John Rickett 102

Thanks again for the suggestions. I can better see the problem now, the sensor element is encapsulated in the ferrule part of the tag so there is probably quite a temperature drop from where the tag is bolted to the head, the tags have now been repositioned. Unfortunately the PowerBox bare sensors (6619) don’t seem to be stocked by any of the large on-line model shops. Before buying directly from PowerBox, I’ll try applying heat conductive silicon glue (arriving tomorrow) and, after degreasing the area, encase the whole tag with a blob of the stuff, hopefully that will conduct the heat to the sensor. Ron, What temperatures are you recording?

Thanks Chaps, You're correct that the temperatures are almost certainly higher than indicated, I'll see what can be done about getting a better contact/heat transfer. The surprise for me though was that the front cylinder is hotter than the rear although the fuel draw is the same, so it doesn't seem as if there's more fuel going through one cylinder and keeping it cooler. I don't have an explanation why there is a temperature difference (which I can feel) and why its the rear that's staying cooler. Regarding the cylinder bolts holes, each bolt is just about touching the cylinder wall so there's no room for the tag unless half of each one was removed. The bare sensors could be an answer though, thanks for pointing out that they exist.

Brian, There is limited scope for fitting sensors, the obvious choice would be under one of the cylinder screws but access is impossible. Each sensor is therefore mounted on the exhaust clip screw, so should be shielded from the propwash. The tag is positioned against the head (under the clip) so at this point, very close to the exhaust, you’d think they would be seeing a hot part of the engine. I tried to mount each exactly the same, see the pics below. After running, both cylinders are hot, I can only touch for a second or so but enough to know that the front cylinder is getting hotter than the rear. Front Cylinder Rear Cylinder

An interesting result – the engine has now had three tankfuls through it and always the front cylinder runs hotter than the rear. At first I thought I had crossed over the temperature sensors, but not so, on stopping the engine the front is hotter to the touch than the rear each time. Typical temperatures are 45C on the front and 34C on the rear (ambient temperature 19C). Another interesting observation was that the temperatures would rise a few degrees when allowed to idle for a few seconds. I don't have reason to doubt the Powerbox sensors as they will both show the same ambient temperature once everything has cooled off. There is a two tank installation with the tanks easy to view (with the cowl off) and both tanks were depleting at just about the same rate and I couldn't differentiate between the amount of the smoke from each exhaust. Peak revs, as best I can measure with my little rev counter, are 6300 on a Falcon 20 x 8 wood prop.

Thanks Ron, Its had two tankfuls through the engine now, another two to go today, then it will be baffles on and cowl fitting before trying again. Plan A was to fit a 20 x 6 prop but the spinner nut I had bottomed out before the prop was tight. I had a 20 x 8 so fitted that and it seems ok. I don't want a quick model so may buy a 21 x 6, how much have you loaded up your engine?

One small feature which I thought worth incorporating, as it could be useful and added only a few grams, was the door retainer (for want of a better expression). Being a tail dragger, to open a door would have meant overcoming some of the door weight as well as a nuisance with the door always trying to close. A spring-loaded cable ran from each door to the engine compartment, the simple geometry causing the spring to have no effect on a closed door, to providing maximum assistance once the door was partially open.

Before reinstalling the engine and wiring up the plugs to put them in series, a quick test was carried out to make sure the powerpanel would drive them and find out what current would be suitable for the OS F plugs. The plugs have a decent glow with 2 Amps set.

From the previous discussion, I thought it was worth ensuring that the anodising had been fully abraded where the copper washer seats. The recess is 12mm diameter, so a short length of 12mm dowel, conveniently lying at the bottom of the scrap wood box, was pressed into service as a simple mill which wouldn’t skid sideways as soon as it was rotated. A glowplug copper washer is 5/16 diameter so a same size bit of 120 grit glasspaper was stuck to the end of the dowel. In for a penny as they say….having plugged the holes with tissue the dowel was put in a portable electric drill and set to the lowest speed. A few seconds later bright aluminium showed around the plug holes, I think that will work.

Ron, I like the idea of the series connection as you've done it, it eliminates a third wire. I'll test to see that the powerpanel will drive two plugs in series, if it does I'll make up a new loom. Paul, in effect the post of one plug is positive and the post of the other is negative, the platinum element is not polarity sensitive. the engine casing acts as the conductor between plugs.

Ok Paul, Would that be a direct fit, even though produced for a spark plug?

Ok Chaps, thanks for the suggestions and I accept Ron that all your engines will conduct as expected. I don't have any slot drills, however I've tried Brian's suggestion of cutting through the anodising by using a brand new 1/4" drill turned by hand. Its obviously done some good as there is now continuity showing on a multi-meter. The contact area must be very small but if its enough to pass the current of a glow plug, then we are back in business. For interest, its not possible to fit a tab washer and fly lead as the plugs are well recessed into Laser heads, the recess is 12mm diameter, only marginally larger than a socket spanner.

Thanks Ron. My engine has all of each head anodised, including the plug threads, consequently the current cannot pass through the engine body. I've tried screwing and unscrewing a plug several times to see if it would wear away the anodising but it hasn't. I recall that on the OS LA (blue) engines, the plug seat and threads were not anodised, presumably because of the problems it would cause, why Laser saw fit to anodise all of the head is a mystery to me!

Thanks for the suggestions. I don't know how thick the plating is, but the plugs will screw in by hand as normal, so the prospect of removing material with a tap sounds as if the plugs could end up a sloppy fit. Ron, I use a 12v powerpanel so can vary the current as plugs require. Regardless of the voltage available if the anodising is acting as an insulator, 2.4v surely isn't going to breakdown that insulation? Do you use remote connections on your engines, and have you found that connecting in series rather than parallel is the answer. I don't understand the logic but could try connecting the plugs in series if you've found it works, I would expect the powerpanel will still drive them.

The engine, tanks and remote glow connections have now been installed ready for ground runs (with the cowl removed). I have a PowerBox temperature monitor which can take up to 5 inputs. The sensor part is a metal tag, to be used similar to a washer under a screw. On a Laser, the closely drilled holes for the cylinder head screws preclude fitting the sensors where ideally they should be. Not wanting to drill holes in a cooling fin (as PowerBox suggest) the exhaust mounting clamp seemed the second best choice. If all is well during the first flights the monitors could be removed, or at least switched off, but should give some indication that better cooling may be needed. Initially I was going to just monitor the cylinders, then thought that since I have the sensors (which each only weigh 4 grams) I might as well track the heated air exiting the engine compartment, so fitted one where the air passes through the main bulkhead and another just inside the rear lip of the cowl, which is about 7” back from the bulkhead. The logic of this being, if there is a significant temperature difference, it may indicate that there is insufficient air passing through. I’m hoping (ever the optimist) this will be a temporary arrangement and that the wiring can come out if all is well. When I think that my first venture into radio control was single channel with a rubber driven escapement, I’m amazed at the advancements which have occurred within the hobby – at least it keeps the mind active when I often think I’m on the verge of that slippery slope of losing my marbles. Keeping the brain active has been necessary trying to get the engine started. A good supply of fuel, but for some reason no glow using my method of a remote set-up, which I’ve used many times before. Having checked all the connections, I abandoned the remote glow and used a glow clip instead – instant success, though only on one cylinder as I’ve only one glow clip. I tried the other cylinder and had the same success. It finally dawned on me that the black anodising was preventing using the engine as an earth return. If there is anyone reading this drivel who has one of these engines and had success with a remote glow connection, I’d be very interested. Also does anyone know how to remove the anodising around the plug hole and threads without causing damage? I can move the earth connection to another point instead of an engine bearer screw but there is only one place a glow plug can be fixed!

Thanks Ron, Hopefully it will, though needs some testing first. I'd like to come along with something which qualifies if not the Gull.