Martin Harris - Moderator
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Posts posted by Martin Harris - Moderator
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2 hours ago, Simon Chaddock said:
I have only once seen this done when a two seat instructional flight just off a 2000' aerotow suddenly went into a true vertical dive to only level off for a direct approach and landing.
While I was learning, my instructor (the legendary John Jeffries) talked me through such an approach from 4000’ after a wave flight. A memorable experience…
After that, I only recall using air brakes at high speed once, while trying to descend below cloud in an ASW20 and getting close to the rough air VNE - the suction was very significant and came as quite a surprise even though I was expecting it!
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In these days of wriggling insurance companies, you would be advised to inform them of this departure from standard as they may regard this as a modification and/or contributory cause to an accident. There will be a recommended maximum width for your wheel rims to avoid handling problems - 30mm is a big difference if you’re using 165s but not so significant on 265s!
Your steering lock stops may also need adjusting and there could be wheel arch clearance issues plus the speedometer calibration may be wrong due to the increased rolling diameter so look out for speed cameras!
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Quite so. There’s often confusion between air brakes and spoilers. Spoilers do what they infer - spoil lift whereas air brakes primarily add drag - although both types have each other’s characteristics as secondary effects
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Conventially, spoilers lie parallel to the wing’s top surface whereas air brakes project vertically. These can be top surface only or, for far better effect, top and bottom, allowing a bleed of high to low pressure air as well as increased drag.
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This is where telemetry is so useful. One cell shorted in a 4S 2P arrangement should allow control but report a low voltage with a sensibly set alarm threshold - worst case scenario is a 2S 2P LiFe but you’re starting from a higher voltage so still within limits for most receivers/servos. I have had this happen on a single 4S 700mAh nicd pack so I speak from actual experience.
If you’re talking about a wiring short after the pack, it’s (a) unlikely and (b) probably going to melt any standard receiver pack wire very quickly, acting as a fuse! If it happens on a single pack set-up, it’s disastrous anyway!
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2 hours ago, Andy Stephenson said:
I'm sure they don't heat up every crankcase in the factory where they make them.
I’m sure you’re right -Jon built all of the later Laser engines (and serviced any that were returned to the factory) so you can take what he says on this matter as gospel. -
Another big step. Delighted for you!
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The only references I can find to the “runaway” situation are when attempting parallel charging, where the voltage depression of the fully charged pack biases the charge current to this pack, resulting in gassing/overheating problems and insufficient charge to the less charged pack.
I don’t believe that anyone is advocating charging in parallel - the packs will only be connected to each other while under discharge where the discharge will bias towards the higher voltage pack, self regulating the overall discharge of the two packs, even with dissimilar capacities.
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1 hour ago, Nigel R said:
No, the scenario I mentioned occurs when connecting in parallel, after charging. Source for me is simply the well known charge curve of nicad/nimh.
I appreciate my previous post was not very clear on that point, apologies for that.
But - as Phil previously stated, cell manufacturers do not recommend parallel connection.
I don't claim any specialist knowledge, but like Simon, some years ago I put some thought and practical testing, using various combinations of equal and unequal charge state packs, into this subject - admittedly using NiCd rather than NiMH and have since used simple paralleling with a variety of cell technologies in many models. On my simplistic level of understanding, the small voltage depression on reaching full charge doesn't appear to be likely to cause any sort of runaway situation. There appears to be only a few millivolts difference which would surely be quickly equalised by the source battery's voltage drop once any energy was being transferred - assuming there was a sufficient imbalance to actually initiate any meaningful chemical reaction.
I suspect it might be that the manufacturers envisaged "in situ" charging of permanent installations when making the recommendation not to make parallel connections.
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Spammer removed. We do vet first posts but his was ambiguous and he was given the benefit of the doubt. Having followed with the one referred to above, he was dealt with appropriately.
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Please be aware of recent legislation changes which make flying of anything but the smallest of models subject to certain legal requirements and restrictions. Full details are available from the BMFA site.
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That sounds far more logical than the disastrous runaway situation suggested. My preference for silicon diodes is based on their forward voltage drop which I feel can be kinder for less voltage tolerant servos. Silicon diodes are noted as reliable components - I don’t know whether Schottky diodes have a similar reputation?
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2 minutes ago, Phil Green said:
Suggested 2nd caveat: diode is supported on a PCB or stripboard - simply soldering them inline under a tube of heatshrink leaves them vulnerable to physical failure - they're not intended to have their legs under any tension, or being subject to repeated bending... 🙂
Oh - not something I’ve considered but I solder mine into one of the wires between the switch and the receiver so I don’t think there’s any bending or tension involved. Much different if soldered to the switch directly though.
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I’ve come across faulty switches during maintenance checks and had a single cell die in flight - early days of telemetry so a beeping alerted me to a problem which became obvious after a rapid precautionary landing.
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I’d like to know a bit more on the scenario above - you would assume it would be a self regulating position if the higher one discharged into the lower - why does this not simply raise the lower pack’s voltage until they equalise? Is there some complication relating to individual cells perhaps or a quirk in the chemistry of NiMHs?
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I’ve long subscribed to the two battery two switch harness set up as the best redundancy solution for my larger models. In my view, simplicity is the key to reliability.
Yes, it doubles the chance of one battery losing connection to the receiver but it drastically reduces the chances of both failing on a single flight. I always do a pre-flight redundancy/individual voltage check and receiver voltage telemetry (on all my receivers) will indicate a cell failure on one pack during flight reducing the effective input voltage.
Additional protection can be provided by using 5 cell Nixx packs, or my preference, 2S LiFe cells, with a silicon diode in series with one leg of each connection to the receiver, which protects against battery failure and reduces the voltage by a little over 1/2 a volt.
I’m unaware of a “special” failure mode on NiMHs but the restriction on making parallel packs is due to the inadvisability of charging them in parallel.
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Yes, shields do not seal against oil migration. Unlike RS (rubber sealed) types, they have no measurable effect on operation. I doubt if the slight extra drag on these is significant for our use though.
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The moderators agree that many posts recently have been in poor taste and in some cases very close to breaking forum rules, risking deletion and possible disciplinary action. Please consider what you’re posting - quality over quantity should be the criteria when pasting the latest thing you’ve seen on Faceache…
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Intriguing…
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How’s the cooling? Do you have an adequate exit route that allow/guide air around the cylinder and head?
Overheating fits your description of it running well initially. Otherwise, check plumbing for problems as the fuel level drops - e.g. pinhole in the tubing.
It’s as well to blank off the “unused” cowl cheek entry if you haven’t already done so.
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We’ve had to remove a post which apparently featured a foreign politician to illustrate a rather well known joke. As it was, no doubt, a cut and paste by a UK based member, we’re sure there was no intent to target the individual but please exercise a little caution where photos of real people are used to illustrate funnies.
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I believe it does, Chris.
Reducing nitro content may help but I added a second shim/gasket to an ASP 180 which was rough and finicky to tune on our “standard” 10% club fuel although I don’t remember it throwing props - but I do tighten my nuts properly with suitable spanners. It transformed it.
Glow ignition timing is purely heat/chemical reaction related. There’s an element of automatic advance built in as revs/load are increased. Changing the plug specification will influence it.
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Good to know that it's sorted. I thought you'd tried changing the plug (in your second post). Was it an old one or was that a faulty new one?
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That takes me back to my apprenticeship when they taught us logic faulting at training school. A test lamp, circuit diagrams and the mk1 brain was all that we were allowed.
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It might be due to the weight of the control surface if it’s particularly free moving. Invert the model and try it - if the overshoot then happens from down elevator it would support this.
If this seems to be the case, confirm by swapping with one of the “good” servos to see if it’s due to the relatively low holding power of these servos around the commanded position but if in doubt, change it.
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Do we overpower our models these days?
in All Things Model Flying
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I think an elderly OS40 FS with little or no obvious compression would be similar to a decent .25 two stroke - in which case, I can confirm that a Puppeteer should fly adequately on one for scale purposes but without the “sparkle” that seems to be the expectation these days.