I am about to get an Extra 300 SP
''Extra 300 SP'' kit for my OS 46AX engine. This will be my first intermediate plane after flying my standard 65'' WS trainer.
I downloaded the construction manual for the above mentioned kit. Since this is a cowled kit which completely encloses your engine, I have a couple of questions answers to which I couldn't find in the manual:
- How am I supposed to fuel up the plane? normally i just use the fuel tank line connected to the needle valve.
It's usual to use a third line to the tank. Plumb it to the bottom of the tank either with a second clunk or a bent fuel tube (don't let the tube rub on the bottom though). You can seal the other end with a screw and let it dangle out the bottom of the cowl, or use a fuel dot to mount to the cowl or fuselage.
You can also buy special 'fuellers' that fit in the line between the tank and the carb if you want to stick with a two line system. These work by blocking off the line to the carb and connecting your refuel system to the tank.
- How can I do the initial priming of the carb (which i do by blocking the carb air inlet and silencer exhaust using my fingers and then hand cranking the propeller 2 or three times)
Sometimes it's possible to poke your finger in the cowl, block the carb, pull the prop through one blade, remove your finger, pull through a bit further, reinsert finger etc. This works but is a pain. Alternatively it should be easy to block the exhaust only and then hand crank six or more compressions. A third method is to squirt a small amount of fuel down the carb from your squeeze bottle or refuel pump.
- How much affect will the side mounting have on the engine startup and normal performance?
Very little as long as the tank is the correct height relative to the new height of the spray bar. Priming is easier, flooding is harder. Performance should be unchanged.
- Lastly will the already available air inlets on the cowl suffice to cool down the engine while in the air and more importantly on the ground during initial startup and checks??? Also how much is flying without the cowl recommended???
If the model is well designed yes (some are better than others). You will more than likely be fine as designed but probably the single best and easiest thing you could do is put a baffle in the cowl. A baffle will ensure that all the air that goes into the cowl will be used for cooling instead of missing the engine all together. The baffle is made from a U shaped piece of balsa glued sideways in the cowl and then sealed with paint or varnish. Position the baffle so that the engine cylinder head fits in the open part of the U. You will have to make cut outs for the engine mount, muffler, throttle link etc. It's a wee bit of mucking around but well worth the effort (use a paper or cardboard pattern to get the right shape).
Air inlet size is important but so is the outlet size to get the hot air out again. Some people will tell you that the outlet has to be three (or two, or four) times larger than the inlet. This isn't quite true but one or other of them will be limiting the air flow. If you need more cooling there's no point in opening up one if the other is limiting the flow eg no point opening up the outlet if the inlets are too small. Having said that the outlet which is parallel to the streamlines, will generally be larger than the inlet, which is at right angles.
A correctly cowled engine will actually provide better cooling, and far less drag, than an engine sticking out in the breeze.
Test flying without the cowl used to be quite common and probably won't hurt anything. It's a good way to eliminate overheating if you are having engine issues
For needle valve & glow plug access i can drill a hole in the cowl so no issues there.
I would highly appreciate a detailed reply.