When the rules for Class B Free Flight limited the displacement of the engine to 0.30 cubic inches one semi-famous modeler used two 0.15s one one plane.
Plans were published. He was sucessful with this scheme.

The model was a pylon ship with one engine a tractor and one a pusher located up at the level of the wing root.

The theory was that two 0.15s would produce more thrust than one 0.29 cu.in engine. I am not sure of that but I know he was using a hot pair of 0.15's.

The swept are of two eight ince diameter props is about 112.5 square inches. The swept area of one nine inch diameter prop is 63.6 square inches.

Today we use larger diameter props on powered sailplanes when compared to the more conventional models. "Bingo".

Maybe some of the armchair gurus have a comment about why a 14 inch prop turning at a significantly lower RPM produces more thrust than an 11" prop.
Is it the "efficiency factor"? Where eficiency is determined by power IN vs power OUT - torque x distance traveled by 1/4th the prop dia. x RPMs which is work per unit time at the crankshaft compared by the work done to pull the plane through the air at a certain speed.

Remember that.Reynolds Number is at work here too. To say just buy lots of props and try them doesn't answer the question does it?

To really simplify the efficiency thing - try this :
compare two props - different sizes
figure the last inch of each plus the an inch at the root of each produces nothing of value
so--- a 4" dia prop has only 2" of effective radius
BUT a 5" prop has 3" effective radius
using this comparison it become obvious that large props have higher percentages of effective area.
My numbers are simply for establishing the basics -they are not part of test results..

Originally Posted by Texas Buzzard

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.................................To say just buy lots of props and try them doesn't answer the question does it?

I would say yes, as one way of getting the end result you want. Which I assume is for the plane to fly how you want it to.

You want speed, you go for a smaller diameter to keep a high rpm and also use a high suitable pitch.

You want slow 3D with loads of thrust for hovering and vertical climb outs, you go for that large diameter and reduce the pitch, (you no longer need the speed).

There are quite a few props out there with identical 'dimensions', i.e. diameter and pitch, by different manufacturer that can definitely give different results. Blade shapes, blade thicknesses, weights, material used, can all have an influence. You wont know which is 'the one', until you have flown it.

As you have mentioned free flight, (FF), look at the comparison in props between a IC powered competition FF that uses a very fast near vertical climb, and a competition rubber powered FF that is looking for duration.

The prop is probably the most important thing in competition powered models, and a lot of flight experimenting goes into selecting, even modifying one, to get the required results.

Looking up a chart or 'calculator' to select a prop may be a good starting point, but the final decision will have to be how the model flies.

TB, you're forgetting about another similar "experiment". The Tri-Fai. It used three Tee Dee .049's in an attempt to try for much the same idea. But in this case all were mounted as tractors on the nose.

Your "Hammerhead" pushme-pullyou version I believe. It doesn't show what engines were specified so I may be mistaken and someone else tried one as well?



And the Tri-FAI three engine oddity;

http://1.bp.blogspot.com/-xpETBByOFs...0/TRI-FAI-.jpg

Oh! Look at that! It's a Bob Hunter design. The same guy that brought us the somewhat famous Sattelite design and that brought the use of CA glue in the form of "Hot Stuff".

As to the idea behind the use of two or three prop discs vs one that's a can of worms. In the Hammerhead pushme-pullyou setup the rear engine is operating in the wake of the front. So some care needs to be taken to prop the rear engine so that it can perform well. Even then we can expect a big reduction in contribution from the rear engine.

Bob's Tri-FAI design seeks to cure that. And in truth it has far more prop disc area than a single 7 to 8 inch prop such as used on the .15's of that day. It may not tell the whole story though. The .15 was able to manage with a 4 to 6 inch pitch depending on the engine and prop choice. On the other hand the smaller half A engines were limited to 2.5 to 3 inch pitch to really sing well. This means that potentially the ultimate velocity could be reduced.

And let's not forget the fuss needed to start and tune three engines all mounted in that sort of close proximity. It must have looked like a "Chinese fire drill" getting them all started and running well. So the whole effort was doomed to be no more than an oddity even when the light bulb over Bob's head first lit up.