you are right, then explain why.  Cite a reference for positive Cm.
Thread: 2.4ghz rx arials
08/02/2009 12:23:53
My (slightly unusual) experience of the 2.4 GHz Tx aerial is as follows:
 
I am right-handed and fly Mode 2 (Throttle left). I find hand-launching left-handed quite hard, so I launch right-handed.  So that's what I did with my electric glider.  But I sent it off a bit too-downhill.  So I quickly grobbed the right-stick and yanked it backwards.  Nothing happened!
 
Yes folks, I was pulling back on the aerial!
Thread: Is 2.4Ghz universal around world?
08/02/2009 12:19:45
As France is in the Eu surely the power limit is the same there as here?  I thought the France switch in the back of my Futaba simly limited the part of the 2.4 GHz spectrum to the bit permitted for this use in France.  Think I read that in the RCM&E review.
Thread: How far away is your plane?
08/02/2009 12:13:49

Thanks for the nice comments, folks.  Appreciated. Hope you find the idea useful in practice.  In my case the reason was to do with flying too far away and getting too close to housing.  So it wasn't radio-range, Owen.  My experience is that R-R is further than you can see the model enough to control it.

Tom: I wear single-vision glasses for flying (I find them clearer than Varifocals) but I think I can answer your question.  When flying, you look at the model and focus on it.  When you bring the Tx aerial-buttton up to compare, you still focus on the model, but as the button's a metre away it is also reasonably sharp, especially on a bright day.  Either way, you are looking through the "distance" bit of the bi-focals.

Phil: best of luck in marketing my idea! As I've "published" it, I have no claim on it.

Thread: What makes models zoom
08/02/2009 10:59:05
JW: you do the Wright Brothers an injustice.  They built a small wind-tunnel to test the efficiency of their wing sections, and much of their work with gliders was to test those results at nearer full-scale before using them on their manned Flyers.  They initially used the results of Lilienthal, taken from (essentially) tethered gliders, but limitations of his work caused them to build the wind tunnel.
08/02/2009 10:50:20

Gemma: I'm sorry if my comments on terminology seemed to be a dismissal of your arguments.  They were intended only to show why CofP is rather a strained concept, and why modern wind-tunnel tests normally show Pitching Moment coefficient (Cm) instead, measured about the Aerodynamic Centre (AC - typically the 25% point).  Note: Wind Tunnel test results often show the Cm graph "upside down" - with negative Cm at the top.

However, let me comment on your arguments themselves.  You say "A flat bottom section displays a nose up couple with increasing alpha".  Well, yes it does if you measure it about the mid-point or centre of area, but not if you measure it about the AC.  In fact almost all sections do as you say, including all symmetric ones - about the mid-point.
 
The AC is defined as the point about which the pitching moment is invariant with alpha (in the usable range), and for symmetric sections the AC is on the 25% point and Cm is zero for all alpha (and hence invariant).  For cambered sections Cm is usually negative.  Negative Cm means a nose-down pitching moment.  Langley data for Clark Y gives a typical Cm of -0.06: nose-down.
 
Here's what Wikipedia says about the theory of thin wing sections: Wikipedia: Airfoil
"  The moment about the 1/4 chord point will thus be,

 \ C_M(1/4c) = - \pi /4 (A_1 - A_2) .

From this it follows that the center of pressure is aft of the 'quarter-chord' point 0.25 c   "
 
As the CofP is BEHIND the AC, the pitching moment about the AC is nose-down.
 
Here's what Wikipedia about Pitching Moment: Wikipedia Pitching Moment
 
The graph shows the Cm for a Cessna 182, consistently negative.  The author quotes from Ira H. Abbott, and Albert E. Von Doenhoff (1959), Theory of Wing Sections, Dover Publications Inc., New York SBN 486-60586-8 :
 
" Pitching moment is, by convention, considered to be positive when it acts to pitch the airfoil in the nose-up direction. Conventional cambered airfoils supported at the aerodynamic center pitch nose-down so the pitching moment coefficient of these airfoils is negative.  "
08/02/2009 08:46:42

TJW: semi-symmetric is a contradiction in terms.  Something is either symmetric or it isn't.  So I agree with Evan.

What's the camber line?  Think of an aerofoil as being built up from two components.  The first is the centre-line - the line which goes from the leading edge to the trailing edge and is always half-way between the upper and lower surfaces.  In a symmetric section this line is flat.  In most other wing sections this line is curved.  The second part is the "streamline" section "wrapped round" this centre-line.
 
The job of this "streamline" section is to stop the airflow becoming turbulent and detaching from the (top surface of) the wing - as happens eventuially in the stall.
 
The job of the centre-line (or camber-line) is to get good lift with little drag, within the design flying speed range (and hence range of angle of attack).  A section with a cambered centre-line will usually give a better lift/drag ratio at normal angles of attack than a symmetric one.
 
Camber-lines can be of many shapes, and so can the wrapped-around "streamline" section.  The "peak" of the camber-line can be well back from the LE or it can be well forward.  Changing  the location of "peak-camber" and the pojnt of maximum thickness of the "streamline" section are the sort of things that alter the conditions where the wing "works best".
 
Camber lines can also have "reverse-camber" towards the trailing edge: a reflex section. 
 
For full-size aircraft and particularly sailplanes the choice of section can make a tremendous difference to performance, but this is less true at model-sizes.  The "best" wing sections lose some of their advantage over "average" ones at model-scale.
Thread: How far away is your plane?
04/02/2009 17:11:24
It's difficult to judge how far away your plane is, even if it's quite close. It's exceptionally hard if it's a long way away. One reason it's hard is because you get no feedback, until it comes down and you have to find it on the ground (or you fly into a tree). Ever found it was a lot further away than you thought? Ever had complaints that your club was flying too close to something, and you all thought you were flying much closer? Here's something I worked out: my Futaba Tx aerial has a button on the top. It's about 1 cm diameter. When my 35 MHz Tx aerial is fully extended, the button is about 1 metre from my eye. So the ratio of button diameter to button eye-distance is about 100:1. So how does this help?

If your plane is flying directly across your field of view and its length looks the same as the button-diameter then it's 100 times further away than its length. So if it's 1.1 metres long and it looks the same "size" as the button then it's 110 metres away. (If it's flying away from you and the wingspan looks the same "size" as the button, it's 100 wingspans away, so if wingspan is 1.8 metres it's 180 metres away. If ANYTHING looks the same size as the button then it's 100 time further away than the thing's size).

If it's flying directly across your field of view and its length looks half the button-diameter then it's 200 times further away than its length; if it's twice as big as the button then it's 50 times further away, and so on.  In this made-up photo, if the plane shown is 1.1 metres long, then the left-hand plane is about 55 metres away, the right-hand one 110 metres and the tiny one above the aerial button is 220 metres away.

The ratio may vary with other makes and models of Tx, so you might like to measure your own.   I'm still thinking about 2.4 GHz!
Thread: LiPo battery packs
02/02/2009 12:57:06

It doesn't say on that screen that you need type in a word (in the title box, I assume you mean).  Apart from that, that's exactly what I did.  I guess the issue is link-wrap and a short title appearing in place of the link prevents that, so I'm going to try that here for the centre (long) link:

 
Though I didn't see distortion before, using IE7
 
OK that seems to work, but as I've discovered in the past you can't delete a posting once you've made it.
02/02/2009 11:45:26
REPOSTED with the links inserted using the chain-link button.
 
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:

for a better photo -but take care: some of the text on this page is generic  and doesn't apply to the Guard series
here

See
http://www.giantcod.co.uk/forum/viewtopic.php?f=9&t=196

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.
 
BTW: I think HobbyWing manufacture the Turningy ESCs, and Nigel Hawes reviewed the Turningy-badged product about a year ago.

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