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Post  roddie Fri Oct 11, 2013 2:31 am

About 20 years ago I took interest in the Mars, after reading an article in Air & Space magazine. I had always wanted to build a R.O.W. R/C model. I perceived a 48" w/s model powered by at least 2 Cox .049 reedies and started to design/build. I have a 575" sq. constant-cord wing built, for a stand "way"-off scale model. The other 2 propulsion units were to be of speed-controlled brushless motors, in the 1000kv range.

I special-ordered 4 props... wood "Zinger" 6 x 3's.... x2 r/h and x2 l/h. I wanted 4 matched props. The full-scale "Mars" had x4 Wright Cyclone Radials. My perception was to have the 2 Cox reedies sharing a 1oz. tank, running at w.o.t. for taxiing...  and the brushless motors for take-off/cruise.

Notice the prop rotation in this vid... this is why Cox reedies/elec. motors w/right/left hand props should prove a good choice for this model's power.


I have the wing framed and pontoons built w/.080" aluminum struts. Engine cowlings cut from the bottoms of plastic 16 oz. soda bottles. The tailplane is not built yet. I need to calculate the moments... but the main fuse structure will be the same as the wing spar... 1" sq. aluminum tubing w/lightening holes.


Martin JRM Mars for x4 reedies Mars_w10


The fuse is channeled and "roughed-out" from CNC router-cut 3" foam, for the tail boom (sq. tubing) and hollowed-out to contain the fuel tank and all radio-related gear.

A "water-drop" would be a very cool function to incorporate... to re-create the aircraft's current full-scale operation. I have given some thought to "misting" nozzles... fed by a w/s washer pump/tank with dishwashing liquid that would produce bubbles/foam... A crop-dusting/agricultural aircraft could also use this system for visual scale realism.

Check out this Martin JRM Mars model...

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Post  batjac Sun Oct 13, 2013 2:52 pm

[quote="roddie"]
Notice the prop rotation in this vid... this is why Cox reedies/elec. motors w/right/left hand props should prove a good choice for this model's power.

I wouldn't use this video as an example of prop rotation on a full size Mars.  The prop rotation in the video is a function of the plane's engine RPM and the shutter speed of the camera filming it. Note that the props reverse directions several times in the video.

The Shuttered Mark
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Post  roddie Sun Oct 13, 2013 3:37 pm

batjac wrote:
roddie wrote:
Notice the prop rotation in this vid... this is why Cox reedies/elec. motors w/right/left hand props should prove a good choice for this model's power.

I wouldn't use this video as an example of prop rotation on a full size Mars.  The prop rotation in the video is a function of the plane's engine RPM and the shutter speed of the camera filming it. Note that the props reverse directions several times in the video.

The Shuttered Mark
Hey Mark, Yes you're probably right... What I plan for this model (or any multi) is alternate prop rotation to eliminate "P"-factor? I was told that the "starboard" engine(s) in a multi is/are the "critical engine(s)", in an "engine-out" scenario because of the torque effect of the c.c.w. prop rotation on the airframe. A starboard "engine-out" causes the "port" engine to roll the airplane and thus cannot sustain powered flight, whereas with a "port" engine-out... the pilot can still maintain flight with aileron/rudder trimming.

Electrics and reedies as you know; will run c.c.w. or c.w... so why not run the starboard engine "conventionally" and the port engine c.w. w/a left hand prop?
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Post  batjac Sun Oct 13, 2013 8:05 pm

roddie wrote:Hey Mark, Yes you're probably right... What I plan for this model (or any multi) is alternate prop rotation to eliminate "P"-factor? I was told that the "starboard" engine(s) in a multi is/are the "critical engine(s)", in an "engine-out" scenario because of the torque effect of the c.c.w. prop rotation on the airframe. A starboard "engine-out" causes the "port" engine to roll the airplane and thus cannot sustain powered flight, whereas with a "port" engine-out... the pilot can still maintain flight with aileron/rudder trimming.

Electrics and reedies as you know; will run c.c.w. or c.w... so why not run the starboard engine "conventionally" and the port engine c.w. w/a left hand prop?
Well, you've got it mostly right.  For this discussion I'll use clockwise/counterclockwise as a pilot would see it from his seat in the cockpit.  On a single engine plane with the prop rotating clockwise as viewed from the cockpit, the "P" Factor makes the plane want to yaw to port.  So, on a twin with both turning clockwise, each engine has a left yaw tendency due to "P" Factor.  Now, since each engine is certain distance away from the centerline, that engine will want to make the plane yaw to the other side.  The right engine's asymetrical thrust makes the plane want to yaw to Port.  The left engine's asymetrical thrust wants to make the plane yaw to Starboard.  So, add in the "P" Factor, and you get one engine more critical than the other.  For example:

Left engine - Port Yaw from "P" Factor plus Starboard Yaw from asymetrical thrust = given amount of yaw to Starboard.  Asymetrical thrust cancels out "P" Factor thrust.

Right engine - Port Yaw from "P" Factor plus Port Yaw from asymetrical thrust = proportionally larger amount of yaw to Port.  Asymetrical thrust and "P" Factor are cumulative.

Therefore, in a twin with both propellors turning conventionally, the left engine is the critical engine.  It takes MUCH more rudder pressure to overcome the engine out than if the right engine is out.  If the engines are counter rotating, then the engine out thrust is equal on each side.

So, if you're going to do counter rotating props, then you'd want the left engines to turn conventionally, and the right engines to turn counterclockwise as seen from the rear using left-handed props.  The way I remember it is that for balanced thrust, the top of the arc of each/all props is towards the fuselage.

The Pilot Mark


Last edited by batjac on Sun Oct 13, 2013 8:50 pm; edited 1 time in total
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Post  roddie Sun Oct 13, 2013 8:43 pm

batjac wrote:

So, if you're going to do counter rotating props, then you'd want the left engines to turn conventionally, and the right engines to turn countrclockwise as seen from the rear using left-handed props.  The way I remember it is that for balanced thrust, the top of the arc of each/all engines is towards the fuselage.

The Pilot Mark
... this is contrary to what I thought... but I get it now, through your explanation... sort of. We can then agree, that it's the "port" engine that is critical, in an engine-out condition... (conventional rotation)? So... given the choice of a subject multi-engine model, looking from the rear; the l/h (port) engine(s) should have conventional (c/w) rotation and the r/h (starboard) engine(s) should have c.c.w. rotation? I'm still confused about prop twist profiles. If you can produce equal/opposite radial forces... why wouldn't you want to?

You gotta love my mental blocks... Thank God my wife loves me!
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Post  batjac Sun Oct 13, 2013 9:03 pm

roddie wrote:... this is contrary to what I thought... but I get it now, through your explanation... sort of. We can then agree, that it's the "port" engine that is critical, in an engine-out condition... (conventional rotation)? So... given the choice of a subject multi-engine model, looking from the rear; the l/h (port) engine(s) should have conventional (c/w) rotation and the r/h (starboard) engine(s) should have c.c.w. rotation? I'm still confused about prop twist profiles. If you can produce equal/opposite radial forces... why wouldn't you want to?

You gotta love my mental blocks... Thank God my wife loves me!
That is correct on the prop rotation.   As for prop twist, are you talking the blade profile fom hub to tip of the prop?

As far as equal and opposite radial forces, I'm guessing that you mean to fly straight when the opposite engine is out.  You CAN angle the engines out to such a degree that the asymetrical thrust is negated.  But the incredible drag and efficieny loss would make the horsepower requirements untenable.  Think "Man-Win Trainer" engine offset.  Well, almost.  I think I've seen one airplane designed that way from the 40's, but it was a failure.  For aerodynamics, planes are designed for flying, i.e. both engines running.  If one engine goes out, the designer just figures you'd better suck it up and have a real strong knee.  The only good solution to this was done by Burt Rutan long ago.  Unfortunately, no aircraft company wanted to pick up on the design and produce it commercially.  The Rutan Boomerang:

http://www.rutanboomerang.com/index.php/multimedia/photo-gallery/early-years/category/5-early-years#

Notice the right engine in the fuselage has more right ofset than normally seen on singles.

The Professor Mark
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Post  kevbo Sun Oct 13, 2013 11:29 pm

Another solution is to reverse the rotation of the rift engine. This was done on the P-38 and on a GA two seated called the derringer.
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Post  batjac Mon Oct 14, 2013 12:23 am

kevbo wrote:Another solution is to reverse the rotation of the rift engine.  This was done on the P-38 and on a GA two seated called the derringer.
But on the P-38, BOTH engines became the critical engine. The rotation wasn't determined for better flight characteristics. The rotation was determined to make the plane a better combat platform.

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Post  roddie Mon Oct 14, 2013 1:02 am

ah yes... there's nothing like a good healthy discussion concerning aeronautics!!! Now that my brain is junk... I need to go to bed!
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Post  fit90 Mon Oct 14, 2013 7:00 am

I thought the original P-38 had contra rotating propellers but that was changed before production. The contra-rotating propellers would have been slightly faster but more difficult to control with either engine out.
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Post  statorvane Sun Dec 22, 2013 5:03 am

Just ran across this thread. Here's a link to a video of the Mars, You can see the props all turn in the same direction. This video may give you some ideas for a water drop feature.

Earthshaking Aricraft

This is a fantastic plane. I believe they were originally built for US Navy to serve as hostpital aircraft.
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Post  roddie Sun Dec 22, 2013 11:11 am

statorvane wrote:Just ran across this thread. Here's a link to a video of the Mars, You can see the props all turn in the same direction. This video may give you some ideas for a water drop feature.

Earthshaking Aricraft

This is a fantastic plane. I believe they were originally built for US Navy to serve as hostpital aircraft.

Thanks for posting this info.! Yes.. these are amazing aircraft to have had a service-life this long. Smaller aircraft (even in numbers..) simply cannot compete with these behemoth's ability to fly straight through the updrafts involved with massive forest fires.. and at lower and more effective altitude. 7000 gallons is a lot of water... Operational costs of the Mars is high though... and forest fires don't happen on a schedule. It will certainly be a gamble to retire both of the remaining aircraft.. without being able to duplicate their effectiveness. Having been a valuable "loss prevention" tool... insurance companies will be concerned with the Mars retirement as well as land owners/residents in heavily forested regions. I'm not what you'd call a "tree hugger"... but uncontrolled massive forest fires are a terrible thing... and unpredictable.

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