A return to model building and flying. Part 37

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March 2007

Wildcat Revisited

widcat mds side

Recently taking advantage of a break in the weather and finding the only dry spot in the CL circle where I didn't sink into the mud, of course it was up wind, I tried out some modifications to the Wildcat.

One of the things I did earlier was to rake the undercarriage forward as much as the structure and ironmongery would permit. This was an attempt to cope with our potholed uneven grass circle, and has been a success. The model has now lost it's tendency to want to flip over on landing every time, which in turn makes it a bit more tolerant on landing approach angles. This may work against me when I finally managed to get the carrier deck up and working, but for now it's working and I can practice. At present the ground is so waterlogged that it doesn't take much of an imagination to think I am flying from a carrier over the sea.

The second modification came about after my SC.15 died a death; it needs a new piston liner, But faced with the usual dilemma of, is it cheaper to repair or acquire a new engine? I decided to not repair it, at least for the present. As my Thunder Tiger.15 had ripped my Hallam Swift around the sky at a great rate of knots, I decided to refit the carb and shoehorn it in as a replacement for the SC. The result was a great dissapointment, the SC.15 had obviously developed a great deal more power. The only thing I can attribute this striking difference to, is the TT carb. In the Swift I was running the engine with a plain venturi which had a bigger cross sectional area, but modifying the carb would not have been as simple as boring out a venturi.

My AP.15 would not fit as the rear mounted pancake silencer would foul the tank. The only other option was to fit a .19, but my two recently purchased ASP.21's (the nearest thing I have to a .19) are earmarked for a Tigercat twin.

wildcat mdsEver since John Walton (Hi John) had shown me an MDS.18 in his possesion, I had wanted one. It has the not unique, but unusual design feature, ala MVVS, of being able to rotate the cylinder to make it into a rear exhaust setup. It also has a raked back needle assembly like more recent Thunder Tiger engines, plus the fact that I like MDS engines. Deciding that this might be a good replacement for the now deceased SC, I found a nearly new one on eBay at the right price.

I have to say that with the engine being a lot heavier than the SC.15, I was not expecting much difference in the performance. This looked like it was probably true when it trundled through the wet grass trying to takeoff; it was not exactly leaping into the air. The SC would have been off in half the distance.

Once airborne things took on a different feel, and the top speed was definitely higher. But the real eye opener was the slow flight. The controllability of the carb was quite astonishing, with no dead spots or sudden transitions, and there is obviously enough power available to hover the Wildcat. I was able to get the model into a vertical position almost at will, and still have enough grunt to open up a rise vertically, even managing to touch the tail on the floor at one point and then rise up and resume level flight. Not with the confidence and aplomb that Jan Odeyn can achieve, but I am getting there slowly.

It's not often that I leave the field with a glow of satisfaction, but this was definitely one of them.

widcat Jan Odeyn

Jan Odeyn showing what can be done with a Wildcat and an O.S.15.

wildcat collage

The only modification I made to the carb, was to cut the internal throttle barrel spring in half to try and reduce it's tension. If you have ever dismantled an RC carb, you will know that most have a spring that biases the throttle barrel outwards, the purpose of which I am guessing is to stop any end float that would affect the midrange needle setting. The only problem is, in doing so it presses the barrel against the retaining screw, and as the barrel has a helical groove to facilitate the movement of the midrange needle in and out of the spraybar, this tends to bias the throttle to full open; a problem that has caused me headaches for other reasons.

Most of these springs are needlessly strong. No problem in an RC model being moved by an actuator, but any extraneous force applied to a CL multi bellcrank system that relies on line tension, can be. Whether this modification accounts for the satisfyingly pleasant throttle responce, I won't know until I can try it on other engines.

Crusader Flight Trial


Ever had one of those mornings when things don't go right from the moment you wake up? Well this was one of them. I had a dull headache from the word go, but it was the first reasonable weather we had had for some time, it wasn't blowing a gale or pouring down with rain, so I threw evering in the car and drove to the field.

It was breezy but not unflyable. Problem number one was I could not adjust the midrange jet on the carb as I had not got a jewellers screwdriver narrow enough to fit in the deeply recessed screw hole. Next was the throttle stop screw jammed the throttle every time I tried to set the correct aperture for a tick over. Oh well, At least I could see if the thing flew even if it was flat out.

I had decided the night before to use the arrestor hook as an anchor for my remote release, not thinking things through. The model of course, has a tricycle undercarriage, and the hook is high up in it's retracted position. When the engine ran it pulled the nose down and released the hook. The take off was a series of ploughing sessions, the prop hurling sods of turf unto the air as the Leo.28 kept struggling manfully to pull the model through a the sodden uneven ground, against the resistance of the hook ploughing a furrow round the circle behind. Somehow the model took off with half the field still attached to the hook, and despite my worries concerning the all moving tailplane, flew quiet normally.

The non-adjustmet of the carb had of course, turned the throttle into an on or off affair, trying to throttle down caused the engine to cut. There followed a bumpy landing with the small nose wheel digging into the ground and flipping the model over. Fortunately the landing was slow and the ground very soft, so no damage done. Even the Wildcat was not flying in it's normal pleasant manner when I gave it a whirl, so it was the time for discretion to be the better part of valour, and call it a day. Our site is peculiar in that if the wind blows from certain directions there is a lot of turbulence, combine that with a blustery wind and things can get, interesting! to say the least This may have been one of those days.

Back home a much bigger nose wheel was fitted, and the carb dismantled. The problem with the carb turned out to a manufacturing one, the Leo carb is a copy of an OS carb but with one lack of attention to detail. An OS throttle stop screw has a reduced diameter end where it fits into the helical groove in the throttle barrel, the Leo just had a plain screw. The result is, it is OK as just a retaining screw, but just jams in the groove if it is used for the purpose it is intended. Grinding the end of the screw end to a shallow taper solved the problem.

Now all I need is some clement weather to try it again.

crusader long shotWell that day arrived, modest wind but cold. The Leo carb continues to be uncooperative, there seem to be too many basic design flaws in it. I ended up with the midrange needle screwed in as far as it will go and it is still not enough. As there is no physical way of moving the spraybar towards the needle to compensate, I am a little screwed. I have other carbs with the same spigot diameter, but all with huge choke diameters, and as my money sitution has become extremely dire at present, life in general is has been very unkind of late and it will take many months to get my finances strait again, so I can't afford to buy yet another carb. This is deeply annoying as the engine is fine, and also nice and light.

The next alternative was to change the engine, the only one I have that remotely resembles the weight of the Leo.28, is an Enya.30 BB, which I hope will be more cooperative in the carb department. All the others I have in my possession and would like to use, are between 50 and 100 grams heavier, which in turn would mean adding a considerable amount of weight to the tail.

I have always thought the fuselage was a little on the short side, probably a hangover from the days when it was designed, when unsilenced light engines were the order of the day, This problem crops up time and time again with older designs, especially ones from the USA, of which the crusader is one example, the Brodak Buster, which I will probably be building next, is another.

I also flew the Wildcat again and proved that the engine throttling was no fluke the first time I used the MDS.18. In a word it's 'wonderful'. Makes me want to cuddle and stroke the engine just to hear it purr. Does anyone have any idea what has happened to MDS engines?

crusader wildcat

The next time I flew the Crusader with the Enya 30 BB I ran into similar problems with the Enya carb that I had with the Leo, it would just jam in the 3/4 closed position. This was getting frustrating. As there was not much I could do at the field I dismantled the carb at home.

I was surprised to find a design/build flaw in the Enya carb, I would have thought Enya would have been above dropping such a clanger. There where actually three faults. The first being, the barrel retention screw; it was too long. Screwed in as far as it would go (as it should be) it jammed in the barrel groove at certain positions. Next was the throttle stop screw, there was no way it could work as it was supposed to. There is a flat ground edge of the barrel on it's inner end. This was ground at the wrong angle, enabling the adjustment screw to lock the whole movement of the throttle. Lastly, as usual, the bias spring was too strong and the barrel would tighten up at certain positions when rotated. This is just not good enough

Cutting the spring in half, refiling the throttle stop flat at a slightly different angle, and shortening the retention screw, has improved things a lot. Things move and stop as they are supposed to, and the closed position can be adjusted now.

I find it ironic that least troublesome carbs I have used have been the cheapest, those from Russia and China (For those that don't know, Leo is Taiwanese, as is Thunder Tiger)

This throttle problem is now beginning to p**s me off. The Enya mod's did make a difference but the throttle responce was awful, with a long lag between opening the throttle and anything happening, followed by an equally long responce going the other way. Clearly unusable. In desperation I replaced the Enya with my MDS.28 which I know works. Did it hell in this model! Yes it did throttle better, but it was far from ideal.

Now I was forced to sit and examine things very closely and carefully. The not quite so obvious thing was that the total throw available in the throttle pushrod was slightly shorter than the throttle arm has to move. There is also a in increasing resistance as the throttle is closed. This was helped by cutting the barrel spring in half. but is still quite noticeable. It seemed to be not too serious on the ground, but maybe it was in flight. Apart from digging holes in the wing to get at the bellcranks, the only solution I could come up with was to make a lever mechanism that would amplify the movement, and remove the Enya throttle barrel spring entirely.

throttle linkage
The full throw was achieved by the lower arm being 50% longer than the top one

It's becoming obvious that I have to pay much more attention to the whole bellcrank system to make sure there is enough movement and travel in future, especially it's all mounted inside the wing. I'm on another steep learning curve at present.

On the bench this all seems to work but the flight test report will have to wait until next month.

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