A return to model building and flying. Part 30

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August 2006

Vertigo Mk III

The Mk III is a modified Mk II, the engine moved forward to counter the exessively rearward CG I had produced, and a counter balance added counteract the weight of the pushrod and keep the elevator neutral in the vertical position, the elevator is restored to it's original size, and the throttle pushrod weighted, to prevent inadvertant throttle opening when picking up the handle..

throttle weight

Throttle pushrod weight is the coil of solder just behind the exhaust pipe.


Elevator mass balance arrangement.

Flights with this Mk. have been quite sucessful, as things are a bit more stable. I am also slowly learning not to over control things, so I can hover for a few seconds at a time. The first second of take off is the most critical, if the model cannot be kept vertical for half a metre the result is a series of tight 'S' shaped manoeuvres that, I may, or may not, survive. Any off centre position with regard to wind direction can also push the model over before it has a chance to accellerate or gain any altitude.

The next thing is to make the teamrace tank I originally pressed into service, into a uniflow system as there is obviously quite a change of fuel head in the vertical poistion. Whist this does not necessarily affect the in flight performance too much, it does affect starting, unless the tank is filled every flight. More rudder offset area and/or more tip weight are also needed as I have to keep taking paces backward sometimes when I can get the model to hover for a few seconds ar a time.

Whatever else, this little project is causing a great deal if amusement amongst other CL fliers at the club, I and I have not had as much fun for years. Once I gain a bit more confidence I will try and video it in action.

Drastic Measures With The Hellcat

After becoming really despondent about the Dutch roll instability on this model and my inability to find a solution. I decided to take some drastic action. I always knew the wing loading was high, but proving that this was causing the other problem was not going to be easy. Apart from ripping the model to pieces and cutting holes in every piece of wood, then recovering the whole thing, the only other way is to use a lighter engine.

I chose the easier option of another engine. Unfortunatly lighter usualy means smaller in capacity. After grafting in an MDS.28 in a manner that allows me to easily change engines in future. I have an SC.32 and an OS 25 FX that both have the same mounting dimensions as the MDS.28 so I can do direct comparisons, once the SC is run in, which may prove interesting.

mds 28

MDS 28 with obligatory needle saver in place
I will have to see if there is a a remote needle assemply available that can be adapted to fit.

I found that the standard silenced would not clear the leading edge of the wing, to the extent of denting the LE when tightening the exhaust bolts. Rummaging through my box of assorted mufflers, you can never have enough mufflers, I found a new Enya one that fitted exactly. It's smaller and probably off something like a .21 but for the moment it has got me out of a predicament.

mds silencer

Difference between the fitted Enya silencer and original MDS can be seen here.

I am beginning to develop a soft spot for MDS engines as this .28 behaves in a simillar manner to my .38, starts reasonably easily, once adjusted correctly throttles well, and is quiet. How much of the quietness is due to the Enya silencer I won't know until I can find a suitable alternative silencer.

Obviously it is not as powerful as the SC.40's, but on 16 metre lines to help compensate a little, I have no problems flying it. Finding the right size of prop and more nitro in the fuel will bring some perfomance improvement; but that is not what I am interested in at the moment.

I am also finding that every time I use a Kavan yellow nylon prop, on whatever engine, they seem to run a lot smoother and throttle better than with an APC. This is a bit of a puzzle, as theory, and every man and his dog dictates they should not?

The first surprise, after the much reduced fast speed (again I do not know how much power is being lost with the small Enya silencer) was the stability in slow flight. There is much improvement, a couple of times I brought the model almost to a standstill into wind hanging on the prop, with no sign of the wing waggling that caused me so much grief before. I only ever managed to get the model this postion once powered by a .40. So it looks like wing loading plays a bigger role than I had imagined it could do.

The fact that I actually felt like I was enjoying myself for once when flying this model, is very encouraging.

Spot the Difference

The pictures on the left are an ASP 36 engine and on the right is an S.C. 32 I recently acquired. Apart from my repositioning of the needle valve assembly on the S.C., can you tell the difference? They appear to be the same right down to the questionable absence of a throttle stop screw on the carb.

asp 36S.C.32
ASP 36S.C.32

Both seem to be slightly altered copy, of an OS SX series engine, but with a more versatile rear needle valve design on the S.C. and ASP. The setup can be changed from a standard carb to remote needle in a couple of minutes, and the backplate mount swung into almost any position. The OS has limited variation in this respect, because NVA mount is part of the backplate moulding; this also leaves the backplate vulnerable to breakage in an accident, which will probably/certainly end up expensive to put right, although I have noticed some of the more upmarket OS designs seem to have addressed this limitation.

OS 32 sx
OS SX series

These Chinese engines are probably not made to the same standards, as powerful, or as long lasting as an OS, but at a fraction of the cost, quite frankly, but I don't care, I can get flight time in that would otherwise be spent saving up for an engine I would be nervous about using in case I damaged it. As the saying goes, "Who needs to pay Roll Royce prices if a Mini will do the job?" Now that I have got used to them, I have an S.C. 15 and .40, and a Thunder Tiger 15 GP, another fine little engine (the only reason I don't have more TT engines is the fact they fetch higher prices second hand), all being use fairly regularly, that start easily, are very reliable, and have plenty of power. The .40 actually seems to be getting even better the more I use it, which reinforces my personal view of not attempting short run in times.and I am sure I could wring even more power out of it, if it were not for the clubs noise restrictions. As soon as I can find a suitable project the .32 will get used as well.

I am sure these oriental engines will increase in price over time as the economic cycle goes through it's inevitable changes, the warning signs are appearing already, so I will grab what I can whilst I can. I have noticed that O.S. and the like can sometimes be tripped off the tongue like fashion statements, but nice if you can afford them all the same. The only thing I have never liked about some Japanese engines is the use of cross head screws, I have a good idea why they are used, but it's not for the end users convenience. If you ever have to work on an engine, cap screws (as, S.C., ASP, AP, MDS etc.) are much easier to work with than the glued in cross head screws employed by OS on a lot of their range, they have caused me a lot of cursing and swearing in the past, as have cross head screws in general. On another practical note, a couple of metric (or any unit) hex keys take up very little room in a flight box.

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