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Reducing power on take off versus thrust line.

 
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phcpilot



Joined: 24 May 2016
Posts: 16

PostPosted: Fri Sep 24, 2021 6:53 am    Post subject: Reducing power on take off versus thrust line. Reply with quote

This topic came up some months back and there was quite a good analysis of the forces involved so I was surprised when John mentioned that after going to his higher horsepower engine he seemed to be okay with having to reduce 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 experienced 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 discovered, with the primary source being a model airplane design site, that providing upthrust on a high, aft mounted engine was exactly the reverse of what was 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 DOWN 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 handling 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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jhauck



Joined: 02 Jan 2020
Posts: 21

PostPosted: Fri Sep 24, 2021 7:53 am    Post subject: Reducing power on take off versus thrust line. Reply with quote

Don’t think I said I was OK with reducing power on takeoff, but it 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. Wink

Well, I think you misunderstood my initial explanation of my experience with my “first” take off after changing out an 80 for a 100 hp rotax. I think I explained something about “muscle memory”. 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’t think the “pitch down” as power comes up is rocket science. It’s 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. Wink

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 have a difficult time explaining that to someone who has little or no experience flying Kolbs, but I try. Sorry you all did not understand what I was trying to mumble.

john h
Titus, Alabama – Kolb Factory Pilot for Homer Kolb, Bruce Chesnut, and Bryan Melborn, Retired Wink

From: owner-kolb-list-server(at)matronics.com [mailto:owner-kolb-list-server(at)matronics.com] On Behalf Of Peter Cowan
Sent: Friday, September 24, 2021 9:53 AM
To: kolb-list(at)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 of the forces involved so I was surprised when John mentioned that after going to his higher horsepower engine he seemed to be okay with having to reduce 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 experienced 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 discovered, with the primary source being a model airplane design site, that providing upthrust on a high, aft mounted engine was exactly the reverse of what was 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 DOWN 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 handling 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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neilsenrm(at)gmail.com
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PostPosted: Sun Sep 26, 2021 10:25 am    Post subject: Reducing power on take off versus thrust line. Reply with quote

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(at)outlook.com (jhauck36(at)outlook.com)> wrote:

Quote:

Don’t think I said I was OK with reducing power on takeoff, but it 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.  Wink
 
Well, I think you misunderstood my initial explanation of my experience with my “first” take off after changing out an 80 for a 100 hp rotax.  I think I explained something about “muscle memory”.  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’t think the “pitch down” as power comes up is rocket science.  It’s 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.  Wink
 
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 have a difficult time explaining that to someone who has little or no experience flying Kolbs, but I try.  Sorry you all did not understand what I was trying to mumble.
 
john h
Titus, Alabama – Kolb Factory Pilot for Homer Kolb, Bruce Chesnut, and Bryan Melborn, Retired  Wink
 
From: owner-kolb-list-server(at)matronics.com (owner-kolb-list-server(at)matronics.com) [mailto:owner-kolb-list-server(at)matronics.com (owner-kolb-list-server(at)matronics.com)] On Behalf Of Peter Cowan
Sent: Friday, September 24, 2021 9:53 AM
To: kolb-list(at)matronics.com (kolb-list(at)matronics.com)
Subject: Reducing power on take off versus thrust line.
 
This topic came up some months back and there was quite a good analysis of the forces involved so I was surprised when John mentioned that after going to his higher horsepower engine he seemed to be okay with having to reduce 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 experienced 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 discovered, with the primary source being a model airplane design site, that providing upthrust on a high, aft mounted engine was exactly the reverse of what was 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 DOWN 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 handling 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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rhalstrick(at)gmail.com
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PostPosted: Tue Sep 28, 2021 3:24 am    Post subject: Reducing power on take off versus thrust line. Reply with quote

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


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chic



Joined: 12 Apr 2019
Posts: 46
Location: Riverside Calif

PostPosted: Mon Oct 04, 2021 9:47 pm    Post subject: Re: Reducing power on take off versus thrust line. Reply with quote

Okay, so I am a little confused here..... If I raise (just) the rear of the engine on my Firestar 2 for more prop clearance off the boom tube, am I adding positively or negatively to the thrust line?

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Richard Pike



Joined: 09 Jan 2006
Posts: 1649
Location: Blountville, Tennessee

PostPosted: Tue Oct 05, 2021 6:36 am    Post subject: Re: Reducing power on take off versus thrust line. Reply with quote

Reading this thread jarred loose a memory from back in the 70's when guys were first starting to put gokart engines on Quicksilvers - Don't remember who it was that said it, but I do remember it was an article in Glider Rider. (Anyone else used to get that one?)
Basically it was that you wanted the engine thrust line to have the prop perpendicular to the horizon when the wing was at it's normal angle of attack in level flight.


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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: Tue Oct 05, 2021 7:20 am    Post subject: Reducing power on take off versus thrust line. Reply with quote

If you pick up the rear, it will push the nose down. Imagine picking up the rear about 2 feet.

On Tuesday, October 5, 2021, 01:48:07 AM EDT, chic <gcechini(at)msn.com> wrote:




--> Kolb-List message posted by: "chic" <gcechini(at)msn.com (gcechini(at)msn.com)>

Okay, so I am a little confused here..... If I raise (just) the rear of the engine on my Firestar 2 for more prop clearance off the boom tube, am I adding positively or negatively to the thrust line?

Read this topic online here:

http://forums.matronics.com/viewtopic.php?p=503414#503414

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lanejones(at)outlook.com
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PostPosted: Tue Oct 05, 2021 8:37 pm    Post subject: Reducing power on take off versus thrust line. Reply with quote

I think maybe it would be a good idea to define front and rear of engine. It sounds to me like everyone doesn’t have the same understanding.
Lane Jones
Kolb Mark III xtra 912 rotax

From: owner-kolb-list-server(at)matronics.com [mailto:owner-kolb-list-server(at)matronics.com] On Behalf Of william sullivan
Sent: Tuesday, October 05, 2021 11:20 AM
To: kolb-list(at)matronics.com
Subject: Re: Kolb-List: Re: Reducing power on take off versus thrust line.

If you pick up the rear, it will push the nose down. Imagine picking up the rear about 2 feet.



On Tuesday, October 5, 2021, 01:48:07 AM EDT, chic <gcechini(at)msn.com> wrote:





--> Kolb-List message posted by: "chic" <gcechini(at)msn.com (gcechini(at)msn.com)>



Okay, so I am a little confused here..... If I raise (just) the rear of the engine on my Firestar 2 for more prop clearance off the boom tube, am I adding positively or negatively to the thrust line?









Read this topic online here:



http://forums.matronics.com/viewtopic.php?p=503414#503414













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PostPosted: Wed Oct 06, 2021 4:20 pm    Post subject: Reducing power on take off versus thrust line. Reply with quote

My comment referred to picking up the prop end of the engine- the end closest to the tail.

Bill Sullivan

On Wednesday, October 6, 2021, 12:37:22 AM EDT, lane jones <lanejones(at)outlook.com> wrote:





I think maybe it would be a good idea to define front and rear of engine. It sounds to me like everyone doesn’t have the same understanding.
Lane Jones
Kolb Mark III xtra 912 rotax

From: owner-kolb-list-server(at)matronics.com [mailto:owner-kolb-list-server(at)matronics.com]On Behalf Of william sullivan
Sent: Tuesday, October 05, 2021 11:20 AM
To: kolb-list(at)matronics.com
Subject: Re: Re: Reducing power on take off versus thrust line.

If you pick up the rear, it will push the nose down. Imagine picking up the rear about 2 feet.



On Tuesday, October 5, 2021, 01:48:07 AM EDT, chic <gcechini(at)msn.com> wrote:





--> Kolb-List message posted by: "chic" <gcechini(at)msn.com (gcechini(at)msn.com)>



Okay, so I am a little confused here..... If I raise (just) the rear of the engine on my Firestar 2 for more prop clearance off the boom tube, am I adding positively or negatively to the thrust line?









Read this topic online here:



http://forums.matronics.com/viewtopic.php?p=503414#503414













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chic



Joined: 12 Apr 2019
Posts: 46
Location: Riverside Calif

PostPosted: Thu Oct 07, 2021 7:44 am    Post subject: Re: Reducing power on take off versus thrust line. Reply with quote

What I meant by the "REAR" of the engine is the prop or tail facing end. I want to put a couple of 1/2" spacers under the mount for boom tube clearance in swinging a 68" prop. Will that adversely effect thrust angle and if so, in what way? Thanks, Chic

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