I am in the process of getting my DO-335 ready for it's first flight.  I felt that It could use a little more thrust.  I was running a COX 5x3 3 blades prop. I had on hand a APC 6x2 2 bladed prop.  Just holding the do-335 in hand the APC prop seemed to pull stronger.  Wanting to make sure I built a thrust stand out of a small electronic scale. Setup pictured below.

On 15% nitro the Cox 5x3 3 blade turned 18200 rpm and produced .59 oz of thrust.
On 15% nitro the APC 6x2 2 bladed turned 20400 rpm and produced .82 oz of thrust

It doesn't show in the photo but the load arm has point contact with the center of the scale.  The scale factor of the resulting lever arm is 2.2:1 .  So the actual readings were 1.3 oz for the COX and 1.8 oz for the APC.  This setup probably doesn't provide absolute readings but I don't care.  all I wanted was an A vs B comparison.  APC wins hands down.  I wounder if I should try an APC 6x3?  LHS is closed till Monday.

PS the white line is a safety line.

I can relate my experience with APC versus Cox or Master Airscrew which are my two go-to props for ripping quick circles on 40-45' of line. If I have to fly in my nearby 35' field, I slow the plane with an APC 6x2. It's like being in first gear and no more. The Tee Dee or Big Mig will turn screaming fast RPMs with the APC, which keeps the engine in its happy spot, but makes the plane slower, but with still authoritative thrust which more manageable for me on the shorter lines. APCs always seem to crank out seriously higher RPMs on my rotary valved engines than the other black props I mentioned. So

For a static thrust check, the Granny gear APC may very well drag the sled harder(at first) than the Wider blade higher pitched Cox prop. My Cox or MA props are between 5 and 5.5". With a light(6oz) plane and an unmuffled Norvel, maybe even 5.75" on 40-45'. Static thrust checks might tell more about acceleration than top end.

Hope that makes some kind of sense. Just my untrained observations.
Rusty

Edit: added a clarification.

I should have realized that less then an oz of thrust was dumb,  dumb & dumber!

         

Nah, John your logic was still in the right place for the sake of a comparison It still told the same story. If it makes you feel any better, I didn't pay any attention to your units either.
Rusty

What you have demonstated is that when you under-rev an engine you lose power....

Especially an engine that needs SPI, which is only effective over a certain number of RPMs.

On a side note, I'm curious as to whether SPI boosts the rotary valve engines as much as the reedies. It seems like the rotary's more direct exposure to the open atmosphere near TDC would vent more(but not all) vacuum from the crankcase that would serve to draw in cold air under the piston. Hope you don't mind me pondering this in your thread. In case it's of interest to other 1/2A engine folks.
Rusty

Hi John,
I really like the work you have done on DO-335, particularly the center mounted engine with the long prop-shaft extension.
I too have been working with a similar application.
The question I have is "do you have a universal joint between the engine and the prop shaft extension?".

Orv.

Hi John,
I really like the work you have done on DO-335, particularly the center mounted engine with the long prop-shaft extension.
I too have been working with a similar application.
The question I have is "do you have a universal joint between the engine and the prop shaft extension?".

Orv.

Yes it is from the nitro RC boat world.  U-joints are getting hard to find because boats have gone electric.  Most boats use flex shafts.  It allows you to adjust the prop angle.  I found mine on eBay a couple years ago.

I suspect that 2" pitch on the front prop and 2.5 on the rear might be good, as the propwash from the front prop presents higher velocity air to the rear one.