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Old 03-31-2013, 09:15 PM   #1
Capt. Midnight
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Default Counter Rotating Props/Motors

Hi there!

I'm building a 1/20th-scale, two motor model of the Boeing Sea Ranger; W/S about 84 inches; AUW about 10-12 lbs). I've been considering whether to incorporate counter rotating props/motors, as the model (like the aircraft) will not have a water rudder.

Any comments/suggestions would be appreciated.

David Plummer

Bellevue, WA
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Old 03-31-2013, 09:25 PM   #2
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Originally Posted by Capt. Midnight View Post
Hi there!

I'm building a 1/20th-scale, two motor model of the Boeing Sea Ranger; W/S about ; AUW about 10-). I've been considering whether to incorporate counter rotating props/motors, as the model (like the aircraft) will not have a water rudder.

Any comments/suggestions would be appreciated.

David Plummer

Bellevue, WA
Counter rotating props are often useful on seaplanes. So if you can find suitable props, you should try them. Differential thrust is also very useful for taxiing on water.

It's never too late to have a happy childhood.
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Old 04-01-2013, 12:10 AM   #3
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Contra-rotaing props are very useful on twins. If done right it adds a huge perforamce gain to the aircraft. Perhaps I should rephrase: if done wrong, the plan's performance sucks. The RAF ordered p-38's without counterrotating props in WWII, and found them nearly useless.
P-factor can be nearly eliminated if the props rotate so that the tips are descending as they pass near the fuselage.

There have been threads here about how to set up differential throttle coupled to rudder. Used by almost all multi-engine seaplanes. AND a helluva lot of fun for aerobatics.

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Old 04-01-2013, 12:32 AM   #4
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Interestingly, the P-38 is one of the few counter rotating twins with tips rotating UP as they pass they fuselage. Something about turbulence trapped between the booms, IIRC...

Generally, the props should rotate inward (tips going down as they pass the fuselage). It does make a difference. I've tried inward and outward and there were definite changes to trim and handling, especially to the planes with a single rudder.

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Old 04-01-2013, 05:03 AM   #5
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Counter-rotating... and mix throttle to rudder (switch to enable/disable the differential thrust)

use the differential thrust to enhance taxi control. Turn the differential off for normal flight.

You can play with the differential thrust in flight with care... I have seen a "stall turn" with the twin doing a 3.5 revolution pinwheel at the top.
First thing to do if in trouble and you have the differential on... is to turn it off.
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Old 04-01-2013, 05:54 AM   #6
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P-factor is eliminated on any counter rotating twin, regerdless of which way the props spin. The reason why they usually spin in the direction so that the blade rotates down as it passes the fuselage is to improve single engine performance. This set up means that neither engine is 'critical'

Interestingly the great majority of twins don't have counter rotating props
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Old 04-01-2013, 08:15 PM   #7
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You can adjust engine thrustline to compensate for the torque/p-factor. That's only "perfect" at one airspeed and AOA. The adjustment would normally be made for best single engine out flight.

I've seen a Free Flight P-38 that used extreme offset and down thrust to ensure that if one (glow) engine quit before the other that the operating engine would not spin the airplane. Something on the order of 9 deg down and 7 deg outboard with counter-rotating props...
Unacceptable for RC. This setup was intended to force the plane to turn toward the operating engine at 100% throttle.
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Old 04-01-2013, 08:53 PM   #8
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Hi there!

Many thanks for the feedback!! If anyone can direct me to the differential-motor-control article/site, I'd appreciate it.

Cheers,

David Plummer
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Old 04-01-2013, 08:59 PM   #9
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What Tx are you using?

It's never too late to have a happy childhood.
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Old 04-01-2013, 09:02 PM   #10
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Originally Posted by Capt. Midnight View Post
Hi there!

Many thanks for the feedback!! If anyone can direct me to the differential-motor-control article/site, I'd appreciate it.

Cheers,

David Plummer
You would need a computer tx with mixing. One ESC would be driven off throttle channel as normal, the other off an auxiliary channel slaved to throttle. You would then slave both throttle and auxiliary to rudder with two separate mixes. Done right this would mean you had both motors operating together off the throttle but rudder would create differential thrust in the direction of the rudder input.
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Old 04-01-2013, 09:16 PM   #11
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I don't know of a site that discusses doing the differential thrust. I have done it for a DC-3.

Exact instructions depend on your transmitter.

Plug one ESC into the normal throttle channel
Plug the other into one of the aux channels (anything past channel 4) typically this will go in 7 or higher since 5 is often a switch operated (gear) channel and 6 is usually used for dual ailerons or as the flap channel.

Program a mix of throttle to the channel selected for the second ESC. This mix is always on, 100%, reverse/norm should match the throttle.
Test to ensure both motors respond the same to throttle. Some computer radios you have to enable the trim of the throttle to affect the second channel.
Do not go further until you have the motors responding just as if you used a Y and a single channel.

Program a mix of rudder to throttle. This should just affect the motor on that channel. Adjust so that when the rudder is deflected toward the side the motor is on throttle is reduced and when rudder is deflected the other way throttle is increased. Neutral rudder should still allow throttle to be off by the stick.
Some radios this mix may have to be adjusted such that 100% throttle is not available with the mix on and rudder neutral.
Assign this mix to a switch.

Now do another mix of rudder to your other motor's channel just as above. (+/- % will be reversed)
Assign to the same switch.

You should be able to "duck-walk' taxi with 0 throttle by stick wagging the rudder. Just the motor you turn the rudder away from will start and increase power proportionally to rudder input.
A small amount of throttle (just a bit more than needed to start the 2 motors spinning at once with rudder neutral) should be able to taxi the plane with the differential thrust controlling it well on the water with no water rudder.

This setup gives a very nice scale taxi effect for a DC3 with a free castering tailwheel.

The result of using the mix set in flight will vary by radio... you may be able to have full throttle at rudder neutral or you may not.

If it prevents full throttle at rudder neutral (rudder-throttle mix active) then it may reduce max power to a level that is too low for flight. The only answer in that case is to only fly with the rudder-throttle mixes off.

Any pair of ESCs can do it... the variable is the transmitter.
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Old 04-02-2013, 03:54 AM   #12
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Capt.....

You might find this interesting........ask the OP a few questions see if he can add a liitle detail.



http://www.wattflyer.com/forums/showthread.php?t=69446

AMA 928214
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Old 04-05-2013, 09:50 PM   #13
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Just as a point of history the early P38 delivered to the RAF, apart from the right hand engines (to make them interchangeable with the RAF P40s) did not have turbo superchargers either.
In this form the Allison was rated at just 1010hp and at only 14000ft. A turbo charged Allison worked well at 30,000ft plus.
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Old 07-18-2013, 04:08 AM   #14
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Default Sea Ranger Update

Hi there!

Here's a photo of the Sea Ranger model showing fairly recent status. If any interest, I can post some additional photos.

Cheers,

Dave P.


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Old 01-26-2014, 10:11 PM   #15
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Default Boeing Sea Ranger

Hi Sea-Planers!

Here's a photo (I hope) of my 1/20-scale Boeing Sea Ranger; has not flown yet - just need some good weather and a competent pilot.

W/S is 84"; length is 57"; AUW is about 11.5 lbs.

Cheers,

Dave P.


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Old 01-27-2014, 01:02 AM   #16
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Capt.,

That is one gorgeous plane. Good luck on the maiden! Post vid if you can.

hawk

Wounded Warrior Fun Fly - Aug 16th ,2014 - Grapevine TX - Info link: https://support.woundedwarriorprojec...ising/RCPilots
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Old 01-27-2014, 05:48 AM   #17
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Yep, she's a beauty. What about some more details and build photos?

Best of luck with the maiden flight.
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Old 01-27-2014, 06:35 PM   #18
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Default Sea Ranger Construction Photos

Hi JetPlane Flyer!

OK, I'll see if I can post a few photos of the construction.

The first image is the cockpit of the aircraft (from Boeing Archive files); the rest are of various construction elements. I'll make another post with a few more.

Cheers,

Dave P.


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Old 01-27-2014, 06:51 PM   #19
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Awesome workmanship!

I like the idea of building the fuselage with the formers mounted on the metal tube. So it looks like all balsa except for foam nacelles?
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Old 01-27-2014, 08:59 PM   #20
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Amazing work man! Beautiful.....
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Old 01-27-2014, 09:27 PM   #21
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Default Sea Ranger Construction Photos

Hi Sea Plane Builders!

Here's one more photo of my Sea Ranger model construction; I tried to load a few more, but failed.

The pontoons were made with a balsa framework (about 10 'bulkheads'), and the inner-bulkhead space filled with blue insulation foam, then sanded to shape and finished with gesso and acrylic paint. The struts are just laminated balsa with a light-ply center sheet (apparently I didn't take any photos during their construction).

I'll see if I can upload a few more pics later.

Cheers,

Dave P.


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Old 01-27-2014, 09:53 PM   #22
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Blue foam is great for supporting the structure... but it is porous and not a good choice for flotation. Works for a while but its hard to get the water out once it gets in.
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Old 01-27-2014, 11:18 PM   #23
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Default Blue Foam

Hi fhhuber!

Thanks for the tip! I have used the stuff before (in some other models, e.g., my P6M-2 wing-tip floats - see photo below), and took reasonable care to fill and seal the foam stuff; I've not had any [I]known[I] trouble with adsorption, but, if fact, I never really took time (or knew how) to really verify that the H2O wasn't being absorbed. What sort of foam (if any) do you recommend, and how do you seal it?

Cheers,

Dave P.


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Old 01-28-2014, 04:41 AM   #24
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When filling for flotation... good old white styrofoam or EPO. These are closed cell foams.
Closed cell foam is very resistant to absorbing water and will maintain buoyancy for many years of continuous immersion. Considered dry completely in minutes if you towel of the water on the surface.

The blue will sink in a couple of weeks even if its thick. And when you are trying to dry it out, some water continues migrating further into the foam. It can take months to dry completely after a few minutes of immersion.
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Old 01-28-2014, 09:05 PM   #25
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Default Blue Foam

Hi Sea Plane Builders!

After getting some good advice from fhhuber (on the type of foam to use), I conducted a NBS-type (nat'l bureau of standards) experiment with a 4" long, 2 x 2-inch piece of blue foam: I weighed it, then immersed it in 1" of H20 for 30 minutes and weighed again: the 'before'/'after' weights were 6g and 7g , which works out to a 17% weight gain! I remeasured the weight about an hour later - still 7g (my scale has a stated linearity of +/-1g). So, clearly, this is something to be considered.

I do believe that if the part of the foam that might be exposed to the H2O is properly sealed/finished, this might not be a fatal problem. Still, if I build another flying boat, I'll use the closed-cell white stuff.

Cheers,

Dave P.
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