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: Micro Rx Battery Packs|
There's some comment about the max current draw from the pack John selected.
Andy Freeman's posting with the link to Component-shop says "if you don't mind soldering your own" but if you look at their site you'll find they do small made-up packs such as this 210 mAH 4-cell pack with Futaba (or other) connectors, and this one has a max. discharge rate of 1.5 A - so plenty to drive several servos. This one weighs 23g. As standard it conmes with a "JST" 2-pin polarised connector (strictly a JST-EHY connector).
The smallest one is 120 mAH (bigger than you requested but it only weighs 15 grammes). It's physically larger than you want (in one direction) but if you cut the heatshrink you may be able to bend the tags and double it up. If not then cut the tags, rearrange the cells and solder the tags back together (directly or with bare wire). Then wrap in tape. Soldering the tags TO the cells is hard (commercial tags are welded not soldered) but soldering tag-to-tag is dead easy.
To work out how long flight-time this will give requires either under-load current measurement or guesswork! Small servos and Rxs don't necessarily mean small current draw. Rule of thumb: 4 servos plus Rx used to run an hour or so on old-type 400 mAH NiCd packs, so 2 servos and an Rx out to run for an hour on this. But check every ten minutes and stop when the pack voltage is under nominal: 4.8 volts.
Edited By John Cole on 17/07/2009 15:30:39
Edited By John Cole on 17/07/2009 15:37:44
|Thread: Cyanoacrylate safety|
I keep a plastic bowl of water to hand when using CA. Yesterday I used it, and I thought I would tell you what it's for:
The top came off my large bottle of CA, and I got quite a bit on my fingers. I immediately put the fingers in the water. If you do that the CA "goes off" instantly, eliminating the risk of your fingers sticking to each other or something else (like the rag you thought you could wipe it off with!).
|Thread: FMS models|
I suggest those of you looking for help say a bit more than "it didn't work". Give the exact error message and say exactly which version of Windows you are using. For instance, I am using XP Home Edition SP3.
|Thread: What does this charge?|
And now IE8's a free upgrade to IE7 (for Xp and Vista). I like it.
|Thread: Learning to fly alone|
I agree that flying a sim is diffrent from flying a real model, but that doesn't mean you can't learn a lot from the sim.
In my opinion, the two main things you can learn are how the controls work when the plane is flying towards you (or flying inverted) and how to match your stick-movements to the way the plane is pointing. These are sort-of model-independent.
There may be better sims than mine but it's main weaknesses are poor representaion of wind and gusts, plus the issue of not properly seeing the model's position in the sky(fly "high" and you lose ground-reference).
Nevertheless I would recommend one to all beginners. There's no queue like there can be for an instructor, and if (when) you make a mistake you don't break anything.
|Thread: Using a seperate cell battery charger on a 4 pack Rx|
Who is the manufacturer, and what's the model number of the charger? That would help me find details of the charger.
But you also need to say what you want to do: is it simply to charge a 4-cell Rx NiMH battery? If so, I think you're going to have a problem.
|Thread: Vac Forming Rig Construction|
|Excellent result. Mass-production next ...|
If you haven't seen it before, this article may be of some help. Interestingly, they use IR heaters. Note the specific point about drying polycarbonate. I see they recommend a higher temperature than I expected: 200 degrees Celsius, but they're also talking about a pretty fierce vacuum at 0.8 bar.
Not sure what kind of mould you are planning to use: from your mention of balsa wood I assume it's a single male mould. But if that's the case, how will you introduce it? The edge of the plastic sheet will need to be flat and reasonably-unwrinkled to give the vacuum seal. In the professional eequipment described in this artice the mould is raised within the vacuum chamber. I guess one option might be to PRESSURISE the cavity initially, so the polycarbonate blows OUT when heated, making space for the mould - and it's then sucked back onto the mould in a second step.
BTW: your original enquiry for sourcing plastic sheets referred to PET. I guess you've changed to PC as it's a lot tougher. But PET's easier to mould (glass temp 75 / melting 260.
Not sure a gas hob is the ideal way to heat the polycarbonate to its softening point. The heat deflection temperature is only about 130 to 145 degrees Celsius, depending on the test used (i.e. the stress level) and the molecular weight. The combustion products from a gas hob are likely to be at a much higher temperature than this locally, and the thermal mass of your sheet is very small so it will heat up locally very easily and may overheat - and tear. It melts at 270 degrees. Clearly it's better to have even heating than localised heating, but you may get OK results with a heat gun. Alternatively, it may help if the edges of the frame are extended downwards, to catch a "pool" of gas.
I've only tried heat-forming polycarbonate once, using a quite different approach (which I WON'T recommend) but I did find one thing which might be relevant: quite a strong tension when I first heated the sheet. You may need to restrain the edges quite hard. You may also find you need to keep the sheet at temperature for a while for it to fully-relax under the vacuum. You'll probably need to take it just above the glass-transition temperature for a few seconds (about 150 degrees).
Edited By John Cole on 14/07/2009 12:28:36
|Thread: Stalling - why is it so dangerous?|
I recommend "model aircraft aerodynamics" by martin simons. The fourth edition is dated 1999, so it doesn't have data on the latest wing sections (more recent ones are easily found on e.g. michael selig's site / university of illinois).
The first edition came out in 1978, and as I recall martin used to write for rcm&e about that time. It's slightly mathematical in the sense that it quotes formulae but I would guess it's easy to follow without going into the maths; just read the text and study the graphs. It is perhaps a bit sailplane-focussed, but then perhaps that's the area where aerodynamics is most important.
Tony: I wrote that the angle of incidence is IN EFFECT increased, and the "in effect" was meant to imply that I was using the term loosely.
What I was trying to convey is as follows: Picture the plane horizontal in both pitch and roll. If the plane is slipping left and the wing has dihedral then draw a new datum which is horizontal but skewed left in the direction the plane is slipping. The angle between the left-wing chord line and this new datum is increased (whereas for the right wing it is decreased). That's just geometry, not aerodynamics. For any point on either wing the AoA is therefore also changed: for the left wing it is increased, and for the right wing decreased. So if the plane is close to the stall, is slipping left and has dihedral, the left wing will stall as it is at the higher AoA.
|Thread: Discharge circuit for li-po batteries|
|Pete: the sources I found suggested 40% charge / 60% discharge for optimal long-term cool storage - so at around 3.75 V per cell, see my earlier posting. And as I said: that's how they come from the manufacturers, and they should know.|
Glasshopper: you still haven't said WHY you are trying to discharge your LiPos. If it's for storage you need only go down to about 3.7 volts / cell, and there's no advantage (and much danger) in taking them down to 3.0 v. If you're trying to do that and exact balance is not needed (because you'll balance them when you recharge) then you need only discharge them through the main output lead. For this all you need is a kitchen timer, a resistive load such as a 12 v bulb (or several in parallel) on your usual power connector, and a digital voltmeter (preferably with a connector that will plug single-cell into your balance lead). Then measure the voltage per cell for each cell, select the lowest, select how much you want to come down, calculate what mAH discharge that means using a volts / capacity chart, set the timer accordingly and plug in. Then disconnect when the timer rings. That's all there is to it.
For a connector to a JST-XH free socket (as is found on many LiPos now) take what's called a JST (2-pin) connector - strictly a JST-EHY connector - as used for the battery power connection on very small models. Cut a few mm off the plug shroud and this will give you a 2-pin connector with exposed pins at 2.5 mm spacing. This will fit into contact pairs in an XH socket (and some others) as well as Futaba Rx battery connectors.
For a LiPo volts / capacity chart, look at my pictures; you'll see one there with the spreadsheet table used to graph it. This is based on third-party data and I use it all the time for checking rsidual capacity / state of charge.
A number of sources recommend partial discharging for long-term storage, and cool storage. Most recommend 3.75 volts, which is about 40% charged / 60% discharged. That's the voltage all my new ones come charges at, and the supplier confirms this is for maximum life. LiPos degrade slowly if full charged and at room temperature.
Here's something I got off the web, from US Army tests.
|Thread: Stalling - why is it so dangerous?|
Martin: a clarification of your (earlier) posting about looking at the model's attitude: your words sound (to me anyway) as if you meant using the horizon as the reference, which is OK if the model's flying level. But is it's descending, this would cause the pilot to underestimate the AoA - and could lead to a stall. So the reference must of course be to the attitude in relation to the flightpath, which I'm sure is what you meant.
Gemma: "is stalling caused by flying too slowly or by excessive elevator?" Yes, of course they come down to the same thing and I know "too slowly" is the usual full-size wording. In fullsize you've got a nice little meter that tells you the airspeed. I used "excessive elevator" simply because a model pilot doesn't really know his model's airspeed (but he can see when he's bending the sticks).
Gemma: you are of course correct about high-speed stalls (incluuding in steep turns) but that was pretty much covered in the original article.
Ken (Anderson): I think there's another point about the difference between models and full-size, and that's the wind - and in particular gustiness. As models are smaller and fly slower they are potentially more affected by any particular speed of wind or gust - as it's a greater proportion of the total airspeed. And I think it's true that we will fly our models in gust-speeds well in excess of the proportionalte gust-speed which would stop full-size flying.
The particular relevance of this ot stalling is that if the plane is flying just above the stall then a change in windspeed can provoke the stall. As I set out above, once a wing starts to stall and drops, the AoA increases further, deepening the stall. So if the change in windspeed reverts (even very quickly) the plane then remains stalled.
Terry: how did you react to the original article? I agree some people like to know how things work, and that others don't find it interesting (and indeed boring).
Martin: I said "avoid a tight turn". This was not meant as a technical term, and I absolutely agree with you that excessive rudder on the turn onto finals is likely to provoke a one-sided stall. I said "tight turn" rather than "steep turn" with exactly that point in mind, but it's good you clarified it.
Gemma: yes, spot on. But before I set out my thoughts on sideslip (or indeed skidding) let me add something to my initial comments - which I really should have put in to start with.
What I was trying to do is explain what (in my opinion) it about stalling that makes it so dangerous. And I missed out something. First let me expand on "tip-stalling".
When a model starts to stall and falls off sideways, we often call this a tip-stall. As I explained in the example above, for a parallel-chord wing, one wing will stall before the other if the plane is turning at the point of stalling. What I left out is what happens next. Partial loss of lift from (in my example) the left wing means that the plane rolls left and the left wing goes down. This action INCREASES the AoA on the left wing (with more increase as go out along the wing) and straightaway the whole left wing is stalled. Conversely, this roll reduces the AoA of the right wing so it remains unstalled. That's why an initial one-sided stall turns into a flick. Of course, all this changes when a quarter-roll is complete, but on the final turn it may not get that far!
Sideslip and skidding. The links to one-sided stalling are I think similar. In both cases the plane is moving sideways through the air, and I'm going to call it "sliding" and not try to distinguish between the two. The link to stalling is I think different depending on whether the wing has dihedral or not. Consider the case where (as in my example) the plane is turning left (and banked left) , and is also "sliding" to the left - so the airflow over the entire plane is coming from a direction slightly left of straightahead.
Now, consider a plane (with dihedral) near the stall, turning and banked left and sliding left: airflow over the left wing will be slightly sideways going in towards the root, increasing the distance travelled from LE to TE and thereby decreasing the local AoA (the "simple sliding effect"). But the dihedral means that the angle of incidence of the left wing is in effect increased (that's what makes rudder/elevator models bank when rudder is applied). So these two effects are in opposition. For the right wing the sliding also decreases the AoA (with the air moving OUT away from the root as it goes back) but so does the dihedral effect (effectively reduced incidence). So a plane with dihedral sounds to be more prone to one-sided stalling in a sliding turn. But the two opposed factors affecting the left wing: which is the bigger effect? The more dihedral, the bigger the "increased incidence" effect, so for any degree of "sliding" there's a dihedral angle beyond which stalling is made MORE likely by "sliding" than not. And that angle is VERY small. Take an arbitrary example: with 10 degrees average AoA and 5 degrees of sliding and 5 degrees of dihedral (that is, each wing points up 5 degrees from the horizontal) the effect of "simple sliding" on AoA is less than 0.1 degree but the dihedral effect on AoA is nearly 1 degree. But it's also affected by the issues that affect a plane with no dihedral
What about a plane with no dihedral? Well, it then all gets a bit messy as the primary effect of sliding is to put the inner part of the right wing into dirty air which has flown round the fuselage. And it's different for a high- or low-wing plane; for a low-wing plane this disturbance primarily affects the flow over the upper surface (where the stall might happen), and for a high-wing plane it affects primarily the flow underneath.
Gemma's right: what matters most is what we need to do to avoid a stall-provoked crash. And it's pretty much down to avoiding excessive elevator (as this is the principal control that stalls the wing), and being particularly cautious about this in a turn. My personal opinion is that it's safer to add power in a turn rather than increase elevator. And of course, keep the CoG well forward! With i/c powered planes landing with a near-empty tank the CoG will be at its rearmost point during the flight, and hence it will be most stall-prone AND elevator-sensitive.
Well, obviously, loss of lift is always bad for a aeroplane, but I don't think Martin Bedding's article (August RCM&E p. 88: Stall School) fully explained why it makes planes crash even though he gives excellent recommendations on how to avoid stalling - particularly on the approach.
It's a problem if the plane's wings both stall, as the plane goes nose-down until (if you release the up-elevator) it reaches flying speed again. Hopefully, this is before the plane "lands".
It's a BIG problem if ONE wing stalls, as recovery is then a bit more difficult.
Let's get some terminology straight. A wing's Angle of Attack is the angle between the (local) airflow and the chord line of the aerofoil; the line drawn between the leading edge and the trailing edge. What makes a wing (or any other flying surface) stall is if the AoA exceeds the stalling angle. I say "local" as the AoA can vary down the wing, and vary between the two wings: see below. When this happens, the (local) airflow over the top surface of the wing goes highly turbulent and fully breaks away from the wing, causing a dramatic loss of lift. What makes it exceed the stalling AoA? Excessive use of the elevator. That's it.
Note I say LOCAL airflow. Here are two key examples of how you get local stalling. Take a parallel-chord wing, the plane flying straight - with up-elevator being gradually fed in (at constant lowish power setting). The plane will slow, nose-up and eventually the wing will stall and nose-down. Now the air flowing over the wing near the root goes more-or-less straight back. The air on top of the wing is at lower pressure (that's what gives the lift); that underneath is at higher pressure. Near the wingtip the air on the upper surface does NOT go straight back but veers inwards towards the root, "pushed" there by air flowing round the wingtip (from bottom to top) because of the pressure difference. So what? Well, as it goes sideways-and-backwards, the air travels further going from the LE to the TE near the tip than it does near the root. What this does is to reduce the local AoA near the tip. So a parallel-chord wing will tend to stall near the root first, and at both roots simultaneously if the plane's flying dead straight. That's good news, as the plane is more likely to stall straight-ahead, with simple recovery.
But only MORE likely. Consider the second example. If the plane is turning - say a level turn to the left - then you can see that by the time it's done a full circle the right wingtip has travelled further through the air than the left wingtip. The difference is good old Pi times twice the wingspan. It's easier to understand the consequences of this if we think of the plane gliding, as during this full turn both wingtips will have descended through the air. The average AoA is then given by the amount of descent and the circumference of the turn. What this means is that the AoA at the left wingtip is greater than at the right wingtip (because it descends the same amount, but over a shorter distance - it's flying at a lower airspeed), and the same applies for any two corresponding points on the two wings. So the left wing stalls first. Lots of lift on the right, almost none on the left, quick quarter-roll left and dive towards the ground. Crunch! So that's why planes crash when you do a turn at low speed, using enough up-elevator to provoke this misbehaviour (termed an Incipient Spin). I explained using the example of a plane gliding; it's essentially the same for under power, but more complicated to explain.
Notice I said nothing specific about tip-stalling. Whether this happens depends on lots of things (spelled out by MB). But if one wing stalls before the other, you're in trouble if you're close to the ground - whether it's tip-stalling or not!
So the recommendations for the final turn on the approach are: don't fly too slowly; keep the power on and don't bank too much - and don't do a tight turn. Or it really will be a final turn! Similar advice applies immediately after take-off.
|Thread: OS 32|
|Advertise it in Classified: one careful owner and starts well.|
Erfolg: thanks for your comments. I agree that a high-mounted motor will generate a nose-down static-thrust pitching moment (though I think the issue is whether it's above the centre of drag rather than the CoG). But what I found with my plane is that increasing power made it pitch UP and climb steeply. And that's what started this thread.
But my question was really about how to solve the problem, and WHY downthrust seems to help. I am by no means sure that the explanation that I've offered (propwash) is correct, but given the results of my trials I can think of no other. And it was to get other people's ideas that I started the thread.
I'm not looking for help on doing the sums, but I am looking for people to give me ideas so I can do the sums myself. Like you, I've quite a strong background in Maths; it was my job before I retired.
Anyway, thanks to all all for the point that I should initially have trimmed for the glide, and then sorted the power-on issues. Thanks Gemma for the point of needing different trim for those two conditions. As well as partially resolving the problem with downthrust (much to my surprise), I also mixed power with elevator, so increased power adds down- elevator (I used a 6% mix) and my plane now flies nicely.
Now for the article in this month's RCM&E about stalling! Gemma: you might like to kick that off.
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