Today's Message Index:
----------------------
1. 04:24 AM - Re: Reducing power on take off versus thrust line. (Richard Halstrick)
2. 05:24 AM - Re: Kolb-List Digest: 1 Msgs - 09/26/21 (Peter Cowan)
3. 05:47 AM - Thrustline again. (Peter Cowan)
4. 06:48 AM - Re: Thrustline again. (John Hauck)
5. 12:13 PM - Re: Re: Kolb-List Digest: 1 Msgs - 09/26/21 (Rick Neilsen)
Message 1
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| Subject: | Re: Reducing power on take off versus thrust line. |
The changes pitch force produced by the engine in an airplane is a result
of the distance ABOVE or BELOW the CG of the plane. Engines mounted above
the CG of the airplane produce a moment (twisting force) opposite to the
force produced by the elevator and airplanes with low mounted engines (jets
with engines under the wings) produce a force that compliments the elevator
.
The moment produced is just the perpendicular component of thrust to the CG
multiplied by the distance from the CG. Tilting an engine up 6 degrees
when it is located above and behind the CG will aggravate the situation by
increasing the lever arm of the thrust to the CG. Tilting it down 6 degrees
will decrease the moment by decreasing the forward component of thrust by
the cosine of the angle (6 degrees would be about 1%).
I=99m new to this forum and in the spirit of full disclosure, I do no
t own a
Kolb aircraft and have never rode in one. I ordered a Twinstar last year
and hope to start building it this month when I receive it from the factory
.
Rich Halstrick
Message 2
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| Subject: | Re: Kolb-List Digest: 1 Msgs - 09/26/21 |
Rick, I get your point about forces at the tail having more leverage than
forces closer to the wings. I don't know how this would compare from an
efficiency point of view. I am not an aero engineer either. But I go back
and forth between pushers and tractors and I'd kind of like to have planes
that fly similarly. So I want a plane where power on makes you go up and
power off lets you go down and it seems to be easy to get that with down
thrust.
If you were into model airplanes you remember that the first thing you'd do
is trim the plane to glide nicely. Then adjust the engine thrust. That's
about what I'm doing.
p
On Mon., Sep. 27, 2021, 2:40 a.m. Kolb-List Digest Server, <
kolb-list@matronics.com> wrote:
> *
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> =================================================
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> HTML Version:
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> Total Messages Posted Sun 09/26/21: 1
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>
> 1. 11:25 AM - Re: Reducing power on take off versus thrust line.
> (Rick Neilsen)
>
>
> ________________________________ Message 1
> _____________________________________
>
>
> Time: 11:25:41 AM PST US
> From: Rick Neilsen <neilsenrm@gmail.com>
> Subject: Re: Kolb-List: Reducing power on take off versus thrust line.
>
> Food for thought. Setting the engine/prop thrust line so that any of it is
> pushing the plane down on a Kolb just doesn't seem to make the best use of
> available thrust. Kolb airplanes have a high thrust line in relation to the
> center of drag. You talk about setting about 6 degrees up thrust so that it
> reduces the pitch down effect of adding power. The amount of thrust
> necessary to push the tail down by the prop thrust must be huge. The prop
> is a short distance or leverage arm behind the center of lift. The elevator
> is a considerably longer leverage arm length behind the center of lift so
> the amount of down force required to cause a pitch up or just counter the
> prop thrust is much less. Seems like you would have reduced forward thrust
> and the wings would have to work harder to counter the added downforce and
> in doing so added drag.
>
> A new pilot needs to adjust to the pitching forces with power changes. It
> is just the way Kolbs fly. My first VW engine mount had a very high thrust
> line. My 72 inch prop had a 7 inch clearance above the boom tube. I
> couldn't go to full power until I got some speed where I had enough
> elevator power to keep from pitching over on the nose. The second mount
> lowered the trust line by 6 inches and that made a huge difference but I
> still have to adjust to pitch force changes.
>
> As usual my advice is worth what you paid for it.
>
> Rick Neilsen
> Redrive VW Powered MKIIIC
>
> On Fri, Sep 24, 2021 at 11:55 AM John Hauck <jhauck36@outlook.com> wrote:
>
> > Don=99t think I said I was OK with reducing power on takeoff, but i
> t did get
> > me out of trouble on my first take off on my very short air strip. Never
> > had to reduce power to takeoff after the first one. Just pull the stick
> > back further. ;-)
> >
> >
> > Well, I think you misunderstood my initial explanation of my experience
> > with my =9Cfirst=9D take off after changing out an 80 for a 1
> 00 hp rotax. I
> > think I explained something about =9Cmuscle memory=9D. I had
> more than 2,000
> > hours flying in front of an 80 hp on my MKIII.
> >
> >
> > My second takeoff and another 1,600 hours in front of the 100 hp engine
> > never proved to be as problem. Actually, never thought about it again.
> It
> > too became muscle memory.
> >
> >
> > I don=99t think the =9Cpitch down=9D as power comes up
> is rocket science. It=99s
> > like pushing on a lever. You can help overcome it in several ways. I
> > changed angle of attack of my horizontal stabiliizers and came up with an
> > effective and simple forced pitch trim system. But on takeoff, just pull
> > the stick back a little more. ;-)
> >
> >
> > Again, my first take off after engine swap caught me by surprise, but not
> > again.
> >
> >
> > Over the 36 years and about 6,000 hours building, experimenting, and
> > flying Kolb aircraft, I have a pretty good idea how they work. I may hav
> e
> > a difficult time explaining that to someone who has little or no experien
> ce
> > flying Kolbs, but I try. Sorry you all did not understand what I was
> > trying to mumble.
> >
> >
> > john h
> >
> > Titus, Alabama =93 Kolb Factory Pilot for Homer Kolb, Bruce Chesnut
> , and
> > Bryan Melborn, Retired ;-)
> >
> >
> > *From:* owner-kolb-list-server@matronics.com [mailto:
> > owner-kolb-list-server@matronics.com] *On Behalf Of *Peter Cowan
> > *Sent:* Friday, September 24, 2021 9:53 AM
> > *To:* kolb-list@matronics.com
> > *Subject:* Kolb-List: Reducing power on take off versus thrust line.
> >
> >
> > This topic came up some months back and there was quite a good analysis o
> f
> > the forces involved so I was surprised when John mentioned that after goi
> ng
> > to his higher horsepower engine he seemed to be okay with having to reduc
> e
> > power to complete a normal take off.
> >
> >
> > Surely this is a condition that is abnormal and one we would not want to
> > have in our aircraft, especially if it is avoidable. And it is!
> >
> >
> > I was experimenting with this myself last March and subsequently notes
> > from Boyd young and Richard Swiderski in April did a pretty good job of
> > analyzing the forces involved.
> >
> >
> > I had recently purchased a Beaver pusher that was designed for a 503 or
> > 582 but had a 912 80 in it. Both the owner and the builder had experience
> d
> > this problem of requiring power reduction in order to rotate and their
> > conclusion was, like Johns, that it was just excess power doing it and
> > there was no alternative. Both suggested I should just go back to lower
> > power.
> >
> >
> > This came as a surprise and a challenge to me. Like the previous posters
> > who provided a sound analysis of the forces involved, I finally discovere
> d,
> > with the primary source being a model airplane design site, that providin
> g
> > upthrust on a high, aft mounted engine was exactly the reverse of what wa
> s
> > required.
> >
> >
> > My one circuit flight with this additional positive thrustline was almost
> > more than I could handle in terms of requiring a huge amount of back
> > pressure to stay airborne. A more positive thrustline actully produced DO
> WN
> > THRUST.
> >
> >
> > After learning how things actually worked and putting in close to minus
> > six degrees down on the thrustline, the plane has become a jem in handlin
> g
> > with full power all through the takeoff roll AND NO STICK FORCE change
> > going from low-power to high-power and back in level flight.
> >
> >
> > I do plan to add a little more down in order to get a nose UP on power
> > application and improve the trim it needs now.
> >
> >
> > Please tell me if I am missing something.
> >
> > Peter
> >
>
>
Message 3
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| Subject: | Thrustline again. |
One last thought on this is that the 503 powered Kolb that I had flew just
fine. And I would bet a 503 Beaver/Skywatch would be the same. But if I'm
going to go wild with power, why not use some of that power to keep the
trim inline and retain reasonable flight characteristics?
On Tue., Sep. 28, 2021, 2:40 a.m. Kolb-List Digest Server, <
kolb-list@matronics.com> wrote:
> *
>
> =================================================
> Online Versions of Today's List Digest Archive
> =================================================
>
> 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 21-09-27&Archive=Kolb
>
> Text Version:
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> http://www.matronics.com/digest/digestview.php?Style=82701&View=txt&Chapter 21-09-27&Archive=Kolb
>
>
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> Total Messages Posted Mon 09/27/21: 0
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> Today's Message Index:
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Message 4
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| Subject: | Thrustline again. |
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| Subject: | Re: Kolb-List Digest: 1 Msgs - 09/26/21 |
Pete
I'm not an engineer of any kind and I really don't want to beat this issue
to death.
There are some basic design elements that make airplanes fly. We can tinker
with airplane designs to get what we want but there are usually negative
trade-offs and sometimes... the effect is beneficial. My Kolb MKIIIC has
super visibility, which is mostly why I built mine. The only trade-off is
the pitch changes with power changes. Homer originally designed a lower
engine mount with a higher boom tube and I assume it had less pitch changes
with power changes. If it was so simple as to pitch the thrust line up then
we all would be flying ours that way. The performance hit with having the
prop thrust push the tail down is too much for most of us. I don't see any
safety issues so whatever makes you happy go for it.
Your model airplane comparison is fine when you have so much extra power
that you can climb vertically and need neutral stability necessary for the
control system.
Also someone one else talked about the cause being CG related. The first
Kolb airplanes had their engines behind the CG but the thrust line was
closer to the center of drag which didn't have the pitch down with power
increases. It was the change to higher thrust line models that now have the
pitch changes. I think the newer designs have the engine further forward
and closer to the CG.
Again worth what you paid for it.
Rick Neilsen
1st Redrive VW Powered MKIIIC
On Tue, Sep 28, 2021 at 8:28 AM Peter Cowan <cowan.phc@gmail.com> wrote:
> Rick, I get your point about forces at the tail having more leverage than
> forces closer to the wings. I don't know how this would compare from an
> efficiency point of view. I am not an aero engineer either. But I go back
> and forth between pushers and tractors and I'd kind of like to have planes
> that fly similarly. So I want a plane where power on makes you go up and
> power off lets you go down and it seems to be easy to get that with down
> thrust.
> If you were into model airplanes you remember that the first thing you'd
> do is trim the plane to glide nicely. Then adjust the engine thrust. That's
> about what I'm doing.
> p
>
> On Mon., Sep. 27, 2021, 2:40 a.m. Kolb-List Digest Server, <
> kolb-list@matronics.com> wrote:
>
>> *
>>
>> ========================
>> Online Versions of Today's List Digest Archive
>> ========================
>>
>> 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 21-09-26&Archive=Kolb
>>
>> Text Version:
>>
>>
>> http://www.matronics.com/digest/digestview.php?Style=82701&View=txt&Chapter 21-09-26&Archive=Kolb
>>
>>
>> ======================
>> EMail Version of Today's List Digest Archive
>> ======================
>>
>>
>> ----------------------------------------------------------
>> Kolb-List Digest Archive
>> ---
>> Total Messages Posted Sun 09/26/21: 1
>> ----------------------------------------------------------
>>
>>
>> Today's Message Index:
>> ----------------------
>>
>> 1. 11:25 AM - Re: Reducing power on take off versus thrust line.
>> (Rick Neilsen)
>>
>>
>>
>> ________________________________ Message 1
>> _____________________________________
>>
>>
>> Time: 11:25:41 AM PST US
>> From: Rick Neilsen <neilsenrm@gmail.com>
>> Subject: Re: Kolb-List: Reducing power on take off versus thrust line.
>>
>> Food for thought. Setting the engine/prop thrust line so that any of it is
>> pushing the plane down on a Kolb just doesn't seem to make the best use of
>> available thrust. Kolb airplanes have a high thrust line in relation to
>> the
>> center of drag. You talk about setting about 6 degrees up thrust so that
>> it
>> reduces the pitch down effect of adding power. The amount of thrust
>> necessary to push the tail down by the prop thrust must be huge. The prop
>> is a short distance or leverage arm behind the center of lift. The
>> elevator
>> is a considerably longer leverage arm length behind the center of lift so
>> the amount of down force required to cause a pitch up or just counter the
>> prop thrust is much less. Seems like you would have reduced forward thrust
>> and the wings would have to work harder to counter the added downforce and
>> in doing so added drag.
>>
>> A new pilot needs to adjust to the pitching forces with power changes. It
>> is just the way Kolbs fly. My first VW engine mount had a very high thrust
>> line. My 72 inch prop had a 7 inch clearance above the boom tube. I
>> couldn't go to full power until I got some speed where I had enough
>> elevator power to keep from pitching over on the nose. The second mount
>> lowered the trust line by 6 inches and that made a huge difference but I
>> still have to adjust to pitch force changes.
>>
>> As usual my advice is worth what you paid for it.
>>
>> Rick Neilsen
>> Redrive VW Powered MKIIIC
>>
>> On Fri, Sep 24, 2021 at 11:55 AM John Hauck <jhauck36@outlook.com> wrote:
>>
>> > Don=99t think I said I was OK with reducing power on takeoff, but i
>> t did get
>> > me out of trouble on my first take off on my very short air strip.
>> Never
>> > had to reduce power to takeoff after the first one. Just pull the stick
>> > back further. ;-)
>> >
>> >
>> > Well, I think you misunderstood my initial explanation of my experience
>> > with my =9Cfirst=9D take off after changing out an 80 for a 1
>> 00 hp rotax. I
>> > think I explained something about =9Cmuscle memory=9D. I had
>> more than 2,000
>> > hours flying in front of an 80 hp on my MKIII.
>> >
>> >
>> > My second takeoff and another 1,600 hours in front of the 100 hp engine
>> > never proved to be as problem. Actually, never thought about it
>> again.
>> It
>> > too became muscle memory.
>> >
>> >
>> > I don=99t think the =9Cpitch down=9D as power comes up
>> is rocket science. It=99s
>> > like pushing on a lever. You can help overcome it in several ways. I
>> > changed angle of attack of my horizontal stabiliizers and came up with
>> an
>> > effective and simple forced pitch trim system. But on takeoff, just
>> pull
>> > the stick back a little more. ;-)
>> >
>> >
>> > Again, my first take off after engine swap caught me by surprise, but
>> not
>> > again.
>> >
>> >
>> > Over the 36 years and about 6,000 hours building, experimenting, and
>> > flying Kolb aircraft, I have a pretty good idea how they work. I may
>> hav
>> e
>> > a difficult time explaining that to someone who has little or no
>> experien
>> ce
>> > flying Kolbs, but I try. Sorry you all did not understand what I was
>> > trying to mumble.
>> >
>> >
>> > john h
>> >
>> > Titus, Alabama =93 Kolb Factory Pilot for Homer Kolb, Bruce Chesnut
>> , and
>> > Bryan Melborn, Retired ;-)
>> >
>> >
>> > *From:* owner-kolb-list-server@matronics.com [mailto:
>> > owner-kolb-list-server@matronics.com] *On Behalf Of *Peter Cowan
>> > *Sent:* Friday, September 24, 2021 9:53 AM
>> > *To:* kolb-list@matronics.com
>> > *Subject:* Kolb-List: Reducing power on take off versus thrust line.
>> >
>> >
>> > This topic came up some months back and there was quite a good analysis
>> o
>> f
>> > the forces involved so I was surprised when John mentioned that after
>> goi
>> ng
>> > to his higher horsepower engine he seemed to be okay with having to
>> reduc
>> e
>> > power to complete a normal take off.
>> >
>> >
>> > Surely this is a condition that is abnormal and one we would not want
>> to
>> > have in our aircraft, especially if it is avoidable. And it is!
>> >
>> >
>> > I was experimenting with this myself last March and subsequently notes
>> > from Boyd young and Richard Swiderski in April did a pretty good job of
>> > analyzing the forces involved.
>> >
>> >
>> > I had recently purchased a Beaver pusher that was designed for a 503 or
>> > 582 but had a 912 80 in it. Both the owner and the builder had
>> experience
>> d
>> > this problem of requiring power reduction in order to rotate and their
>> > conclusion was, like Johns, that it was just excess power doing it and
>> > there was no alternative. Both suggested I should just go back to lower
>> > power.
>> >
>> >
>> > This came as a surprise and a challenge to me. Like the previous
>> posters
>> > who provided a sound analysis of the forces involved, I finally
>> discovere
>> d,
>> > with the primary source being a model airplane design site, that
>> providin
>> g
>> > upthrust on a high, aft mounted engine was exactly the reverse of what
>> wa
>> s
>> > required.
>> >
>> >
>> > My one circuit flight with this additional positive thrustline was
>> almost
>> > more than I could handle in terms of requiring a huge amount of back
>> > pressure to stay airborne. A more positive thrustline actully produced
>> DO
>> WN
>> > THRUST.
>> >
>> >
>> > After learning how things actually worked and putting in close to minus
>> > six degrees down on the thrustline, the plane has become a jem in
>> handlin
>> g
>> > with full power all through the takeoff roll AND NO STICK FORCE change
>> > going from low-power to high-power and back in level flight.
>> >
>> >
>> > I do plan to add a little more down in order to get a nose UP on power
>> > application and improve the trim it needs now.
>> >
>> >
>> > Please tell me if I am missing something.
>> >
>> > Peter
>> >
>>
>>
>>
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