The_Mongrel

Hey All,

I have just finished building a Catalina with 2 motors and have a question....

She flies fine, but does have some torque roll problems due to both props spinning the same way... is there any props on the market designed for rotating in the other direction??

I can reverse the motor without issue, but the prop will then push, not pull.

Any suggestions would be great.

Cheers
-J

Tall Paul

There are pusher props available, but maybe not in the size you need.
Look at what GWS and APC provide.

GaGeeBees

My Feedback: (23)

Check out E-Flite as well.

The_Mongrel

Ok Thanks guys... are the called anything special??

jdetray

Hi Mongrel -

They are known as "pusher props" or "reverse pitch props." Here are some links.

[link=http://www.modelflight.com.au/graupner_propellers_reverse.htm]Graupner Reverse Pitch Props[/link]
[link=http://www.apcprop.com/cgi-bin/store/agora.cgi?maxp=400&ppinc=6a&product=+]APC Pusher Props[/link]
[link=http://www.allerc.com/index.php?cPath=36_38&sort=products_sort_order&page=2]GWS Reverse Pitch Props[/link] (only two props available here)

- Jeff

The_Mongrel

You are a Legend!! Thanks heaps!!

Matt Kirsch

My Feedback: (21)

What do you mean by "torque roll problems?"

Nearly all full-scale, multi-engine prop planes have all their props spinning in the same direction, for simplicity in manufacturing and maintenance.

Handling issues can be overcome with adequate vertical fin area, and adjusting the down/right thrust on each individual motor. A degree or two of down thrust on the starboard (right) engine will take a huge bite out of the plane's tendency to pitch up and/or roll to the left under power.

Before I go giving you all kinds of thrust angle advice, though, I probably should get my theory straight... Thrust angles are to counteract three factors, P-factor, gyroscopic precession of the prop, and engine torque. A conventional engine will tend to roll a plane left and lift the nose when mounted square to the airframe due to these factors. Down and right thust counteract these factors.

I believe you want MORE down and right in the starboard engine(s), both to counteract the natural tendencies and to assist in engine-out situations. The starboard engine(s) should be set to counteract the three factors for all the engines, and the port engine should be mounted pretty much square IIRC.
- If you lose the port engine, the starbord engine will want to yank the plane around to the left. Right thrust in that starbord engine will counteract that.
- If you lose the starbord engine, the port engine will want to yank the plane around to the right. Natural P-factor, gyroscopic precession, and engine torque will go a long way in counteracting that.

Now, take this all with a grain of salt because I might have it completely backwards...

-pkh-

My Feedback: (2)

I believe if you use a pusher prop as a reverse rotation tractor prop, you'll have to mount the prop backside out to have the airfoil going in the proper direction. With a pusher, the leading edge of the airfoil is at the back of the prop, and as a tractor, you'll want the leading edge of the airfoil at the front of the prop. Otherwise you'll rotate the pusher prop's airfoil backwards (trailing edge first), which will be less efficient.

GaGeeBees

My Feedback: (23)

E-Flite's P-38 has counter-rotating props.

http://www.hobbyzone.com/rc_planes_e...htning_400.htm

The_Mongrel

Matt,

Your theory does make sense, and I have thought about adjusting the thrust angles to try and counter this problem. If you take a look at this video and forward to where I land on the water, you will see that when I try to taxi, she does a u-turn. It takes full rudder right to even begin to get her to taxi in a straight line. under normal flight - a fair bit of right trim is required to keep her level.

http://www.youtube.com/watch?v=b2p5Z3nJNEM

I will look into the thrust angles and report back

thanks again
-J

The_Mongrel

WAY COOL! My plane runs 10X8s and a local supplier has the E-Flight spare parts... so I can get them locally!

THANKS!!!

-J

Tall Paul

Not enough water rudder!

GaGeeBees

My Feedback: (23)

What water rudder? All it's got is that little bitty fin

(manufacturer's photo)

Tall Paul

And that's the problem!
The full-scale used differential motor power.
On the water, models need a water rudder.
My Gentle Lady boat has the rudder extended into the water for water steering.
Haven't had a chance to try the twins yet this year.. here flying with "land keels".

The_Mongrel

hey Guys,

Mine is not the great planes model.. Mine is from an Australian Importer called Aussie Flight and made and made by GLM.



No water rudder at all.. and in fact the rudder on this model is quite small.

Cheers
-J

Matt Kirsch

My Feedback: (21)

That starboard motor appears to have some up and left thrust in it from the picture. It may be the cowling is on crooked, or an optical illusion.

Up thrust and left thrust would explain the problems you are having.

BTW, the Catalina's engines both rotate in the same direction on the full scale version.

The_Mongrel

hey Mate - This photo is not my bird. It is the photo from the importer's web site.

I am going to try and have a look at the thrust angles and also going to look at counter rotating.

Thanks again
-J

Flypaper 2

Full size Cat had quite a bit of out thrust on both engines. Flew quite happily on one engine. In ww2, in the Caribean, a bet was put up whether it could get off the water on one engine. Took about two miles but it did get off. It then proceeded to fly all the way to England on one engine.There is a good book about the cats in ww2. If interested, I'll check in my library for the title.

aarowsmith

Yes, but what about the starboard flux capacitance, or did you forget the angle of the dangle is critical here.
In theory the pitch of the angle can be triangulated to the passerby???

All I know is when I peg the throttle, it go way fast!!!





Say what??? I am lost here', and do I know what the heck' we are talking about???"[sm=drowning.gif]

The_Mongrel

heheh a funny read!! But the seriousness of fixing this issue is real. I landed on the dam yesterday and had serious trouble getting it back because it just wanted to go around in circles!!

I need to find a fix

Flypaper 2

Mongrel:
Used to have an old Zagi flying wing where I had two motors mounted on the ailerons. Sort of a poormans vectored thrust. Anyway I had the throttles mixed with the rudder channel, so that left rudder would speed up the right motor and slow down the left one. With the stick in the centre, both throttles worked together. Had a Futaba 9C back then that had good programming in it. I have the Spektrum DX7 now.

Swift427

deleted

The_Mongrel

Hi Swift,

don't mean to contradict you but you cant simply reverse a prop and turn it into a pusher. It will not work. The airfoil on the prop will still face the wrong way.

Cheers
-J

Swift427

deleted

Tall Paul

Mounting a normal prop backwards on a motor merely degrades the performance of the prop.
Free flighters do this all the time for that critical first flight.
Mount a normal prop backwards AND reversing the direction of the motor rotation makes the prop push the air opposite to the "normal" direction.
Easy Star and ZAGI do this.
To achieve counter-rotation with a twin, tractor motors as with the PBY, the motor with the pusher prop needs to rotation oppositely.
All that takes is swapping any two of the three wires to a brushless motor.
Or rewire the brushed motor with the pusher prop.
It's no biggie.
This one uses brushed motors. I reverse the plug-in extension depending on which motor turns which way.