REVERSE PITCH PROPS AND PLANE TRIM

[jose modesto]

I have been building gas stunt planes for 44 years and have developed systems that help with the trim and set ups of these models
1) reverse bellcrancks rear leadout down
2) rabe rudders
3) equal wings
4) engine offset
5)rudder of set
i converted one of the gas models to electric, all was well with counter clock(TRACTOR) turning props when i switched to counter  rotating props(13x4.5) the plane was not happy. These reverse pitch props require a little thinking outside the normal BOX.
Now i had to much line tension on outside maneuvers and minimum on insides the normal trimming options were not available
1) leadouts back only gave to much yaw(trying to move up leadout back it also moved the down line further back more yaw on outside)
2) less rudder of set helped outside but not inside
3) removal of Rabe rudder helped with excessive yaw
4) additional tip weight to get inside line tension hurt the outside turns
So now i have to rethink my approach to Estunt and counter rotating props.
1)non reverse bellcranck
2) reverse the Rabe rudder to give more line tention for inside turns
3) a little inside Engine offset
Have you guys run into this and how are you trimming your EMODELS.
JOSE MODESTO

[Howard Rush]

You have to fly left-handed.  I think that's the only solution.  Hope this helps.

[Dennis Adamisin]

Hi Jose
I think you are on to something - in the sense that the Pusher prop would lead to diffenent trim settings, however most folks have simlly plugged & played without much change.  However, taking your list

1. Precession now reverese, should revert to old style UP line in back (but effect of this is pretty small to begin with)
2. Precession effect is reversed, movement of Rabe rudder must also be reversed. (huge effect!)
3. Equal wings - no change
4. Engine offset - less
5. Have not changed any but I use little to start with.  Torque is OUT so less offset is PROBABLY the right way

We are been moving leadouts forward

Have not tried Howard's suggestion yet...

[WhittleN]

Bart (AKA Tempest) needs to fly electric with pusher props. He would be unbeatable. Left handed you know.

Norm

[Igor Burger]

Our models are deveped for tractor props for decades, so it will need little rework with open mind. Most was already written, I will only extend little:

1) reverse bellcrancks rear leadout down
- this is questionable, the theory for up line front is cancellation of precession yawing, but several of my models needed just opposite, up line back, the theory says that up line front can lead to oscillation, while up line back is damping .. I always try both during trimming of new model

6) thrust line OVER the wing line, landing gears under the wing and elevator coincidence
- shold be also reverted, all 3 effects cancel the precession and p-factor which permanently pushes nose up, reversed pitch will need it just opposite ... BUT ... I still think that fraction of that pitching comes from asymetric stab position on place of downwash which is little directed down on wing airfoil surface, so may be pusher prop will work well with 0 incidence, but landing gear is definitelly on wrong side of fuselage. 

[jose modesto]

Dennis what I'm finding is that now i have to trim for inside line tention, and this is so different from the norm. Yes reverse pitch gives better line tention on vertical eight,hourglass and all outside maneuvers but it comes with a loss of inside maneuver line tention,Is this better or just different.
I can no longer use the reverse bellcranck with the pusher props as this puts the down line in back.
Now when i build a new Eplane it has to be designated for pusher or tractor and i don't think that the differences in trim can be tolerated for the highest level of competition.
Paul Walker briefly mentioned the trim differences from pusher to tractor props. Paul can you elaborate on your findings between the pusher and tractor props and the trim changes to your model.
Dennis is the solution a tween with tractor and pusher props.This type of model will have neutral line tention on inside and outsideturns.With this type of set up we can simply trim with ledouts for inside and outside line tention.
Josee Modesto

[Dean Pappas]

Hi Jose,
Hi Gang,
Yes, the reverse rotation prop will involve a re-trim.
Backwards Rabe rudder?    Maybe,
In-thrust?   I doubt it.
Equal versus unequal wing panels?    Same problem just a different flavor.
Forward UP line?           I'm with Igor: every ship seems different.

Yes, less right thrust and maybe less right rudder are needed, as a result of the favorable spiral airflow into yaw effect, but ...

Here's the one that no one is taking into effect, and I think it is the real cause of the "poor" tension in some insides.
With a tractor prop, spiral airflow exerts an outward-rolling torque on the wings and tail.
(You don't believe that I have the direction correct? Go ask a good local pattern flier which way his plane rolls when the throttle is reduced to idle!
They roll left because of the subtle aileron trim used to counterract the effect of spiral airflow on the wings/tail.)
That's why it seems that virtually all trim tabs end up on the underside of the outboard wing, unless the wing has an otherwise obvious warp.
Simply doing a tractor versus pusher test without going through a trim tab adjust or flap re-tweak exercize gives an incomplete picture.

Of course, now the motor current tells us how much torque and how much spiral airflow correction is needed.

Hey, if it was easy, everybody would do it.

Dean P.

[Wynn Robins]

I am waiting in for some more discussion on this - I have a feeling that the pusher I am running on the T-rex I just finished may be causing the issues I am having - mostly light tension on insides .......   

[Dean Pappas]

Hi Wynn,
Spend some time in clean air really looking at wings level upright and inverted.
Dean

[Paul Walker]

Paul Walker briefly mentioned the trim differences from pusher to tractor props. Paul can you elaborate on your findings between the pusher and tractor props and the trim changes to your model.


I have been using the pusher prop for most of the flights on the new plane. I have come to really like it.

However, it didn't start off that way. I started by making a tractor and pusher prop of the same brand and size. With those, I got the lap times within 0.10 sec/lap. I flew the tractor for a number of flights to adjust to is since it was different than the prop I was using previously. Once used to it, I switched to the pusher. On the very first flight, I could feel the advantage of the pusher in most outside maneuvers. I could also feel the change on the insides. At that point, I wasn't sure I was going to adjust to the lighter insides.

I thought about changing the trim to compensate, but only brought the leadouts closer to each other. Then I simply pounded in a couple dozen flights to "adjust" to it. After that time, I could no longer feel the insides getting loose.  I paid close attention to what was going on, and decided that there was proper balance between the insides and outsides. I simply fly it now, and do appreciate the advantage in the hourglass and clover.  Thanks Dennis for getting APC to make that prop.

My conclusion is that we are used to a certain feel with tractor props, and switching to the pusher does reduce the tension on the insides, but in my case it made them more balanced.  However, your results may vary.

[Howard Rush]

How's the first loop of the clover in the wind?

[Paul Walker]

How's the first loop of the clover in the wind?

I does not seem to matter. It still pulls through just fine.

You'll have to fly it before you start building.

PW

[Shultzie]

Paul...
Thanks for the photo of that great new flying field of yours...
''BUILT IT...AND THEY WILL COME!"

[Leo Mehl]

You have to fly left-handed.  I think that's the only solution.  Hope this helps.

You must have been watching me fly!

[Russell Bond]

One thing I found was I had to give UP THRUST (not down thrust) on the motor to make my plane more stable in level flight.
Bruce had the same effect on his plane. In fact, giving it a touch of down thrust made the "hunting" worse.