Usually models of radial-engine planes end up with absurdly small props.  Are you sure you're not using way too much motor?

Here are a couple of screen shots of motocalc output, these are only a few of an almost infinite selection of parameters

Usually models of radial-engine planes end up with absurdly small props.  Are you sure you're not using way too much motor?

Tim:
It is interesting that the real plane, with a radial engine, has the propeller come within 6" of the fuselage. They actually paint a res stripe artound the fuselage at that point to warn the ground crew of the proximity of the propeller.

I usually build somewhat on the stout side. I am guessing that the model will come in at 6 pounds+ (retracts, flaps, batteries). So, with the drag of the flying lines, I am anticipating that the motor equivalent of a .50 or .60 (total) is about what I should need.

Jim Fruit

Jim,

As a couple of alternatives, if you can't get a decent performance compromise with the a two-blade prop of the scale diameter you could:

As you mentioned use a multi-blade prop, or,

Cut down a larger diameter prop to get more blade area in your 8 inch diameter.  Also, I would expect you will need a fairly low pitch, say a 4.5 to 5.5 range.  You will sacrifice some prop efficiency, but should gain in overall thrust.  Prop efficiency is not an issue in scale, generally speaking.

I hope you will keep us posted on your project as the build comes along.  The twin Beech is one of my favorite planes and one that I've always wanted to model.  The yellow one that is in the NASM is just gorgeous.

John

Jim, I looked up the dimensions of your model in your previous post. I fly a dH Hornet at 57" span and approx 7.5# with two OS 30's and 10-5 props. No retracts, but flaps and bomb drop with electronic control. Wing and power loading are such that it won't do high flight or fly on one engine. Twins pay a weight penalty vs a single engine model. I suggest you pay close attention to weight control. You should have enough power if you can find a prop to absorb it. I use large diameter, low pitch props on all my scale models.
Chuck

I think the Navy designation for this plane is: JO

Chuck:

You are absolutely correct. That "C-47" did not look right when I typed it. I should quit relying upon memory. Just getting too old, I guess. That is why I ask questions.

The actual plane designation is RC-45J Navigator, tail number 9771. It has an identical sister ship, tail number 9585, that is on loan to Pima.

JHF

I'm leaving for Muncie on Monday and will have two electric power models you can look at, FWIW. We can talk about it there, if you like.  There are several interelated variables:

The motor kV
Battery voltage
type of ESC
wing/power loading
and propellor

Keep in mind that you're probably better off to be over powered since you have throttle control and can dial it back.  That said, the motors will like to run somewhere near their kV rating when making flight power.  kV is the nominal rpm per volt the motor is wound for.   14V x 1100 kV = 15400 rpm You also want to stay within the current limits for the motors.  Fortunately with electrics you can calculate how much power you have at the limit and get some idea of what capacity you need in the speed control/battery combo.

Read through the stunt set up section and look at the variety of applications and plane size and weight.

John

My Jenny, for instance, uses a 6S pack, nominally 22 volts with a 550 kV motor.  I don't know what the cruise RPM is, but it works nicely with a 17-5 prop at about 50% throttle for level flight and  about 80% throttle for high flight.  This isn't directly comparable to your situation, but maybe will give a little feel for where you're headed.

I'm leaving for Muncie on Monday and will have two electric power models you can look at, FWIW. We can talk about it there, if you like.  There are several interelated variables:

The motor kV
Battery voltage
type of ESC
wing/power loading
and propellor

Keep in mind that you're probably better off to be over powered since you have throttle control and can dial it back.  That said, the motors will like to run somewhere near their kV rating when making flight power.  kV is the nominal rpm per volt the motor is wound for.   14V x 1100 kV = 15400 rpm You also want to stay within the current limits for the motors.  Fortunately with electrics you can calculate how much power you have at the limit and get some idea of what capacity you need in the speed control/battery combo.

Read through the stunt set up section and look at the variety of applications and plane size and weight.

John

My Jenny, for instance, uses a 6S pack, nominally 22 volts with a 550 kV motor.  I don't know what the cruise RPM is, but it works nicely with a 17-5 prop at about 50% throttle for level flight and  about 80% throttle for high flight.  This isn't directly comparable to your situation, but maybe will give a little feel for where you're headed.

John:

Looking forward to seeing you at the NATS.

Jim Fruit

Jim
One of the things I watch for is disk area of the prop.  The bigger the disk the more thrust.  Remember the Disk Area goes up at the Square of the 1/2 Diameter.  A three bladed prop of the same diameter and pitch will have almost 50 % more thrust at the same RPM as a similar shaped two bladed prop.

Some examples: An 8 In. prop has 50 Sq. In. of Disk Area, a 10 In. prop has 78 Sq. In. of Disk Area, an 11 In. prop has 94 Sq. In. of disk Area.

Therefore an Eight inch three bladed prop will have almost the thrust of a 10 inch two bladed prop.
Clancy

Clancy:

I knew that when you got wind of the question, you would come up with some pretty solid resolutions. Thanks a bunch. See you at the NATS.

JHF

Everybody:

Thank you so much for all of your help. This thread is the greatest.

JHF

Jim
Some time run an engine on a test stand and monitor the Max RPM. 
Switch from a two bladed prop to a Three Bladed prop.  Compare the max RPM (Power available) verses diameter of each prop.  If loaded to the same Max RPM then you have an indication of the relative efficiencies of two and three bladed props of the same blade design. 

Consider the performance increase experienced by the P-47 with the paddle bladed props or the Corsair with three and four bladed props.

Clancy