Forces and Moments

The propeller's main purpose is to create thrust in a direction parallel to the propeller's axis of rotation. This thrust force is directly related to how much the propeller accelerates the air that's within its reach. We have all stood behind a running propeller and felt the wind that it creates but rarely are we able to see or feel the rotating motion of the slipstream behind the prop. This swirling of the air in the propeller's wake is one of the things that can affect our airplane's handling qualities especially at low airspeeds and high power settings. So what does this "spiraling slipstream" really do to the airplane? One of the most visible effects is the tendency of the airplane to yaw nose-left when the engine is at full throttle and the airspeed is near zero. This can happen on takeoff, at the top of a Hammerhead, when you get too slow on a vertical upline and especially when your doing 3D maneuvers. Have you ever held a 40-size airplane vertical while setting the high-speed needle valve? Try it some time and see if you can feel the yawing moment created by the prop's slipstream when you pulse the throttle. If you don't have right thrust in the setup it will tend to yaw the airplane's nose to the left ...but why? If you take a look at your airplane you'll notice that for the most part it's a symmetrical animal when viewed from the front or the top but when you look at it from the side you see that this symmetry disappears. We immediately notice that the vertical tail doesn't have a mirror image like the wing and horizontal tail. Try to mentally visualize the spiral slipstream that develops behind a clockwise (as seen from the cockpit) rotating propeller and follow its helical path until the propeller wake reaches the vertical tail. The streamlines in the slipstream will be shaped like the coils of a spring. The faster you fly the more the spring will appear to be stretched and the smaller the angle of attack of the vertical tail. If you look at the approaching wake from the vertical tail's point of view you will see the air approaching from the left. All of the vertical tails that I'm familiar with have symmetrical airfoils and must at some angle of attack to produce a force.

[Figure 1] shows two streamlines being shed off the propeller and the angle of attack seen at the vertical tail due to the path of the propeller's wake. This local angle of attack causes the vertical tail to produce a side force behind the center of gravity, which results in a nose left yawing moment. Notice what would happen if the vertical tail stuck out the underside of the airplane instead of the top (dotted streamline). The force would then be in the opposite direction and tend to cause the nose to yaw to the right. This same nose right yawing moment would happen if your propeller spun in the opposite direction (clockwise) like it does on the full-scale Sukhoi. I would dare to say that the spiral slipstream is by far the most visible propeller effect that we deal with while flying aerobatics. The spiral slipstream is also the ONLY reason we need to put right thrust in our engines. This effect is pretty much the same whether the airplane is flying upright, inverted or on its side. It always tends to make the nose yaw to the left and to compensate for this we offset or angle the engine's centerline. This solution to offset the engine is not a perfect one but the side effects are small. We typically choose to alter the angle of the engine rather than slide the engine over to one side because the moment arm we can generate is much greater that way. When we angle the engine we create some amount of side force that typically makes the pilot use less right rudder than left in knife edge. What if your design had a symmetrical vertical tail assembly (same amount of vertical tail above the CG as below)? With this arrangement you could eliminate the need for right thrust altogether. While playing around in the wind tunnel with various propeller-powered models, I've been able to add sub-rudders and underbody fins that removed the spiral slipstream effect. Too bad we don't see any symmetrical vertical tails on full-scale aerobats...I suppose that could make takeoff and landing a real hassle.