A poser for Paul Walker

[peabody]

I am asking on the forum because I believe that a lot will be interested in the answer(s).
Paul's "trim chart" is considered by many the "bible" of trimming.
My questions:

1) In retrospect, would you change anything in the process?

2) Is anything in the chart different with electric power?

Thanks!

[Dane Martin]

Thanks for asking what a lot of us were thinking. I'll be watching this also, as the next plane (my stuka) will be electric. I'm especially interested in the lead out / cg positioning. Is it different on planes with electron based fuels vs glo fuels?

[Peter in Fairfax, VA]

You've got to think that CG will differ, based on IC fuel consumption changing CG during the flight.  Also interested to hear.

[Paul Walker]

Yes, there are some differences.

I am on the road currently. Will respond when home in another day or so.

[peabody]

Thanks Paul!
I am interested, too, if you would change anything in trimming an IC....it's been a long time, and I wonder?

[Paul Walker]

First off, my comments reflect the latest version of the flow chart that was in Stunt News several years ago. That was updated from the version of several decades previous.

As far as IC vs Electric in terms of using the chart, there is no difference on how to work through it. However, that doesn't mean they will end up the same!

But, there are differences between IC and electric in how they seem to trim out. The following are the significant differences I have found, and many other electric pilots as well: CG, leadout position, and engine offset.

CG....my data from years of using both.
IC balances at 22 to 26 percent of MAC, without fuel. Around 17 to 20 percent if a full tank of fuel is considered.
Electrics (mine) balance between 8 to 12 percent of MAC. That is a considerable difference!

Leadout position...IC is from 1.0 to 1.5 inches aft of the CG. I have even seen experts fly with the LO's in front of the CG. Oh my!!! On electric, they range from 3.0 to 4.0 inches aft of their CG. Again, that is a huge difference.

Engine/motor offset. IC "usually" is set at ZERO degrees offset. There are exceptions, but most use no offset. Electrics tend to use 2 degrees offset.


So why is this? The CG is related to the motor. Electric has around a dozen torque impulses each revolution versus the IC of one every other revolution. This once every other revolution impulse is much larger and may be contributing to the rate at which the plane can rotate, or not rotate. However my experience with 4 strokes opposes this theroy as they wanted the CG more forward as well. None the less, if you try to fly an electric plane with an IC CG, it won't fly up to its potential. The P-47 had the CG at 8 percent and still cornered tightly.

I have found that engine offset in an electric powered plane is a no brainer. I have tried over and over to attempt to use no offset, but the offset helps in so many places without any negative impacts that I just build it into the plane from the start.

Leadouts...similar to engine offset, I have tried over and over to make the leadouts work in a more "traditional" position. They will work there, and one could convince onesself that it was good, BUT, there is one, or two significent benefits of being aft. Number one is that aft leadouts resist the speeding up in consecutive maneuvers in a good wind. This effect is repeatable, and many other electric fliers that have tried this have found this same benefit. The other bonus is that it tends to damp the wiggles after corners MUCH better than with the leadouts forward. Give it a try, you might like it.

So, for the purposes of the trim chart, these effects can be incorporated into the bench trim before even hitting the field, thus reducing some of the trial and errors the chart can cause.

So, the bottom line is, I see no difference in the way the chart is used but the starting trim is different for IC and eletric.

[peabody]

Thanks Paul! I appreciate your wisdom!

[Crist Rigotti]

Thank you Paul. 

[Peter in Fairfax, VA]

Paul,

Thanks for yesterday's write-up, as well as the trimming guide here:

http://flyinglines.org/pw.trimflow1.html

Some comments/questions:

1. Yesterday's write-up had some excellent "rule of thumb" estimates regarding initial CG and leadout positions, important to first flight setup.  Does the trimming guide include any initial estimates?

2. When you mention leadout position, are the measurements to the midpoint between the two lines?  Is there a suggested spacing width between the two leadouts?

3. Another trimming aspect is the range of throw / ratio between flaps / elevator.  Are there any rules of thumb here, or trimming considerations?

My apologies if these topics have been already addressed and my first reading missed them.  I'm re-entering control-line competition after a long break and have one simple goal - get my pattern points once this season.

thanks,

Peter

[Paul Walker]

Paul,

Thanks for yesterday's write-up, as well as the trimming guide here:

http://flyinglines.org/pw.trimflow1.html 

Some comments/questions:

1. Yesterday's write-up had some excellent "rule of thumb" estimates regarding initial CG and leadout positions, important to first flight setup.  Does the trimming guide include any initial estimates?

[ ] No, the guide did not. There are many different designs, all with different requirements. I would start by setting it up per the plans/kit recommendations.

2. When you mention leadout position, are the measurements to the midpoint between the two lines?  Is there a suggested spacing width between the two leadouts? 

[ ] Yes, mid point. No suggestion.
[ ] I don't think you asked the correct question. The question should be: which line should be in the front? For a conventional prop, counter clockwise looking from the front, the up line should be in the front. For a clockwise rotation, the down line should be in front. This is to counteract the precession from the prop rotation in a corner. The spacing between the two then becomes a function of how much precession your plane has. The spacing can be adjusted to balance your precession levels. Now, you do have independent leadout guides, don't you?

3. Another trimming aspect is the range of throw / ratio between flaps / elevator.  Are there any rules of thumb here, or trimming considerations?

[ ] Again, I would default to the plan recommendations. However, if none exist, I would start at 1:1, same elevator and flap deflections.
[ ] As far as throw goes, you certainly don't need more than 30 degrees travel.

My apologies if these topics have been already addressed and my first reading missed them.  I'm re-entering control-line competition after a long break and have one simple goal - get my pattern points once this season.

thanks,

Peter

Good luck on your goal Peter!

[Gary Dowler]

Paul,
Thank you very much for posting all of that. Most helpful.  My profile P-47 is getting closer. Got the engine yesterday.  Paint will be done soon.

Gary

[James Mills]

Paul,

Thanks for posting, good info.

James

[Vitalis Pilkionis]

For a conventional prop, counter clockwise looking from the front, the up line should be in the front. For a clockwise rotation, the down line should be in front.

I suppose this is only valid for a conventionally mounted bellcrank. For an inverted bellcrank things should change accordingly.

This is to counteract the precession from the prop rotation in a corner. The spacing between the two then becomes a function of how much precession your plane has. The spacing can be adjusted to balance your precession levels. Now, you do have independent leadout guides, don't you?

Sorry, but I can't get it. How can tinkering with leadouts (which affect only elevator) interract with the precession induced yawing. Rabbe rudder certainly can, that I understand. Any explanation please.

Regards,
Vitalis Pilkionis

[Paul Walker]

I suppose this is only valid for a conventionally mounted bellcrank. For an inverted bellcrank things should change accordingly.

I don't know what you are describing here.


Sorry, but I can't get it. How can tinkering with leadouts (which affect only elevator) interract with the precession induced yawing. Rabbe rudder certainly can, that I understand. Any explanation please.

The leadouts apply a fore-aft force on the wingtip. A fore-aft force there translates to a yaw moment, one that acts just like a rudder. That force is dependent on the location of the leadouts.

The flying lines take a natural curved shape based on the drag force of the lines and the line tension you feel at the handle. That natural curve would extend from the aircraft CG to your handle. If the leadouts were positioned exactly at that location, then they would produce no fore-aft force on that wing tip. However, move them either forward of backward and they will produce that fore-aft force. Using this information will produce yaw moments just like a rudder.

Regards,
Vitalis Pilkionis

[Tim Wescott]

I suppose this is only valid for a conventionally mounted bellcrank. For an inverted bellcrank things should change accordingly.

I'm not quite sure where this is coming from, but I believe you're confused on a bit of basic classical mechanics.  Absent of the effects of internal friction and mechanical compliance (springiness), the location and orientation of the bellcrank doesn't matter.  As long as the internals are without significant friction or spring, all that matters is where the the leadouts exit the wings and what they do to the flaps and elevator.  For the purposes of this discussion, the mechanicals inside the airplane can be treated as a black box.  So if the leadouts happen to cross inside the wing, that's immaterial (unless you lose track and wrap them around each other -- then you can't ignore internal friction anymore).

[Ken Culbertson]

Absent of the effects of internal friction and mechanical compliance (springiness), the location and orientation of the bellcrank doesn't matter. 

But since internal friction will be present, I have always tried to get the bellcrank as close to the CG as is practical.

ken

[Joe Ed Pederson]

Paul,

Thanks for yesterday's write-up, as well as the trimming guide here:

http://flyinglines.org/pw.trimflow1.html

Some comments/questions:

3. Another trimming aspect is the range of throw / ratio between flaps / elevator.  Are there any rules of thumb here, or trimming considerations?

[ ] Again, I would default to the plan recommendations. However, if none exist, I would start at 1:1, same elevator and flap deflections.
[ ] As far as throw goes, you certainly don't need more than 30 degrees travel.

For Paul, Ted, or Brett,

Does Paul's answer, "As far as throw goes, you certainly don't need more than 30 degrees travel" apply to flapless models as well?

Thanks,

Joe Ed Pederson

[Vitalis Pilkionis]

I don't know what you are describing here.

I'm sorry for my limited english. Ofcourse I mean reversed, not inverted (see picture below). So has the up line be in the front no matter how the bellcrank is mounted?

For a conventional prop, counter clockwise looking from the front, the up line should be in the front.

Again, sorry, but that is somehow contradicts to what Igor says in this topic: https://stunthanger.com/smf/open-forum/suspended-bellcrank-questions-44764/

"That was already several times analyzed - rear line up and tractor prop is the way"

"If you put up line front, it will, as any other positive feedback in controlls, make positive feedback what will make hunting worse and also more difficult fly offs from corners."



Regards,
Vitalis Pilkionis

[Tim Wescott]

But since internal friction will be present, I have always tried to get the bellcrank as close to the CG as is practical.

Absolutely.  There are folks who believe, however, that bellcrank position by itself has an effect on stability, and lose track of the "as is practical" part of the above guideline.  I wasn't sure if that was the case here, which motivated my comments.

[Tim Wescott]

For Paul, Ted, or Brett,

Does Paul's answer, "As far as throw goes, you certainly don't need more than 30 degrees travel" apply to flapless models as well?

Thanks,

Joe Ed Pederson

It depends on the model, but mostly yes.  And if it's a Ringmaster or other old beast with a huge elevator, you need even less.

[Paul Walker]

I'm sorry for my limited english. Ofcourse I mean reversed, not inverted (see picture below). So has the up line be in the front no matter how the bellcrank is mounted?


Again, sorry, but that is somehow contradicts to what Igor says in this topic: https://stunthanger.com/smf/open-forum/suspended-bellcrank-questions-44764/

"That was already several times analyzed - rear line up and tractor prop is the way"

"If you put up line front, it will, as any other positive feedback in controlls, make positive feedback what will make hunting worse and also more difficult fly offs from corners."



Regards,
Vitalis Pilkionis

Well, that is clearly Igor's preference.

My preference is to do what I suggest.