A return to model building and flying. Part 43
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Engines, Damned Engines
My own problems with the SC.40 I had to change before the Nat's, seem to be partly solved by a process of elimination The culprit seemed to be the carb. Quite how or why it has started to misbehave I have now idea, as the object in question seems to be perfectly sound.
Grafting on another SC carb (read that as a generic ASP/Magnum/SC carb), from a smaller engine, the choke is a ½mm smaller in diameter than the original, the engine ran, but took a lot of tweaking to find the midrange settings. This I believe, is something to do with the difference in midrange needle design, or lack lack of a needle in this case, which seems to be a feature the later designs. The mid range needle has been replaced with a tube that slides over a slit in the spraybar, quite how this is an advantage over a needle I can't quite see at present.. The machining would be the same, so I don't thinks it a cost saving exercise, and this arrangement has been used on Super Tigre engines for some years.
Flown over a few days it seemed to be behaving itself, until I eventually encounter the same weather conditions that produced the problem in the first place. Sure enough back came the problem. Not as bad as the original but bad enough to make me loose confidence in the engine again.
This has got to be some fundamental problem that I have not figured out yet and it is extremely frustrating. Ignoring the carburettors, two different ones and the same behaviour would seem to rule out that as a problem, and an engine that is sound, plenty of compression and nothing mechanically wrong with it. (I have run far worse engines with success), I am running out of ideas.
I replaced the original crankshaft and bearings over a year ago, the bearings where shot and the crankshaft was a mass of rust; strangely, everything else was as new. I suspect the previous owner had been using a lot of nitro, probably 100% synthetic oil and left it standing for a long period. There is only one aspect of the engines behaviour that has puzzled me since the rebuild (apart from the carburation problem). At peak revs there is an intermittent rattle that I put down to conrod wear, I have noticed this before on certain engines; but I am beginning to wonder if it is pre-ignition. If the compression is too high for the fuel/nitro content, this would be likely to occur. Even doubling the nitro content from 5% to 10% made no discernible difference. 0% nitro just makes the carb and needle settings slightly bit more critical. A change like this has made noticeable differences in the performance of my MDS or Super Tigre engines.
As I use the same fuel almost constantly, and it's changes in the weather that seem to make the problem show up badly, I can only assume that the moisture content in the air is enough to trip things over from critically OK to the engine knocking. This may also explain why the engine has always had an excess of power on a good day, but is almost unusable on a bad one. Lowering the compression by shimming the head should prove a point one way or the other.
I still can't see how this would affect the carburation? Unless the engine is so busy struggling to try and run with the ignition too far advanced that the carb becomes irrelevant., as the engine can't pick up revs.
Making a temporary head shim from some 0.03mm Litho Plate, this was all I could find in my bits and pieces, and it is a little on the hard side to cut out., I tried it again. The extra shim seems to have made the engine loose some of it's harsh occasional rattles; but as usual, testing on different days the unpredictable carb behaviour showed up yet again.
All is sweetness and light until the tank reaches about ½ - ¼ full then the carburation goes to pot. Running the engine rich enough to overcome this means it is not on song for the first seven laps of a carrier flight, just when I need it to be so. combine this with a, works OK one day then not the next problem and the engine becomes a liability no matter how powerful it is. I have tried almost every conceivable arrangement of tank vents, all to no avail; but other engines work fine with the same tank, so I think that can be eliminated from the equation.
As I have another competition to fly in, it's back in with the older SC.40 which has not shown any similar characteristics: at least not yet.
Strange Stunt Behaviour
Last month I mentioned feeling foolish yet again flying a stunt model, a Brodak Buster to be precise, and I won't repeat it all again. Suffice to say that after a frustrating time trying to find out what was wrong with it over a period of months, the solution was to dig into the wing and replace the original with a bigger bellcrank.
The original bellcrank had no provision for fitting the elevator pushrod closer to the pivot. Which means either altering the length of the elevator horn or the line spacing on the handle. The first I tried and didn't see much change, the second was not feasible without a new handle.
Original kit bellcrank top. Replacement bottom.
I am still not sure what effect offsetting the leadout holes in relation to the pivot on the alloy bellcrank had on it's behaviour. There was always a strange feeling off looseness around neutral and the movement would come in very quickly once the handle was moved. I almost felt like something was binding then freeing up suddenly, although I could never repeat the effect on the ground. I have spent enough time messing with the model so I am just glad it now works and feels like it should and I actually feel in control of the model for once.
Removing some of the lead, and having a trial flight gives the impression that I will be able to remove all the lead and it may still be a little on the nose heavy side. I still can't quite believe that changing the bellcrank could make this much difference.
Those that have followed my exploits may have read about the problems I encountered with a Dumas Crusader carrier model, it's all here if you didn't. It looks a bit more war torn now than in this picture taken after it was built.
Well I decided to give it another airing after fitting a Magnum.36 I wanted to give some air time to after a modest bit of bench running. All went well for quite a few flights over a couple of separate days, except for the leaden flight characteristics of the model. I had even got to the point where I was beginning to get used to it enough to start thinking I might be able to do something with it after all, when disaster struck.
One of the problems I have found with an all flying tailplane, is not the control response, but the fact that the elevator itself fouls the pushrod at extreme angles of movement. This is something that oddly, would not be a problem in a built up fuselage, because the pushrod could run centrally between the two halves of the tailplane.
To get round the fouling I had been forced to make a bend in the rear of the pushrod to clear the leading edge of the tailplane at full up deflection; this meant that it hung about 2-3cm below the bottom of the rear fuselage. and up until now had not caused any serious problems.
I was aware that the tricycle undercarriage was forcing the tail into the ground on take off or after a touch and go, and was causing the pushrod to hit the ground; but it had never caused more than a moments rattle and shake before it was airborne.
However, like all good problems waiting to happen, it bit me just when I was lulled into a false sense of security. I was practising putting it down in a precise spot, a process that involved continuous take off's and landings each lap; when after a particularly heavy thump the model nosed up, and kept going up. I knew from the feel of the lines whilst I still had tension, that there was something seriously wrong. Holding full down was doing nothing and I could see the other lines go slack .
A Magnum.36 develops quite a lot of power flat out, easily enough to pull the model up vertically through a wing over that would no disgrace a stunt model, followed by the inevitable plunge on the other side. As the elevator was stuck in the full up position, something you cannot miss with an all moving tailplane, the model did try to pull it's nose up, so the descent was not truly vertical; but very nearly. The resultant crunch made me cringe. and bits seemed to fly all over the place.
Regaining some of my composure, I walked over to what seemed like a write off, only to find the model was almost unscathed. The only damage was a broken prop and one half of the tailplane detached from the wire joiner. The wing covering also seemed to suggest damage internally in one place, but after a bit of surgery turned out to be perfectly sound. When the advertising blurb for this model said it was tough; they meant it.
What I think happened, is the blow to the pushrod forced the bellcrank round until it reached a position that made it jam on the undercarriage wire, and that was it, no control at all.
So, after a rethink I have replaced the elevator bellcrank with a larger one, which should help two problems, reduce the chance of jamming and make the controls a little less sensitive, which in turn may enable me to move the CG back a little further, helping hold a higher angle of attack at slow speed; and at the same time re-routing the pushrod to a gap between the elevator and the fuselage in the hope that it will be less vulnerable to knocks on landing.
As always time will tell.
And it did tell. The day before my next, and probably last carrier comp of the year dawned, almost flat calm and warm. The next days forecast was for strong winds, even stronger gusts, and occasional showers; so it was a bit pointless practising with my Hellcat as the next day would be totally different..
I did however, want to see if any of the modifications to the Crusader had made any difference. The flying is similar, and re-routing the pushrod has moved it out of harms way, as was proved when the tail end was repeatedly forced into the ground during touch and goes.
The throttle was another matter, after a lot of fiddling I finally managed to fit a carb with a stop adjustment screw, doing away with the need for the temporary collet I had employed to jam the throttle pushrod and stop it closing altogether. So far so good.
What caused problems, was an elevator pushrod guide I had made to keep the rod from fouling the inner edge of the all moving tailplane. I had not allowed enough sideways movement of the rod, which in turn stopped the throttle from fully closing. This caused some unpredictable throttle responses.
I could only shut the throttle by jerking on the lines. I had spent an awful amount of wasted time trying to start the engine, so nothing was set up right. After one such jerk, the model almost fell out of the sky because of the sudden reduction in revs, and hit the ground hard. The lines hit the ground at the same time releasing the line tension, causing the engine to rev up and the model to take off. Unfortunately the lines caught in the grass, and I was presented with a free flighting Crusader going full chat at about 1 metre above the ground, coming roughly in my direction....! Fortunately the down line dragged first and dug the nose into the ground stopping the engine. I did observe in one of those slow motion surreal sort of moments, that it seemed very stable in free flight...?
At that point I thought that it might be a good idea to call it a day, whilst I still had a model in one piece, and go home to work on some modifications.
Breaking News: I won the carrier event..........! See here for some details of the day. There are also a few additional pictures added of myself flying at the previous meeting: pictures courtesy of Andy Housden.