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Thrust line...?

 
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nickc(at)mtaonline.net
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PostPosted: Sat Apr 03, 2021 4:50 pm    Post subject: Thrust line...? Reply with quote

Kolbers,

I have been interest in the talk about desired thrust line. I took a piece line and attached to the front of my engine mount plate, I then walked it back to the tail, and lowered the string until it was making contact with the whole upper surface of the mount plate. The string ends up 4 inches above the end of the boom tube. The center of the prop is 8 inches above the string line…i.e.12’’ above the top of the boom tube end.

My Kolbra is the prototype, of the Kolbra’s.

Nick Cassara
Palmer, Alaska


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neilsenrm(at)gmail.com
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PostPosted: Sun Apr 04, 2021 8:27 am    Post subject: Thrust line...? Reply with quote

Nick/All 

It seems like John Hawk did some testing to see the effects of thrust line changes and found no differences. From that I decided that the best thrust line would be directly in line with the direction of flight.
 
There are two factors that are part of the equation, center of drag and center of balance. The prop on most Kolbs is above the center of drag so we get pitch changes with power changes. Hang your plane from the center of balance (CG) position. The CG is forward of the position of the prop but also fairly close to the CG. Press down on the prop and the plane pitches up, lift and it pitches down. So if you set the thrust line a bit down from the direction of flight the plane will pitch down more than it already does with power increases, not good. Set the thrust line a bit up and the plane will pitch down less with power increases which is good but it pushes the plane down and that is really not a good thing.
 
The moment arm between the CG and the prop is very short in comparison to the moment arm of the horizontal stabilizer and elevators so any pitch considerations are most efficiently handled by the tail. So I suggest that the prop thrust line be set directly in line with the direction of flight which  is something around 3-5 degrees up from the bottom of the wing.
  As usual this is worth what you paid for it.
Rick Neilsen
Redrive VW Powered MKIIIC


On Sat, Apr 3, 2021 at 8:52 PM Nick Cassara <nickc(at)mtaonline.net (nickc(at)mtaonline.net)> wrote:

Quote:
--> Kolb-List message posted by: Nick Cassara <nickc(at)mtaonline.net (nickc(at)mtaonline.net)>

Kolbers,

I have been interest in the talk about desired thrust line. I took a piece line and attached to the front of my engine mount plate, I then walked it back to the tail, and lowered the string until it was making contact with the whole upper surface of the mount plate. The string ends up 4 inches above the end of the boom tube. The center of the prop is 8 inches above the string line…i.e.12’’ above the top of the boom tube end.

My Kolbra is the prototype, of the Kolbra’s.

Nick Cassara
Palmer, Alaska

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Richard Pike



Joined: 09 Jan 2006
Posts: 1669
Location: Blountville, Tennessee

PostPosted: Sun Apr 04, 2021 6:36 pm    Post subject: Re: Thrust line...? Reply with quote

Appreciate your thoughts Rick. After a couple weeks of experimenting with this rig, puzzling over it, and reading this, I think tomorrow I will order a couple more longer bolts so that I can get the engine and thrust line back to what it was originally (like Kolb had it), just a couple inches higher, because so far, the closer I get it to original stock, the better it flies. It just needs a bit more elevation so the prop will clear the tube when the thrust line is stock.

Wind has been too contrary & trashy to get any accurate readings yet, but so far the 3.47:1 box & 72" three blade prop looks like an amazing improvement.


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_________________
Richard Pike
Kolb MKIII N420P (420ldPoops)
Kingsport, TN 3TN0

Forgiving is tough, being forgiven is wonderful, and God's grace really is amazing.
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williamtsullivan(at)att.n
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PostPosted: Mon Apr 05, 2021 6:33 am    Post subject: Thrust line...? Reply with quote

Richard- I believe Jack Hart did a lot of experiments with this, and also made a simple jig to cut down a prop (if memory is correct).
Here is the link to his stuff: http://www.jackbhart.com/firefly/fireflyindex.html
Everybody on the List ought to bookmark it.

Bill Sullivan

On Sunday, April 4, 2021, 10:37:18 PM EDT, Richard Pike <thegreybaron(at)charter.net> wrote:




--> Kolb-List message posted by: "Richard Pike" <thegreybaron(at)charter.net (thegreybaron(at)charter.net)>

Appreciate your thoughts Rick. After a couple weeks of experimenting with this rig, puzzling over it, and reading this, I think tomorrow I will order a couple more longer bolts so that I can get the engine and thrust line back to what it was originally (like Kolb had it), just a couple inches higher, because so far, the closer I get it to original stock, the better it flies. It just needs a bit more elevation so the prop will clear the tube when the thrust line is stock.

Wind has been too contrary & trashy to get any accurate readings yet, but so far the 3.47:1 box & 72" three blade prop looks like an amazing improvement.

--------

Richard Pike

Kolb MKIII N420P (420ldPoops)

Kingsport, TN 3TN0

Forgiving is tough, being forgiven is wonderful, and God's grace really is amazing.

Read this topic online here:

http://forums.matronics.com/viewtopic.php?p=501264#501264

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