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                           Kolb-List Digest Archive
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                     Total Messages Posted Sun 09/26/21: 1
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Today's Message Index:
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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
>

 
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