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Nigel R

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Everything posted by Nigel R

  1. It does sound a little high for a "locked" position.
  2. Yes... but with a bit of "it depends". 😀 I'd say, given that electric motors attempt to be "close to" a constant speed device (and apply as much torque as they need to, to get to that speed), that you've simply had "too little servo for the job" - and caused the servo to apply all its torque without much movement. This is inefficient, and inefficiency = heat. So yes, it is the extended time with the very high current draw that did the damage, all through the excess heat that generated. As you say - beyond capability for added length of time. If the servo could "get moving" at the ends of the movement (where the linkage / geometry is often more favourable), then it can "keep moving" through the difficult bit (see dave windymiller's airbrake). But this is absolutely indicative of a marginal servo. In dave's case, very marginal indeed, as it stopped and couldn't restart at the midpoint. In your case even more so as the servo gifted you some magic smoke. If you can stand the graph, a DC motor behaves as: Torque is basically linear to current. Right hand side of graph - full (stall) torque, the current is at the maximum, the speed at zero, nothing is moving, but everything is soon burning and smoking, not a good place to be! If the servo is beefy enough to get the load moving, then the speed can rise a bit, and the torque (and current) can drop, everything starts coming down to within the servos 'happy place'. Note the point of max efficiency - it is around 80% of "unloaded speed" with around 20% of maximum torque/current. Flight surfaces, being used all the time, we wouldn't want to be going past that max efficiency point (and probably sitting below it, as a safety margin, truth be told). Something like retracts, or brakes, with 'occasional operation', we can load up the servo a bit more (perhaps going closer to the 'max power', in the middle) - but never to the point where the servo cannot restart movement. That said, a retract or brake won't be spending much time in these heavy load situations as it is cycled just a handful of times in a single flight. Short version - use a big enough servo*. ( * way too big becomes inefficient - however, that's an easier problem to solve, and involves less smoke and busted airframes )
  3. Have to ask - do you mean each retract servo draws 0.8A with the wheels fully up and/or fully down? This seems quite a lot. What does the servo geometry look like with the new servos? Is it a full 180deg range of motion? Servo slow was not directly responsible - these features only changes the rate at which the TX changes from up to down, and vice versa. What it will have done, is expose an inadequate servo - one which now had to spend far more time in the portion of the movement where the current draw was very high. Some other servo characteristics (inherent to the servo's particular feedback amp) will apply when servos move slowly against a large load, and in your case may have helped tipped something marginal 'over the edge' so to speak (more obvious in a case like dave windymiller describes where the servo could no longer "get going" against its load).
  4. For commercial alternatives, Dubro Lazer Rods are still "reasonably priced", and are as good as the Sullivan fluted snakes (which are now silly money). The SLEC fluted blue/yellow ones are nearly as good but seem to have a slightly 'looser' fit by comparison, but still perfectly good for straight control runs. No connection beyond happy customer, etc.
  5. Nope, torque never equals power... different units, ask any GCSE/O level physics student... Power is the rate at which "work" is done, i.e. time / speed is important. "Work" is force times distance; and force is linearly related to rotational torque (e.g. typical hobby servo). No speed or time is involved. More torque will move a given load quicker, this much is true. So power and torque are (indirectly) related, but, not the same thing. The big difference with digitals vs analogue is that they are (typically) capable of applying full torque very quickly In theory it could be possible to have an analogue amp with the same kind of performance as a digital amp, but you'd need to start adding very expensive and very accurate components to a typical servo feedback amp, and in the end it is much simpler, easier, and more repeatable to do all the required sums inside a digital widget. Short version - digital feedback amp beats analogue for performance/cost.
  6. They still use a linear pot for position feedback.
  7. My take, on mechanical retracts, I'd put a 'proper' retract servo (they have the correct range of motion to properly lock with no servo load) - and live with whatever speed they operate at. You could go to air-up/air-down retracts, with a valve in the line if you want slow/asymmetric. That works pretty well. Fancy control valves are available that can do this in one single part. I've no experience with electronic retracts. You can definitely get a sequencer to get one up before the other, not sure if that can do "different speed".
  8. As Paul says - paper gaskets are easy. Use a scalpel blade in a compass to cut out the centre circle. Put engine back together, trim the excess off. Quicker to do, than to find and order a fancy one. Exhaust gaskets, I don't bother. For two strokes, clean up as best you can then use a smear of slow epoxy when you assemble the thing, it will work far better and makes a perfect seal. Gaskets tend to allow the exhaust to loosen off.
  9. Agreed. I only had one of the gutless HP25 four strokes in mine, outside loops were off the menu, but rolls are easy, loops are easy, and I used to fly it inverted a fair amount. That said, it's "a bit" twitchy when upside down. Rolling circle is messy, but end of day, with something like this, you're "just" dabbing in course corrections with the elevator, while rolling. "Trainers" can be a lot of fun. I keep one in the hangar now as a go-to if I haven't flown for a while.
  10. https://aerofred.com/details.php?image_id=97487 Limbo Dancer on aerofred.
  11. And some more plans links here, more recent design:
  12. It depends what you want your trainer to train you on. Orientation and basic control, sure, flyweight foamie ticks the box. Heavier trainers also make you learn energy management. Some funflys... Another good example of the stick/boom type is 'Stickit IV'. plan / article availabile on outerzone: https://outerzone.co.uk/plan_details.asp?ID=7904 And another 90s design, Fan Dancer: https://outerzone.co.uk/plan_details.asp?ID=15020
  13. Not sure about where to go for good reads. Fun fly recipe - lots of power, little weight, structural simplicity - aerodynamics take a back seat. Pilot skill is very much prioritised over "fancy" airframes.
  14. https://outerzone.co.uk/plan_details.asp?ID=5496 Plan of CG Ultimate on Outerzone here.
  15. £13.61 for the header, £27 for silencer, slightly less than £50? Justengines baffled silencers are effective, I'm using that exact one on a similar size four stroke. No connection, satisfied customer, etc.
  16. If you read the above or even just search it you can quickly see it is still 30%.
  17. If your Hurri wasn't 'hunting' or showing any kind of pitch oscillation during normal flight I'd be surprised if your landing issue was affected by the CG move. A fair sized push of down during inverted seems as expected - I'm assuming (always dangerous) the Hurri has a 'semi-symmetrical' or Clark Y type airfoil?
  18. I'm assuming this means you get issues with the carb seals / diaphragms or similar, due to the water absorbtion? Would continued use prevent this?
  19. Which probably means a CG approaching (or even on) the neutral point. At this stage you will need to start trimming the thrust line to minimise pitch changes with throttle. Even then it might not be possible to trim out the desire to pitch up/down - it depends on the airfoil in use and how the centre of pressure moves with angle of attack. Lots of variables in play. (note, none of the preceding is getting into discussion of stall, or tip stall - still talking tendency for the airframe to start 'hunting' in pitch). Symmetric airfoils display the least of effects. Heavily cambered airfoils display the most. Some airfoils have inherently unstable connection between the centre of pressure, and pitch; meaning with increased angle of attack they show an increased desire to pitch up (whereas ye olde Clark Y is the reverse, with more angle of attack, it tends to pitch down, and is inherently stable). I am not an aerodynamicist, I may have very imperfect understanding of the above - take with pinch of salt. Wikipedia link... https://en.wikipedia.org/wiki/Longitudinal_stability
  20. Weston Prosynth 5% nitro is currently a nose under £20. Four gallons comes with free shipping. I have found their fuel to be perfectly ok.
  21. Seems like a difficult quest to make an engine make a lot of mess. Weston will do it for you if you really want, note minimum post quantity is 2 gallons.
  22. Petrols in my neck of the woods are running large props at low revs and some fancy extra silencers. Not exactly "out of the box". IME, for the same kind of power region, neither a glow 10cc two stroke (on and off a tuned pipe), nor a 15cc glow four stroke, were a problem to get past the magic 82dB, but both were very close to or on the limit.
  23. Some thoughts / my tuppence. It strikes me that one of the problem scale models have is that scale models don't operate on scale grass. We can't fix the grass problem, so we might want to angle the gear forward "a bit" to counter that. I'd guess that can be accommodated by different pintle angles and whatnot on a scale model, all whilst keeping the scale wheel well position. The CG shouldn't need to be much different to the full size. Warbirds were not designed to be inherently unstable (unlike a fly by wire jet).
  24. But that doesn't mention or address stability. It does mention "high speed stall", as in, a heavy model stalls at a higher speed, but that's a different subject to stability. The stall angle is not affected by CG position. As GG says, happy to be educated.
  25. Stability has (possibly) a few meanings. Aerodynamically (we need an aerodynamicist here) I don't believe the characteristics of the airfoil, in terms of CofP movement etc etc, will vary by scale or reynolds. I could be wrong. However - taper, wing loading, tailplane size, neither factor in the airfoil parameters. At an airframe level, a tapered wing will exhibit more tip stall at lower scale due to behaviour at lower reynolds. I guess that could be 'stability'. How the tip stall problem is countered is a different question I guess.
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