R/CFlorin

I'm not sure if this is the right forum to post this question...
I've got 2 cox TD 09 engines wth throttle/carburetor and i'm thinking to put them inline, with a puller and a pusher prop. I've done a bit of research and the second propeller (pusher) would have a higher pitch so the incoming air can be accelerated even more.
The front pulling propeller could have more blades to get a more amount of air, say 3 blade pull prop at front and 2 blode higher pitch pusher prop at the back.
The throttles would be controlled by a single servo between the engines.
Would this setup work, or is it worth to try out of curiosity?
Thanks.

w8ye

My Feedback: (16)

Sounds pretty good to me

But one .20 engine will have more power and be more efficient than two .09's

pe reivers

Why not!
The front prop should be a larger two blade though to adapt the prop diameter to the prop wash, which narrows down behind the first prop due to Bernoulli's law.

blw

My Feedback: (3)

Pe,

At first, I agreed with you. But, my guess is that the rear prop could end up working with only disturbed air from the forward prop. I made a quick sketch to show the reduced column of disturbed air from the pressures of the forward prop. My guess is that this column of air would be the swirling slipstream, or P factor air, also further disturbed by the fuselage. This could mean that the rear prop would experience uneven pressures around the disk. This could lead to vibrations and unequal efficiencies.

I've not seen this explained, so it is all a guess. It would seem that an equal sized or larger rear prop would bring in clean air near the blade tips to equalize the airflow more. I'm sure this has all been figured out much better.

downunder

Except he's talking Cox Tee Dee 09's which are really gutsy engines

The pull/push system works on full size planes so I guess it'll work fine with models. My only reservation is on the rear engine with its plain bearing where the thrust is pushing backwards on the crankshaft.

pe reivers

a counter rotating of the tractor and the pusher tend to even out the swirl. Yet efficiency goes down the drain because a narrower column of air is accelerated, much increasing the apparent prop disk load. It was no success in German WW2 fighters, but that is no reason not to do it in a model airplane.
After a quick failure mode analisys I think, the Cox pusher needs a hardened steel or phosphorbronze shim between the prop drive washer and the crankcase.

pe reivers

edited:

a counter rotating of the tractor and the pusher tend to even out the swirl. Yet efficiency goes down the drain because a narrower column of air is accelerated, much increasing the apparent prop disk load. It was no success in German WW2 fighters, but that is no reason not to do it in a model airplane.
After a quick failure mode analisys I think Downunder is right, the Cox pusher needs a hardened steel or phosphorbronze shim between the prop drive washer and the crankcase.
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gkamysz

My Feedback: (19)

Efficiency of counter rotating props close together actually improves slightly. There are NACA documents that are available. In the case of a model were the prop selection is limited, se what works best. A conventional twin setup would outperform what we are discussing by quite a bit.

R/CFlorin

Thank you for all your replies.
I have some things unclear though - shouldn't the rear prop be smaller in diameter and higher in pitch? If the first prop already accelerates the air, it would be normal to have the second propeller move at higher RPM that the first one to accelerate the air even more right?
Since the engines are cox td 09, i've heard that the best propeller for that is a 7x3.5 (i already have a 7x3), so I am thinking to get a 6x4 pusher propeller for the rear engine.
As for the reinforcement between the prop drive washer and the crankcase for the rear engine, does the incoming air create that much pressure to affect the crankshaft of the rear engine?
thanks.

pe reivers

The conclusions are correct.
It is not a reinforcement for the crank that you need nor the inrushing air creating problems; just a washer to take the thrust of the rear engine. As a pusher, you will get aluminum to aluminum for thrust bearing material. This is not the best combination, hence the need for a thrust washer made of steel or bronze.

IronCross

Apparently Cessna engineers weren't aware of all the above problems when they designed the 337 Tractor/Pusher config you describe... You might check out KMP's model of the 337 [link=http://www.kmp.ca/product_info.php?cPath=22&products_id=29&osCsid=e9b42ebc92bd13b3573e500322b99009]KMP[/link],, I talked to them at the West Coast IMAA flyin at Castle.. They seemed to think any too engines within the specified range would work just fine... No big deal... I think there are a couple threads on here about the plane and engines also... Been awhile though...

MarkNovack

My Feedback: (3)

I was thinking Dornier Arrow.

blw

My Feedback: (3)

Ironcross- I was thinking the same thing. This is much different than a counter rotating prop system.