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james.vanlaak(at)gmail.co
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 Posted: Tue Oct 05, 2021 12:22 pm Post subject: Thrust line |
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I have been biting my tongue on this thread for a while but have to say a few things. And yes, I am an engineer.
Remember that thrust losses from pitching the engine are going to be very small. The thrust loss varies with the cosine of the angle. So for small angles the loss is small.
The pitching force of the airplane results from the sum of the forces acting on it at that moment. The sum yields a resultant force vector and its relationship to the center of gravity tells whether it is up or down.
On takeoff, there is negligible aerodynamic drag so the thrust line is a major force to accelerate the airplane. If it is not pushing through the center of gravity there will be a pitching moment. On Kolbs, it is obviously down.
During the takeoff roll, there is a force from the rolling friction of the wheels. It is obviously well below the center of gravity, so it also pitches down.
Early in the takeoff the drag on the wing is relatively low so its contribution is fairly low, but as airspeed increases it grows. At maximum level speed the drag of the wing is a major part of the total drag that obviously equals the thrust from the engine (hence maximum level speed). Other sources of drag, like the fuselage and landing gear legs will tend to pitch nose down. But remember that the thrust of the engine on a Kolb is pushing pretty close to the centerline of the wing, so those two forces are largely offsetting.
Finally, the downforce on the tail is what we use to balance the rest. If we had the horizontal tail in the wake of the prop we would see a downforce proportional to the power setting. High power with high tail downforce. Look at certified pusher prop planes, mostly amphibians. Lake Amphibian, Seabee, and others all have big tails in the middle of the propwash. There is a reason for that.
I built an original Firestar and owned a Mark 3X. Both were nice flying airplanes with large pitching moments. I also owned a Buccaneer amphibian, Rans S-12, Sportflight Talon, Drifter, CGS Hawk, and Super Hornet. All had similar but different pitching responses. I thought the S12 was the worst, but the Buccaneer was close behind.
In general it is not hard to learn to control this, but it does require attention. A power loss on takeoff may require a strong nose down push to keep from stalling. If you forget that you will be in trouble. I never worried about it for myself because I flew the planes a lot, but I did not want to loan them to someone who was not familiar with the characteristics.
Frankly, I love pushers but not this pitching moment. Someday I will build a custom machine with the horizontal tail in the slipstream to minimize the effect.
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jaydub
Joined: 07 Jan 2020
Posts: 74
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| Posted: Tue Oct 05, 2021 12:37 pm Post subject: Thrust line |
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"Someday I will build a custom machine with the horizontal tail in the slipstream to minimize the effect."
I was glad to see you had a CGS Hawk. They still have a high thrust line but the curved boom tube was to get the tail into the prop wash so you had some response once you added some power. Also, in my Hawk the engine sat a lot lower than on the Firestar 2 I'm building. My prop centerline was about even with bottom of the wing on my 1991 Hawk Arrow. The drag on the landing gear definitely gave a pitching down moment but I'm surmising the Firestar I'm building will be a lot worse since the engine will sit so high in comparison.
Jay
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Jerry-TS-MkII
Joined: 23 Aug 2017
Posts: 79
Location: Indiana
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| Posted: Tue Oct 05, 2021 3:26 pm Post subject: Thrust line |
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Very Weird! I only see and reply to the list via my email. I no longer
log on. I only saw the response from James Vanlaak JUST NOW.. and very
much appreciate what he wrote! What I found odd, was having not seen
his post before I wrote mine! So I was not trying to trump his words.
Thanks James for the much better (technical) description!
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