Nigel R
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Posts posted by Nigel R
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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?
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7 minutes ago, Don Fry said:
won’t start as it was not winterised)
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?
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2 hours ago, Chris Freeman 3 said:
A tail heavy aircraft will show that it is very hard to trim in pitch and will tend to climb or dive.
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...
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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.
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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.
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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.
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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).
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1 hour ago, Chris Freeman 3 said:
David Boddington wrote this in AMI August 1996
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.
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1 hour ago, Chris Freeman 3 said:
A big difference between a scale aircraft with a higher wing loading than non scale lightly loaded aircraft. Scale or semi - scale aircraft normally have smaller tail surfaces and tapered wings which also has a bearing on the stability.
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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Perry pumps IMO:
- good but quite pricey by comparison to the extra tank
- quite small and easy to retrofit, especially in a tight install where a tank won't fit
- not every carburetor will be 100% happy with a pump, although most twin needle can be made to work acceptably well *
* When I converted a pump engine (with a flaky pump) to regular operation, the main difference in parts was that the carb barrel was different, which had the effect of altering the mid-range running characteristics. Without the different barrel, the midrange would have been lean without the pump.
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Can't recall the name of it now, but they're a way you can set up a small auxiliary tank in the correct place.
Chicken hopper?
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Super nylon are fairly decent props. I prefer them to apc.
I wouldn't go out and get a new apc prop yet, use what you have.
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Hello all
I've inadvertently ended up with some Robart units (the 1/8" OD tubing, air up/spring down) and some 4mm OD tubing control clobber. The 4mm stuff looks like the black horse clobber (but is some old Eurokit stuff).
I guess I simply need some way to convert between the two tubing sizes - does such a widget exist?
Clearly the easy option is to buy the Robart control gear bundle, but this seems an expensive answer...
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Covering.
Have to force myself to start. Whatever method I'm using.
I'm fine with woodwork and kit install.
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ED, Eneloops are gold standard, as RX batteries are a single point of failure, I spend out on the best here. Of course, options exist to have multiple packs with failover type setups, but the best option is almost always a simple setup with reliable parts. Plus, Eneloops seem to have good shelf life. Anywhere stocking ripmax clobber will carry eneloop packs:
http://www.ripmax.com/Item.aspx?ItemID=O-4EN2000AASF&Category=090-010
I expect someone will be along to say how good LiFe or LiPo packs are soon... 😈
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2 hours ago, Engine Doctor said:
Hi paul . Yes pinking thats what i said. Its nothing to do with injectors but compression and burn speed of the fuel being used in the cylinder.
As for how our glow engine ignition works i can only give my practical or real world opinion as I have no degree and im not into the theory side of things.
If you run an engine where you can see inti the exhaust port , something like a cox 049 you can see the reflection of the glow plug. As you increase speed you can see the gp coil gets much brighter/ hotter . Connect a battery and it will glow almost white hot. Its the extra heat causing a quicker reaction with the fuel and platinum coil that advances the ignition process. The ignition ofmethanol is partly heat and partly due to a catalytic reaction with platinum and as we all learnt ar school heat speeds up these reactions.
Diesel engines or compression ignition work similarly but with a much higher compression ratio typically 16:1 or higher. The faster the engine revs the hotter the cylinder gets so advancing the ignition point /timing. If our model diesels get too hot they over compress and will eventually stop. You may also have noticed if a vintage model when a diesel starts to over compress they eiund heavy and laboured. A long shallow dive often cools it enough to run well again and even induce a occasional misfire or under compression as timing is retarded again.
Also worth adding;
at idle - with the engine running at a vacuum in the intake, there is comparatively little air/fuel charge to be compressed, with a low amount of compression comes a low amount of heating of the intake charge
at full throttle - the engine is running wide open, a much larger air/fuel charge enters the motor, thus more compression occurs, and with more compression comes a larger increase of charge temperature
the hotter mixture contributes to the advance of ignition
"Diesel engines or compression ignition"
Technically, glow engines are compression ignition - aided and abetted by the catalyst in the glow plug of course.
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I'd only somewhat agree Martin - I've several engines that I've tweaked and tweaked to get "perfect" transition and they've wound up in a slightly different needle setting to the "perfect" idle. It's a balance, as ever.
I'd add that carbs with an old or busted O ring are never going to be right, so there's that, too.
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Just checked the stats on this, its listed as 61" and 9lbs finished weight. I'd go for a 120 glow four stroke. Plenty on Ebay as a second hand.
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As noted above, glow motors often lose RPM at low speed without the driver. If the idle is reliable without it... well, who cares? Worry about nailing the full throttle tuning, and getting idle + transition reliability, everything else is not that important.
Just thinking aloud about factors affecting the ignition timing on our glow motors, there are a lot - heat of engine, heat of plug element, element wear, element composition, fuel/air mixture, how well the carb atomises the fuel... adding or removing a glow driver to the mix changes that balance by adding heat to the glow element + advancing the ignition point.
Worth noting that automotive diesel engines - which also rely on compression ignition - use direct injection at terrific pressure and can time the injection squirts to affect ignition timing. We have none of that control!
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What vapex cells are they? They do "standard" and low discharge types (marked as "instant" i think).
Your results look about expected for standard type. Although the variance is quite large.
Low discharge are better for our purposes, in my opinion. Those should show 80pc of the rate capacity when cycling at 2C.
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Used to? Still do on some of my needles...
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Model Technics Laser 5 - is 5% nitro 15% synthetic
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Enya are good motors, but less common, and I think spares may be an issue.
Os and saito are far more common.
You pays your money etc.
Glow fuel cost?
in IC Engines
Posted
If you read the above or even just search it you can quickly see it is still 30%.