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smgates
Joined: 13 Apr 2019
Posts: 13
Location: Perth, Australia
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 Posted: Sun Apr 04, 2021 10:51 pm Post subject: Kolb-List Digest: 3 Msgs - 04/04/21 |
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Thanks Rick - that does make sense.
Steve Gates
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slingshot003
Joined: 10 Mar 2017
Posts: 28
Location: FL USA
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| Posted: Mon Apr 05, 2021 9:44 am Post subject: Kolb-List Digest: 3 Msgs - 04/04/21 |
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It seems like there is a lot of confusion regarding the effect of the thrustline on our Kolbs. I like to take things to their extreme to better visualize how things work, so bear with me here. Let's assume the engine/redrive have no weight & we can place them wherever we choose in relation to the cg. If we place it at the nose of our plane, a downward thrustline will clearly produce a similar downward force.  Similarly, an upward thrustline will produce and upward force. Now if we place the engine at the tail, all of the above thrustline adjustments will have an opposite effect, eg, downward will produce an up force & upward a down force. Therefore, since the engine is in reality, behind the cg, changing the thrustline downward will add to the upward force on our Kolbs. The only thing questionable is "how much change?". For the sake of this exercise, we can ignore the effect of the moment arm of our raised engine above the center of drag, because it is a constant, it is built into our design -we are only considering changes due to thrustline change. If our engine is 1.5 feet (guessing) behind cg; & if we have 250lbs of thrust (guessing); &  we point the thrustline 90 degrees down then we will have 350 lbs of force trying to rotate our plane upwards. But if we only change the thrustline 3 degrees downward (3.33% of 90) then there is only 11.7 lbs of upward rotational force being added to whatever other forces are in reality present. What I think we can safely take away from this thought experiment is that the effect of a small downward change in thrustline should produce a small upward change. How significant this change is, I don't know, but intuition tells me it at most would be felt at the level of a trimming force & probably not noticeable at all. Â
I have personally been thinking about this a lot as I am installing a Hirth 3202 which has a .75 to 1 inch higher thrustline than the Rotax. I plan to test the effect of changing the thrustline myself. But then a harder question still remains (assuming the change is noticeable): since any change of thrustline off of the actual line-of-flight is wasted energy, we have to ask if this "waste" of energy to accomplish better trim would be more efficiently delt with by the induced drag of changing trim at the tail? Which is the most effective way of extracting the most energy from our thrust, as both methods of trim take away from our forward speed and upward lift? This is a more formidable question to answer for me. I am sure there is an answer, but I haven't put in the effort yet to find it. Maybe someone here already has this knowledge to share? -Richard Swiderski
On Mon, Apr 5, 2021 at 2:39 AM Kolb-List Digest Server <kolb-list(at)matronics.com (kolb-list(at)matronics.com)> wrote:
| Quote: | *
 =========================
  Online Versions of Today's List Digest Archive
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Today's complete Kolb-List Digest can also be found in either of the
two Web Links listed below. The .html file includes the Digest formatted
in HTML for viewing with a web browser and features Hyperlinked Indexes
and Message Navigation. The .txt file includes the plain ASCII version
of the Kolb-List Digest and can be viewed with a generic text editor
such as Notepad or with a web browser.
HTML Version:
  http://www.matronics.com/digest/digestview.php?Style=82701&View=html&Chapter=2021-04-04&Archive=Kolb
Text Version:
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  EMail Version of Today's List Digest Archive
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      ----------------------------------------------------------
              Kolb-List Digest Archive
                   ---
           Total Messages Posted Sun 04/04/21: 3
      ----------------------------------------------------------
Today's Message Index:
----------------------
   1. 09:27 AM - Re: Thrust line...? (Rick Neilsen)
   2. 07:37 PM - Re: Thrust line...? (Richard Pike)
   3. 09:32 PM - Re: Kolb-List Digest: 2 Msgs - 03/30/21 (chic)
________________________________Â Message 1Â _____________________________________
Time: 09:27:35 AM PST US
From: Rick Neilsen <neilsenrm(at)gmail.com (neilsenrm(at)gmail.com)>
Subject: Re: Thrust line...?
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:
>
> Kolbers,
>
> I have been interest in the talk about desired thrust line. I took a piec
e
> 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 linei.e.12=99=99 above the top of the boo
m tube end.
>
> My Kolbra is the prototype, of the Kolbra=99s.
>
> Nick Cassara
> Palmer, Alaska
>
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>
________________________________Â Message 2Â _____________________________________
Time: 07:37:12 PM PST US
Subject: Re: Thrust line...?
From: "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
________________________________Â Message 3Â _____________________________________
Time: 09:32:36 PM PST US
Subject: Re: Kolb-List Digest: 2 Msgs - 03/30/21
From: "chic" <gcechini(at)msn.com (gcechini(at)msn.com)>
You have a 750 LB gross weight and that is what you should watch along with the
weight and balance which is critical......How much does your plane weigh? Mine
is about 400lbs empty so that will leave about 350lbs for people, fuel, & any
luggage/coolers ect. 10 gal gas is 60lbs so that leaves 290lbs in mine for pilot/pass.
I am 6'4" and I fit in mine so your height should work and if you are
290lbs or less you would work in mine! Hope that helps
Read this topic online here:
http://forums.matronics.com/viewtopic.php?p=501265#501265
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lcottrell
Joined: 29 May 2006
Posts: 1494
Location: Jordan Valley, Or
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| Posted: Mon Apr 05, 2021 11:04 am Post subject: Kolb-List Digest: 3 Msgs - 04/04/21 |
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I've followed this thread through all the different replies, and I once rode in a Mark three that dived for the ground when full power was pushed. Didn't like it at all, and didn't feel that it was airworthy either. Anyway I was curious about the difference in my HKS on my firestar. So I stuck the ?protractor? on the bottom of the wing as it sat in my hanger and found that the bottom of the wing measured 38 degrees, I then stuck it on the engine plate and found that it read 30 degrees. So my 60 horse engine is 8 degrees flatter than the wing.
With this configuration and trimmed for straight and level flight at 5400 rpm's. Max sustained cruise is 5800 for the HKS. If I go to full throttle of 6100 rpms the plane climbs, rather than nosing down. In other words I can climb at neutral input from the stick.
Not real sure if any of this answers your question or not, but it seems to me that a difference of 8 degrees flatter compared to the wing should get one to where the plane operates best.
for what its worth.
Larry
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jhauck
Joined: 02 Jan 2020
Posts: 26
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| Posted: Mon Apr 05, 2021 12:36 pm Post subject: Kolb-List Digest: 3 Msgs - 04/04/21 |
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Wing and horizontal stabilizer, prop wash, all play into the picture.
My Ultrastar reacted to power as your FSII. Increase power and it would climb. Of course the US had a low thrust line.
Had my original FS rigged and trimmed to climb with power, descend with power reduced.
Not with the MKIII.
The MKIII flew straight and level at idle with zero nose up trim. Come in with the power and it took a lot of nose up trim to relieve pressure on the stick.
I did a lot of experimentation over the years with thrust lines, changing vertical stab position, horz stab angle of attack, and on and on. Discovered it best to leave thrust line as designed, change angle of attack of horz stab, and have a good pitch trim mechanism, basically nose up trim.
Nose down pitch when power is applied to a high thrust line engine is normal. Got to do something to react to that force.
John h
Honest John’s Fish Camp
Melbourne Beach, Florida – 1,080 miles into our second Great Loop adventure.
From: owner-kolb-list-server(at)matronics.com [mailto:owner-kolb-list-server(at)matronics.com] On Behalf Of Larry Cottrell
Sent: Monday, April 5, 2021 3:04 PM
To: kolb-list(at)matronics.com
Subject: Kolb-List: Re: Kolb-List Digest: 3 Msgs - 04/04/21
I've followed this thread through all the different replies, and I once rode in a Mark three that dived for the ground when full power was pushed. Didn't like it at all, and didn't feel that it was airworthy either. Anyway I was curious about the difference in my HKS on my firestar. So I stuck the ?protractor? on the bottom of the wing as it sat in my hanger and found that the bottom of the wing measured 38 degrees, I then stuck it on the engine plate and found that it read 30 degrees. So my 60 horse engine is 8 degrees flatter than the wing.
With this configuration and trimmed for straight and level flight at 5400 rpm's. Max sustained cruise is 5800 for the HKS. If I go to full throttle of 6100 rpms the plane climbs, rather than nosing down. In other words I can climb at neutral input from the stick.
Not real sure if any of this answers your question or not, but it seems to me that a difference of 8 degrees flatter compared to the wing should get one to where the plane operates best.
for what its worth.
Larry
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neilsenrm(at)gmail.com
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| Posted: Mon Apr 05, 2021 1:18 pm Post subject: Kolb-List Digest: 3 Msgs - 04/04/21 |
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Larry
This maybe this is where real world smacks theory. Ok so put that protractor on the wing in flight at cruise. If you have around 4 degrees of pitch on the wing to maintain level flight then you have your thrust line pushing down and that would explain why you aren't getting pitch trim changes with power changes. I would think that you would need more than 4 degrees upthrust to get there but what do I know.
I set my pitch trim so my MKIIIC will cruise with just a bit of down pitch because I like it that way. Full power requires a bit of back pressure. When I pull power to land I need to push to get the nose down till I add a notch of flaps then it is neutral again. It is the way it is and feels normal to me. My thrust line is app. 4 degrees flatter or up from my wing. My motor mount is welded at 4 degrees with no way to change it.
Ok so what is the best thrust line? It would seem that a thrust line that pushes down on the plane would require a bit more pitch up and more drag and maybe some wasted thrust but what do I know?? If you get no pitch changes with power changes that is a good thing too. Theory vs real world?
I think John H is out playing on his boat doing the 2nd great loop.
Rick Neilsen
Redrive VW Powered MKIIIC
On Mon, Apr 5, 2021 at 3:07 PM Larry Cottrell <lcottrell1020(at)gmail.com (lcottrell1020(at)gmail.com)> wrote:
| Quote: | I've followed this thread through all the different replies, and I once rode in a Mark three that dived for the ground when full power was pushed. Didn't like it at all, and didn't feel that it was airworthy either. Anyway I was curious about the difference in my HKS on my firestar. So I stuck the ?protractor? on the bottom of the wing as it sat in my hanger and found that the bottom of the wing measured 38 degrees, I then stuck it on the engine plate and found that it read 30 degrees. So my 60 horse engine is 8 degrees flatter than the wing.
With this configuration and trimmed for straight and level flight at 5400 rpm's. Max sustained cruise is 5800 for the HKS. If I go to full throttle of 6100 rpms the plane climbs, rather than nosing down. In other words I can climb at neutral input from the stick.
Not real sure if any of this answers your question or not, but it seems to me that a difference of 8 degrees flatter compared to the wing should get one to where the plane operates best.
for what its worth.
Larry
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lcottrell
Joined: 29 May 2006
Posts: 1494
Location: Jordan Valley, Or
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| Posted: Mon Apr 05, 2021 1:38 pm Post subject: Kolb-List Digest: 3 Msgs - 04/04/21 |
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Perhaps a bit better measurement of the angles will tell us what we need to know. One gets used to what ever we use often enough, I just like the fact that when I input throttle the only thing that happens is I go faster or slower depending on which way I push it. At my age simple things are better.
John is back on the Great Loop and moored close to the intercoastal waterway on the East side of Florida at the moment enjoying life to the fullest.
Larry
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