I have read on here that the GP 42 makes a decent C/L engine. Well I have a R/C version I have been thinking about converting for a brodak P-40 ARF. Would This be a good match for this plane? If so does anyone have a ventury/nva for it?  It has a mounting like a Fox [plate style.] Thanks Rick

Rick, i agree with Andrew. I have 2 gp42's that i converted, they run great and pull great for the money. 10/22/  50/50 fuel with tt 11x4.5 prop.

Remember that tank height is a consideration. Most NVAs are located about 0.5" above the mounting faces of the flanges on the crankcase. What do people do about tank height and adjustment when using a modified GP 42?

Oops! Wrong pic, that was the previous engine. This is the one presently on that plane that pulls it well.

Hello Rick,
Afraid I do not have any of Len's venturis. I have always machined up my own and they are a direct bolt on, in place of the R/C carb. I am close to getting my lathe up and working, so I can make some more this winter. As far as tank height vs spray bar position. For some very strange reason, which I have been totally unable to fathom, it doesn't seem to make much difference! I have never come across an engine that is so insensitve to tank height. I have flown must be 8 or 9 setups and I have never had to shim the tank to get equal upright to inverted performance. I am sure some experts will disagree and say it isn't so, but I believe Len has observed the same behaviour. If anyone has any explanation I would like to hear it, because the GP42 is just an engine like all the rest, so I am dumbfounded at the behaviour I have seen!

Regards,

Andrew.

We have a very good stunt flier who made up a telescopic tube that carried the spray bar through a range of heights and he detected no run differences for that engine - from memory it was a Tigre 46.

Other engines will confound that completely, go figure!

Having been stung once or twice, at least, by wrong tank v. engine alignment, I chose to do the stupid, simple(?) thing about it...

My test-bench/break-in stand has a rotating baseplate, parallel to the shaft centerline. A (too complicated...) tank bracket allows adjusting the relationship during bench running.

The Tatone, or is it J'Tech these days? mount on the baseplate - with the tank carrier, can be rotated while the engine is running. The tank can be mounted to simulate the in-flight relationship to the engine in upright, inverted, head-out profile and head-in profile arrangements.

You may have read my "wig-wag" method of checking a model for similar or identical "run" upright and inverted?  I can simulate those gyrations on this bench.

It works for me, to at least 90% effectiveness. As I often mention re: the "wig-wag:" FINAL tank position must be proven in flight, but the bench "pre-test" gives me confidence I won't flood or flame out in an early flight.

Again, this is stupid, simple, do-it-until-it's-done technology. I have found that similar engines may need very different "tank height" solutions... And it's not predictable by spraybar height...

Applying the "wig-wag" technique to the test bench: -

The tank carrier is lined up with the engine at the presumed relationship. Engine is started and set to a solid 'medium' run that will respond to tank height. (Tank too high: gravity feed adds to the suction feed, and vice versa.) Position the engine with the tank vertex vertically down. Rotate the baseplate to an angle that simulates upright flight - I use about 45° from the vertex down position...

Note RPM, or engine sound if you don't have a tach.

45° Rotate it 90° to 45° off vertical to the other side - which in this example would simulate loads on the fuel in inverted flight. Note RPM - or listen to the engine. The angle where it runs richer is where the tank is "too high," which will be confirmed by it going leaner in the other angled position...

Stop the engine, shift the tank according to what you observed. Crank it up again, and check the two angled positions. Repeat as necessary to reach a layout where there is no - or at least minimal - RPM change at the two angled positions.

  I subscribe to the 'center of pumping action' (or more correctly 'center of pumping flow') theory but believe that it includes all ports, cavities and channels between spray bar and exhaust.
And the spray bar point simply describes one of the end parameters for this theory.

 And as such I believe that the 'center' of the flow is an elastic AREA not a hard point or line drawn on the crankcase due to the fact that the ports or channels containing gas flow share duties between the lower and upper crank case volume, so there is a varying overlap depending upon design, engine speed and orientation etc.

Some engines obviously have a larger center, sweet spot or shared area provided by their multi-tasking ports than others.

Can't prove any of this of course but its what I believe at the moment.