MB setup

1.Props:
Have you test how small prop dia is too small? I mean standard CF prop (no russian superwide)
And what is max dia that you think it´s still ok?

It is not easy to say when a prop is too small. From simple impulse theory you could go under 150 mm without loosing too much efficiency, but the fuselage interferes, and that could be a reason not to go smaller. I am preparing to build a propeller windtunnel in the next year and then we are going to find answers to this question. Our experience so far is that props of 162 – 165 mm seem to work well but with wider props at 155 mm we could go as fast. The advantage of the small prop being of course that mach numbers ( now around 0.9) at the tips go down, a wider prop can also be made with a thinner section, more optimal for the high speeds and compressibility effects.

Pitch: What do you think is min. and max pitch to use? Min pitch around 180 mm, max around 210 mm. We use normally 190 – 205 mm.

2.The volume of the combustion chamber with the piston at top dead centre: 0.48 – 0.51 cc
I am planning to use “big pipe” I know that volume depends how big prop you use.
What is the min. volume and what is max that is still OK.

Standard for us is 0.49 with the fat pipe. For bigger prop you need the smaller volume, for lighter prop you could use the higher. In theory more volume makes it possible to get more power at the same combustion chamber pressure ( and temperature), but in practice this gives too lean needle settings, which burns pistons. So the smaller volume ( higher compression) is necessary to be able to set the engine rich enough to keep it cool. Strange but necessary compromise.

3. Pipe lenght. orginal 273mm. Can you tell me what is optimum RPM with orginal 273mm pipe (on the air). How may mm. you have to shorter the pipe if RPM rise 1000RPM.

273 is pretty standard. I personally use 276 mm by adding a ring behind the exhaust stack. The engine runs easier, cooler, but less fast with a longer pipe.

4. Exhaust port timing:
What is normal timing on MB FIRE engine. How to change it if RPM rise 1000rpm.

Standard for the MB40 is 192 degrees. Rpm does not simply rise with the pipe length in a proportional way, since there is a very complex interaction and feedback with the way the engine works. Within a normal working condition of the engine ( around 32.000 – 33.500 rpm) 2 degrees of exhaust timing ( 0.2 mm exhaust height) with the same pipe length will give appr. 500 rpm, but a shorter pipe would have given almost the same effect. Higher exhaust means of course higher gas temperature ( less expansion of the gas) and therefore more rpm until the engine won’t do it anymore or runs too hot. It is a long way to find the new optimum after changing exhaust timing for pipe, prop, compression and settings. We never found ( for F3D engines) a net improvement with more exhaust timing, that is why the MB40 is quite moderate in exhaust, probably due to the fact that we have to run the engine on the pipe on the ground for 15 seconds. This determines the compromise.

5. glowplug.
I have a friend who can make plugs to me. What do you think is good lenght and thickness for glowplug element?
He has many different thickness 0,1mm – 0,3mm

We use 0.25 mm diameter, length of wire appr 18 mm, ranging between 15 mm and 20 mm. Material: 80% Pt 20 Rh.
Shorter or thicker wire is faster but more difficult to handle the engine. The ranga of workable seetings becomes smaller.