While these fixes above can help, some may introduce other slight problems. Since I started using downthrust, Hunting has all but disappeared for me.
I don't like to have any looseness in my controls, though back in the late 50's, we all thought that was the solution. It worked slightly, helped, but didn't cure. Still better than nothing though. Later on, I used to like a thicker stab than the elevator. Of course, the idea was to help cure hunting by blanking out the elevator authority around neutral. It also worked slightly. Here's where one of those slight problems comes in. We began noticing a tendency to develop a flat spot at the intersections of the Eights. Equalizing the thickness, tightening the controls, and some downthrust seemed to cure a lot of problems when I finally had them all in place.
I have a theory why downthrust seems to work, and why you don't use upthrust when you switch to a pusher prop.
I was thinking about why downthrust, tight controls, with no slop or delay, seemed to work for me. While deep in thought, I remembered the lessons from full-scale flight school. I'm not trying to say that I am a great pilot in a Cessna, I soloed, but never got my license. it's just the early lessons on what makes an airplane fly help lend a possible solution to our hunting problems.
Remember the four forces that affect an airplane in flight, Gravity, Lift, Drag, and Thrust? These are solidly connected with each other and must balance out to attain smooth LEVEL flight. This relationship exists in all objects while in flight. I also remembered that, as far as I know, all single engine full-scale airplanes use downthrust. Why, was my question. There are more possible answers than I realized. Also, there're lots of bits and pieces happening up front when you try to set, or trim, for smooth and level flight, but achieving, and maintaining level light may well be the greatest benefit of downthrust.
First, we should all agree that a positive angle of attack is needed to create lift. Why? The AOA is locked and related to the other 3 forces, the amount of the angle is controlled by the weight, (Gravity) and includes how much power from the Thrust is needed to overcome Drag, which couples with Lift to create, through speed and angle of attack, enough lift and thrust to overcome drag and gravity. When that occurs, you are in a stable flight envelope. If the thrust and flight surfaces are at 0-0-0 incidence, all the speed you can attain in your vehicle will just make it go faster. Unless there's lift component in the picture, that dog won't fly.
But that, I believe, is only a part of the problem. If I remember flight school correctly, when establishing a level flight cruising altitude, you have to set your speed first when you get to the altitude you want. That Cessna has built in downthrust, but you'll often find that you need to trim your elevator, usually just enough to balance out the forces. We won't go into rudder effects because our models are flying in a circle. When the plane is trimmed out, you can climb or descend just using the throttle. Interesting, speed, thrust, can cause the plane to raise or fall from the increase, or loss of that thrust.
Now, after all that, I'm finally going to get to the point.
Here's our special stunt ship, cruising along in level flight. Speed, about 55 mph, altitude, 4-6 feet. Like most well-built and designed modern stunters, it's been built and set up with 0-0-0 incidence. The thrust line, chord line of the wing, and chord line, of the stab-elevator, are all with the same incidence relative to each other. So why does it try to hunt?
Here's where my theory comes into play.
Remember, lift is created by the AOA, which is locked in with Gravity, Thrust, and Drag. Because of the 0-degree thrust line, our airplane will wind up flying with the nose up, by whatever degree the AOA is, making the thrust vector slightly up also. If your incidences are set at 0-0-0. Your airplane has a tendency to want to climb slightly if anything even slightly upsets any one of the four forces. I believe this has relevance with Dave Fitzgerald's published description of the positive effects of a bit of positive stab incidence. It creates a slight downward pressure at the nose to help keep the nose from rising with every slight upset in one of the four forces.
Better, in my thinking, is to do as the full-scale guys do, and add some downthrust. In fact, I've had good results using about 2 degrees of downthrust and a little positive stab incidence. The thrust line now is more closely aligned in the direction of flight and there's a small component of down pressure on the nose that couples to the speed of flight. The stab.
Due to being confused over the prop and gyro effects, I thought that these were the forces causing the nose to rise when using a tractor prop. This made me believe that I would need to use upthrust when using a pusher prop because the prop effects would be reversed. I was called on that mistaken belief. I was sure I was right, so I took a deep dive into the subject. After some time, and finally using my computer and AI to get to the answer, I found I was wrong. Those prop effects are mostly showing up in yaw, left or right based on the direction of rotation. Yaw also shows when the nose is raised or lowered. The Rabe Rudder was developed to help counter the nose-in yawing from deploying down elevator.
So, it's my belief that since much of the hunting problem is from creating lift, stabilizing, balancing out level flight at our cruising speed, and has little or no bearing on the direction of prop rotation. Upthrust is wrong for planes using a pusher prop. Downthrust is the solution since you must have a positive AOA to create lift regardless of the direction of prop rotation.
It was Bob Whitely and his article, "Things that always work." that began the process of opening my eyes. So far, he's been right.
Topic: Porpoising ? (Read 5533 times)