Push pull efficiency

[LaGaffe]

I hope I'm asking this question in the right place.

How much less efficient is a push pull arrangement a la Savoia-Marchetti SM-55, LeO H-24, Dornier Wal, ... than two tractors? A reasonable approximative guesstimate to get into the right ballpark is what I would be interested in. And what can you do to make it the beast of it (given the resources of a modeller's kitchen)? Counter rotating props (long live varioprop!), higher pitch/revving pusher, more blades, ...?

Cheers

[Sparky Paul]

I don't know why full-scales go to push-pull, other than removing concern about an engine failure and running out of control power to keep the plane flying.
Efficiency really isn't a concern for a model unless you're going for a lot of speed, or a lot of distance.
Here's a variation on push-pull full-scales haven't tried.. :)
And the usual Cessna type, which perform well with both motors, and adequately on either motor alone.

[Jimmbbo]

I hope I'm asking this question in the right place.

How much less efficient is a push pull arrangement a la Savoia-Marchetti SM-55, LeO H-24, Dornier Wal, ... than two tractors? A reasonable approximative guesstimate to get into the right ballpark is what I would be interested in. And what can you do to make it the beast of it (given the resources of a modeller's kitchen)? Counter rotating props (long live varioprop!), higher pitch/revving pusher, more blades, ...?

Cheers

FWIW, the Cessna 336/337 Skymaster ("Mixmaster" or "Skysmasher" among airport habitues ;) http://www.airliners.net/info/stats.main?id=150 ) was reported to have better single engine performance on the aft engine vice the front..

Did a consulting job on a late-'70s USAF O-2 (337) crash after an engine failure, and recall the performance data substantiating that, but my file lost in a move somewhere in between.

Agree with Sparky Paul that the theory was that the airplane would be easier to fly after an engine failure, and if you get certified in the 336/337, your priveleges are limited to "center line thrust only". Also, in model-scale power loading, any performance gain is a practical non-issue.

Cheers!

Jim

[LaGaffe]

Let my rephrase, since "efficience" seems to make everyone take off in the wrong direction. If I'm doing a push-pull installation of the kind that can be found on any number of old flying boats, LeO H-24, or for instance the four engine Farman F-121 'Jabiru', and I know/guess (old rule of thumb or from experince) that X Watts (input) would be sufficient for an all tractor installation, howe many Watts would be sufficient given that a push-pull installation (of this kind) is less efficient? Ballpark rule of thumb?

(That is, the goal is not efficiency or improved one-engine-out behaviour, but selecting the right motors etc for a scale(ish) model or a parody of such a plane. And to satisfy my curiosity.)

[Jimmbbo]

Let my rephrase, since "efficience" seems to make everyone take off in the wrong direction. If I'm doing a push-pull installation of the kind that can be found on any number of old flying boats, LeO H-24, or for instance the four engine Farman F-121 'Jabiru', and I know/guess (old rule of thumb or from experince) that X Watts (input) would be sufficient for an all tractor installation, howe many Watts would be sufficient given that a push-pull installation (of this kind) is less efficient? Ballpark rule of thumb?

(That is, the goal is not efficiency or improved one-engine-out behaviour, but selecting the right motors etc for a scale(ish) model or a parody of such a plane. And to satisfy my curiosity.)

Looks like all my thumbs have been broken - can't come up with a rule ;) ... My guess would be if you assumed the combination loses 30% over a twin tractor configuration, you'd be conservative, but at best that's a WAG. I suspect the data's available, but it doesn't flop out on a google...

Suggest you set up a simple test stand with a pair of motors you have around (or can borrow from a cohort) and do some thrust testing with both configurations, using a fish scale to measure the thrust...

Cheers!

Jim

[peterangus]

I assume that the fwd and aft motors are identical.

For the motors to pull equal input power, the aft prop will have to have a higher pitch . [same diameter assumed]

Bench tests of the combination should be interesting. You can evaluate the overall efficiency [of motors + props]. See attachment.

For the bench test you should place a flow guide in front of the fwd prop. Without it, the airflow approaching the aft prop will be severely distorted, and not representative of flight conditions. There would be a very slow onset flow near the tips, and a much faster onset flow elsewhere.